THE AMERICAN JOURNAL

ANATOMY

EDITORIAL BOARD

CHARLES R. BARDEEN,
University of Wisconsin.
HENRY H. DONALDSON,
Wistar Institute of Anatomy.
THOMAS DWIGHT,
Harvard University.
JOSEPH MARSHALL FLINT,

University of California.

SIMON H. GAGH,
Cornell University.

G. CARL HUBER,
University of Michigan.

GEORGE S. HUNTINGTON,
Columbia University.
FRANKLIN P. MALL,
Johns Hopkins University.
J. PLAYFAIR McMURRICH,
University of Michigan.
CHARLES S. MINOT,
Harvard University.
GEORGE A. PIERSOL,
University of Pennsylwania.
HENRY McH. KNOWER, SECRETARY,
Johns Hopkins University.

VOLUME VI
1906-1907

THE AMERICAN JOURNAL OF ANATOMY
BALTIMORE, MD., U.S. A.

The Lord Baltimore Press

BALTIMORE, MD., U.S. A.

pep bro eels

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JU

TAY

VI:

Vit;

CONTENTS OF: VOLIWE

JosePH MarsHaut Frinr. The Development of the Lungs
With 4 Plates and 29 Text Figures.

GrorcE L. StreeTeR. On the Development of the Mem-
branous Labyrinth and the Acoustic and Facial Nerves
in the Human Embryo.

With 2 Plates and 8 Text ih eunes

Benson A. Conor. The Finer Structure of the Glandula
Submaxillaris of the Rabbit
With 6 Figures.

Harry Lewis Wireman. The Relation Between the Cyto-
reticulum and the Fibril Bundles in the Heart Muscle
Cell of the Chick

With 2 Diagrams Ana) 17 Hiswves

Basin C. H. Harvey. A Study of the Structure of the
Gastric Glands of the Dog and of the Changes Which
They Undergo after Gastroenterostomy and Occlusion of
the Pylorus set) ay» Shea Ss

With 5 Figures.

Witsur L. Le Cron. Experiments on the Origin and Dif-
ferentiation of the Lens in Amblystoma
With 5 Plates.

CuHarLes R. BarpEEN. Development and Variation of the
Nerves and the Musculature of the Inferior Extremity
and of the Neighboring Regions of the Trunk in Man.

With 10 Plates and 7 Text Figures.

139

= 167

aebon

. 207

. 245

. 259

lv Contents

VIII. G. Cart Huser. The Arteriole Recte of the Mammalian
Kerdmey. 0 oe ee eke eat ee, ies ie re
With 4 Text Figures.
IX. J. Piayratr McMurricu. The Phylogeny of the Plantar

MimSGubatatels ses. c) oe yee) bk 5 Ms. cee ee
With 9 Text Figures.

X. Epsren C. Hitt. On the Gross Development and Vascular-
imadOrokmtheesbhis:. {2° 3, . 4°.) eee mesos
With 14 Text Figures. —

XI. Warren Harmon Lewis. Experimental Evidence in Sup-
port of the Theory of Outgrowth of the Axis Cylinder. . 461
With 21 Figures.

XII. Warren Harmon Lewis. Experimental Studies on the
Development of the Eye in Amphibia. III. On the
Origin and Differentiation of the Liens. 2 73 55 Ge
With 83 Figures.

XIII. Cuartes R. Stocxarp. The Embryonic History of the
Lens in Bdellostoma Stouti in Relation to Recent
EIR PeRIMEMISS eo ek kn. ss. <+ 3s, Seen eg enc
With 3 Text Figures.

The first four numbers of THe ANAToMIcAL RecorD were
issued with this volume of The American Journal of
Anatomy. ‘THE PROCEEDINGS OF THE ASSOCIATION OF
AMERICAN ANATOMISTS, Twenty-first Session, December,
1906, and Twenty-second Session, March, 1907, are in-
cluded in Nos. 8 and 4 of the Recorp.

THE DEVELOPMENT OF THE LUNGS.

BY
JOSEPH MARSHALL FLINT, M.D.,

Professor of Anatomy in the University of California.
(From the Hearst Anatomical Laboratory of the University of California.)

WITH 4 PLATES AND 29 TEXT FIGURES.

It requires only a cursory inspection of the literature on the lungs to
show the unsatisfactory state of our knowledge concerning the develop-
ment of these organs. In the first place, the ontogeny and phylogeny
of the mammalian lungs have stood in apparent conflict. There are,
moreover, few features of their anatomy upon which there is any agree-
ment among the various investigators who have contributed to this field.
As a reworking ‘of the entire subject has seemed desirable, the author
was guided in choosmg the pig, first of all, by the practically unlimited
supply of the different embryonic stages and, secondly, by the fact that
the artiodactyls possess in well developed form, all of the most discussed
types of bronchi.

METHODS.

For the study of the early stages of the development of the respiratory
system, the Born reconstruction method was employed. Fruitful sug-
gestions for its use have been obtained from the contributions of Bar-
deen and Huber, whose applications of the Born method have been
followed in this study. Sections of a series of pigs were cut at 20 micra
and stained in hematoxylin and congo red. The reconstructions were
made at a magnification of 100 diameters. In order to obtain an ac-
curate orientation of the subdivisions of the bronchi, the piling of the
plates according to the external form of the lung was controlled by
dissections of the lungs of a series of embryos of a corresponding age as
those, used for reconstruction after the method suggested by Minot.
Liberal use has been made of the various corrosion methods to follow
the evolution of the bronchial tree in pigs from 4 cm. to those of adult
life. The use of Wood’s metal and of celloidin corrosions gave fruitful
results, although the majority of the stages were obtained by the use

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

2 The Development of the Lungs

of celluloid corrosions. For this purpose celluloid is dissolved in ace-
tone and injected from aspiration bottles into the lungs through the
trachea. Like the cellodin corrosions these were digested or macerated
in concentrated hydrochloric acid. The advantage of celluloid over cel-
loidin casts lies in the fact that the former, hke Wood’s metal, may be
left in the air and handled freely without the disadvantages of the
glycerine bath, which often makes it either difficult or impossible to
study certain parts of the celloidin preparations. For the study of the
development of the respiratory lobules a combination of celluloid and
Wood’s metal preparations proved most advantageous. Preparations of
the entire embryonic !ung cleared in oil of cloves were also found ser-
viceable as control preparations for the reconstructions. They are, how-
ever, of doubtful value save for this purpose as the young dorsal and
ventral buds on the stem bronchus are almost invisible until they have
reached a considerable size.

The organogenesis was followed in a series of stained sections from
embryos and lungs hardened in Zenker’s fluid and stained by Mallory’s
method. At the period of birth the alveoli were distended by injecting
them, under low pressure, with Zenker’s fluid, thus obviating the obscure
and uncertain pictures which are obtained when the lung is collapsed
and contracted. In following the development of the epithelium, the
well-known silver nitrate method has been used.

REVIEW OF THE LITERATURE.

To von Baer, 28, we are indebted for the first description of the de-
development of the pulmonary apparatus. In the chick it consists of two
small hollow swellings about the middle of the head gut, which ap-
pear on the third day. These projections give rise to the lungs, while
the hollow cavities represent the rudiments of the bronchi although
the trachea up to this time is unformed. On the fourth day the lungs,
still in connection with the «esophagus, he more ventralwards, but the
bronchi in growing backwards have dilated into small sacs. Anteriorly,
however, the bronchi join each other at an acute angle and terminate in a
short canal, the anlage of the trachea which communicates with the ceso-
phagus behind the pharynx. These observations were amplified by the
work of Remak, 55, Selenka, 66, Gotte, 67, and especially His, 68, who be-
lieves the larynx and trachea arise from a ventral groove in the head gut.
Caudalwards, this structure has two lateral projections representing the
rudiments of the bronchi which are bilateral and paired in contra-
distinction to the unpaired anlage of the larynx and trachea. Less in

Joseph Marshall Flint 3

accordance with our modern ideas on the development of the lungs are
the papers of Rathke, 28, and Seessel, 77, while more recent contributions
are those of Fischelis, 85, and Kastschenko, 87. The work of the latter
has been especially emphasized by Weber and Buvignier, 03, who support
his views on the serial homology of the lungs with the branchial
pouches. They believe, from their work on the duck, that in birds as
well as mammals the anlage of the lungs are paired derivatives of the
respiratory tube. The lungs, therefore, while not representing actually
existing branchial pouches, indicate the reappearance of endodermic
evaginations of the head gut which has carried gills among the ancestors
of vertebrates.

The study of the development of the amphibian and reptilian lung
was taken up somewhat later when Rathke, 39, in Coluber natrix de-
scribed its appearance from paired projections from the head gut. He
states that’ the right lung increases in size until it is larger than the
stomach while the left remains, in consequence of regressive changes, as a
slight appendix of the trachea. Baumann, o2, in Tropidonotus natrix
confirms these observations of Rathke by finding the right lung is three
times larger than the left in an embryo 3 mm. long, while at 5 mm. it is
some forty times larger. But he is inclined to believe, however, that the
discrepancy in size is due to arrested development of the left lung sac
rather than a true regressive process. Betrachians were studied by
Remak, 55, who found the first rudiments as paired buds from the
head gut passing laterally and caudally, while Gétte, 75, describes
the origin of the lungs in Anura from endodermal projections imme-
diately behind the last branchial pouch. Gé6tte, in Anura, suggested
the possibility of transformed branchial pouches taking part in the
formation of the lungs, before Kastschenko described the origin of the
avian lung from the respiratory tube. Naturally, the observations of
Gotte, like those of Kastschenko, are supported by Weber and Buvig-
nier, 03, while Gétte, 04, himself, more recently reaffirms that theory.
Greil, 05, however, who also worked on Anurans comes to the opposite
conclusion from these investigators. Primitively the lungs appear, ac-
cording to Greil, in the form of two bilaterally symmetrical grooves in the
ventral wall of the heat gut about the time the first four gill pouches are
formed. The fifth and sixth pouches appear later and are separated
from the lung anlage by an appreciable space which is greater than the
interval between the individual pouches. He concludes, therefore, that
the gill pouches have nothing whatever to do with the formation of
the lungs. In subsequent stages the pulmonary grooves deepen and are
covered with a thickened splanchnopleure to form the primitive lung sac.

4 The Development of the Lungs

Between these structures a transverse gutter appears, while the portion
of the head gut anterior to this, produced by the narrowing of its lateral
walls, forms a longitudinally placed laryngo-tracheal groove, which gives
rise to the trachea and larynx. The separation from the cesophagus
then begins at the caudal extremity and proceeds forwards.

Among the earlier investigators there was an apparent unanimity of
opinion that the subsequent differentiation of the amphibian and reptil-
ian lung was due to a centripetal ingrowth of septa from the lung wall
dividing and subdividing the primitive lung cavity into a series of smaller
peripheral spaces. Furthermore, as early as the middle of the last century
Leydig, 57, taught that the complicated lungs of the higher vertebrates
represented a complex of a series of simpler lungs, or, in other words,
that the infundibulum of the mammalian lung might be compared with
an entire frog’s lung with its parietal alveoli. Miller, 93, in a compara-
tive study of the reptilian, avian, and mammalian lung, states that the
complexity of the reptilian lung is due to a system of septum formation
while the process of budding plays a secondary réle. In the avian lung,
however, budding becomes more important and septum formation is
secondary. Thus Miller looked upon the avian lung as a transition stage
between the reptilian lung with its septum formation and the mammalian
lung produced by the budding process.

In an extensive study of the dried lungs of adult reptiles Milani,
94, 97, emphasizes the importance of septum formation for the differen-
tiation of the pulmonary apparatus as one ascends the animal scale. The
formation and enlargement of primary septa upon the dorsal and ventral
walls of the lung cavity which extend horizontally from the median to
the lateral wall of the lung as well as the further subdivision of these
spaces by secondary septa is responsible for the gradual evolution of the
complex from the simple lung.

Ever since the work of Kolliker, 79, the architecture of the mammalian
lung has unanimously been conceeded by all who have worked upon the
embryonic stages to rest upon a process of centrifugal budding. The
centripetal formation of septa, apparently, plays no part in its evolution.
There has been, therefore, a great gap between the developmental pro-
cesses in the reptilian, amphibian, and avian lung, on the one hand, and
the mammalian lung on the other, for, as Gegenbaur has pointed out,
ontogeny and phylogeny have apparently stood in conflict, as the pul-
monary apparatus in the ancestors of the mammals was produced by a
process exactly opposite to that which ontogeny shows is responsible for
the growth of the mammalian lung.

The first work which has offered us a suitable explanation of this

Joseph Marshall Flint 5

apparent discrepancy between the ontogeny and phylogeny of the mam-
malian lung is that of Moser, 00, who, in studying the comparative
embryology of the respiratory apparatus in vertebrates, comes to the
important conclusion that all vertebrate lungs are formed by a common
growth process. In birds the respiratory apparatus is developed from
a projection of the head gut, and its bronchial system results solely from
a process of budding. In reptiles the growth process is exactly like
that in birds, namely, by a bronchifugal system of sprouts while the
septa are produced by relatively resistant points in the lung wall remain-
ing between two of its outgrowing portions. This same method of
growth, furthermore, is again repeated in a less localized and more diffuse
form in amphibians where it gives rise, in the first place, to the dilated
lung cavity, and, later, to the semispherical projections on the peripheral
wall of the lung. In amphibia, as in reptilia, septa are formed by more
resistant points in the lung wall remaining between two projecting por-
tions.

Any doubts of Moser’s method or results seem to be effectually silenced
by the appearance of Hesser’s, 05, careful and convincing paper on the
development of the reptilian lung. MHesser finds the endodermal anlage
of the reptilian lung appearing as a fold projecting from the head gut
immediately behind the last gill pouch. This separates from the cesopha-
gus in a caudocranial direction. From the cranial portion the trachea
is formed, while the caudal part gives rise to the bronchi. ‘The latter
grow out as long, narrow tubes, at first in a dorsolateral direction, and,
later, parallel to the median plane of the embryo. In the lizards, the
bronchi begin to widen at the lateral side, making a sharp distinction
between the extra-pulmonary bronchus and the future lungs. In species, —
however, where there is no extra-pulmonary bronchus, the dilatation
affects the whole tube. We have then, at this stage, a respiratory anlage
consisting of a long narrow trachea with two narrow bronchi arising from
it. These terminate in two enlarged primitive lung sacs. At this point
the inner surface of the lung becomes complicated by the more rapid
growth of certain portions of the wall of the lungs by a hernia-like pro-
duction of buds. This process begins in a cranial portion of the lung
and proceeds gradually to its caudal extremity until finally a large num-
ber of buds surround the sac. In Tarentola, the most prominent group
appears along the dorsal side of the stem bronchus, while the remaining
sprouts occupy transverse rows alternating with the dorsal series.

While, in lizards, the stem bronchus is dilated, in turtles (with the
exception of the caudal end which contains a large lumen) it remains a
relatively small tube. The bronchi grow to considerable length before

6 The Development of the Lungs

branches appear. These are produced by buds or hernial projections from
the wall of the bronchus. Upon the stem bronchus are produced, ac-
cording to Hesser, a lateral and medial row of buds, a result in which he
is not in accord with Moser, who believes that there are three series, a
lateral, ventral, and dorsal. Especially noteworthy is the fact that in
land turtles the lateral bronchi form dilated sacs which later grow into
wide ducts, while in the sea turtles the buds grow out as small tubes
somewhat dilated at the ends.

The question of the unequal development of the snake’s lung has re-
cently been taken up again by Schmalhausen, 05, who finds in Tropi-
donotus natrix an unpaired pulmonary anlage. From its caudal end,
appears later the two projections for the lungs, which grow unequally
but continuously throughout embryonic life. The enormous overgrowth
of the right lung leaves the left as a slight appendix upon it. There is,
apparently, no regressive change such as Rathke supposes takes place
in Coluber natrix. Schmalhausen’s observations support Baumann’s
supposition on this point. More important, however, is a still further
confirmation of the work of Moser and Hesser as the lung of Tripido-
notus natrix grows not through the development of axipetal septa pro-
duction but from an outward budding of the lung wall.

In view of these researches of Moser, Hesser, and Schmalhausen, then
we may look upon respiratory apparatus of vertebrates as the resultant
of a common principle of growth, and, in turning to the consideration
of the ontogeny of the mammalian lung, there is good ground for believ-
ing that its developmental processes no longer conflict with its phylo-
geny. The evolution of the pulmonary system of mammals was first
studied by Kolliker, 79, who traced the development of the organ in
rabbits. It appears from an unpaired anlage which arises behind the
gill pouches. This is produced by longitudinal furrows which separate
the head gut into a dorsal and ventral portion from the latter of which
the lungs arise, while the former forms the cesophagus. On the tenth
day, the lower part widens so that the lung anlage forms a half canal
which ends caudalwards in two round depressions. Through a longi-
tudinal fissure, the anlage is still in communication with the cesophagus,
while both structures are surrounded by a mass of mesoderm. The
projections forming the rudiments of the lungs grow rapidly and bend
dorsalwards, and, at the same time, the trachea and cesophagus begin
to separate. This process starts at the posterior end of the juncture and
progresses towards the head.

A few years later Uskow, 83, confirmed Kolliker’s observations on
the rabbit by finding on the tenth day evidences of separation of the

Joseph Marshall Flint 7

head gut into dorsal and ventral portions. From the ventral segment
arises the respiratory system, while the dorsal is transformed into the
cesophagus. About the level of the sinus venosus, the lungs appear as an
unpaired dilatation of the ventral section and, synchronously, the trachea,
also unpaired, is developed from the head gut just above it. Although
the two structures appear simultaneously, the anlages, according to
Uskow, are quite independent. Fol, 84, finds the origin of the lungs in
a human embryo 5.6 mm. long as lateral diverticule on the head cut just
behind the series of gill pouches. He is inclined to believe with Gotte
in the transformation of the last pair of gill pouches which have dis-
appeared in the phylogeny of vertebrates into the respiratory apparatus.

His, 87, recognized the anlage of the human lung before the flexion
of the embryo, that is to say, about the third week. It appears as a
groove in the ventral part of the anterior segment of the intestine which
becomes flattened just below the Fundus branchiales into a sagittal
fissure and divides into an anterior and posterior half. From the former
the trachea is formed, while the latter develops into the cesophagus.
The respiratory portion begins above as a groove and ends below at the
level of the auricles in a widened projection. From the latter, the lungs
are evolved, while the former yields the trachea. At first, there is no
medial division of the unpaired anlage which, save through the thickness
of its epithelial lining, it is difficult to differentiate in the early stages.
At the end of the first month the separation of the trachea from the
cesophagus, beginning at the caudal extremity and proceeding upwards,
is complete. And, as this separation takes place, there is a bilateral
division of the anlage, which yields the primitive bronchi. These bend
sharply dorsalwards, like a horseshoe, to embrace the cesophagus. The
dilated primary lung sacs formed on these divisions are asymmetrical,
the cause of which is probably to be sought in the first anlage of the
lungs, which, according to His, does not show bilateral symmetry.

Up to this time Kélliker, Uskow, and His have agreed in their
observations that the respiratory apparatus of mammals is derived from
an unpaired anlage, but Willach, 88, in following the pulmonary system
- of the mole believes the trachea arises from an unpaired anlage, while the
lungs originate as paired structures. The asymmetry of the anlage
according to Willach is probably responsible for the greater development
of the right over the left lung. In rats and mice, the process of develop-
ment as described by Robinson, 89, agrees, in general, with the results
obtained by His, Kélliker, and Uskow. Stoss, 92, and Bonnet, 92, in
the study of sheep give results which accord with the findings of Uskow
and Kolliker in rabbits, while Minot, 92, in his account of the evolution

8 The Development of the Lungs

of the pulmonary system in man, differs from His in looking upon the
first anlage as symmetrical. Its subsequent asymmetry Minot believes
is due to the unequal development of the heart.

In sheep, Nicholas and Dimitrova, 97, find by the reconstruction
method in an embryo of 5 mm. the main bronchi resulting not from a
bifurcation of the primitive pulmonary projection, but as asymmetrical
buds on its lateral face. Later stages, 7-9 mm., show an exaggeration
of the precocious asymmetry as the right side is considerably more
developed than the left, and the two primitive bronchi with the trachea
form an inverted T.

Narath, o1, followed the development of the lungs in rabbits and
guinea pigs. In the latter, the development begins as a lateral flattening
of the head gut just under the Fundus branchiales. This process con-
tinues until the lumen of the head gut forms a sagittal fissure just above
the lower anlage, which, as it passes upwards, soon resumes its rhom-
boidal form. The ventral groove deepens and thickens, while, at the
same time, the dorsal groove becomes narrower. Lungs and trachea
arise from the ventral, while the dorsal part yields the cesophagus.
Somewhat later a longitudinal furrow separates the two and the pro-
jection at the most caudal portion of the ventral groove, forming the first
unpaired anlage of the lungs, shows a slight asymmetry as the right
side is somewhat larger than the left. The lung anlage increases in
size, ventrally, but even more markedly to the right and left. These
two outgrowths, the anlage of the bronchi, show different relationships,
as the right bends dorsally and caudally, while the left remains practi-
cally transverse. About this time begins the separation of the trachea
from the cesophagus, which proceeds in a caudocephalic direction until
the mesoderm surrounding the lung sacs not only projects into the cavity
of the ccelom, but also passes in and separates the respiratory from the
digestive portion of the head gut. The end of the lung sacs dilate, while
still maintaining a marked asymmetry and, as this takes place, they ex-
tend dorsalward and embrace the cesophagus. In the development of the
cat’s and rabbit’s lung, the transformation in general agrees with the
conditions in the guinea pig so that Narath finds himself in accord with
the earlier researches of Ko6lliker and Uskow, who also worked on the
latter animal. Somewhat later Weber and Buvignier, 03, in a com-
parative study of the origin of the lungs, especially in Muinopterus
Schreibersii, followed, by the reconstructive method, the lateral flatten-
ing of the post branchial region of the head gut. They describe a bran-
chial crest, which descends from the last pair of gill pouches and ter-
minates just before reaching the region in which the pulmonary appa-

Joseph Marshall Flint 9

ratus appears. The latter is formed from two asymmetrical thickenings
of the lateral wall of the head gut, the left of which appears first in an
embryo with 18 primitive vertebrae a little- below and ventralwards to
the last trace of the branchial crest. A constructive process, which these
authors hypothecate, isolates the entire ventral segment of the head gut
carrying with it the rudimentary lungs and extending as far cephalad
as the last gill pouches. Weber and Buvignier obviously abandon the
idea of the primitive unpaired anlage described by Kolliker, Uskow, and
His, and with it the conception of a pulmonary groove formed syncro-
nously with or before the lungs. Thus, the trachea is post-pulmonary in
origin and is formed by this constructive process involving the ventral
part of the head gut in the region behind the gill pouches. Like Gotte
in Anura, Kastschenko in the chick, and Fol in man, Weber and Buvig-
nier look upon the pulmonary apparatus as diverticule of the head gut
serially homologous with gill puoches.

Very briefly Blisnianskaja, 04, describes the paihes of the human
lung as a projection in the ventral portion of the foregut, which, in an
embryo of 4.5 mm., shows by two lateral grooves the beginning separa-
tion of the respiratory from the digestive system. At this stage, how-
ever, the two systems are still in open communication.

It is apparent that here is a practical unanimity of opinion among
those who have contributed to our knowledge of the development of the
mammalian lung as to the nature of the anlage and the process by which
the primitive lung sacs are produced. Slight differences of opinion may
be explained by the nature of the material and the methods by which
the different observers have worked. Fol, who believes in a_ paired
anlage for the human lung, studied an embryo somewhat older than the
specimens of His and Blsnianskaja, while Weber and Buvignier and
Willach, with this single exception, stand alone in regarding mammalian
respiratory apparatus as arising from primitively paired structures. In
turning, on the other hand, to the consideration of the organogenetic pro-
cesses by which the bronchial tree is produced not only are few authors in
accord, but, also, there is scarcely a chapter in the whole of embryology in
which we find so many different opinions based apparently upon objective
work. It will be wise, therefore, to consider briefly first the results which
have been obtained by the different contributors to this chapter on the
development of the lungs, and then attempt to make therefrom a fair
statement of our knowledge of the architecture and origin of the bronchial
tree at the present time.

Before the appearance of Aeby’s paper we had no general conceptions
concerning the architecture of the bronchial tree. According to the cur-

10 The Development of the Lungs

rent belief, as he himself points out, the division of the bronchi was
dichotomous. Little of the origin, the relations, and mode of division
of the bronchi was known and even less of the significance of the lobes
either to each other or to the species in which they were found. Aeby,
80, graphically describes the darkness which surrounded our knowledge
of the lung and blames the widely-accepted dogma of dichotomy for the
condition. It is noteworthy, however, how the few objective investigators
whose publications immediately preceded Aeby’s also held his conception
of the growth process of the tree. Among the first of these was Kiittner,
76, who followed certain stages of the growth of the bronchi in the older
stages of cow embryos and described the method of their proliferation as
undivided from the end, that is to say, monopodial. From the stems
of the bronchi, he says, lateral buds appear having their axes directed
at right angles to the mother bronchus. By the subsequent rapid growth
of these branches the monopodial character of the division is lost and an
apparent dichotomy ensues. A year later Cadiat, 77, in sheep embryos
measuring 12-15 mm. and upwards finds the trachea and main bronchi
already well formed and describes the growth process as occurring not
from the dilated ampullae at the end of the bronchi but rather from
lateral outgrowths from their walls. In a slightly different way Stieda,
78, who also used sheep embryos supplemented by rabbits, came to prac-
tically the same conclusion.

In the year preceding Aeby’s publication, Kolliker, 79, describes the ap-
pearance of secondary branches upon the primitive lung sacs in rabbits
on the 12th day, when the stem bronchus of each lung has three pro-
jections. From this period the subdivisions become so numerous that it
is difficult to follow them step by step, but, in general, the first branches
pass dorsalwards and lateralwards. This branching, according to K6l-
liker, occurs from hollow buds or projections from the epithelial tube
which multiply rapidly until each lung consists of a small tree of hollow
canals with swollen terminal buds.

From these citations it is of course obvious that the idea of monopody
was not new at the time Aeby wrote, and so the ignorance of the times
concerning the architecture of the pulmonary tree was not, as Aeby sup-
posed, so much due to the dogma of dichotomy as to the lack of a thorough
piece of objective research such as he himself attempted to supply.
And while many of his conclusions may find no place in our final con-
ceptions concerning the structure of the lung, still they must always re-
ceive the credit of having furnished us with a working hypothesis by
the aid of which the problem might be attacked by objective methods.
His suggestive appeal to embryologists, of which His, 87, speaks later,

Joseph Marshall Flint 11

indicates his belief in the final solution of the question through embryo-
logical investigations. An interesting parallel, in a more limited way,
might be drawn between the effects of Aeby’s stimulating paper and
the energetic investigations in the field of experimental biology which
followed the annunciation of Weismann’s views on heredity.

Aeby abandoned entirely any idea of dichotomy and substituted in its
place a strict monopodial explanation of the arrangement of the branches
of the bronchial tree. Each lung, according to this author, possesses a
stem bronchus which forms its axis and leaves the lung at the hilum to
fuse with its mate on the opposite side as they join the trachea. Of great
importance is the relationship which the pulmonary vessels, especially the
arteries, bear to the bronchial tree. The veins run in front of the
bronchi, the arteries behind, as the latter are forced in leaving the heart
to cross over the large air passages to reach their place. This crossing
occurs near the upper end of the stem bronchus and divides the tree into
two distinct segments of different importance. These are termed epar-
terial and hyparterial, according to their position with reference to the
point where the pulmonary arteries cross the bronchi.

The arrangement of lateral bronchi is throughout typical and regular.
Few occur in the eparterial while most are in the hyparterial zone. The
former may be absent, but the latter are always present. The hyparterial
systems of both lungs are symmetrical, but the eparterial systems, on the
contrary, are ordinarily asymmetrical. The hyparterial bronchi always
appear in two series, a dorsal and a ventral, which usually alternate and
have their origin from the stem bronchus relatively close to each other,
leaving the greater portion of the large bronchus free from branches.
This forms then the angle of a three-sided prism from which the two
series of lateral bronchi extend into the adjacent space bounded by the
chest wall. The dorsal bronchi are shorter. The lateral bronchi give up
some of their branches to the stem bronchus, a process which may be fol-
lowed, according to Aeby, step by step, with the greatest clearness. These
wander medialwards and finally cover the previously naked portion of the
stem bronchus with dorsal and ventral accessory bronchi. These either
remain close to the parent stem or else wander downwards. ‘Their de-
velopment begins usually quite far down the left lung, while in the right,
they appear higher up and often produce a special bronchus supplying
the Lobus infracardiacus known as the Bronchus cardiacus.

Eparterial bronchi are always single and never give off accessory
branches. They arise from the stem bronchus at a point midway between
the sites of origin of the lateral bronchi and divide generally into dorsal
and ventral branches. One, especially the left, or both may be absent,

12 The Development of the Lungs

thus giving to us three principal forms to the bronchial tree, namely, (1)
Lungs with an eparterial system on both sides; (2) Lungs with an epar-
terial system on the right side; (3) Lungs without an eparterial system.
In some instances the eparterial bronchus is shifted back on to the trachea
while in certain lower animals, especially the birds and reptiles, the
eparterial system is more highly developed than in mammals. In the
phylogeny of the lung, however, it becomes smaller until it may dis-
appear entirely in some of the higher series.

In the further development of the lung sacs in the human embryo as
described by His, 87, all secondary bronchi arise from the first five
primary divisions. Three of these occur on the right lung sac and two
on the left. On the right side they are termed upper, middle, and end
buds while those on the left are respectively lateral and end buds. With
Aeby, His finds the primitive lungs prismatic in transection with one
attached and two free angles between which lies its dorsal or costal sur-
face. The stems give rise to the so-called ventral bronchi, which, His be-
lieves, should have been termed lateral bronchi. Owing, however, to the
general acceptance of Aeby’s nomenclature, he has followed it. From
the stem bronchus dorsal branches appear which like the ventral group
subdivide regularly. These secondary branches are accordingly desig-
nated as follows:

1. Bronchus dorsalis posterior.
. Bronchus dorsalis lateralis.

. Bronchus ventralis lateralis.
. Bronchus ventralis anterior.

H= ©9 a

His agrees with Aeby with reference to the interpretation of the epar-
terial bronchus and looks upon it as an unpaired branch which, if it were
in the hyparterial region, would divide into dorsal and ventral elements.
As a matter of fact, after its appearance in the human embryo, it gives —
off branches which have these two general directions. On the other
hand, he looks upon the Bronchus cardiacus as a true side bronchus,
which, in opposition to the dorsal series, passes in a ventral direction.
Its independence is shown in its early appearance as well as by the dis-
tance which separates it from the first and second ventral bronchi. It is
regarded by His as an element which appears out of the schematic order
and follows its own development. In the left lung, cardiac and eparterial
bronchi are lacking, but the first ventral bronchus sends up a strong
dorsal branch, which mounts up into the apical region of the left lung
and is designated the Bronchus ascendens. In this way a substitution is
made for the eparterial bronchus of the right side which, with the

Joseph Marshall Flint 13

absence of the Bronchus cardiacus, destroys the absolute symmetry of the
hyparterial region. His followed the successive appearance of the chief
bronchi and their main branches by the reconstructive method as far as
embryos of the second month.

The growth of the tree occurs according to His by an extension of the
root branches and a division of the end buds. In no place did he find
evidences of lateral budding. ‘The end buds during the process of divi-
sion lose their conical form and flatten to some extent, while an elevation
appears on one side which through the formation of a furrow leads to the
outgrowth of two separate enlargements from the original bud. By the
acquisition of cylindrical status on the part of these secondary buds the
process can repeat itself. Below the region of the 3d hyparterial bronchus
a point is reached where one cannot hold strictly to the principle of
monopodial division, for it is impossible, His believes, to make as Aeby
does the principles of monopodial and dichotomous division mutually
exclusive. This, His remarks, is a conception of a somewhat transcen-
dental nature, which leads the zealous investigator to personify his own
ideas in the organ. The causes which control the form development of
a growing tissue need not always remain the same, but may change its
character once or several times. Accordingly, His summarizes the growth
process from the unpaired anlage of the lung, which extends to either
side in paired dilatations. From these primary sacs, lateral sprouts
appear by monopodial growth. Further division is by dichotomy and
finally a point is reached where the division occurs by more or less
abundant lateral budding.

Willach, 88, studied several stages of the development of the lungs in
the mole and pig, but his material, however, was not sufficient to give
him a very complete picture of the gradual evolution of the pulmonary
apparatus so he used the findings of other investigators to fill the gaps.
Although Willach’s own specimens did not include the stages of the first
division of the primitive bronchi he believes the growth from first to
last is monopodial, the end bud developing a lateral bud before its lumen
narrows. ‘These lateral buds become cylindrical as the parent bronchus
continues to grow. Willach concludes from a study of the illustrations
in His’ paper that the eparterial bronchus is a derivation of the first
ventral bronchus and looks upon it as an accessory branch in the sense of
Aeby. He likewise believes that the apical branch of the Ist left ventral
bronchus is analogous to the eparterial branch because, on its side, it
bears the same relationship to the first lateral branch of the pulmonary
artery that the eparterial does on the left. Willach follows the ideas
of His in believing the Bronchus cardiacus is an independent lateral

14 The Development of the Lungs

bronchus and not an accessory bronchus in the sense of Aeby. In the
case of the other so-called accessory bronchi, however, this author is in
accordance with the views of the latter. Robinson, 89, studied the de-
velopment of the lungs in rats and mice, and finds about the eighth day
the primitive lung sacs growing lateralwards and dorsalwards, forming
the bud-like projections into the ccelom from which the primitive and
stem bronchi arise. The eparterial bronchus, according to Robinson, arises
as the first division of the right lung bud. As a distinct branch, it is
absent on the left side, although it is compensated for by a branch of the
first lateral hyparterial bronchus, which is totally unrepresented on the
right side and passes up to the apex of the lung. Robinson, in this
view, 1s in accord with the findings of His. He believes the growth of
the tree occurs by a flattening of the terminal bud opposite the axis of
the bronchus and a subsequent division into two unequal segments of
which the smaller becomes the lateral branch giving rise to what he terms
an unequal or sympodial dichotomy. Robinson also describes branches
arising as hollow buds from the main bronchus after it has resumed its
cylindrical form, allowing the interpolation of secondary bronchi between
those already existing, while the dorsal accessory bronchi of Aeby arise,
according to Robinson, by a division of the primary dorsal bronchi, not
by budding but by having the dorsal stalk split from the point of origin
of the first median bud as far back as the stem bronchus, allowing this
medial bronchus to obtain a secondary origin from the stem bronchus
itself instead of from the primitive dorsal branch. The bronchus infra-
cardiacus is ontogenetically a derivative of the main stem bronchus, but
phylogenetically it is, as Aeby suggests, an original branch of the Ist
hyparterial bronchus.

With the exception of the Bronchus cardiacus, Robinson has nothing
to say concerning the ventro-accessory bronchi of Aeby. He calls them
ventral bronchi, but it is not clear whether either ontogenetically or phylo-
genetically, as in the case of the most prominent one of the group, he
considers them accessory branches of his lateral bronchi.

Ewart, 89, published a large monograph containing a criticism of
Aeby’s ideas on the architecture of the lungs. Ewart, like Aeby, used
material consisting of dissections and corrosions of the adult lung, but
only of one species, namely, man. Apparently this author did not per-
ceive as clearly as Aeby that the hyparterial and eparterial theory was
in reality a working hypothesis, which could only receive from embryo-
logical investigations the evidence necessary for its final substantiation
or disproof. From his investigations Ewart believes that dichotomy,
more or less equal, is the principle governing the division of the bronchi

Joseph Marshall Flint 15

from beginning to last. He abandons the distinction between the hypar-
terial and eparterial regions as well as Aeby’s simple nomenclature and
substitutes in its place a method of topographical designation which,
besides going into endless detail, is constructed entirely independent of
embryological considerations and has received, thus far, no support from
subsequent investigators.

In a series of papers the first of which appeared the same year, Zum-
stein, 89, 91, 92, 00, by the study of corrosion specimens of the lungs of
a series of mammals and birds in which the pulmonary artery as well
as the bronchial tree was injected is unable to support Aeby’s conclusions
with reference to the influence of the pulmonary artery on the archi-
tecture of the bronchial system. The division of the tree into eparterial
and hyparterial bronchi according to Zumstein is not based on sound con-
clusions as he finds a series of variations in both arteries and bronchi,
indicating that a formative influence in the sense of Aeby cannot exist.
At the same time Zumstein siudied the development of the lungs in the
mole and the duck by the Born reconstruction method. With other in-
vestigators, he agrees in the precocious development of the right lung
He does not describe in detail, however, the gradual evolution of the
mammalian lung but simply states that the dorsal and medial bronchi
arise later than the lateral branches but do not attain the extensive de-
velopment of the latter. Whether or not he considers them accessory
bronchi in the sense of Aeby is not clear from his description. The
Bronchus infracardiacus may originate, according to Zumstein, either
from the stem bronchus beneath the second lateral bronchus or from this
bronchus itself. The eparterial branch of Aeby he designates as the first
lateral bronchus. In the early stages the Arterie pulmonales originate
far cranialwards and accompany the trachea ventro-lateralwards on both
sides. The left is more dorsal even before the trachea is reached while
the right artery passes ventralwards of the first lateral branch of the
right bronchus (Aeby’s eparterial). It is scarcely possible, Zumstein
concludes, for the arteries to have an influence upon the structure of the
tree as the first bronchi have appeared on the stem bronchus before the
arteria pulmonalis can be traced into the lung.

In a preliminary note Narath, 92, published a résumé of a large
monograph upon the embryology and comparative anatomy of the bron-
chial tree of the mammalian lung, which appeared some nine years later,
o1. Before this work was published, however, Narath, 97, described the
development of the lung in Echidna aculeata. In all of the papers, he
takes exception to Aeby’s fundamental conception of the architecture of
the bronchial tree. From a rich embryological material, echidna, rab-

16 The Development of the Lungs

bit, and guinea-pig, he describes the growth of the tree after the forma-
tion of the primitive lung sacs as taking place by monopodial growth
with acropetal development of lateral twigs. In this process the stem
bud is the principal structure, which grows on undivided with the ventral
bronchi originating as lateral outgrowths upon it. The primitive lung
sacs are to be looked upon, according to Narath, as the first stem buds.
By this process arise from the stem bronchus two series of lateral
branches, the ventral and dorsal bronchi. While the former are true
derivatives of the stem bronchus, the latter, Narath is inclined to
regard, as branches of the ventral bronchi which in course of ontogenetic
and phylogenetic development are given up to the stem bronchus. From
his embryological investigations, Narath supports Aeby’s conclusions
with reference to the dorsal and ventral accessory bronchi. They are
formed first on the ventral and dorsal branches and then wander to their
positions on the inner and ventral side of the stem bronchus. In this
group and in complete accord with Aeby, he would also classify the
Bronchus cardiacus except that, unlike Aeby, he believes it can arise in
some instances from the second or third ventral bronchus. The pulmon-
ary artery according to Narath’s view has no great influence on the growth
of the bronchial tree as he, like Zumstein, has found a whole series of
variations in the artery without any important changes in the bronchi.
Furthermore, he reiterates Zumstein’s view that, both at the time the
primary bronchi are formed, as well as later, the pulmonary arteries are
thin, weak vessels of insufficient strength to influence these relatively
thick and well-developed epithelial structures. Of equal importance in
this connection is the observation that the arteries cross over the bronchi
to pass down on its lateral, instead of its dorsal, side. Only at the end
of the stem bronchus is its position distinctly dorsal. In consequence
of this course, it forms a half spiral round the stem bronchus. Of a
crossing in the sense of Aeby no true case exists. Narath accordingly
proposes to abandon the distinction between the so-called eparterial and
hyparterial regions of the bronchial tree.

The eparterial bronchus of Aeby has, according to Narath, the same
area of distribution as a dorsal bronchus. He not only regards it such,
but believes it is in reality, the first dorsal bronchus. ‘To emphasize its
special meaning for the topography of the lung, he terms it the apical
bronchus. It is never suppressed nor does it degenerate in certain ani-
mals as Aeby suggests. It is, furthermore, always present normally as
a lateral branch of the first ventral bronchus and possesses, moreover,
the power of wandering up either onto the stem bronchus or the trachea.
In speaking of his conviction that it is a real dorsal bronchus he con-

Joseph Marshall Flint 17

tinues: ‘“* Mit dieser einen Thatsache fallt die ganze Aeby’sche Theorie
von den ep- und hyparteriellen Bronchien ein- fiir allemal.” This view
for which Narath has apparently received the entire credit in the litera-
ture was, as we have already -seen, first announced by Willach. Na-
rath’s single addition to Willach’s statement is in the designation of the
eparterial branch as a dorsal element in conformity with his idea as to
the possible derivation of the whole series of dorsal bronchi. In his
belief, that the eparterial bronchus has the area of distribution of a dorsal
bronchus, his observations are not in accord with those of Aeby, His,
and Robinson.

Minot, 92, thinks the ideas of Aeby and His are erroneous with ref-
erence to the monopodial growth of the tree. He, on the other hand,
looks upon the branching as characteristically dichotomous, describing
the branches as having rounded ends. After division they develop un-
equally with the ventral fork, as a rule, serving as the stem. The first
branches correspond to the lobes, but he does not agree with the findings
of His and Aeby with reference to the presence of a bronchus in the
right lung which is not represented in the left. With Willach and
Narath he regards the eparterial bronchus of the right side and the
apical branch of the first ventral on the left as homologous. The dif-
ference between the two, Minot holds, is due to the more precocious de-
velopment of the right side and the secondary modifications in the arteries.
The relationship of the veins confirms this view. The peculiar course
of the right pulmonary artery is due to the abortion of the 5th arch on
the right side and the subsequent transfer of the origin of the artery to
the left.

In a series of papers d’Hardiviller, 96, 1, 2,; 97, 1, 2, 3, describes in
the rabbit and sheep, the evolution of the tree after the trachea and
main bronchi are laid down. There is, according to this author, a stem
bronchus which transverses the whole lung and from which all of the
primary bronchi are derived by means of collateral ramifications, that
is to say, through epithelial herniw from the walls of the stem bronchus,
a process in which the terminal bud of the bronchus takes no part. In
this way appear, in the rabbit, two buds on the right side and one on the
left which, with the stem bronchi, enter into the formation of the five
lobes of the lungs and produce all further ramifications. In the sheep,
on the other hand, there are, including the stem bronchi, four buds on
the right and two on the left giving rise to the six lobes of the sheep’s
lungs. The primary branches of the stem bronchus occur in four series,
external, internal, anterior, and posterior, according to their position

2

18 The Development of the Lungs

on the stem bronchus. Of the four series, the primary, external, and
posterior are the most important and are extensively developed, forming
the principal bronchi of the adult lung. On the other hand, the anterior
and internal proliferate to some extent but do not form extensively
developed branches of the adult tree and are, therefore, termed by
d’Hardiviller accessory bronchi using a similar nomenclature with a dis-
similar meaning from Aeby and Narath. The further growth of the
tree after the origin of the principal bronchi by collateral ramification,
is by unequal dichotomy at first, and later, equal dichotomy. The pro-
cesses differ with the different primary bronchi and appear earlier in the
sheep than in the rabbit. The cardiac bronchus, according to d’Hardi-
viller, arises from the stem bronchus and, in this animal, remains inde-
pendent. In the sheep, it emigrates on to the 1st lateral bronchus.
The bronchus on the left side, he believes, always originates on the stem
bronchus and wanders onto the 1st lateral thus forming the Bronchus
cardiacus of Hasse. In the rabbit, d’Hardiviller finds the eparterial
bronchus originating on the right side by collateral ramification, but
unlike other investigators, he believes there is also an eparterial bronchus
on the left. It appears on the 13th day and in 24 hours begins to degen-
erate and remains as a solid epithelial mass in connection with the
mother bronchus. In consequence of his belief of the presence of this
left eparterial element, d’Hardiviller thinks Aeby’s classification of the
lungs of mammals is only of secondary value. It also emphasizes its
independent character and forces him to conclude that it is independent
of Narath’s apical bronchus as it is not a lateral branch of the first
ventral bronchus.

d’Hardiviller’s series of papers was interrupted by the appearance
of a study by Nicholas and Dimatrova, 97, upon the development of
the lungs in sheep by the Born reconstruction method in which they sup-
ported, in most respects, his observations. In an embryo of 5 mm. they
find the main bronchi appearing as asymmetrical buds on the lateral
faces of the anlage. In their later growth, this asymmetry is exaggerated.
After the origin of the primitive pulmonary sacs two buds appear on
their lateral walls (embryo 9 mm.) representing the first two lateral
bronchi while simultaneously the tracheal bronchus is seen as an elon-
gated projection on the right side of the trachea. No trace of a sym-
metrical bronchus, however, is found on the other side. They regard
this element as being entirely independent of the bronchial system which
must be regarded as a supernumerary bronchus originating from the
future trachea just as the collateral bronchi are formed from the stems.
The collateral bronchi, of which there are three sets, a lateral, a dorsal,

Joseph Marshall Flint 19

and a ventral, originate in the form of buds upon the bronchial stems.
Each is an independent structure and does not show any ontogenetic
relationship with the other bronchi, indicating a wandering of the acces-
sory bronchial groups as described by Aeby, Willach, and Narath or
d’Hardiviller, in the case of the cardiac bronchus of Hasse. From
the division of the first lateral bronchi, a branch passes up towards the
head on the left side which is unpaired, for on the opposite side this
region is supplied by the tracheal bronchus. The infracardiac bronchus,
Nicholas and Dimitrova regard as an unpaired precocious ventral branch
for which there is no symmetrical structure in the left lung. The
remaining ventral bronchi appear later as in an embryo of 18 mm. they
find one between the second and third, and another between the third
and fourth lateral element.

Huntington, 98, in studying the eparterial system of a series of adult
mammals, comes to the conclusion that the right and left lungs agree
morphologically in the type of their bronchial distribution and that
the asymmetry is apparent and not real. These apparent differences are
due to the shifting of a branch of the upper bronchus (cephalic trunk)
which wanders up and becomes topographically eparterial. At times, the
asymmetry may be more exaggerated by the migration of the entire
branch. As the factor involved in this change is the bronchus itself and
not the pulmonary artery, Huntington proposes to abandon Aeby’s dis-
tinction between the hyparterial and eparterial regions of the bronchial
tree except in a topographical sense. In the left lung there is a morpho-
logical equivalent for every eparterial element that may occur in the
right lung and, accordingly, this author believes in the equivalent mor-
phological value of the upper and middle lobes of the right side with
the upper lobe on the left. This, it will be remembered is the conclu-
sion of Willach and Narath except that Huntington, lke Willach, does
not believe that the eparterial element is primarily a dorsal bronchus.
As the pulmonary artery does not run dorsal to the stem bronchus, but
lateral, or dorsolateral, as Narath has shown, Huntington proposes
to abandon also the distinction made by Aeby between the dorsal and
ventral bronchi. From the study of his corrosions this author believes
that the primitive type of division is practically dichotomous and later is
changed into the monopodic system. Phylogenetically, the primitive
type is the so-called bilateral hyparterial form, while the symmetrical
eparterial type represents the end stage in the process of evolution and
not the beginning as Aeby and Wiedersheim believe.

An ingenious effort is made by Guyesse, 98, to support the monopodial
theory of growth. This author has studied the transformation of the

20 The Development of the Lungs

tracheal musculature into the muscle of Reisseissen in the successive
branches of the bronchial tree. He finds the entire stem bronchus until it
is past the divisions of the upper and middle lobe and projects well into
the lower lobe has a musculature like the trachea. On the other hand, the
bronchus of the upper, middle, and then lower part of the stem has the
muscle of Reisseissen. These findings, Guyesse believes, give evidence
that the production of the main bronchi is by monopodial growth.

Miller, oo, while working chiefly on the anatomy of the lobule, agrees,
apparently, with Aeby’s division of the eparterial and hyparterial region
of the human lung, and, furthermore, he also speaks of monopodial
division of the tree.

According to Justesen, 00, who studied the branching of the bronchial
tree chiefly in cow embryos of well-advanced stages and in post-natal life,
the division of the bronchi from first to last takes place by undoubted
dichotomy after which the asymmetry is produced by unequal growth of
the stem. This author approves of His’ attitude towards Aeby’s theory
of monopodial development in general, but criticises his belief in the
production of the first branches of the tree by monopody without having _
the material to follow their successive development. It seems rather
strange, therefore, that Justesen, who was himself without these stages,
should attempt to prove from His’ illustrations in which these branches
were already formed, that they originated by sympodial dichotomy
especially after remarking so wisely, “ Es ist kein Versuch, die Frage
durch unberichtige Analogie folgerungen zu lésen. Ich will nur behaup-
ten, dass die Frage nicht gelést ist, weitere Untersuchungen dagagen
nétig sind.” Justesen does not believe in the production of bronchi by
lateral outgrowths of the mother stem. He believes, therefore, Stieda’s
observation was faulty and states that no other investigator has since
repeated this observation. He is ignorant, apparently, of the work of
Robinson, d’Hardiviller, and Nicholas and Dimitrova.

Justesen does not accept Aeby’s distinction between the eparterial and
hyparterial regions of the bronchial tree and looks upon the accessory
bronchi of Aeby as independent structures. Their irregularity he
ascribes to the presence of the heart and vertebral column.

Merkel, 02, agrees with His, that the first divisions of the stem
bronchi are produced by monopodial growth and that the later divisions
arise by dichotomy. With Narath, he abandons Aeby’s distinction be-
tween the eparterial and hyparterial region as resulting from the in-
fluence of the pulmonary artery on the architecture of the tree, and
looks upon the right apical bronchus, the so-called eparterial, as a
derivation cf the first ventral and homologous with the apical branch of

Joseph Marshall Flint 21

the 1st ventral or lateral bronchus on the left side. Concerning the
so-called accessory bronchi, Merkel seems to be in accord with the older
observers in looking upon them as derivations of the dorsal and ventral
lateral bronchi, and apparently follows Narath, instead of His, regarding
the Bronchus infracardiacus as a possible derivative either of the first,
second, or even third ventral bronchus instead of an independent branch
of the stem.

The comparative embryology of the lungs in vertebrates has been
studied by Moser, 00, whose material consisted chiefly of the lower
vertebrates amplified to some extent by sections of rat, mouse, and rabbit
embryos. All vertebral lungs, according to Moser, are developed through
a common principle consisting in a general increase in size due to an
increase of their constituent tissues. The epithelium is the principal
factor which originates from the endoderm and passes as a single tube
into a solid mass of connective tissue forming the framework of the
lung. If this connective tissue is thin, the growth of the epithelium
produces a widening of the intrapulmonary bronchus with simple pro-
jections on its walls as in amphibia. On the other hand, if the con-
nective tissue is dense and resistant, the epithelial increase is localized
in certain places, the cells are packed together until they force their
way into the connective tissue forming buds such as we find in the lungs
of all vertebrates from reptiles up. Certain points on the walls of the
lung are more resistant and remain in the lung cavity as septa. At the
same time, as we ascend the scale, the number of buds of the second
order constantly increase. According to Moser, we may also observe
at this time a gradual increase in the mass of connective tissue in pass-
ing from lower to higher vertebrates, and we obtain, in consequence, a
system of long canals or bronchi passing through a connective tissue sac.
The division of the bronchi is always and exclusively by monopodial
growth, and is a main bronchus, the intrapulmonary bronchus, which is
a direct continuation of the extrapulmonary bronchus passes through
the lung from the root to its distal end.

By means of the reconstruction method, Bremer, 04, studied the
lung of the young opossum (Didelphys virginiana) and compared it
with older stages. His youngest specimen measured from 10.5 to 12.5
mm. and were taken from the same pouch. Older specimens, 14 cm.
long, and three adults were also used for comparison. In five out of
six of the new-born animals Bremer found an eparterial bronchus on
both sides, except that the one on the left bronchus is always smaller
and placed slightly lower than the eparterial branch on the right. The
air chambers supplied by it, however, do not form the apex of the lung.

22 The Development of the Lungs

In spite of its small size and low position, it is above the first ventral
bronchus and behind the artery and thus, according to Bremer, makes
the right and left side of the lung symetrical and reptilian in type as no
placentalian lungs are. The complete symmetry of the young lung is
marred by the presence of a cardiac lobe on the right side which is
unrepresented in the left. Bremer states that the reptilian lung has the
double eparterial bronchus and thus the lung of the opossum is reptilian
in type. In its later phases, the lung is changed from the reptilian to
the mammalian form by the loss of the left eparterial bronchus, the
multiplication of its bronchi and the acquisition of a new type of air
chamber. In a 14 cm. opossum no trace of the left eparterial bronchus
remains but Bremer states he is unable to follow the degeneration of
this element from lack of necessary stages. He believes, however, with
Selenka, that in the opossum we have an epitome of the evolution of the
reptilian lung to the mammalian lung by means of the changes noted
above.

The observations of Bremer at once recall the views of d’Hardiviller,
who believes the left eparterial bronchus is always present in rabbits
but subsequently degenerates. If this observation is confirmed it would
seem to support d’Hardiviller’s contention, although Narath, it will be
remembered, believes that d’Hardiviller was dealing with a variation.
From Bremer’s statement that no other lungs of placentalia have the
double eparterial system, it is apparent that he has overlooked Aeby’s
description of the lungs of Phoca vitulina, Bradypus tridactylus, Didel-
phinus delphis, Auchenia lama, Equus caballus, and Elephas Africanus,
and some other nine species described by Narath and two species of
Cebus by Huntington, making in all seventeen species where the condi-
tion described by Bremer as exceptional in mammalia is permanent.
We must also consider the possibility that Bremer is dealing with a
dorsal bronchus placed abnormally high on the stem bronchus, especially
as he states this bronchus did not supply the apex of the lung. The
observations of Narath, 96, on Echidna aculeata are also suggestive in
this connection as he states the relationships of the vessels, while young
marsupialia are in the pouch suffer no further change either in the case
of the arteries or the veins. Furthermore, Narath does not support Se-
lenka, 87, with whom Bremer is, more or less, in accord in his observa-
tions on the opossum lung as he finds the lung of Echidna develops like
other mammalian lungs and is not differentiated from the developmental
processes which are active in the production of the placentalian lung.
He, therefore, does not approve of a comparison of the lung of marsupials
with that of reptiles. Moreover, Hesser was unable to find an eparterial

Joseph Marshall Flint 23

bronchus or a bronchus whicn corresponded to it in his extensive work
on the reptilian lung. (Personal communication. )

Blisnianskaja, 04, from the study of a series of models of the lungs
of human embryos concludes that His’ criticism of Aeby’s nomenclature
is correct, and accordingly divides the branches of the main bronchus
into two groups, namely, a dorsolateral representing Aeby’s dorsal
series, and a ventrolateral including Aeby’s ventral group. She states
that this revision is justifiable even from a study of Aeby’s own illustra-
tions. These two series originate so that a line connecting their roots,
from two more or less spiral lines on the stem bronchus. The eparterial
bronchus, according to Blisnianskaja, is a dorsal branch of the first
ventrolateral bronchus, which emancipates itself and wanders up on
the stem bronchus according to the ideas of Willach, Minot, Narath,
and Huntington. The entire dorsolateral group are similarly placed
originally upon the ventrolateral group, they separate and wander up
on the stem bronchus to receive a separate origin. As the eparterial
on the right side is the first dorsolateral bronchus, Aeby’s first dorsal
bronchus becomes Blisnianskaja’s second dorsal element. The apical
bronchus on the left side is homologous then to the eparterial on the
right side. The Bronchus cardiacus is also a division of the 1st ventro-
lateral bronchus on the right side, which separates from the mother
branch, passes downwards, and receives a final origin upon the stem
bronchus. Since the eparterial bronchus arises from the 1st ventro-
lateral, Blisnianskaja believes that the upper and middle lobe with the
cardiac bronchus on the right side are equivalent to the upper lobe on the
left side, and that the lower right lobe is equivalent to the left lower lobe.
The form of the embryonic lung is influenced by the large fcetal heart
and by the long development through which the human trunk, especially
the thorax, passes. Blisnianskaja believes the method of division is
sympodial or unequal dichotomy. She has never observed a bronchus
originating from the complete bronchial tube by the monopodial growth.

A glance at this review of the literature shows a unanimous agree-
ment among the various investigators only upon the independence of the
lateral group of bronchi (ventral of Aeby, His, and Narath). There
is, however, with the exception of Willach and Fol a general recognition
of the fact that the mammalian lung arises from an unpaired anlage.
Although supported by objective investigations, the interpretation of the
origin of the other groups of bronchi, the method of their growth, and
their significance for the architecture of the bronchial tree have varied
within wide latitudes. We may be said at the present time to have no
settled views upon the development of the bronchial system. In view of

24 The Development of the Lungs

the work of Moser and Hesser, the student of the mammalian lung, how-
ever, may look upon its phylogeny as being no longer in conflict with its
ontogeny, and may also state his problem in the following series of
questions:

1. Is the anlage of the lung unpaired or paired?
2. Is it symmetrical or asymmetrical ?
3. Does the pulmonary artery exert any fundamental influence upon
the growth of the bronchial tree, separating it into two regions of un-
equal significance as expressed in Aeby’s Ep- and Hyparterial theory?

4. Is the “eparterial bronchus” an independent structure or a de-
rivation of the 2d lateral bronchus? Is it an unpaired or paired ele-
ment? Does an “eparterial bronchus” always form on the left side
and then degenerate or undergo atrophic changes ?

5. Is the Bronchus ascendens of His, or the left apical bronchus of
Narath, the equivalent of the ‘‘ eparterial bronchus ” ?

6. Are the lateral bronchi independent structures ?

7. Are the dorsal bronchi independent structures or derived from the
lateral group ?

8. Are the ventral bronchi independent structures or derived from the
lateral group ? .

9. Are the medial bronchi independent or derived from the dorsal
group?

10. Is the Bronchus cardiacus an independent or accessory bronchus ?

11. In what way do the bronchi grow? Does one system of growth
predominate throughout the whole development of the bronchial tree?

12. What is equivalent value of the lobes of one lung in terms of the
other ?

THe ANLAGE OF THE LUNGS.

The development of the respiratory apparatus begins in a pig by a
lateral flattening of the head gut just below the Fundus branchiales. At
the age represented by an embryo, 3.5 mm. nape breech measurement,
the last gill pouch has in transsection (Fig. 1)* a flattened rhomboidal
form with dorsal, ventral, and lateral angles. Below this gill pouch,
lying behind the Sinus venosus, which already shows evidences of the
increasing asymmetry of the heart, the ventral angle as it deepens to form
the pulmonary groove (PI. I, Fig. 1) is pushed somewhat to the right
of the median plane (Fig. 1). The head gut in passing caudalwards,

* References to Text-Figures may read simply Fig. 1, or Fig. 2, or Fig.
3, etc., but every reference to figures on plates is accompanied by the proper
plate number.

Cav)
Or

Joseph Marshall Flint

narrows gradually until its lumen in cross-section forms an asymmetrical
sagitally placed fissure. A short distance above the Ductus hepaticus
(Fig. 3 DH) the pulmonary groove terminates caudalwards in an irregu-
lar enlargement (Fig. 2 PA), the asymmetrical pulmonary projection
forming the first unpaired anlage of the lungs. As yet, there is no trace
of the main bronchi nor any evidence of a division. Ventralwards,
it projects somewhat from the level of the ventral margin of the intestine
below it (Pl. I, Fig. 1), while laterally it is more marked on the right
than on the left side, an asymmetry more apparent from a transverse
section (Fig. 2) or a dorsal view of the reconstructed intestine (PI. I,
Fig. 2). Whether the cause of this asymmetry lies primarily in the
anlage itself or is due to the influence of the heart as Minot suggests, it
is impossible to determine from these specimens. Below the pulmonary
projection, the head gut while still asymmetrical lies more in coincidence
with the median longitudinal plane.

PA.
Text Fig. 1. Trexr Hie. 2.

TExT Fie. 1. Section of embryo pig 3.5 mm. long, showing head gut in the
region of the upper part of the Mesocardium posterior. C=Celom. SV=
Sinus venosus. VM= Mesocardium posterior.

TExT Fic. 2. Section of embryo pig 3.5 mm. long, through the pulmonary
anlage. C—Celom. PA= Pulmonary anlage.

At this stage, the epithelial lining of the head gut is quite variable in
thickness. In the pulmonary enlargement (Fig. 2. PA) it is clothed
by a columnar epithelium of several layers with mitoses taking place
chiefly in the innermost row. In the dorsal segment of the head gut
at this level, it is considerably lower especially at the dorsal angle where
it consists of a single layer. Above the projection it is thinner in the
bottom of the groove and thicker at its sides. The Mesocardium pos-
terior (Fig. 1 Vif) begins just below the last gill pouch and extends
down to a short distance below the pulmonary anlage. Between these
points, the entire head gut is surrounded by a mesoderm composed of
anastomosing cells in which the exoplasmic or fibrillar portion of the
mesoderm is not well differentiated (compare Mall, 02, and chapter on

26 The Development of the Lungs

organogenesis). In the upper part of the gut just below the gill
pouches, the mesoderm, covered by ccelomic epithelium forms slight
asymmetrical projections into the ccelom (Fig. 1 ¢), while at the level of
pulmonary swelling, the anlage of the mesodermic portion of the lung
wings (Fig. 2) takes the form of two irregular lateral projections into
the coelomic cavity. The one on the right is much larger than that on
the left (Fig. 2), so much so that at this stage the latter is only faintly
shown. This results in a marked asymmetry of the primitive lung wings
themselves. The mesoderm in the two wings is characterized by the rich-
ness of its cellular content, as the portion behind the intestine already
shows a differentiation preceding the stages of chondrification of the prim-
itive vertebre. The mesoblastic anlage of the lungs arises from the
general mesoderm of the head gut. Just below the pulmonary anlage

TExT FKig. 3.

Text Fig. 3. Section of embryo pig 3.5 mm. long, through Ductus hepaticus.
C=Celom. DH =—Ductus hepaticus.

Text Fig. 4. Section of embryo pig 4 mm. long at the beginning of the
Mesocardium posterior. C—=Celom. VM—=Mesocardium posterior. SV =
Sinus venosus. O=(£sophageal portion of head gut. PG Respiratory
portion of the head gut.

on the left side are evidences of the Recessus pleuroperitonalis which,
as described by Stoss, 92, may at this stage be followed through a few
sections.

In a slightly later stage, 4 mm., for example, the embryo shows the
next step in the development of the respiratory apparatus. The head
gut is more symmetrical with reference to the median longitudinal plane
(Figs. 4, 5, 6). In the upper portion below the gill pouches, a longi-
tudinal fissure appears on either side dividing it now into well-marked
dorsal and ventral segments giving the gut in the respiratory level, more
or less of an hour-glass appearance in transsections. These fissures mark

Joseph Marshall Flint 27

the line of separation between the respiratory (Figs. 4 Pg, 5'PA) and
digestive systems (Figs. 4, 50) and extend from the region just below
the gill pouches to the pulmonary anlage. In the upper portion, near
the gill pouches, the lumen of the cesophageal part is somewhat larger,
while, at the level of the pulmonary anlage, the respiratory segment is
markedly dilated (Fig. 5 PA). Between these levels, the relationship
between the two is practically equal (Fig. 4). Above, the epithelium is
lower in the dorsal and ventral angles, shghtly so in the lateral fissures
but somewhat thickened at the sides of both dorsal and ventral segments.
In these thickened portions there is a double layer, in the angles a single
layer of epithelium. In passing caudalwards, the epithelium of the
respiratory anlage thickens as its lumen increases in size until a double
row of columnar cells line the floor of the pulmonary groove (Fig. 5 PA),

Thxt Wie. 5: Text Fic. 6.

Text Fic. 5. Section of an embryo pig 4 mm. long, through the upper
part of the pulmonary anlage. C—=Celom. PA=Pulmonary anlage. O=
Digestive portion of the head gut.

Text Fic. 6. Section of an embryo pig 4 mm. long through the lower
portion of the pulmonary anlage. C—Celom. O= Digestive portion of the
head gut. BD—Right stem bronchus.

while at the sides, they are three cells deep. At the level of the pulmon-
ary anlage, the asymmetry is again evident. The projection has now
begun to extend lateralwards on each side to produce the main bronchi.

To the left, the evagination is considerably higher than on the right
and also less prominent. At the same time the asymmetry is exagger-
ated by the anlage of the right bronchus (Fig. 6 BD) which points some-
what caudally. The epithelium lining the two primitive bronchi is
columnar and consists of several layers. Rapid mitosis is taking place
chiefly in the inner row of cells.

With the more marked symmetry of the head gut itself, there is also a
greater symmetry of the mesodermal anlage (Figs 5, 6) of the lungs.

28 The Development of the Lungs

While the two wings still show the influence of the asymmetry of the
bronchial projections, they are somewhat more regular than in the
preceding stage. The anlage of the right wing is larger than the left
and the Mesocardium posterior is also pushed slightly to the right. The
character of the mesoderm remains about the same as in the last stage,
that is to say, rich in cells with scarcely any differentiation of the exo-
plasm into primitive connective-tissue fibrils. Below the lung anlage,
the Recessus pleuro-peritonealis is patent on the right side.

In a still later stage, 4.5 mm., the conditions remain practically as
in an embryo of 4mm. ‘The most apparent differences lie in the further
development of the two main bronchi. That on the left (Fig. 8 BS)
grows practically at right angles to the axis of the pulmonary groove,
while the right bronchus is directed laterally and caudally (Fig. 9 BD)

Thx Wie. 8.

Text Fic. 7. Section of an embryo pig 4.5 mm. long at the beginning
of the Mesocardium posterior. C—=Celom. PG=Respiratory portion of the
head gut. O=Digestive portion of the head gut. SV—Sinus venosus.

Text Fic. 8. Section of an embryo 4.5 mm. long, through the anlage of the
stem bronchi. C=Celom. PA=Pulmonary anlage. BS=Left stem
bronchus. O = (Msophagus.

and extends through a number of sections after the other has disap-
peared.” From the anlage at the point of origin of the bronchi, there is
a crest-like projection of the epithelial tube in the midline which is
exaggerated by the slight dorsal flexure of the two main bronchi. This is
scarcely seen in cross-sections, but can be made out easily in embryos
cut longitudinally. At this stage, we also note the beginning of the pro-
cess of separation of the respiratory from the digestive tract in a sulcus
(Fig. 8) formed below the pulmonary anlage just behind it and in front
of the ventral part of the cesophagus which is continuous above with the
lateral fissures. In this particular embryo, the process seems a little

Joseph Marshall Flint 29

precocious as I possess later stages where the two systems are in open
communication at a lower level than is shown in this specimen. At the
level where the Mesocardium posterior begins (Fig. 7 VM), the epithe-
lium lining the fore gut is columnar and consists, except in the ventral
and dorsal angles, usually of a double layer of cells. In the anlage of the
lungs (Fig. 8), it is slightly higher and shows a more active karyoki-
netic process. A similar layer of endoblast extends out into the primitive
bronchi. At the tips, cell division is proceeding rapidly. 'The mesoderm
of the lungs remains, so far as its differentiation is concerned, practi-
cally unchanged, but the lateral extension of the left bronchus now
makes the projection into the ccelom at this level more marked than on
the right side as the right bronchus, lying in a caudo-lateral direction
nearer the median plane, does not carry the mesoderm quite so far into
the right coelomic cavity. On both sides, the Recessus pleuroperitonealis

Tix HGS 9)

Text Fic. 9. Section of an embryo 4.5 mm. long, through the lower part
of pulmonary anlage. O=(Mophagus. C=Celom. BD=—Right stem
bronchus. RD=—Right Recessus pleuroperitonealis. RS—=Left Recessus
pleuroperitonealis. VM—Mesocardium posterior.

may be seen. It is larger and extends higher on the right than on the
left (Fig. 9 RD,RS). In Fig. 9, the beginning of the formation of the
dorsal mesentery at the lower level of the lungs is apparent.

By the reconstruction process, the changes which have been occurring
in the two preceding stages are demonstrated beautifully in a pig 5 mm.
long where they are also considerably accentuated. Above (Pl. I, Figs.
3, 4) is seen a segment of the last gill pouch, while below it, the head gut
narrows rapidly to a sagittal fissure forming the ventral respiratory and
the dorsal digestive portion. The pulmonary groove, still in open com-
munication with the cesophagus, terminates below in the asymmetrical

30 The Development of the Lungs

right and left bronchi. Of the two, the left (Pl. I, Fig. 3s) passes
lateralwards almost at right angles to the axis of the groove, while the
right (Pl. I, Fig. 3d) extends caudalwards and lateralwards, giving
a sharp asymmetry to the fork which they form with the trachea (PI. I,
Fig. 37’). From the slight crest in the midline which is not seen in
the ventral view, both bronchi bend slightly dorsalwards. At the ends,
there is a slight increase in the caliber of the bronchi, but end buds are
not yet formed upon them. Underneath the point where the two unite,
the sulcus from which the separation begins is already present, but it
does not extend quite as far cranialwards as in the preceding stage.
Viewed in profile, the whole anlage now extends somewhat ventralwards
from the head gut, an extremely important relationship as we shall see
in the chapter on the relation of the blood-vessels to the bronchial tree
(cf. Schema A). The head gut below the origin of the two bronchi bends
slightly ventralwards and to the left. In this region, which may be
considered the anlage of the stomach, a noticeable dilatation of the gut
is taking place (PI. I, Figs. 3, 4).

In this stage the character of the mesoderm has not changed, the
Mesocardium posterior begins at a lower level owing to the descent ox
the heart, while the dorsal mesentery is now well marked above the
level of the lower extremity of the right bronchus. The two lung wings
are more symmetrical and project further into the ccelom than in the
preceding stage. Nevertheless, they are still asymmetrical in so far as
the projection forming the left lung is higher than that of the right.
Both on the right and left sides, the Recessus pleuroperitonealis is well
marked. In another embryo of the same measurement, but evidently
somewhat better developed, the process of separation of the bronchi from
the cesophagus is well started. The sulcus between the trachea and the
cesophagus extends just above the level of the origin of the two bronchi.
This is filled with mesoderm of a nature similar to that about the head
gut. The mechanical factors involved in the process are difficult to
make out, but it begins by an approximation of the epithelium along the
line of the two lateral fissures and then proceeds upwards from the
suleus formed behind the primitive bronchi which is filled with mesoderm.

At this stage the following formula of the derivatives of the pulmonary
anlage may be made:

TRACHEA.
Right bronchus. Left bronchus.

At 6 mm. (PI. I, Figs. 5, 6) the process of separation is practically
complete, the trachea and cesophagus remaining in communication only
at the upper end. At the point of origin of the two bronchi, the cesa-

Joseph Marshall Flint 31

phagus and trachea are separated by a mass of mesoderm filling the
intervening spaces. The simple bronchial system has increased in length
and caliber, but the relationships are practically the same, save for the
appearance of the rounded terminal buds on the end of the stem bronchi
(PL. I, Figs 5, 6ds). While, in this embryo, the two bronchi still lie
ventralwards to the head gut, they now begin at the ends to bend more
dorsalwards than in the preceding stage, the right a trifle more than the
left. The Mesocardium posterior is still lower than in the preceding
stage, its upper level now beginning only a short distance above the
origin of the left bronchus. The mesodermic syncytium is unchanged.
The lung wings are fairly symmetrical as they project on either side
into the ccelomic cavity. The difference, however, between the right and
left lung bronchi still suffice to give the two lungs a slight asymmetry.
The Recessus pleuroperitonealis is marked on the right side and ex-

Thx Bre, 10:

Text Fig. 10. Section through the primitive lung sacs of an embryo 7.5
mm. long. C=Celom. BD=Right stem bronchus. BS—=Left stem
bronchus. DM Dorsal mesentery. VM —=Mesocardium posterior.

tends some distance above the lower end of the right bronchus, while the
left recessus is almost obliterated.

The next stage in the development shown in an embryo 7.5 mm. long
consists in the complete production of the primitive lung sacs through
the dilatation of the buds on the end of the right and left bronchi (PI. I,
Figs. 7, 8sd). The size of the branches of the primitive tree have
increased markedly, the two dilated lung sacs while still lying ventral-
wards of the cesophagus now bend sharply backwards forming a horse-
shoe-like curve around it (Fig. 10). The left still preserves its position
at right angles to the trachea with a slight growth caudalwards at the
bottom of the sac. On the right side, the general direction of the
bronchus is lateralwards, dorsalwards, and caudalwards. The form of

32 The Development of the Lungs

the dilated sacs is different on the two sides, that on the right is larger
and more nearly triangular in transsection (Fig. 10 BD). It projects
further dorsalwards than the left (Fig. 10 BS). As yet there are no
marked evidences of the production of lateral branches except a slightly
more prominent angle at the upper lateral wall of the right sac and a
similar irregularity of contour on the upper wall of the left. From these
points, as we shall see in the next stage, the paired second lateral bronchi
arise. Just above the origin of the stem bronchi, however, on the right
side of the primitive trachea, one observes a slight bulging or outgrowth
of its wall. At this level, the epithelium is a trifle thicker and numer-
ous mitotic figures occur. The projection extends over an area of about
80 mikra and represents the anlage of the first lateral bronchus (PI. I,
Figs. 7, 8, L. 1). The process by which this structure is produced is
apparently a simple evagination to be compared, perhaps, with the evag-
ination of the pulmonary swelling from the primitive head gut, on the
one hand, and the primitive bronchi from the pulmonary anlage on the
other. ‘Thus we may consider the same process as repeating itself in the
development of the first stages of the pulmonary apparatus. No similar
evagination, however, can be observed on the left side.

In the mesoderm of the lungs, the dorsal mesentery (Fig. 10 DM)
now reaches as high as the forking of the trachea, while the Mesocardium
posterior (Fig. 10 VM) extends as high as the anlage of the tracheal
-bronchus. The mesodermic syncytium itself shows some differentiation,
particularly under the pleura and in the region of the mesocardium and
dorsal mesentery. Here the cells branch and anastomose and the differ-
entiation of the exoplasmic portion into fibrils is in progress. About the
cesophagus and pulmonary epithelium, however, there are dense masses
of mesodermal cells without much differentiation. This group of cells
is engaged in the production of the young basement membranes as the
stems continue in their growth. In consequence of the more equal dila-
tation of the sacs, the simple lung wings are more symmetrical than at
any other period of early embryonic life. Differences, however, between
the two sides on inspection of the reconstructions are readily made out.
The right Recessus pleuroperitonealis extends slightly above the level
of the lower end of the stem bronchus, while the left has disappeared.
At this age we may express the derivations of the pulmonary anlage in
the following tabulation :

TRACHEA.
Lateral 1.
Right bronchus. Left bronchus.
Right lung sac. Left lung sac.

At 8.5 mm. the irregular contour of the lung sacs is lost and the two

Joseph Marshall Flint 33

bronchi continue their growth after the production of the first two paired
lateral bronchi. ‘These appear as lateral evaginations from the walls of
the primitive sacs. On the left side, however, the bud is directed more
cranialwards owing to the horizontal position of the left stem. The
trachea increases in diameter and length; the bronchi, however, still
maintaining the same general relationships, have grown in both caliber
and thickness. Now, the very slight evagination of the tracheal bronchus
has increased considerably in size and projects from the right wall so
as to be noticeable particularly in longitudinal sections from which the
model shown in Pl. I, Figs. 9 and 10, was reconstructed. It is quite
as apparent as the paired Lateral 2 and is separated from the one on the
right side by a distance of approximately 380 mikra. ‘These three bronchi
may be considered as practically contemporaneous branches of the prim-
itive tree with the tracheal bronchus appearing as a very faint evagina-
tion before the lateral bronchi as such can be definitely seen in the
primitive lung sacs.

The two stem bronchi now extend more caudalwards than in pre-
ceding stages; of the left particularly is this true. They also preserve,
although not to such a marked extent, the horseshoe-like dorsal curvature
observed in a pig 7.5 mm. long. On their lateral surfaces are two slight
evaginations, the anlage of the second lateral bronchi (PI. I, Figs. 9, 10,
L. 2). Of these the right project lateralwards, while the left points
upwards. These two projections do not appear from a terminal portion
of the end bud, but from its lateral surface. They are, therefore, the
productions of a monopodial growth The epithelial lining in these
primitive buds is a trifle deeper than in the other parts of the tubes and
in the inner row karyokinetic figures are more numerous than in the
other parts of the respiratory endoderm. 'The mesoderm about the buds
does not appear either thicker or thinner than that on other parts of the
respiratory tube. It is impossible, therefore, that this tissue can exert
any marked growth influence in the production of these lateral branches.
Much more probable are the space relationships to which the tube adapts
itself as, lateralwards in the ccelom, we have one point of least resist-
ance, while caudalwards between the thorax wall and the liver, is another.
The bending of the stomach anlage to the left (PI. I, Figs. 9, 10) for a
time may have some influence on the growth of the left bronchus holding
it in its more horizontal position. From this point the consideration of
the development of the mesodermic portion of the lungs will be dis-
cussed in a separate chapter.

3

a
34 The Development of the Lungs

The branches of the primitive bronchial tree in a pig 8.5 mm. long,
then, may be tabulated as follows:

TRACHEA.
Lateral 1.
Right stem bronchus. Left stem bronchus.
Lateral 2. Lateral 2.

At 10 mm., the trachea (PI. I, Figs. 11, 12 7) has increased consider-
ably in size and Lateral 1, which appeared as a simple swelling in the
earler stages, has now grown to a button-like enlargement (PI. I, Figs.
11, 12, L. 1) sharply constricted from the wall of the trachea. It points
lateralwards and also shghtly ventralwards. The division of the trachea
into right and left bronchi shows still the asymmetry of the preceding
stages as the plane of the left stem is still more transverse than the right.
At the same time the general direction of the right bronchus does not ex-
tend so far dorsalwards, as the growth of the right bronchus has appar-
ently been directed more towards the tail end of the embryo. Just at
the point of bifurcation, the second lateral bronchi on either side are
seen; the one on the right is somewhat larger than the corresponding
branch on the left. Both, however, are now fairly symmetrically placed,
although right Lateral 2 is shghtly more ventral and the left more apical
in its direction. Beneath right L. 2, a slight bulging is visible on the
axial bronchus directed ventralwards. This is the anlage of Ventral 2,
the infracardiac bronchus (Pl. I, Fig. 11, V. 2) which arises directly
from the stem bronchus and not from L. 2. At the same time, directly
opposite the anlage of Ventral 2, there is also a slight dorsal evagination
of the stem, indicating the first traces of Dorsal 2 (Pl. I, Fig. 12, D. 2)
on the right side. The appearance of Ventral 2 (Bronchus infra-
cardiacus) and Dorsal 2 is accompanied by an apparent lateral flatten-
ing of the stem bronchus due to the extension of the buds dorsalwards
and ventralwards from the axis of the mother branch giving it, in cross-
section, a marked oval shape, while above and below, it resumes its
cylindrical form. This may be nicely seen in Figs. 11, 12, and 13, where
11 shows a transsection of the stem bronchus above, 13 below, and 12
at the level of the primitive dorsal and ventral branches (Fig. 12, V. 2,
D. 2). In the inner row of epithelium in these projections, karyokinetic
figures are much more numerous than in other parts of the stem bronchus
save in the neighborhood of the terminal bud. At the same time there
is a packing of the nuclei at the base nearer the basement membrane
which is now less distinct and gives the epithelium the appearance of
having an extra row of cells at this point.

The left bronchus is considerably shorter than the right and projects

Joseph Marshall Flint

ise)
OL

more lateralwards. Its stem is cylindrical in form and it terminates
im a rounded bud-lke swelling in which mitoses are numerous. No
evidences of Ventral 2 or Dorsal 2 are seen. If we turn for a moment to
the consideration of the origin of L. 1, we find the bronchus is a trifle
more precocious, but practically simultaneous with the second lateral
branch in its origin. It is separated from Lateral 2 by a considerable
distance. If the views of Willach and Narath were correct, this branch
should not appear until later, and should be traceable, step by step, from
the bud forming right Lateral 2 to its final position on the trachea. Its
direction is practically lateralwards with a scarcely visible tendency to
point ventralwards. It would not then, from the topography of its ori-
gin, bear any analogy to a dorsal bronchus. From this distinctly lateral
position of its origin, I have classed it among the lateral group of bronchi,
although, in its subsequent growth, one of its branches extends down into

TXT IGse alent 2 sand ls:

Text Fias. 11, 12, and 13. Sections through the right stem bronchus of
an embryo 10 mm. long. Fig. 11 above, Fig. 12 through, Fig. 13 below the
origin of Ventral 2 and Dorsal 2. V=—=Ventral. D=Dorsal. C= Pleural
cavity. S—=Stem bronchus. V.2—Ventral 2, the Bronchus cardiacus. D. 2
= Dorsal 2.

the dorsal region giving the bronchus a certain superficial resemblance to
that series. On the other hand, the lower lateral elements grow ventral-
wards in the later embryonic stages and thus also lose their early strictly
lateral character. This much is certain; if L. 1 arises phylogenetically
from the dorsal group, a view for which there is no convincing proof,
absolutely all trace of the migration is lost in the pig. It originates like
one of the lateral bronchi and subsequently develops its superficial re-
semblance to the dorsal elements. Whatever support for the relation-
ship of the broncnus to the dorsal series, can be drawn from this fact, is
multiplied by the behavior of a lateral branch of left L. 2, which does
exactly the same thing in an adaptative process on the part of the bron-
chus to a relatively unobstructed environment.

Similarly, Ventral 2 is produced after the formation of Lateral 2
simply as an evagination of the walls of the stem bronchus. It occurs at

36 The Development of the Lungs

a level below the point where the stem bronchus has already regained its
cylindrical form after the production of the second lateral bronchus on the
right side. Of the possibility of its being a branch of Lateral 2, in these
specimens, there is not the shghtest evidence. In this particular lung, D.
2 and V. 2 are given off at practically the same level. This is, however,
not always the case as one, usually the ventral, may arise higher up.
It is this variability in the origin of these branches which gives rise in
the adult tree to the series of stages, which simulate a transplantation
of the Ventral 2 from Lateral 2 to the stem bronchus. They represent,
however, simply a normal range of variation in the origin of the bronchus.
Narath states the wax-plate method is not adapted to the study of these
branches and has, for the most part, used specimens cleared in oil of cloves.
In my experience, the latter method is valuable only for the lateral bronchi
where the buds are seen in profile and, therefore, are sharply outlined.
In such specimens, either the dorsal or the ventral series must be studied
not only through the mesoderm, but also through the entire thickness of
the stem bronchus. In looking upon the surfaces of such buds as D. 2
and V. 2 in an embryo like that represented in Pl. I, Figs. 11 and 12,
the slight projections forming the anlagen of these branches are invisible
because they cannot be studied in contour. After they have developed
into well-formed buds, they are quite apparent in cleared preparations,
particularly when the stereoscopic microscope is used. By that time, how-
ever, the important stages of their origin are lost. So far as is known
to me, reconstructions, controlled and supplemented by cleared and dis-
sected specimens afford us the only opportunity to see the first traces
of these branches. For such schematic pictures as shown by Narath,
96 (Text Figs. 1, 2, 3), which represent schemata of the origin of his
apical bronchus and V. 2 from the bud of L. 2, I can find, in the pig, no
parallel. Furthermore, the bud of V. 2 is shown in the schemata before
the apical bronchus appears, while in the pig the latter is either the
independent precursor or the contemporary of Lateral 2, while Ventral
2 is not formed until after the other two branches are well developed.

At this stage the following divisions have appeared in the primitive
bronchial tree:

TRACHEA.
Lateral 1.
Right stem bronchus. Left stem bronchus.
15 Py. Mee
WVa2e
IDE

In a reconstruction of the bronchial tree of a pig 12 mm. (PI. I,
Figs. 13, 14) the trachea and stem bronchi have increased considerably

Joseph Marshall Flint 37

in size. At the same time, Lateral 1, the tracheal bronchus (PI. I, Figs.
13, 14, L. 1) has grown further lateralwards. Its terminal bud beyond
the constriction near the point of origin bends somewhat ventralwards
in conformation to the topography of the environment of the thoracic
cavity at this level. Its general course after its origin is dorsalwards
causing its lower extremity to overlap the upper part of L. 2 (Pl. I,
Figs. 13, 14, L. 2). The asymmetrical characteristics of the two-stem
bronchi are also maintained, the right extending lower and nearer the
midline than the left, which projects more lateralwards. They also bend
slightly dorsalwards. It is probable, however, that the asymmetry of this
specimen is extreme, as I possess other specimens at this age in which
the two sides, while markedly asymmetrical, are more nearly enanteo-
morphic than this one. In order to control this specimen, it was re-
constructed a second time with exactly the same results. Of the two
second lateral bronchi (Pl. I, Figs. 13, 14, L. 2), the right extends a
little farther lateralwards and ventralwards than the left, its growth
being influenced at this stage by the presence of L. 1 above and behind
it. The left, however, with practically unobstructed environment grows
lateralwards and dorsalwards and upwards at this period. Both are
terminated by the end buds, which lke that on L. 1, are in a stage pre-
paratory to division. On the right side, Ventral 2 (Pl. I, Fig. 13, V. 2)
the Bronchus infracardiacus has developed to a button-like bud on the
ventral portion of the stem bronchus separated from it by a sharp con-
striction at the base. It is not so well developed as the two second
lateral bronchi or L. 1. On the corresponding portion of the left stem
bronchus, no analogous branch has appeared. It remains, in fact, naked
through the whole future development of the tree. Neither is there in
the pig, at this or later stages, a branch which forms at this point and
subsequently wanders up on left L. 2, as d’Hardiviller suggests, to form
the so-called cardiac bronchus of Hasse. On the lateral sides of both
stem bronchi, buds forming Lateral 3 (Pl. I, Figs. 13, 14, L. 3) have ap-
peared. These extend directly lateralwards for a short distance to termi-
nate in swollen bud-like extremities, while the portion near the stem
bronchus has a definite constriction. Of the two, the right is shghtly larger
than the left. From this point on, the stem bronchus continues caudal-
wards to terminate in the enlarged end buds. On the right side, the axial
bronchus extends considerably lower than on the left. On the dorsal
side of the stem between L. 2 and L. 3, appears on each side, the bud
representing Dorsal 2 (Pl. I, Fig. 14, D. 2). That on the right side ap-
pears before the left and is a trifle more developed. The left, however, is
quite apparent. It is also possible that either of these buds may not be

38 The Development of the Lungs

formed, in which case this area of the stem remains naked throughout
life. This state of things, while occurring seldom, is found oftener on
the right than on the left side and the cause may possibly be due, in this
particular instance, to the presence of the rapidly growing Ventral 2, to-
gether with the presence of L. 1 above, or otherwise simply to the general
tendency for the tree to vary within wide limits. As in the case of the
ventral and lateral group, the position of these dorsal buds may vary from
complete suppression to a position on the stem at the level of L. 2, or to
one opposite Lateral 3. The usual situation is about midway between the
second and third lateral branches. These buds are the same as Narath’s
Dorsal 2 and Aeby’s Dorsal 1. Our results agree with Aeby’s designation
as Narath, in considering Lateral 1 and a dorsal branch, was forced ac-
cordingly, to change the denomination of his dorsal series. Like Ventral
2, I have designated the first dorsal bronchus as D. 2, simply to keep it
in harmony with the lateral series.

At this period the following branches of the bronchial tree have de-
veloped:

TRACHEA.
1 ae
Right stem bronchus. Left stem bronchus.
Ibe Ibs 2
Wind: :
1D); 2 D. 2.
Ibe, BF Ibe Bk

13.5 mm. (Pl. II, Figs. 15, 16). At this stage the trachea is slightly
larger and somewhat longer than in the preceding embryos. On its right
side passing dorsolaterally is found L. 1 (Pl. II, Figs. 15, 16, L. 1)
which has undergone division and yielded two practically equivalent
branches, one of which passes downwards and dorsalwards (PI. II, Figs.
15, 16 di) and the other lateralwards and slightly upwards (Pl. I,
Figs. 15, 16 vs). These primary subdivisions, terminating in rounded
buds, represent in the adult the dorsoinferior and the ventrosuperior
branches of L. 1. At this stage, the two halves of the lung are much
more symmetrical than we have seen them in any of the preceding recon-
structions. The trachea and two main bronchi denuded of their side
branches, now have more or less of a wish-bone shape. The trachea
passes ventralwards to the origin of the stems and then, as the two axial
bronchi diverge from the poiat of union, they also pass somewhat dorsally
and reach their maximum point of separation at the level of the third
lateral bronchi.. From this point, as the end buds are approached, they
again converge towards the median line. The right is only slightly larger

Joseph Marshall Flint 39

and more developed than the left. At the same time, there has been a
more symmetrical readjustment of the two second lateral bronchi, making
them both with reference to their direction and the distance which
separates them from the trachea practically mirror images of each other.
L. 2 on the right side passes laterally and somewhat superior, undergoing
like the tracheal bronchus a division into two practically dichotomous
branches. Of these, one branch, which will continue as the main
bronchus (PI. II, Figs. 15, 16 i) les ventralwards, while the other is
directed dorsally and slightly inferior. The latter is the dorsal inferior
branch (PI. II, Figs. 15, 16 di) of the right L. 2 in the adult, and its
downward course is due, as we shall see later, to the presence of L. 1
above, which prevents its growing upwards to the apex of the lung like
the corresponding branch of the left side (Pl. II, Figs. 15, 16ap). In
comparing the growth of the three first divisions of the bronchial tree
until they have reached their present development, it is possible to note
in the progress of L. 1 and L. 2 on each side their passage through
practically the same stages simultaneously. If the apical branch of
L. 2 on the left side is equivalent of L. 1 or the tracheal bronchus as
Willach, Narath, and others suggest, it is difficult to explain the tardy
appearance of the left element and to give a reason why the right should
be so well developed. As a matter of fact, this apical branch of the left
Lateral 2 is not the homologue of L. 1, but of the dorsoinferior branch of
right Lateral 2, a branch, which, in the adult lung, is practically but not
quite as well developed as the apical branch itself. The difference be-
tween the two lies in the different nature of the environment in which
they grow. Of equivalent age and value in the bronchial tree, the dorso-
inferior branch on the right side, influenced by its space relationships
and the presence of L. 1 above is forced to grow downwards and back-
wards, while on the left side, the corresponding branch, unobstructed
through the absence of L. 1, mounts upwards to the apex of the lung
to supply the territory through which the tracheal bronchus runs on
the opposite side. This power of substitution, which the bronchi pos-
sess is not confined to this branch alone, but may take place in many
other parts of the tree, as we shall see in the later stages. In my corro-
sions, I have never found an instance of the suppression of L. 1 in the
pig. Narath, or (Pl. VII, Fig. 5), however, shows a case in the human
lung which indicates how, under these circumstances, this dorsoinferior
branch of right L. 2 with an unobstructed environment may take a
course almost exactly like the corresponding branch on the opposite side.

Arising as in the preceding stages from the axial bronchus between
L. 2 and L. 3, Ventral 2 on the right side has increased considerably in

40 The Development of the Lungs

length and passes ventralwards, medianwards, and caudalwards. At its
terminus there is a definite bud. The corresponding portion of the stem
bronchus on the left side, however, remains nude. In seeking for an
explanation of the cause for the extreme development of Ventral 2 on the
right side and its usual absence on the left, I have been impressed with
the extreme adaptability of the lung to its environment and the way in
which the bronchi follow mechanical principles in growing along the
lines of least resistance. We realize, of course, the fact that the lungs
are relatively late accessions to the animal economy, that they also, ex-
cepting possibly in marsupials, are functionless until the period of
birth. It is natural, therefore, to find them secondary to and influenced
by such organs as the heart and liver, as well as the chest wall by which
they are surrounded. These, moreover, have chronologically the de-
velopmental precedence and are of definite functional use during the
embryonic life of the organism. For the suppression of left V. 2 there is
an explanation as we shall see in the chapter on the development of the
pulmonary vessels and we may look upon the hyperdevelopment of right
V. 2 as an effort to fill up the space which exists especially in quadrupeds
between the heart and diaphragm in the region of the median plane.
Dorsal 2 (Pl. I, Fig. 16, D. 2), situated between L. 2 and L. 3, shows a
slight growth over the preceding stages, but still persists simply as a
slight projection from the axial bronchus. The third lateral (PI. IT,
Figs. 15, 16, L. 3) has increased in size over the corresponding branch
in a younger embryo, and now possesses a more distinct terminal bud.
There is, however, no indication of division as yet. On the ventral side
of the axial bronchus, just beneath L. 3, there appears a slight swelling,
indicating the origin of the third ventral bronchus (Pl. II, Fig. 15,
V. 3). Directly behind it, on the dorsal surface of the stem, is a pro-
tuberance showing the point of origin of Dorsal 3 (Pl. II, Fig. 16, D. 3).
Below these two branches, there is on the lateral side of the axial
bronchus a bud indicating the point of origin of Lateral 4 (Pl. II, Figs.
15, 16, L. 4), while the axial bronchus continues downwards and ends in a
terminal swelling on which some signs of the origin of L. 5 (PI. II,
Figs. 15, 16, L. 5) are already shown.

At this level, an evagination (Pl. II, Figs. 15, 16 MS) appears on the
inner side of the end bud pointing medialwards and slightly dorsalwards
just opposite the bud of Lateral 5. This is the first one of the medial
series to appear on the reconstructions. ‘They are, however, extremely
variable both in their constancy and origin. In some trees they are en-
tirely absent, in others they may occur with great regularity, but never
in my specimens, which included sections and corrosions of over one hun-

Joseph Marshall Flint 41

dred lungs, do they occur higher than a short distance above the level of
Lateral 4. They may exist only on one side or else on both. Like the
other series, they arise as lateral outgrowths of the bronchial stem, not as
secondary derivations of the dorsal series, according to the processes de-
scribed by either Narath or Robinson. It is interesting, moreover, to note
the relation of this group to the cesophagus. In the higher levels where
the cesophagus lies between the stem bronchi, no medial bronchi occur, in
the lower levels, however, as the cesophagus passes ventralwards to the
stems, leaving the medial surfaces of the lung free, these branches are
produced. Text Fig. 26 shows these conditions well. The edge of the
cesophagus is seen in cross-section, while from the median wall of the end
bud below it an evagination which will form a Medial 4 or 5 is clearly
seen (Fig. 26M). This would seem to indicate another adaptation on
the part of the tree, to its space relationships.

On the left side, L. 2 (Pl. II, Figs. 15, 16, L. 2) is directed lateral-
wards and slightly dorsalwards. Like the corresponding bronchus on
the right side, there has been a dichotomous division, which has yielded
two branches, one directed dorsally and superior (PI. II, Figs. 15, 16 ap)
and the other lateral and ventral. The latter is the continuation of the
main bronchus, while the former constitutes the apical branch, or
Bronchus ascendens of His, of L. 2 on the left side. Owing to the
unobstructed possibility of its growth upwards, inasmuch as there is on
the left side no L. 1, this branch, as we have seen, grows in a slightly dif-
ferent direction from the corresponding division of the same lateral
bronchus on the right side, but, for the reasons given above, it must be
viewed as distinctly homologous with the dorsoinferior branch of right
L. 2. This branch and its relationships may be seen in many of Narath’s
illustrations, from which the nature of its origin is as clearly shown as
in the pig’s lung. On the left side, there is no V. 2, but between L. 2
and L. 3, the Dorsal 2 (Pl. II, Fig. 16, D. 2), which already appeared
in the earlier stages, is now well marked.

Lateral 3 (Pl. I, Figs. 15, 16, L. 3) is directed laterally and possesses
a distinct bud at the end. It is also directed slightly dorsalwards, occu-
pying a plane almost identical with the second lateral branch above. On
the ventral surface of the axial bronchus, just below the point of origin
of the third lateral, a small projection indicates V. 3 (PI. II, Fig. 15,
V. 3), while behind the stem bronchus, but somewhat lower, a similar
projection marks the origin of the D. 3 (Pl. II, Fig. 16, D. 3). The
fourth lateral bronchus (Pl. I], Figs. 15, 16, L. 4) exists at this stage

42 The Development of the Lungs

simply as a shght projection from the lateral wall of the axial bronchus
as it continues downwards and ends in a terminal bud. The following is
a tabulation of the tree in a pig of this age:

TRACHEA.
1, ale
(2) DE:
(2) VS.
Right bronchus. Left bronchus.
1.2: Ibe},
(2) DI. (2) Ap.
(2) LI. (2) Gh:
Were:
1D} 2 D. 2
ie Ibe &
Weed Wee
Drs Deo
L. 4 L. 4
15
M. 5

In a pig 15 mm. long (PI. I, Figs. 17 and 18) the trachea has in-
creased in size and passes somewhat ventralwards to the point of bifurca-
tion. On the right side and directed slightly dorsal and inferior, is Lat-
eral 1 (Pl. II, Figs. 17 and 18, L. 1). A short distance from its point
of origin, the ventral superior (Pl. II, Figs. 17, 18, vs) and dorsal in-
ferior (Pl. II, Figs. 17, 18, di), branches are seen. These, in turn, now
give rise to secondary branches. On the dorsal inferior branch, the first
division (Pl. II, Fig. 18, d) is directed dorsally and somewhat medially
This is the first main dorsal branch of the dorsoinferior in the adult lung,
The other division continues on as the stem branch. The ventral superior
bronchus passes laterally and superior. It now possesses a branch (PI. II,
Fig. 18, d) passing dorsally and slightly upwards. This is the dorsal
branch of the ventrosuperior division of L. 1, and is found usually in
the adult lung. ‘The more general symmetry of the two main divisions of
the trachea noted in the last reconstruction persists, the trachea passing
downwards to the point of division and the right and left bronchi, as in
the last stage, form with it a structure suggestive of a wish-bone. The
axial bronchi bend laterally, dorsally, and medially, their point of widest
divergence being now opposite the fourth lateral bronchi, a relation which
persists in adult life, and with which the cesophagus, passing ventralwards
at this level, probably has something to do. On either side, the second
lateral bronchi pass lateralwards, then bend slightly dorsalwards and
finally at their tips begin to bend ventralwards again. This indicates the

Joseph Marshall Flint 43

first appearance of the folding of the lung wings around the heart and
liver, a process which is naturally directed largely by the form of the
chest wall and shows another adaptation of the bronchi to the space in
which they have to grow. As yet, however, the remaining lateral bronchi
have not developed sufficiently to. bend towards the ventral side of the
body. On the right side, L. 2 has increased considerably in length, but
possesses no more branches than the reconstruction of the preceding stage.
The dorsal inferior branch, however, is considerably longer, and now
grows distinctly downwards and lateralwards. Owing to the presence of
Lateral 1, with the Lobus superior above and a consequent lack of space,
this branch does not grow as rapidly as the relatively unobstructed corre-
sponding branch on the left side, which, at this stage, is somewhat further
advanced in its development. V. 2 (Bronchus infracardiacus) passes
from its point of origin on the ventral side of the axial bronchus between
L. 2 and L. 3, downwards, ventralwards, and medialwards. It is divided
into branches of equal size, the first passing somewhat inferior (PI. II,
Fig. 17, 7) and somewhat lateral, forms the inferior branch of the infra-
cardiac bronchus in the adult. The other division passing more medial-
wards, is the continuation of the main bronchus. From the dorsal side of
the stem, D. 2 (Pl. II, Fig. 18, D. 2) arises and subdivides into two short
branches, the upper and median of which forms the median branch
(Pl. II, Fig. 18 D. 2, m) of this trunk, while the other continues
as the main bronchus. L. 3 (PI. I, Figs. 17, 18 L. 3) passes lateral-
wards and shghtly dorsalwards and, while considerably longer than in the
preceding stages, it possesses as yet no secondary divisions. V. 3 on the
right side is, in this specimen suppressed. It is noteworthy that next to
Ventral 2 of the right side, this element of the ventral series is most often
missing, a fact which may easily be accounted for by the hyperdevelop-
ment of Ventral 2, which does not, as a rule, leave much territory in this
region to be supphed by a ventral bronchus in this segment of the tree.
Dorsal 3 (Pl. IH, Fig. 18, D. 3) has grown considerably in size and now
possesses a terminal bud. The fourth lateral (Pl. I], Figs. 17, 18, L. 4)
shows a marked growth and is provided with an end bud, while between it
and L. 5, on the ventral side of the axial bronchus a small projection indi-
cates the fourth ventral bronchus (PI. I, Figs. 17, V. 4). Immediately
opposite it, D. 4 (Pl. II, Fig. 18, D. 4), arises as a small bud from the
dorsal aspect of the axial bronchus, while Lateral 5 (Pl. II, Figs. 17, 18,
L. 5) originates from the outer side of the stem as a small bud-like pro-
jection. From this point, the axial bronchus passes downwards and
terminates in a slight end bud. On the left side, Lateral 2 shows a

44 The Development of the Lungs

marked growth of its apical branch (PI. II, Figs. 17, 18, ap), which
passes upwards and dorsalwards and terminates in two branches, one
of which passes dorsally and inferior and indicates its first dorsal branch
(Pl. II, Fig. 18, L. 2, d), while the other continues upwards as the
extension of the stem of this bronchus. Near the extremity of L. 2
another branch is given off, which extends ventralwards and inferior
(Pl. II, Fig. 17, L. 2, vt). This corresponds to the ventroinferior
division of the bronchus in the adult lung. As there is no ventral
bronchus between L. 2 and L. 3 on the left side, the axial bronchus
remains at this point perfectly smooth. The second dorsal bronchus
(Pl. II, Fig. 18, D. 2) of this embryo is placed somewhat lower than the
corresponding branch of the opposite series and arises just above the
point where Lateral 3 originates. Like its homologue, it shows a subdivi-
sion into two secondary branches, one of which is the regular medial
branch (PI. I], Fig. D. 2, m), while the other forms the stem of Dorsal 2.

Texr Hie. 14.

TEXT Fic. 14. Section through the left lung of a pig 14.5 mm. long, showing
the median evagination of the end bud to produce Medial 5. V= Ventral.
D=Dorsal. M=Medial 5. S=lumen of end bud.

The third lateral bronchus (Pl. I, Figs. 17, 18, L. 3) grows lateralwards
and dorsalwards, and is not provided with secondary branches at this
stage. Appearing as a small bud from the ventral aspect of the axial
bronchus, a short distance above L. 4 is Ventral 3 (Pl. II, Fig. 17, V. 3),
while at a point about opposite this branch and a little above, Dorsal 3
(Pl. II, Fig. 18, D. 3) also originates as a small bud from the posterior
surface of the stem bronchus, approximately midway between L. 3 and
Lateral 4. The latter (Pl. II, Figs. 17, 18, L. 4) is somewhat shorter
than the third, and has no secondary divisions. Ventral 4 (PI. I, Fig.
17, V. +4) appears as a very faint swelling of the ventral aspect of the
axial bronchus below L. 4+. In a corresponding position on the opposite
side of the main bronchus Dorsal 4 appears (PI. II, Fig. 18, D. 4) also
in the form of a slight evagination from the stem. Lateral 5 (PI. II, _
Fig. 17, L. 5) is merely indicated by a slight swelling on the side of the

Joseph Marshall Flint 45

terminal bud of the axial bronchus. About opposite it on the inner side
of the stem bronchus, is an evagination marking the anlage of a bronchus
of the medial series (Pl. II, Figs. 17, 18, M. 5) like that seen on the right
side at a similar point on the tree in the reconstruction of the preceding
stage. Fig. 14 shows a section through the end bud where this element
is in process of formation. The numerous karyokinetic figures and the
definite extension of the evagination from the median portion of the
lumen of the bud (Fig. 14, 8) is clearly shown. This picture, when com-
pared with the reconstruction and Text Fig. 26, indicates that there is
no essential difference in the method of formation of these branches of
the stem. Like the dorsal, ventral, and lateral elements, they are pro-
ducts of monopodial growth. |

Following is a tabulation of the derivatives of the bronchial tree at
this stage:

TRACHEA.

Ideally

(2) DI.

(33) J2)
(2) VS.
(ep

Right bronchus. Left bronchus.
1652 ree

(2) DF. (2) Ap.

(3) D.
Can

WV. 2:

(2) £.
ID), Dp. 2.

(2) MU. (2) M.
105 8} Ike, ot
V.3 Vira
L. 4 L. 4
D. 4 D. 4
V.4 vV.4
5 Ibn &

M.5

In a pig 18.5 mm. long (PI. II, Fig. 19, Pl. III, Fig. 20) the trachea
is only a little larger than in the preceding embryo. It still passes
slightly ventralwards from the upper end to the point of bifurcation.
On the right side passing downwards and slightly dorsalwards, one finds
Lateral 1 (Pl. II, Fig. 19; Pl. III, Fig. 20, L. 1), which divides almost
at right angles into its main divisions, the dorsoinferior (Pl. II, Fig. 19;
Pl. III, Fig. 20, L. 1, dv) and ventrosuperior (Pl. I, Figs. 19; Pl. ITI,

46 The Development of the Lungs

Fig. 20, L. 1, vs) branches. The dorsoinferior passes downwards and
dorsalwards and terminates in the neighborhood of D. 2, a relationship
which persists to the adult stage as its further growth downwards is now
checked by the series of dorsal bronchi below. This branch shows new
divisions over the preceding stage as we find besides the dorsal branch,
which passes dorsalwards and medialward, a lateral branch (PI. III,
Fg. 20, 1) arising about the same level, which passes laterally and dor-
sally. Both of these divisions terminate in end buds. The main stem
of the bronchus continues downwards to its termination, which is marked
by slight end swelling. The ventrosuperior or apical branch (Pl. U1,
Fig. 19; Pl. III, Fig. 20, vs) of L. 1, extends further cephalad than in
the earlier stages. Besides the dorsal branch indicated in the preceding
reconstruction, which shows signs of division, a lateroinferior branch
(Pl. II, Fig. 19; Pl. III, Fig. 20, 1) is given off somewhat further on,
which passes at this time downwards and shghtly outwards, and forms
the first lateroinferior branch on this bronchus of the adult tree. The
main stem continues upwards and ends in a terminal bud. The trachea
and the stem bronchi still preserve the characteristic wish-bone appear-
ance noted in the two preceding reconstructions. The two axial bronchi
bending lateralwards, dorsalwards, and medialwards, the point of widest
separation being, as in the earlier stages, about the level of the fourth
lateral bronchi. In the preceding reconstruction, the beginning of the
ventral growth of the two wings of the lung were apparent on Lateral 2.
This action is now also shown on the third lateral branches. The first
pair, however, curve around the heart, while those of the lower series
follow the chest wall and the curvature of the diaphragm over the
liver. The fourth, fifth, and sixth lateral divisions still pass outwards
and slightly backwards without showing this bending at the extremities. -
On the right side, the second lateral bronchus arises about the point of
bifurcation of the trachea, and passes slightly ventralwards, then runs
upwards, slightly dorsalwards, and again ventralwards, preserving its
course practically in one horizontal plane. In this specimen the first
branch is a ventroinferior (Pl. IJ; Fig. 19, L.2, vt), which extends
downwards and ends in a bud, while the dorsoinferior branch (Pl. II,
Fig. 19; Pl. Ill, Fig. 20, L.2, dv), which is scarcely larger than the
preceding stage, is the second branch of Lateral 2. This condition indi-
cates one of the very important factors in the growth of the bronchi,
namely the ability of either branch after a division to continue on as a
stem. In nine out of ten cases, the ventral fork, after the first division
of Lateral 2, produces the main trunk, leaving the dorsal fork as the large

Joseph Marshall Flint 47

dorsoinferior branch, which is the equivalent of the apical branch on L. 2
of the left side. In this specimen, however, the ventral fork becomes the
ventroinferior branch and the dorsal fork continues as the main bronchus,
giving rise to the dorsoinferior branch only after undergoing another
subdivision. In a much smaller percentage of lungs, the same thing
happens on the left side, the ventral fork giving rise to a ventroinferior
branch, while the dorsal grows on as the stem, producing the apical or
stem only after passing through another division at the end. When this
state of affairs occurs, we have the so-called “ cardiac bronchus of Hasse,”
which d’Hardiviller believes is formed on the stem bronchus in the space
for left V.2, and then wanders up to Lateral 2. Of course in some
animals Ventral 2 is formed regularly on the left side, and in others as a
variation which establishes the symmetry of this segment of the tree.
In the pig, however, owing to the relations of the pulmonary vein to
this part of the stem (see chapter on pulmonary vessels), I have never
seen a left Ventral 2. This power of the bronchi gives us a suggestive in-
sight into the adaptations of the growing branches. The selection of the
division to continue as the stem is probably governed largely by the physi-
cal environment in which the branches find themselves. As the conditions
are usually the same, the same branches ordinarily become the stem, but
if these are changed, what generally forms the stem is shunted off to
become a side branch of relatively small size, while the division which
usually constitutes the side branch, grows out as the stem and produces a
numerous progeny of lateral divisions. In other words, the extent of the
growth of a branch depends to some degree upon the nature of its physi-
cal environment. As I have stated above, owing to the generally fixed
conditions, the major branches, especially such important ones at Lateral
2, have ordinarily a fixed type of division, but further out on the laterals
or in the lower divisions, like Lateral 4 or 5 for example, this interchange
of forks frequently takes place, as almost every specimen shows variations
in the order of the branching.

The next division of the L. 2 is the ventrosuperior (Pl. II, Fig. 19;
Pl. III, Fig. 20, L. 2, s), projecting from the main bronchus just ex-
ternal to the dorsoinferior branch, while a short distance lateralwards and
dorsalwards is given off a dorsosuperior branch (PI. II, Fig. 19; Pl. II,
Fig. 20, L. 2, ds), which already shows indications of division. These
branches represent apparently branches of the second order, but in reality,
after a dichotomous division, each segment of the stem between the suc-
cessive branches is equivalent in its order to that of the last lateral
division. In the adult lung these branches are all easily recognizable.

48 The Development of the Lungs

Ventral 2, the infracardiae bronchus, has grown markedly, and presents
a long inferior branch (PI. I, Fig. 19, V. 2, 7), which passes downwards
and ventralwards and is indicated in the architectural history of the
younger stages. The next division is a small bud from the upper por-
tion of V. 2. (Pl. IL, Kiss 19) V2 2) us) which has a yentrosupenor
direction and is found in specimens of the adult tree. This branch is
small and at this stage consists simply of a shghtly marked bud from
the main bronchus. In most of the corrosions I have made of the lungs
of older embryos it always shows by its flattened spreading branching that
it is more or less influenced by the presence of the heart above it. The
ventroinferior branch (Pl. II, Fig. 19, V. 2, vt), which is the next in
order, is a slight bud, passing downwards and slightly ventralwards, and
which, it may be worth while observing, with the inferior branch, some-
times substitutes for Ventral 3, when it is suppressed. After this branch,
the main bronchus continues on to terminate in slight end swelling.

Here we are able to observe again the mechanical influence of environ-
ment on the growth of a bronchus. The inferior group of branches of
Ventral 2 have space in which to grow and are accordingly of exaggerated
size in comparison with the superior group, which cannot attain such ex-
tensive development, owing to the presence of the heart above them. In
this bronchus, as well as in the laterals, we also have the possibility of
propagation of the stem through either branch of a dichotomous division,
as I have a number of specimens on which the ventrosuperior division
arises before the inferior, indicating in these specimens, the use of the
latter as the stem with the inferior branch arising from a subsequent
forking. Right Dorsal 2 (Pl. III, Fig. 20, D. 2) of this specimen has
not developed as far as the corresponding bronchus in the preceding stage,
the terminal bud merely suggesting an approaching division, which was
already well advanced in the bronchial tree from a 15 mm. pig. Such
variations, however, are not uncommon. ‘The third lateral bronchus (PI.
II, Fig. 19; Pl. III, Fig. 20, L. 3) passes outwards and slightly ventral-
wards. From its dorsal aspect, a dorsal branch (PI. I], Fig. 19; Pl. III,
Fig. 20, L. 3, d) originates, which terminates in the swelling already
showing signs of division. The third ventral bronchus (PI. I, Fig. 19,
V. 3) arises from the ventral aspect of the stem, between Lateral 3 and 4
and grows downwards, apparently influenced by the marked development
of Ventral 2 above it. Dorsal 3 (PI. III, Fig. 20, D. 3), passes dorsal-
wards and lateralwards, and has a well-marked median branch (PI. III,
Fig. 20, D. 3, m) which terminates in a large bud, while the main
bronchus points somewhat dorsally and laterally. Lateral 4 (Pl. II,

Joseph Marshall Flint 49

Fig. 19; Pl. III, Fig. 20, L. 4) has a definite ventral bud and at its
ends is undergoing division. The fourth ventral bronchus (Pl. II,
Fig. 19, V. 4) is somewhat smaller than the V. 3, and appears as a
constricted button-like bud from the ventral aspect of the axial trunk,
while Dorsal 4, arising at a somewhat higher level on the opposite side of
the stem ends in a relatively large bud, which is as yet undivided. From
the lateral aspect of the axial bronchus Lateral 5 (Pl. I], Fig. 19; Pl. III,
Fig. 20, L. 5) takes origin, and ends in a terminal bud without division.
D. 5 (Pl. Ill, Fig. 20, D. 5) is the smallest of the dorsal branches
on this side, and appears simply a pedunculated projection from the
dorsal aspect of the main stem, while the fifth ventral bronchus is present
solely as a slight elevation or projection (Pl. II, Fig. 19, V. 5) from
the ventral wall of the axial bronchus which, continuing caudalwards,
ends in a terminal bud.

On the left side Lateral 2 (PI. II, Fig. 19; Pl. III, Fig. 20, L. 2), which
was practically symmetrical with the corresponding branch on the right
side in a pig 13.5 mm. long has now, in the rapid development of its
main branch, lost even more than in the preceding stage its symmetrical
relationships with right L.2. The ventrosuperior or apical branch (PI.
II, Fig. 19; Pl. III, Fig. 20, L. 2, ap) is markedly increased in size,
and now arises from the more superior aspect of the bronchus and passes
superiorly and slightly dorsalwards. Its termination has reached a
height equal to the point of origin of the tracheal bronchus on the right
side. From its dorsal aspect, the first dorsal branch (PI. III, Fig. 20,
L. 2, d) is derived, which is now subdivided into two regular buds. A
little higher, the lateral branch (PI. III, Fig. 20, L. 2, 1) is seen, while
the apical end of the bronchus is in the stage of division. Further lateral-
wards, on L. 2 a dorsosuperior branch (PI. III, Fig. 20, L. 2, ds) origi-
nates, which has a marked bud and is in process of division, while the
next is an inferior or ventroinferior branch (Pl. II, Fig. 19; Pl. III,
Fig. 20, L. 2, vt) existing simply as a small pedunculated projection from
the under surface of the bronchus. Lateral 2 terminates in a bud, which
has undergone definite division, but the resulting branches are not yet
sufficiently characteristic to be placed with reference to the adult tree.
Inasmuch as Ventral 2 on the left side is always missing on the pig’s
lung, that aspect of the main bronchus remains perfectly smooth. At this
period, however, the Vena pulmonalis already overlies this portion of the
axial stem, but, for the sake of clearness in the illustration, it has been
placed in approximately the median plane. Dorsal 2 (Pl. III, Fig. 20,
D. 2) arising just above L. 3 passes dorsalwards, and has two marked

4

50 The Development of the Lungs

bud-like projections, one of which represents the median branch (PI. ILI,
Fig. 20, D. 2, m), usually the first branch of the dorsal series, which is
already indicated in the preceding construction. Lateral 3 (Pl. II, Fig.
19; Pl. III, Fig. 20, L. 3) passes lateralwards and slightly ventralwards.
It has a well-marked dorsal (Pl. III, Fig. 20, L. 3, d) and somewhat
further out a ventrosuperior branch (Pl. II, Fig. 19; Pl. Il, Fig. 20,
L. 3, vs), both of which are represented in the adult lung. The continu-
ation of the bronchus ends in a bud, which is already undergoing further
division. At a point just above the fourth lateral, Ventral 3 (Pl. II,
Fig. 19, V. 3) arises, and ends in a slight terminal swelling. Dorsal 3
(Pl. III, Fig. 20, D. 3) is considerably smaller than D. 2, and also
smaller than the corresponding branch on the opposite side, but is
already divided into two buds, one of which represents the median branch
of this bronchus, while the other forms the stem. Such variations
in size as are shown in this instance, however, occur very frequently.
Lateral 4 is somewhat shorter than L. 3, and has a well-marked ventral
and a less marked dorsosuperior branch. The fourth ventral bron-
chus (Pl. II, Fig. 19, V. 4) is slightly smaller than the third and
arises from the corresponding position in this interspace, while D. 4
(Pl. Ill, Fig. 20, D. 4) is considerably longer than the third, and
ends in a bud which is not yet divided. Lateral 5 (Pl. Il, Fig. 19;
Pl. Ill, Fig. 20, L. 5) terminates in an undivided bud, and V. 5 (PI.
II, Fig. 19, V.5) consists simply of a slight bulging of the epithelial
wall of the axial bronchus. Similarly the fifth dorsal (Pl. III, Fig. 20,
D.5) is merely suggested by a faint projection from the epithelial tube.
Lateral 6 (Pl. II, Fig. 19; Pl. III, Fig. 20, L. 6) is the smallest of the
lateral series and ends in a slight swelling, while the axial bronchus con-
tinues downwards, terminating in an end bud. At this point the division
of the stem is practically dichotomous. This specimen has no medial
bronchi and is especially characterized by the lack of variations, for all
of the bronchi, excepting the medial group, are present in almost
schematic order. The entire absence of the medial group, however, must
be regarded as exceptional for most trees, either on one side or both, have
medial branches in some of the interspaces below the level of Lateral 4.
While we have seen in the reconstructed series, examples of variations
caused by the suppression of either a dorsal or ventral bronchus, another
type occurs, not represented here, of which I have several specimens in
my corrosions of the embryonic lung, namely, a reduplication of either
the dorsal, ventral, or the medial bronchi in any one interspace. This
may or may not be accompanied by a simultaneous suppression of one

Joseph Marshall Flint 51

of the adjacent elements of the same series. Following is a tabulation
of the branches of a tree in an embryo 18.5 mm. long.

TRACHEA.
Iyale
(2) ID e
(3) D-L.
(2) VS.
(3) DS-LI.
Right bronchus. Left bronchus.
Ta 2. Ibe 74
(2) VI.
(2) DI. (2) Ap.
(3) D-L.
(2) LI. (2) ia.
(3) DI-S-DS. (3) DS-VI.
V. 2.
(A) ik
(2) VS.
(2)) VE.
ID KA IDE Ze
(2) MW.
iby By 1 3s
(2) D.: (2) D.
(2) VS.
(Cire
Wer3 Wiss
Das: DFS:
(2) M. (2) M.
ae Ae L. 4.
Gav: (Cave
(2) DS.
D. 4 D. 4
V.4 vV.4
1G. & 15 15)
1D), 5) D.5
V.5 Wotd
L. 6

Owing to the increasing complexity of the tree, it becomes almost
impossible to reconstruct it by Born’s method after this stage. At the
same time I have not succeeded in getting good celluloid corrosions
younger than 4 cm. pigs. This gap, however, has been partially bridged
by drawings of the serial sections of the lung of a 23 mm. pig, aided
by specimens cleared in oil of cloves, or injected and subsequently
cleared according to the suggestion of Hochstetter, 98. By these methods,
it is possible to follow the main divisions of the ramifications consider-

52 The Development of the Lungs

ably beyond that of the last reconstruction. With reference to the smaller
buds, however, it is impossible either in sections or in clear specimens
to determine definitely their course and final relationships. Neverthe-
less, as shown in these specimens, the bronchial tree evolves along the
same lines. The tendency for the tips of the wings of the lungs to fold
ventralwards around the heart and liver also becomes more exaggerated
than in the case of the lung of a 18.5 mm. pig.

With the exception of the smaller buds, following is a tabulation of
the main branches of the lung at this age.

TRACHEA.
1b, ae
(2) DI.
(3) D-L-M-L.
(2) VS.
(3) DS-LI-LI-D.
Right stem bronchus. Left stem bronchus.
1b PA, Tere
(2) DI. (2) Apical.
(3) D-I-D. (3) D-L-M-D.
(2)04T. (2) eLE:
(3) VI-DS-I. (3) DS-VI-DI-DS.
Ware
(2) cE
(3) De
(2) VS.
@invwae
1D, 2s 1057
(2) UM. (2) M.
(3) 8S.
(2) L. (2) SD.
(2) M. (2) L£.
Tiere Ib, By
(2) V. (2) D.
(2) 5D: (2) V.
(2) SV. @)ee
(2) D. (2) S.
We ok Wars
(CA Op (2) 8.
(2) M. (2) M.
1D), Drs:
(2) M. (2) M.
(2) Z. CANE:
(2) M.
L. 4. L. 4.
C2) V7. (2) D.

Joseph Marshall Flint 53

(2) D. (2) 8.
(2) V (2) D
(2) V
D. 4. D. 4
(2) M. M.
(2) L (2) L
V. 4. Vv. 4
(2) M. (2) M
Tes 5 L. 5
(2) V. (2) D
(2) DS. (2) V
(2) I (2) DS.
IDES D5
(2) M. (2) M.
V.5 Wa 15)
M. 5 M. 5
L. 6 L. 6
(2) V
(22D
D6: D. 6
V.6 V.6

In a pig 5 em. long, the bronchial tree can be studied by celluloid cor-
rosions (Pl. IV, Fig. 21), but perfect specimens of the air passages in
these small embryos are extremely difficult to obtain. The main features
of the tree remain practically the same as in the earlier stages, save
that it has increased markedly in the complexity of its branching. 'l'he
trachea with its main bronchi maintains the wish-bone appearance op-
served in the reconstructions of younger embryos, but a marked difference
is noted in the lateral bronchi, which now bend sharply ventralwards as
the lung folds around the heart and liver, following the curve of the
thoracic wall. The first lateral bronchus, while showing the chief charac-
teristics observed in the younger stages, has a more complicated system
of branches. It extends lateralwards and posterior, and divides into 1ts
two main branches, the dorsoinferior and ventrosuperior. The former
runs dorsalwards, ventralwards, and posterior, while the latter brancn
passes anterior, ventralwards, and slightly medianwards. The maim
branches of the dorsoinferior bronchus are, at this stage, seven in number,
and extend dorsally, laterally, and medially. Their serial arrangement
may be determined from the tabulation at the end of this section. 'I'here
are five main branches of the ventrosuperior or apical division, which
have chiefly a dorsosuperior and a lateroinferior course.

Lateral 2 on the right side shows a marked increase in the complexity
of its large dorsoinferior bronchus, which now shows six subdivisions.

54. The Development of the Lungs

The lateroinferior branch which serves as the continuation of the main
bronchus, runs lateralwards, ventralwards, and slightly posterior. This
has five main divisions, which have, in general, a ventroinferior and
dorsosuperior course. V.2, the Bronchus infracardiacus, passes median-
wards, ventralwards, and slightly posterior. The main divisions noted
in the earlier stages show an increase in their branching. Dorsal 2 ex-
tends in a dorsoposterior direction and its main branches radiate medial-
wards, lateralwards, and superior. The third lateral bronchus passes
lateralwards, ventralwards, and slightly posterior. Its branches run ven-
trally, dorsally, and in a ventrosuperior direction. V.3 bronchus in this
specimens is not present. Dorsal 3 has four main branches, which have
the same general direction as the second dorsal, namely, median, lateral,
and superior. ‘The fourth lateral bronchus has, at this stage, six main
divisions, extending superiorly, laterally, and medially. D. 4 runs lateral-
wards, ventralwards, and slightly posterior, and has seven main branches
passing in a ventral, dorsosuperior, and dorsoinferior direction. In this
tree there is a median branch, M. 5, rising from the main bronchus op-
posite L. 5, the branches of which run in an ventrosuperior and a dorso-
inferior direction. ‘This bronchus is fairly constant, and is met with
frequently in corrosions of older lungs. Its origin has been traced in the
series of reconstructions of embryonic lungs from a medial evagination
of the wall of the stem bronchus. D.5 passes dorsalwards and slightly
inferior. It has three main divisions extending medially, laterally, and
inferior.. The Ventral 5 runs ventralwards, medialwards, and slightly
posterior, and has a medial and a lateral branch. Lateral 6 passes
lateralwards, posteriorly, and to a slight degree ventralwards. It is,
as yet, not long enough to show the ventral curvature, which is more
marked in the lateral branches of the higher orders. Its branches, at
this stage, run chiefly ventralwards and dorsalwards. Dorsal 6 projects
dorsally and slightly posterior and has a single median division, while
Ventral 6 as yet, has no branches.

Lateral 2 on the left side, owing to the further apical growth of its
main division which passes up to the apex of the lung varies even more
than in the preceding stage from the corresponding branch on the right
side. ‘This bronchus supplies the apical region of the left ling, which,
in general, is taken by L. 1 and L. 2 on the opposite side, although the
total volume of lung tissue is not nearly as great as that combined in
the territory tributary to right L. 1 and L. 2. The apical branch grows
almost directly superior, and has six main branches that run chiefly in
dorsal, lateral, and medial directions. Its first main dorsal branch ex-

Joseph Marshall Flint 55

tends dorsalwards and slightly posterior, and bears a strong resemblance
to the series of dorsal bronchi from the stem bronchus. Its branches
run medially, laterally, and dorsoinferiorly. The continuation of the
main bronchus, the lateroinferior branch, corresponds in its course prac-
tically to the main branch of the opposite side. It possesses seven main
divisions, which run dorsosuperiorly, ventroinferiorly, and dorsoin-
feriorly.

There is, as usual, no Ventral 2 on the left side. Lateral 3 runs later-
ally, ventrally, and shghtly posterior. At this stage it has seven main
branches, which pass dorsally, ventrally, superior, and inferior. While
the remainder of the branches on the left side below this point show
many asymmetrical arrangements from the corresponding divisions on
the right, the architectural characters are sufficiently similar to avoid a
repetition of the description. The main idea of these tabulations is to
show the successive appearance of the chief bronchi of the adult lung
and to indicate how the divisions are adapted to the space relationships
to which the growing tree must adapt itself. It is not to be supposed
that simple mechanical conditions govern entirely the growth of the
bronchi, as its chief architectural features are undoubtedly phylogenetic.
This much, however, is certain, that there remains always a considerable
adaptability on the part of the growing branches, which is shown in their
substitution power when one of the usual elements is suppressed, and
apparently by the ability of either fork from a division to serve as the
stem.

Following is a tabulation of the branches of the tree at this stage:

TRACHEA.
Ie ale
(Ay toe
(3) D-L-M-L-D-L-M.
(2) VS.
(3) DS-LI-LI-DS-LI.
Tae de Ta. 2:
CALE (2) Apical.
(3) D-I-D-V-DI-D. (3) D-L-D-M-V-D.
C2) ear. (2) BT.
(3) VI-DS-I-DS-VI. (3) DS-VI-DI-DS-VI-D-I-D-S.
Wied: ;
C2) er
(3) 2DIr
(2) VS.
(ie
(2) VE.
(3) LI-VI.

(2) SV.

@inVve

The Development of the Lungs

(3) 8.

(3) S-D-V.

(3) D-V-S.

V. 3 suppressed.
1D), By

L. 4.

(2) M.
(2) L.
(2) M.
(2) 8.

(2) V.

(3) S-M.

D. 2.

(2) M.

(3) SL-IV.
(2) SL.

(3) S-LI-D.
(2) LD.
(2) M.
(2) 8S.

(2) D.

ox
wo cw

(3) SM-IM-L-M.
(2) V.
(3) S-M.
(2) ele
(3) D.
(2) 8.
(3) D.
(2) 8.
(3) D-V-S.

(2) DI.
Ae

. Suppressed.

(2) M.

(2) L.
(2) M.

(A) We

(3) S-M.
(2) D.
(3) S-M-L.
(2) LS.
(3) D-V-D-V-S-1.
(2) D.
(2) V.
(3) M-L.
(2) 8.
(3) V-D.
(2) V.
(2) D.

(2) M.
(2) L.
(2) I.
(2) 8.
(2) M.
(2) Ri:

Joseph Marshall Flint 57
V.4 Wey4:
(2) M.
(2) 8. (2) ZL.
(2) L. (2) UM.
(2) M. (2) 8.
(2) ZL.
(2) M.
M. 4 between L. 4 and L. 5.
10 5% 1, 155
(2) V. (2) D.
(3) S-M-L. (3) L-M-L.
(2) DS. (2) V.
(3) S-M. (3) L-M-L.
(2) DI. (2) D.
(C)nVe (C2 ave
(2) 8. (2) 8.
(2) DI. (2) V.
CV
M.5 opposite L. 5.
D5: 1D); fy.
(2) M. (2) M.
(2) Z. (2) L.
(74) He (2) MS.
(2) L.
Weroe V.5.
(2) M. (2) M.
(2) ZL. (2) S.
L. 6. 1.6:
(2) V. (2) Dt
(2) D. (2) V.
(2) V. (2) I.
(2) D.
D. 6. DAG:
(2) M. (2) M.
WG: Ve 6:
(2) LB.
(2) M.

In the study of the further development of the bronchial tree, I have
made corrosions of the lung in a series of pig embryos of increasing age
increments represented by a centimeter of growth up to and beyond the
time of birth. From this series of corrosions it would be possible to
tabulate the history of each bronchus until the full growth is attained.
The results would be too detailed, however, to be of any value. More-
over, the wide range of variation of the branches destroys the absolute
sequence of the branches in a successive series giving the formule only

58 The Development of the Lungs

an average relative value. Those which have preceded are, however, suffi-
ciently constant to serve as a general guide to the direction taken by the
main branches of the adult tree.

It may be well, however, to show pictorially the subsequent evolution
of the tree without taking up the details of the branching, as a good
corrosion of the bronchial system holds the general form of the lung
quite as well as a hardened specimen of the lung itself. The tree of a
pig 7 cm. long is shown in Pl. IV, Fig. 22. Besides the increasing com-
plexity of the branching, one notes the ventral curvature of the lateral
bronchi parallel with the chest wall. This is most marked in Lateral 2,
less so as we proceed to Lateral 6. There are some peculiarities on this
tree which are of great interest, for Ventral 3 on the left side is sup-
pressed and in its place a prominent division of the second ventral or in-
fracardiac branch has grown medianwards to take its place. A branch
from Lateral 3 also runs to this region, giving an appearance as though
it might be a ventral bronchus which had not left the lateral series. It
is, however, a simple substitutive process on the part of the lateral branch
for an element which has not developed in the earher stages. This speci-
men also shows an instance where the dorsal fork of the first division of
Lateral 1 continues as the stem, leaving the ventral fork, which usually
serves that purpose, as a ventrosuperior branch, while the large dorso-
inferior branch which is usually comparable to the apical branch on the
opposite side rises from the next division. A median bronchus occurs
on the left side opposite Lateral 4. On the right side, median divisions
are not present.

In the corrosion of a tree from the lung of a pig 18 cm. long (PI. IV,
Figs. 23, 24) a number of interesting features may be observed, which
serve to illustrate some of the developmental characteristics of the grow-
ing bronchial tubes. In the first place, we ordinarily have five paired
lateral bronchi, while in this specimen there are but four. This indi-
cates the suppression of the last of the lateral elements which is com-
pensated for by an hyperdevelopment of Lateral 5 to supply the region
usually tributary to Lateral 6. Accordingly the terminal forking of
the stem bronchus, which usually occurs between Lateral 6 and the
continuation of the stem, takes place in this instance between it and
Lateral.5 (Pl. IV, Figs. 23, 24). While this tree shows the suppres-
sion of one of the lateral branches, I also have some specimens which
present a series of six paired lateral bronchi below L. 1, indicating a
possible variation in these elements between these limits with 5 as the
average. Ventral 3 is suppressed on both sides, on the right it is

Joseph Marshall Flint 59

substituted for by inferior branches of Ventral 2 and partly by one of
the branches of the first ventroinferior division of Lateral 4. On the
left side, the ventroinferior divisions of Lateral 3 and Lateral 4. send
branches to this region. Median 4 occurs on both sides opposite Lateral 4.
It is particularly interesting to note the effect of the presence of median
branches upon the dorsal series. Where median bronchi are present the
median branches of the adjacent dorsal elements are very small and
poorly developed, owing to the usurpation of their territory by this
series. This naturally gives rise to the pictures which make it appear as
though the median series might be transplanted elements from the dorsal
bronchi. This relationship, however, is only another indication of the
adaptability of the branches of the tree, for in this instance, had the
median branches been suppressed, the median branches of the neighbor-
ing dorsal series would have grown over to occupy the territory in which
the former are found.

In this specimen the ventral curvature of the lateral series is much
more marked than in the preceding stage and now affects, to some ex-
tent, the whole lateral series, although Lateral 5 bends slightly, while Lat-
eral 2 (Pl. IV, Fig. 23) shows an extreme ventral curvature, a character-
istic which is progressively diminished until Lateral 4 is reached. This
unequal. bending has a marked effect on the stem bronchus and its other
branches, and is responsible for the characteristic spiral-like insertion of
the lateral and dorsal scries upon the stem of adult lungs which has been
observed but not explained by most of the investigators since Aeby. As the
lateral bronchi turn ventrally more rapidly in the upper than in the
lower series, the stem bronchus and its branches twist with them. Thus
in the adult lung Lateral 2 appears to rise on the ventrolateral aspect of
the stem and each successive element of the lateral series is inserted
shghtly more lateralwards. Similarly, on the adult tree, Dorsal 2 appears
to originate somewhat on the dorsolateral surface of the stem, and the suc-
ceeding elements are successively inserted more directly dorsalwards. ‘The
spiral line connecting the origins of these two series of bronchi simply
represent the degree of torsion of the stem bronchus as the lateral
‘bronchi, in following the curvature of the chest wall, bend around the
heart and liver. This is also nicely shown by the course of the pulmonary
artery which, naturally, is mechanically influenced by the twisting of
the stem bronchus as it is held in the angle formed between the laterai
and dorsal series of bronchi. It is, of course, this secondary relation-
ship of the lateral bronchi which led Aeby to term them ventral. In

60 The Development of the Lungs

their origin, however, they are, as we have seen, distinctly lateral, and I
have applied to them, therefore, the genetic nomenclature.

The condition of the tfee a few days after birth is shown in PI. IV,
Fig. 25. In order to show the three chief series of bronchi in a single
illustration, Ventral 2, the Bronchus infracardiacus, has been broken off
near the root. The tip of the ventrosuperior branch of the tracheal bron-
chus, owing to an accident, was also broken and should extend upwards
and ventralwards for a considerable distance. Although the general form
of the tree has not changed to any marked extent, besides the increase in
the branching, the second laterals extend far ventralwards so as to em-
brace the heart. The effect of the presence of the heart on the tree, as
in earlier stages, is shown particularly well by the direction of the
branches of the tracheal and second lateral branches. The portions of
these bronchi, which come in relation to the heart are nude, their
branches extend so as to occupy the remainder of the chest cavity in their
neighborhood, a relationship, which may also be seen by an inspection of
the tables in the younger stages. Below Lateral 2, however, owing to a
freer environment, the bronchi show the power of branching in any
direction. In this specimen a few interesting variations are shown, one
of which is of particular importance for comparison with the conditions
shown in the preceding stage, namely, in the presence of seven lateral
bronchi on the right side and five on the left. On the right side the
whole ventral series is present, while on the left, two ventral bronchi
occur between Lateral 5 and Lateral 6, a fact which would be difficult to
explain if we viewed these branches as derivations of the lateral series
since the entire group is complete from Ventral 3 down. Dorsal 3 on the
right side is hyperdeveloped, while Dorsal 4 is quite small, a not unusual
variation. None of my other specimens show such a marked aevelop-
ment of the medial bronchi as Medial 4, 5, and 6, present on the right
side, as well as an element of this series opposite Lateral 5 on the left side.

RELATIONS OF THE BLOOD-VESSELS TO THE BRONCHIAL TREE.

In tracing the angiogenesis of the vascular system in the submaxillary
gland and the suprarenal body, the author, 00, 02, 03, showed that some
of the mechanical principles, which Thoma, 93, in his well-known re-
searches found were involved in the development of the blood-vessels in
the Area vasculosa of the chick, might be applied to vascular systems
developing in three dimensions in the growing organs of mammals.
Thoma found in the chick, that arteries and veins are originally simple
capillaries. The subsequent transformation of the latter into arteries

Joseph Marshall Flint 61

on the one hand and veins on the other, is due to their fortuitous location
with reference to the primitive aorte and the venous ostia of the heart.
Their growth in size bears a definite relationship to the velocity of the
current in them, while their arterial or venous nature is determined by
the character of that current, a high pressure pulsating column of blood
giving rise to an artery, a low pressure constant current forming a vein.
The nature of the current depends, naturally, mechanically upon its
position on the arterial or venous side of the capillary plexus. In con-
sidering the problems of angiogenesis in mammals, I called attention to
the fact that Thoma’s principles do not explain all the facts of vascular
development nor do they entirely accord with them. For example, the
statement that a new growth of blood-vessels follows a rise of blood
pressure in a capillary area must be considered only an hypothesis and
not a demonstrated fact, for this would make the vascular system the
stimulus for the new growth of cells, while it is much more probable
that cells give the stimulus for the production of new capillaries. It is,
of course, obvious that the principal factors that govern organic growth
are resident in the cells rather than the blood-vessels as is indicated by
their behavior in the embryo before the vascular system is laid down.
In tracing the development of the intrinsic vascular system of the
mammalian lung, it is also obvious that the vessels follow the same histo-
mechanical and histogenetic principles which are active in forming the
vascular systems of such organs as the Gl. submaxillaris and the Gl.
suprarenalis. Different conditions in the chief cells of the lung, namely,
those of the bronchial tree, and different relations of the arterial supply
and the venous drainage, give rise to different relationships on the part
of the arteries and veins in the pulmonary apparatus. In the suprarenal
body, we have the formation of a blood vascular system with a well-
marked capsular plexus from which the blood supply of the organ is
derived, and in the submaxillary gland an organ, where the blood-
vessels, as in the lungs, accompany the ducts. In the latter instance,
however, the conditions are such as to give rise to a venous system where
the blood is drained by Vene comites of the main arteries, while in the
pulmonary circulation, a relationship exists in which the arteries and
veins are separated from each other by means of the bronchial tubes.
According to the studies of Bremer, 02, which have also been confirmed
by Sakurai, 04, the pulmonary arteries in the pig appear to originate sym-
metrically from the pulmonary arches like those of other mammals. At
first they remain comparatively parallel and later (7-8 mm.) bend
towards each other, sending out at the same time small branches which

62 The Development of the Lungs

finally fuse into transverse anastomoses which yield ultimately a common
trunk with two origins above and two main pulmonary arteries below.
Bremer suggests that the bending of the arteries towards each other may
be caused by the growth of the right and left auricles. This state of
affairs occurs in the pig 11 mm. long. Later, the upper part of the
right artery degenerates, and, with it, finally the right pulmonary arch.
Thus we have the next stage where both arteries arise as a common trunk
from the left pulmonary arch.

In the earlier pig’s embryo (5 mm.), the arteries arising from the
pulmonary arches on each side may be followed caudalwards a short
distance from their origin on the arches, but only in particularly good
specimens, as they are soon lost in the irregular capillary plexus sur-
rounding the head gut to which, in their course, they give off frequent
branches. At the same period, it is also possible to note the ingrowth of
the pulmonary vein from the yet undivided portion of the auricle. It
may be seen in a few sections running dorsalwards in the Mesocardium
posterior towards the pulmonary anlage, which is, as yet, only partially
separated from the cesophagus. It is asymmetrical as it les shghtly to
the left of the medial plane. Its branches connect with the capillary
plexus about the head gut and pulmonary anlage, establishing a venous
outflow on the ventral side of the respiratory apparatus. Concerning
the early appearance of the Vena pulmonalis in the pig, my observa-
tions are in accord with those of Narath on the rabbit for in these ani-
mals, the Vena pulmonalis is apparently evident at a much earlier stage
than His, 87, or Schmidt, 70, were able to observe it in man.

At 6 mm. after the formation of the primitive lung sacs is well under
way, the pulmonary arteries may be seen (PI. I, Figs. 5, 6 ad. as.) run-
ning in approximately parallel courses until they diverge and are lost
behind the right and left bronchi in the capillary plexus about the primi-
tive lung sacs. Their course, however, on the two sides is different owing
to the horizontal position of the left stem bronchus, the artery on that
side (Pl. I, Fig. 5as) is forced to turn dorsalwards in order to pass
behind the left sac sooner than the right pulmonary artery, which main-
tains its more ventral course and, finally, at a lower level descends behind
the right stem bronchus.

The factors which determine the course of the pulmonary artery in
passing behind the lung sacs are, first of all, the ventral position of the
venous outlet into the Sinus venosus, leaving the arteries to develop
from behind. That is to say, with the increasing size of the right and
left stem bronchi and the consequent enlargement of the capillary plexus

Joseph Marshall Flint 63

about them, it is natural, with the venous outlet already established on
the ventral side of the sacs, that the capillaries on the dorsal side should
enlarge into arteries. Furthermore, after its origin and partial separa-
tion from the cesophagus, the terminal part of the entire pulmonary
apparatus extends somewhat ventralwards from the head gut making it
additionally easier for the arteries to form on the dorsal than the ventral:
surface of the anlage. ‘These factors are responsible for the course,
which the arteries and veins take with reference to the bronchial tree,
while the asymmetry of the stem bronchi appears to cause the chief dif-
ference in the course of the arteries on the two sides. It is, further-
more, possible that some of this irregularity is also due to the medial

ScHEMA A.

Schema to show the origin of the relations of the pulmonary vessels to the
lungs. LA=Lung anlage. AP=Arteria pulmonalis. VP—=Vena pulmon-
alis. L.1= Site of origin of Lateral 1 the “ eparterial bronchus.” L.2—=Site
of origin of Lateral 2, the first bronchus in the “ hyparterial region.” L=
Liver anlage.

bending of the right artery in preparation for its transfer from the
right to the left pulmonary arch according to the suggestion of Bremer,
although in Bremer’s descriptions, with which my specimens agree, this
actual transfer is made at a much later period, and I am accordingly
inclined to minimize the possible influence of this factor. It is also
worthy of note that we have no crossing of the bronchi by the arteries
in the sense of Aeby. As they run down, they gradually turn dorsal-
wards to take up a position behind the primitive sacs and are lost in the
capillary plexus, which surrounds them. The pulmonary vein, scarcely

64 The Development of the Lungs

longer than in the preceding stage, through the further growth of the
auricular septum now empties into the left auricle.

In a pig 7.5 mm., the arteries (Pl. I, Figs. 7, 8 ad. as.) maintain the
same relationship as those in the preceding stage, namely, the right
lies more ventral than the left and also somewhat nearer the median
line. Behind it, however, the evagination for the formation of Lateral 1
has appeared. At this time, the artery consists simply of an endothelial
wall supported by the surrounding mesoderm. Situated some distance
from the trachea, it is absolutely impossible that such a structure should
have a determining influence upon either the production or position of
this or other branches of the bronchial tree. Furthermore, it is now well
known that such vessels do not influence mechanically the growth of
organs which they supply, but follow the developmental processes which
are inaugurated in the chief cells of the organ itself acording to defi-
nite histodynamic and histomechanical principles.

By a glance at the schema which elucidates this point, we see how the
two factors outlined above have worked to bring about the relationship of
the artery to the primitive lung sacs. After its origin during the pro-
duction of the primitive lung sacs, the lung anlage (Schema LA) ex-
tends ventralwards. The Vena pulmonalis (Schema VP) in growing in
from the auricle has established the venous outflow ventral to the an-
lage, leaving the pulmonary arteries (Schema AP) to form on the
dorsal side of the primitive stems. This relationship occurs, however,
before there is the slightest indication of the presence of any of the main
bronchi. Later as they appear, Lateral 1, the so-called “ eparterial
bronchus” (Schema L. 1) develops behind the artery and Lateral 2
(Schema L. 2) in front of it. Sometimes Lateral 1 is higher up,
where it appears on the trachea, sometimes lower down where it forms
on the stem, often where it forms on both sides, the left is lower than the
right. The most important element in determining the position of
Lateral 1 is the point at which the trachea separates into the two stems.
As we have seen, when this is high, taking Lateral 2 on each side as the
fixed topographical point, Lateral 1 is on the stem; when it is low, as
in the pig, Lateral 1 forms on the trachea.

It is also important to observe that the relationship between the
Arteria pulmonalis and Lateral 2 is not “ eparterial” as Aeby suggests ;
the artery in the embryo simply runs ventralwards to Lateral 1 and then
passes gradually behind the stem. The “ eparterial and hyparterial 4
topography of the bronchi is due to the descent of the heart in the later
stages of embryonic life and to the degeneration of the Ductus arteriosus
after birth when the entire circulation from the right ventricle, conse-

Joseph Marshall Flint 65

quently, is transferred from the systemic into the pulmonary system.
Until this occurs, the pulmonary arteries do not even approximately cross
the stem bronchi as Aeby suggests. Apparently, as we shall see later, he
recognized this fact. Furthermore, my observations in older stages are
in accord with the findings of Zumstein and Narath, who hold that, in
the sense of Aeby, a true crossing on the part of the artery never exists.
It seems to me important, therefore, for a logical conception of the
architecture of the bronchial tree, that the terms “ eparterial and hypar-
terial” or, at least, all that they imply should be abandoned.

The pulmonary vein (Pl. I, Fig. 7 v) is seen at this stage with two
small tributaries, one from the head and another from the caudal
region running in the Mesocardium posterior. They are in connec-
tion with other dilated capillaries which may be seen in the neigh-
borhood of the lung sacs, but the latter have not become large enough as
yet to form definite veins. The vascular apparatus of the lungs, then,
at the period of the formation of the two lung sacs, consists in two small
asymmetrical arteries passing down behind the primitive stem bronchi
ending in an irregular capillary plexus about the dilated epithelial tubes
from the ventral side of which run enlarged capillaries emptying into
the pulmonary vein in the Mesocardium posterior.

No particular change is observed in the next older embryo 8.5 mm. in
the relationships of the arteries (Pl. I, Figs. 9, 10 ad. as.). With the
lengthening of the stem bronchi, however, owing to the increased capil-
lary field about the bronchi, the right and left pulmonary veins (PI. I,
Fig. 9v) may be seen emptying into the common trunk which, in turn,
now opens into the left auricle. In a pig 10 m. long, the pulmonary
arteries maintain their general relationship to the trachea, the right
passing ventral to Lateral 1 (Pl. I, Figs. 11, 12 ad). Continuing down-
wards, they gradually extend behind the stem bronchi giving off branches
to the irregular capillary plexus which surrounds the primitive tree, ele-
ments of which may be seen, here and there, in well-prepared cross-
sections of the lung. As a rule, the arteries lie on the dorsolateral
aspect of the stem. At this stage, it is quite evident that the three first
branches of the tree, practically in the same period of development, are
growing without reference to the arteries as they are surrounded only by
a capillary plexus derived from branches of the arteries and from which
dilated capillaries empty into the veins. As they increase in size, the
arteries and veins, which follow the various ramifications of the tree
are formed from the capillary plexus according to the regular histo-
mechanical laws. The two main tributaries of the vein (PI. I, Fig. 11 v)

5

66 The Development of the Lungs

forming the right and left stem veins, run on the ventromedial aspect of
the stem originating from the plexus about the main bronchi. In this
way, we have established the regular alternation of artery, bronchus, and
vein which persists throughout the life of the tree, although it will be
remembered that this relationship is due primarily to the position of the
vein with reference to the anlage.

At 12 mm. (PI. I, Figs. 13, 14) the vessels have followed the natural
growth of the bronchi. From the capillary plexus on the dorsal surface
of Lateral 2 on each side, the artery to that branch is formed. The
vein (Pl. I, Fig. 13) by the rapid development of Ventral 2 is pushed
somewhat medialwards at this point. With the marked development of
Lateral 1, the tracheal bronchus, in a pig 13.5 mm. long, a branch (PI.
II, Fig. 15) is given off from the right pulmonary artery, which runs up
along the ventral surface of the bronchus to end in the plexus about that
branch. Continuing downwards, the arteries (Pl. II, Fig. 16) on both
sides run on the dorsolateral aspect of the stem. The branches to Lateral
2 have increased somewhat in length, and from the right pulmonary
artery a new branch is formed, which, passing under the root of right
Lateral 2, ends on the lateral and under aspect of Ventral 2, the Bron-
chus infracardiacus. The artery still maintains its position with refer-
ence to the stem, which causes it to lie in the angle between the lateral
and dorsal bronchi. Thus, the artery itself, however, is not responsible
for the division of these two groups from the stem as Aeby implies when
he says in speaking of Lateral 1, “In ihm hat offenbar die Scheidung
des hyparteriellen Gebietes in zwei streng geschiedene Bezirke noch nicht
stattgefunden, ein Thatbestand, der wohl damit in Verbindung gebracht
werden darf, dass die Lungenarterie nicht sondernd einzugreifen ver-
mocht hat.” Should we still suspect a causal relationship here, it is only
necessary to glance at the ventral bronchi, particularly Ventral 2, to see an
element not only originating from the stem away from the influence of the
artery but also with its growth, developing from its capillary plexus an ar-
tery which passes around the stem and rests on its lateral side. Interesting
changes, at the same time, are occurring in the veins (PI. II, Fig. 15).
From the tracheal bronchus, a branch may be observed passing down to
the common pulmonary vein running still more ventral than the artery
to Lateral 1, another one of the final adult relationships in the pig’s lung.
Here, however, we have an exception to the general relationships of the
vessels to the bronchi due to the more ventral position of the veins and
the failure of right pulmonary artery to form behind Lateral 1, which,
in this particular instance, gives us a Vena comes to the artery to the
tracheal bronchus instead of the usual alternation found in other portions

Joseph Marshall Flint 67

of the tree. On the ventral surface of Lateral 2, veins originate, which
empty into the right and left pulmonary veins, while medialward and
above Ventral 2 lies the vein of that bronchus which joins the right pul-
monary just below the tributary from Lateral 2. In this stage, either
owing to the hyperdevelopment of Ventral 2, or the increasing asym-
metry of the heart, or both, the pulmonary veins are shifted somewhat to
the left, causing them to he somewhat beyond the median line. At the
same time, the veins in these young stages are frequently reduplicated
as the final channels are not always definitely selected. In order to show
the different branches of the tree without extra illustrations, in this and
the succeeding reconstructions, the pulmonary vein has been kept in the
median line, and only the chief channels are shown in the case of redu-
plication, which is a frequent occurrence.

In a pig 15 mm. long, the pulmonary artery (Pl. I, Fig. 17) on the
right side still has a more ventral and medial position than that on the
left, a fixed relationship from embryos 12 mm. in length as the arteries
both rise from a common trunk originating from the left pulmonary arch.
Just below the point of origin of Lateral 1, the artery to that trunk is
observed (Pl. I, Fig. 17), which passes up and divides with it into its
ventrosuperior and dorsoinferior branches. The two pulmonary arteries
bending dorsalwards pass back of the right and left bronchi, giving off
the branches to the second lateral bronchi, which lie on their dorsal and
superior surfaces. On the right side, the artery to the second ventral
bronchus (PI. II, Fig. 17) has increased in length with the growth of that
branch, while arteries to the second dorsal bronchi (Pl. II, Fig. 18)
are observed passing along their lateral walls. From this point, the
pulmonary arteries continue on in the angle between the dorsal and
lateral bronchi, giving off branches to the third and fourth lateral ele-
ments (Pl. II, Fig. 18) on each side which lie above and behind them.
From the capillary plexus around the termination of the right and left
stem bronchi, the beginnings of the pulmonary veins (PI. II, Fig. 17)
are seen as in the preceding stage. From the fourth lateral and third
lateral branches on either side, veins are formed which lie below and in
front of these bronchi and pass in front of the stem bronchi to empty
into the pulmonary veins, which lie upon their median and ventral as-
pects. The vein from the second ventral bronchus (PI. II, Fig. 17), as
in the younger stage, is placed medially to it and empties into the right
pulmonary at the base of the third lateral bronchus. The veins from
the second laterals have increased considerably in length, and lie on the
ventral aspect of these divisions, while the Vena pulmonalis, formed by
the confluence of the two right and left veins, lies ventral to the trachea

68 The Development of the Lungs

just below the point of bifurcation. On the right side, the vein from
Lateral 1 passes downwards and medianwards to empty into the Vena
pulmonalis at a point just above the confluence of the two vessels which
accompany the stem.

In a pig 18.5 mm. long, the relationships of the pulmonary arteries
to the trachea (Pl. II, Fig. 19) remain the same. Just above the point
of bifurcation, they pass gradually behind the main bronchi to take un
their dorsolateral position. No marked changes are observed in the
arteries to Lateral 1, save in an increase in length. The second lateral
branches present no changes, except on the left side where a branch runs
up on the dorsolateral aspect of the apical division of Lateral 2 (Pl. I,
Fig. 19). The artery to Ventral 2 arising just beneath the Lateral 2
on the right side and passing around the stem and under the root of
Lateral 2 to run along the outer aspect of the second ventral bronchus,
now shows a secondary branch which follows the inferior division (PI.
II, Fig. 19) of Ventral 2. Small arteries are given off to right and
left Dorsal 2 which run along their lateral superior aspect. On either
side, branches to Lateral 3 (Pl. III, Fig. 20) run from a point just
below the origin of the arteries of Dorsal 2. Beneath the third lateral
bronchi, arteries arise which pass around the axial bronchus, and run
lateralwards to Ventral 3. Below this level, branches are given off on
both sides successively to Dorsal 3, Lateral 4, Ventral 4, Dorsal 4, and
Lateral 5 (Pl. III, Fig. 20). The pulmonary veins (Pl. II, Fig. 19)
lie medialwards and yentral to the main bronchi. Besides the branches
from the lateral bronchi, which have been observed in the preceding
stages, venules, lying on the medial surface of the dorsal bronchi, pass
around the median aspect of the main bronchus and empty into the pul-
monary veins. Similar veins from the ventral bronchi run along their
median aspect, and empty into the Venze pulmonales on both sides.
Otherwise, there are no marked changes in the yenous system at this
stage save that the veins from the Lateral 2 and Lateral 1, on the right
side now empty into the Vena pulmonalis by a common trunk. The
second lateral vein on the left and with it a vein from the apical branch,
which joins it about the root of Lateral 2 empties into the main pulmon-
ary vein at a level somewhat higher up than the one which accompanies
the left stem bronchus. The two veins from the stems join about the
point of origin of the main bronchi and are continuous with the Vena
pulmonalis above. From the infracardiac bronchus, a vein empties into
the right stem vein just above the level of L. 3.

At this stage the main characteristics of the pulmonary vessels are
established for life. The arterial branch to Lateral 1 runs upwards

Joseph Marshall Flint 69

from the right pulmonary artery along the ventral surface of the bron-
chus and then follows the main divisions of the bronchi. Both arteries
pass down behind the stem, lying on their dorsolateral surface in the
angle between the dorsal and lateral bronchi. From it, three series of
vessels arise, namely, those to the lateral bronchi, which run on the dorso-
superior surfaces ; those to the dorsal bronchi, which pass backwards from
the stem artery on the laterosuperior aspect of the bronchus; and those
to the ventral bronchi, which pass lateralwards around the stem bronchi
to the lateral surfaces of the ventral group. Owing to the suppression
of median bronchi on the tree of the 18.5 mm. embryo, the origin of the
vessels to the median bronchi will be studied later in the corrosions of
older embryos.

The veins have two chief branches accompanying the stem bronchi
on their ventromedial surfaces. They receive as tributaries, veins from
the lateral bronchi, which run along their ventroinferior surfaces and
join the stem vein by passing above the corresponding ventral elements.
Branches from the dorsal series of bronchi run along the medial surface
of the bronchi across the median aspect of the stem to empty into the
veins on either side. A series of tributaries are also derived from the
ventral bronchi, which, after a short course on the medial aspect of these
bronchi, terminate abruptly in the stem veins. The vein from L. 1 hes
ventral to the corresponding artery and empties into the vein of Lateral
2 in the Vena pulmonalis. Thus we have the veins from the upper and
middle lobe emptying together into the main Vena pulmonalis on the
right, while the single vein from the upper left lobe joins the main trunk
on the opposite side. Below, the veins accompanying the stem fuse just
below the division of the trachea and empty at this point into the Vena
pulmonalis. The moving of the veins towards the left, due up to this
time to the asymmetry of the heart and the hyperdevelopment of Ventral
2, is now somewhat exaggerated by the development of the inferior vena
cava on the right side of the infracardiac lobe, which also presses this
structure to the left and, accordingly, must be looked upon as a factor in
increasing the asymmetrical position of the pulmonary veins.

The next period of growth in the vascular system can be easily fol-
lowed in specimens of the entire embryonic lung which, after fixation in
some fluid like formalin or corrosive acetic to preserve the blood in the
larger vessels, are subsequently cleared in oil of cloves or creosote. If
the vessels are not too full both series are easily traced, but, in any case,
the veins stand out distinctly. Owing to the complicated structure of
the tree, however, the exact relationships of the arteries and veins to
the bronchi are best seen in double corrosions in which, either the bronchi

70 The Development of the Lungs

and arteries or the bronchi and veins are injected, or else, in triple in-
jections where all three systems are filled with different masses. Prepa-
rations with the artery and veins filled with one color and the bronchi
another, are relatively easy to obtain, but the more instructive triple
injections are extremely difficult to make. The changes gradually taking
place with the growth of the tree, may be followed step by step in these
cleared and corroded specimens, but they need not be described in detail
until they are more exaggerated, as shown, for example, in triple corro-
sions of a pig 15 cm. long. Owing to my inability to find an artist who
could draw these complicated structures, the reader may perhaps find it
convenient to follow the following descriptions by means of the metal
corrosions shown in PI. IV, Figs. 23, 24. The common pulmonary artery
now divides to the left of the trachea a short distance after its origin from
the pulmonary arch. The branch to the tracheal bronchus is given off
from the right pulmonary artery at the left margin of the trachea and,
after crossing ventralwards to it. divides with Lateral 1 into a dorso-
inferior and a ventrosuperior branch. The latter passes ventralwards
to the tracheal bronchus, and, at its point of division, mounts up over
the ventrosuperior branch and comes to occupy a position dorsal, slightly
medial, to this bronchus. The dorsoinferior branch passes beneath, and
runs dorsal to the dorsoinferior bronchus. The right pulmonary artery
then passes downwards in front of the trachea, and turns back and out
to oceupy a dorsolateral position to the axial bronchus. Just above the
second lateral bronchus, the branch to that division of the tree is given
off, which courses a little above and behind the bronchus sending rami-
fications to accompany its side bronchi. The dorsoinferior branch crosses
behind the main bronchus, and runs dorsal to the branch which it
supplies, leaving that structure between it and the corresponding vein.
In the remainder of its course, the second lateral branch lies dorsal to
the bronchus with the bronchus between it and its accompanying vein.
The branch to Ventral 2 originates just below Lateral 2 and, passing
underneath its root, winds around the axial bronchus to gain the lower
and lateral aspect of the Bronchus infracardiacus, which it accompanies
in its ramification. The dorsal branch to Dorsal 2 runs on the lateral
surface of the bronchus and is given off from the right pulmonary artery
near the origin of the bronchus. The third lateral branch hes dorsal-
wards and sligthly superior to Lateral 3, and ramifies with its branches.
The branch to the third ventral bronchus arises in a manner similar to
that of the second, and winds underneath the third lateral bronchus
around the stem to the lateral aspect of Ventral 3. The artery corre-
sponding to Dorsal 3 has a similar distribution to the one above. The

Joseph Marshall Flint ak

fourth lateral lies above and behind the bronchus, while the fourth ventral
passes in a similar manner to those supplying the same series of bronchi
in the upper part of the tree. The fourth dorsal runs backwards just
lateral to the bronchus, maintaining, in general, this position as it rami-
fies. In cases where there are median bronchi, as in this specimen, the
artery passes medianwards around the dorsal surface of the stem and is
placed dorsal to the bronchus during its ramification. The fifth lateral,
ventral, and dorsal have corresponding positions to those of the higher
orders, an] occupy the same relative positions. On the left side the
pulmonary artery passes down without crossing the left bronchus at all
to take its dorsolateral position to the stem. Just above the point of
origin of left Lateral 2, the corresponding artery arises, and after pass-
ing a short distance dorsosuperior to the bronchus, almost immediately
divides, sending a branch to the apical bronchus which continues up-
wards, placed laterally and dorsally to it. The remainder of the arteries
on the left side have the same course as the corresponding branches on
the right. In this description, I have followed strictly the typical
specimens, although it is well to bear in mind that here, as in other
parts of the vascular system, frequent variations are encountered. The
veins still unite to empty into the left auricle through a common Vena
pulmonalis. Branches from Lateral 1 and 2 form a common, large
venous trunk on the right side, emptying directly into the Vena pulmo-
nalis, while the vein from the left Lateral 2 joins the latter at a corre-
sponding level on the opposite side. Below, the veins accompany the stem
bronchi and their tributaries form a common trunk at the level of Lat-
eral 3, which, crossing the ventral part of the stem bronchus between
Lateral 2 and 38, empties into the pulmonary vein from below. The
further growth of Ventral 2 on the right has gradually pushed the
veins from the lower portion of the bronchial tree much more to the
left, so that the large common trunk from the portion of the tree below
Lateral 2 lies directly over the left axial bronchus at a point where the
second ventral bronchus on that side would originate if the latter were
present. It is this fact, as we have pointed out above, which has such
great significance in explaining the suppression of that branch. From
the ventrosuperior branch on the tracheal bronchus, the vein hes ventral
and medial to it, receiving tributary vessels placed somewhat below the
side branches of this bronchus. The vein from the dorsoinferior branch
of Lateral 1 is placed ventralwards to that branch, and passes upwards
to join the main trunk at a higher level. The main vein from Lateral 1,
then passes down ventral to the artery and bronchus to form a common
trunk with that from Lateral 2 as we have described above. The latter

~
cae)

The Development of the Lungs

is placed above and ventralwards to the bronchus, receiving tributaries
from its side branches. The main dorsoinferior branch of Lateral 2
lies ventralwards to its bronchus, while the corresponding artery is
placed dorsalwards and above. This vein crosses behind Lateral 2 to
join the main venous trunk, which accompanies Lateral 2 until, in com-
mon with the vein to the tracheal bronchus, it empties into the common
pulmonary.

The veins from Lateral 3, 4, and 5 have shifted now so that they lie
a short distance ventralwards from the corresponding bronchi. They
pass medialwards under the ventral bronchi and empty into the right
pulmonary stem vein; those from Ventral 3, 4, and 5 lie medial to the
respective bronchi and run dorsalwards to the stem vein. Two veins
now accompany Ventral 2, one above running medialwards and upwards
and emptying into the large trunk formed by the fusion of the two stem
veins, and another lying behind the branches of Ventral 2 which passes
upwards and joins the common vein from the lower part of the tree on
its right side at the point of junction of the veins from the right and
left stems.

From Dorsal 2, 3, +, 5, and 6 the veins, lying medial to their stems,
run ventralwards past the stem bronchus to empty into the large stem
veins opposite their corresponding branches. The veins from the medial
branch lie yentralwards to them and pass lateralwards to the stem vein.
The relationships of the veins on the left side of the tree below Lateral 2
are, with the exception of those from the Lobus infracardiacus, similarly
arranged to those on the right.

Throughout the whole tree to this stage, we note with the single ex-
ception of Lateral 1 the constant relationship, which was indicated in
the earlier embryos, of the regular alternation of artery, bronchus, and
vein. In the earlier stages, the vessels were placed relatively close to the
bronchi; but with the increasing age of the embryo, the position of the
artery and vein has gradually shifted giving them a position at some dis-
tance from it. In some cases, this wandering may be so marked, especially
below Lateral 2, that the main veins and their chief branches may
occupy a position approximately midway between the adjacent bronchi.
The arteries, however, always lie closer to the air passages.

In the first part of embryonic life, the left pulmonary arch with a por-
tion of the right connects the right ventricle and the aorta, and the pul-
monary arteries, after the manner described by Bremer, finally take
origin from the left by a common stem arising from its under surface.
The aortic arch lies above, and both arches are situated superior to the

Joseph Marshall Flint 73

point of origin of Lateral 1, the tracheal bronchus. This relationship
and the subsequent behavior of the two arches as the heart moves down
affords us, I believe, some suggestive hints to explain the suppression
of Lateral 1 on the left side and its unusual low position in those ani-
mals in which it is present. Through all of the stages we have followed
hitherto, both the aortic and pulmonary arches, and the origin of the pul-
monary arteries lie well above the origin of Lateral 1. As shown by a
corrosion of the bronchi, arteries and veins in an embryo 15 cm. long,
the pulmonary arch is exactly opposite the site of origin of Lateral 1,
while the aortic arch is still higher. At the age represented by a pig
20-21 cm. long, the heart and vessels have descended further caudal-
wards, leaving the pulmonary arch well below the root of Lateral 1 and
the aortic arch exactly at its level. At the time of birth, both arches
have descended still more and pass dorsalwards in the interval between
the trachea, the stem bronchus, and the apical branch of Lateral 2
(cf. Pl. IV, Fig. 25). Now, had a symmetrical branch to Lateral 1
developed on the trachea, it is obvious that the descent of the great vessels
and heart would have been prevented. Instead of reaching their final
resting place just above the division of the trachea, they would have
been left hanging above the level of Lateral 1. It is thus reasonable to
suppose that the failure of this branch to form is due to a phylogenetic
provision on the part of the tree to leave a passage for the descent of
the heart and its great vessels.

A similar state of affairs is met with in the suppression of Ventral 2
on the left side. As the pulmonary vein forms approximately in the
median line in the younger stages, the hyperdevelopment of right Ventral
2, the development of the inferior vena cava on the right side, and the
shifting of the origin of the pulmonary vein from the site of its forma-
tion near the center of the undivided portion of the auricle to the left
auricle, together with the increasing asymmetry of the heart, tends to
carry the vein to the left. From its primitive approximate midline posi-
tion in the earlier embryos, it is found with the increasing age of the
embryo gradually passing to the left. In a pig 15 cm. long, we have
the vein for the entire lower segment of the tree lying over the portion
of the stem where left Ventral 2 should develop. Later still, in an
embryo 20-21 cm. long, the descent of the heart has changed once more
these relationships leaving this area of the stem bronchus covered by the
root of the pulmonary vein as it empties into the left auricle. As in the
case of Lateral 1, this suppression represents a provision on the part of
the tree to leave a space for the pulmonary veins.

74 The Development of the Lungs

We are forced, however, to consider those animals in which these
commonly suppressed elements are present. In these relationships we
can see a reason why no Lateral 1 and Ventral 2 should form, but
whether this stands absolutely in the relation of cause and effect, it is
impossible from my material to say, as it is conceivably possible, although
less probable for the condition te represent an adaptation on the part of
the vessels to the use of unoccupied space. For either its absolute affir-
mation or disproof, therefore, a series of animals, in which these ele-
ments occur, must be examined from this standpoint during their
developmental stages. This much may be said, however, in all of the
lungs objectively pictured by Aeby, Huntington, and Narath where Lat-
eral 1 is present on both sides, the one on the left-is usually lower than
the corresponding branch on the right. In the instances where they
are on the same level, both are so low that they do not interfere with
the descent of the heart and great vessels. Similarly, a bronchus that is
not situated on the left stem in the segment between L. 2 and L. 3 can-
not be considered as the homologue of V. 2, the Bronchus infracardiacus.
All other cases are substitution branches of the lateral bronchi or the
stem. In the lungs which have been well pictured in the literature,
where a real Ventral 2 occurs on the left stem, they are usually small
and poorly developed and would not materially influence the migration
of the Vena pulmonalis. It is also possible in these cases, as the veins
are never drawn, that the latter have different relationships from those

Influence of the Vessels upon the Architecture of the Bronchial Tree.—
After following the development of the vascular system, we may con-
sider now the possibility of the influence of the vessels upon the archi-
tecture of the bronchial tree. Concerning the general asymmetry of
the lungs, many of the older investigators have looked upon the heart
or the great vessels as being responsible for this irregularity. Thus
Bichat, 29, and Riidinger, 73, thought the left bronchus owed its greater
length to the asymmetry of the heart, while Meyer, 61, looked upon the
aortic arch as the factor which drew it out to greater length. In review-
ing these statements, Aeby felt these authors passed over the most
weighty relationship in overlooking the crossing of the bronchi by the
arteries at a particular point on the stem to run down on its dorsal sur-
face. This crossing enables the artery in the “hyparterial” to divide
the side bronchi into a dorsal and ventral series, while the “ eparterial ”
bronchi, situated above this separating influence of the artery, have their
dorsal and ventral branches arising from a common stem. In quoting
Kolliker’s observations on a 35-day human embryo, Aeby calls attention

Joseph Marshall Flint

=z
Cr

to the origin of the pulmonary arteries in the embryo above the lungs,
and states: ‘“ Hin spater eparterieller Bronchus muss somit so lange
hinter ihr legen, als nicht in Folge des hohern Aufsteigens des Organs
eine bogenformige Ablenkung derselben tiber den ersten Ventralbronchus
hinweg nach vorn hin stattgefunden.” While Aeby looked upon the
lungs instead of the heart as the movable factor in establishing the adult
relationships of the arteries to the tree, he recognized notwithstanding
this misinterpretation, the necessity of the embryological topography of
the “ eparterial ” or first lateral bronchi to produce the conditions which
we find in later stages. It is clear from the above account of the de-
velopment of the pulmonary arteries that these delicate vessels which
regularly follow the growth of the bronchi and do not, in fact, appear
in any part of the lung until after the respective branches which they
supply are present, have no formative influence on either the structure
or relationships of the bronchi, but are simply passive followers of their
development produced by histomechanical principles from the capillary
plexus which surrounds them. Finally, a crossing of the stem bronchus
by the artery does not occur until after birth when all of the bronchi
are laid down, and even then, in the strict sense of Aeby, does not exist
as Zumstein and Narath have already shown. It is thus most difficult
to determine just what led Aeby to lay such stress upon the adult rela-
tionship of the artery to the stem when he obviously, as the above quota-
tion shows, clearly recognized that it was not associated with the earlier
formation of bronchi, but was due, as he supposed, to the later ascent of
the lungs. Furthermore, the pulmonary artery is not responsible for
the dorsal and ventral divisions of the stem bronchi as we have ventral
and medial elements also arising from the stem away from any possible
influence of the artery.

Miller, 98, brings forward an interesting suggestion with reference
to the effect of the pulmonary arteries on the tree dependent upon the
descent of the heart in mammals which have had the form of their chest
wall altered by their life in water. The pulmonary arteries, according
to Miiller, following the descent of the heart tend to drag the “ Ventral
bronchi” caudalwards, leaving the dorsal bronchi free and uninfluenced
by the arteries to wander up on the stem bronchus or trachea to form
the so-called ‘ eparterial” bronchi. This ingenious suggestion is not
borne out, however, by the facts of embryology, for as we have seen, all
the bronchi are well formed before the heart in its descent reaches a
level where the pulmonary arteries could exert such a traction upon the
lateral bronchi.

76 The Development of the Lungs

Huntington, 98, says: “If we seek for an explanation of the cause
which leads to the migratory changes of the cephalic bronchus (Lateral
1), I admit that we enter the realm of pure hypothesis. At the same
time, the very general development throughout the mammalia of this
type, with the resulting greater respiratory area of the right lung, may, I
think, not improperly be referred to the development of the mammalian
form of the systemic and pulmonary arteries. On the left side, the
greater quantity of blood thrown from the right ventricle into the left
pulmonary artery passes through the Botallian duct directly into the
aorta, only a small portion traversing the left pulmonary circulation.
On the right side, however, with the early obliteration of the dorsal seg-
ment of the fifth arch, all the blood entering the right pulmonary artery
is forced to traverse the entire pulmonary circulation returning to the
left auricle by the pulmonary veins.” This explanation, according to
Bremer’s description of the development of the pulmonary arteries, could
not account for the increased size of the right lung, especially in the pig
where all of the blood to the lungs is forced to pass through the left
pulmonary artery after the establishment of the transverse anastomoses
and the subsequent degeneration of the proximal portion of the right
pulmonary artery.

We may say then in conclusion, that there is one simple possible expla-
nation for the general asymmetry of the mammahan lung which lies
in the asymmetry of the anlage. Owing to the fact, however, that the
pulmonary anlage in lower animals is frequently symmetrical, it seems
more probable to look upon this characteristic as an adaptation on the
part of the pulmonary apparatus to its environment which may reach
such extremes as we find in the lung of the snake. It is more probable
then, that, with the necessity of an increased respiratory surface as we
ascend the animal scale, the asymmetrical heart and the development
of its adult form gives us adequate ground for a normal asymmetry of
the respiratory apparatus, especially as the heart and liver, forming the
principal environment of the lungs, have phylogenetic precedence and
are of more physiological importance during intrauterine life. In its
final form, this asymmetry consists, in the vast majority of lungs, in a
suppression of left Lateral 1 to leave space for the descent of the aorta
and pulmonary arch with the heart and a suppression of left Ventral 2
to provide room for the pulmonary veins from the lower lobes. In ani-
mals, however, where these branches are formed they are so placed that
they do not interfere with either of these features of the development of
the vascular system.

Joseph Marshall Flint att

LoBE FORMATION IN THE LUNGS.

The relation of the mesoderm to the primitive tree has been described
in connection with the appearance of the bronchi, largely because it
arises from the general mesoblast of the head gut and takes part in the
separation of the pulmonary anlage from the cesophagus. The meso-

lu
i Le
ST ST
Thx) HEiGs. db:

Text Fic. 15. Outline drawing of the lungs of an embryo pig 10 mm. long.
Ventral view. (Figs. 15-19-24, inclusive, drawn with a camera lucida from
cleared preparations.) L.1, L.2—=Swellings, limited by shallow grooves,
over Lateral 1 and Lateral 2. S7’—=Mesoderm over the caudal portion of the
stem bronchi. Also L.1—=Lobus superior. L.2—=Lobus medius (right)
and Lobus superior (left). SZ—Lobus inferior.

derm, it will be remembered, shows the influence of the first irregularity
of the early branches of the tree and forms two indefinite unequal
rounded projections into the primitive ccelom on either side. These

Li li
(Le L2 é ie
L3 \

V2 L3
i ST L3 V2 Ls
| ST ST
ie B
ixeT IG slo

Text Fic. 16. Outline drawing of the lungs of an embryo 12.5 mm. long.
A. Ventral view. B. Dorsal view. 2.1, L.2, L.3, V.2, and ST =Swellings
over the several bronchi and the stem designated by these abbreviations. At
this stage the anlagen of the lobes are complete. L.1—=Lobus superior, L. 2
= Lobus medius (right), Lobus superior (left). V.2—Lobus infracardiacus.
L.3 and ST =Lobus inferior.

are the anlagen of the two lung wings. On both sides the Recessus
pleuroperitonealis projects upwards and somewhat medialwards to the
bronchi; the left, however, is very poorly developed. Ventralwards the
mesoderm continues forwards into the Mesocardium posterior.

~
(oa)

The Development of the Lungs

At 10 mm. the two simple lungs are quite asymmetrical (Fig. 15).
Increasing in size with the growth of the bronchi, they also follow their
asymmetrical development. The fain swellings observed in the preceding
stage have become so exaggerated that we have on the surfac of the lung
marked rounded elevations indicating the presence of Lateral 1 (Fig. 15,
L. 1) on the right side, and Lateral 2 on both sides (Fig. 15, L. 2).
These projections are limited by shallow groves. From above down-
wards, the trachea and hence the mesoderm extends ventralwards until
the point of bifurcation is reached, when, following the course of the
stem bronchi, it passes dorsalwards on either side of the cesophagus.

At 12.5 mm. (Fig. 16) these characteristics are exaggerated. On
the right side, high up, we have the projections over the bronchi, which
have been found before this stage. ‘They have increased in size with the

7 l1
1
Le Le Le
Le |
fi Vo L3 13 (2 L3
3 4
L4 te 14
14
A.

1B}.
Adios 1a alir/

TExt Fic. 17. Outline drawing of the lungs of an embryo pig 13.5 mm.
long. A. Ventral view. B. Dorsal view. The letters represent the mesodermic
swellings over the bronchi designated by the abbreviations. Designations
the same as in Fig. 16, except that L.3 and all swellings below that order
unite in the pig to form the Lobus inferior.

growth of their respective elements; also there is now a well-marked pro-
jection over the newly-formed V. 2 (Fig. 16, V. 2) and a less apparent
swelling, the bud representing Lateral 3 on each side (Fig. 16, L. 3).
The furrows have deepened, and the lower part of the wings below Ven-
tral 2 now embraced by the Wolffian body and chest wall dorsally, the
heart, liver, and diaphragm ventrally, and the mesoderm of the ceso-
phagus medially, have already in cross-sections an irregular prismatic
form. At this stage we may say, the anlagen of the lobes are com-
plete. From each of these main projections, a lobe is produced and the
shallow grooves deepen with the further growth of the lungs to form
the interlobar fissures. That is to say, on the right side the swellings
over Lateral 1, Lateral 2, Ventral 2, and the stem produce respectively

Joseph Marshall Flint 79

the Lobus superior, Lobus medius, Lobus infracardiacus, and Lobus in-
ferior, while Lateral 2 and the stem bronchus produce the Lobus superior
and Lobus inferior on the left. At 10 mm. the swelling over L. 1 is
practically in the same lateral plane as L. 2, while at 12.5 mm. it is
crowded slightly dorsalwards by the further growth of the latter.

In a pig 13.5 mm. long (Fig. 17), the characteristics of the lobe
formation are intensified. On the right side, the upper lobe containing
Lateral 1 is pushed still more dorsalwards, while the middle lobe con-
taining Lateral 2 is, at the same time, forced shghtly ventralwards by
the antagonism in the growth of their two main bronchi. The Lobus
infracardiacus, containing Ventral 2, extends downwards and medial-
wards, while the lower lobe extends more caudalwards and is now,
through its whole extent, distinctly prismatic in cross-section. On the

Text E1q@. 18.

Text Fic. 18. Outline drawing of the lungs of an emtryo pig 14.5 mm.
long. A. Ventral view. B. Dorsal view. Designation of lobes as in Fig. 16.

left side, the Lobus superior, owing to its more unobstructed environ-
ment, extends somewhat higher than its homologue, the Lobus medius, on
the right side. The Lobus inferior is not quite so large or well devel-
oped as the corresponding right lobe. The primary fissures between the
several lobes have deepened and now extend well into the substance of
the lung. With the division of Lateral 1 and Lateral 2 on each side, the
secondary branches also raise secondary projections on these surfaces of
the lobes between which are slight secondary furrows. Similarly the
Lobus inferior on each side shows slight swellings limited by shallow
grooves over L. 3 and L. 4. In the pig, these swellings and grooves,
however, under ordinary circumstances, never lead to a separation of
the lung substance into extra lobes.

Fig. 18 shows the lungs of an embryo 14.5 mm. long. The Lobus

80 The Development of the Lungs

superior on the right side (Fig. 18, L. 1) is now pushed dorsalwards
by the presence of the heart and the Lobus medius (Fig. 18, L. 2), so that
its caudal portion now hes above the series of swellings over the dorsal
bronchi (Fig. 18 6, D. 2). On the left side, the Lobus superior now
shows a dorsoapical swelling over the apical branch of L. 2 (Fig. 18,
L. 2), which indicates the beginning of the portion of the left upper lobe,
which substitutes for the Lobus superior on the right side. The fissure
between L. 2 and L. 3 on each side deepens, while the Lobus inferior on
both sides shows a series of projections over the several branches of the
stem. On the ventral surface, V. 3 is indicated; on the lateral border,

Tuxr Hie 9:

TExT Fic. 19. Outline drawing of the lungs of an embryo pig 18.5 mm.
long. A. Ventral view. B. Dorsal view. The abbreviations on the swellings
represent the order of the bronchi beneath. Designations as in Fig. 16.

L. 3, L. 4, and L. 5; while, on the dorsal border, swellings for D. 2,
D. 3, and D. 4 are present.

In a pig 18.5 mm. long (Fig. 19), the right Lobus superior contain-
ing Lateral 1, projects upward some distance beyond the tip of the upper
lobe on the left side. The fissure separating it from the Lobus medius
has deepened. Its lower portion now passes behind the medial lobe,
although the two are united at their roots, that is to say, the ventro-
medial aspect. The Lobus infracardiacus projects ventralwards and
medialwards until it extends over the median line above the cesophagus.
The lower lobe on the right side shows projections along the lateral
border for L. 3, L. 4, and L. 5, and, on the dorsal border, for D..2, D. 3,
and D. 4. The ventral surface, likewise, has very slight swellings for

Joseph Marshall Flint 81

V. 3 and V. 4. , The latter, however, are very faint and are separated
from the rest of the lobes by very shallow grooves. On the left side,
the Lobus superior (Fig. 19, L. 2) is separated from the lower lobe by
a deep cleft, while the development of the apical branch of L. 2 has
pushed up with it a segment of this lobe which also grows backward
until it lies above the series of dorsal swellings (Fig. 19 B) and bears a
marked resemblance to the Lobus superior on the opposite side. Ex-
cepting for the Lobus infracardiacus, the lower lobe has characteristics
practically homologous to the corresponding lobe of the right side. The
dorsal flexion of both lower lobes still persists and the lateral tips or
margins of the median lobes now begin to show, at their lateral extremi-
ties, a shght bending ventralwards as they begin to fold around the
heart.

As the lung continues to grow, with the successive appearance of new
branches, new elevations are formed on the surface of the primitive lobes
until finally, as Narath describes, they have an appearance like the sur-
face of a mulberry. The primitive lobes, however, keep their independent
character: and alter in form by two chief factors, namely, the intrinsic
growth of the lung itself, and the change in its environment formed by
the chest wall, heart, liver, and diaphragm. Narath has given as the
cause of the lobe formation, the extremely rapid growth of the first
branches of the tree, while the later branches of slower growth fail to
form furrows in the mesoderm deep enough to subdivide the lung further.
With this view, I am in complete accord, but it ought, it appears to me,
to be extended to include the character of the mesoderm. In the early
stages, this is in extremely plastic form, which easily moulds itself to the
pressure of the growing bronchi beneath. Up to 10 mm. there is scarcely
any differentiation in the mesoderm into distinctly fibrillar and cellular
portions, while at 12 mm. this change is inaugurated and fibrils appear
particularly in the region of the root of the primitive lung. At 20 mm.
the whole mesodermic portion is composed of young connective tissue
with well-marked fibrils. As the mesoderm differentiates, therefore, it
becomes firmer and is less easily influenced by the growth of the young
bronchi.

Fig. 20 is an outline drawing of the lateral and diaphragmatic aspects
of the lings of an embryo 19 em. long. At this time, all of the important
adult topographical features of the lungs are present. A., shows well how
the right Lobus superior has grown down and back into the dorsal area,
moulding itself even more than in an embryo 18.5 mm. long (Fig. 19)
to that portion of the thoracic cavity and extending now up over the

6

82 The Development of the Lungs

base of the heart beyond the midline making the sum of lung tissue in
L. 1 and L. 2 considerably greater than that in L. 2 on the opposite side.
Owing to this growth, the Lobus medius is pressed ventralwards, its
dorsal segment lying in the angle between the Lobus superior and the
Lobus inferior. It may be interesting to note, that the portion of the

Abiae Ine, 7A0),

Text Fic. 20. Outline drawings of the lungs of a pig 19 cm. long.
A. Right side. B. Left side. C. Diaphragmatic surface. At this stage, the
surface of the lungs is smooth. The topography of the bronchi beneath,
taken from corrosion specimens of the same age, is indicated by dotted lines
and letters.

lobe which lies in this angle, is supplied by the large dorsoinferior
bronchus. It is, therefore, ontogenetically equivalent (vide Pl. II,
Figs. 15, 16) to the apical segment of the Lobus superior on the other
side. Nothing could indicate clearer the adaptation of the growing

Joseph Marshall Flint 83

bronchi to their environment, or the possible influence of environment
upon the branches of the tree. The tips of the Lobus medius have
grown around the heart until they have almost met in the midline. On
the undersurface, the unpaired Lobus infracardiacus (Fig. 20, V. 2)
is clearly seen particularly in its relationship to the Vena cava inferior.

With the increase in size between this and the last stage, the swellings
over the various bronchi have disappeared and the surface of the lobes
become smooth. The topography of the Lobus inferior on both dia-
phragmatic and lateral surfaces is indicated on the surface of the lungs
by dotted lines. By a comparison with Fig. 19, the origin of these
topographical relations are clear.

With the further development of the pig’s lung which has been de-
scribed by Narath, I cannot agree. In the account of the form relation-
ships, his work is accurate, but in the interpretation of the relative sig-
nificance of the different parts of the lung and the equivalent values of
the lobes on each side, our results differ chiefly with our derivation of the
principal bronchi. That is to say, according to his view the Lobus
superior and the Lobus medius on the right side are equivalent to the
Lobus superior on the left. They are almost or completely separated
through an accessory fissure, making the Lobus superior correspond to
the dorsal or apical area in his preparations and equivalent to the cephalic
or apical projection of the Lobus superior of the left lung. The latter,
as we have seen, is only a secondary substitution product of a branch
of left L. 2, ontogenetically equivalent to the region of the Lobus medius
on the right side which is supplied by the large dorsoinferior bronchus.
On the other hand, the right Lobus superior, supplied by L. 1, is totally
unrepresented in the left lung. This unpaired lobe, therefore, and also
the cephalic portion of the upper lobe on the left, properly belong not to
the dorsal area, as Narath suggests, but to our lateral and his ventral
region. The fissure between Lobus superior and Lobus medius on
the right would be primary and not accessory in the sense of Narath.

In recapitulating the development of the lobes, we may say, then. that
the mesodermic portion of the lungs, derived from the general mesoderm
about the head gut, is pushed out by the growing bronchi to form
irregular asymmetrical swellings in the coelom. These are the anlagen
of the primitive wings of the lungs. With the appearance of L. 1 on
‘the right side of the trachea, and L. 2 on each stem bronchus, primary
swellings are formed in the two wings over these bronchi, giving rise to
the simplest form of the Lobus superior, Lobus medius on the right side,
and the Lobus superior on the left. The remainder of the mesoderm

84 The Development of the Lungs

about the stem bronchi form the anlage of the Lobus inferior on each
side. With the appearance of V. 2, the Bronchus infracardiacus, on the
right, a swelling forms over it yielding the anlage of the Lobus infra-
cardiacus. These swellings are at first surrounded by shallow grooves,
which, with the rapid growth of the bronchi beneath, develop into the
fissures separating the various lobes. With the further growth of these
chief bronchi and the appearance of the series of bronchi on the stem,
a series of swellings and fissures are formed over and between them.
These are equivalent, in all senses except in age and size, to the earlier
fissures and swellings, but, under ordinary circumstances, never deepen
into distinct lobes. ‘This is partly due to the more rapid growth of the
first bronchi, to the gradual increasing density of the mesoderm, and,
lastly, to the environment of the several lobes of the lung. That is to
say, the Lobus superior with L. 1 has the territory between the chest
wall and the upper part of the heart on the right side. The right Lobus
medius and the left Lobus superior, with L. 2, have the large space be-
tween the chest wall and the angle formed between the heart and liver
on each side. It is important, however, to note on the left side, owing
to the absence of L. 1, the Lobus superior sends up the apical segment
of the lung containing the left Bronchus ascendens. The Lobus infra-
cardiacus, with V. 2, grows out into the space left between the heart
and liver and the two lower lobes, while the Lobus inferior on each side
lying in the more or less triangular space between the chest wall and
liver and diaphragm becomes prismatic in cross-section and grows caudal-
wards and lateralwards to fill up the rest of the pleural cavity.

In the pig, then, we have a series of primary projections limited by a
series of fissures some of which give rise to the permanent pulmonary
lobes. Those projections and fissures which take part in the lobe forma-
tion in the pig, it is well to observe, are the first to form, but in other
animals these same conditions do not appear to obtain. In Hystrix
cristata, for example, not only the primary fissures between practically
all of the principal bronchi may give rise to a series of lobes, but these
may even be subdivided by the secondary fissures formed by the secondary
branches of these elements, while in other animals, as for example man,
the deepening of the fissures about V. 2 usually do not produce a separate
lobe, leaving this region of the lung included in the right Lobus inferior.
Between these forms we have extensive individual and general variation.:

The drawings in Fig. 20 may be used conveniently to explain the
lobe production in all mammals; A represents the conditions in animals
where L. 1 is present on one side or both; B, the conditions where L. 1

Joseph Marshall Flint 85

is absent on one side or both; C represents lungs where a Lobus infra-
eardiacus is present, and by eliminating this lobe and altering the topo-
graphy of the ventral bronchi, it may be used for lungs where V. 2 is
either absent or included in the Lobus inferior. For example, B repre-
sents the conditions found in Hystrix cristata in both lungs where not
only all of the primary bronchi in that animal have produced lobes, but
some of them are still further partially subdivided. There is also a
type of lung represented by Phoca vitulina where L. 1 is present on both
sides, but L. 2 in this species is thrown into the Lobus inferior. For this
state of affairs 4 would suffice if the permanent fissure between L. 2
and L. 3 were replaced by a dotted line. The suppression of the lobes
indicated in Phoca vitulima may involve all fissures giving us a lobeless
lung like those of Delphinus delphys and Pithecus satyrus.

It is, of course, clear from the above description how we regard the
equivalent values of the lobes on the two sides, but they may be simply
stated in two simple formule of equivalence which will fit the lungs of
most animals depending upon the presence of L. 1 and V. 2 on one or
beth sides. Type 1 includes the great majority of mammalian lungs.

Type 1. Type 2.
L.1 present only on the right side. L. 1 present or absent on both sides.
Right Side. Left Side. Right Side. Left Side.
Lobus superior — O. Lobus superior — Lobus superior.
Lobus medius = Lobus superior. Lobus medius = Lobus medius.
Lobus inferior = Lobus inferior. Lobus inferior = Lobus inferior.
or or
Lobus inferior + V.2—=Lobus in- Lobus superior — Lobus superior.
ferior + V.2 or O. Lobus inferior — Lobus inferior.

While lobe production in the lungs is obviously dependent on the
growth of the bronchi in the. majority of instances, the number of lobes
is apparently without definite morphological significance. It may vary
in animals from multilobed lungs like those of Hystrix to lobeless lungs
like those of Pithecus satyrus. The common relationships, however, are
expressed in the types given above.

THE ORGANOGENESIS OF THE LUNGS.

In turning to the organogenesis of the lungs from the period of the
formation of the Anlage until the adult stage is reached, the first interest
settles in the chief cells of the bronchi and the pulmonary connective
tissue. Both of these structures have been followed up to the age repre-
sented by a pig 10 mm. long, in the chapter on the development of the

86 The Development of the Lungs

bronchi. From this time, it is more convenient to consider these stages
by themselves. In the description of the differentiation of the frame-
work, I have taken as a basis the work of Mall, 02, who has described in
the pig the origin of the connective tissues from a common mesodermic
syncytium. By a differentiation of this syncytium into an endoplasmic
and exoplasmic portion, the connective tissues are produced. The former

ESP TE. Zils

Text Fic. 21. Longitudinal section of the left lung of an embryo pig
13 mm. long. Fixed in Zenker’s fluid and stained by Mallory’s Fuchsin-
Anilin blue method. X70. b=Stem bronchus. p=—pleura. a= Young
connective tissue. =syncytium. m—evagination forming medial bronchus.

remains as the protoplasm about the connective tissue cells, the latter
forms the various fibrils. The author, 03, has traced the development of
the framework of the submaxillary gland in the pig, where, in the earlier
stages, the process of differentiation is the same as in the lungs. By

Joseph Marshall Flint

oo)
-2

way of review, suffice it to say that the syncytium forming the primi-
tive framework of the lungs differentiates slowly until 10 mm. is reached
when, in the neighborhood of the root of the lung and the Mesocardium
posterior, the fibrils begin to appear and the cells become more isolated
from each other. About the young bronchi, however, they are still in
close apposition during the formation of the reticulated membrane about
the tubes, which, in Mallory preparations, may be seen as a dark blue line.

At 13 mm. (Fig. 21) these conditions are well shown. The stem
Lronchus (Fig. 210) and its chief lateral branches is seen in longi-
tudinal section lined, by an epithelium consisting of a row of inner

nxn Ge 22.

Text Fic. 22. Section of the lung of an embryo pig 3 cm. long. Same
preparation as used with tissue shown in Fig. 21. xX 70. p=pleura. a=
connective tissue. 6 —bronchus.

columnar cells with smaller polygonal cells beneath them. The epithelial
tube is surrounded by a simple reticulated membrane which is in pro-
cess of formation. Above, at the root of the lung (Fig. 21 a), the trans-
formation of the exoplasm into young connective-tissue fibrils has taken
place, while in the lower portions of the Lobus inferior (Fig. 21 ¢), the
framework consists of a mass of anastomosing syncytial cells without
any particular differentiation. About the basement membrane, the cells
are thickly packed and under the primitive pleura (Fig. 21p) the
epithelium of which has begun to flatten, we have a distinct blue line
indicating the formation of a membrana propria.

o/2)
(0/6)

The Development of the Lungs

In a pig 30 mm. long (Fig. 22), the framework of the entire lung
shows a differentiation into primitive fibrils. The young fibrils are
more distinct and less granular, while the spaces between are larger
than in the preceding stage. With the differentiation, the relative quan-
tity of endoplasm has diminished in the loose part of the syncytium,
leaving in some places isolated connective-tissue cells (Fig. 22.¢), or in

TexT WIG. Zo.

Text Fig. 23. Section of the lung of an embryo pig 5 cm. long. Same
preparation as used with tissue shown in Fig. 21. x 70. p=pleura. a=
connective tissue. = bronchus.

others they are multipolar in appearance with branching and sometimes
anastomosing processes. Immediately about the trachea and large
bronchi, the cells are closely packed together preparatory to the pro-
duction of the various coats of these structures. The basement mem-
brane is distinctly fibrillated as is seen at points where the plane of sec-
tion is tangential to the bronchi. About the larger bronchial elements a

io)
ito)

Joseph Marshall Flint

group of elongated fusiform cells having a distinctly circular arrange-
ment may be noted, representing the earlier stages of the production of
the muscular coat.

The epithelium in all the large and in the majority of small bronchi
still consists of two layers of cells, the inner columnar, the outer poly-
gonal in form. But in the youngest branches of the oldest bronchi,
namely Lateral 1 or 2, there is now a reduction to a single layer of
columnar cells (Fig. 220). Cilia are as yet invisible in these speci-
mens, but the cuticula at the inner margin of the cells is already differ-
entiated. At the root of the lung, a few dilated lymphatics may be noted
near the bronchi and pulmonary vessels; they have not, however, grown
beyond this point into the substance of the lung wings.

Embryo 5 mm. long (Fig. 23). The general framework (Fig. 23 a)
of the lung at this period has undergone a further differentiation over
the preceding stages, consisting in an increasing density and complexity
of the young fibrils, which now possess a more distinctly fibrillar appear-
ance, while the quantity of endoplasm about the connective-tissue cells
has slightly diminished, except in the immediate neighborhood of the
larger bronchi. The pleural epithelium (Fig. 23 p) is much more flat-
tened and the nuclei of the individual cells consequently further apart.
As shown by points where the plane of section falls tangential to its
surface, the basement membrane beneath this epithelium is distinctly
reticulated. About the larger bronchi, there is a distinct circumferential
arrangement of the exoplasmiec fibrils in which are imbedded a great
many cells. The basement membrane is slightly thickened and just
beneath the latter there is now a well-marked layer of fusiform cells
with elongated nuclei running circularly about the bronchial tube. Ex-
ternal to this stratum, is a looser circular arrangement of the exoplasmic
fibrils as well as the cells embedded in it. When the bronchi are cut
longitudinally, these circumferential lamelle of cells and exoplasm run
parallel to the long axis of the tube. The epithelium, as in the preced-
ing stages, shows a distinct division into two or three layers, with the
nuclei situated approximately in the middle of the cell. The thickening
‘on the edge of the cell lining the lumen is apparent, although cilia are as
yet unformed. As the branches of the tree are followed towards the
periphery, the layers of circularly directed syncytial cells disappear and
we have simply the primitive basement membrane with the connective-
tissue cells immediately about it. In the most terminal parts of the
air passages, the double layer of epithelium has been replaced by a single
layer of lower columnar epithelium (Fig. 230). All of the bronchi

9C The Development of the Lungs

from the first to last possess marked lumina. From the root of the lung,
the lymphatics have now grown some distance into its substance. They
have thin walls composed of young fibrils lined by endothelium with
occasional valves. They are confined, however, to the immediate neigh-
borhood of the main bronchi and their chief subdivisions.

Text Fig. 24.

Text Fig. 24. Section of the lung of an embryo pig 7 cm. long. Same
preparations as used with the tissue shown in Fig. 21. X70. p=pleura. Db
=pbronchus. a—connective tissue. /—lymphatics. This stage shows the
beginning of the lobulation.

Pig 7 em. long (Fig. 24). A number of interesting changes have
taken place in the evolution of the lungs since the last stage other than
in a further differentiation of the framework, which at this time is con-

Joseph Marshall Flint 91

siderably denser. The circularly arranged fusiform cells noted in the
earlier stages about the main bronchi are collected into bundles to form
the muscular layer outside of the mucosa, while still external are stages
in which the chondrification of the syncytium is progressing as the latter
passes over into the precartilage stage at the periphery, and into young
cartilage in the center to form the simple chondral rings of the trachea
and larger bronchi. The epithelium of the latter is sometimes thrown
into folds, is cylindrical, and composed of a double layer of cells. As one
follows the branching to the end buds, it first becomes single layered and
then of a low columnar type (Fig. 246). Chondral rings and bronchial
cartilages are present only around the trachea and the upper part of the
stem bronchi; the muscular coat, as one passes peripheralwards, thins
out until it first consists only of a single layer of cells, and finally at the
smaller branches and end buds is replaced by the young connective tissue,
which, in the latter region, is engaged in the formation of the reticulated
membranes.

The most interesting change, however, lies in the further growth of the
lymphatics, which, in the earlier stages, are found in the root of the
lung in the neighborhood of the pulmonary vessels and large bronchi.
As they grow in, they accompany these structures for a distance, then,
approaching the end branches, they leave them and run in a plexiform
manner midway between the bronchial tubes (Fig. 247) until they reach
the pleura (Fig. 24). This gives the lung now an indefinitely lobu-
lated appearance, in which the periphery of the simple lobule is indi-
cated by the lymph vessels and the center by the bronchi. The lymphatics
are lined by flattened endothelium, their walls are formed by the young
connective-tissue fibrils, and, here and there, valves are beautifully shown,
which, in general, point away from the pleura. The pleural epithelium
(Fig. 24 p) is much flattened and now rests upon a thickened layer of
young connective-tissue fibrils.

Pig 13 em. long (Fig. 25). At this stage, we have the whole lung
subdivided into a series of connective-tissue lobules with essentially the
same characteristics as those shown in the preceding stage, namely, a
peripheral plexus of lymph vessels with the bronchus in the center. The
growth is centrifugal in so far as the bronchi are concerned and, in this
sense, the lung at this stage may be compared in some respects with
the younger stages of the salivary glands for example, where similar
lobules without peripheral lymphatics are also formed from a centrifugal
growth of the ducts. The framework at this stage (Fig. 25a) is con-
siderably thicker than in the preceding embryo, the fibers denser and, at

92 The Development of the Lungs

the same time, there are more connective-tissue cells. Under the pleura
(Fig. 25 p) and in the interlobular spaces, the fibrils are gathered into
slight trabecule, which limit small spaces in the connective-tissue net-
work.

The larger bronchi show an increase in the characteristics indicated in
the last stage. The epithelium is thrown out into longitudinal folds,

Texr Wig. 25.

Text Fic. 25. Section of the lobule of the lung of a pig 13 cm. long
Same preparation as used with the tissue shown in Fig. 21. <X 70. p—=pleura.
a@—=connective tissue. b—=bronchus. c—end bud. /=lymphatics.

which are accompanied by folds of the basement membrane and sub-
mucosa. This is composed of trabeculae formed from the young con-
nective-tissue fibrils. In the young submucosa, the simple muscle bundles
lie, and still external to the muscularis the cartilagenous rings are in
process of formation. Proceeding peripherally, the bronchi grow essen-
tially younger and the epithelium is first reduced to a single columnar

Joseph Marshall Flint 93

layer, which then becomes lower until, in the lobules, it forms a lower
columnar epithelium. Still further out in the growing terminal buds
(Fig. 25 ¢), it now has a distinct cubical form. About these, the mem-
brana propria is formed from the connective tissue of the lobule.

The lymphatics (Fig. 257), forming a plexus around the bronchial
veins and arteries at the root of the lung, accompany them towards the
periphery, giving off branches to the interlobular spaces en route. Their
walls, owing to the increasing differentiation of the framework, are
thicker. On reaching the periphery of the lung, they leave these struc-
tures and pass out as in the preceding stages to the pleura. They have
a plexiform arrangement and may (Fig. 25) be traced at times into the
substance of the lobules. This course may also be observed in the deeper
lobules of the lung as well as those on the surface under the pleura.

In the period of embryonic hfe between pigs 13 and 19.5 em. in length
there are no marked changes of the relationships we have,thus far de-
scribed. In the larger bronchi, a gradual development has occurred.
The epithelium now possesses well-marked cilia springing from the
cuticular border of the inner layer of epithelhum, between the elements
of which, goblet cells appear here and there, partly filled with mucous.
These are clearly seen first in the stem bronchi of pigs between 15 and
17 cm. long. The folds, which have already been described running
longitudinally with the bronchus, now look in cross-sections hke regular
papille with a core of submucosa. That they are regular structures of
the bronchi and not shrinkage products is shown by the impressions they
leave on corrosion specimens which are injected under considerable
pressure as well as their appearance in distended lungs. The muscu-
laris mucose is more developed and the bronchial cartilages are well
formed. In general, the relations of the lymphatic system has not
changed; lymph glands may be observed forming in the neighborhood of
the root of the lung, and large bronchi in pigs as young as 12 cm. ‘They
naturally increase in size and number with the age of the embryo. With
the other changes, there has been a gradual flattening of the epithelium
in the growing ends of the tree, until, in an embryo 18 cm. long, the
end buds are lined by a very flat form of cubical cells with spherical
nuclei. The cytoplasm, which in the earlier stages was granular, is now
clear and transparent.

At 19 cm. (Fig. 26), some notable changes have been inaugurated in
the structures. The ciliated epithelium of the stem bronchi possesses
a great number of goblet cells. In the submucosa, the muscularis has
gathered into distinct bundles, while from the fundus of the crypt-like

94 The Development of the Lungs

invaginations between the mucosal folds appears an ingrowth of glands,
containing partly serous cells and partly mucous cells which penetrate
sometimes as far as the muscularis and sometimes between its bundles
into the submucosa between it and the bronchial cartilages. In general,
the relations of the lymphatic system (Fig. 26/) have not changed, but

Thxt EKie. 26.

Text Figc. 26. Lobule of the lung from a pig 19.cm. long. Same preparation
as used with the tissue shown in Fig. 21. X70. b=bronchus. p—pleura.
i1=lymphatics. c—end buds. a@—connective tissue.

the connective-tissue lobules (Fig. 26) containing the growing ends of
the bronchial tree have increased considerably in size. The framework
(Fig. 26a) is denser around the end buds (Fig. 26 ¢), which, while still
lined by flat cubical epithelium, now show a dilatation of their lumina
preparatory to the formation of the respiratory lobules of Miller.

In pigs about 22 cm. long (Fig. 27), the chief changes are in the grow-

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Joseph Marshall Flint

96 The Development of the Lungs

ing end buds (Fig. 2771) which now have an extremely complicated con-
tour and show widely dilated lumina. As they begin to pack together
in the lobule, the connective tissue is compressed between them, and its
nuclei in consequence appear more numerous. From the low cubical

Texe Hie. 28.

Text Fie. 28. Section of a lobule of the lung of an embryo pig 27 cm.
long. Same preparation as used with the tissue shown in Fig. 21, except that
the lung was distended with the fixing fluid. 130. b=bronchiolus. br=
bronchiolus' respiratorius. i1=ductulus alveolaris. a — atria. 1= lym-
phatics.

epithelium of the smaller bronchi (Fig. 270) the transition is easy to
follow over into the irregular flattened epithelium that now lines the
young respiratory lobules. The nuclei are pressed against the sides of

Joseph Marshall Flint 97

the lobules and the relatively slight amount of clear cytoplasm extends
between them. The Bronchioli respiratorii (Fig. 27 br) are now readily
recognized leading off from the bronchioli (Fig. 27>). They open into
the dilated Ductuli alveolares (Fig. 277) from which the primitive Atria
may be seen as lateral outgrowths.

Shortly before birth, in a pig 27 cm. long (Fig. 28), the framework
of the lung at the root, between the lobules and under the pleura, consists
of definite trabecule composed of fibrils in the meshes of which le the
connective-tissue cells. In the neighborhood of the root, the trabecule
are thick and firm and thin out as the periphery is reached. The struc-
ture of the stem bronchi is on the same plan as in the earlier stage, but
the epithelium submucosa, muscularis, and cartilages are more developed.
As the periphery is approached in this, as in the younger stages, they
become essentially younger in structure, loosing first their cartilages, then
the muscularis, and finally, before terminating, have only a thickened
basement membrane which contains connective-tissue cells (Fig. 28D).
The respiratory lobules are now fully formed, but are not as large or as
complicated as in the stages after birth. In this section there are two
Bronchioli respiratorii (Fig. 28 br) from the ends of which the Ductuli
alveolares (Fig. 287) lead. These terminate in dilated Atria (Fig. 28 a)
on the walls of which the Sacculi alveolares are now indicated as shght
irregular outgrowths. While complete corrosions of the lungs in which
the respiratory lobules are injected are of great service in interpreting
the pictures found in sections, I have feared to trust these preparations
for an exact description of the growth of these structures, owing to the
possibility of artefacts. The nuclei of the respiratory epithelium now
project often into the lumen of the air spaces. In general, the cells are
extremely flattened and the nuclei elongated. A flat sheet of protoplasm
extends out from either pole of the nucleus resting upon the mebrana
propria. Here and there, where capillaries project into the lumen of the
air passages, the nucleus lies in the angle formed by the capillary and the
basement membrane with the protoplasmic portion of the cell projecting
up over the capillary, like a non-nucleated plate.

Adjacent Lobuli respiratorii impinge on each other, pressing the loose
connective tissue, which has hitherto existed between the lobules into a
thin membrane in which the capillaries run. This interalveolar mem-
brane now consists of the membrana propria of the adjacent lobules, to-
gether with the interalveolar connective tissue. The lymphatics in the
various parts of the lung still show essentially the same relationships.

After birth (Fig. 29) the development of the lung has advanced along

‘

98 The Development of the Lungs

the same lines followed in embryonic life. The chief changes occur in
the respiratory lobules. The bronchiolus (Fig. 290) is clothed by
eubical epithelium surrounded by a well-marked basement membrane

Trexr Fic. 29. Sections of a portion of the lobule of the lung of a pig, two
days old. Same preparation as used with tissue shown in Fig. 28. X 130.
?=lymphatics. b—bronchiolus. br=—bronchiolus respiratorius.
tulus alveolaris. a=atria. sa—sacculi alveolares.

i= duc-
c—alveoli pulmonaris.

Joseph Marshall Flint 99

about which are numerous connective-tissue cells. There is as yet, how-
ever, no differentiation of this layer into muscle fibers. From this arise
the short Bronchioli respiratorii (Fig. 29 br) where the cubical epithe-
lium flattens as the passages run into the Ductuli alveolares (Fig. 290).
From these structures, the Atria (Fig. 29a) are formed, which in turn
produce the Sacculi alveolares (Fig. 29sa). The air sacs which were
only indicated in a pig 27 cm. long are now distinctly seen. It is pos-
sible that they are even more developed before birth than is shown in
Fig. 28, as I have frequently found embryos in utero 29 cm. long.
Unfortunately, I have been unable to obtain good sections from speci-
mens of this age. This makes, however, no essential difference as the
whole respiratory lobule is produced before the pig is born. Following
the use of the lungs for respiration, there is a dilatation of the various
structures of the lobule (cf. Figs. 28 and 29) which is accompanied by a
still greater flattening of the connective tissue between the alveoli, yield-
ing practically a single membrane containing the blood-vessels between
the two layers of respiratory epithelium. ‘This, however, as we have seen,
ontogenetically consists of the two basement membranes and the inter-
alveolar framework of the adjacent alveoli. The larger connective-tissue
lobules still retain their general relationships, increasing in size with
the growth and dilatation of the respiratory lobules of Miller. The
lymphatics (Fig. 297) still have their regular relationships.

In a half-grown pig, one observes the thickening of the framework,
which in the main septa at the root and under the pleura is now made up
of well-formed trabecule, consisting of connective-tissue fibrils. The
bronchi have developed peripheralwards taking on an older type, 1. @.,
adding muscular layers, submucous glands, and bronchial cartilages,
which may be traced as far as the larger intralobular branches. From
this point peripheralwards, gradually thinning, the muscle layer extends
to the opening of the atria in the Ductul alveolares. The lymphatics
in the interlobular septa are difficult to see as they are pressed together
by the growth and distension of the connective-tissue lobules. No
marked changes occur between this and the adult stage, save that the
lobules are sometimes less apparent owing to their larger size and the
fact that the septa may become thinned out in the later stages of growth.
They may be demonstrated as definite anatomical structures in the pig
by thick sections stained by Mallory’s method or better still by complete
Wood’s metal injections. When a lung has been distended for a short
time with air to its maximum, Wood’s metal will pass into all the indi-
vidual alveoli. After digestion, we have a cast of granular appearance

100 The Development of the Lungs

which maintains absolutely the form of the lungs. This may now be
broken up into the lobules, as the splitting always occurs along the septal
lines and, thus, the entire connective-tissue lobular system may be re-
vealed. It should be observed that the lobules may become compound
through a failure of the septa to persist, a process similar to that which
takes place in the submaxillary gland where the whole series of primitive
lobes, which are first formed in the embryo and separated by well-marked
septa, disappear and are indicated in the adult only by irregular septa,
without distinct relationships, passing in from the capsule. Usually,
however, these lobules in the pig’s lung not only persist, but may be
easily demonstrated by any of the ordinary connective-tissue stains.
Recapitulation of Organogenesis.—In recapitulating the growth of the
main structures of the lungs, we have stem and main bronchi origi-
nating in the primitive lung sacs as an epithelial tube with a double
layer of epithelium, the inner of which is columnar, while the outer
is composed of smaller polygonal cells. This simple tube is surrounded
by a membrana propria formed by a deposit of fibrils from the exoplasm
of the connective-tissue syncytium. As the bronchi grow, a layer of
spindle cells differentiates from the mesoderm, which is transformed
into the muscular coat of the bronchi. Later still, a chondrification
of the perimuscular syncytium takes place from which the cartilla-
ginous rings of the trachea and the bronchial cartilages are formed.
With these changes the connective-tissue fibrils become grouped into
trabecule about the bronchi and in the submucosa. Later, the mucosa
is thrown into a series of longitudinal folds, while from the cuticular
border of the inner row of cells, cilia develop. From the bottom of
the erypt-like invaginations formed by the longitudinal folds of epith-
elium, glands begin to grow down into the submucosa, which some-
times pass between the developing muscle bundles into the deeper layers
of this coat. As this process takes place, there is a differentiation of
some of the epithelium into goblet cells, a process which also takes place
in the glands, giving rise to a series of submucous glands with partly
serous and partly mucous cells. While these changes are taking place
in the mucosa, the cartilages are also growing, and with them, a further
differentiation of the framework into distinct fibrous trabeculae. As we
follow the bronchi peripheralwards, they become simpler and essentially
younger in structure and yet develop their adult characteristics in pre-
cisely the same way. The epithelium soon becomes single layered of a
columnar type, and then of a distinct, flat, cubical form. The Lobuli
respiratorii begin to develop in pigs about 19 cm. long by a slight dila-
tation of the growing ends of the bronchi. These represent the bron-

Joseph Marshall Flint 101

chioli. Later, the Bronchioli respiratorii are formed which have a
progressively flattened epithelium, running over into Ductuli alveolares.
These are present at the age represented by a pig 22 cm. long. Subse-
quently, Atria, Sacculi alveolares, and Alveoli pulmonis form in the
prenatal period, all of which have the characteristic flattened respiratory
epithelium. After birth there is a dilatation of the lobules and a further
flattening of the epithelium occurs, and before the pig is half grown, a
muscle layer develops about the air passage as far as the Atria, where it
stops in sphincter-like bands.

The framework of the lung develops from a general syncytium form-
ing the mesodermic anlagen of the two lung wings. By a gradual differ-
entiation of connective-tissue fibrils from the exoplasmic part of the
syncytium, the framework becomes denser and, finally, at 8 cm., a sug-
gestion of lobulation is obtained about the end branches of the growing
bronchi. Within the lobules the framework differentiates as the embryo
grows, forming simultaneously basement membranes for the young
bronchial buds. At the same time, the interlobular fibers, and those
beneath the pleura, unite to produce trabecule. As the lobuli respira-
torii towards the end of foetal life begin to impinge on each other, the
interalveolar framework and the two adjacent basement membranes are
pressed together into a single wall or septum in which the blood-vessels
run. These lobules remain until adult life, and correspond in the pig
apparently to those described by Laguesse and d’Hardiviller, 98, and
Councilman, o1, in the human lung. Noteworthy, however, is the fact
that they may become compound by the loss of the interlobular septa and
the subsequent confluence of several adjacent lobules. This usually
takes place at the base leaving the periphery of the compound lobule
separated by partial septa.

The lymphatics appear at the root of the lung in an embryo 4-5 cm,
in length. Accompanying the bronchi and vessels, they gradually grow
in for some distance and until the smaller air passages are reached,
they leave these structures and grow towards the pleura in the inter-
spaces between the smaller bronchi, aiding in the differentiation of the
connective-tissue lobules. The reason for this course is not entirely clear,
but it may be due to the increasing density of the framework about the
bronchi, which forces the later-appearing lymphatics into the interlobu-
lar spaces as a Locus minoris resistentia. Upon reaching the pleura,
they turn and form a plexus in the subpleural connective tissue. Here
and there they may be seen penetrating into the lobules, but cannot be
followed for any distance in them. At 23 cm. the first evidence of the

102 The Development of the Lungs

submucous lymphatic plexus is seen in the stem bronchi. It may, how-
ever, be found earlier, but the vessels are difficult to follow in unin-
jected specimens.

It would seem, thus, that we have in the pig’s lung, besides the lym-
phatic plexuses accompanying the bronchi, arteries and veins, an inter-
lobular system which Miller has been unable to find in the human lung.
Injections pointing to such a relationship he has interpreted as arte-
facts. If Miller’s conclusions prove to be correct, then the lymphatics
of the human lung must develop so far as the interlobular septa are
concerned in some other way.

In following the organogenesis of the lungs in the pig, one finds at no
period in their life history, openings, or fenestre, which suggest a com-
munication between adjacent respiratory units. They form, as we have
seen, independently at the growing ends of the tree and as they approxi-
mate each other, it is always possible to demonstrate the interlobular or
interalveolar framework without interruptions suggestive of fenestrae
offering a communication between adjacent alveoh. Furthermore, in all
my corrosions, many of which are complete enough to fill completely the
Alveoli pulmonis and maintain the entire form of the lungs, no instance
was found of an interalveolar communication. Ruptures frequently
occur forming irregular extravasations, but in the most complete injec-
tions, one is always able to isolate completely the individual Lobuli
respiratoru. The results of this paper, then, support the conclusions of
Miller, Laguesse, and Oppel, and are not in accord with the views of
Hansemann, Zimmermann, Merkel, and Schulze with reference to the
presence of these foramina in the walls of the alveoli of the mammahan
lung.

DISCUSSION OF THE LITERATURE.
THE ANLAGE OF THE LUNGS.

As in the case of the early stages of the amphibian and reptilian lung,
there is a general agreement among most authors who have worked upon
the mammalian lung that the respiratory apparatus arises from an un-
paired anlage, which the majority regard as asymmetrical. Of these
investigators, His thinks the future asymmetry of the lungs is to be
sought in this characteristic of the anlage, while Minot looks upon the
asymmetry of both anlage and lungs as secondary to changes taking place
at this time in the heart. Fol believes the anlage is paired and regards
it, moreover, like Gétte and Weber and Buvignier as associated with the ©
gill pouches. The anlage, in the pig, arises from the ventral portion of

Joseph Marshall Flint 103

the head gut as a ventral groove with a more marked projection at the
caudal extremity, which becomes separated from the dorsal segment of
the gut by two longitudinal fissures, along the line of which the final
separation occurs. The upper part of the anlage gives rise to the trachea,
the lower to the lungs. If the pulmonary apparatus in mammals should
finally be shown to have a serial relationship with the gill pouches, all
trace of the process is certainly lost in the pig. From the first, the anlage
is asymmetrical. Whether this is a characteristic of the respiratory appa-
ratus or is due, as Minot suggests, to the influence of the heart, it is
impossible, from my material, to say. Suggestive, however, is the fact
that the pulmonary anlage in many of the lower animals is symmetrical.

THE GROWTH OF THE BRONCHIAL TREE.

Few of the many characteristics of the bronchial tree have given rise
to more discussion than the method of its growth. Between the two
extremes of dichotomy and monopody, most of the possible intermediate
processes have been described. A special review of the literature on this
point seems desirable to see what harmony can be drawn from the differ-
ent observations. So far as possible when space permits, the process will
be described in the words of the various contributors to this field.

If we recapitulate the history of the several series of bronchi it may be
said that all of the chief bronchi are produced in the same manner, that
is to say by monopodial growth. Even the formation of the stem bronchi
from the pulmonary anlage does not differ in any material way from the
subsequent formation of the products of the stems themselves. As the
tree grows, there is no definite division of the end bud as the main
branches are outgrowths of the walls of the trachea or the two stem
bronchi. In the pig, the trachea produces only a single element, namely,
Lateral 1. The process of growth is successive, that is to say, the ele-
ments are produced one after another from above downwards, recapi-
tulating the manner of growth shown in simpler animals like the reptiles,
for example. When a new element is about to be formed, one notes an
increase in the number of karyokinetic figures in the epithelium in the
region of the new branch. The basement membrane becomes much less
distinct and the connective-tissue nuclei in the surrounding mesoderm
are more closely packed together. In this region, a slight bulging of the
epithelial wall is then noted, as is shown, for example, in Fig. 12, which
increases in size until a small elevation is raised on the surface of the
stem. This subsequently grows, yielding a rounded projection on the
stem, which gradually emancipates itself and gives rise to a new bronchus.

104 The Development of the Lungs

The process is essentially the same whether it occurs either in the neigh-
borhood of the terminal bud, higher up on the stem, or on the trachea.
In general, we may say that the lateral and medial bronchi are produced
nearer the terminal end of the main bronchus, while the dorsal and
ventral elements are produced somewhat higher up from the stem, often
where the latter has regained its cylindrical form. .

If Narath’s interpretation of the bud as reaching up to the last appar-
ent lateral branch is allowed to stand, then all of the branches except the
tracheal bronchus must be considered in the sense of Narath as lateral
productions of the end bud. Narath’s distinction, however, does not
seem to be well made for, in the pig’s lung at least between the last
lateral branch and the tip of the stem bronchus, there is always a con-
siderable portion of the main stem which has a definite cylindrical form
and terminates in a distinct dilatation at the end. Much as Narath’s
view would tend to simplify the question, there is little justification,
therefore, in looking upon the entire distal part of the stem bronchus as
the terminal bud. On the other hand, there is no essential difference in
an evagination taking place at ihe bud and in one taking place on the
stem.

It may be well to notice certain differences in the behavior of the stem
at different periods in the life of the organism as well as differences be-
tween different species. For example, in the pig, the stems seem rela-
tively more irregular and dilated in size in embryos between 10 and 13
mm. long, but on the whole are fairly cylindrical throughout the growing
period. On the other hand, in some species the stems, particularly at
the growing ends, are quite irregular in shape and may be considerably
dilated, suggesting somewhat pictures corresponding to the growing lungs
of reptiles.

After the formation of the chief branches has occurred, the primitive
monopodial system may persist for a few generations on the side branches.
The principal method of division is, however, by dichotomy equal and
unequal. Apparently the selection of the method depends somewhat on
the physical conditions of the space in which the bronchi are forced to
divide. In the case of the first divisions of Lateral 1, of Lateral 2 on
each side, and Ventral 2 on the right side, the division is of practically
equal dichotomy, as they have a relatively free space about them. When,
however, the direction is more or less controlled by the limited environ-
ment of the bronchi, it becomes unequal, one fork growing on so rapidly
to become the stem, that the other is left either as a small bud or a small
side branch, which develops further when the space relations permit.

Joseph Marshall Flint 105

Later still when the total volume of the lung is such that each bronchus
is more or less equally surrounded by mesoderm, the dichotomy is equal,
although of the two forks resulting from a division, one becomes the
stem and the other is shunted off as a side branch. The point, however,
where monopody ceases and dichotomy begins is apparently different in
different species and may be different in different parts of the lung. In
the pig it is below Lateral 6 while, in man, according to His, the transfer
is made at Lateral 4. It must be remembered in this connection, how-
ever, that the space relations in this region of the human lung are quite
different from those in the pig owing to the different position of the
heart, diaphragm, and liver.

The bronchi, apparently, show great adaptability both in the power
and direction of their growth. This interesting characteristic is best
shown when one of the chief bronchi are suppressed. Adjacent branches,
while still rooted firmly at their point of origin, then grow into the area
of the lung usually supplied by the suppressed element, a process which,
taken in connection with the extreme variation of the point of origin of
the bronchi, give rise, in the adult tree, to the series of pictures which
suggest a wandering of the branches. In my whole series of specimens
numbering ten reconstructions and many cleared specimens 3 to 18.5
mm., and about 100 corrosions of pigs from 4 cm. to the half-grown
stage, I have never found any evidence which pointed to a wandering of
any elements of the tree. The bronchi remain attached to their stems
where they are formed, although their branching is controlled to a great
extent by the space in which they have to grow. When this is altered
by the suppression of one of the usual elements, adjacent branches show
a power of substitution which is perhaps best exemplified in the fate of
the two dorsal forks of the first division of the right and left Lateral 2.
On the right side, this branch, owing to the presence of the Lateral 1
above it, is forced to grow downwards and posterior to form a dorso-
inferior branch of Lateral 2, while on the left side, this same fork, un-
obstructed by the absence of Lateral 1, grows upwards to substitute for
the suppression of the lateral element above.

In turning to the literature we find that between such outspoken de-
scriptions as those of d’Hardiviller for example, on the one hand, and
Justesen, on the other, it is not difficult to differentiate, but in the cases
where terms like sympodial dichotomy and monopody with acropetal
development of the lateral buds are used, it is not always easy to deter-
mine whether the authors have not been describing the same process with
different words. At the outset, therefore, it may be well to state that

106 The Development of the Lungs

by monopody we understand lateral outgrowths from the wall of the
bronchus whether they occur on the side of or above the terminal bud,
and by dichotomy we understand an undoubted division of the terminal
bud. In equal dichotomy the two divisions grow for a time equally but
later may give rise to a system of monopodial appearance by the selec-
tion of one branch to continue as the stem, while in unequal dichotomy
the two buds develop unequally from the first. In the case of dichoto-
mous divisions, however, it is obvious the portion of the stem between
two side branches is genetically equivalent to the side branch of the
lower order.

Since one can explain theoretically the entire bronchial tree equally
well by either a monopodial or a dichotomous process of growth, it is
not surprising to find different views among those who have studied only
the finished bronchial system. This is well shown among modern in-
vestigators in the work of Aeby, 80, and Ewart, 89, the former of whom
believed in monopodial growth from first to last, while the latter says
“ Dichotomy is the alpha and omega of bronchial division.” Hunting-
ton, 98, also in working upon comparative material of adult stages finds
a double system primarily dichotomous with a subsequent monopodial
type of branching in the development of the stem bronchus. In a sys-
tem thus capable of two explanations, obviously, the only observations
which will really aid in solving the question come from those who have
studied the lungs during the process of their growth.

If we turn to this series of investigations we find Kiittner, 76, stating
that “Das Wachsen ist monopodisch, d. h. das Epithelrohr wachst an
seinem Scheitel ungetheilt fort, wihrend seitliche Sprossen am Stamm
desselben hervortreten und mit ihrer Lingsaxe zu der des erzeugenden
Furthermore, he states that these
buds grow and divide rapidly, giving rise to so many more lateral
branches than the principal axis that it is difficult in the adult tree to

39

Rohres rechtwinkelig gestellt sind.

recognize its primitive monopodial character.

Cadiat, 77, describes the process as follows, and it is important to
remember he is speaking of solid buds: “Il est facile de comprendre
maintenant comment se produisent les ramifications bronchiques. Un
premier bourgeon se forme plein et se développe en longueur, l’ampoule
se produit a Vextrémité. Alors son evolution est arrétée; sur les parois
naissent des bourgeons secondaires qui se terminent de méme, et ainsi
les canaux bronchiques vont sans cesse en se multiphant, mais toujours
dans des directions différentes.”

Stieda, 78, states: ‘“ Zuerst ist der Canal einfach, dann theilt er sich

Joseph Marshall Flint 107

in Aeste, welche sich abermals theilen, so dass sowohl durch fortgesetzte
Theilung des auch durch seitliche Sprossenbildung im epithelialen an-
fangs noch leicht tibersehbaren Canalsystem ensteht, dessen blinde Enden
etwas leicht erweitert sind.”

Kolliker, 79, describing a 12-day rabbit embryo, says: “ Das innere
Epithelialrohr, das nun Bronchus heissen kann, hat in jeder Lunge drei
Ausbuchtungen und werden von nun an mit dem Grdésserwerden des
Organes die Veristelungen bald so zahlreich, dass dieselben nur schwer
Schritt fiir Schritt zu verfolgen sind.” Further, in speaking of the in-
erease of the bronchi in man and animals, he says in general: ‘ Das
innere Epithelialrohr hohle Aussackungenen oder Knospen erzeugt,
welche, rasch sich vermehrend, bald in jeder Lunge ein ganzes Biiumchen
von hohlen Kanalen mit kolbig ansgeschwollenen Enden erzeugen, von
welchen aus dann durch Bildung immer neuer und zahlreicher hohler
KKnospen endlich das ganze respiratorische Hohlensystem geliefert wird.”
His, 87, in working on the development of the human lung, describes the
process of growth as follows: The first branches as far as Lateral 4
arise by monopodial division, which he describes in the following terms:
“An keiner Stelle findet sich eine Andeutung, als ob aus den einmal
eylindrisch gewordenen Wurzelrohren Seitensprossen zu entstehen ver-
mochten. Die einzige Productionstitte neuer Formbestandtheile sind
die Endknospen, und zwar erfolgt die Umgestaltung auf dem Wege
dichotomischer Theilung. Die Knopsen verlieren ihre kugelige Grund-
form, indem sie an der Anheftung gegentiberliegenden Seite sich abplat-
ten und zugleich in transversalem Sinne strecken. Bald tritt eine tren-
_nende Furche auf, wodurch die urspriingliche einfache Knospe in zwei
getrennte Verwolbungen auseinander geht. Allmahlich emanzipiren sich
diese letzteren und bekommen auch ihrerseits cylindrische Stiele, worauf-
hin derselbe vorgang von Neuem Platz greifen kann.” In summarizing
the process he continues: “ Nach erfolgter Trennung der beiderseitigen
Anlagen bildet eine jede derselben einen gebogenen und zugleich birn-
formig ausgeweiterten Schlauch, mit einzelnen schirfer markirten Vor-
treibungen. Aus diesen treten die primaren Seitensprossen als mono-
podische Bildungen im Sinne yon Aeby hervor und ihre fiir beide Seiten
asymmetrische Anlage bestimmt auch die Differenzen spiaterer Ausbil-
dung. Der weitere Verzweigungsmodus bleibt nun waihrend geraumer
Zeit der dichotomische. Zuletzt tritt aber ein Zeitpunkt ein, wo die
Endknospen aufhéren sich dichotomisch zu theilen und wo sie wieder in
ein System mehr oder minder ausgiebiger Seitenknospen auslaufen.”

In mouse, mole, and pig, Willach, 88, describes the process as follows:

108 The Development of the Lungs

“Ich glaube vielmehr, dass beim Menchen, wie bei den Siugethieren, die
Sprossung eine sogen monopodische ist, welche darauf beruht, dass das
Mutterrohr vor seinem kugeligen Endblischen eine Verengerung seines
Lumens erfahrt, wahrend das Lumen des Endblischens sich erweitert
und seitliche Ausbuchtungen treibt, jene Knospen, die wieder zu Réhren
werden, und das Mutterrohr weiter fortwichst. Das Tochterrohr ist
enger als das Mutterohr.”

The growth process is described by Robinson, 89, in these words: “ In
the rat and the mouse, the ramification of the bronchi is produced princi-
pally by dichotomy. The germ of each bronchus, as it grows outwards
and dorsally, becomes expanded at its termination; this expansion is
gradually constricted into two portions of unequal size, that is the dicho-
tomy is in the form described by botanists as unequal or sympodial.”
Further he states: “ Although most of the branches are produced by
dichotomous division of terminal expansion, certain of the dorsal branches
arise as hollow buds from the wall of the stem bronchus after it has as-
sumed its cylindrical form, and these buds are interpolated between pre-
existent branches.” He describes the origin of our median bronchi in the
rat as follows: “The second dorsal branch immediately after its origin
is similarly divided, and the constriction passes rapidly towards the axial
stem, until its apex reaches the level of the circumference of the main
bronchus. Thus, from the dorsal bud, a dorso-internal (median) branch
is formed.” Robinson apparently does not believe that the branches
are successive in their formation.

Minot, 92, states that “the branching occurs in a highly characteristic
manner, for the stem always forks, but the forks develop unequally, one
(terminal bud) growing more rapidly and becoming practically the con-
tinuation of the main stem, while the other (lateral bud) appears as a
lateral branch. Speaking in general it may be said that the ventral fork
serves as the stem. In consequence of this method of growth the adult
lung consists of main stems with lateral branches. . . . But it is erro-
neous to suppose, as did Aeby, that the system of growth is strictly
monopodial, it being in reality a modified dichotomous system. The
branches all arise by terminal forking, never as outgrowths from the side
of a stem.”

d’Hardiviller, from his studies on the rabbit and sheep, announces the
following law of development: “Toutes les bronches primaires, princi-
pales ou accessoires, naissent en divers points des bronches souches par
ramification collatérale, le bourgeon terminal des bronches souches ne
prenant aucune part a leur formation.” ‘These principal branches then,

Joseph Marshall Flint 109

according to d’Hardiviller, give rise to secondary branches by the produc-
tion of lateral buds as well as by equal and unequal dichotomy. d’Hardi-
viller does not believe that all branches of the stem are successive in their
formation.

Nicholas and Dimitrova, 97, in the sheep, describe the growth of the
main bronchi as lateral buds which appear successively on the terminal
portion of the stem bronchus.

The results of Justesen, oo, contained in an extensive paper devoted
entirely to the method of growth of the bronchial tree, may be given in
one sentence, “ Die Bronchialverzweigung ist also eine dichotomische,”
in which process he would include all branches of the tree from first to
last.

The process of growth of the bronchial tree according to Narath, g2,
96, o1, is a rather complicated process. He looks upon the primitive
lung sae as the first production of a stem bud. When a side branch is
produced from the end bud a slight swelling is observed on its lateral
side, emphasized by the occurrence of mitosis in this region. In conse-
quence of the greater pressure at this point, the end bud bends slightly
in the opposite direction, that is to say, medialwards. As the new bud
grows, this process continues until there is a distinct kink in the axis
of the stem opposite the new element. As it increases in size, the side
bud takes first, the form of a cone-like projection with a rounded summit,
as the stem bud grows on, then the epithelial wall about its base sinks
somewhat towards the axis of the stem, until the daughter bud is isolated
from the stem and then grows on. It is important to note, furthermore,
that Narath considers the end bud the entire terminal part of the stem
up to the last well-formed lateral branch.

In reference to the origin of the dorsal bronchi, Narath states from
his observations on the rabbit, that they are produced without partici-
pation of the stem bud and that they appear later than the corresponding
lateral bronchi. Furthermore, the comparative anatomy of the tree
suggests to him that the dorsal series are primarily side branches of the
lateral bronchi which, in course of ontogeny or phylogeny are placed
back on the stem. In support of this view, he finds the dorsal buds
arising at the same level as the lateral and, apparently, in communica-
tion with the contour of the latter. Then, he continues, if lateral bronchi
are able to give up dorsal branches to the stem, this process repeats itself
with the latter series in giving rise to the median bronchi. While he is
not absolutely certain that this process takes place in the origin of the
dorsal elements, he states that it can be proved with certainty in the
formation of the medial series. He shows a schematic series of draw-

10) The Development of the Lungs

ings of the median branches of D. 2, D. 3, and D. 4 in their different
stages, giving an apparent transplantation of this median branch upon
the stem bronchus. Like the median series, Narath also believes that the
ventral bronchi (the Ventro-accessory of Aeby) are branches which are
given up from the lateral branches to the stem. In one rabbit embryo
Narath was able to show a relationship between Ventral 1, the infra-
cardiac bronchus, and Lateral 1. He says further: “ Der Zusammen-
hang der Knospen ist ein primires Verhiltnis und kein sekundires.
Und wenn weiter eingewendet werden sollte, die Knospen hingen deswe-
gen so innig zusammen, wei bei der erwachsenen Lunge die Bronchien go
enge beisammenstehen, so wiirde ich auch wiederum gerade diesen Befund
bei der erwachsenen. Lunge als fiir die Aeby’sche Ansicht sprechend
verwerthen.” In a word, while not absolutely pledging himself to this
view, Narath believes that there are but one primary set of bronchi,
namely the lateral, and that the other three series, the dorsal, ventral,
and medial originate either directly from these branches as in the case
of the dorsal and ventral groups, or the median branches of the dorsal
series as in the case of the median bronchi, and are then given up on to
the stem bronchus.

Moser, 02, says for the vertebrate lung in general that “ Das Verzwei-
gungssystem der Kanale innerhalb der Lunge ist stets und ausschliess-
lich ein monopodiales.” It must be remembered, however, that Moser’s
material on the mammalian lung was very limited and confined to older
embryos which were studied by means of sections instead of corrosions
and reconstructions. Some criticism might be made of her comparative
material especially in view of the more exact methods used by Hesser in
the same field.

Blisnianskaja, 05, in the human lung states that “ Die Bronchial-
verzweigung geschieht nacht dem dichotomischen.Typus, der durch
ungleiches Wachstum der Gabeliiste ein monopodisches Aussehen erhilt.”

Hesser, 05, in his important work on the reptilian lung states that
“ausser allem Zweitel, bei niederen wie bei hoheren Reptilien die erste
Aste aus dem Stammbronchus monopodial angelegt werden. Tarentola,
Anguis, Chrysemys u. a. zeigen dies unzweideutig. Die Bronchien haben
eine ansehnliche Linge erreicht, bevor noch Seitenaste auftreten, und
wenn die erste Knospe sichtbar wird, tritt sie aus der Seite des Bronchus
hervor, und zwar in einer bedeutenden Entfernung von dessen kaudalem
Ende.” In speaking of the further growth of the branches, he continues.
“Denn dadurch, dass das Lingenwachstum der Aste nicht porportional
zur Vermehrung der Anzahl ihrer Knospen ist, geht die Monopodie all-
miahlich in Dichotomie tiber. . . . Also besteht zwischen Monopodie und

Joseph Marshall Flint ft

Dichotomie nur ein gradueller, aber kein wesentlicher Unterschied, und
es wiirde daher kein Erstaunen hervorrufen diirfen, wenn in der Archi-
tektur des Bronchialbaumes sowohl die eine wie die andere Weise zur
Anwendung gekommen ist.”

If we attempt to tabulate these views on the growth of the bronchial
tree, the results may be placed in three main divisions as follows:

1. Dichotomy. Older authors, Ewart, Minot, Justesen, Blismianskaja.

2. Monopody. Kiittner, Cadiat, Kolliker, Aeby, Nicholas and Dimi-
trova, Willach, Narath, Moser.

3. Monopody and Dichotomy. Stieda, His, Robinson, Huntington,
d’Hardiviller, Hesser, Flint.

It is also possible to subdivide them still further in the following way:

1. Dichotomy. Older authors, Ewart, Justesen, Minot.

2. Unequal Dichotomy. Robinson (?), Blisnianskaja.

3. Monopody. Aeby, Moser.

4. Monopody with participation of the end bud. Willach, Narath,
Nicholas, and Dimitrova.

5. Mixed Monopody and Dichotomy simultaneously. Stieda, Robin-
son.

6. Monopody and Dichotomy successively. His, @’Hardiviller, Hunt-
ington, Hesser, Flint.

While we have already called attention to those who have only studied
the branching from the finished tree, to which class belong Aeby, Ewart,
and Huntington, there is still a group, in the series of authors given
above, who have not followed the lungs through the development of the
stem and its chief branches in mammals, that is to say, their material
consisted of embryonic stages after the formation of the principal bronchi
was complete. The observations of these investigators are only import-
ant for the specific fields in which they worked, for it goes without say-
ing, as His has suggestively remarked, the conditions which govern the
form development of a growing part need not necessarily remain the
same through the different phases of its evolution. It may change its
character either once or more than once.

Thus for a series of animals covering amphibia, reptilia, birds (Moser,
Hesser, Schmalhausen), man (His), rats and mice (Robinson), mouse,
mole (Willach), rabbit (d’Hardiviller), sheep (Nicholas and Dimitrova),
rabbit, Echidna, cat (Narath), pig (Flint), we have a general agree-
ment, that the stem and its principal branches are produced by mono-
podial growth. I have placed Robinson in this group, partly because
he believes some of the chief branches are monopodial in nature, but
largely because, notwithstanding his own use of the term “sympodial

112 The Development of the Lungs

dichotomy,” his own description of the process of division appears to me
to be esentially of a monopodial character. Against these views we have
the outspoken description of Minot for dichotomy, in the human lung,
as well as that of Blisnianskaja. The latter does not describe the process
in detail and her illustrations appear to me to be capable of a monopodial
interpretation, especially in view of the careful work of His on the same
material. It is also noteworthy that she quotes the statements of Juste-
sen in supporting her ideas on the sympodial development of the chief
divisions of the stem. It may be recalled, however, that this author did
not possess in his material stages which showed the development of these
particular branches.

While it is possible to draw much harmony from the verbal descrip-
tions of the process of division which I have given above, there are, of
course, many exceptions and different complexions to these views. Since,
in my opinion, it makes little difference whether the monopodial out-
growths take place from the end bud or from the stem a little higher
up, we may justifiably say that among those who have studied the pro-
duction of the chief bronchi of the vertebrate lung, the following stand
for an absolute monopodial system: Moser, Hesser, Schmalhausen, His,
Wilach, Robinson (?), @Hardiviller, Nicholas and Dimitrova, Narath,
and Flint. ‘This series includes obviously all who have worked on the de-
velopment of the lung during this period except Minot, Blisnianskaja,
and Robinson, whom I have placed in both lists. Of these authors, Wil-
lach, Narath, Minot, and Blisnianskaja believe that our Lateral 1, the
so-called ‘ Eparterial or tracheal bronchus,” is a derivation of our
Lateral 2, which wanders up on the stem bronchus or trachea, the others
look upon it as an independent and unpaired element. Narath and
Blisnianskaja regard the other chief bronchi as secondary derivatives of
the lateral group as “ accessory ” in the sense of Aeby. Willach believes
the ventral and median groups as accessory, that is to say, derived from
the lateral and dorsal bronchi respectively, while Robinson thinks the
chief bronchus of the ventral series, Ventral 2 (the Bronchus infra-
cardiacas) is ontogenetically independent, but phylogenetically accessory.
The latter describes the origin of the medial bronchi, his dorsointernal
group, from the dorsal by a process of progressive splitting of the first
medial branch of the dorsal bronchi until it comes to have an independent
origin on the stem, a view which is advanced in greater detail by Narath.

All of the arguments of Narath and Blisnianskaja concerning the
derivation of the ventral, dorsal, and medial series either primarily or
secondarily from the lateral bronchi are quite unconvincing, for like

Joseph Marshall Flint 113

the support, which Narath brings from the comparative anatomy, the
facts are capable of a simpler explanation, 7. ¢., a wide variation in the
position of the buds and the power of one bronchus substituting for
another. These two factors which I have followed in detail in the pig’s
lung, will explain all of the conditions in the adult tree which led first
Aeby and then Narath and their followers to look upon the ventral and
medial groups as derivatives of the lateral series. It may also be well
to call attention to Hesser’s pointed criticism of Narath’s view when he
remarks that the lateral buds of Narath when they have only reached the
development of a low round cone with a broad base, represent the anlagen
of four different branches, namely the dorsal, lateral, ventral, and medial
bronchi which must isolate themselves and take their places on the
stem. And lastly, we cannot help noting the lack of the one convincing
argument which should come from comparative anatomy consisting in a
primitive lung that possessed only lateral bronchi.

Furthermore, the series of schematic figures, which Narath gives to
show the origin of the medial from the dorsal bronchi are objectively
correct and agree with the conditions found in the pig’s lung not only
in the embryonic stages but in the adult tree as well. He finds the first
median division of the dorsal bronchi as one descends from D. 2 to D. 5,
is placed successively nearer the stem bronchus until, at the latter point
buds are seen on the dorsal and medial sides of the stem. He interprets
this condition as indicating a wandering of this median branch to the
stem. Asa matter of fact, however, this is the normal relationship for
the grown lung, and, as I have pointed out above, the medial series do
not occur higher than Lateral 4. It is scarcely justifiable, therefore,
to interpret the successive change in the insertion of this median branch,
together with the appearance of the medial buds in their usual position
as evidence of wandering on the part of the median bronchi.

In reference to the further division of the tree after the principal
branches are laid down, Moser, Willach, Narath, Cadiat, Kiittner, and
Kdlliker believe in a monopodial propagation, while His, Minot, d’Hardi-
viller, Hesser, and Flint believe in the dichotomous form either equal,

unequal, or both.

AEBY’S EP- AND HYPARTERIAL THEORY.

The substance of Aeby’s views with reference to the influence of the
pulmonary artery upon the bronchial tree has been given in the abstract
of his monograph. This theory, which has influenced, more or less, the
work of all subsequent investigators has been accorded a varied reception.

8

114 The Development of the Lungs

His, Willach, Robinson, d’Hardiviller, and Miller, either actively or
passively, support the views of Aeby, while Ewart, Zumstein, Narath,
Minot, Huntington, Justesen, and Merkel have abandoned them. In
some cases it is difficult to ascertain just what position an author takes
concerning the theory for some of them use indiscriminately the terms
hyparterial and eparterial in describing the tree. These terms, of course,
may have only a simple topographical significance, as in the case with
Huntington, without implying the meaning which Aeby attaches to
them. Of all the authors who are considered as supporting Aeby’s
theory His, alone, is outspoken in his belief that the eparterial bronchus
is a dorsoventral bronchus which if it were in the hyparterial region
would divide into dorsal and ventral branches. Willach, who first de-
scribes the eparterial branch as arising from the first ventral bronchus,
apparently accepts the theory, although Narath, a few years later advo-
cating the same view, states that this single fact is sufficient to disprove
Aeby’s hypothesis once and for all. Zumstein attacked the theory from
another point of view, namely, by failing to find in corrosion specimens
the relationship, which Aeby describes, and by noting variations in the
pulmonary artery which, apparently, had no influence on the archi-
tecture of the tree. In these observations Zumstein is supported by
Narath, who also describes such specimens. Both observers also call
attention to the fact that, at the time the primitive bronchi are formed,
the pulmonary artery is a fine, delicate vessel which would have no in-
fluence on the larger, firmer epithelial structures. Huntington attacks
the theory from another point of view in looking upon the wandering
of bronchi as the chief factor in the formation of the eparterial bronchi
to which the relationship of the artery is simply secondary and topo-
graphical.

From the results recorded in this paper, it would appear that the rela-
tionship of the arteries to the tree and the differentiation of two sets of
bronchi with different relationships to the pulmonary arteries are pri-
marily due to the topography of the anlage with respect to the Vena pul-
monalis and the projection of the anlage ventralwards from the head gut.
In consequence, the arteries form behind the primitive stems before any of
the side branches are produced. Later the first lateral bronchus develops
above and behind the artery, while the remainder of the series are formed
below and in front of it. As the heart descends, the topography of the
arteries to the stems changes, but in no way and at no time have the
arteries a fundamental influence in differentiating two segments of the

Joseph Marshall Flint 115

tree. On account of the association of this influence with the terms
“ eparterial ” and “ hyparterial ” it is, perhaps, well to abandon them as
Zumstein and Narath have suggested. However, this much is certain:
The theory ought not to be abandoned without an acknowledgment of
our indebtedness to it. That the theory would stand or fall from the
results of embryological research, Aeby clearly recognized, much more
clearly apparently than some of his critics. As a working hypothesis,
his view was generally accepted from the time of its publication until
the appearance of Narath’s paper.

1sT LATERAL BRONCHUS. ‘“‘ EPARTERIAL”’ BRONCHUS OF AEBY.

This, Aeby regards, as a dorsoventral bronchus which lies above the
pulmonary artery and, therefore, not under its influence. If it were
in the hyparterial region the artery would divide it into dorsal and
ventral bronchi, a view in which Aeby is supported by His. It is an
independent structure; it may be either paired or suppressed. ‘These
characteristics form the basis of Aeby’s classification of the mammalian
lungs. Willach first proposed the idea that this was a branch of the Ist
ventral bronchus, while Robinson, like His, believes it is an unpaired
and independent branch. Zumstein in abandoning the eparterial theory
terms this the first lateral bronchus. Narath uses the expression Apical
bronchus and takes the same view as Willach inasmuch as he considers
it a branch of the 1st ventral bronchus. The former, however, goes
further in regarding this element as a definite dorsal bronchus. This is
compatible with his tentative view of the whole series of dorsal bronchi
arising probably primarily from the ventral group. Minot supports Wil-
lach, while d’Hardiviller thinks it is an independent element arising
from the trachea in sheep and the stem bronchus in rabbits in which
view he is upheld by Nicholas and Dimitrova so far as his observa-
tions in the sheep are concerned. Justesen, Merkel, and Blisnians-
kaja follow Willach. The unique and remarkable observation of
d@’Hardiviller, who states that in the rabbit there exists primitively an
eparterial bronchus on each side, is the only suggestive evidence of the
degeneration of an eparterial bronchus taking place during the ontogeny
of the embryo. For a time each develops symmetrically and then later
the left atrophies and disappears. Upon this observation d’Hardiviller
concludes that Aeby, His, Robinson, Narath, Nicholas, and Dimitrova
are mistaken in stating no bronchus arises at this level on the left side,
and, believes in consequence, Aeby’s classification of mammalian lungs
is only of secondary value. In certain species they may both develop,

116 The Development of the Lungs

in others the left only may atrophy, while in still others both may undergo
the atrophic changes leaving the tree consisting only of a symmetrical
hyparterial system. This observation of d’Hardiviller has only received
a single supporting observation in the whole literature and that is by
Bremer in the opossum lung. Bremer finds in embryos of 12.5 mm.
what he calls an eparterial bronchus on the left side. In his specimens,
14 cm. long, this is absent and, therefore, he presumes the bronchus has
degenerated between the two stages he has been able to observe. Narath,
in the possession of two adult rabbit lungs with left eparterial bronchi
as variations, is inclined to believe d’Hardiviller is dealing with an
abnormality, and, furthermore, in view of the unique nature of the
observation, adds that absolutely indisputable histological preparations
must be produced to show the degeneration of a bronchial bud which has
once been formed. This criticism of Narath would, in part, apply to
Bremer’s observation. The production of the bronchial tree in Echidna,
according to Narath, follows the same principles which we observe in
other mammals and the lung of the adult is not differentiated from that
of placentalia. Moreover, the vessels and their relationships undergo no
further changes while the young are in the pouch either in respect to the
artery or the veins. It is thus hardly possible in these observations of
d’Hardiviller and Bremer that we are dealing with a true regressive pro-
cess. In fact, it is more probable that in both cases we are either dealing
with a variation or a dorsal bronchus which is placed higher up than
usual upon the stem bronchus. This assumption is made quite probable
by Bremer’s statement that his left eparterial bronchus did not supply
the apex of the lung.

This bronchus is undoubtedly one of the lateral series as Zumstein
and Nicholas and Dimitrova hold. It, like the remainder of the lateral
series, originates from the lateral wall of the trachea or the stem. The
fact that it is usually unpaired and has a different topography to the
pulmonary artery does not separate it from this group. It is true, the
bronchus originates a little more dorsalwards than the remainder of the
series, but this is due partly to the different space relationships in the
upper part of the thorax and partly, to the ventral torsion of the lower
lateral bronchi, which exaggerates the slight difference that occurs be-
tween Lateral 1 and the remainder of the series in the embryo.

Inasmuch as a bronchus corresponding to Lateral 1 has never been
described in Reptilia or Amphibia, it must be regarded as peculiar to
mammals. The great rarity in the occurrence of paired first lateral
bronchi suggests that no more morphological significance can be laid on

Joseph Marshall Flint 117

its presence on both.sides than its absence. The unpaired Lateral 1
on the right side must be regarded as the normal condition for mammalia,
due to a phylogenetic provision for the descent of the heart and great
vessels through the suppression of the element on the left side. In cases
where it is formed bilaterally, no instance of a left Lateral 1 on the
trachea has yet been described. As Narath shows, it is always some-
what lower on the left side than the right when the element is bilaterally
present. From Narath’s tables, the bronchus is unpaired on the right
side in 199 species, is bilateral in 15 species, and is absent on both sides
regularly in 3 species. These three types, apparently, obey no definite
law ; in the same order of animals, all three types may be found in nearly
related species.

In some instances, Lateral 1 arises from the trachea, in others from
the stem bronchus. When, however, we observe the conditions in those
animals where it is formed on the trachea, we find the bifurcation occurs
near the second pair of lateral bronchi. On the other hand, where
Lateral 1 is produced on the stem, the division of the trachea takes
place high up, throwing it on to the main bronchus. Its dorsal char-
acter, in which Narath believes, is, however, secondary, as its lower
branches are forced backwards by the presence of L. 2 below it and the
relatively free space beside the vertebral column just above the dorsal
bronchi.

APICAL BRONCHUS OF WILLACH AND NARATH.

So general is the acceptance of the view that Narath is the author of
this idea, it may be well to quote his own words in which he gives the
eredit to Willach: “Ich bin ganz der Meinung Willach’s, dass der
apicale Bronchus ursprunglich ein Seitenast des 1. ventralbronchus sei,
der auf den Stammbronchus geriickt ist.” Willach explains himself
thus: “Man konnte also den von Aeby als eparteriell bezeichneten
Bronchus als Nebenbronchus zum ersten Ventralen derselben Seite im
Sinne Aeby’s auffassen, der, wenn bronchial, an den Stammbronchus,
wenn tracheal, an die Trachea abgegeben worden ist.” Further, he says:
“Andrerseits diirfte aber der erste ventral Seitenbronchus der linken
Seiten dem der rechten plus dem eparteriellen Bronchus entsprechen.
Der erste linke ventralbronchus zeigt nimlich einen nach yvorwarts stre-
benden Ast, der in seiner Gestalt nicht allein Aenlichkeit aufweist mit
dem eparteriellen Bronchus bei verschiedenen Thieren; sondern er ist
auch geradezu in einem eparteriellen Gebiet gelegen, wenn man von einem
ahnlichen, aber doch etwas verainderten Gesichtspunkte aus, als es Aeby

118 The Development of the Lungs

gethan, zwischen dem eparteriellen und hyparteriellen Bronchialgebiet
unterscheidet.”

Aeby looked upon this apical branch of the 1st lateral on the left side
as a simple side branch, which extends up into the apex of the lung
having a certain outward similarity to Lateral 1, which might, he point-
edly remarks, lead to erroneous assumptions. This branch was named by
His, the Bronchus ascendens, an element, which substitutes in the left
lung for the unpaired eparterial bronchus in the right, a view in which
he is supported by Robinson. Narath and Willach, on the other hand,
as stated above, look upon it as the equivalent of the eparterial bronchus,
a homology which is affirmed by Minot, Huntington, Merkel, and Blis-
nianskaja, but d’Hardiviller, and Nicholas and Dimitrova accept the
conclusions of Aeby, His, and Robinson. That is to say, d’Hardiviller
accepts them in so far as they regard the left apical bronchus of Narath,
a true side branch of the 2d lateral trunk and not the equivalent of the
eparterial bronchus on the right side.

In following, step by step, the appearance of the secondary divisions
of Lateral 2, in the pig, we find on the right side the dorsal fork is
turned downwards and outwards owing to the presence of Lateral 1
above it, in consequence of which, it becomes the large dorsoinferior branch
of L. 2. On the right side, however, this unobstructed branch extends
upwards toward the apex of the lung and substitutes, as Aeby-and His
pointed out, for the suppression of left L. 1. It is, however, a true side
branch of Lateral 2 and is not to be regarded as the homologue of right
Lateral 1, which in the vast majority of cases is unpaired.

LATERAL BRONCHI.

Kolliker, who worked on the rabbit, agrees with the observations of
Remak on the chick in finding the first branches of the stem bronchus
growing lateralwards and dorsalwards. He did not, however, give the
lateral group a special name. Aeby, whose observations were made upon
full-grown material, designated them ventral bronchi in contradistine-
tion to the dorsal group, both of which arise in the hyparterial region
from independent origins, while in the eparterial region the dorso-
ventral bronchus uninfluenced by the pulmonary artery has a common
origin from a point on the stem bronchus or trachea midway between the
origin of the dorsal and ventral bronchi in the hyparterial group. Al-
though His would have preferred the term lateral bronchi, he follows the
description of Aeby, while Robinson is really the first to take his term
lateral bronchi from the topography of the embryonic lung. Zumstein

Joseph Marshall Flint Tag

and Nicholas and Dimitrova have accepted Robinson’s terminology,
while Willach, Narath, Merkel, and Bremer have followed Aeby.
Although he believes the selection an unhappy one, Narath, like His,
uses the term “ventral” simply because it has received general accept-
ance in the literature and because the bronchi run to the ventral part of
the lung. All of the lateral group receive a topographical nomenclature
from Ewart, while d’Hardiviller calls them “external bronchi,’ and
Blisnianskaja “the ventrolateral” group. Curiously enough, these are
the only branches of the entire bronchial tree which all authors unani-
mously agree, despite the different terminology, are wholly independent
derivatives of the stem bronchus.

Owing to the topography of the origin of this series of bronchi from
the lateral wall of the stem, the author has followed Robinson, Zum-
stein, and Nicolas and Dimitrova in their nomenclature instead of Aeby
and His. This is quite logical for, as His has pointed out, all of the
ventral characteristics of this group are secondary to their later growth
ventralwards in the space between the diaphragm and chest wall. The
spiral line formed by joining the origins of the lateral bronchi on the
stem represents the extent of ventral growth of these bronchi, as the
upper elements reach farther ventralwards than the lower and conse-
quently the torsion of the stem is greatest above and gradually dimin-
ishes as the lower elements are reached. These occupy practically the
lateral plane of their origin. Finally, the presence of a real set of ven-
tral bronchi in many species renders the change in the nomenclature
urgent. .

DORSAL BRONCHI.

With the exception of Ewart, d’Hardiviller, and Blisnianskaja, all
authors designate this group the dorsal bronchi. d’Hardiviller calls
them posterior bronchi, while the latter classifies them as a dorsolateral
group. ‘There is also a general agreement that they are independent
derivations of the stem bronchus, although Narath, without absolutely
pledging himself to this view, is inclined to look upon them as a group
primarily derived from the lateral series. He reaches this conviction
partly because he regards the “ Eparterial ” bronchus as the first dorsal
bronchus and a definite dorsal branch of Lateral 1 and partly because
they bear a certain similarity to branches of the lateral group. In conse-
quence of the shifting of his Dorsal 1 up on to the trachea or stem
bronchus, Narath regards Aeby’s D. 1, D. 2, D. 3, etc., as D. 2, D. 3,
and D, 4, respectively. In looking upon the dorsal group as derivatives

120°? The Development of the Lungs

of the lateral bronchi, Narath has the support of Blisnianskaja, who
argues if the “eparterial” is a dorsolateral bronchus, it is reasonable
to suppose the remainder of the series are similarly derived. Neither
of these authors, however, have followed the wandering step-by-step either
of the eparterial or the dorsal branches on to the stem bronchus. They
are, on the contrary, independent derivatives of the stem and, like the
lateral series, are to be considered as a group of principal bronchi.
Phylogenetically they are one of the most sharply differentiated groups
of the stem. We have designated the dorsal series, D. 2, D. 3, D. 4, ete.,
to keep their numerals in harmony-with that of the larger lateral bronchi,
although it is clear, of course, that our D. 2 is the first element of the
dorsal series.

VENTRAL BRONCHI.

Because of their extreme variability, Aeby looked upon this group as
accessory bronchi, which had their origin in the lateral series and subse-
quently wandered to take up a position on the stem bronchus. Among
this group he classifies the Bronchus cardiacus. These conclusions were
obtained from the study of adult specimens, so Aeby brings no definite
proof of their wandering. His does not mention them, while Willach,
also without evidence, seems to accept Aeby’s view. They are, according
to Robinson, a definite group of independent bronchi, which he terms
ventral. Narath accepts the older view of Aeby, but like that author,
his conclusions, with the exception of the infracardiac bronchus, are
drawn from comparative study of corrosions of the adult lungs. More-
over, even in the case of the Bronchus cardiacus, Narath acknowledges
embryology brings no direct proof of a wandering in the sense of Aeby.
d@Hardiviller clings to the expression accessory, although he regards this
group, which he terms anterior bronchi as independent derivations of
the stem bronchus. In the latter view he is supported by Nicholas and
Dimitrova who, like Robinson, term them ventral branches of the stem.
The results obtained from the pig indicate that the ventral bronchi are
independent derivatives of the stem and do not form first on the lateral
series and then secondarily become transplanted on to the main bronchus.

VENTRAL Ep BRONCHUS CARDIACUS.

This bronchus Aeby looked upon as the most important of the ventro-
accessory group. Derived primarily from the second lateral bronchus,
it takes its place upon the stem bronchus between it and L. 3. In many
species it supplies a separate lobe, the Lobus infracardiacus instead of

Joseph Marshall Flint 121

being included in the Lobus inferior. In his investigations on the
human lung, His, from its size, the position of its origin, and its pre-
cocious development looks upon the Bronchus cardiacus as an inde-
pendent element which appears out of the regular schematic order, a
view with which Willach agrees. Robinson, while accepting the onto-
genetic interpretation of His, believes with Aeby in its phylogenetic
derivation from the Lateral 2. In holding that it may arise either from
the second lateral or the stem bronchus, Zumstein takes a combined
view, that is to say, in some instances it is an accessory bronchus and in
others it is an independent structure. Narath is a most decided sup-
porter of Aeby’s doctrine, both from an embryological and a comparative
point of view, but thinks L. 3 and L. 4, as well as the second lateral
bronchus may give rise to this trunk, a view in which he is supported
by Merkel and Blisnianskaja. d’Hardiviller and Nicholas and Dimi-
trova, however, look upon it as one of the principal branches of the stem
bronchus. In the pig, the independence of this element is shown with
great clearness where it forms the largest element of the ventral group
of bronchi. Its hyperdevelopment apparently results from the increase
in the respiratory surface by the utilization of the space between the heart
and liver medialwards to the two stem bronchi for lung tissue. It is
unpaired, like Lateral 1 and with that element destroys the symmetry
of the tree.

MEDIAL BRONCHI.

Aeby’s idea in classifying this group as dorsoaccessory, that is to say,
branches originating on the dorsal bronchi and wandering on to the
stem bronchus was practically the same as in the case of his ventro-
accessory group, namely, their inconstancy and the existence, in a
series of adult lungs, of bronchi, which looked like transition stages
between the origin of a medial element on a dorsal trunk and its final
position on the stem bronchus. Willach, without definite observations,
supported this view, while Robinson, who calls them dorsointernal
bronchi and believes them accessory, in the sense of Aeby, describes their
origin by means of a splitting of the division between the two buds of a
dorsal bronchus down to the main bronchus leaving the inner one of the
buds with an independent origin on the stem. Zumstein speaks of them
as medial and independent in which he has the support of Nicholas and
Dimitrova and d’Hardiviller, although the latter designates the group
as an internal series. Merkel accepts the older doctrine of Aeby.
Narath, also, believes from both embryological grounds and from com-

122 The Development of the Lungs

parative anatomy that these bronchi can be traced as branches of the
dorsal group. A criticism of his view has already been given. In the
pig, they are irregular, but independent products of the stem. As they
never occur more than a short distance above L. 4, we find the reason lies
in the presence of the cesophagus, which prevents the development of
medial bronchi above that level.

The main results of the preceding paper may be expressed in the fol-
lowing:

Résumé.

1. The anlage of the lungs in the pig is unpaired and asymmetrical.
It arises from the ventral part of the head gut behind the Sinus venosus,
as a ventral outgrowth, preceded by a lateral flattening of the foregut
below the gill pouches and the appearance of longitudinal furrows, which
divide the fore gut into two parts, a ventral respiratory portion and a dor-
sal digestive segment. From the lower part of the anlage the lungs arise,
from the upper the trachea. If there is a serial phylogenetic association
between the pulmonary anlage and the gill pouches, as some authors
maintain, the connection is lost in the pig, for the lungs originate well
below the gill area and distinctly ventralwards to the series of bronchial
pouches. From the caudal extremity of the pulmonary anlage, arise
two lateral outgrowths, giving rise to the stem bronchi. These, like the
anlage itself, are asymmetrical, the right growing lateralwards and
caudalwards, while the left extends almost directly horizontal. Then
the respiratory and digestive portions begin to separate, a process, which
begins from the caudal end of the anlage and extends upwards along the
line formed by the two longitudinal furrows, freeing the respiratory
apparatus from the cesophagus. In its subsequent growth, the pulmo-
nary anlage enlarges, the tips of the stem bronchi dilate, and begin to
bend dorsalwards around the cesophagus. ‘This results in the formation
of the primitive lung sacs. At this time, the production of the bronchi
begins. They are readily divided into four series from the topography
of their origin, namely, lateral, dorsal, ventral, and medial.

2. The first lateral bronchus, the so-called “ eparterial bronchus,” is,
in the pig, unpaired and arises as a lateral outgrowth from the right
side of the trachea, just above the roots of the two stem bronchi. It
is distinctly lateral in origin and bears a serial relationship to the re-
mainder of the lateral bronchi. Its position in mammals varies, some-
times it is on the stem bronchus, but it is often situated on the trachea.

Joseph Marshall Flint 123

This difference can usually be explained by the point of origin of the
two stem bronchi with reference to the pair designated as Lateral 2.
If the stems originate low down, then Lateral 1 is thrown on to the
trachea, while if their origin is higher up, the first lateral arises from
the stem bronchus. Apparently Lateral 1 is characteristic of mammals
and, according to Aeby, of birds. A bronchus corresponding to it has
not been found either in reptilia or amphibia. In almost all mammals
it is an unpaired element. No satisfactory proof has even been brought
to show a bilateral development of Lateral 1 with a subsequent degenera-
tion of the left bronchus, notwithstanding the fact that this process has
been described in two species. At no time in the life history of the pig
is there a Lateral 1 formed on the left side. There is furthermore no
embryological evidence to show a relationship between Lateral 1 and
the dorsal series of bronchi. These characteristics are secondary and
result from the antagonistic effects of the growth of Lateral 1 and
Lateral 2. The latter is foreed somewhat ventralwards, while the former
is pressed dorsalwards, until its lower branches le above the dorsal series
of bronchi.

3. The remainder of the lateral series originate in succession from
the lateral side of the stem bronchus as lateral outgrowths or hernia-
like expansions of the wall of the stem bronchus near the terminal bud.
These elements in their growth outwards finally reach the chest wall.
Here they are compelled to grow in the space between the ribs and the
liver and consequently follow the curvature of the chest wall which
ultimately gives them, more or less, the appearance of ventral bronchi,
a fact which led Aeby, who studied only the finished tree, to call them
the ventral series.

4. The dorsal series of bronchi, originating like the lateral group as
outgrowths from the stem bronchus, are usually paired. They alternate
with the paired lateral bronchi and are independent productions of the
stem. They do not either ontogenetically or phylogenetically originate
from the lateral bronchi. For convenience, the first pair are called
Dorsal 2, to keep the designation harmonious with the larger series of
lateral bronchi. .

5. The ventral bronchi originate as outgrowths from the ventral sur-
face of the stem. They, like the other series, are independent produc-
tions of the main bronchus. They are not originally formed on the
lateral bronchi and subsequently transferred to the stem bronchus. Con-
sequently, they are chief bronchi and not accessory in the sense of Aeby.
In the pig and in the great majority of mammals, left Ventral 2 is

124 The Development of the Lungs

suppressed. With the absence of left Lateral 1, it destroys the absolute
symmetry of the mammalian lung. The cause for the remarkable hyper-
development of the Ventral 2 on the right side in most mammals is
undoubtedly due to the effort to increase the respiratory area by filling
the space that intervenes between the heart and diaphragm with the
Lobus infracardiacus. The remainder of the ventral series are usually
paired in the pig and like the dorsal series ordinarily alternate with the
larger lateral bronchi. As a rule their roots are placed on the ventral
surface of the stem midway between the adjacent lateral elements and
opposite the corresponding dorsal bronchi. The first ventral element is
designated Ventral 2 on account of its topographical relationship to
Lateral 2.

6. The medial bronchi are, like the other series, produced by medial
outgrowths from the stem. They are not formed on the dorsal bronchi
and then transferred to the stem. They rarely occur higher than the
level of Lateral 4 and are extremely irregular in their arrangement.

7. Noteworthy are the great variaticns found in the production of the
various bronchi. ‘The lateral series are by far the most constant ele-
ments of the tree. Still, it is not uncommon to find either an extra
element formed or else to see one of the usual elements suppressed. As
the common number of lateral elements is six on the right side and five
on the left, the extremes may vary between five and seven on the right
and four and six on the left. In the case of the dorsal series, the
variation is even more marked than in the lateral, thus, one element may
be suppressed, leaving the dorsal area between two adjacent lateral
bronchi naked or, else, an extra element may be formed, giving two dorsal
elements in a single interspace. The ventral series is still more variable
than the dorsal, so much so, in fact, as to make it uncommon even in
the pig where these elements are unusually well developed, to find a
series complete, of course, with the exception of left Ventral 2, which is
always suppressed. It is not uncommon to find several elements of
this series absent at once. Like the dorsal bronchi, they may also be
reduplicated in a single interspace. The medial bronchi are the most
variable of the four types. They may not be present at all, they may be
present only on one side, or they may be reduplicated in a single inter-
space, but, in the pig, they never occur higher on the stem than the level
of the fourth lateral bronchus. The reason for this fact lies in the
presence of the cesophagus above this point, which allows no space for
the development of medial elements from this portion of the stem
bronchus.

Joseph Marshall Flint 125

8. The following formula would represent the complete series of prin-
cipal bronchi in the lung of the pig:

TRACHEA.
Lateral 1.
Right Stem Bronchus. Left Stem Bronchus.
Lateral 2. Lateral 2.
Dorsal 2. Dorsal 2.
Ventral 2.
Lateral 3. Lateral 3.
Dorsal 3. Dorsal 3.
Ventral 3. Ventral 3.
Lateral 4. Lateral 4.
Dorsal 4. Dorsal 4.
Ventral 4. Ventral 4.
Medial 4. Medial 4.
Lateral 5. Lateral 5.
Dorsal 5. Dorsal 5.
Ventrai 5. Ventral 5.
Medial 5. Medial 5.
Lateral 6. Lateral 6.

It is extremely rare to find a tree as complete as the one expressed in
this formula. A number of bronchi may be missing or else some may be
reduplicated.

9. The whole series of bronchi show a most remarkable adaptation to
the space in which they have to grow. This is true of both the chief
bronchi as well as their smaller subdivisions. When, for example, a
bronchus is suppressed, an adjacent branch will grow into the area
usually supplied by the missing element, substituting for its loss. It is
in this way that we obtain the large series of pictures which suggest a
wandering of the secondary branches from the lateral and dorsal ele-
ments on to the stem bronchus. After a careful study of this point, it
may be definitely stated that bronchi never wander. They remain firmly
fixed on the stem or side branches where they originate. Not uncom-
monly their direction may be altered, however, by changes in the space
in which they develop.

This response on the part of the growing bronchi to their space rela-
tionships is also shown in the course or direction of the principal elements
as well as their secondary branches. We have, therefore, Lateral 1
produced and growing into the area between the upper part of the heart
and chest wall. Owing to the larger space just beside the vertebral
column and the antagonism between it and Lateral 2, the lower branches
of Lateral 1 are forced dorsalwards until it resembles superficially a

126 The Development of the Lungs

dorsal bronchus. The second lateral bronchi develop in the region be-
tween the chest wall, heart, and liver. The area in which the remainder
of stem has to grow has in cross-section practically the shape of an
isosceles triangle. The stem, occupying a point about the middle of the
base, sends three sets of branches, namely, dorsal, lateral, and ventral,
directed into the angles of the triangle where they would have the most
freedom to develop. Between the roots of the two stem bronchi runs
the cesophagus, leaving no place for the development of median branches
in this region. At the level of Lateral 4, however, below the cesophagus
more room occurs and, consequently, we observe in this region the forma-
tion of medial bronchi. Undoubtedly the difference in the branching of
the stem in the Lobus inferior of the human lung when compared with
the pig may be sought in its altered topography owing to the erect posture
which changes principally the position of the liver.

This adaptation on the part of the lungs to their environment is to be
expected for they are relatively late accessions to the animal economy and
are of no known use to the organism during the period of gestation.
Accordingly as the heart and liver are both phylogenetically older than
the lungs and also are of known functional value during feetal life, it is
natural that the latter should adapt themselves to the early needs of
older organs.

10. The growth of the main series of bronchi is monopodial in char-
acter, that is to say, they are produced without a definite division of the
end bud. New elements are not always produced from the end bud, but
may be formed from the stem some distance from its terminus.
The process is successive, that is to say, the elements are produced one
after another from above downwards, recapitulating the method of growth
shown in simpler animals like the reptiles, for example. When a new
element is about to be produced, on2 notes an increase in the number of
karyokinetic figures in the epithelium in the region of the new branch.
The basement membrane becomes less distinct and the connective-tissue
nuclei in the surrounding mesoderm are more closely packed together.
In this region a slight bulging of the epithelium is then noted, which
increases until a small elevation is raised upon the surface of the stem.
This increases in size, yielding a rounded projection, which gradually
emancipates itself and gives rise to a new bronchus. The process is
essentially the same whether it occurs in the neighborhood of the termi-
nal bud or higher up on the stem. In general, we may say, the lateral
and medial elements are produced nearer the terminal end of the main

Joseph Marshall Flint 127

bronchus, while the dorsal and ventral elements are formed somewhat
higher up, often where the stem has regained its cylindrical form.

Subsequent division of the branches may occur either by monopody or
dichotomy. Often monopodial production of buds persists for one or two
generations on the main bronchi, then the method becomes dichotomous,
either equal or unequal in nature depending somewhat on the space in
which the bronchi have to divide. In the case of equal division of the
bud, however, one fork grows on to become the stem while the other re-
mains as the side branch. The first division of the main bronchi may,
it is well to note, be dichotomous as in the case of Lateral 1 and Lateral
2. Thus in its growth, the mammalian lung recapitulates the history
of the simpler lungs of lower animals.

11. The pulmonary arteries in the pig arise from the pulmonary arches
as Bremer has described. At first, they run parallel, then bend towards
each other, sending out anastamoses, which yield finally a common trunk
with two origins above and two arteries below. Later the upper part of
the right artery degenerates and with it the right pulmonary arch. At
5 mm. before the pulmonary arteries may be followed as far as the
anlage of the lungs, the pulmonary vein may be seen as a slight ingrowth
from the undivided portion of the Sinus venosus, passing through the
Mesocardium posterior towards the pulmonary anlage. It forms almost
in the medial plane. With this establishment of the venous outlet
ventralwards to the anlage, the arteries, as the growth of the organ
proceeds, are naturally developed from the capillary plexus on the dorsal
side of the primitive bronchi. This fixes the arteries with reference to
the stem bronchi before any of the side branches are produced. As the
pulmonary anlage projects some distance ventralwards from the head
gut, Lateral 1, the “eparterial” bronchus, develops above the artery,
while Lateral 2 and the remainder of the principal branches originate
below. Thus, the two regions of the tree have a different topography
with reference to the pulmonary artery, but this vessel has no funda-
mental influence on the structure of the two parts, nor does it differen-
tiate the tree into two regions of different morphological significance as
Aeby has maintained.

The entire primitive tree is surrounded by a capillary plexus. As the
bronchi grow, and produce new branches, arteries are developed from
this plexus on the dorsal side of the tree as the artery lies dorsalwards
and lateralwards to the stem. From this position, arteries to the lateral
bronchi run out above and behind them. The branches to the dorsal
bronchi pass dorsalwards along the lateral aspect of these elements. To

128 The Development of the Lungs

the ventral series, arteries pass around the lateral aspect of the stem
bronchus beneath the root of the corresponding lateral bronchus to gain
the outer aspect of the ventral bronchus along which they run. The
medial bronchi receive their supply from branches that originate from
the main artery and pass around the dorsal aspect of the stem to run on
the dorsal surface of the medial bronchi. As the right pulmonary artery
runs ventralwards to Lateral 1 the artery to that bronchus develops on
its ventral surface.

In the younger stages, both the aortic arch and the Ductus arteriosus
he well above the level of Lateral 1. As the embryo increases in age,
there is a gradual descent of the heart and with it, the great vessels.
At 15 cm. one observes the Ductus arteriosus at the level of Lateral 1;
at 22 cm. the aortic arch reaches this point, while at birth both vessels
le below the bronchus.

12. The pulmonary vein develops in pigs about 5 mm. long as an in-
growth from the undivided portion of the Sinus venosus at the level of
the pulmonary anlage. As the stem bronchi increase in size, right and
left pulmonary veins develop from the capillary plexus which surround
them. ‘These, naturally, form on the ventral surface, with the bronchi
between them and the arteries. Similarly, as the various principal
bronchi are produced from the stem bronchus, veins are formed from
the capillary plexus. The veins from the lateral bronchi lie below and
ventralwards to the bronchi, those from the dorsal elements run along
the medial aspect of the air passages to empty into pulmonary veins
lying ventralwards to the stems. The veins from the ventral bronchi
extend along the medial aspect of the bronchus and terminate directly
into the pulmonary veins; those from the medial bronchi extend along
their ventral surface to empty in the larger veins accompanying the
stems. ‘The vein from Lateral 1 runs along the ventral aspect of the
bronchus somewhat ventralwards to the corresponding artery. This
forms the single exception to the general alternation of artery, bronchus,
and vein. As the embryo increases in age, the Vena pulmonalis, which
originates near the midline, is gradually pushed to the left by the
increasing asymmetry of the heart, until it finally comes to he over the
area of the stem bronchus where a left Ventral 2 would have developed
if such a bronchus were present. ‘The hyperdevelopment of the Bronchus
infracardiacus associated with the development of the Vena cava in-
ferior to the right of that bronchus aids in pushing the Vena pulmonalis
to the left.

13. The asymmetry of the mammalian lung is associated with the

Joseph Marshall Flint 129

asymmetrical development of the heart and its great vessels. In the
descent of the aortic arch and the Ductus arteriosus during embryonic
life from a point above the origin of Lateral 1 to a point below, we have
an explanation for the suppression of this element on the left side, for
if this bronchus were formed, both aorta and the Botallian duct would be
caught upon it and their descent prevented. Likewise the Vena pul-
monalis appears in the midline and is carried to the left until it finally
rests on the portion of the stem where a left Ventral 2 should develop.
The usual suppression of these two elements, therefore, must be looked
upon as a phylogenetic provision to allow for the descent of the great
vessels on the one hand and the shifting of the Vena pulmonalis on the
other. It is noteworthy that in those animals where these bronchi are
formed on both sides, they are so situated as to offer no resistance to
either of these features of the development of the great vessels.

14. The mesodermic portion of the lungs is derived from the general
mesoderm about the head gut. As the bronchi appear, this is pushed
out into the primitive coelom to form two irregular swellings, marking
the anlagen of the two wings of the lungs. With the appearance of
Lateral 1, on the right side, and Lateral 2, on each stem bronchus,
swellings are observed on the two simple lungs just over these bronchi,
giving rise to the simplest forms of the Lobus superior, Lobus medius,
on the right side, and the Lobus superior on the left. The remainder
of the mesoderm about the stem bronchus forms the anlage of the Lobus
inferior on each side. With the formation of Ventral 2, the Bronchus
infracardiacus, a swelling from the mesoderm forms over it which is the
anlage of the Lobus infracardiacus. These swellings are first surrounded
by shallow grooves, which with the rapid growth of the bronchi beneath,
rapidly develop into deep fissures separating the various lobes from each
other. With the further growth of these bronchi and the appearance of
the series of bronchi on the stem, projections and fissures are formed over
and between them and in the mesoderm. These are equivalent in all
respects except in age and size, to the earlier fissures and swellings, but,
under ordinary circumstances, never give rise to distinct lobes. This is
due to the more rapid growth of the first bronchi, to the gradual in-
creasing density of the mesoderm, and, lastly, to the environment of the
several lobes of the lung. The right Lobus superior, containing Lateral
1 does not belong to the dorsal region of. the lung as some authors hold,
but to the lateral. The characters which make it appear as a dorsal
segment are secondary and not primary. Likewise the portion of the left
Lobus superior containing the apical bronchus belongs to the lateral

H)

130 The Development of the Lungs

region and not to the dorsal. As in the case of the right Lobus superior,
its dorsal characteristics are secondary. This segment is to be com-
pared to the portion of the right Lobus medius which contains the main
dorsoinferior bronchus. Moreover, the entire left Lobus superior is the
ontogenetic equivalent of the right Lobus medius. The right Lobus
superior is an unpaired lobe and has no equivalent in the left lung. The
same thing is true of the Lobus infracardiacus.

Lobe formation varies greatly in different species. In the majority
of mammals, there are three or four lobes on the right side, arising from
Lateral 1, Lateral 2, Ventral 2, and the stem bronchus, while, on the
left side, there are ordinarily two formed from Lateral 2, and the stem.
Extremes of variation occur, however, between a lobeless lung in which
none of the bronchi subdivide it and a multilobar lung in which most
of the principal bronchi have segmented the wing into a series of small
lobes. Apparently, the division of the lung into lobes is of no general
morphological significance.

15. In the light of recent researches on the reptilian, amphibian, and
avian lung, it is possible to take a new viewpoint for the development of
the mammalian lung. The lungs of lower animals, we now know, are
products of monopodial growth. 'The simple lungs of reptilia are capa-
ble of producing monopodially outgrowths in any direction (Hesser).
These may become specialized in certain species and have a definite topo-
graphy. As we mount the animal scale, the necessity of an increased
respiratory surface finally results in the transformation of the original
simple lung into a conducting apparatus, which is represented in the
mammalian lung by the stem bronchus and its chief branches. The
simple lungs may no longer be compared to the Lobuli respiratorii of the
mammalian lung, for the latter represent new elements which with the
increased respiratory surface are added peripherally to the simpler
lungs as these become transformed into bronchi. With the addition
of these new elements, the respiratory function also wanders periph-
eralwards, so that the portion of the mammalian tree which represents the
simpler lungs undergoes a change of physiological function. Its phylo-
genetic relationship to the simple lungs is shown by the monopodial
growth of the mammalian stem bronchus and its principal branches,
which recapitulate ontogenetically the growth process of the simple
lungs before producing dichotomously the prepheral respiratory struc-
tures which are used in mammalian respiration. In certain animals,
moreover, the stem bronchus and its branches retain for a period in their
life history their respiratory function. In monotremes and marsupialia,

Joseph Marshall Flint alesyil

the young are transferred to the pouch and compelled to carry on their
own respiration when only the stem bronchus and its chief branches are
formed. The ordinary respiratory structures used in the adult stage,
are produced at a later period. We have, thus, both a physiological and
an ontogenetic proof that the simple lungs correspond, in mammals,
only to the stem bronchus and its chief branches.

The great majority of mammalian lungs are asymmetrical, the asym-
metry consisting in the presence of an unpaired Lateral 1 and an un-
paired Ventral 2, both of which occur on the right side. Some mam-
malian lungs are symmetrical and considerable effort has been made to
explain all the asymmetrical lungs on the basis of the minority of sym-
metrical ones. The asymmetrical lung, however, must be regarded as
typical for mammals. The two bronchi responsible for the asymmetry
are, so far as we know, characteristic of the mammalian and avian (Aeby)
lung as similar bronchi have never been described in the lungs of lower
animals. The cause for the asymmetry, apparently lies in the necessity
of leaving space for the descent of the heart and great vessels, by the
suppression of left Lateral 1, on the one hand, and to allow room for
the shifting of the heart which draws the Vena pulmonalis to the left by
the suppression of left Ventral 2, on the other. In those lungs where
these two elements, which are usually missing, are found, they are appar-
ently so placed as not to interfere with these features of the development
of the heart.

16. In the organogenesis of the lungs, we have the stem and main
bronchi consisting of simple tubes lined by a double layer of epithelium,
the inner of which is columnar, while the outer is composed of smaller
polygonal cells. This simple tube is surrounded by a membrana propria
produced largely by the deposit of fibrils from the exoplasm of the con-
nective-tissue syncytium, composing the mesoblastic portion of the lungs
at this early stage. As the bronchi grow, a layer of spindle cells differ-
entiate from the mesoderm, which are transformed into the muscular
coat of the bronchi. Later still, a chondrification of the perimuscular
syncytium takes place from which the cartilaginous rings of the trachea
and the bronchial cartilages are formed. With these changes, the con-
nective-tissue fibrils become grouped into trabecule about the bronchi
and in the submucosa. Later, the mucosa is thrown into a series of
longitudinal folds, while from the cuticular border of the inner row of
cells, cilia develop. From the bottom of the crypt-like invaginations
formed by the longitudinal folds of epithelium, glands begin to grow
into the submucosa, which sometimes pass between the developing muscle

132 The Development of the Lungs

bundles into the deeper layers of this coat. As this process takes place,
there is a differentiation of some of the epithelium into goblet cells, a
process, which one also observes in the glands, giving rise to a series of
submucous glands with partly serous and partly mucous cells. While
these changes occur in the mucosa, the cartilages are also growing, and
with them a further differentiation of the framework into distinct fibrous
trabecule takes place. As we follow the bronchi peripheralwards, they
become simpler and essentially younger in structure and yet, develop
their adult characteristics in precisely the same way. The epithelium
soon becomes single layered and of a columnar type as the periphery is
reached. Finally it takes on a distinct, flat, cubical form. The Lobuli
respiratorii begin to develop in pigs about 19 cm. long by a slight dila-
tation of the growing ends of the bronchi. These represent the bron-
chioli. Later Bronchioli respiratorii are then formed, having a pro-
gressively flattened epithelium, which runs over into Ductuli alveolares.
These are present at the age represented by a pig 22 cm. long. Subse-
quently, Atria, Sacculi alveolares, and Alveoli pulmonis form in the
prenatal period, all of which have the characteristic flattened respiratory
epithelium. And finally, after birth, there is a dilatation of the lobules
and a further flattening of the epithelium occurs, and before the pig is
half grown, a muscle layer develops about the air passages as far as the
Atria, where it stops in sphincter-like bands. One finds at no period in
the life history of the pig’s lung, openings or fenestree which communi-
cate between adjacent respiratory lobules. The latter form independ-
ently at the growing ends of the tree and as they approximate each other,
the interalveolar framework can always be demonstrated between them
without interruptions suggestive of fenestrae connecting adjacent alveoli.

17. The framework of the lungs develops from a general syncytium
forming the mesodermic anlagen of the lung wings. By a gradual differ-
entiation of connective-tissue fibrils from the exoplasmic part of the
syncytium, the framework becomes denser and, finally, at 8 cm., a sug-
gestion of lobulation is obtained about the end branches of the growing
bronchi. Within these connective-tissue lobules, the framework differ-
entiates as the embryo grows, forming simultaneously basement mem-
branes for the young bronchial buds. At the same time, the interlobular
fibers and those below the pleura, unite to form trabecule. As the Lobuli
respiratorii, towards the end of foetal life, begin to impinge on each
other, the interalveolar framework and the two adjacent basement mem-
branes are pressed together into a single wall or septum in which the

Joseph Marshall Flint 133

blood-vessels run. These lobules persist until adult life, although they
may become compound by the rupture of the interlobular septa and the
subsequent confluence of several adjacent lobules. ‘This process ordi-
narily takes place at the base, leaving the periphery of the compound
lobule separated by partial septa.

18. The lymphatics appear at the root of the lung in an embryo 4-5
em. in length. Accompanying the bronchi and pulmonary vessels, they
gradually grow in for some distance until the smaller air passages are
reached, when they leave these structures and grow towards the pleura
in the interspaces between the smaller bronchi, in what represent the
primitive interlobular spaces. In this way they aid in the differentiation
of the connective-tissue lobules. The reason for this course is not entirely
clear, but it may be due to the increasing density of the framework about
the bronchi, which forces the later-appearing lymphatics into the inter-
lobular spaces as a locus minoris resistentiz. Upon reaching the pleura,
they turn and form a plexus in the subpleural connective tissue. Here
and there, they may be seen penetrating the lobules, but cannot be fol-
lowed for any distance in them. At 23 cm., the first evidence of the
submucous plexus is seen in the stem bronchi.

LITERATURE.

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American Journal of Anatomy. Vol. III, 1904.

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(2) Anatomischer Anzeiger. 1906.

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134 The Development of the Lungs

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|

Joseph Marshall Flint 135

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EXPLANATION OF THE PLATES.
leye/ANGu Ie
Figs. 1-14.
Fics. 1-20 are magnified 50 diameters. Pulmonary arteries red, pulmonary
veins blue, bronchi white.
Fic. 1. Reconstruction of a portion of the head gut of a pig’s embryo 3 mm.
long. Ventral view.
Fic. 2. Dorsal view of the same reconstruction.

Fic. 3. Reconstruction of a portion of the head gut of a pig’s embryo 5 mm.
long. Ventral view.

Fic. 4. Dorsal view of the same reconstruction.
Fic. 5. Reconstruction of the bronchial tree of a pig 6 mm. long.

Fie. 6. Dorsal view of the same reconstruction.
Fic. 7. Reconstruction of the bronchial tree of a pig 7.5 mm. long.
Fie. 8. Dorsal view of the same reconstruction.
Fic. 9. Reconstruction of the bronchial tree of a pig 8.5 mm. long.

Fic. 10. Dorsal view of the same reconstruction.

136 The Development of the Lungs

Fic, 11. Reconstruction of the bronchial tree of a pig 10 mm. long.
Fic. 12. Dorsal view of the same reconstruction.

Fic. 13. Reconstruction of the bronchial tree of a pig 12 mm. long.
Fic. 14. Dorsal view of the same reconstruction.

PEAT Hels

Figs. 15-19.
Fig. 15. Reconstruction of the bronchial tree of a pig 13.5 mm. long.
Fie. 16. Dorsal view of the same reconstruction.
Fic. 17. Reconstruction of the bronchial tree of a pig 15 mm. long.
Fig. 18. Dorsal view of the same reconstruction.
Fic. 19. Reconstruction of the bronchial tree of a pig 18.5 mm. long.

PLATE ITI.
Fig. 20.

Fic. 20. Dorsal view of the same reconstruction.

PEATE AVE
Figs. 21-25.

Fia. 21. Celluloid corrosion of the bronchial tree of a pig’s embryo 5 cm.

long. X 2.
Fig. 22. Celluloid corrosion of the bronchial tree of a pig’s embryo 7 cm.
lone x2: °

Fig. 23. Wood’s metal corrosion of the bronchial tree of a pig’s embryo
18 cm. long.

In this specimen one lateral bronchus on each side is suppressed, giving
five laterals on the right and four on the left, instead of the usual complement
of six and five respectively. Ventral 3 on both sides is suppressed. Sub-
stituting for these branches are ventral branches of the adjacent lateral
bronchi, while on the right side a lateral division from the inferior branch
of V. 2 also extends into the region usually supplied by right V. 3.

Fic. 24. Dorsal view of the same preparation.

Dorsal 3 on the left side is suppressed. It is compensated for partly by
Dorsal 2 growing lower than usual and partly by branches from Medial 4 on
that side. On the right side Dorsal 4 is reduplicated, the upper element
growing dorsolateralwards, the lower directly dorsal. Medial 4 and 5 are
present on both sides.

Fig. 25. Wood’s metal corrosion of the lung of a suckling pig two days
old. Ventral view. X 2.

Ventral 2 is broken off to show the dorsal bronchi. In places where the
metal has passed into the smaller bronchi, the dichotomy is well shown.
The branches are schematic in their arrangement with the exception of
Ventral 5 on the left side, which is reduplicated, and an extra irregular lateral
branch is interpolated on the right side.

Joseph Marshall Flint

ABBREVIATIONS.

b = Gill pouch.
a= Head gut.
c= Pulmonary anlage.
h = Ductus hepaticus.
o = Msophagus.
ad = Arteria pulmonalis dextra.
T = Trachea.
d= Right stem bronchus.
$= Left stem bronchus.
as = Arteria pulmonalis sinistra.
v = Vena pulmonalis.

Bol, G2, b:3, G4, &. 5, L.0, ete: The lateral series’ of ‘bronchi.
D2. D.3, D.4, D:5, D. 6, ete, = The dorsal series of bronchi.
V.2, V.3, V.4, V.5, V.6, etc.= The ventral series of bronchi.

M.4, M.5, etc.= The medial series of bronchi.

ap = Apical branch of left L. 2.
m= Medial branch.
d= Dorsal branch.
1= Lateral branch.
v= Ventral branch.
$= Superior branch.
4= Inferior branch.
In the combined abbreviations:
di = Dorsoinferior branch.
li = Lateroinferior branch.
vs = Ventrosuperior branch, etc.

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AMERICAN JOURNAL OF ANATOMY. VOL. VI.

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ON THE DEVELOPMENT OF THE MEMBRANOUS LABY-
RINTH AND THE ACOUSTIC AND FACIAL NERVES IN
THE HUMAN EMBRYO.

BY

GEORGE L. STREETER, M.D.,
Associate, Wistar Institute of Anatomy.

From the Anatomical Laboratory of Johns Hopkins University.

WITH 2 PLATES AND 8 TEXT FIGURES.

In the following paper some observations are reported concerning the
embryonic morphology of the acoustic nerve and the development of
the ganglion mass incorporated in its trunk. The differentiation of
this latter mass, the ganglion acusticum, and its subdivision into the
ganglion vestibulare and the ganglion spirale present several features of
interest ; and deserving of especial attention is the additional light which
the study of this process throws upon the question of nerve supply of
the saccule, and the ampulla of the posterior semi-circular canal. It
is found, namely, that these two portions of the membranous labyrinth
are not supplied by the cochlear nerve, as described in English and
German text books, but are supplied by the vestibular nerve, as has
been maintained by some of the French writers. This brings all of the
ampulle together with the utricle and saccule under control of the
same nerve, and leaves the cochlear nerve as a specialized and distinct
nerve for itself, supplying only the cochlear duct. This arrangement is
one which should be gratifying to the physiologist, for it draws a
definite line between that portion of the nerve complex which controls
the analysis of sound and that which controls equilibrium.

1Preliminary reports concerning this investigation were read, and the
models demonstrated, at the International Congress of Anatomists at Geneva,
August, 1905, and at the meeting of the American Association of Anatomists
at Ann Arbor, December, 1905.

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.
10

140 Development of Har and VII-VIII Cranial Nerves

This investigation was originally concerned only with the acoustic
complex, later it was extended to the ear vesicle, and it was found
possible to add several new features concerning the development of this
structure and the formation of the membranous labyrinth to that which
was already known from the work of His, Jr., 89, who, as far as could
be learned, is the only investigator that has made a direct attack on
this region in the human embryo since the introduction of wax plate
reconstruction methods. It is, of course, to be remembered that in his
work attention was mainly directed toward the nerve and ganglion
masses, while the finer structure of the ear vesicle was not considered in
detail.

The contributions here reported include both additional early stages in
the development of the ear vesicle and further details in the formation
of the individual parts of the labyrinth. Also some apparently funda-
mental errors in the work of the above investigator have been here
corrected. One of these regards the saccule, which as represented by His,
Jr., develops as a compartment pocketing out from the upper end of
the cochlea, but which in our specimens develops as a com-
partment or subdivision of the utricle. Instead of the saccule developing
from the cochlea, the cochlea develops from the saccule, though this
occurs at a considerable time before the separation between utricle and
saccule is complete.

The facial nerve, and especially its sensory division or pars intermedius,
bears such a close relation to the auditory apparatus that it was found
convenient to include it in some of the reconstructions. It was possible
to identify conditions in the embryo confirmatory of what is now the
generally accepted opinion as regards the adult, 1. ¢., that the nervus
intermedius is the dorsal and sensory root of the seventh, its fibers
arising in the geniculate ganglion and continued peripherally in the
chorda tympani and great superficial petrosal.

MATERIAL AND METHODS.

This work was made possible through the kindness of Professor Mall,
who gave the writer, for the purpose of this investigation, free access
to his large collection of human embryos. In the following list are
tabulated the embryos which were selected for reconstruction :

George L. Streeter 141

List of Embryos Reconstructed.

| Length in mm. | Section.
Number of | Probable age
embryo. N. B. V.B. | ECE Thickness. Direction.
148 4.3 | 3 20 10 uv Coronal.
Bee LG 6.5 6.6 26 15 Sagittal.
2 7 6 26 15 Coron-trans,
163 9 | 9 30 2 Transverse,
109 10.5 Di 33 20 Transverse.
1% 13 lames 36 20 Transverse.
144 12 | 14 37 40 Sagittal.
22 18 20 44 50 Transverse.
229 — 21 44 50 Sagittal.
86 20 30 54° 50 Coronal.

One or more wax plate reconstructions were made of each embryo
after the method of Born. In most cases the models included the
membranous labyrinth with the acoustic and facial nerves, and a portion
of the central nervous system. Of these models seven were selected for
illustration and are shown in Plates I and II. The form of the models
has been controlled in all cases by dissections of pig embryos of cor-
responding stages of development, prepared in the manner described in
a previous paper (Streeter, 04, p. 87). Such: comparison was of
particular assistance in the study of the nerves and ganglion masses.
The value of these dissections was greatly increased by previously staining
the embryos, in toto, with alum cochineal (powdered cochineal 6 gm.,
ammonia alum 6 gm., and distilled water 200 ce.), which produces a
brilliant differentiation of the tissues. In the same way that a micro-
scopical section is improved by staining so is a stained microscopical
dissection that much better than an unstained one. In studying these
a strong, direct illumination of the specimen is necessary.

Whenever the size of an embryo is expressed by a single dimension it
refers to its greatest length, and the age is that as determined by Mall’s
rule, 7. ¢., age in days equals the square root of the greatest length times
one hundred.

The drawings for Plates I and II were prepared under the guidance
and assistance of Mr. Max Brodel, for which the author derives pleasure
in taking advantage of this opportunity to acknowledge his appreciation.

MemBranous LABYRINTH.

The auditory organ is generally described as developing phylogeneti-
cally from the lateral line organs of the marine vertebrate, which sink
beneath the surface of the body and develop a cartilagenous or bony

142 Development of Ear and VII-VIII Cranial Nerves

capsule, and become incorporated in the underlying head skeleton, the
communication with the surface being maintained by a specially devised
accessory apparatus.

In the embryo the first sign of the auditory organ, according to
Krause, 03, and Poli, 97, consists of a thickening of the ectoderm, the
auditory plate, which is seen lateral to the still open medullary groove
in the region of the future third brain vesicle. In vertebrates having two
layers of ectoderm the thickening involves the inner layer, the outer not
being affected. Owing to the fact that the growth of cells shows greater
activity in the deeper strata of the auditory plate it soon becomes con-
verted into a cup shape depression and is then called the auditory fossa
or auditory cup. By the folding in and closure of its edges the auditory
cup is in turn converted into the auditory vesicle, which, however,
remains attached to the surface for a longer or shorter period by means
of an epithelial stalk or canal being finally separated from the surface,
in mammals much earlier than in lower vertebrates.

It is at this point, just after the ear vesicle has been pinched off from
the ectoderm, that my own observations begin. This stage corresponds
to the “ primitive ear vesicle” of Krause, 03, and will be described under
that heading here.

The primitive ear vesicle-—The reconstruction of the ear vesicle of an
embryo 4.3 mm. long, No. 148, shown in Fig. a, Plate I, represents
our youngest stage. This is considerably younger than the youngest
human embryo described by His, Jr., 89. It is about the same age
as shown in Krause’s, 03, Fig. 82, a model from a rabbit embryo, and is
younger than the first stage of the series of models of the ear vesicle of the
bat recently published by Denis, 02.

The ear vesicle consists at this time of a slightly elongated, oval sac,
having the following diameters: dorso-ventral, .39 mm.; caudo-cephalie,
:26 mm., and transverse, .28 mm. It lies closely against the neural tube,
and is connected with it by the acoustic ganglion, similarly as is shown by
Mall, 88, in the dog, figured in his Fig. 4, Plate XX, and is surrounded
on all sides by a thin layer of mesodermal tissue.

On the dorso-lateral surface, above that portion which is to become
vestibular pouch and near where the endolymphatic appendage is to be
separated off from the rest of the vesicle, there is a shallow groove. This
groove, as seen in the sections, is cut transversely and consists of a seam,
or the meeting point of the former edges of the auditory cup whose
approximation completes the closure of the vesicle. This closure seam
shows various degrees as regards the completeness of fusion, manifested

George L. Streeter 143

by a difference in the thickness of the opposite edges, and the degree of
obliteration of the line of juncture. The remainder of the vesicle wall
is everywhere quite uniform in appearance, consisting of 2-3 layers of
slightly elongated epithelial cells, without any apparent differentiation to
indicate points of future nerve endings.

ac. endolympn.

ECOCMIEa

mea. aud. ext.

motor VI.
\_ chora. tymp.
\

\

\
petros. sup. maj.

Fig. 1. Profile reconstruction showing the membranous labyrinth and its
relative size and relations to the brain and the fifth and seventh cranial
nerves. Human embryo 14 mm. long, Mall Collection No. 144, magnified about
8 diams.

No epidermal stalk could be detected connecting the vesicle with the
surface, or persisting beneath the surface epithelium, as observed in the
rabbit by Krause, 03, p. 88. Evidently in the human embryo such a
stalk must be either very temporary or else never present, as here we
have to do with a vesicle whose closure and detachment from the surface
must be regarded as only just completed.

144 Development of Ear and VII-VIII Cranial Nerves

The development of the endolymphatic appendage and its relation to
the epithelial stalk formed during the detachment of the ear vesicle from
the epidermis has excited a considerable controversy out of which certain
facts have become definitely established. In the first place it is evident
(Keibel, 99; Alexander, 01; Krause, o1, and 03) that in the chick the
appendage is formed out of the original union region between epidermis
and labyrinth anlage, and corresponds to the closing place of the ear
vesicle, and is its last point of attachment to the surface. On the other
hand it is also established (Corning, 99; Peter, 00, and Krause, or)
that in reptiles and amphibia the tip of the appendage does not coincide
with the point of detachment of the ear vesicle, but is situated somewhat
more dorsal and proceeds in a course of independent development before
the detachment of the vesicle is complete.

In the human embryo the endolymphatic appendage approaches in
its development more nearly the type seen in amphibia than that in the
chick. It is not developed until the epidermal stalk, if there ever is
any such in man, has disappeared. Its anlage is formed by that portion
of the vesicle wall just dorsal to the seam of closure, forming a rounded
point on the dorsal edge of the vesicle, thus its tip cannot coincide with
the point of detachment. Its situation is indicated by the external form
before there is any apparent differentiation of the wall and can be seen
in Fig. a, Plate I. By comparison of Figs. a-f, Plate I, it will be
noticed how, by a process of extension, this diverticulum becomes con-
verted into the endolymphatic appendage. In the second stage, Figs. D
and c, the external form of the appendage is more distinctly outlined,
as a short diverticulum opening widely into the rest of the vesicle.
In the next older embryo, Figs. d, e, and f, by extension of the tip and
constriction of its base the appendage begins to assume a typical form.
The last step in its differentiation consists in the widening of the distal
end into a flattened pouch or sac, in contrast to the remainder, which
persists as a narrow duct connecting it with the vestibule, indicated
in Figs. 1, m, n, Plate I, and well marked in Figs. a, b, c, Plate II. .
These are the two divisions of the appendage that are distinguished by
the names endolymphatic sac, and endolymphatic duct.

During this process of expansion the wall of the appendage which
originally, like the rest of the primitive vesicle, consists of an epithelium
of 2-3 layers, is thinned out to a single layer. The thinning out com-
mences in embryos of about 6 mm. It is at first limited to the lateral
surface and the extreme tip’ of the appendage, while the median wall
continues to be 2-3 cells thick. It is not until the embryo is about 18

— sacc. endolymph.

eie\= eanalis post.

= \==cochilea:

‘chord. tymp.

Fig. 2. Profile reconstruction showing the membranous labyrinth and the
fifth and seventh cranial nerves. The sensory part of the seventh is indicated
by solid black. The great superficial petrosal nerve extends from the geni-
culate to the spheno-palatine ganglion. Human embryo 30 mm. long, Mall
Collection No. 86, magnified about 7 diams.

146 Development of Ear and VII-VIII Cranial Nerves

mm. long that the whole appendage wall is thinned out to a single
layer. It seems probable that the thick median wall in embryos of 6-18
mm. constitutes a germinating bed which furnishes the cells needed for
the rapidly expanding appendage. It is only these cells that continue to
multiply, and they can be imagined as moving around toward the lateral
surface in a single layer in the order in which they are derived from
their focus of growth.

The diverticulum stage-—Between the primitive vesicle just described
and the labyrinth possessing cochlea, semi-circular canals, and accessory
recesses, there is a stage through which the ear vesicle passes which can
be characterized as the diverticulum or pouch stage. It is represented
by the embryos 6.6 mm. and 9 mm. long, shown in Figs. b-f, Plate I. In
these two embryos the vesicle may be said to consist of two pouches, a
large, bulging triangular one above, with the endolymphatic appendage,
the vestibular pouch, and opening into it from below the more slender
and flattened cochlear pouch. Where these two pouches meet, there is
a portion of the vesicle which is destined to form the utricle and saccule.
It can be distinctly seen in Fig. f, Plate I. This was observed in the
bat by Denis, 02, who called that part of it which projects toward the
median surface the diverticule utriculo-sacculaire. The space concerned,
however, involves also a part of the anterior and lateral walls of the
vesicle and perhaps it would be advantageous to include this whole region
under the concise and descriptive name atriwm. This atrium is properly
a subdivision of the vestibular pouch. It is in fact all that part of
it which is left after the separation off of the semicircular canals and
their ampulle. It is not to be confused with the cochlear pouch, which
is phylogenetically a secondary diverticulum, which buds out from the
atrial portion of the vestibular pouch. The embryonic relation is in-
dicated in the following table:

{ endolymphatic duct.

endolymphatic appendage, Cndol aipneuiemeet

semicircular canals.
ampulle.

7 utricle.
atrium, {

canal pockets, 4
primary vesicle, + vestibular pouch,

saccule.

cochlear pouch, cochlea.

If the words pars superior and pars inferior were substituted for the
two pouches this conception would then be at variance with Krause, 03,
only as regards the saccule which he describes as belonging to the pars
inferior. This will be again referred to later.

George L. Streeter 147

The surface markings of the vesicle during this stage assume a signifi-
cant character. In the first place the vestibular pouch at once takes
on a triangular shape with the apex toward the appendage. The three
borders of this triangle form the anlages of the semicircular canals
(see Fig. d, Plate 1), which bear the same inter-relation as the canals
in later stages. A second feature which is apparently constant and
important is the sharp, vertical groove, which cuts in between the anlage
of the posterior canal and the posterior end of the lateral canal. This
we may call the lateral groove. It was not represented by His, Jr., 89,
but can be seen in the model from the 8 mm. rabbit of Krause, go, p. 296,
and still better in models 3, 4, and 5 of Denis, 02, which were taken
from the bat. The latter author mentions it in his text.

Ventral to the anlage of the lateral canal, on the lateral surface of
the vesicle there is a rather large depression or fossa, which becomes
more marked in proportion to the increasing projection of the lateral
canal, which overhangs it like a shelf. This fossa forms the lateral wall
of the atrium from which the utricle and saccule are to develop. The
cochlear portion of the vesicle is limited to its ventral tip and extends
up along the rounded posterior border nearly to the prominent anlage of
the posterior canal. There intervenes between them that portion of
the wall that is to become the posterior ampulla. The tip of the
cochlea begins to bend forward practically as soon as the cochlear pouch
can be distinguished as such.

The changes in the structure of the wall of the ear vesicle which ac-
company the pouch formation are limited to the thinning out of certain
areas on the dorso-lateral surface of the vestibular pouch, and the lateral
surface of the appendage as has already been referred to. The remainder
of the vesicle wall is of the primitive type; there were no areas that could
be recognized as nerve endings. In embryo No. 163, 9 mm. long, how-
ever, protoplasmic nerve processes extend from the ganglion and lose
themselves in the vesicle epithelium. The branch destined to become
the posterior ampulla nerve could be seen with great distinctness; but
where it ended there was no reaction to be seen on the part of the
epithelium.

The period of semicircular canal formation is shown in Figs. g-k,
Plate I. The process consists in the expansion of the edges of the ves-
tibular pouch, 7. e., the canal anlages, and the coincident absorption of
the intermediate vestibular walls, as was essentially described by Boéttcher
in his monumental work of 1869, and to some extent by other observers
even previous to that. Since then further details have been worked out

median _ (section Aa.)
ye
/

ant. post.

— ant.
surface

Ab.

utr
d.endolymph. /
\ /
\ /
\ /
/
absorption focus
NX
NEN
Noo
\
\ NS
Cpe: 82s ENS SS i eD fe Steen ee ee
\
Ba.
crus comm. ——
KBAR TY / area semi
QQ E CT en
outer fe SS Ty a s Oy,
(a WON Vege, ; he,
edge os. *, d ewes, ; j
et oF ey 69 a” :
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pete focus

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aY

fey

AAS

outer jse/

outer z =

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ee . er Fi

D. 8weEks.

Fic. 3. Development of a semicircular canal. A, B, C, and D represent
transverse sections of the superior canal (x 240), taken at corresponding
points from embryos No. 163, 9 mm.; No. 109, 11 mm.; No. 229, 21 mm., and
No. 86, 30 mm. Aa, Ab, Ba, and Bb are explanatory drawings of lower
magnification to show the ear vesicle and the situation and shape of sections

from which A and B are taken.

George L. Streeter 149

by various investigators, notably by Krause, go, and 03, who approached
the problem along the whole line of vertebrates. He demonstrated that
the canals are formed one after the other in definite sequence, the superior
first then the posterior and lastly the lateral. Our information con-
cerning the human semicircular canals is based principally on the work
of His, Jr., 89.

An interesting interval, which was left open by His, Jr., between his
stages shown in his Figs. 6 and 7, Plate I, is filled in by my models,
made from 11 and 13 mm. embryos (Figs. g-k, Plate I). What is to
be particularly noted is the change occurring in the structure of the
vestibular wall which can be seen from a surface examination of the
model. Those areas which are to persist stand out prominently ‘and
present a fairly definite outline of the future labyrinth, while the inter-
mediate areas, which are destined to be absorbed, collapse before the
advancing mesoderm; this is well shown in Figs. 7 and k. It might
be thought that the absorption of epithelium in Fig. 7 had been com-
pleted as far as the superior canal is concerned, and that the remaining
epithelium would go to make the canal wall, necessarily stretching out
to obtain the diameter represented by the same canal in Fig. m. This,
however, is not the case; it is only the thickened edge of the pockets of
the vestibular pouch that becomes canal wall. In Fig. 7 there still
remains a large area of epithelium that is to be absorbed before the inner
rim of the superior canal is reached.

The histogenesis of the semicircular canal is shown in the accompany-
ing Text Fig. 3, in which A, B, C, and D represent transverse sections
of the superior canal in four stages of differentiation, taken at corres-
ponding points and magnified the same number of diameters., The
striking feature of the process is the persistence in the canal anlage of
the primitive epithelium of 2-3 layers until after the canal is closed
off, evidently being a factor in its rapid growth. Section A is taken
from the ear vesicle of a 9 mm. embryo, the same as shown in Figs. d,
e, f, Plate I. Aa shows the entire section of which A is a portion, and
Ab indicates the direction of the section as regards the ear vesicle. Sec-
tion B is from a 11 mm. embryo. The entire section is represented by
Ba, whose position as regards the ear vesicle is shown on Bb, which is
from the same model shown in Figs. h, i, 7, Plate I. Sections through
the vestibular region at this stage are very interesting, as they show
by the thickness of the wall which are the persistent areas; section Ba
is made in such a way as to include the anlages of two canals, the lateral
wall of the crus commune and a part of the utricle and the ductus

150 Development of Ear and VII-VIII Cranial Nerves

endolymphaticus, all of which stand out prominently. The intervening
vestibular epithelium, which is doomed to absorption, consists of a single
layer of cuboidal cells, as shown in B, in contrast to the thick outer edge
which is to become canal.

This process of absorption may be described histologically as a con-
version of the definite epithelial membrane into a line of cells which
seem to fuse with and cannot easily be distinguished from the adjacent
mesodermal cells, the line finally becoming broken and irregular. The
transition from one step in this procedure to the next is quite abrupt;
thus in B the thin membrane is sharply cut off from the absorption focus.
Several specimens were examined of about this age, and in one case,
embryo No. 175, 13 mm. long, it was found that absorption of the
epithelium was going on before the lateral and median walls of the
vesicle had actually come together. So it is possible that during this
process the vesicle cavity is in some cases left temporarily in open com-
munication with the spaces of the adjacent mesoderm. The final curling
in of the edges and closure of the canal tube repeats in a way the pro-
cedure which we have already seen in case of the auditory cup during
its conversion into the auditory vesicle. It is probably likewise mechani-
cally brought about by the arrangement of the epithelial cells. Section
C shows the canal after the formation of the closure seam, the so-called
raphe of Hasse. The thickness of the epithelium of the outer edge and
presence of division figures indicate that the activity of growth still
continues. Section D shows a canal in an embryo 30 mm. long, the
same stage as that shown in Figs. a, b, c, Plate II. Here the epithelium
is reduced to a single layer and division figures have disappeared. It
can be seen, however, that traces still exist of the thickened outer edge
and the raphe of Hasse. This stage differs from the adult canal prac-
tically only in its diameter, which there is 3-4 times greater. Doubtless
this growth is in large part accomplished simply by the flattening out and
expansion of the individual cells.

The formation of the ampulle can be seen by comparing the figures
on Plates I and II. It will be noticed that their development proceeds
simultaneously with that of the canals. In their histogenesis they re-
semble the canals, in having a thin single layer of epithelium on the
inner rim and the thick 2-3 layered epithelium on the outer surface. It
is out of the latter primitive epithelium that the macule are developed,
and they make their appearance before ampulle and canals are com-
pletely separated from the remainder of the vestibular sac; they can
be seen in the 11 mm. stage, but a high degree of differentiation is not

George L. Streeter 151

found until we come to embryos 20 mm. long. It will be remembered
that His, Jr., 89, represents ampulle as forming on both ends of the
superior and posterior canals. This was not confirmed in our models;
the ends of these two canals where they unite to form the crus commune
show no such enlargement. Each canal possesses but one ampulla.

The development of the utricle and saccule is dependent on the sub-
division of the atrium into an upper and lower compartment. The
atrium, as has already been described is that ventral part of the vestibular
pouch into which the endolymphatic appendage opens, and into which
the cochlear pouch opens from below; in Fig. 7, Plate I, it is marked
utric-sacc., and in Fig. 7 the lateral surface of it is marked sacc., and
in Fig. & a partial median view of it is marked utric. In Figs. 7 and k,
though the canals and ampulle are already completing their separation
from the vestibular pouch, the atrial region has not yet begun its sub-
division. It, however, suggests by its outer form the future saccule and
utricle. The actual subdivision begins in embryos between 18 and 20
mm. The initial ingrowth of the membranous partition can be seen
in Figs. J and m, where it can be distinguished as a horizontal cleft which
forms in front between the utricular and saccular parts of the atrium.
Strictly speaking we cannot speak of a saccule and utricle until the inter-
vening partition is complete. It is practically complete in Figs. a, b, c,
Plate II; here it reaches back to the entrance of the ductus endolym-
phaticus. It later divides the orifice of that structure, thus affording
it separate openings into the utricle and saccule, the two openings con-
stituting the so-called ductus utriculo-saccularis.

In the meantime the utricle itself has developed a definite shape. As
can be seen in the Figs. a, 6, and c, a transverse constriction divides it
into an anterior or cephalic part and a posterior or caudal part. The
anterior part constitutes the general utricular cavity, in the floor of
which the nerve ends. In front, just ventro-median to the ampulla of
the superior canal, a distinct diverticulum extends forward from it
which is called the recessus utricularis. The posterior part consists of
a central sinus utriculi communis, into which opens from above the
crus commune, laterally the sinus utriculi lateralis of the lateral canal,
from below the sinus utriculi inferioris of the posterior canal, and on
the median side the ductus endolymphaticus.

If one compares Figs. a, b, c, Plate II, with pictures of adult prepara-
tions such as found in the beautiful atlas of Schédnemann, 04, it is
apparent that the labyrinth of the 30 mm. embryo has practically com-
pleted its gross development. In its further expansion all parts of it

152 Development of Ear and VII-VIII Cranial Nerves

become relatively more slender and the saccule draws away from the
utricle and becomes flattened as well as biconcaved or saucer-shaped.

The cochlea as compared with the derivatives of the vestibular part of
the ear vesicle is less complicated in its development, presenting only
the peculiarity of spiral growth. The cochlea has already been referred
to as the pouch which forms the ventral tip and part of the posterior
border of the vesicle, as seen in Figs. b-f. In Figs. g, k, it is partly
demarcated from the saccular region by a broad fossa. At 20 mm.,
Fig. J, a sharp constriction separates it from the saccule, and this be-
comes in the 30 mm. embryo the ductus reuniens, and in the meantime
the cochlea has become a spiral of two turns.

As regards the relation of cochlea to saccule we differ from the descrip-
tion given by His, Jr., 89, who represents the saccule as budding off
from the upper end of the cochlea, which is just the reverse of our own
interpretation and what might be expected on the ground of the com-
parative anatomy of these structures. We know that in certain fishes
the ear vesicle consists of a simple utricle into which the semicircular
canals empty. In certain other fishes pockets bud out from the utricle
analogous to the saccule. When we come to animals that leave the
water, the amphibians, there develops from the saccule a secondary pocket,
which in birds and reptiles takes on the characteristics which identify it
with the mammalian cochlea. That is to say, first utricle, then utricle
and saccule, and finally utricle, saccule, and cochlea. The phylogenetic
development presents here, in discrete steps, the process which we find
in the human embryo, but in the latter case it is a matter of simultaneous
growth of all three structures.

A resumé of the development of the labyrinth is presented in the form
of a diagram in the adjacent Fig. 4, which illustrates the successive steps
by which the simple ear vesicle enlarges and becomes differentiated into
the group of connected individual compartments which characterize the
adult ear.

The ear vesicle very early (6-7 mm. long, 3 weeks) assumes the form
of two communicating pouches, the vestibular -pouch, with its endolym-
phatic appendage, and the cochlear pouch. ‘The first gives origin to the
semicircular canals, ampulle, utricle, and saccule. The semicircular
canals, in consequence of the approximation and absorption of the inter-
vening wall of the vesicle, make their appearance between the fourth and
fifth weeks, 9-14 mm. (only one is shown in the diagrams). That
portion of the vestibular pouch that is not involved in the formation of
the canals and their ampulle may be called atrium, to indicate that

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154 Development of Ear and VII-VIII Cranial Nerves

it forms at this time a common meeting place into which open the different
compartments, including the endolymphatic appendage. At six weeks,
20 mm., the atrium becomes separated into an upper and lower division
by an ingrowth of its wall, thus forming the utricle and saccule. This
partition continues inward in such a way as to split the orifice of the
ductus endolymphaticus, the divided ends of which form the ductus
utriculo-sacculus. The cochlear pouch opens directly into the atrium,
and as the development proceeds it can be seen that it is into that part
of the atrium which is destined to form the saccule. At the fifth week,
14 mm., a beginning constriction appears between the cochlea and the
saccular region. This constriction corresponds to the ductus reuniens
and gradually narrows down until in the adult in many cases the com-
munication between cochlea and saccule is obliterated. It is very ap-
parent that the saccule is not developed from the cochlea, but the cochlea
may be said in a certain sense to develop from the saccule.

N. VESTIBULARIS AND N. CocHLEARIS.

The earlier anatomists described the auditory nerve as being made
up of two main divisions. One of these, according to their plan, supplied
the utricle, saccule, and the ampulle of the three semicircular canals,
while the other division they considered to belong exclusively to the
cochlea. This description prevailed up to the time the exhaustive mono-
graph was published by Retzius, 84, upon the comparative anatomy of the
membranous labyrinth and its nerves. This investigator, by means of
careful dissection of a great variety of vertebrate material, was able to
present a much more minute description of the n. acusticus than had
previously existed. In mammals, according to his view, the anterior
division or ramus vestibularis supplied the utricle, and the superior and
lateral ampulla, while the posterior division or ramus cochlearis supplied
the saccule, the posterior ampulla and the cochlea. This classification
was substantiated not long after by His, Jr., 89, in his paper on the
development of the human acoustic complex, in which he also represented
the cochlear division as supplying not alone the ductus cochlearis but
also the saccule and ampulla of the posterior canal. From that time
until now the classification made by Retzius has been the one generally
adopted by both English and German text books. Certain French
writers (Cannieu, 94, 04, and Cuneo, 99), however, have come back to
the original conception of the cochlear nerve and its individuality. They
point out that Retzius fuses in his ramus cochlearis the inferior branch
of the ramus vestibularis and the cochlear nerve proper. They admit

George L. Streeter 155

that these two lie side by side and are closely united, but further than
that deny any anatomical or physiological relation. A similar conclu-
sion has also been reached by Alexander, 99, who studied serial sections
of the acoustic ganglion mass taken from various adult mammals.

My own observations concerning the development of these structures
in the human embryo are quite contrary to those of His, Jr., and as will
be immediately seen, they seem to indicate that the cochlear division
of this complex has nothing to do with the nerves to the saccule and
posterior ampulla, but possesses its own specialized characteristics which
distinguish it from all the rest of the acoustic mass. Embryologically,
therefore, it seems well to follow Canniew’s, 94, lead and adopt the
following classification :

N. Octayvus (N. AcustTIcus).
r. ampul. sup..
pars superior, r. ampul. ext.

r. recess. utric.
n. vestibularis,

F A . sacc.
pars inferior, Fn Bae
r. amp. post.

n. cochlearis, }ramuli spirali.

The form and branches of the acoustic mass in its different stages and
its relation to the labyrinth is shown in the figures on Plate I and II.
Two colors are added so that the cochlear division can be distinguished
from the vestibular; the former is colored yellow and the latter light
red. The same ganglion mass is shown more diagrammatically in the
accompanying Fig. 5, showing its appearance in embryos 4, 7, 9, 20, and
30 mm. long. The vestibular part is indicated by fine dots and the
cochlear by coarse dots. The drawings on the left present a median
view and those on the right a lateral view.

In the youngest stage, embryos of about 4 mm., Mall collection, No.
148, the outlines of the ganglion mass are indefinite, particularly the
peripheral border. The central end is more distinct and the protoplasmic
cell processes can be seen leading to the wall of the neural tube. This
is somewhat younger than the earliest stage of His, Jr., 89. In the next
stage, embryos of about 7 mm., the outlines of the ganglion can be clearly
made out. A section through such a ganglion is shown in Fig. 6. It
les closely against the front edge of the vesicle, its lower end migrating
around on the median side. In its outer form it consists of an upper
and lower part, pars superior and pars inferior, each of which develops
its own separate group of peripheral nerve branches; the central root

11

156 Development of Ear and VII-VIII Cranial Nerves

of the ganglion connecting it with the brain consists of a single stem.
Owing to the proximity of the ganglion mass to the ear vesicle the nerves
uniting them are at this time very short. His, Jr., 89, p. 6, regards that
portion of the ganglion which we have called the pars inferior as the
ganglion cochleare. What I regard as the ganglion cochleare or ganglion
spirale does not make its appearance until a trifle later, in embryos of
about 9mm. ‘There can be seen then a group of ganglion cells massing
themselves on the ventral border of the pars inferior, which corresponds
completely to the future spiral ganglion and may be considered as its
anlage. This anlage develops into a derivative which buds off from
the pars inferior and then follows an individual course of growth in-
dependent of the latter, and this is analogous to the way in which we
have already seen the membranous cochlea bud off from the saccule and
develop independently.

That part of the pars inferior which does not participate in the forma-
tion of the spiral ganglion remains closely related to the pars superior,
and supplies the saccule and posterior ampulla. It is this that His, Jr.,
describes in a later stage as the Zwischenganglion, and whose centripetal
fibers he joins to those of the main cochlear trunk.

In embryos of 20 mm. (compare Figs. 1, m, n, Plate I) the pars
superior has increased greatly in size, and its peripheral nerves, which
before were massed together, have become separate and distinct branches.
The pars inferior, from which the spiral ganglion is rapidly separating,
consists of a connecting strand of ganglion cells giving off separated
branches to the saccule and posterior ampulla. The fibers extending to
the posterior ampulla are at first (embryos of 11 mm.) loosely spread
out and give the appearance of more than one nerve, but later, either by
atrophy of some of them or by becoming bundled together more closely,
they constitute a single compact nerve. It is possible that here we have
to do with temporary fibers representing branches to the additional nerve
endings which are found in this region in lower forms.

- The cochlear nerve can be distinctly seen collecting its fibers from the
spiral ganglion and extending up toward the brain. The exact manner
in which this nerve reaches the neural tube proved difficult to determine.
It apparently sprouts out from the spiral ganglion and travels up on
the median surface of the vestibular ganglion until it reaches the brain.
To be certain of this would require a greater number of stages between
8 and 10 mm. than were available. In the embryos studied the proximal
end of the nerve could be made out almost as soon as the distal. So it
is possible that the cochlear trunk consists originally of a column of

Um.ve stib,---

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4Amm. 7mm

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4

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ramp. sup... rei = ir tes
--—n.vestib— —- -r amp. lat.
ramp. lat-.__ A eee B
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tS. pe Sacc.
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Gutric<_

30mm:

MEDIAN VIEW LATERAL VIEW

Fig. 5. Stages in the differentiation of the acoustic nerve complex. Vesti-
bular ganglion shown by fine dots, and spiral ganglion by large dots.

158 Development of Har and VII-VIII Cranial Nerves

ganglion cells connecting the anlage of the spiral ganglion with the
brain, and the conversion of this column into fibroblasts produces the
early fibers of the trunk; this would explain the abrupt appearance of
the nerve trunk in all parts of its course at once.

Proceeding to embryos 30 mm. long, the same as seen in Figs. a, b, ¢,
Plate I, we meet with conditions which are practically those found in
the adult. There is the vestibular nerve, on whose trunk is situated its
ganglion mass, consisting of an upper and lower division. ‘The upper
division is connected with the labyrinth by the branches supplying the
anterior and lateral ampulle, and the utricle; the lower division gives
off branches to the sacculus and posterior ampulla. In the adult the
division between the pars superior and pars inferior is still more pro-
nounced and the separation can be even seen in the trunk of the nerve.
The ganglion mass is completely divided except for a bundle of anasto-
mosing fibers, which according to Alexander, 99, may also be accompanied
by a chain of ganglion cells—a persistence of the embryonic connection
between the pars superior and pars inferior.

The cochlear nerve lies on the median surface of the two divisions of
the vestibular ganglion, but is connected with them only by contiguity.
This can be demonstrated by dissection methods in pig embryos; the
cochlear trunk can be easily lifted off, leaving the vestibular ganglion
mass and all of its branches in position, both of the pars inferior and
pars superior. At the point where they enter the central nervous system
the cochlear and vestibular trunks are in the 30 mm. embryo closely
united; they run into each other slightly more than is shown in Fig. 5,
and it is not easy to distinguish at what point the one stops and other
begins. Their separation is brought about by the increase in size of
the restiform body which develops in between them. During this process
it could easily happen that some of the cochlear fibers should become
grouped in with the vestibular, or that some of the vestibular should
become grouped in with the cochlear; in both cases finally reaching their
proper destination. If the individual fibers were traced in a large num-
ber of cases undoubtedly a considerable variation in this respect would
be found. An important stride in this direction was made by Held, 93.

The twisted, rope-like character of the cochlear nerve, as indicated
in Fig. 5, is easily identified in the 30 mm. embryo. An effort was
made in the pig to determine the number of turns and their relation to
the number of turns of the spiral ganglion. There is apparently a
certain amount of correspondence, but the fibers were too tightly adherent
to admit of a satisfactory unrolling of the nerve.

i

George L. Streeter 159

In the summary of these nerves it should be emphasized that we have
distinct points of difference between the vestibular and cochlear divisions
of the acoustic complex, both as regards the ganglia and the nerve trunks
themselves. The ganglia, belonging to the vestibular division, are spread
along the trunk of the nerve; the ganglion of the cochlear division is
situated at the extreme distal end of the nerve, and lies directly on the
membranous labyrinth, being closely incorporated with it later in the
cartilagenous capsule. The vestibular terminal branches develop as dis-
erete and fairly long nerves; the cochlear terminal branches are short
and freely anastomose. The main trunk of the cochlear division is
characterized by the compactness of its fibers and their spiral arrange-
ment; while in the vestibular division the fibers are less compactly
bundled, showing a tendency to subdivision, and do not have the spiral
character.

N. Factatis AND Pars INTERMEDIUS.

The facial nerve is so closely united in position with the acoustic
ganglion that it was found advisable to include it in the reconstructions.
For reason of simplicity it is not shown in Plates I and II, but its form
and general position at two different stages is shown in the Text Figs.
1 and 2. It can be seen how it is divided into ventral and dorsal (motor
and sensory) roots, and the situation of the geniculate ganglion on the
latter.

Particular attention was given to the ganglion geniculatum and pars
intermedius in order to bring their early morphology into accord with
the conditions found in the adult. Though the “portio media inter
communicantem faciei et nervum auditorium” was described by several
writers over a century ago, and in the intervening time frequent reference
and much speculation has been made concerning it; yet it has turned out
that none of this was actually in advance of the original description until
there appeared the paper of Sapolini, 83, who was the first to give a
detailed report of its deep origin and terminal distribution. This inves-
tigator succeeded in dissecting out the pars intermedius in adult human
material throughout its whole course. He describes it as arising in the
floor of the fourth ventricle, from where it ascends as a nerve band which
runs along the median edge of the acoustic area. At the level of the
inferior cerebellar peduncles it makes its exit through the side of the
pons, between the seventh and eighth cranial nerves, and then passes
through the geniculate ganglion, and is continued into the chorda tym-
pani, which in turn joins the lingual branch of the fifth nerve and forms

160 Development of Ear and VII-VIII Cranial Nerves

a terminal plexus in the substance of the tongue. We are thus indebted
to this writer for establishing the fact that the nerve of Wrisberg, or pars
intermedius, and the chorda tympani, are two continuous parts of the
same nerve, connecting the anterior part of the tongue with the floor of
the fourth ventricle, an essential fact which has been very slow in
getting into our text books. Further details regarding the central path
of this nerve is given in a supplementary note in a paper of His, go,
in description of a 17 mm. human embryo. He reports this nerve as
extending from the geniculate ganglion as an independent bundle into
the brain, where it can be seen running along the median edge of the
acoustic area spinalwards until it reaches the entering fibers of the

Ty?
@ ef eg * fy ayy

erat (
ZO if
. ° t
e°o0 °

G. genic. et pars intermed.

Fic. 6. Sagittal section of a 7 mm. human embryo (B. 17), showing the
relations of the facial-acoustic complex.

glossopharyngeus. It joins with these fibers and together with them
takes part in the formation of the tractus solitarius. His felt justified
in assuming that we have here to deal with a union within the neural
tube of the taste fibers from the anterior and posterior portions of the
tongue, one group coming through the chorda tympani and nerve of
Wrisberg, and the other through the glossopharyngeus. This view has
since then been vigorously supported by Dixon, 99, who pictures the
facial nerve as a typical branchial nerve developed in connection with
the ear cleft, having a motor part behind the cleft supplying the muscles
developed from this arch, and a sensory part made up of the chorda
tympani and great superficial petrosal which consist almost entirely of

George L. Streeter 161

taste fibers. Reasoning from the development and comparative anatomy
of the chorda tympani he points out the improbability of the presence
of any taste fibers in the trigeminal nerve as had been maintained by
many. That the fifth nerve pair contains few or no taste fibers has been
further established by the careful observations of Cushing, 04, who, after
the removal of the Gasserian ganglion, fund in all of his cases that
the anterior part of the tongue retained in greater or less degree the
sense of taste as well as common sensation.

If after the work of Sapolini there remained still doubt regarding
the anatomical identity of the pars intermedius, geniculate ganglion, and
chorda tympani, it was removed by the tonvincing dissections of Penso,
93. This worker investigated these structures in man and a considerable
variety of other mammals, using principally dissection and teasing
methods, though his observations were made also in part from serial

auric. vag!

iz
\
|
'
\
|

|
! } :
gang. genic.
petros. sup. maj.

\- -- -motor VW.
n.chord. tymp.

Fig. 7. Drawing made from a dissection of the facial nerve in a 20 cm.
pig embryo. At this stage the sensory and motor divisions can be easily
separated.

sections. He showed conclusively that.the geniculate ganglion is primar-
ily connected with the motor trunk of the facial only by contiguity, its
fundamental attachments being a central one, the pars intermedius, and
two peripheral ones, the chorda tympani and the great superficial petrosal.
He also describes a number of small anastomoses existing between these
branches and the surrounding structures, including the motor portion
of the facial, the acoustic ganglion, the spheno-palatine ganglion, and
the auricular branch of the vagus. He therefore represents the great
superficial petrosal and chorda tympani as composite nerves, which are
made up, in the first place of fibers from the geniculate ganglion, and
in addition to these the fibers from the above-mentioned anastomosing
branches. During the past year a paper confirming Penso’s work in its

162 Development of Ear and VII-VIII Cranial Nerves

essential points has been published by Weigner, 05, who studied serial
sections of rodent and human material, and found it possible to trace
to their destination the fibers of the n. intermedius by their histological
character; that is to say, from the frequency of the sheath nuclei, the
small size of the fibers, and the presence of scattered ganglion cells lying
along the course of the fibers.
The common identity of the pars intermedius geniculate ganglion and
chorda tympani, as described by Sapolini and Penso, in the adult is less
easily seen in the early embryo owing to the incomplete differentiation
of these structures. In fact, His, Jr., 89, states that up to the third
month no nerve is to be seen arising between facial and acoustic, and
he concludes that the intermedius must until that time run in the
trunks of these two nerves. In our embryos, however, it was found
earlier than that. The Fig. 6 represents the pars intermedius and geni-
culate ganglion in an embryo of 3} weeks, and- they can also be dis-

Fic. 8. Sagittal section through the geniculate ganglion and facial nerve
of a 30 mm. human embryo, No. 75, Mall Collection, showing how they are
separated by a connective tissue partition.

tinguished in embryo No. 148, Mall collection, which is about 20 days
old. The ganglion can be made out first, and shortly after that the
path of loose fibers connecting it with the neural tube. As is seen in
Fig. 6, the ganglion and its proximal root are at this early period dis-
tinctly separate from the acoustic mass, and it is only as the acoustic
mass increases in size that it could be said to fuse with the geniculate
ganglion; the existence even here of a real fusion is doubted, for no
appearance was observed in any of our embryos that could not be ex-
plained by mere contiguity. The separation existing between these
ganglia, in the early stages, merits the attention of those who would

eee EEE

George L. Streeter 163

believe that they have a common origin, as was thought by the younger
His.

In Fig. 6 the ventral or motor division of the facial nerve can be
seen cut obliquely at the ventral edge of the geniculate ganglion. In
this embryo the dorsal root or pars intermedius is fully as large as the
ventral or motor root; but the proportion is gradually reversed as one
looks through older stages, due to the more rapid growth of the ventral
root. The pars intermedius is in this sense a more prominent structure
in foetal life than in the adult, indicating that phylogenetically it has
played a more important role in lower forms than in man. With the
development of the connective tissue the geniculate ganglion becomes
inclosed in a sheath, and is walled off from the motor root, against which
it continues to lie, as may be seen in Fig. 8. In this figure the ganglion
is cut somewhat obliquely, and at the two ends can be seen the central
and peripheral fibers of the ganglion. To determine the destination of
the peripheral fibers dissections were made of embryonic pigs and it
was found easy to demonstrate in 3-20 cm. embryos that the distal fibers
leave at one corner of the ganglion by the great superficial petrosal nerve
and at the other by a bundle that runs along the motor division until it
leaves it as the chorda tympani. Fig. 7 represents such a dissection
made in a 20 em. pig, showing the nature of the anastomosis with the
auricular branch of the vagus, and following essentially the arrangement
described by Sapolini and Penso.

In these dissections of the seventh nerve of pig embryos a ganglion
cell mass, connected with the pons ganglia, was seen extending caudal-
wards as a surface ridge which could be traced beginning at the fifth
nerve and then passing in between the seventh and eighth nerves and
finally ending on the dorso-lateral surface of the restiform body. This
is apparently the same structure that has been found by Mr. C. R. Essick,
of the Johns Hopkins Medical School, in human adult material, and
a full description of which he has now ready for publication.

REFERENCES.

ALEXANDER, G., g9.—Zur Anatomie des Ganglion vestibulare der Saugethiere.
Sitz. d. Kais. Akad. d. Wiss., math.-naturw. Classe, Bd. 108, Abth.
III, 1899.

o1.—Zur Entwicklung des Ductus endolymphaticus. Arch. f. Ohren-
heilkunde, Bd. 52.

BOrTcHER, A., 69.—Ueber Entwick. u. Bau des Gehorlabyrinthes. Verhandl.

d. Kaiserl. Leop. Carol. deutschen Akad. der Naturforscher., Bd. 35.

164 Development of Ear and VII-VIII Cranial Nerves

CANNIEU, A., 94.—Recherches sur le nerf avditif. Thése doctorat en medi-

cine. Bordeaux. (Quoted from Cannieu, 04.)

o4.—Oreille interne. Traité d’anatomie humaine. Poirier et Charpy,

Paris, Tome V.

Corninc, H. K., 99.—Ueber einige Entwicklungsvorgaénge am Kopfe der
Anuren. Morph. Jahrb., Bd. 27.

Cunto, B., 99.—Nerfs craniens. Traité d’anatomie humaine. Poirier et
Charpy, Paris.

Cusuine, H., 04.—The sensory distribution of the fifth cranial nerve. Johns
Hopkins Hospital Bulletin, Vol. XV.

Dents, P., 02.—Recherches sur le developpement de l’oreille interne chez les
mammiféres. Arch. d. Biol., Vol. XVIII.

Drxon, A. F., 99.—The sensory distribution of the facial nerve in man. Jour.
of Anat. and Physiol., Vol. X XXIII.

HELD, H., 93.—Die centrale Gehorleitung. Arch. f. Anat. und Physiol., Anat.
Abth.

His, W., 90.—Entwickelung des menschlichen Rautenhirns. Abhandl. d. S.
Kgl. Sachs. Gesell. d. Wiss., Bd. 17.

His, W., JUN., 89.—Zur Entwickelungsgeschichte des Acustico-Facialgebietes
beim Menschen. Arch. f. Anat. u. Physiol. Anat. Abth. Suppl.

KEIBEL, F., 99.—Ueber die Entwicklung des Labyrinthanhanges. Anat. Anz.,
Bd. 16.

KRAUSE, R., g90.—Entwicklungsgeschichte der hautigen Bogengange. Arch. f.

mikr. Anat., Bd. 35. :

or-—Die Entwickelung des Aqueductus vestibuli s. Ductus endolym-

phaticus. Anat. Anz., Bd. 19.

03.—Entwickelungsgeschichte des Gehororgans. Handbuch der Ent-

wickelungslehre, Lief. 4 and 5, O. Hertwig, Jena.

Matz, F. P., g8.—The branchial clefts of the dog. Studies from Biolog.
Lab. of Johns Hopkins Univ., Baltimore.

PENSO, R., 93.—Ueber das Gang. geniculi u. die mit demselben zusammen-
hangenden Nerven. Anat. Anz. 8.

Peter, K., 0o.—Der Schluss des ohrgriibschens der Hidechse. Arch. f. Ohren-
heilk., Bd. 51.

Pout, C., 97.—Zur Entwickelung der Gehorblase bei den Wirbelthieren. Arch.
f. mikr. Anat., Bd. 48.

ReEtzius, G., 84.—Das Gehorergan der Wirbeltiere. Stockholm.

SAPOLINI, 83.—Etudes anatomiques sur le nerf de Wrisberg et la corde du
tympan on un treiziéme nerf cranien. Journ. de Med. de Bruxelles.
Vol. LXXVII.

ScHONEMANN, A., 04.—Die Topographie des Menschlichen Gehérorganes. Wies-
baden.

SrreeTeR, G. L., o4.—The develophhent of the cranial and spinal nerves in
the occipital region of the human embryo. Amer. Journ. of Anat.,
Vol. IV.

WEIGNER, K., 05.—Ueber den Verlauf des Nervus intermedius. Anat. Hefte. 87.

George L. Streeter 165

DESCRIPTION OF PLATES I AND II.

The reproductions shown on these plates represent different views, in most
eases lateral, front, and median, of seven selected models showing the mem-
branous labyrinth and acoustic complex reconstructed from the following
human embryos: No. 148, 4.3 mm.; No. B17, 6.6 mm.; No. 163, 9 mm.; No.
109, 11 mm.; No. 175, 13 mm.; No. 22, 20 mm.; No. 86, 30 mm. The colors,
yellow and red, are used to indicate respectively the cochlear and vestibular
divisions, and in general nerve fibers can be distinguished from ganglion
cell masses by their lighter tone. The pictures represent a magnification of
. 25 diams.

The following abbreviations are used:

absorpt. focus =area of wall where absorption is complete.
amp.— ampulla membranacea.
crus = crus commune.
c. 8c. lat. = ductus semicircularis lateralis.
c. sc. post. = ductus semicircularis posterior.
c. $C. sup. = ductus semicircularis superior.
coch. or cochlea = ductus cochlearis.
duct. endolymph. = ductus endolymphaticus.
d. reuniens = ductus reuniens Henseni.
endol. or endolymph. = appendix endolymphaticus.
rec. utr. = recessus utriculi.
sacc. = sacculus.
sac. endol.—=saccus endolymphaticus.
sinus utr. lat.= sinus utriculi lateralis.
utric. = utriculus.
vestib. p. = vestibular pouch.

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AMERICAN JOURNAL OF ANATOMY=-VOL VI

THE FINER STRUCTURE OF THE GLANDULA SUBMAXIL-
LARIS OF THE RABBIT.

BY

BENSON A. COHOE.
From the Hull Laboratory of Anatomy, University of Chicago.

WITH 6 FIGURES.

Since the early researches of Boll, 69, the glandula submaxillaris of
the rabbit has been the subject of many investigations on the part of
histologists. This observer believed that the gland might well be re-
garded as the type of a whole important series of glands, formed on
the same plan. The character of the cell, with its polygonal form,
prominent nucleus and abundant cytoplasm, which after treatment with
osmic acid became dark and granular, led him to consider it a pure
serous gland. Later observers, notable among whom were Nussbaum,
Langley, and Miiller, were able to confirm his opinion as to the pure
serous nature of the gland, and noted in addition that certain of. the
cells were loaded with granules, while others possessed a clear cytoplasm
free from granulation. The weight of opinion was that these cells,
unlike as to their cell contents, simply represented different physiological
phases of one type of cell. The evidence presented so far. to substantiate
this view has not been altogether convincing, and in order to test the
validity of such an opinion, a considerable number of glands, both in
normal resting conditions and in various stages of activity has been
studied during the present investigation in order to establish the identity
or otherwise, the specificity of the different cells in this gland.

HISTORICAL.

The minute anatomy of the gl. submaxillaris of the rabbit was studied
by Erik Miller, 96, in a series of investigation upon the relation of
gland structure to physiological activity. He found that after sublimate
fixation and staining with Heidenhain’s iron hematoxylin, alternating
transparent and darkly stained areas of gland tubules could readily be
distinguished even with low magnification. By careful technique and

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

168 Finer Structure of the Glandula Submaxillaris

thin sections (2-4 micra), he was able to demonstrate certain structural
differences in these two cell complexes. The cells of the darkly stained
areas showed a cytoplasm filled with large, deeply stained granules.
In some cases the granules filled the entire cell, while in other cells
could be seen a lightly stained, fibrillar network, within the meshes of
which the distinctly colored granules were held. The “ transparent
gland tubules” he found to consist of cells, the cell substance of which
was formed of a fine network with thickened nodal points, forming
regular clear spherical meshes. This second variety of cell did not fix
so readily nor so well as the granule-holding cell and was frequently
found shrunken and with irregular meshes, while at other times, under
better conditions of fixation, the meshes appeared as only the expression
of clear, round, unstained granules separated from one another by the
colored network. Apart from cellular structure he believed that these
two cell groups differed in no way from each other. “ Only,” he added,
“it must be mentioned, that the deeply stained tubules are found gen-
erally near an intercalated duct.” To this rule, however, he claimed
exceptions.

In order to explain the occurrence of these two varieties of cell forms,
Miiller considered two possibilities. Either the two forms of cells were
of different kinds and yielded different secretions or they were identical
and elaborated the same secretion, the apparent differences in structure
resulting from different stages of activity. The second view appeared
to him to be the correct one. There was not, he believed, a difference
in kind between the cells but a difference in the state of function. Ex-
perimental evidence derived from an artificial stimulation of the gland,
seemed to him to substantiate his opinion, when he found that after
active. secretion, the deeply colored areas of the gland parenchyma under-
went a change as the granules disappeared. Here he was able to find
transition stages between the light and dark cells. In case these granules
might be simply artefacts in the sense of Fischer, he examined fresh
tissue and found a similar and even more striking condition of differ-
entiation between clear and dark cell areas, and furthermore that both
kinds of cells contained granules, those of the dark areas being more
highly refractive.

In the light of these observations, he formulated an hypothesis of
the process of secretion of saliva in this gland. According to his view
the cells of the clear areas contained the finished products of secretion.
The unstained granules in these cells were transformed into the secretion
vacuoles and these discharged their contents as saliva into the secretion

Benson A. Cohoe 169

canaliculi, whence they passed into the lumen of the gland tubule. New
granules were formed within the cell meshwork to replace those thus
exhausted. These regenerated granules, lying within the meshes, at
first small, increased in size and gave rise to the stainable granules of
the dark cell areas. The latter, after attaining a certain size, lost their
power of taking up stains and were transformed into the unstainable
granules of the clear cells. In this manner arose the portions of the
gland tubule containing clear cells.

Miiller was not the first to observe that the cells of the submaxillary
of the rabbit were wanting in homogeneity. Nussbaum, 77, in an in-
vestigation of the process of ferment formation in glands chose this
gland as the type of a pure serous salivary gland. After treatment
with osmic acid he found that certain groups of cells assumed a deep
black stain which distinguished them from other groups of cells within
the gland. These dark granular cells he believed to be particularly
rich in ferment.

Langley, 78, also employed the submaxillary of the rabbit in his studies
of ferment formation, and gave us valuable data concerning the secretory
stages of this gland. He treated the gland with osmic acid, as did
Nussbaum, but was unable to entirely confirm the results obtained by
the latter observer. Nussbaum represented the intercalated ducts or
ductules as composed of small, elongated cells, from which without
intermediate forms there is a sudden change into the large cells of the
alveolus. In this view he was supported by von Ebner, 72. Langley
found that, while it was true that certain cells forming the part of the
alveolus immediately succeeding the ductule were more deeply stained
than those of the more peripheral part of the alvolus, yet there was no
section in which any marked difference in coloration between the cells
lying next to the ductules and the cells of the ductules proper could
be discerned. In all of his sections the intralobular ducts were stained
most deeply, the ductules and neighboring cells less deeply, and the
distal alveolar cells least of all. His conclusion was: “It appears to
me, on the contrary, that the cells composing the ductule are but slightly
elongated or not at all, and graduate as to size and appearance into the
alveolar cells, so that of particular ones it is difficult to say whether
they belong to a ductule or an alveolus, hence I name them transitional
cells.” ;

Held, gg, in a series of researches upon granules and gland protoplasm
studied the granule forms in the submaxillary of the rabbit. He dis-
tinguished three forms of cells characterized by three kinds of granules.

170 Finer Structure of the Glandula Submaxillaris

The first form of cell was filled with very highly refractive granules
(“ dunkle Zellen” of Erik Miiller) ; another kind of cell contained very
transparent granules, with a refractive index about equal to that of
the protoplasm (“helle Zellen” of Miiller). The third form of cell
occurred least frequently and contained granules of low refractive power,
the so-called “ ring granules.” Such forms were found in fresh sections
of the gland taken from a rabbit after a fast of twenty-four hours. He
employed various fixing reagents for studying stained sections. With
alcoholic fixation he found that all three forms of granules had dis-
appeared. With acetic acid the clear cells were destitute of granules,
while the granules in the dark cells remained undissolved. After fixation
in formalin and staining with Heidenhain’s iron hematoxylin, he was
able to distinguish: two kinds of cells: (1) clear cells which contained
only isolated granules; (2) cells, which in addition to granules, con-
tained filaments comparable to the vegetative fibers of Altmann.

More recently Gerhardt, 03, has observed the granule complexes of
this gland. He made a study of the changes in the salivary glands
which followed upon section of the secretory nerves and found these
granular areas in a gland sixteen weeks after section of the sympathetic
in sections hardened in sublimate and stained with iron hematoxylin.
He noted the occurrence of groups of granule-containing cells around
the intercalated ducts but offered no explanation as to their nature
beyond the suggestion of a possibility of their being artefacts.

Illing, 04, in an article upon the comparative anatomy of salivary
glands regarded the submaxillary of the rabbit as a pure serous gland
of the tubular variety. He noted a peculiar appearance in the structure
of the gland in the fact that in each cell a border and a central zone
could be distinguished: “In der Randzone erscheinen die Zellen vollge-
pfropft mit Kornchen, wihrend nach dem Lumen hin die K6rnchen
sich mehr und mehr lichten und immer sparsamer werden, bis sie nahe
dem Centrum schhesslich vollstandig verschwunden sind.”

MATERIAL-TECHNIQUE.

In the preparation of a series of submaxillary glands, which might
be regarded as representative of the different stages of physiological
activity, pilocarpine was used to stimulate a flow of saliva. In each case
the rabbit was deprived of food for a period of twenty-four hours prior
to the administration of the drug. It was determined by experimentation
upon several rabbits, that the greatest exhaustion of the gland appeared

Benson A. Cohoe 171

at the end of from six to nine hours, with a dosage of 0.014 gms. per
kilo of body weight of the animal. Profuse salivation commenced as a
rule in from three to five minutes after the subcutaneous injection of this
amount of pilocarpine, although certain of the animals manifested a
marked idiosyncrasy towards the drug, as a result of which the stages
of secretion as evidenced in sections of the gland revealed considerable
variation at the end of the same number of hours, and with equivalent
doses. In this manner a series of stages, taken at intervals of two hours
was obtained, varying from the normal resting gland, through the
phases of secretion, to a time twenty-four hours after the first admin-
istration of pilocarpine. At the end of this length of time it was
found that the gland had again assumed a resting condition and that
its microscopic appearance was that of a normal, unstimulated gland.

In each case fresh sections of the gland were examined in a medium
of aqueous humor of the eye, which furnished a fluid which was at
once isotonic and did not dissolve the granules within the cells. In
addition to aqueous humor various reagents, referred to later, were used
upon the fresh sections in order to study the conduct of the granules
in different media. Small pieces of the remainder of the gland were
hardened in different fixing reagents such as alcohol, formalin (10 per
cent solution), Carnoy’s fluid, aqueous sublimate, Kopsch’s fluid, Zenker’s
fluid, and others.

The majority of these hardening fluids, with the exception of Kopsch’s
fluid, afforded little or no granular fixation. A modification of Kopsch’s
fluid, suggested by Dr. R. R. Bensley, consisting of a mixture of equal
parts of Kopsch’s fluid and saturated alcoholic sublimate, to which is
added an equal volume of distilled water, gave better results than the
simple Kopsch’s fluid and served to fix the granules in a striking
manner. A saturated solution of sublimate in normal salt solution gave
good cellular fixation with scarcely any shrinkage, but did not preserve
the granules. The most satisfactory results were obtained by the use
of Bensley’s alcoholic-bichromate-sublimate, which in addition to
preservation of the granules, gave excellent cytoplasmic fixation. Even
with this fluid, however, as with all of the reagents before mentioned,
some shrinkage of the distal tubular cells was produced. rik Miiller, 96,
has called attention to the difficulty in avoiding shrinkage in the fixation
of these cells. After fixation the tissue was imbedded in paraffin and thin
sections made varying from 2-5 micra in thickness.

For staining purposes Heidenhain’s iron hematoxylin, followed by

12

72. Finer Structure of the Glandula Submavxillaris

eosin, erythrosin, or acid fuchsin and orange G. was found to give best
results for the study of cytological structure. Various other special
stains, referred to later, were employed for the differentiation of the
cellular elements in the gland.

THE STRUCTURE OF THE NoRMAL Restinc GLAND.
If a thin, fresh section of the normal resting gland be examined in
aqueous humor, with low magnification, the two cell groups described
by Miiller can readily be discerned. Many cells, occurring always in groups

twenty-four hours. From a preparation, hardened in Bensley’s alcoholic-
bichromate-sublimate, and stained in hematoxylin and eosin. X 70.

are found to contain highly refractive granules, while other less con-
spicuous groups of cells are filled with granules of a low refractive index.
Light and dark areas in the gland are thus mapped out with sharp
distinctness, even with slight magnification. An examination under high
magnification reveals the fact that all of the cells of both groups are
loaded with granules. From this it can be at once inferred that the
granules found in sections of the fixed gland are not artefacts produced
by the precipitating action of the hardening fluid, but are in reality
pre-existent in the cells. In the fresh preparation, a careful ‘examination

Benson A. Cohoe Tye}

discloses the fact that the cells of the dark groups occur invariably in
close relation to the ductules. Repeated observations, both in fresh and
hardened sections, fails to show any exception to this rule.

The granules seen in the dark cells in a fresh section appear to be
smaller in size than those in the light cells. Considerable variation
is to be noticed in the size of the individual granules in the dark cells.
The nuclei in both kinds of cells, in a resting gland, are obscured by
the great abundance of granules. The action of certain reagents on
these unfixed granules gives striking evidence of the ready solubility of
the granules in both kinds of cells. Those found in the clear cells are
especially soluble in most reagents. With isotonic salt solution the
granules are not preserved but slowly dissolve. If placed in a one
per cent solution of acetic acid, the section at once becomes opaque and
the granules cease to be spherical, although remaining visible, while
the two cell groups can be no longer distinguished. Glycerine, in con-
centrated solution, also dissolves the granules. After treatment with
osmic acid (1 per cent solution), the lumina of the ducts are seen to be
crowded with dark brown granules, while the granules in the dark cells
assume a brownish black coloration. When treated with alcohol, or with
aqueous sublimate, the granules are readily dissolved. Solger, 96, more-
over, has called attention to their ready solubility in dilute solutions
of chromic acid, potassium bichromate, acetic acid, and pure water.

In a thin section (2-3 micra) of a resting gland, fixed in Bensley’s
fluid, and stained with hematoxylin and eosin, there can be readily
observed, even with low magnification, in the portion of the gland near
an intercalated duct, a group of cells loaded with granules, which stain
intensely with eosin. (Fig. I.) These centrally disposed cells are fol-
lowed by a second group of cells, the cytoplasm of which stains indiffer-
ently with eosin and is destitute of granules. At no time does this
second variety of cell lie in close relation to an intercalated duct, while
the granule-holding cell, upon repeated observations, is found always
opening directly into a ductule. After staining with Heidenhain’s iron
hematoxylin the granules assume a deep blue-black color and are seen
to be crowded towards the lumen of the tubule. The nuclei, when visible
among the mass of granules, appear oval in form and are situated towards
the base of the cell. This proximally disposed group of granular cells
is of constant occurrence throughout the lobule and has a_ variable
diameter of from 85-200 micra. The individual cells of the group have
an average diameter of 24 micra, while the diameter of the nucleus
averages about 11 micra.

174 Finer Structure of the Glandula Submaxillaris

The clear cells of the distal group present a well-marked cytoplasmic
net-work, forming large meshes. (Fig. II.) The cytoplasm stains
feebly with most of the ordinary stains. The nuclei of these cells are

Fic. II. A portion of the same gland shown in Fig. I more highly magnified.

Preparation stained in iron-alum hematoxylin. Leitz Homog. Imm. 1/12.
Och 4.

on the whole smaller and less spherical than those of the granule cells,
and are located nearer to the base of the cell than in the other variety
of cell. These cells, even after the shrinkage with which their fixation
is always attended, possess a somewhat larger diameter than the granule

Benson A. Cohoe 15)

cell. Their average diameter is about 28 micra, while that of the
nucleus is 9 micra. Secretory canaliculi occur abundantly in connection
with these outer cells, and are seen to ramify between the cells. (Fig. III.)

The evidence adduced by a study of sections of the normal resting
gland, stained with certain selective dyes, is most convincing of the
specificity of these two kind of cells. Sections hardened in Bensley’s
fluid, and stained with iron hematoxylin and orange G., show the cyto-
plasm of the granule-holding cells deeply stained by the orange G., while
the cytoplasm of the clear cells takes on a bluish tint from the hematoxy-
lin. The granules stain a deep blue-black. With Bensley’s neutral
gentian method the granules are colored an intense violet. A sharp
differentiation between the two kinds of cells is afforded by means of
Mann’s methyl-blue eosin, the granules and cytoplasm, of the granular

Fig. III. A section through a group of distal alveolar cells, showing secre-
tion canaliculi. From a preparation stained in iron-alum hematoxylin.
Leitz Homog. Imm. 1/12. Oc. 4.

and duct cells staining a deep red, while the cytoplasm of the clear cells
is colored by the methyl blue. With Nissl’s methylene blue the intra-
lobular duct and granular cells stain metachromatically, a reddish violet,
and the clear cells are stained blue. Dahlia (1 per cent aqueous solution)
also stains the cytoplasm of the granular cells metachromatically; the
eranules assume a decided reddish color. With this stain the clear cells
and nuclei are blue, and the cytoplasm of the intralobular duct cells takes
on a violet color. By means of iron hematoxylin and erythrosin, a
clear differentiation can be established. The cytoplasm of the granular
cells stains a deep pink in contrast with the black granules, while the
cytoplasm of the clear cells is stained a feeble pink by the erythrosin.
Orcein stains both kinds of cells, the cytoplasm of the intralobular duct
cells taking up the stain intensely, while the granular cells also stain
strongly; the cytoplasm of the clear cells displays much less affinity for

176 Finer Structure of the Glandula Submavxillaris

the stain. Unna’s polychrome methylene blue stains the clear cells
blue and leaves the granular and duct cells with scarcely any coloration.

In sections stained with hematoxylin and Congo red, the cytoplasm
of the duct and granular cells is strongly colored by the Congo red, while
the clear cells take up the blue of the hematoxylin. By means of the
Ehrlich-Biondi stain, the intralobular duct cells are dyed an orange-
yellow, the granules and cytoplasm of the granular cells an intense red,
and the clear cells a violet-blue. This stain was employed by Krause, 95,
in his study of the glandula submavyillaris of the hedgehog (erinaceus
europeus) and the results obtained bore a striking similarity to the
conditions found in the same gland in the rabbit with the use of the
same stain. With toluidine blue the granular cells and granules stain
a bluish-green, while the clear cells take on a fairly strong blue color.
After fixation in Kopsch’s fluid, and staining with a mixture of lichtgriin
and safranin, the clear cells and all of the nuclei take up the brilliant
red of the safranin, and the granular cells, as well as the intralobular
duct cells, assume a violet blue. With Ehrlich’s neutral solution the
granules stain an intense red, as does also the “rodded” cytoplasm of
the cells of the intralobular ducts; the cytoplasm of the clear cells stains
blue.

By the use of the so-called specific mucin stain of Krause, thionin,
followed by potassium ferrocyanide, the clear cells stain meta-
chromatically, giving the red color characteristic for mucous cells.
The granular and duct cells stain with the thionin and remain unchanged
after treatment with the potassium ferrocyanide. A similar and sug-
gestive result may be obtained by staining sections hardened in Kopsch’s
fluid with Mayer’s muchematein, as modified by Bensley, 03. The clear
cells when thus treated give a well-marked “ mucous” reaction, while
the granule and duct cells remain unstained. With other ordinary
methods of fixation, this so-called mucous reaction cannot be obtained
in so striking a manner. After formalin fixation, however, the clear
cells give a feeble reaction with muchexmatein.

In order to study the relation of the cells of the granular areas to the
intercalated ducts, a number of Golgi preparations of the gland were
made. With the resulting impregnation, the granular cell complexes
appeared as dark brown masses, while the clear cells were stained a light
yellow. The injected secretion canaliculi were found present in both
varieties of cells. In the cells of the granular areas the canals appeared
short, broad and unbranched, a condition which can readily be observed
in sections of the gland, hardened in aqueous sublimate, and stained with

Benson A. Cohoe Ue

iron hematoxylin. In these Golgi preparations the cells of the granular
area were found invariably lying next to an intercalated duct. They,
in turn, were constantly succeeded by a group of clear, non-granular
cells, occupying a distal position in the tubule.

The intralobular ducts of this gland are lined by a single layer of
epithelial cells of a high columnar type. This layer rests upon the base-
ment membrane, outside of which there is a scanty amount of connective
tissue. The nuclei of the cells are large and spherical and occupy a
position central in the cell. A well-marked nucleolus is present, but
the chromatin is less abundant than in the nuclei of the gland cells.
The cytoplasm is abundant and stains readily with acid dyes. Towards
the base of the cells it is arranged in the form of longitudinal striations,

Fic. IV. A gland alveolus, with its accompanying intercalated duct leading
into an intralobular duct. Preparation stained in iron-alum hematoxylin.
Leitz Homog. Imm.1/12. Oc. 4.

giving rise to the “rodded” variety of cell constituting the “ salivary
tubes,” described by Pfliiger, who believed that this form of cell was
actively concerned in the secretory processes of the gland. In an ex-
hausted gland, after prolonged stimulation, the cytoplasm is observed
to stain more feebly than in the resting gland. Large secretion granules
are never found in these cells, but the cytoplasm itself manifests a finely
eranular appearance.

The ductules, or intercalary ducts, open into the intralobular ducts,
obliquely or at right angles, with an abrupt change from the low, flat
cubical cell of the ductule to the high columnar, rodded cell of the intra-
lobular duct. (Fig. IV.) The cells are elongated in form, contain but
little protoplasm and are devoid of granules. The reaction of the

178 Finer Structure of the Glandula Submavxillaris

cytoplasm towards stains is similar to that of the cells of the ducts and
of the granular areas. The nuclei are oval and are situated towards
the central part of the cell. The change from the elongated, non-
granular ductule cell to that of the large cubical, granule-holding cell
of the granular area is also an abrupt one. Langley, 78, working from
a physiological point of view, believed that there was a gradual transition
from the cells of the ductule to the cells of the alveolus. He treated
sections with osmic acid, which affords at best a poor protoplasmic stain.
Nussbaum, 77, represented the ductules as formed of small, elongated
cells, from which without intermediate forms, there is a sudden change
to the large alveolar cell. By the use of osmic acid he found them to
contain granules. Von Ebner, 72, had previously supported the view
of a sudden change from the low, cubical cell of the ductule into the
large alveolar cell. Lastly Klein, 82, has noted that the flattened,
elongated, epithelial cells of the ductules pass directly into the columnar
secreting cells of the large alveoli. An examination of preparations of
thin sections, stained with iron hematoxylin, confirms the correctness of
the observations of Nussbaum, von Ebner, and Klein. One hesitates to
dispute an opinion, entertained by so distinguished an observer as
Langley, but his technique was for physiological, not cytological purposes.
In a later paper, 79, his opinion in regard to a graduation in form and
size of cell was not so positive; he says, “Some of the cells, which, from
their shape, certainly would be called ductule cells, certainly contain
granules, though they are, I am inclined to believe, absent from the
ductule cells, springing immediately from the ducts. The absence of
cell outlines and the difficulty of obtaining thin sections of the fresh
gland, makes a decision on this point difficult to arrive at.”

The question of the existence of a differentiated, tubular portion of
the gl. submaxillaris, designated by the name of Bermann’s gland, has
been the subject of much controversy among investigators of salivary
glands. Bermann, 78, described in connection with the submaxillary
gland of the rabbit, a small tubular portion of the gland enclosed in the
same connective capsule. This part of the gland he believed to be an
“organ sui generis.” In the new born rabbit he found it to be very
small and lying in close proximity to the -hilus of the gland. According
to him a similar tubular portion occurs in the submaxillary glands of
the guinea pig, bat, dog, cat, and fox.

A number of other authors have confirmed, in the main, the observa-
tions of Bermann. Langley, 79, found that sections of the gl. sub-
maxillaris of the rabbit, made near the entry of the ductus Whartonianus.

Benson A. Cohoe 179

usually displayed a lobule or a portion of a lobule, consisting of branch-
ing tubules in place of the ordinary alveoli or ducts. W. Krause, 84,
believed that this gland was undoubtedly a morphologically and fune-
tionally differentiated portion of the submaxillary gland, tubular in
form, which represented a rudiment of the gl. sublingualis, the con-
nection of which with the ductus submaxillaris was altogether secondary.

The mass of evidence, brought by the majority of investigators, 1s
opposed to the existence of a clearly differentiated portion of this gland
in the sense of Bermann. Beyer, 79, after .a series of researches upon
this subject, maintained that Bermann’s gland was none other than
a gl. sublingualis. Von Ebner, gg, as a result of experimentation, re-
garded Bermann’s gland as being produced by a stasis of secretion
in the broad lumina of the gland tubules. Illing, 04, after a comparative
study of the gl. submaxillares of the domestic animals, could not confirm
the presence of this gland: “ Eine sog. Bermannsche Driise, wie sie von
Bermann als eine deutlich differenzierte, besondere, zusammengesetzte
tubulése Driise in der submavxillaris beschrieben wurde, habe ich weder
bei Hunde und Katze noch beim Kaninchen oder irgend einem anderen
Tiere konstatieren kénnen.” In order to substantiate his opinion, he
sectioned the entire submaxillary glands of several rabbits, and recon-
structed models of the same. In no case did he find a differentiated
tubular portion of the gland. He concurs with the view, expressed by
a number of other observers, that Bermann’s gland is simply a portion
of the gl. sublingualis (polystomatica), which frequently hes in such
close proximity to the gl. submaxillaris as to be sectioned along with it.
S. Mayer, 94, in his Gland Studies, has noted that forms conforming
with Bermann’s description (“ rein tubuldsen Driise ”) may be found in
the gl. submaxillaris and parotis of the rabbit, and in the gl. parotis
of the dog. Among the structures occurring in the region of the sub-
maxillary gland, presenting sources of errors for observers, he mentions
the “ Winterschalfdriise,’? found in many animals in this region. In
the female rat he has noted the presence of mammary gland tissue ex-
tending into the submaxillary region, and he suggests the possibility of
Bermann having mistaken degenerated mammary gland tissue for a
differentiated portion of the submaxillary: “ Es ist leicht modglich, dass
Bermann in den von ihn geschilderten, in der Nachbarschaft der Sub-
maxillardriise gelegen ‘ rein tubulosen Driisen, auch riickgebildete Milch-
driisen lippchen vor sich hat.”

In a study of many glands during the present investigation a specially
differentiated part of the gland, as described by Bermann, was in no
instance observed.

180 Finer Structure of the Glandula Submavxillaris

Tue STAGES OF PHYSIOLOGICAL ACTIVITY.

The microscopic appearances of the hardened and stained sections of
the gland, taken at any stage of activity, simulated closely the conditions
found in the preparation of the fresh gland of the corresponding stage,
with this notable exception, that while in fresh sections of the gland,
granules were found present in both varieties of cells, in the fixed
sections they occurred only in the group of cells lying next to an inter-
calated duct. No reagent so far employed has had the property of fixing,
to any extent, the granules in the clear distal group of cells. Always
associated with this lack of granule fixation, there was present more or
less shrinkage of the cell cytoplasm. It was, moreover, to be observed,
that the newly formed granules, in a gland recovering from exhaustion,
were more readily soluble than the granules of a resting gland, for which
reason, while in the fresh tissue abundant granules might be present,
the corresponding fixed specimen might display scanty granulation.

After a period of three hours’ stimulation with pilocarpine a thin,
fresh section of the gland, examined in aqueous humor, showed all of
the cells still loaded with granules. In the cells of the dark areas,
close to the ductules, the granules varied much in size and occurred in
irregular masses. The cells of the clear areas were still filled with
granules. The nuclei were discernible with difficulty. In a fixed prepa-
ration of the gland, stained with Heidenhain’s iron hematoxylin and
eosin, the cells were seen to be in a condition of partial exhaustion.
There were fewer granules present in the cells of the dark areas and the
cytoplasm of these cells was clearly differentiated from the more lightly
stained cytoplasm of the cells of the clear areas. With eosin the cyto-
plasm of the granular cells was stained a lively red, while that of the
_distal, clear cells was feebly colored by this stain. The nuclei of the
granular cells were now more readily visible than in the resting gland,
and showed a tendency to assume a position more central in the cell.
Secretion canaliculi could now be seen in both kinds of cells, but were
observed to be more numerous in the clear cells. In the granule-holding
cell they were found to be short and broad without any apparent branch-
ing.

At the end of four hours in fresh sections, the blood vessels of the
gland were seen to be much congested, and both cell complexes showed
fewer granules. It was to be noted that the granules in the cells of
the dark areas appeared to persist longer under stimulation than those
in the cells of the clear areas. After seven hours the gland was found
shrunken, with the blood vessels engorged. The two cell complexes could

———

Benson A. Cohoe ; 181

be differentiated only with difficulty, while all of the cells appeared ex-
hausted and almost destitute of granules. In the cells of the dark
areas some few small granules were still present, and in some cells border
fat granules could be seen. The gland tubules appeared shrunken while
the nuclei were readily visible and were found to be round and small.
The hardened and stained specimens of the gland taken from these
stages revealed conditions which corresponded closely to those of the
fresh sections.

The stage of most complete exhaustion was obtained from an animal
killed nine hours after stimulation. The gland here appeared pale

LHWILDER”

Fic. V. A section of an exhausted gland of a rabbit, after a fast of twenty-
four hours and subsequent stimulation with pilocarpine for a period of nine
hours. From a preparation stained in iron-alum hematoxylin. Leitz Homog.
Immo. 1/125 Oc! 4.

and shrunken. In fresh sections, examined in aqueous humor, the cells
were found to be free from granules, with the exception of the dark cells
in which a few still persisted. Atno time did the granules entirely disap-
pear from these cells, a fact that accords with the observations of Langley,
78. The few granules remaining in the dark cells were irregular in form
and of unequal size. The cells bore evidence of shrinkage and the nuclei
had lost their rotundity. In a fixed and stained specimen of this stage
(Fig. V), the distinction between the two kinds of cells was less
characteristic than in the resting gland. The clear cells appeared even

182 Finer Structure of the Glandula Submavxillaris

more shrunken than in the normal gland. The protoplasmic network
was formed of larger and more irregular meshes, which did not stain
so readily as in the resting gland. The nucleus was observed to be large
with no apparent shrinkage, and with nuclear membrane and nucleolus
well marked. The chromatin, on the other hand, was not so abundant.
The location of the nucleus was more central in the cell and further
removed from the basement membrane, than in the resting gland, while
its form was more spherical with less apparent distortion.

The cells of the granular areas in the exhausted gland were recognized
with greater difficulty in stained sections on account of the loss of
secretion granules through active stimulation. Even here, however, a
few granules still persisted. Those remaining were smaller than the
eranules found in a normal gland and were found wholly in the distal
part of the cell, lying in close proximity to the lumen of the tubule.
The cytoplasm was diminished in amount and revealed a reticular mesh-
work. It stained feebly in marked contrast to the strongly staining
character of the cytoplasm of the resting granular cell. The nuclei
were located towards the center of the cell and showed little evidence of
shrinkage. ‘They were observed in this stage, as in the resting gland,
to be slightly larger than the nuclei of the clear cells, and to be poor in
chromatin.

A section of the gland in the exhausted stage, stained with iron hema-
toxylin, afforded a clear view of the secretion canaliculi. The lumen of
such a canal, as well as the lumen of a tubule, appeared broader than
in the resting gland. Ramifications of the canaliculi could plainly be
observed among the clear cells but were absent from the granular cells.
The cells of the intercalated and intralobular ducts were somewhat
shrunken and fallen away from the basement membrane; the proto-

“rodded ” duct cells were found to stain less

plasmic striations of the
intensely than in the resting gland. The blood-vessel in the interlobular
connective tissue were engorged with red corpuscles.

The next stage was taken from a gland twelve hours after stimulation,
when both kinds of cells were found to again contain granules in a fresh
section. In the cells of the dark areas many granules were present, while
the clear cells were seen to contain fewer and smaller granules. It
was, moreover, quite obvious that the dark cell groups occurred in close
relation to the ductules, in the same position as in the resting gland.
After a period of twenty hours, the gland, although somewhat shrunken,
had again assumed in a large degree, the appearance of a normal resting

Benson A. Cohoe 183

gland. In fresh sections the cells and nuclei were found to be but little
shrunken. Both areas could be distinguished but not so readily as in
the normal unstimulated gland. The tubules surrounding the ductules
contained many granules, none of which were of great size. In a hard-
ened and stained section of this stage, the cells of the dark areas were
found to again contain many granules, the nuclei being for the most
part obscured. The clear cells were now less shrunken, and the cyto-

“
»
aw
bey
s
L

See DS EH ine R

Fig. VI. A section of the gland of a rabbit killed twenty-four hours after
stimulation with pilocarpine. Preparation stained in iron-alum hematoxylin.
Leitz Homog. Imm. 1/12. Oc. 4.

plasm stained more vigorously. The nuclei gave the appearance of
again being displaced towards the base of the cell by the abundant
formation of secretion, and were more or less irregular in contour. It
was, moreover, to be noticed in stained, just as in fresh preparations,
of this gland, returning to a normal resting condition, that the granule
cell-complex was again intermediate in position and situated, in every
case, in close relation to an intercalated duct.

The final stage taken for the study of secretion phases, was from a

184 Finer Structure of the Glandula Submavwillaris

gland stimulated twenty-four hours previously. Here the appearance
was found to be that of a normal resting gland. In fresh sections all
of the cells were found loaded with granules. he dark cell areas could
be readily detected, and were seen to occur regularly around the ductules,
just as in the unstimulated resting gland. The cells of these areas were
filled with granules, much larger than those in the cells of the clear
areas. ‘These latter cells were loaded with small granules of low refrac-
tive power, and while some of the cells were completely filled, in others
the granules were found only towards the lumen of the tubule. The
nuclei in most of the cells were not visible, owing to the mass of granules.
When distinguishable, they were found close to the base of the cell, and
were spherical in form. There was no evidence in the gland of degen-
erative processes. In a fixed and stained specimen, taken from this
stage (Fig. VI), there could be observed little or no deviation from the
appearance of a normal resting gland. The cells of the dark areas,
occurring around the ductules, were found loaded with granules, while
the granules in the cells of the clear areas remained unfixed, just as in
the normal gland.

DISCUSSION.

After an examination of the different physiological phases of the gl.
submaxillaris of the rabbit, conducted in the manner described, and a
study of the morphology and microchemical reactions of the cells of
the gland, one is forced to abandon the view held by some earlier observers,
that it is a pure serous gland formed of cells of one and the same type.
The following observed facts justify us in regarding it as a composite
gland with at least two kinds of cells in the secreting tubules, and further-
more, that these two varieties of cells do not represent different secretion
phases of the same cell but are morphologically and physiologically
distinct:

(1). In fresh sections of the gland, two distinct areas can be observed,
each composed of a group of cells containing granules with a refractive
index different from that of the granules in the cells of the other group,
indicating a difference in chemical composition. The granules in the
cells of the two groups behave in a different manner towards various
reagents.

(2). The cells forming the “ dark” areas in hardened sections always
contain granules and are always associated in close relation with an
intercalated duct. In such fixed and stained specimens the distal group
of cells, composing the “ clear” areas are always destitute of granules.

Benson A. Cohoe 18

Ou

(3). An examination of the secretion phases, observed in fresh sections,
reveals the fact that under stimulation, the granules of the clear cells
disappear earlier than those of the dark cells, while in a gland recovering
from exhaustion, those in the clear cells are the later to reappear.

(4). In stained preparations of stimulated glands, the granules are
found to have disappeared from the cells of the dark proximal group,
and to have reappeared in a gland recovering from exhaustion, in one
and the same region, around an intercalated duct. The cells of the
distal clear group, in a section prepared after fixation in any of the
fluids employed, are never found to contain stainable granules during
any phase of secretion.

(5). The cytoplasm of the two kinds of cells exhibits different staining
properties and affords distinctly different microchemical reactions. The
proximally disposed granule-holding cell conforms to the classic descrip-
tion of a serous cell, while the clear distal cell resembles in some degree
the appearance of a mucous cell.

The significance of the presence of these two types of cells in this
gland is as yet doubtful, and must remain so until our knowledge of
the chemistry of the submaxillary saliva of the rabbit is more exact
and the microchemical methods at our disposal further developed. The
role of each cell in the production of the secretion is unknown and,
indeed, even the presence of a diastatic ferment in this saliva is still a
debated question. Many suggestive, but by no means convincing, results
were obtained concerning the nature of these cells through their staining
and microchemical properties.

The cytoplasm of the cells afforded many valuable criteria for differ-
entiating the two types. In the clear cells it is less abundant than in
the granular cells and contains a basophilic recticulum. It possesses a
substance which has a marked affinity for hematoxylin and basic stains.
With acid stains it does not stain so readily as the cytoplasm of the
granular cell. With P. Mayer’s muchematein, as modified by Bensley,
03, in sections fixed with Kopsch’s fluid, a typical “ mucous” reaction
was obtained in these clear cells. With Krause’s so-called mucin stain
(thionin followed by potassium ferrocyanide) these cells also behaved as
mucous cells and gave a metachromatic red color. Neither of these
stains, however, can be regarded as specific for mucin. According to
Hoyer, go, a metachromatic color with thionin is a positive indication of
mucin or a mucin-like body. Krause, 95, tested the validity of this
assertion by successfully staining sections of hyaline cartilage and of

186 Finer Structure of the Glandula Submavillaris

the epithelium of the gall bladder, and concluded that the reaction is
not specific for mucin. Although such reactions, along with the accom-
panying morphological structure of a large, clear cell, of indifferent
staining power, and a somewhat flattened nucleus lying in the proximal
zone of the cell, is suggestive of the mucous nature of these distal tubular
cells of the rabbit’s submaxillary, we are not warranted in interpreting
them as such, at least in the light of our present meager knowledge of the
chemistry of the mucins. The researches of Langley, 86, and Ham-
marsten, 85, have shown that the elaboration of mucins by the secreting
cells is a process involving several stages. Mayer, 97, constructed a
graded series of mucins, ranging from those which stained with difficulty
in muchematein to mucin which stain readily and deeply with the same
dye. Conversely he was able to obtain a typical mucin reaction in the
cells of the gl. submaxillaris of the hedgehog, a gland which, according
to Krause, does not secrete mucin. Bensley, 02, in a study of the cardiac
glands of mammals has called attention to the fact that it is unreasonable
to expect that the different mucins and different stages of the elaboration
of mucin would present always the same staining properties. It is
possible that future researches may prove that these cells secrete a mucin-
like substance.

The second type of cell is found in the group of granule-containing
cells in the region of an intercalated duct and presents the well-known
picture of a serous cell. The cytoplasm is abundant and stains readily
and deeply with ordinary dyes. ‘The nucleus is prominent, lics towards
the center of the cell, and is spherical in form. If a ferment were
elaborated by this gland, it would be natural to conclude that the cells
of this second type gave rise to it, and that the granules found within
them were zymogenic in character. From the fact that the granules
stained black with osmic acid Nussbaum, 77, concluded that these cells
were particularly rich in an amylolytic ferment and that the distal
tubular cells were not concerned in its formation. Langley, 78, and
Griitzner, 78, were unable to confirm his observations and believed that
an amylolytic ferment was absent from this gland. With Macallum’s,
95, microchemical test for iron, a negative result was obtained. The
presence of iron either in the granules or in the cytoplasm could not be
demonstrated. The nitro-molybdate reaction, employed by Macallum, 98,
to detect phosphorus microchemically in the tissues, also gave a negative
result. Prozymogen, either in a diffused form in the proximal zone of
the cell, or irregularly distributed in the basal cytoplasm in the form
of “basal filaments” of Solger was accordingly regarded as absent.

I a ee

Benson A. Cohoe 187

A comparison of the cellular structure of this gland with the gl. sub-
maxillaris of the hedgehog (Hrinaceus ewropeus) discloses a similarity
of structure which is at once both striking and suggestive. This gland
in the hedgehog was first described by Kultschizky, 85, and later by
Krause, 95. In the tubules of the gland are found granular and non-
granular cells. With Biondi’s stain the granules assume an intensely
red color, as does also the meshwork found within these cells. The
granule-containing cell in the rabbit’s submaxillary react similarly with
Biondi’s stain. The cells destitute of granules, which occur in the peri-
pheral part of the tubule in the hedgehog’s gland are stained blue, while
the clear, non-granular cells of the rabbit’s submaxillary are colored a
violet-blue with this stain. The cytoplasm of these outer peripheral cells
possesses ‘a well-marked network, within which granules are never found.
The nucleus is often irregular and lies close to the basement membrane.
Kultschizky described the granular cells as serous, and the cells lying
outside of the latter and not containing granules as mucinoid. He was
able to show that the two kinds of cells in this gland were arranged in
regular groups and that the serous cells did not appear in the form of
demilunes, as in the mixed salivary glands of other animals but occupied
once in a while a great part of the gland lumen.

Krause also called attention to the similarity existing between the
submaxillary glands of the rabbit and hedgehog: “Es lag natztirlich
sehr nahe der Submaxillaris des Kaninchens, in welchemn wie Nussbaum
zuerst beschrieben hat, die Endstiicke der Ausfiihrungsginge zahlreich
sich in Osmium sadure intensiv schwirzende Granula enthalten, welche
der erwahnte Autor fiir Ferment Kornchen halt.”

The relation of the two kinds of cells to each other in this gland of
the hedgehog, while quite simple does not display the constancy nor
regularity of position found in the corresponding gland of the rabbit,
where the granule-containing cells always occupy a position intermediate
between an intercalated duct and the group of clear or non-granular
cells. In the hedgehog the tubule of granular cells branches simply,
and its end piece is covered with these mucinoid cells. The latter are
related in such a way to the former that mostly the middle of a small
lobule contains only granular cells, which are surrounded by a garland
of non-granular cells. This relationship Krause verified by means of
Berlin blue injection.

The similarity in the behavior of these cells of the two glands towards
stains and in their microchemical reactions is very marked. In both

13

188 Finer Structure of the Glandula Submaxillaris

glands the granule-containing cells are oxyphilic and the non-granular
basophilic. When fresh sections are placed in a one per cent solution
of pyrogallic acid in both glands the rodded epithelium of the ducts and
the granule-containing cells are stained a lively brown, while the second
kind of cell manifests no reaction. Merkel, 83, employed pyrogallic
acid to demonstrate the presence of calcium salts microchemically in the
salivary glands. Hence Krause concluded that the granule-containing
cells secrete the bulk of the lime salts and albumin of the submaxillary
saliva of the hedgehog. On the other hand sections of the rabbit’s sub-
maxillary when treated with a solution of ammonium purpurate for the
detection of calcium salts failed to show the presence of calcium either
in the cells of the ducts or of the granular areas. A brown coloration
with pyrogallic acid cannot be regarded as a test for the presence of
calcium.

That both glands when treated with Krause’s mucin stain give a
similar reaction has been already mentioned in the present paper. In
the hedgehog the granular (serous) cells stain a weak blue and the non-
granular mucinoid cells take on the simple metachromatic red color
characteristic of mucin. The submaxillary of the rabbit when treated
with this stain gave a similar result; the granular cells stained blue
and the clear cells a metachromatic red. Although this stain, as before
pointed out, is not specific for mucin, it nevertheless serves to indicate
a similarity in the chemical composition of the cytoplasm in the corres-
ponding cells of these two glands.

In concluding it is a pleasure to thank Professor R. R. Bensley for
the many valuable suggestions offered during the preparation of the
present paper.

LITERATURE CITED.

BENSLEY, R. R., 02.—The Cardiac Glands of Mammals. Am. Journ. Anat.,

Baltimore, Vol. II, No. 1, pp. 105-156, 1902.

03.—The Structure of the Glands of Brunner. The Decennial Publica-

tions of the University of Chicago, Vol. X, Chicago, 1903.

BERMANN, I., 78.—Ueber die Zusammensetzung der Glandula submaxillaris
aus verschiedenen Driisenformen und deren funktionelle Struktur-
verinderungen. Wurzburg, 1878.

Bryer, G., 79.—Die Glandula sublingualis, ihr histologischer Bau und ihre
funktionellen Veranderungen. Inaug. Diss., Breslau, 1879.

Bott, F., 69.—Die Bindesubstanz der Drtisen. Arch. f. mikr. Anat., Bonn, Bd.
5, S. 334-355, 1869.

Jenson A. Cohoe 189

EBNER, VON V., 72.—Ueber die Anfange der Speichelgange in den Alveolen
der Speicheldriisen. Arch. f. mikr. Anat., Bonn, Bd. 8. S. 481-513,
1872.

99.—Koelliker’s Handbuch der Gewebelehre des Menschen, Bd. 3,

Leipzig, 1899.

GERHARDT, U., 03.—Ueber die histologische Veranderungen in den Speichel-
driisen nach Durchschneidung der secretorischen Nerven. Arch. f. d.
gesammte Physiol. Bd. 97, S. 315-334, 1903.

GRUTZNER, P., 78.—Ueber Bildung und Ausscheidung von Fermenten. Arch.
f. d. gesammte Physiol., Bd. 16, S. 105-123, 1878.

HAMMARSTEN, O., 85.—Studien tiber Mucin und mucinahnliche Substanzen.
Arch, f. d. gesammte Physiol., Bonn, Bd., 36, S. 373-456, 1885.

HELD, H., g9.—Beobachtungen am tierischen Protoplasma. I. Driisengranula
und Drtisenprotoplasma. Arch. f. Anat. u. Physiol., Anat. Abt.,
S. 284-312, 1899.

HOYER, 90.—Ueber den Nachweis des Mucins in Geweben mittelst der Far-
bemethode. Arch. f. mikr. Anat., Bonn, Bd. 36, S. 310-374, 1890.

ILLING, G., 04.—Vergleichende makroskopische und mikroskopische tber die
Submaxillaren Speicheldriisen der Haussaugetiere. Anatomische
Hefte, Bd. 26, 1904.

Kien, E., 82.—On the lymphatic system and the minute anatomy of the
salivary glands and pancreas. Quart. Journ. Microsc. So., Vol. XXII,
pp. 154-175, 1882.

KRAUSE, R., 95.—Zur Histologie der Speicheldrtisen. Die Speicheldrtisen des
Igels. Arch. f. mikr. Anat., Bd. 45, S. 93-133, 1895.

KRAUSE, W., 84.—Die Anatomie des Kaninchens. 2. Aufl., Leipzig, 1884.

KuLtscuizky, N., 85.—Zur Lehre vom feineren Bau der Speicheldrtisen.
Zeitschr. f. wiss. Zool., Bd. 41, S. 99-106, 1885.

LANGLEY, J. N., 78.—Some remarks on the formation of ferment in the sub-

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Leipzig, Bd. 12, S. 303-330, 1897.

190 Finer Structure of the Glandula Submaxillaris

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(

THE RELATION BETWEEN THE CYTO-RETICULUM AND
THE FIBRIL BUNDLES IN THE HEART MUSCLE CELL
OF THE CHICK.

BY

HARRY LEWIS WIEMAN.

From the Biological Laboratory of the University of Cincinnati.

WirH 2 DIAGRAMS AND 17 FIGURES.

In the great mass of literature that has appeared on the subject of
the striated muscle, no account apparently exists of the histogenesis of
the heart muscle cell of the chick. This seems rather strange in face
of the fact that the chick always has been the classic subject for embryo-
logical research. The present study was undertaken for the purpose of
determining the structures existing in the heart muscle cell of the chick,
especially for comparison with results of similar work that has already
been done on other vertebrate forms.

The particular phase of the histogenesis treated in this paper is the
relation between the cyto-reticulum of the embryonic cell and the fibril
bundles as found in the adult muscle tissue. To avoid any possible con-
fusion or misunderstanding, a definition of terms here at the outset will
perhaps not be amiss.

By cyto-reticulum, the writer means the deeply staining network found
traversing the cytoplasm of early embryonic cells. The fibril bundles,
corresponding to the “ Muskelsiulchen ” of Koelliker, 02, are the striated
longitudinally disposed masses running the length of the adult cell.
Each fibril bundle is composed of more elementary parts called fibrils.

On account of the abundance of material and the ease with which
the various stages in the development of the cell can be secured, the
chick is very well adapted to a study of this kind. The one serious ob-
jection is that the various structures of the cell are not as well differ- -
entiated as in some other forms.

To Professor Guyer, at whose suggestion this work was taken up, the
writer is much indebted for valuable assistance.

AMERICAN JOURNAL OF ANATOMY.—VoOL. VI.

192 The Fibrils of the Heart Muscle Cell of the Chick

METHODS.

A detailed study of the cytoplasmic structures of the heart muscle cell
requires the use of a very high magnification. To secure good defini-
tion, it was found necessary to make sections 3 p» in thickness. Sections
of 4 to 10 » thickness were used for general structure and form of the
cell. Sections were cut in paraffin.

Of the different fixing agents used, none gave more satisfactory prepa-
rations than Kolossow’s, 92, solution. None was found to surpass it
in faithful preservation and differentiation of the cytoplasmic structures.
The tissue is killed and hardened by treatment for 15 minutes with a
1 per cent osmic acid solution. This is followed by immersion for the
same length of time in a reducing mixture of pyrogallol and tannin.
The tissue is next washed in a 0.25 per cent solution of osmic acid, and
then in water. After being passed through graded alcohols, the tissue
is cleared in xylol, and embedded in paraffin. This method followed by
Delafield’s hematoxylin or methylene blue, counterstained by 0.5 per
cent solution of acid fuchsin in 70 per cent solution of alcohol gave
excellent results. The chromatin structures of the nucleus and the cyto-
reticulum appear almost black, while the undifferentiated cytoplasm is
red. Acid fuchsin alone without the nuclear stains also yields good
preparations.

One objection to Kolossow’s solution is that it does not penetrate
rapidly enough, and as a result, especially in adult tissue, the structure
of the cells in the interior could not be made out as well as that of those
in the peripheral portions of sections.

Another killing fluid which gave good results was Gilson’s mercuro-
nitric fixing mixture, followed by iron hematoxylin for staining. This
method produces more uniformly penetrated preparations, but does not
show the cytoplasmic reticulum to such good advantage as the osmic
acid method.

Hermann’s platino-aceto-osmic mixture was also tried. After treat-
ment with this solution the tissue is stained with alcoholic safranin for
24-28 hours. The preparation is then treated with gentian violet ac-
cording to Gram’s method. This method was not as satisfactory as
either of the foregoing.

Picrosulphuric-acetic acid was used with good results where general
structure was the object sought. This fluid is made by adding a 5 per
cent solution of acetic acid to Kleinenberg’s picrosulphuric acid solution.
Treatment with the above is followed by staining successively with
carmalum and Delafield’s hematoxylin.

Harry Lewis Wieman 193

Besides section methods, maceration was employed. This process was
used with satisfactory results in studying the general structure of the
entire cell of the adult tissue. Of the various methods tried, treatment
for twenty-four hours with a 20 per cent solution of nitric acid, fol-
lowed by staining with Delafield’s hematoxylin and acid fuchsin, gave the
best preparations.

THe ApuLtt Herart-Muscie CELL.

Examination of the adult tissue shows it to be composed of an anas-
tomosing network of fibers, the demarcation of which into cells is rather
uncertain. Apparently a fiber is arranged into cylindrical cells the
length of which exceeds many times the diameter. The tapered ends
of the cells seem to fit together much in the manner of a dove-tailed .
joint. The nucleus is oblong in shape and occupies a more or less central
position, although it is sometimes seen very close to the periphery of
the fiber.

The contractile substance, the fibril bundles, consists of deeply staining,
longitudinal masses running the length of the cell. Surrounding the
fibril bundles and separating each one is the undifferentiated sarcoplasm.
Fig. 15 represents portions of two fibril bundles, each with its envelope
of sarcoplasm, from the outer wall of the ventricle of the adult fowl.
The fibril bundles are divided by cross striations at right angles to the
longitudinal axis into alternating broad, deeply staining bands, and
narrower bands more lightly stained (Fig. 15, b/). The broad, heavily
stained bands correspond to the “ Querscheibe,” or “ Briicke’s doubly
refractive substance” of mammalian heart muscle (MacCallum, 97).
One of these bands is shown in Fig. 15,a. By carefully focussing up
and down, a very narrow, deeply staining band can be seen crossing the
light bands. This is the “ Zwischenscheibe,” or “ Krause’s membrane,”
which structure is not very distinct nor readily made out. The con-
tinuation of this line, however, can be plainly seen in the sarcoplasm
surrounding the fibril bundle (Fig. 15, c). The Querscheibe, as is readily
shown in Fig. 15, are slightly larger in diameter than the more lightly
stained parts between.

The sarcoplasm surrounding the fibril bundles is not homogeneous,
but is divided into disc-like parts (Fig. 15,d), which could be especially
well seen in peripheral cells when partially macerated. In preparations
of this kind, the discs were found separate from each other, while the
fibril bundles remained intact, that is, they did not break up into corre-
sponding lengths. This it seems would militate against the idea of

194 The Fibrils of the Heart Muscle Cell of the Chick

Krause’s membrane being continuous with the line of demarcation be-
tween two adjacent sarcoplasmic discs. If the latter were the case, it
would seem that the fibril bundles should show a tendency to break and
separate along the line of Krause’s membrane. This did not happen.
Hence it may be that what appears to be Krause’s membrane may only be
the line of demarcation between two successive sarcoplasmic discs seen
through the lightly staining parts of the fibril bundles.

Teased tissue showed the fibril bundles each to be made up of smaller
longitudinally disposed parts, the fibrils. The fibril bundles were in all
cases surrounded by sarcoplasmic discs.

In the adult tissue the fibril bundles are more abundant in the periphery
of the cell than toward the center, which is composed for the most part
of a network of undifferentiated protoplasm (Fig. 16). This figure
represents a longitudinal section through the center of the cell. Part
of the sarcoplasmic discs of a fibril bundle is marked sa in this figure.

Cross sections show cells of widely varying diameter. Fig. 17 repre-
sents three cells of a medium type. Cells of three and four times the °
diameter of these are also found. Fig. 17 is exceptional in that it shows
the cell boundaries very distinctly. In most cases the cells are so closely
applied that it is difficult to distinguish cell walls. These sections show
the fibril bundles as dark, deeply staining patches a, surrounded by the
sarcoplasmic discs, (sb) (Fig. 17). Some of these discs are seen to
be further subdivided (sd), into what MacCallum, 97, has described as
“small sarcoplasmic discs.”

Between the cells as represented in Fig. 17, and connecting them, may
be noticed a number of threads ¢, which resemble very much the strands
of the reticular structure of the interior of the cell. Just what the origin
and nature of these structures are, could not be determined. A study of
these structures would doubtless throw some light on the question of
whether the heart muscle fiber is a syncytium or not. No structure
analogous to the “ protoplasmic bridges” of mammalian heart tissue or
the “stratum granulosum terminale” (Prezwoski, as quoted by Mac-
Callum, 97, p. 611), of human heart muscle, was found connecting the
ends of the cells.

Such, in brief, are the structures met with in the heart muscle cell
of the adult chick.

EMBRYONIC DEVELOPMENT.
To study the different stages through which the cell passes in its
development, sections from embryos varying in length from 8 mm. (15
somites), to 22 mm. (8 days), were examined. Cells characterized by

Harry Lewis Wieman 195

a certain structure are not confined to any one stage. In the following
account cells are said to belong to a certain stage because they were
first noticed in that stage, although they may be, and frequently are,
seen in sections from tissue several days older.

Thirty hours (15-somite stage).—Cells of this stage exhibit in longi-
tudinal section a characteristic short, broad, cylindrical form (Fig. 1).
The ends of the cell taper rather abruptly from the center. The nucleus
(Fig. 1,d) is large and oval; its short diameter being but slightly less
than the diameter of the cell. Large chromatin masses (Fig. 1, b) may
be seen as heavily stained, irregularly shaped clumps, and a very fine
network traverses the nucleus.

The cytoplasm exhibits a pronounced reticular structure, with large
and irregular meshes. At the intersections of the threads of this network,
the staining is somewhat heavier, marking off these parts more distinctly
than the rest of the reticulum.

Fig. 2 is a cross section through the region of the end of the nucleus.
The cytoplasm shows the same reticular structure, the nucleus appearing
oval in outline.

Cells of this general description are typical of the heart tissue in
its very earliest state of formation. They can be readily distinguished
from the unmodified mesenchyme cells, in that they do not show the
branched structure of the latter.

Seventy-two hours (3-day stage).—Up to this period in its develop-
ment, the cell differs but slightly from the description given above. At
this time, however, several changes are to be noticed. The cell has become
larger and the tapering ends have increased in length (Fig. 3). The
points of intersection of the threads of the cyto-reticulum are more
distinctly marked than before by accumulations of heavily staining
material, spherical in form (Fig. 3,a@). These are also shown in cross
sections (Fig. 4,a). The nucleus is oval in form though somewhat
smaller than. in earlier stages. The chromatin masses (Vig. 3,0; Fig.
4,b) have decreased in size.

Ninety-six hours (4-day stage) —Fig. 5 represents a longitudinal
section of a cell of this period. The cell has increased enormously in
size, especially in its longer diameter. ‘The deposits on the cyto-
reticulum are larger and stand out very clearly and distinctly (Fig. 5, a).
The meshes of the cyto-reticulum are still very irregular in form. The
nucleus does not seem to have undergone the same increase as the cyto-
plasm, and its chromatin masses have become smaller and more evenly
distributed (Fig. 5,b; Fig. 7,6). Fig. 6 shows a cross section above

196 The Fibrils of the Heart Muscle Cell of the Chick

or below the nucleus. Some of the meshes of the cytoplasmic network
are divided into smaller parts, the small sarcoplasmic discs of MacCallum,
97 (Fig. 6, sd).

120-130-hours’ stage—In Figs. 8 and 9 are. shown characteristic
longitudinal sections of the cell at this stage in its development. The
eyto-reticulum has undergone a striking change. Instead of the irregular
structure of the preceding stages, we find a definite arrangement of the
network into rectangular meshes. The deposits on the reticulum have
increased in bulk in such a way that in longitudinal section they appear ~
as oval-shaped bodies (Fig. 9,a). In Fig. 9, which represents a slight
advance in the development of the cell over that shown in Fig. 8, inter-
posed between every two adjacent longitudinal threads of the network
bearing the deposits, is to be seen a longitudinal thread the intersections
of which with the transverse threads are not marked by heavily staining
deposits (Fig. 9, ba).

This re-arrangement of the meshwork is apparently the first step in
the laying down of the fibril bundles. Subsequent development shows
that the longitudinal strands bearing the deposits represent the axes
of the fibril bundles, and the deposits, the Querscheibe of the adult tissue.
The transverse threads (Fig. 9,¢) according to MacCallum, 98, give rise
(in the mammal) to Krause’s membrane. However, this will be referred
to again later.

Cross sections (Fig. 10) show the meshes of the reticulum to be
further subdivided, but few of the original size remaining. It is to be
noted that one or two very large, irregularly shaped meshes are present
in this section (Fig. 10, Ja). Apparently these areas, in later stages,
become divided into smaller parts, just as the neighboring cytoplasm has
already become divided. Evidently this is the manner in which the
erowth process in the cell takes place, the cytoplasm in the smaller
meshes increasing greatly in bulk and then, by subdivision, producing
a number of meshes approximately the size of the first.

The nucleus is oblong in longitudinal section (Fig. 8,d; Fig. 9, d),
and roughly circular in cross section (Fig. 10, d). The chromatin
masses are small, although Fig. 10 shows one of considerable. size.
It appears that Eycleshymer, 04, in his work on skeletal muscle-cells
of Necturus, found just the opposite change to take place in the size
and distribution of the chromatin granules, 7. e., in younger stages the
karyosomes were evenly distributed in the nucleus, and in later stages
collected in large masses.

130-140-hours’ stage——The most interesting phase of the entire devel-

Harry Lewis Wieman ON

opment is seen at this period. The evidence met with in this stage
furnishes the most decisive proof in favor of a definite relationship ex-
isting between the cyto-reticulum of the embryonic cell and the fibril
bundles of the adult. Longitudinal sections present a very marked
appearance of cross striations (Fig. 11,a; Fig. 12,a). On examination
it is found that these striations are produced by a growth, principally
in length, of the deposits on the cyto-reticulum of previous stages (Fig.
9,a; Fig. 3,a). These heavily-stained bands stand out as clearly as if
stamped with a die. Sections at this period present all gradations of
striations as may be seen from Figs. 11 and 12. The transverse strands
of the cyto-reticulum are not very prominent. The tapering ends of
the cells show an enormous increase in length. It is to be noted that
the striations first appear in the elongated ends, at least they show a
greater degree of development in this part of the cell. These markings

Qa

ee

3

cause the ends of the cell to stand out more prominently than the other
regions, and it is now very difficult to distingush cell walls in longitudinal
sections. However, by means of the heavy striations, one can see how
these slender projecting ends have made their way between other cells,
and thus trace the formation of the syneytium-like structure of the
adult tissue. In later stages, when the cell exhibits uniform striation,
the course of the ends of the cells cannot be so well followed. The
accompanying diagram would then represent the structure of the adult
fiber in Jongitudinal section. This would also explain the fact that
a cross section of the adult tissue, as for example, from a to b, shows
cells of widely varying diameter. A similar explanation was offered by
MacCallum, 98, in the case of the striated muscle of the pig.

Fig. 12 shows the ends of two adjacent cells seen in the same section
as Fig. 11, but which have apparently proceeded further in their develop-
ment. The striations are well marked. In the figure several fibril
bundles are represented (a). The structure found here corroborates the
statement that the formation of the Querscheibe starts in the ends of
the cells.

198 The Fibrils of the Heart Muscle Cell of the Chick

Cross sections (Figs. 13 and 14) show an increased number of the
smaller meshes of the cyto-reticulum, while the deeply staining deposits
(a) which are cross sections of fibril bundles have become greatly en-
larged. These heavily staining patches are best developed toward the
periphery of the cell. The nucleus is roughly oval in outline.

From this structure to that of the adult is but a step. While all the
intermediate stages between this and the adult were not examined,
sections from stages beyond this up to the eighth day, showed the same
structure in various degrees of development. In the adult the sarco-
plasmic discs are better developed. The Querscheibe are broader and
resemble thick, flat plates. The length and diameter of the cell is greatly
increased.

SUMMARY.

The facts which appear to be of importance as set forth in the fore-
going, are as follows:

1. The cytoplasm of the early embryonic heart cell is traversed by an
irregular network, the nodes of which are marked by heavily staining
deposits.

2. This network tends to become more and more regular, until its
strands are longitudinally and transversely disposed.

3. The heavily staining deposits on the primitive network develop into
the Querscheibe of the adult fibril bundle.

4. The longitudinally disposed lines of, the network represent the axes
of the fibril bundles of the adult.

5. The sarcoplasmic discs of the adult develop from the inter-reticular
cytoplasm of the embryonic cell.

GENERAL DISCUSSION ON THE RELATION BETWEEN THE CYTOPLASM AND
THE FIBRIL BUNDLES.

According to Ranvier, 89, the myocardium of the mammal is com-
posed of rhomboidal branching cells. The nucleus is centrally placed
and is surrounded by a granular mass stretching out in the axis of the
cell. Surrounding this granular mass is the contractile element which
shows longitudinal and transverse markings. This element is the fibril
and is made up of successive segments having the same structure as
voluntary muscle.

Koelliker, 02, describes the mammalian heart muscle as composed of
an interlacing network of cells having centrally placed nuclei. The
contractile substance, as in voluntary muscle, consists of fibril bundles,
the so-called “ Muskelsaéulchen,” which show a definite transverse striping.

SE————_-

ce ee ee ee

Harry Lewis Wieman 199

Of the later workers, Godlewski’s, 02, description is interesting, because
of his denial of cell. structures in the heart muscle of the rabbit. Ac-
cording to him the heart muscle is a syncytium in which there is no
cell demarcation. The contractile substance is the fibril which shows the
various markings described by other authors. -

Thus, in general, in addition to the above, the work of investigators
goes to show that the contractile substance in the muscle tissue is the
fibril. A number of these in turn compose the fibril bundle. Workers
are not agreed as to the origin of the fibril bundles. The older hypothe-
sis that they are extra-cellular structures is now refuted and the generally
accepted opinion is that they are intracellular.

Two current views exist as to the nature of the fibril bundles. The
first is that these structures are coagulation products, and that the living
cell contains neither cyto-reticulum nor fibrils (Englemann, 73-81).
The second view is that the fibrils are differentiated structures which
are formed in the living cell. The latter theory is the one which the
results of many workers seem to verify. Of the latest workers Hycle-
shymer, 04, reports having observed the fibrille (fibrils) in the living
muscle cells of Jarval necturus.

Concerning the origin of the fibrils, there are what is known as the
network theory and the fibrillar theory. The upholders of the network
theory maintain that the muscle cell contains a contractile reticulum, the
longitudinal threads of which form the fibrils, the meshes being filled with
a more fluid substance. Others consider that the fibrils are produced by
the coagulation of the fluid substance, as a result of the action of various
reagents. Later work apparently refutes the latter idea. The advo-
eates of the fibrillar theory maintain that the fibrils are the contractile
elements, and further, that they arise independently of the cyto-reticulum.
Hycleshymer’s, 04, work on necturus, supports this idea, and Godlewski,
o2, in his work on the striated muscle cell of the rabbit, has been able
to find no trace of a cytoplasmic network.

In the theory urged by MacCallum, 98, we have a combination of the
network and the fibrillar theories. In this author’s words (Goby. aalke
simplifies the conception of the structure of striated muscle fiber greatly,
to consider the fibril bundles and the membranes bounding the compart-
ments in the sarcoplasm as derived from the primitive network found in
the muscle cells of very young embryos.” (p. 209) “This network
tends to become more and more regular until the meshes are of the
form of large discs. Some of these break up into smaller ones and in
the nodal points of the network there is an accumulation or differentiation

200 The Fibrils of.the Heart Muscle Cell of the Chick

of its substance, giving rise to longitudinally disposed masses. These
become what in the adult are known as fibril bundles and the discs are
the sarcoplasmic discs.”

Now the question is, How do these theories apply to the conditions met
with in the heart muscle of the chick? We will consider MacCallum’s
theory first.

The occurrence of the breaking up of the meshes of the cyto-reticulum
into smaller parts was noticed in the case of the chick. That the fibril
bundles arise from the center of the meshes at the nodal points of the
network appears, however, to be untrue. Here, in fact, it seems that the
nodes of the original network mark the positions of the fibril bundles;
in other words, that the primitive longitudinal threads develop into these
structures. ‘To illustrate, the accompanying figure represents a diagram-
matic cross-section of the cyto-reticulum. The unbroken lines represent
the threads of the original network. The circular masses at the inter-

sections of this network (a, b, c, ete.) represent the heavily-stained
deposits. The dotted lines divide the large meshes or discs into the
“small sarcoplasmic discs” of MacCallum (boc. cod, etc.). Now ac-
cording to the author just mentioned, a fibril bundle arises in the center
of any large disc at the point of intersection of the dotted lines (0, 2, etc.).
The full lines would then represent boundaries of sarcoplasmic discs.
However, in the case of the chick the facts seem to indicate that the fibril
bundles arise at the points of intersection of the lines of the original
network (a, e, b, etc.). Then the sarcoplasmic disc surrounding any
one particular fibril bundle, as, for example, that one the cross section
of which is represented by a, would be the area bounded by yboearzk.

Harry Lewis Wieman 201

The writer’s preparations clearly show that the nodal points of the
original cyto-reticulum of the embryonic heart cell are marke: by more
heavily staining deposits, both in longitudinal and in cross section (Figs.
3, 4, 5, 6). The deposits can be followed in successive stages, and are
always identified with the longitudinal and transverse threads of the
network. Eventually they develop into the Querscheibe, and the longi-
tudinal threads of the network become the axes of the fibril bundles of
the adult tissue. If this be true, the fibril bundles cannot originate from
the centers of the meshes seen in cross sections of the cyto-reticulum.

Suppose now that fibril bundles were to form at the points a and e
of the diagram. Then we would have the two fibril bundles surrounded
by one set of sarcoplasmic discs (odtparzhyb), a condition sometimes
found in the adult; also mentioned by MacCallum, 97 (p. 613), in the
human heart muscle. However, were these two sets of sarcoplasmic
discs to become separated by a plane of division along a line from 0 to z,
then each fibril bundle would have its own set of sarcoplasmic discs,
the structure more generally met with in the adult.

If the fibril bundle in its development follows the method suggested
by the writer, another difficulty is encountered with the results of Mac-
Callum’s, 98, work. This worker states that the transverse membranes
of the cytoplasmic reticulum of the myoblasts of man and of pig (repre-
sented in the chick in Fig. 8, a; Fig. 9, ¢), give rise to Krause’s mem-
brane in the adult. Attention is called to the fact that (as seen in Figs.
8 and 9) the intersections of these transverse lines with the longitudinal
lines of the reticulum mark the positions of the deeply-staining substance
which later becomes the Querscheibe. Now, in the adult, Krause’s mem-
brane is found as a narrow transverse band across the lightly-staining
portions of the fibril bundles, and not at all connected with the Quer-
scheibe. Thus it is readily seen that MacCallum’s explanation of the
formation of this structure does not apply in the case of the chick, nor
could its origin be determined satisfactorily, for, as was remarked in
another place, it is not very well differentiated in the heart muscle of
the adult chick.

At this point it is interesting to consider the work of Eycleshymer, 04,
on the striated muscle cell of Necturus. This author states that in the
study of the striated muscle cells of Necturus, he has been unable to
find any evidence of a definite or fixed relation between the cytoplasmic
network and the fibrille (fibrils). Further, as serious objection to
the existence of such a relation, he says that the fibrille are unstriated
for some time after their appearance.

202 The Fibrils of the Heart Muscle Cell of the Chick

In the case of the chick, however, the above is not true. For if we
consider the longitudinal threads of the network of the stage represented
in Figs. 8 and 9, as the incipient fibril bundles, then the deposits mark-
ing the intersections of the threads would represent the striations, since
later these develop into the Querscheibe of the adult tissue. The question
is just what is to be understood by “ first appearance of fibrille.” If
by this we mean the earliest stage in the development of the fibril bundles
at which there is any resemblance to the adult structure, we may say
that the fibrillee are striated from the start in the chick.

With reference to another point in this connection, the writer here
quotes a passage from the same author, pp. 298, 299: “A point of
capital importance is found in the fact that in Necturus, Amia Lepi-
dosteus . . . . as my own observations show, and in other forms
as Kaestner, 92, has found, the beginning of fibrillation is coincident
with the first contractions. The movements of the embryo first begin
in the anterior of the mid-dorsal myotomes and in these the myoblasts
are first fibrillated. The above considerations led the writer to support
the theory that the fibrille are pre-existent structures and represent the
principal contractile element.”

The same argument cannot be applied to the heart muscle of the chick,
because the first contractions occur at the time when about 15-17 myo-
tomes have been formed in the embryo. As may be seen from the
representation of the heart muscle cell at this stage (Fig. 1), no
structure which might be truly called a fibril is present. The first
appearance of anything that in the faintest way resembles the adult
is not seen before the 120-130-hours’ stage (Figs. 8 and 9).

If the fibrille are not pre-existent structures, what then are the
contractile elements in the very early embryonic stages? In view of the
above facts the explanation offered by MacCallum, 97 (p. 620), seems
plausible. This author suggests that the contractile elements in the
early embryonic heart would be represented by the irregular network
seen at that stage before true fibrils exist. This, it seems, would lend
support to the writer’s suggestion in regard to the origin of the fibrils.
For if, as MacCallum says, the cytoplasmic network represents the con-
tractile element in the early stages, it seems reasonable at least to con-
sider the longitudinal lines of this network as developing into the fibrils
rather than to suppose the latter to arise from accumulations of the
network-substance in the cytoplasm contained between the meshes.

In conclusion, the results of the work embodied in this paper point
to the existence of a definite relationship between the cytoplasmic reti-
culum of the early embryonic cell and the fibril bundles of the adult cell.

Harry Lewis Wieman 203

LITERATURE CITED.

ENGELMANN, TH. W., 73-81.—Mikroskopische Untersuchungen tiber die quer-

gestreiften Muskelsubstanz. I u. II Pfliigers Arch., Bd. VII, 1873.

Kontraktilitat und Doppelbrechung. Pfitigers Arch., Bd. XI.

Neue Untersuchungen iiber die mikroskopischen Vorgange bei der

Muskelkontraktion. Pfliigers Arch., Bd. XVIII, 1878.

—— Mikrometrische Untersuchungen an _ kontrahierten Muskelfasern.

Pfitigers Arch., Bd. 26, 1881.

Bemerkungen zu einen Aufsatz von Fr. Merkel: Ueber die kontraktion

der quergestreiften Muskelfaser. Pfitigers Arch., Bd. 26, 1881.

Ueber den faserigen Bau der kontraktilen Substanz mit besonderer

Beriticksichtigung der glatten und doppelt schrag gestreiften Muskel-

fasern. Pfliigers Arch., Bd. 25, 1881.

EYCLESHYMER, A. C., o4.—The Cytoplasmic and Nuclear Changes in the
Striated Muscle Cell of Necturus. Am. Journ. Anat., Vol. III, No. 3.

GopLEWSKI, E., oz—Die Entwickelung des Skelet- und Hertzmuskelgewebe
der Sdugethiere. Arch. f. mikr. Anat., Bonn, 1902, Bd. LX.

KOELLIKER, 02.—A. Koelliker’s Handbuch der Gewebelehre des Menchen. 6.
Anlage, p. 609, Leipz., 1902.

Koossow, A., 92.—Ueber eine neue Methode der Bearbeitung der Gewebe mit
Osmiumsdure. Zeitschr. f. wissenschaftl. Mikroskopie Bd. 9, 1892, p.
38.

MacCatLum, J. B., 97-—On the Histology and Histogenesis of the Heart

Muscle Cell. Anat. Anz., Jena, 1897, Bd. XIII, S. 609-620.

98-—On the Histogenesis of the Striated Muscle Fiber and the Growth

of the Human Sartorius Muscle. Johns Hopkins Hosp. Bull., Balto.,

1898, Vol. IX, pp. 208-215.

RANVIER, 89.—Traité technique d’histologie. Paris, 1889.

EXPLANATION OF FIGURES 1 TO 17.

ABBREVIATIONS USED.

a= Heavily staining deposits at the nodal points of cyto-reticulum of early
embryonic cells.

6 = Karyosome masses.

c= Transverse threads of cyto-reticulum.

d=Nucleus.

bl1= Lightly staining bands on the fibril bundles. ~

sd =Small sarcoplasmic discs of MacCallum, 97.

sb = Sarcoplasmiec disc.

sa = Part of a sarcoplasmic disc seen in longitudinal section.

bn = Cyto-reticulum of the interior of the adult cell.

bz= Points of intersection of longitudinal threads with transverse threads,
not marked by heavily staining deposits.

7a= Enlarged cytoplasmic area.

14

204 The Fibrils of the Heart Muscle Cell of the Chick

MIGss Tome:
The accompanying figures are camera lucida drawings made at table level.
In all cases the magnification is about 2000 diameters.

Fie. 1. Longitudinal section of heart muscle cell from a 30-hour embryo.
The cytoplasm shows an irregular network.

Fig. 2. Cross section at 30-hour stage.

Fig. 8. Longitudinal section, 72-hour stage.

Fig. 4. Cross section, 72-hour stage.

Fic. 5. Longitudinal section, 96-hour stage.

Fics 6 and 7. Cross sections, 96-hour stage; Fig. 6, above or below the
nucleus; Fig. 7, through the region of the nucleus.

Fig. 8. Longitudinal section, 120-130-hour stage.

205

Harry Lewis Wieman

eS PASE RSS ¢

Se
RE a :
F. AAS ISS REE
SERS SEG
Beri wen Gord
ae eee 3

Kies. 9 ro 17.
The accompanying figures are camera lucida drawings made at table level
In all cases the magnification is about 2000 diameters,

Fig. 9. Longitudinal section, 120-130-hour stage.

Fie. 10. Cross section, 120-130-hour stage.
Fias. 11 and 12. Longitudinal sections, 130-140-hour stage.

Fics. 13 and 14. Cross sections, 130-140-hour stage.
Longitudinal section of two adjacent fibril bundles of adult tissue.

Hie. 15;
Fic. 16. Longitudinal section through the reticular structure found in the

center of the adult cell.
Fig. 17. Cross section of the adult heart muscle cell.

A STUDY OF THE STRUCTURE OF THE GASTRIC GLANDS
OF THE DOG AND OF THE CHANGES WHICH THEY
UNDERGO AFTER GASTROENTEROSTOMY AND OCCLU-
SION OF THE PYLORUS.

BY

BASIL C. H. HARVEY,
Hull Laboratory of Anatomy, University of Chicago.

WITH 5 FIGURES.

New methods of fixation and staining introduced by Bensley in his
work on the alimentary tract, have demonstrated new facts in the minute
anatomy of this region. ‘They have been especially valuable in their ap-
plication to the study of the cellular elements of the various digestive
glands, and he has already apphed them to the study of the stomach in
many animals. His work was confined, however, to normal anatomy,
and by this investigation of the structure of the mucous membrane when
placed under abnormal conditions, and subjected to the action of external
influences different from those normally acting upon it, it was hoped,
with the assistance of these improved methods, to learn something which
might still further advance our knowledge of the nature and relations
of its highly differentiated cells.

TECHNIQUE.

The methods of investigation adopted were as follows. The dogs were
killed by illuminating gas at various times after operation, always when
they were in the fasting condition, at least ten hours after feeding. The
stomach was immediately opened and its condition noted. Strips of
mucosa at the sites selected for study were then removed. Parallel
incisions 1 cm. apart were made with a razor through the tunica mucosa,
so as to isolate strips 1 em. wide and 2 em. long, including 1 em. of
gastric and 1 cm. of duodenal mucous membrane. These were then
dissected free from the muscular coat with razor and scissors, and laid
down in a drop of fixing fluid on a piece of sheet cork with the free
surface next the cork. It was pinned in place by porcupine quills, and

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

208 Gastric Glands of Dog after Gastroenterostomy

a little fixing fluid applied to the muscularis mucose by a pipette. After
two or three minutes the pins were removed and the strip dropped into
a bottle of the fixative. This method prevents the curling of the strip
due to the contraction of the muscularis mucosx, which otherwise disturbs
the relations of the glands.

Small pieces of mucous membrane were removed at the time of the
operations and fixed in various fluids. ‘These served as controls with
which glands modified as the result of the operations were compared
later, and also as material in which the normal, healthy structure of the
mucous membrane was studied.

Experiments were made with very many fixing fluids and it was
found that the one which gave the best all-around results was made of
formalin, 3 per cent aqueous solution of potassium bichromate, saturated
aqueous solution of mercuric chloride, and water, in equal parts. Strips
of mucosa were left for two hours in this fluid, then washed and
dehydrated. It is difficult to fix both the zymogen granules and the
cytoplasm in the cells of a dog’s gastric glands. The zymogen granules
are more difficult to fix than in cats or rabbits, and are not fixed at all
in Zenker’s fluid, and very poorly in Bensley’s fluid or Bouin’s. Kopsch’s
fluid fixes the granules well and the cytoplasm poorly. Bensley’s and
Bouin’s fluids fix the cytoplasm well but the zymogen granules poorly.
The above solution gave very good results for both.

Paraffin sections three or four micra thick were cut, and fixed to the
slide by the water method. The following stains were found most in-
structive. Neutral gentian as used by Bensley, 00, gives a very definite
blue stain of the zymogen granules. In material fixed in Bensley’s fiuid
it does not stain the prozymogen, but in formalin fixations I have
usually found it to stain the prozymogen purple. A mixture of equal
parts of saturated aqueous solutions of acid fuchsin and orange G stains
the granules of the parietal cells a definite pink in a minute or two,
and if it be followed by a saturated aqueous solution of toluidine blue
for one or two minutes, there is produced in addition a very pretty
metachromatic pink tint in the surface mucus and in the thece of the
cells of the surface and stomach pits, and also a deep blue color of the
zymogen granules and the prozymogen. Mayer’s muchzematein and muci-
carmine were also used to stain mucus. The former gave the more
constant and the latter the more beautiful results. The modifications
of its preparation suggested by Rawitz, 99, who evaporated the mixture
of carmine, aluminum chloride and water to dryness at a low tempera-
ture before dissolving in alcohol, and by Bensley, who used Mayer’s stock

Basil C. H. Harvey 209

solutions instead of the diluted forms which Mayer employed, were used
with advantage. The following copper-chrome-hematoxylin stain per-
sonally communicated to me by Professor Bensley and reported here for
the first time, has been very convenient and valuable. Sections are
placed for one minute each in a saturated solution of neutral copper
acetate, a 3 per cent aqueous solution of potassium bichromate, and a
saturated aqueous solution of hematoxylin crystals, being washed in tap
water after each. This round is once repeated and then the stain is
differentiated in Weigert’s borax-ferricyanide. It stains the granules
of the parietal cells black and this stain is very persistent, not being
quickly removed by the borax-ferricyanide. It stains the chromatin
black. From the latter, however, the stain quickly disappears in the
differentiating solution, leaving the parietal cell granules clearly differen-
tiated (Fig. 5). This method was very satisfactory for the detection
and study of parietal cells. Paracarmine and iron-hematoxylin were
also used as nuclear dyes, and very satisfactory preparations were
obtained by following them with muchematein and mucicarmine re-
spectively.

NorMAt ANATOMY.

As has been shown by various investigators from Cobelli, 66, on, the
entire stomach exclusive of the pars cesophagea may be divided through-
out the mammalia into three parts, cardiac, fundus, and pyloric, in
accordance with differences in character of the tunica mucosa.

I. The cardiac region is occupied by glands which are fully described
by Edelmann, 89, Bensley, 02, Haane, 05, and others, and which are
not especially concerned in this investigation.

Il. The fundus region is occupied by mucous membrane which is
chocolate pink in color and is composed of glands opening into a cylindri-
cal pit, which is merely a depression of the surface extending through
one-fifth to one-fourth of the thickness of the mucous membrane. The
epithelium lining it passes by a very gradual transition into that of the
general gastric surface on the one side and into that of the gland necks
on the other. Its cells are cylindrical, containing a round or oval nucleus
in the attached half, and at the free end a theca containing a rounded
mass of mucous secretion which becomes thrown out during digestion.
Bensley, 98, has described a second globule of mucus near the nucleus in
the cat, and Holmgren, 02, and others, describe as trophospongium,
structures appearing in these cells between the nucleus and the free end.
The cytoplasm in fixed specimens presents a fibrillar element and an
interfibrillar substance.

210 Gastric Glands of Dog after Gastroenterostomy

The glands proper are made up of two parts, the necks opening into
the foveole and the bodies opening into the necks. The neck occupies
about two-thirds of the length of glands near the lesser curvature and
one-third of those near the greater curvature. There are very considerable
individual variations in the relative lengths of these two portions, the
variations being due mainly to differences in length of the body. Both
parts are made up of parietal cells and chief cells.

(a) Parietal cells. These are relatively more numerous at the foveolar
end of the neck, and they become relatively less numerous toward the
muscularis mucose. They have ordinarily a diameter two or three times
as great as that of neighboring chief cells. They are separated from
the lumen by a layer of chief cells, between which lie the intercellular
ducts, putting them in communication with it, although they occasionally
extend to the lumen themselves, especially in the neck region. Their
outline in sections is usually round or oval. The nucleus is central
and is often double or even triple, and there is evidence of direct division
of these nuclei without corresponding division of the cytoplasm. (Cade,
or.) This author observed near the nuclei, bodies which he regards
as debris of broken-down nuclei which have been replaced by new ones
arising by direct nuclear division. The cytoplasm contains many fine
granules, staining pink in the fuchsin-orange G mixture, dark brown or
black in copper-chrome-hematoxylin (Fig. 5), and remaining unstained
by neutral gentian or toluidine blue. Some cells are packed with them.
Vacuoles, which Sachs, 87, regarded as indications of pathological proc-
esses, and which Schmaus and Albrecht, 95, regard as indicative of
degeneration, appear in many of my preparations of the apparently
healthy resting stomach. A granule free zone, more or less closely sur-
rounding the nucleus and described by Zimmerman, 98, Kolossow, 98,
and others, is occasionally present. It, as well as the vacuoles, may be in
part due to the presence of the system of intracellular canals demon-
strated by Eric Miiller, 92, Golgi, 93, and Langendorf and Laserstein,
94, communicating with intercellular ducts or directly with the gland
lumen, and appearing in various parts of the cell section as granule free
areas.

Spirilla staining purple with neutral gentian, and black with
copper-chrome-hematoxylin, described first by Bizzorzero, 93, appear
constantly in the gland lumina and are closely associated with the parietal
cells. According to Theohari and Babes, 05, they were not to be found
actually within these cells in dogs till after treatment with their gastro-
toxic serum. Bizzozero found them, however, in the healthy stomach and

Basil C. H. Harvey 211

I have been able to confirm his observations, seeing them frequently in
parietal cells, occupying the intracellular canal system, or contained
in vacuoles, or sometimes, also, apparently in the cytoplasm itself, al-
though in the latter case it must be borne in mind that they may be in
extremely small ramifications of the intracellular canals. They do not
appear to have any association whatever with the chief cells.

A few parietal cells appear in all my preparations of material fixed
in aqueous bichromate solutions, which take a yellowish brown or yel-
lowish green color with this fixative and take no further stain whatever
with acid fuchsin, orange G, Ehrlich’s triacid mixture, eosin, mucicar-
mine, muchematein, neutral gentian, or paracarmine. With copper-
chrome-hematoxylin they stain like other parietal cells but give up the
stain very readily, so that in sections properly differentiated they are
unstained with this method also. They are unaffected by hydrochloric
acid or ferricyanide of potassium even after prolonged exposure, and
they do not give the iron reaction of MacCallum. They do not blacken
in osmic acid nor take any stain with Sudan III. In material fixed in
solutions which do not contain salts of chromic acid they do not appear,
the parietal cells being nearly uniform in color and staining reactions.
Occasionally they present the full and rounded outline common to
parietal cells, but many are irregular in shape and many are shrunken.
Their nuclei are spherical and contain a normal amount of chromatin
staining in hematoxylin. The karyoplasm is yellowish green. The
cytoplasm contains fine granules which stain in hematoxylin but give up
the stain much more readily than the granules of other parietal cells.
They retain it, however, more firmly than the cytoplasm about them, so
that in sections stained with this method there sometimes appear cells
in which the cytoplasm is yellowish green while the granules contained
are black. Cells somewhat similar to these were described by Popoff, 98,
in material taken from stomachs in inflammatory conditions and fixed in
Flemming’s fluid. He reports similar staining reactions, but found in
the cells karyokinetic figures and net formations which I was quite un-
able to find in my preparations, although I employed the methods which
he recommends. These peculiar parietal cells resemble very closely the
chromaffine cells described by Kohn, oz, in the suprarenal gland, the
glomus caroticum and in various parts of the sympathetic nervous system.
Their staining reactions are similar, the most important one, namely,
the selective affinity for salts of chromic acid, being very distinct in
my preparations, so that the parietal cells are separated into two classes
by it. The granules of these cells in the stomach seem to possess

212 Gastric Glands of Dog after Gastroenterostomy

somewhat less susceptibility of staining with basic dyes than those he
describes, and they are not, as in his cells, the only part of the cell
structure possessed of the affinity for chrome salts, as is clearly shown in
those of my preparations in which the granules are slightly stained by
hematoxylin while the rest of the cytoplasm is yellowish green. In the
last particular they agree with Rabl’s, g1, description of the chromaffine
cells of the suprarenal gland.

Mitotic divisions are extremely rare among the parietal cells. Tortora,
99, reports their presence in the neck region. Popoff, 97, describes them
as occurring in inflammatory conditions. I was not able to find them in
any of my preparations.

The parietal cells are sometimes invaded by lymphocytes, which may
then appear in the interior of them, as has been reported by Sewall, 79,
Hamburger, 89, Bonnet, 93, and others. I have frequently seen them in-
vaded by mast cells, both in healthy glands and in those modified as a
result of operations. Stinzing, 99, reports similar observations of mast
cells, which were especially abundant twelve hours after ingestion of
food.

(b). Neck Chief Cells. These have been well described by Bensley,
98, as forming a layer of pyramidal cells immediately surrounding the
lumen with their large end lying next it. The nuclei lying near the
attached end are oval in cells which have thrown out their secretion,
and flattened in those yet full of it. The cytoplasm consists of a tra-
becular framework containing in its interstices the cell secretion. This
is poured out during several hours after ingestion of food, but accu-
mulates during rest near the free end of the cell, where it forms definite
spherules in preparations of material fixed in alcoholic solutions, but is
precipitated as an irregular mass upon the trabecular framework in
material fixed in watery solutions. In whatever form it is present, he
has shown that it stains differentially with mucin dyes, pink with muci-
carmine, blue with muchematein, blue in Ehrlich’s indulin-aurantia-eosin
blood-staining fluid, and takes a metachromatic pink tint in aqueous
solutions of toluidine blue. For these reasons he regards it as mucus.
It differs, however, in its reaction to mucus stains, from the mucus in
the foveolar cells and on the surface. While occasionally mucus in both
regions is stained in the same section, usually when one region is well-
stained, the mucus in the other is faintly or not at all stained. Bensley
explains this difference on the basis of the multiplicity of the chemical
constitution of mucins, which is described by Mayer, 97, and others. The
neck chief cells pass by a gradual transition into those of the foveolar

Basil C. H. Harvey 213

epithelium, the intermediate zone presenting frequently cells undergoing
mitotic division, and it is probable, as Bizzozero pointed out, that the
cells of both these regions arise in this zone, some new cells forming
foveolar epithelium, and some passing into the gland to become neck
chief cells.

(c). Body chief cells. These are similar in shape to the neck chief
cells though somewhat larger. The spherical nucleus near the attached
end is never flattened by accumulated secretion. The cytoplasm con-
tains a fibrillar reticulum presenting a strong affinity for basic stains
due, as Bensley has shown, to the presence in it of a substance particularly
concentrated in the attached end of the cell, giving a strong reaction for
masked iron and corresponding in this respect to the prozymogen demon-
strated by MacCallum in the pancreatic and other glandular cells. It
often presents the appearance of bands extending toward the nucleus,
staining dark brown or black with copper-chrome-hematoxylin and in-
tensely blue with toluidine blue (basal filaments of Solger, 96, ergasto-
plasm of Garnier, 97, and Cade, or). At the free ends of these cells
are many coarse granules occupying meshes formed by the cytoplasmic tra-
becule. They stain blue with toluidine blue, as does also the cytoplasm, but
they are stained differentially an intense blue by neutral gentian. These
are the zymogen granules of Langley, 81.

In my preparations I was quite unable to find mitotic divisions of the
body chief cells, a result agreeing with that reported by Cade, or. Biz-
zozero, 93, reports that in young dogs he found them occasionally, but
extraordinarily seldom.

There is no gradual transition from neck chief cells to body chief
cells but an abrupt change, and cells possessing fully the characters of
ferment cells lie next to cells possessing fully the characters of mucous
cells. The two varieties were regarded by Bensley, 96, and Zimmermann,
98, as specifically distinct. No part of the body chief cells is stained
by mucus-staining dyes, and therefore it may be concluded that they
take no part in the formation of mucus. ‘They are specialized for the
formation of ferment. In this function there is no evidence to show
that the neck chief cells have any part unless the presence of a very faint
iron reaction in their bases may be interpreted as such. They are
specialized for the formation of mucus. Trinkler, 83, reports that he
saw what seemed to be transition stages, and Cade, o1, states that in the
zone where the body of the gland meets the neck he observed what he
thought might be transition forms, but says they were not sufficiently
distinct and definite to permit of a positive statement of such transition.

214 Gastric Glands of Dog after Gastroenterostomy

Bensley and Zimmermann did not find them, and my own observations
have been quite in accord with theirs. No chief cells appear in any of
my preparations made by the methods introduced by Bensley, which are
not either clearly mucous or clearly ferment cells.

While the body chief cells are almost exclusively ferment cells, there
exist among them occasional though rare mucous cells. Somewhat similar
cells described by Trinkler, 83, were interpreted by him as transition
forms between parietal and chief cells, but as Bensley has pointed out, the
selective action of mucicarmine demonstrates clearly their true nature.

III. The pyloric region is occupied by mucous membrane, which is
whitish yellow in color. It extended in several dogs to a distance of 6 to
8 cm. from the pyloric valve, the extent being usually about 1 cm.
greater along the major curvature than along the minor. ‘These measure-
ments agree with those reported by Deimler, 04, for the same animal.
When long strips of mucous membrane extending from the pyloric valve
into the fundus region are removed from the stomachs of dogs and
flattened out, the distance from the pyloric valve to undoubted fundus
mucosa is from 10 to12 cm. An intermediate zone 2 to 3 cm. wide exists
between the two regions. In the pyloric region the foveole are wider
than in the fundus region and each extends through from one-third
to two-thirds of the thickness of the mucous membrane and each, as has
been shown by the reconstructions of DeWitt, receives the openings of
several branched glands. These have a larger lumen than the fundus
glands, and are lined by cells of one kind only which resemble
accurately the neck chief cells. Like them they secrete mucus accu-
mulating during rest in spherules in the free end of the cell, flattening
the nucleus against the attached end and being poured out after inges-
tion of food. It occupies the meshes of a delicate cytoplasmic framework.
There appear among these cells, as Hamburger, 89, has pointed out, the
small pointed cells of Stohr which Bensley found also among the neck
chief cells of the fundus region. Stéhr, 82, describes also the occasional
presence of parietal cells in the pyloric glands of man. I did not find
them in my preparations of the dog’s stomach.

In the intermediate zone, as one studies parts successively nearer the
pylorus, the parietal and ferment cells gradually become fewer and
disappear. The disappearance of the two kinds of cells seems to pro-
ceed pari passu, and I did not find the parietal cells extending more
than one or two glands beyond those in which ferment cells are to be
found. Across this zone the gland bodies become progressively shorter

Basil C. H. Harvey 215

till they disappear, while the length of the gland necks is very little
altered. From a study of the intermediate zone it appears that the
pyloric glands correspond to the neck region of the fundus gland without
the parietal cells.

Capr’s EXPERIMENTS AND OBSERVATIONS.

In 1901, in a paper dealing with the normal anatomy of the gastric
mucous membrane, and with the changes which it undergoes in various
physiological conditions and after its subjection to various operative
procedures, Cade gives an account of the alterations in its structure in a
dog and a cat near the site of gastroenterosomies which he had performed.
His observations were made on only one animal of each species and
the changes were studied between six and seven months after the opera-
tion. In the dog he effected a gastrojejunostomy connecting with the
stomach in the fundus region, by which part of the food might pass
into the intestine, but since the normal pylorus remained open, part
of the food might pass by the normal way. In the cat he made a
similar anastomosis, and in addition divided the mucous membrane in
the middle of the stomach, making two sacs, one proximal communicating
with the cesophagus and jejunum, and the other distal communicating
by way of the pylorus with the duodenum but never containing any food.
The animals were killed after six and a half months and the changes
at the site of the gastroenterostomies studied.

He found the foveole at the wound margin enlarged and deepened,
due to the absorption into them of parts of the gland necks. In their
deeper parts were seen occasionally cyst-like dilatations, the walls of
which were made up of cells shorter and flatter than those normally
constituting this epithelium. Otherwise they were unchanged.

The glands were sinuous in outline and their lumina irregularly en-
larged. The gland cells were of one kind only and resembled closely
the neck chief cells of normal glands. They were cylindrical or cubical
in outline, with nuclei poor in chromatin and often flattened against
the attached ends of the cells. The cytoplasm was formed of a network,
with fine meshes and stained a light pink tint with carbol-thionin, similar
to, but fainter than that which he observed with this dye in pyloric
gland cells and other undoubtedly mucous cells. They did not, how-
ever, take the metachromatic pink tint with toluidine blue which he
obtained in clearly mucous cells. They did not take any stain at all in
muchematein, mucicarmine, or indulin, but did not differ in this respect
from normal neck chief cells or pyloric gland cells, for Cade did not

216 Gastric Glands of Dog after Gastroenterostomy

obtain satisfactory results with mucous staining dyes in any of these
cells. 'Toluidine blue failed completely to demonstrate the presence of
prozymogen or zymogen granules.

Notwithstanding the failure of these cells to take any stain with
mucous staining dyes, he considers the glands so modified to be muci-
parous in function and similar to normal pyloric glands, basing his
conclusion upon their general appearance with large lumina and sinuous
outlines, upon their connection with large foveole, upon similarities of
structure of their cells, and upon the absence from them of zymogen
or prozymogen. He finds such modifications only in the immediate
vicinity of the anastomosis. At some distance from it the mucous
membrane is unaltered and preserves all the characters of normal fundus
mucosa. Between this normal mucosa and the line of operation is a
transition zone in which as the area examined is progressively farther
from the new pylorus, the following changes were observed: ‘The lumina
become successively narrower and more regular; parietal cells appear,
at first pale and poor in granules, gradually becoming more granular and
richer in nuclear chromatin. Ferment cells appear in the ends of the
glands with zymogen staining in toluidine blue and prozymogen staining
in Bismarck brown, constituting a steadily increasing proportion of the
chief cells of the gland body until the structure normally present in the
fundus region is attained.

His observations may be summarized in the statement that at the
site of a gastroenterostomy made in the fundus region, there is formed
a new pylorus which is similar to the normal pylorus in the general form
of the glands present, in their relation to the foveolz and in the character
of the glandular elements. Upon this observation he bases the con-
clusion that these structures possess a morphological flexibility and an
ability to transform themselves in response to the influence of new and
altered conditions of existence and to assume the performance of new
functions quite different from those which they usually perform.

DISCUSSION OF CADE’S WorK.

These observations and conclusions have bearings upon several im-
portant theories, first, that of the specificity of cells. They indicate
that for these cells, at any rate, specificity in the sense in which Bard, 98,
uses the term, does not exist. The second is that of the cause of differ-
entiation of the pyloric mucous membrane. If it can be produced at
will by the surgeon from fundus mucosa by subjecting it to the influence
of mechanical conditions which he can at any time induce, a strong

Basil C. H. Harvey 217

suggestion is afforded that somewhat similar mechanical influences,
naturally produced, may account for the ontogenetic production of this
special form of mucosa. Quite in accord with this suggestion is that
offered by Bensley, 02, as explanation of the differentiation of the cardiac
mucous membrane. He thinks it may be effected by the action of me-
chanical forces naturally produced, operating through many successive
generations and associated with natural selection.

Cade’s work, while extremely important and significant, seemed to
leave unanswered several questions which naturally arise in connection
with it. First: Are these changes which he observed constant and
permanent? He observed them in a single dog and a single cat, and at
only one stage after operation, namely, that of six and a half months.
He believed they were permanent, but lamented the lack of material in
which to study later stages. He was quite sure, however, that at such
stages the changes would be yet more marked than in those which he
studied. Second: What becomes of the parietal cells which have dis-
appeared? Do they become disintegrated or transformed into other
cells? No suggestion of an answer to these questions was offered.
Third: What becomes of the ferment cells which disappear? Are they
disintegrated or transformed? He thought they were transformed into
the mucous cells which replaced them, but no evidence beyond the fact
of replacement is adduced. Fourth: Whence come the new mucous
cells which occupy their places? He believed they arose by transforma-
tion of the ferment cells, but an equally legitimate conclusion, as far
as his experiment could explain the question, would be that they arose
by the division of previously existing mucous cells found constantly,
though in small numbers, in the gland bodies. It is true that evidences
of division in these cells are reported as being exceedingly rare by Biz-
zozero, 93, and that none were actually found in Cade’s preparations,
but it seems quite possible that the stimulus of the operation and of
the altered conditions produced by the operation might perhaps have
caused their production in extraordinary numbers soon after it took
place and before six and a half months had elapsed there might have
arisen in this way a sufficient number of new cells to form a complete
epithelial lining for the gland bodies and the process may thereupon
have ceased. It is also possible that the mucous cells of similar character
in the neck region, which are seen frequently undergoing mitotic division,
might have produced many new cells which moved down into the gland
bodies and replaced the ferment cells which were disappearing. His
assumption that the new mucous cells arise by transformation of zymo-

218 Gastric Glands of Dog after Gastroenterostomy

genic cells does not therefore seem necessary to account for the phe-
nomena he observed, and it would require for its establishment the
study of stages other than that of six and a half months after operation.
Fifth: Do changes of a reverse nature go on in the pyloric region of the
stomach when the pylorus is occluded? In the light of Cade’s conclusion,
one might expect such changes to occur, when the mucous membrane of
this region is placed in a position where it is made part of the fundus
and no longer performs its usual functions. This can be effected by
artificial occlusion of the normal pylorus, and the establishment
of a new one by gastroenterostomy in another part of the stomach.
The development in the pyloric glands under these conditions of changes,
which would tend to make them resemble more closely the fundus gland,
would seem the more probable, since occasional fundus elements have
been found in the pyloric mucosa. Stohr, 82, found parietal cells in
human pyloric glands. They have been observed by Cade, o1, and
Renaut (reported by Cade) in human pylori partially occluded by
carcinoma. In man Bensley, 03, has found them beyond the pylorus in
the glands of Brunner.

It was obvious that these questions could be answered only by the
study of postoperative changes in both regions of the stomach at many
stages, and in view of the importance and significance of the conclusion
reached, and the interesting nature of many of the questions arising,
it seemed worth while to extend experiments along this line and to
study with the newer technical methods these changes at many stages.

PERSONAL EXPERIMENTS.

I performed gastroenterostomies on a large numberof dogs, and in
those which were to be kept one month or longer I occluded the pylorus
also. At the time of operating, several small pieces of mucosa were
removed from the area involved and fixed in various solutions, with a
view to using them as controls with which the mucous membrane modified
as a result of the experiments might be compared Jater. They served
also for the study of the structure of the normal, healthy, gastric mucous
membrane. The gastroenterostomies were performed on the anterior
surface of the stomach by Woelfler’s* method of suture only, and there
was little trouble in getting satisfactory results. One or two made on
the posterior surface by Von Hacker’s* method were equally satisfactory.
Operations on the anterior surface in the dog produce a new pylorus in
a part of the stomach which is dependent when the dog is on his feet,

‘Described in Bryant’s Operative Surgery, o5.

Basil C. H. Harvey 219

thus facilitating the passage of food into the duodenum. The anas-
tomosis was 4 to 5 cm. long and was distant from the pylorus at least
12 cm. on the stomach, and 20 em. on the duodenum. It is advisable that
the gastric opening be distant at least 10 em. from the normal pylorus,
in a dog of medium size, in order that it may be undoubtedly in the
fundus region. In one animal one end of the anastomosis was inten-
tionally disposed in the pyloric region about 6 cm. from the pyloric
valve.

More trouble was experienced in occluding the pylorus. After hgature
with heavy silk its lumen was re-established in a month. Then the
pylorus was plicated, so as to produce a longitudinal fold on its ventral
surface. ‘The edges of this fold were united by a few fine silk sutures,
and two ligatures of heavy silk were tied around the whole about 1 em.
apart. After 10 months the lumen was partially established. The
following method finally adopted was always successful. A longitudinal
incision 2 to 3 cm. Jong was made on the ventral and another on the
dorsal surface of the pylorus. They extended through the serous and
muscular coats. ‘The mucous tube was then freed and cut across. The
ends were closed with fine silk and pushed into the stomach and duo-
denum. The edges of the wound were then pulled out laterally so that,
while originally longitudinal, it became transverse. The edge in front
of the stomach was then sutured firmly with heavy silk to that behind
it, and the same procedure followed with the anterior and posterior
duodenal edges. It is necessary that these sutures be made with heavy
silk and that a large piece of tissue be included, because of the powerful
contraction of the m. sphincter pylori. By this method the continuity
of the vascular and nervous structures at the margins of the pylorus are
preserved.

The dogs were killed at stages after the operation of 2, 4, 7, 9, 12, 14,
and 15 days, and of 1, 2, 3, 4, 514, 6144, and 10 months. The stomach
was opened and the condition of the anastomosis and pylorus noted.
The area of the anastomosis selected for study was that farthest removed
from the pylorus in order to be sure to avoid the intermediate region.
In one case which will be especially described later, material was taken
from the pyloric end of the anastomosis also. This was the case in which
one end of the anastomosis was actually in the pyloric region. For
the study of the results following occlusion of the pylorus the material
was selected from the center of the pyloric area. The changes occurring
in the duodenum were not especially studied. Search was made, how-

15

220 Gastric Glands of Dog after Gastroenterostomy

ever, in nine or ten cases for ulcers in the part of the duodenum proximal-
ward from the anastomosis, where they have been reported after similar
operations on the human subject. They did not appear in my dogs.

RESULTS.

1. Degeneration.—Examination of sections shows that immediately
after the operation there is degeneration of the glands which are struck
by the knife or included in a suture. This process is limited to a very
few glands which lie next the line of operation. Not more than three
or four are affected. Sometimes the glandular elements undergo degen-
eration in situ but frequently are cast off, some of them preserving the
structure characterizing them in the healthy mucous membrane, but
most of them showing evidence of degeneration. In preparations of two
and four day stages there are constantly found in immediate proximity
to the lines of incision, masses of débris which give evidence of their
derivation from all forms of glandular elements. They occupy the
lumina of glands or le free on the surface of the mucosa. Occasionally
they contain intact cells retaining their nuclei and cytoplasmic structure,
but these are not numerous and for the most part the cell contents have
no enclosing membrane or definite outline, and those of one cell are not
marked off in any way from those of neighboring cells, but all form a
common mass in which elements of various cells are found distributed,
retaining their power of acting specifically with stains. Zymogen
granules, usually extremely coarse, and resulting perhaps from the con-
fluence of smaller ones, appear in neutral gentian preparations in con-
siderable numbers, and copper-chrome-hematoxylin preparations show
many characteristic parietal cell granules, while in many places the sub-
stance between the granules shows a positive reaction with muci-carmine.

The body chief cells are the first of the glandular elements to show
evidence of degeneration. Their nuclei become swollen and pale, and
poor in chromatic material, each containing usually a single small mass
of chromatin, although a few nuclei contain several small chromatic
bodies. In a few the nuclear membrane is broken down and the karyo-
plasm distributed throughout the cell. The cytoplasm usually contains a
few large zymogen granules but no basal filaments or organized prozy-
mogen. As Garnier, 97, has pointed out, prozymogen is probably com-
plex and unstable, and in cells injured by the operation, whatever of it is
present at the time is probably at once transformed into zymogen gran-
ules and no more is produced. The cytoplasm of most cells contains also
many vacuoles, and in all cells it is reduced in amount.

The parietal cells are more resistant than the chief cells and con-

Basil C. H. Harvey 221

sequently appear relatively more numerous in areas of degeneration than
in the healthy mucous membrane. This relative stability of the parietal
cells and their resistance to destructive influences has been noted in
the human subject by many observers. Béckelman, 02, Hammarschlag,
Koreynsky and Jaworsky, 02, and Bouveret, 93, mention it in gastric
ulcer. Popoff, 97, states that it is every evident in inflammatory con-
ditions of the gastric mucous membrane in the dog. Their stability
was very evident in my preparations, as parts of the two or three degen-
erating glands were composed almost exclusively of parietal cells, all
the other glandular elements having degenerated and disappeared. Many
of the parietal cells remaining retained a full and definite outline and ap-
peared little affected by degeneration. Many others of them, however,
lost their regularity of outline, becoming oblong, square, or pyriform
in sections and presenting changes in both the nucleus and cytoplasm.
The former, while partaking sometimes in the irregularity of the cell
outline, are usually spherical, being pale and swollen and containing
two or three small masses of chromatin. Occasionally the karyoplasm
takes a diffuse and homogeneous nuclear stain, showing that the nucleus
is undergoing degeneration by karyolysis. In nearly all of them, how-
ever, the nuclei degenerate by karyorhexis, although the pyknotic stage
which is described by Schmaus and Albrecht, 95, as developing at the
termination of this process and in which the chromatic material is
massed into a ball, does not appear. The earlier stages, however, were
common in which the chromatin disappears from the interior of the
nucleus and hes upon the nuclear membrane, which later becomes broken
down and the nuclear material distributed throughout the cell. The
cytoplasm commences to degenerate earlier than the nuclei. It becomes
reduced in amount and its granules become fewer and paler. Commonly
a circular area appears about the nucleus, from which the granules have
either disappeared completely or become unstainable. This area appears
absolutely empty and often it extends throughout the whole of the
cytoplasmic part of the cell.

A feature of these degenerative processes is the resistance of the
nuclei. Although this appears sometimes in the chief cells it is much
more marked in the parietal cells, where the nucleus usually persists
pale and swollen, but entire, after most of the cytoplasm has disappeared.
They often form masses in which the individual nuclei are separated
by only a very little cytoplasm which, retaining its distinctive character,
determines the nature of the cell. This same nuclear stability is reported
in the parietal cells of the human stomach in cases of ulcer, where
Bockelman, 02, found that sometimes the cytoplasm had so completely

222 Gastric Glands of Dog after Gastroenterostomy

disappeared. that it was very difficult to distinguish such parietal cells
from lymphocytes. The same difficulty has presented itself in my work.

Another feature is the infiltration of the interglandular connective
tissue by small round cells and the very great increase in the number
of red blood cells in the capillary vessels. The latter is probably due
to inflammatory venous stagnation. It is most marked in the region
of the gland necks and the deeper parts of the foveole, where the vessels
are always greatly distended and where there are sometimes extravasa-
tions.

The degenerative process goes on during a few days following the
operation. At stages of two and four days it is very marked, being
less evident in later stages until in fourteen day stages it hardly appears
at all. It is always limited to three or four glands at the most, lying
in immediate proximity to the line of incision and suture. The loss
of these glands by degeneration does not as a rule leave a gap in the
mucous membrane, because the redundancy of the gastric fundus mucosa
tends to keep the margin of the undegenerated portion opposed to unde-
generated duodenal mucosa, with which it becomes continuous.

2. Changes in gland lumina and foveole.—In the glands surrounding
the anastomosis, which are not injured by the knife or sutures, no acute
or extensive degenerative processes lke those described above go on.
There are, nevertheless, very considerable postoperative changes. The
foveole and gland lumina become dilated to form cystlike structures.
These appear at first within two or three days after the operation in
in the bottoms of the foveole only, but by the fourth day appear also
in the gland necks and bodies. They are formed first in the glands
next the line of operation and in later stages involve other glands as
well, being found progressively farther away from it up to two week
stages, when they exist throughout a zone 1 em. in width surrounding
the anastomosis and including over one hundred glands. In early stages
these dilatations appear as spherical or oval structures distended with
secretion. A little later they are often pear-shaped, and at two weeks’
stages they are usually more or less completely collapsed and confined
for the most part to gland bodies, appearing in every fifth or sixth gland
only. They usually form disc-like structures at the end of the gland
lying next the muscularis mucose, between it and the ends of other
glands not so enlarged. After two weeks they begin to disappear, first
from the glands farthest from the anastomosis, and by three months
they are only occasionally present. One very large closed cyst appeared
at the line of union in one of my six and a half months’ preparations.
It was approximately spherical, and extended throughout the thickness

Basil C. H. Harvey 223

of the mucosa. In nearly all preparations of late stages small cysts
appeared occasionally. In sections of material taken from the pyloric
end of the anastomosis, which extended into the pyloric region, and
studied five and a half months after operation dilatations in the gland
lumina appeared throughout the section. A section from the cardiac
end of the same anastomosis was clearly in the fundus region and
showed cystic dilatations in three glands only next the line of union of
gastric and intestinal mucous membrane. Similar cystic structures ap-
peared also in the duodenum. Such cystic dilatations appear constantly
in considerable numbers in the healthy mucous membrane of the cardiac
region (Schaffer, 97, and Bensley, 02), but in the fundus region they
seem to be associated always with subacute or chronic inflammation. In
my preparations they occur in a place where the mucous membrane has
been subjected by the operation to a very considerable irritation which
is perpetuated by the abnormal conditions introduced by the operation.
Such cystic dilatations appear in gastric ulcer (Krukenberg, 88) and
some dilatation of gland lumina has been reported in ulcer, carcinoma,
and chronic gastritis by nearly every pathologist who has written upon
these subjects. Occasionally they are filled with a homogeneous, or
sometimes (in alcohol hardened material), granular substance which,
because it stains in mucus staining dyes I consider to be mucus. Fre-
quently they are empty or contain only a very small quantity of mucus
as a layer upon their walls. Their primary formation probably depends
upon the increase in the secretion of mucus which goes on during most
inflammations of the stomach, and probably also on an associated increase
in its tenacity interfering with its rapid removal and producing accumu-
lations in these dilatations. The fact that some are empty may be ex-
plained by the digestion in situ of the mucus and its replacement by a
watery substance, which does not remain in the section. Cade, o1, points
out that by this dilatation of their lumini the glands approximate the
pyloric type, and makes of this point a confirmation of his conclusion
that the glands near the anastomosis are changing in nature and becoming
true pyloric glands, but since such a change occurs generally in gastric
inflammations, and since an inflammatory process exists in the glands
under consideration, that seems to afford a reasonable and _ sufficient
explanation of the dilatation.

In the cells of the foveolar wall there are no marked post-operative
changes. Where the foveole are greatly distended the cells become
shorter and may be cubical or even flattened with corresponding altera-
tions in the form of the nuclei which he in their attached ends. Other-
wise these cells retain their normal characteristics.

224 Gastric Glands of Dog after Gastroenterostomy

3. Changes in the parietal cells—The parietal cells, which Cade did
not find in the glands next the anastomosis, appear in all my preparations,
extending quite to the line of union. Up to stages two months after
operation they are sometimes, though not always, less numerous than
normally, in the two or three glands nearest the line of union. At
stages of three months and later they appear in nearly or quite normal
frequency in all the glands of the section, including those near the
anastomosis. But while their number does not appear materially altered,
some of them present changes after the operation, which may be attributed
Losses

In many glands there seems to be more variation than normally in
the size of these cells. Four months after operation most of the parietal
cells in glands near the anastomosis are smaller than in glands farther
away. This I thought due possibly to pressure atrophy, since in this
preparation the gastric mucosa was overlapped for a short distance by
the duodenal. As in many other cellular structures affected by inflam-
mation there is among the parietal cells a tendency to hyperplasia. Some
few become quite large, possessing a diameter two or three times as great
as that of ordinary parietal cells and provided with multiple nuclei,
three appearing sometimes in my preparations. They are rich in granules
taking the characteristic stain. They often contain vacuoles and fre-
quently a very large number of spirilla (Fig. 1). They are found
usually near the muscularis mucose and exist long after the operation,
being found in preparations of ten months’ stages. Such large parietal
cells are described by Bockelman, 02, in gastric ulcer in man attaining
sometimes a diameter of thirty micra and containing two to four nuclei.

The yellowish-green cells described above, which show an affinity for
chromic acid salts, appear in somewhat larger numbers than in the
normal mucosa, particularly during the week after operation, when
nearly every gland section contains three or four cells of this type.
Although most frequent in the gland bodies they are occasionally present
in the necks. ‘They are often more irregular in outline than the neigh-
boring parietal cells. Their nuclei are usually spherical and poor in
chromatin. In the two or three glands struck by the knife which undergo
degeneration they are not more frequent than in glands at some litlte
distance from the incision. But from their irregular shape and their
increased frequency in areas of subacute inflammation they would seem
to be degenerative forms.

In preparations six and a half months after operation I found in the
parietal cells of gland necks close to the anastomosis a very distinct,
cloudy area, near the nucleus (Fig. 4). Its outline was usually irregular

Basil C. H. Harvey 225

and it occupied from one-third to two-thirds, or even more, of the cyto-
plasmic area of the cell. It seemed nearly always closely associated with
the nucleus and was frequently applied about it in a crescentic manner.
It stained purple with neutral gentian in which stain it is very definitely
and distinctly marked, but was not clearly demonstrable by any other
method. Every parietal cell in the neck region contained it, but it was
present in only a few cells of the gland bodies. The nature of this sub-
stance is in doubt, but its appearance does not at all suggest that it is
constituted by the remains of broken down nuclei, which Cade describes
as existing in these cells.

In the walls of cystlike dilatations many parietal cells are found flat-
tened and extended, constituting part of the cyst wall and lying next the
lumen, between chief cells. They have flattened nuclei and are some-
what poorer in granules than normal cells, those present, however, show-
ing clearly the staining reactions of characteristic parietal cell granules.

In a dog in which a small stomach pouch had been isolated from the
fundus region and made to communicate with the surface by the Khigine-
Pawlow operation, Cade, 03, examined the glands at the surface opening
of the fistula at a stage six and a half months after the operation. He
reports that parietal cells, while absent from a few glands, were found
in most of them, and while he believed that those which he found were
“in the way of disappearing,” their presence in that situation seems to
be in accord with my findings in the neighborhood of a gastroenteric
anastomosis. He was not able to find them, however, in the neighborhood
of the gastroenteric anastomosis, which he made in a dog and examined
at the same stage after operation, a result which seems surprising since
if they exist in fundus glands at the surface of the body, where they
must be subjected to great irritation, it would seem unlikely that they
would disappear from an internal part of the stomach where the irrita-
tion must be much less. This result seems also difficult to understand
in view of the fact that I have found them constantly in my preparations.

4. Changes in the neck chief cells—The neck chief cells, like the
foveolar cells, are sometimes flattened by reason of the enlargement of
the lumina. A few of them contain vacuoles in their cytoplasm. Other-
wise they preserve their normal structure.

5. Changes in the body chief cells—The body chief cells show very
striking changes in the neighborhood of the anastomosis. Immediately
after the operation the cytoplasm becomes vacuolated. This was espec-
ially marked at four days after the operation, but in the stomach from
which my preparations of this stage were made the wound margins did

226 Gastric Glands of Dog after Gastroenterostomy

not appear healthy. In other stages soon after the operation vacuoliza-
tion, although present in the chief cells, was not nearly so well developed.

The ferment forming function of these cells is completely lost. Abso-
lutely no zymogen granules can be demonstrated in their cytoplasm
by the neutral gentian or any other stain which I have employed. They
assume, however, the appearance and properties of mucus forming cells,
and contain in their free ends masses of substance staining in muci-
carmine and muchematein. There is a transformation of ferment-form-
ing cells into mucus-forming cells. The change is a gradual one, com-
mencing within the first week at the line of anastomosis and extending
radially during the following two or three weeks into the mucous mem-
brane around it. It reaches its widest extension at some time between
two weeks and a month after the operation and at the end of two weeks
no zymogen granules whatever can be found within a zone 7 mm. wide
surrounding the line of union and including within it fifty or fifty-five
gland bodies to the radial section. In these glands the bodies are formed,
in addition to parietal cells, of chief cells containing mucus and differing
in no demonstrable respect from the neck chief cells (Fig. 2).

Outside this zone the chief cells of the gland bodies contain zymogen
granules, but in the cells of glands immediately outside it, lying at a
distance of about 7.5 mm. from the line of union, they are present in much
smaller numbers than normally appear. They gradually increase in num-
ber, however, as one studies glands successively farther removed from the
line of union. The first gland in which they appear contains often only
one or two ferment cells and these contain only a few granules in each.
Beyond this gland the number of ferment cells and granules steadily
increases until the body chief cells are packed with granules between
the nucleus and the free end and contain prozymogen between the
nucleus and the attached end. It must be borne in mind, however, that
in the normal mucous membrane there are found among the body chief
cells occasional mucous cells between the ferment cells.

At the end of one month after operation the ferment forming function
of the modified body chief cells farthest away from the line of union com-
mences to reassert itself and a few zymogen granules begin to appear in
them again. This process extends gradually toward the anastomosis
until, at six and a half months after the operation, the chief cells of the
gland bodies next in line of union contain zymogen granules in quite nor-
mal numbers and do not differ in any respect from the corresponding
cells in parts of the fundus region remote from the site of operation.
While this process is going on it may sometimes be observed that the fer-
ment cells nearest the line of union are somewhat distant from the other

Basil C. H. Harvey 227

ferment cells and appear as small islands in an area in which all the
other cells contain mucus. This occurred in my preparations of three
and four month stages.
The exact extent of this change at the various stages after the opera-
tion is shown in the following table:
At 7 days it extends through 6 glands, with very few ferment cells in the
next 40 glands.

At 9 days it extends through §&8 glands.
At 12 days it extends through 22 glands.
At 15 days it extends through 55 glands.
At 1 month it extends through 50 glands.
At 2 months it extends through 36 glands.

At 3 months it extends through 30 glands, except for two glands near the

anastomosis, which contain three or four ferment cells in the ends of
each.

At 4 months it extends through 18 glands.

At 5 months it extends through 4 glands.

At 6% months it extends through 3 glands. (In one case.)

At 6% months it extends through no glands. (In a second case.)
At 10 months it extends through no glands.

In the last two cases ferment cells were found in glands next to the
line of union.

This table shows the extent to which zymogen granules are entirely ab-
sent from body chief cells. This extent can be easily determined, since
there is no difficulty in detecting these granules in neutral gentian
preparations even when only one or two are present in the cell. The
intermediate zone immediately outside it, across which the number
of granules gradually increases till the normal is attained, is wide
enough to include ten to thirty glands to the radial section.

Just as in the loss of the zymogenic function the prozymogen dis-
appears before the zymogen granules, so in the reassumption by these
cells of their zymogenic function, the prozymogen appears in the basal
part of the cell before zymogen granules can be demonstrated in the
free end, and in advance of those glands in whose cells zymogen granules
appear are found glands whose cells, while containing in the free ends
a substance showing an affinity for mucus staining dyes, contain in their
attached ends a substance showing an affinity for toluidine blue and which
resembles prozymogen. Such cells occasionally form the walls of small
eysthike dilitations relatively remote from the line of union.

There is, therefore, during the first two or three weeks after the
operation, in the bodies of many glands immediately around the anasto-
mosis a gradual replacement of the ferment cells by mucous cells, extend-

228 Gastric Glands of Dog after Gastroenterostomy

ing peripheralward, and a reversal of this process during the following
five or six months, during which there is a replacement of these mucous
cells by ferment cells, extending centralward. The same phenomena
followed the suturing of simple incisions which I made through the
gastric mucosa in the fundus region. Throughout all stages of these
processes the increase in number of one kind of cell is exactly pro-
portional to the decrease in number of the other kind, a fact which
suggests strongly that the new mucous cells formed in the first month
and the new ferment cells formed in the following months arise each by
transformation of previously existing cells of the other variety. This is
confirmed by the fact already noted that when the process of ferment
formation in cells is extending toward the anastomosis during the four
or five months following the one immediately after the operation, the
most advanced ferment cells, that is those nearest the line of union,
contain only a very few zymogen granules. During the first month,
‘too, while the mucus forming process is extending into gland bodies
farther and farther from the anastomosis, the most advanced mucous
cells contain only a little mucus, which stains feebly, while those near
the line of union are almost full of mucus and take a very strong stain
with mucicarmine.

A consideration of the other possible modes of derivation of these cells,
namely, division of previously existing cells of the same kind, or trans-
formation of parietal cells, shows that they do not arise in either of
these ways. There are no indications that the new mucous cells which
are formed within a month after the operation arise by division of pre-
viously existing mucous cells in the gland bodies. Mitotic divisions
among these cells are extremely rare and there is no appreciable increase
in the number of these mitoses after the operation. It is also very
unlikely that many of them arise by proliferation of the mucous chief
cells of the gland necks and travel down the glands to replace the
ferment cells of the bodies, because there is very little, if any, increase
‘in the number of mitoses in the neck region and there is no evidence
of active migration of cells at this time. Further, there are no evidences
of degeneration or disappearance of body chief cells except in the two
or three glands which degenerate on account of injury by the knife or
sutures. The new ferment cells which appear in gland bodies between
the first and seventh months after operation cannot arise by transforma-
tion of previously existing ferment cells, because examination of pre-
ceding stages shows that absolutely no ferment cells existed in any part
of the glands in the situation where new ferment cells arise. The trans-
formation of parietal cells into chief cells, while suggested by Trinkler,

Basil C. H. Harvey 229

83, and Pilliet, 87, has not been confirmed by other observers. Bensley
has not been able to find it and in my preparations I have found abso-
lutely no evidence of their transformation into chief cells of either
variety. Further, while new mucous or ferment cells are being formed,
the parietal cells of the gland bodies are not diminished in number nor
are there mitoses among them, so that new chief cells cannot arise from
them. There remain only the ferment cells from which new mucous
cells can be derived, and only the mucous cells from which new ferment
cells can be derived, and so by a process of elimination of every other
source possible, there is afforded a very strong confirmation of the indi-
cation that cells of these two varieties are transformed one into the other.

Stages of the transformation may be actually observed, in which the
same cell contains both mucus and zymogen. In a preparation of the
stage of two weeks after the operation I stained a section with neutral
gentian and selected in it a gland containing cells with only a few
zymogen granules in them. Between this gland and the line of operation
all gland bodies contained chief cells which were exclusively mucus-
forming. A few cells of this gland were drawn showing zymogen
granules in their free ends. The section was then put in alcohol until
all the neutral gentian was extracted and then stained in muchexmatein.
The same cells were then found to contain considerable numbers of
mucous granules.

From all these facts it seems necessary to conclude that ferment cells
may be transformed into mucous cells, and that these same cells may
later lose their mucus-forming function and reassume the morphological
characters and function of ferment-forming cells, which they possessed
before the operation.

The replacement of ferment cells by mucous cells was observed by
Cade, oz, who assumed that it was a transformation and interpreted it
as evidence of the formation of a new pylorus, and he concluded there-
from that the body chief cells are not specifically set apart for the
performance of one function, but that they possess a morphological flexi-
bility and take on a pyloric character when made to perform the part
of pylorus. In view of the fact that in addition to the alteration of
function caused by the establishment of the new pylorus, there is
affecting these cells, also an inflammation produced by the mechanical
violence of the operation and by the tension which the attachment to
the duodenum makes upon this area of mucous membrane, it seems to
me that this explanation is incomplete, since it takes account of only one
of the two influences which are operating to produce changes in the cells.
The change might be merely the expression of the cellular reaction to

230 Gastric Glands of Dog after Gastroenterostomy

the inflammatory process, which might disappear upon the termination of
the inflammation. Since these cells afterward resume their former
characters and function in spite of the persistence of the new pylorus,
it seems necessary to conclude that they undergo a mucous transforma-
tion simply because they are in an inflammatory condition, and not
because they play the part of pylorus and therefore, in response to a
great biological law which requires the morphological adaptation of struc-
tures to correspond to the performance of new functions, undergo a
transformation making them like pyloric cells. Such a mucous trans-
formation is common in inflammatory conditions in the stomach. Meyer,
89; Hayem, 92; Ewald, 93; Boas, 94; Schmidt, 95; Popoff, 97, and
many others report it as occurring in gastritis. Sachs, 87; Schmidt, 96;
Leuk, 99, and many others report it in ulcer. Bockelmann, 02; Lubarsch,
and others have described it in carcinoma. Cade, 03, has described it
at the surface opening of a fistula leading into a stomach pouch isolated
from the general gastric cavity of a dog.

Most pathologists regard this mucous transformation as a retrograde
change, and it is frequently referred to as a mucous degeneration.
Letulle, o0, speaks of it as a “ probably irreparable” change. Cade be-
lieved it to be permanent. He observed it six and a half months after
the operation of gastroenterostomy in the dog and laments the lack of
preparations of later stages, in which he expected to find the changes
much more pronounced. I made two preparations of the six and a
half months stage, in one of which the ferment cells extended to within
three glands of the anastomosis, and in the other they extended quite
up to it. In the ten months’ preparation they extended quite to the
anastomosis, and the glands containing them, although lying next to
duodenal glands and not separated from them by more connective tissue
than is usually found between adjacent gastric glands, nevertheless did
not differ in any essential particular from normal gastric glands in
the fundus region. (Fig. 3.) Cade, 03, reports also certain other
observations which he made of conditions in which this mucous trans-
formation of ferment cells was found persistent for long periods. In
the experiment mentioned above, in which a fistula was made in a dog,
putting a stomach pouch made in the fundus region in communication
with the surface of the body, he found, six and a half months after
the operation, that at the orifice of the fistula the chief cells of the
bodies of the glands had become transformed so as to resemble closely
the chief cells of the gland necks or of the pyloric glands. The mucous
membrane of the pouch walls generally was not modified but preserved
the character normally found in gastric mucosa of the fundus region.

Basil C. H. Harvey 231

Cade and Latarjet, 05, observed also a case in which these transforma-
tions of ferment cells into mucous cells persisted for many years. An
infant had had a gastric hernia which opened to the surface spontaneously
during the first year of its life. The part of the stomach with which
it communicated was later separated from the general cavity by strangu-
lation of the hernia and constituted a separate gastric pouch. About
the margins of the fistula by which this pouch communicated with the
surface, the mucous membrane was found, after nineteen years, to
contain glands derived clearly from fundus glands but containing large
lumina and formed of a single layer of rather cubical cells which were
in all essential points like pyloric gland cells. The walls of the small
stomach were formed of normal fundus mucosa. In this case the fer-
ment forming chief cells of the gland bodies had been replaced by mucous
cells probably during the first year of life, and the mucous character
had been preserved for nineteen years. But in both these cases of
gastric fistula the conditions present are not those existing at a new
pylorus formed by gastroenterostomy. 'The opening to the surface sub-
jects the glands near it to irritation from garments, bacteria, friction,
etc., and to excoriation from drying combined with friction, all cf which
combine to produce a continued irritation which must tend to keep the
glands affected in a condition of chronic inflammation, and so perpetuate
in them phenomena which we have seen to be characteristic of inflam-
matory conditions.

In view of the subsequent reassumption by these cells, in my experi-
ments, of their original function, after performing another function
for several months, it seems necessary to believe that in the dog’s
stomach the mucous transformation is not necessarily an irreparable,
retrogressive change.

And since such transformations are only temporary after simple
incisions and after gastroenterostomies, it seems quite justifiable to
conclude that they may be only temporary after inflammations from
other causes also, that they are not necessarily degenerations nor even
final differentiations, but are merely a passing stage of the cytomorphosis
of these cells into which it is quite within their capacity to pass for
months, and from which they may return to the assumption of their
former characters and the performance of their former function. It
should, therefore, be possible that in stomachs in which chronic inflam-
mation has resulted in the change of ferment cells into mucous cells
there may occur a changing back again of these same cells into ferment
cells and a resumption of their original activity. This may serve to

232 Gastric Glands of Dog after Gastroenterostomy

explain to some extent certain clinical phenomena, for example, those
reported by Einhorn, 02, in two patients in whose stomachs he found the
ferment cells transformed into mucous, cells but whose health and diges-
tion were comparatively good two years later.

The transformation of mucous cells into ferment cells has not been
commonly observed. Cade, o1, suggested the possibility of such trans-
itions but says the facts are not enough to enable one to aftirm them.
Bensley and Zimmermann considered the two forms of cells specifically
distinct, and therefore transitions from one form to the other and back
again were not expected a priori by the writer.

6. Results of occlusion of the pylorus——After occlusion of the pylorus
so that no food passes through it, a new pylorus having been formed by
gastroenterostomy in another part of the stomach, I was not able to find
any changes whatever indicative of the assumption by the mucosa in
this region of the characters present normally in the fundus region.
There was sime infiltration by round cells, but there was not at any stage
up to and including that of ten months after the operation, any trans-
formation or replacement of pyloric gland cells by ferment cells. No
zymogen granules or prozymogen in any form appeared in the cells, nor
could I find any parietal cells. The glands were always typical pyloric
glands, in the relative depth and breadth of foveole and of gland lumina,
in the general character of the glands and in the structure of the
glandular elements constituting them. When, therefore, the conditions
of existence of the pyloric mucous membrane are altered and it is placed
as far as possible in the position normally occupied by fundus mucosa,
it does not show any tendency to assume the morphological characteristics
of the fundus mucosa. It must be borne in mind, however, that in
this experiment the gastric musculature is unaltered and, it may be
presumed, acts in exactly the same way as before the operation. So
that while no food actually passes the pylorus, the mucous membrane
there is still subjected to its mechanicai action to a very considerable
extent, as the muscles of the stomach wall force the contents along the
old course against the pyloric mucosa.

7. Significance of results observed—The retention by pyloric gland
cells of their original characters after occlusion of the pylorus, and the
complete return of the cells of the glands near a new pylorus to their
original form and function even after a transformation of over six and
a half months duration, and in spite of their subjection to the action
of external conditions, which are very considerably changed, seems to
indicate for these cells a certain specificity. It does not seem to support

Basil C. H. Harvey 233

in full the contentions of Bard, 98, who believes that there is inherent
in the cell by its heredity a predestination to a certain form and function
unalterable by the action of external conditions except within the nar-
rowest limits. But it indicates that cells, which from whatever cause,
either inherent or external, have reached a certain stage of differentiation
and specialization, do not possess the power to respond readily to the
action of external influences. They tend to retain the form and function
which they have attained, even when subjected to the action of external
conditions very different from those which have previously been acting
upon them. The ferment cells, however, though very highly specialized,
frequently in response to changed conditions assume a form through
which they have probably passed during the development of their
special characters. This retrograde step is not necessarily a degeneration
or final differentiation, for they possess still a tendency on the removal
of the disturbing conditions to become again specialized. And _ this
further development tends to proceed by lines along which the cell has
previously travelled in its cytomorphosis. ‘'To this extent my observations
show that such highly differentiated cells possess a kind of specificity.
But the tendency to develop along such lines is not an imperative necessity
to the cell. It retains the power of developing along other lines also,
if certain other external conditions be present, and the number of trans-
formations presented by gastric chief cells is very considerable. Schmidt,
96, reports that he found these cells changing into intestinal epithelium
consisting of goblet cells and cylindrical cells with a striated cuticle, at
the margins of ulcer and in chronic inflammation. Leuk, 99, found
goblet cells among them. Hari, o1, reports them in both healthy and
pathological conditions in various parts of the human stomach, often
in the fundus region. Schaffer also reports the existence in the human
stomach of similar patches of intestinal epithelium which he regards
as elements dislocated from the duodenum. But the fact that he found
them in the part of the stomach farthest removed from the duodenum,
coupled with their absence from a healthy human stomach taken from an
executed man and examined by Bensley and Revell, and with the fact
that they have not been observed in stomachs of other mammals, not-
withstanding the very great number which have been examined, but
usually in a healthy condition, seems to justify the conclusion that they
are not dislocated duodenal epithelium but are transformations of gastric
epithelium under the influence of pathological conditions. Dean D.
Lewis has shown me such intestinal epithelium taken from the fundus
region of a human stomach which was affected with carcinoma. Ham-

234 Gastric Glands of Dog after Gastroenterostomy

merschlag, 96; Cohnheim, 96; Bockelman, 02, and others report similar
observations. Lubarsch has seen it in cases of achylia gastrica. Ewald,
93, and Meyer, 89, have illustrated it as occurring in chronic gastritis.

Such transformation seems to a certain extent in accord with the
conclusions of O. Hertwig, 98, according to whom cells of various kinds
inherit equivalent potentialities, many of which they appear to lose
gradually during the successive stages of their cytomorphosis, while as-
suming more completely and exclusively the performance of special
functions, but retaining throughout a large part of their life the ability
to assume forms and functions quite different from those ordinarily asso-
ciated with them, when subjected to the influence of varying external
conditions. And while it seems quite possible that cells may attain so
high a degree of specialization as to lose completely the power of under-
going any further transformations, except degenerative ones, it must be
borne in mind that, unfortunately, we possess no criterion by which this
condition can be definitely determined. And since, as in the case of
the ferment forming grandular elements of the dog’s stomach, many
cells which appear to perform one very highly specialized function, and
under normal conditions, present invariably associated morphological
characteristics, may nevertheless retain the power of performing actively
very different functions after undergoing great transformations, it is
not possible to conclude that any cells are specific before subjecting them
to prolonged study by experimental methods.

In conclusion, it is a pleasure to express my gratitude to Prof. Bensley,
under whose direction this work was done.

SUMMARY.

Cells possessing a special affinity for chromic acid salts and resembling
closely the chromaffine cells, described by Kohn in the suprarenal gland
and elsewhere, exist normally among the parietal cells of the dog’s
stomach.

After gastroenterostomy the mucous membrane within 7 mm. of the
line of operation undergoes the following changes.

The body chief cells, which are normally ferment-forming, become
transformed into mucous-forming cells. This is a gradual process be-
ginning immediately after the operation, at the line of suture, and
extending radially about the anastomosis. It reaches its maximum extent
of 7 mm. about three weeks after operation. Ferment cells in the next
3 mm. show a tendency to this change, but do not undergo a complete
transformation. After one month a reverse transformation commences,

Basil C. H. Harvey 235

and the same cells again become ferment cells. This process is com-
pleted by six and a half months after operation. After that the gastric
glands next to duodenal glands at the anastomosis do not differ materially
from those remote from it. This is a transformation of cells, not a
replacement. Cells were found containing both mucus and zymogen.

Many parietal cells enlarge; many become vacuolated. The number of
cells with an affinity for chromic acid salts is considerably increased,
especially within a week after the operation.

When a new pylorus was formed in the fundus region of the stomach
by gastroenterostomy, the glands around it did not show any permanent
changes indicative of their assumption of the character of pyloric glands.

When the pylorus was occluded also, the pyloric glands did not show
any tendency to assume the characters of fundus glands.

The gastric glands tend to retain their normal characters even under
the influence of changed external conditions. If they undergo trans-
formations in response to the action of such changed conditions, they
tend later to resume their original characters. But they do not always
do so. Metaplasia of gastric glandular epithelium has been observed to
a considerable extent in various conditions, mostly pathological.

Since cells very highly specialized may undergo such transformations,
the term “specific” must be applied to them with care, and only after
prolonged study by the experimental method.

AUTHORS CONSULTED.

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— o2.—The Cardiac Glands of Mammals, Am. J. Anat., Baltimore, II,
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16

236 Gastric Glands of Dog after Gastroenterostomy

Boas, I., 94.—Diagnostik und Therapie der Magenkrankheiten, 3 Aufl., Leipz.

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EBSTEIN, W., 70.—Beitrage zur Lehre vom Bau und den physiologischen Func-

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Er1nuHoern, M., o2.—A further Contribution to our Knowledge of the Histology
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Basil C. H. Harvey

rar)
Se)
~

HAANE, G., 05.—Ueber die Cardiadriisen und die Cardiadrtisenzone des
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238 Gastric Glands of Dog after Gastroenterostomy

MacCatium, A. B., o5.—On the Distribution of Assimilated Iron Compounds
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Basil C. H. Harvey 239

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240 Gastric Glands of Dog after Gastroenterostomy

Mucous Ce//s.

Parietal cel//
Sprri//um.

EG.

Fic. 1. Part of the bodies of glands near the line of union three months
after gastroduodenostomy. The parietal cells are enlarged and many con-
tain spirilla. Fixation: Kopsch’s fluid. Stain: Neutral gentian by Bensley’s
method.

Fic. 2. Section of mucous membrane including the line of union four
months after gastroduodenostomy. Fixation: Bouin’s fluid. Stain: Muci-
carmine. The gland cells which stained red with mucicarmine are shown
black in the figure. (See opposite page.)

Basil C. H. Harvey 241

oes *___ Duodenurn

SS
BQ

SNS Sle
S388 Re
TARA ose
Hes S>
Sau Qos
QQIS RASS
TRS ot
au >

EiGa2:

242 Gastric Glands of Dog after Gastroenterostomy

Duodenal ¥ands.

Gastric glands.

Parretal cells.

ferment ce/!s

LYMOSE/
§ranules.

Frozy/O3en.

Fic. 3. Section of mucous membrane, including part of the line of union
six and a half months after gastroduodenostomy. Fixation: Kopsch’s fluid.
Stain: Neutral gentian by Bensley’s method. Cells containing ferment
granules are seen extending quite to the anastomosis. The glands containing
them present the characteristics of normal glands of the fundus region.
Ferment granules which stained blue in neutral gentian are shown black in
the figure.

Basil ‘CC. H. Harvey 243

S Spiritla.

Eireas3

Fic. 4. Part of the bodies of glands near the line of union six and a half
months after gastroduodenostomy. The parietal cells contain an irregularly
distributed substance staining purple with neutral gentian. Fixation:
‘Kopsch’s fluid. Stain: Neutral gentian by Bensley’s method.

Fig. 5. Part of the bodies of glands near the line of union ten months
after gastroduodenostomy. Fixation: Kopsch’s fluid. Stain: Copper-chrome-
hematoxylin (Bensley).

oe
4

a

EXPERIMENTS ON THE ORIGIN AND DIFFERENTIATION
OF THE LENS IN AMBLYSTOMA.
BY

WILBUR L. LE CRON,

Student of Medicine.
Anatomical Laboratory, Johns Hopkins University.

WitTH 5 PLATES.

Spemann’ destroyed the rudiment of the optic vesicle on the wide
open medullary plate of Rana fusca with a hot needle. When the eye
failed to regenerate the lens was wanting, although the normal lens
forming ectoderm had not been injured.

Lewis * cut away the optic vesicle in Rana palustris at a later stage,
shortly after the closure of the neural folds, but before there were any
signs of lens formation. The normal lens forming ectoderm was unin-
jured, but it failed to give origin to the lens, unless there was sufficient
regeneration of the eye to bring it into contact with the ectoderm.

Lewis also transplanted the optic vesicles so cut away, beneath the
ectoderm in other regions of the head. In embryos where such trans-
planted eyes came into contact with the overlying ectoderm, lens forma-
tion often occurred. His method of operation consisted in making an
incision caudal to the eye region, turning the skin flap forward and cut-
ting away the exposed optic vesicle, and then replacing the skin flap into
its original position.

These experiments indicate very clearly that the lens is dependent for
its origin upon the contact influence of the optic vesicle upon the ecto-
derm ; the lens, in other words, is not a self-originating structure.

The beginning then of lens formation, namely, the thickening of the
inner layer of the ectoderm to form the lens-plate, is dependent upon
some influence exerted on this ectoderm by the optic vesicle. Is the con-
tinued differentiation of this lens-plate into the lens-bud, the lens-vesicle,
and lastly the lens independent of any farther influence of the optic
vesicle, or is the normal differentiation of the lens-plate dependent upon
the continued influence of the optic vesicle and optic cup?

At the suggestion of Dr. Lewis, I undertook in the spring of 1905 an

1Ueber Correlationen in der Entwickelung des Auges. Verhandl. der Anat.
Gesellschaft, 1901.

2Hxperimental Studies on the Development of the Eye in Amphibia. I.
On the Origin of the Lens. Rana palustris. Am. Jour. of Anat., III, 1904.

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

246 Origin and Differentiation of the Lens

experimental study of the question of the self-differentiation of the lens
in Amblystoma punctatum. It seems possible that by removing the
optic vesicle or optic cup at various stages before and during lens forma-
tion, one would be able to determine whether the continued influence of
the optic vesicle was necessary for the normal differentiation of the lens.

The operation of cutting out or removing the optic vesicle without
injury to the lens forming ectoderm or to the developing lens rudiment,
is a simple one. The embryos were operated upon in tap water under
the binocular microscope. They were held in position with a small pair
of fine forceps, and a semi-circular incision was made, with a very finely
pointed needle, through the ectoderm a little caudal to the bulge made
on the side of the head by the developing eye. The skin flap was then
turned forward, exposing the rounded optic vesicle. The latter was cut
off from the side of the brain, and then when the skin flap was turned
farther forward the optic vesicle was carefully pulled away from the
ectoderm, sometimes as a whole, or else in small pieces. Great care was
taken, however, not to injure the developing lens or surrounding ecto-
derm. After removal of the optic vesicle or the optic cup, the skin flap,
which in later stages had attached to it the lens rudiment, was turned
back into its original position, and held there by turning the embryo over
on its side, with the skin flap against the bottom of the dish. The mere
weight of the body sufficed to hold the flap in place. Healing was rapid;
one to two hours generally being sufficient for complete closure of the
wound. The operations were all made on the right side, while the left
remained intact for purposes of comparison. The older embryos were
first anesthetized in acetone chloroform in order to keep them quiet dur-
ing the operation.

The embryos thus operated upon were allowed to live from two hours
to thirty days, then killed in Zenker’s fiuid, thoroughly washed, embedded
in paraffin, and cut into serial sections 10 micromillimeters in thickness.
They were stained in hematoxylin and Congo red.

It has already been noted that in Rana fusca as well as in Rana palus-
tris a lens will not arise from the normal lens-forming region of the
ectoderm, if the optic vesicle is removed about the time of, or shortly
after, the closure of the neural folds. Likewise in Amblystoma punc-
tatum the lens fails to arise when the optic vesicle is removed at an early
stage.

The optic vesicles were removed, by the operation already described,
from embryos of Arablystoma shortly after closure of the neural folds
(see Fig. 1). At this age there is not the slightest visible trace of lens
formation or of any changes in the ectoderm leading to lens formation.

Wilbur L. Le Cron 247

The optic vesicle is in contact with the ectoderm, but is not adherent to
it (see Fig. 2).

Five embryos (Experiments VII,, 75 2s 35 32) of this stage (VII) thus
operated upon and killed 2, 4, 9, 10 and 12 days later, show no regenera-
tion of the right eyes and-likewise no signs of lens formation. In two
embryos killed seven and nine days after the operation there appear
partially regenerated eyes, which, owing to their small size and lack of
contact with the overlying ectoderm, have failed to stimulate lens forma-
tion. Other embryos (Experiments VII,,,,7; ) operated upon and al-
lowed to live five, seven, and two, eight days, show regenerated eyes
with developing lenses. In these the regenerated eyes were of sufficient
size to come into contact or to remain in contact with the ectoderm long
enough to cause lens formation. These latter experiments thus indicate
that the operation of turning back the skin flap does not interfere with
lens development, provided that the flap, when returned to its original
position, comes into contact finally with a regenerated optic vesicle.

On the left or normal side of the embryo referred to above, which was
killed two days after the operation, there is still no trace of the lens-
plate, but on the normal side of the one killed four days afterwards, the
lens-bud is well advanced, and in the embryos allowed to live five days,
the lens-buds are still attached to the inner layer of the ectoderm and
are about 110 microns in diameter. Embryos killed eight and nine days
after the operation, however, show that the normal lens has completely
pinched off from the ectoderm, and that the lens-fibers are beginning to
differentiate at the medial pole. At eight days the lens is about 140 p
in diameter. The normal lens in the embryo allowed to live 10 days is
about 130) in diameter and has pinched off from the ectoderm, and
lens-fibers are fairly well developed. The normal 12 day lens is about
150 » in diameter and shows still further differentiation.

The contrast, then, between no lens at all on the right side, where the
optic vesicle is small or wanting even 12 days after the operation, and
the normal lens on the left side is very marked indeed, and leads to the
conclusion that the lens in Amblystoma as in Rana is not a self-originat-
ing structure.

In one embryo (Experiment VII,,) which was allowed to live 30 days
after the operation, the right eye is entirely wanting, but in the region in
which the lens would have formed under normal conditions, there seems
to be a lens-bud rudiment (see Fig. 3). Its small size and rudimentary
condition is in marked contrast to the normal lens on the opposite side
of the head (compare Fig. 9). This little lens-bud, if it is one, and this
seems somewhat doubtful, has probably arisen because the embryo at
the time of the operation had advanced, as regards the formation of the

248 Origin and Differentiation of the Lens

lens, farther than the other embryos of this series. That this
is possible without any indication on the surface is very
probable. In the examination of a number of embryos of Rana palustris,
it was noted that embryos, which so far as external features were con-
cerned seemed of the same age, often differed considerably in the amount
of differentiation of the lenses. If this is true for Rana palustris it
probably holds also for Amblystoma. Consequently some variations
must be expected in the results obtained from embryos which are alike
so far as their external features are concerned. And so, I am inclined to
believe that the rudimentary lens-bud in the above experiment indicates
that the optic vesicle had already exerted some influence on the ectoderm
leading to lens formation, but when this influence was removed those
ectodermal cells possessed but very little power of self-differentiation,
and hence the development of the lens soon came to a standstill. That
this is true will become more evident when we consider the results ob-
tained after removal of the optic vesicle at later stages.

The foregoing experiments indicate very clearly then, that the lens
is not a self-originating structure, and that it is dependent for its origin
upon the influence or stimulus of the optic vesicle. If the optic vesicle
is necessary for the starting of the lens, it may also be necessary for its
differentiation, and if so, the removal of the influence of the optic vesicle
during the various stages of lens development, should be followd by re-
tardation and abnormal growth of the lens.

The optic vesicle was next removed from embryos at a stage (VIII)
slightly older than those first operated upon. At this stage the tail-bud
is just beginning to show and in most of the embryos the lens-plate ap-
pears as a very slight thickening of the inner layer of the ectoderm,
where the optic vesicle comes into contact with it (Figs. 4 and 5). |

Three embryos (Experiments VIIT,, ,, ,) thus operated upon and
killed two, five and twelve days afterwards, were without regenerated
eyes on the right sides and without traces of lens formation. On the
normal or left side of the embryo killed two days after the operation
there is a marked thickening of the ectoderm forming the lens-plate.
The five-day embryo shows on the left or normal side a well-formed Jens
separated from the ectoderm and with the beginning of the formation
of lens fibers. This normal lens is about 130 in diameter. The em-
bryo that was allowed to live 12 days has on the normal side a well dif-
ferentiated lens, about 190 in diameter, with long lens-fibers. Al-
though the external appearances of these embryos would have indicated
that the lens-plate had begun to form, nevertheless such thickenings on
the under side of the skin flap were not observed at the time of the op-

Wilbur L. Le Cron 249

eration. It is evident that the impulse before removal of the optic vesicle
was not sufficient to bring about any visible signs of self-differentiation.

In another embryo (Experiment VIII,,) operated upon at this stage
and killed three days afterwards, there is found on the left side a normal
lens about 130, in diameter which has separated from the ectoderm,
and on the right side a lens-plate—like thickening. At the time of the op-
eration a very slight thickening of the ectoderm .in the right lens region
was noted. In this experiment no regeneration of the eye took place.
There has evidently, then, been great retardation or almost complete
stoppage in the growth and differentiation of this lens-plate, owing, I
believe, to the absence of the continued influence of the optic vesicle.

Another embryo (Experiment VIII,,) of this stage operated upon
and killed four days later has on the left side a lens-bud about 100 m in
diameter. The latter is still attached to the ectoderm. The right eye
is wanting, but there is a thickening of the ectoderm in the normal po-
sition. The differentiation and growth of the right lens-plate has been
much retarded, or perhaps has come to a standstill, evidently again
through the loss of the influence of the optic vesicle.

Another embryo (Experiment VIII,,), killed five days after only par-
tial extirpation of the optic vesicle, shows on the left side the normal
lens 130 » in diameter, just about ready to separate from the ectoderm.
The regenerated right eye is deeply seated and separated from the ecto-
derm by mesenchyme. The ectoderm over the eye is thickened into a
lens-plate, but is much checked in development. Evidently the deeply
situated, regenerated eye has exerted no influence upon the lens-plate.
At the time of the operation a slight thickening of the ectoderm was to
be seen.

Another embryo (Experiment VIII,,) of this stage (VIII), from
which the right eye was entirely removed, was allowed to live six days.
At the time of the operation there was to be seen on the right side a
slight thickening of the ectoderm for the lens-plate. The normal lens
on the left side, about 150 in thickness, has separated from the ecto-
cerm and shows considerable differentiation of its lens-fibers (Fig. 6).
In the normal position on the right side there is a vesicular body sep-
arate from, but close to, the ectoderm (Fig. 7). It consists of a single
layer of high columnar epithelial cells surrounding a central cavity, and
is about 110 in diameter. The appearance and general arrangement
of the cells indicate very clearly that it is a lens-vesicle considerably re-
tarded in development. It is also abnormal in that the medial pole
shows only slight indication of the beginning of lens-fibers. Such elon-
gation of the cells of the medial pole is always found, even before the
nermal lens separates from the ectoderm.

250 Origin and Differentiation of the Lens

Practically the same conditions are to be found in an embryo (Experi-
ment VIII,,) killed nine days after the operation. There is no sign
of a right eye, the normal left lens is about 160» in diameter, and is
farther advanced than the one above. On the right side in the normal
position for the lens there is a spherical body about 100 » in diameter.
It consists of a single layer of high columnar cells surrounding a cavity.
‘It is entirely separate¢ from the ectoderm, but lies near to it. The
medial pole shows no indication of the formation of lens-fibers beyond a
slight elongation of the cells. At the time of the operation a thickening
of the ectoderm was noted.

In another embryo (Experiment VIII,,), killed 30 days after the
operation, there is complete absence of the right eye. Beneath the ecto-
derm of the lens region on this side is a very degenerated-looking lens,
about 110 » in thickness (Fig. 8). It consists of a layer of flat epithelial
cells surrounding a spherical mass of irregularly arranged, degenerating
lens-fibers. ‘The mass is much vacuolated, but stains rather like the
lens-fiber part of the normal lens. The latter is 260m in diameter
(Fig. 9). This experiment affords a marked instance of incomplete de-
velopment and differentiation on the part of the lens when the influence
of the optic vesicle is removed.

From another embryo (Experiment VIII,,) of this stage, the optic
vesicle was partially removed, and 10 days after the operation a well de-
veloped eye had regenerated. The right lens has normally grown and
differentiated, being almost as fully developed as the lens on the left
side. Such instances where regeneration has occurred show that the
operation itself of turning the ectodermal flap forward, does not inter-
fere with lens development, provided the flap, when returned to its
original position, comes into contact with a regenerated optic vesicle.
Hence, whatever retardation of the lens may occur, must be explained by
reasons other than by the immediate results of the operation.

A series of 11 experiments was made upon embryos of a stage (1X)
somewhat older than the above. A thickening of the inner layer of the
ectoderm was perceptible, and the invagination for the developing lens
could be seen from the surface (Figs. 10-11).

Two embryos (Experiments IX,, ,,), killed eight days after the opera-
tion, show lenses on the left sides normally developed, being about 170 p
in diameter. On the right sides are small spheroidal bodies consisting
of one or two Jayers of columnar epithelium surrounding central cavi-
ties. Figures 12 and 13 are from mesial sections of the abnormal and
normal lenses of one of these eight-day embryos. The right abnormal
lens shows that the lens-plate (as seen in Fig. 11) continued its devel-

Wilbur L. Le Cron 201

opment for a short time after the operation to the formation of a lens-
bud, and to its separation from the ectoderm as a lens-vesicle. Here its
development was ultimately checked and no lens-fibers were formed.
This lens-vesicle is much smaller than the normal lens, and its lack of
differentiation is also very evident.

This abnormal lens-vesicle is about the same size as a normal lens
which has just separated from the ectoderm (see Fig. 19). It does not,
however, show nearly the amount of differentiation of such a lens,
especially in the region of the median pole, which shows no formation of
lens-fibers, as does the normal lens. The cells of the median pole of this
abnormal lens are somewhat elongated, as compared with those of the
lateral pole, but this difference already existed in the lens-plate at the
time of the operation. It seems, then, that the development of the lens-
plate has been more in the change of form than internal differentiation.

In two experiments (IX,, 4,) in which the embryos were allowed to
live 11 and 14 days, the lenses of the corresponding sides are approxi-
mately of the same size, and about the same amount of difference is
found between the right and left lenses of each embryo. The lenses on
the left measure about 190 » (compare Fig. 39), while those on the
right are much smaller, being only about 100, in diameter. The latter
(Fig. 14) are small spheroidal vesicles, consisting of a single layer of
high columnar cells, and lying close to the ectoderm. In the cavity of
the vesicle there are a few detached ectodermal cells, showing signs of
degeneration. The cells of the epithelial wall of the vesicle are healthy in
appearance, and more like those of the anterior epithelial layer of a
younger normal vesicle (as in Fig. 19), than the more flattened cells of
the left normal lens of the same embryo. The cells of the median pole
of the abnormal vesicle, as in the preceding experiment, are somewhat
elongated, as compared with those of the lateral pole. This lens-vesicle,
although six days older than the one shown in Fig. 12, is much smaller.
This diminution in size may have been caused by injury to the lens-
plate, although at the time of the operation no injury was noted. On
the other hand the lens-plate may not have been quite so far advanced
at the time of the operation as the lens in the preceding experiment, and
hence fewer cells of the inner layer of the ectoderm were influenced to
take part in the formation of the lens-vesicle. Or, again, it may be after
a certain time these abnormal Jens-vesicles decrease in size, perhaps by
the migration of some of the cells into the vesicle. This point can only
be settled by further experimentation.

When the optic vesicle is removed after lens formation has begun, the
development of the latter is not checked directly after, or by the im-

itty

252 Origin and Differentiation of the Lens

mediate effects of the operation, but retardation results only later from
the lack of influence of the optic cup. In other words, after removal of
the optic vesicle, lens development still continues for a short time, the
extent of the development depending upon the size of the lens-plate at
the time of the operation. The lens seems to receive a sort of momen-
tum from the contact influence of the optic vesicle, and this impulse or
stimulus is sufficient to give the lens a limited amount of self-differen-
tiation, even in the absence of the optic cup. In all cases where long
enough time is allowed, the process of separation of the lens from the
ectoderm takes place, although generally the lenses lie nearer to the ecto-
derm after their separation than the normal ones.

Three other embryos (Experiments IX,,4.4,) of this stage (IX),
which were killed six, eight, and thirty days after the operation, show
slight regeneration of the right eyes. In the six-day embryo, the right
lens contains a mass of fibers surrounded by a complete layer of epi-
thelium, and no medial pole is distinguishable. In the other two in-
stances the regeneration of the eye was very slight, and the lenses are
not only considerably retarded in growth, but also in differentiation. In
the 30-day embryo the difference in size of the two lenses is 120 p, the
left measuring 260 and the right only 140 in diameter. The latter
contains some fibers and material staining like the normal lens. This
material seems to be degenerated lens-fibers. Owing to slight regenera-
tion of a few optic vesicle cells, the lens was perhaps at first rather
normally developed and differentiated, but these few cells were not
sufficient to influence the lens completely, and hence after a short time
degeneration occurred.

In the next stage (X) the gill mass is large but shows no division.
The lens-bud is well marked when the skin flap is turned forward in the
operation (Figs. 15-16).

In one embryo (Experiment X,,) that was allowed to live 14 days
after the operation the normal lens measures 160 and the right one
100 » in diameter, thus showing a difference of 60 » in size. The right
lens has separated from the ectoderm but the eye is entirely wanting.
A complete layer of epithelium surrounds the lens, which contains be-
sides degenerated material a few healthy fibers in the region that had
once been the medial pole (Fig 17, and compare Fig. 27).

In three other experiments (X,, 55.4), 11 which the embryos were
allowed to live seven, nine, and eleven days, slight regeneration of the
right eyes has taken place. In the seven-day experiment the regenera-
tion is very slight indeed. The right lens is fairly normal in appear-
ance, but somewhat retarded (Figs. 18-19). It has separated from

Wilbur L. Le Cron 253

the overlying ectoderm, and is of good size, but is not so far advanced
ir point of differentiation as the normal left lens. A well defined me-
dial pole is to be seen, but this appears to have stopped development
when compared with the rapid advancement of the pole of the opposite
lens to form fibers. Had the embryo lived longer the lenses would have
shown a much greater difference, due, as in other instances, to the absence
of the influencing optic cup. In the nine- and eleven-day experiments
the right lenses have separated from the ectoderm. They have epithelial
coverings one layer in thickness, and in the centers are masses of lens-
fibers more or less degenerated. The normal lenses on the left sides
measure about 170, while the right lenses are much smaller, being
only 120 » in diameter.

In such instances, where only a slight regeneration of the eye occurs,
the lenses may at first be rather normal in appearance, but later they
invariably show degeneration, which is due, I believe, to the removal of
the optic cup. The partial influence of the bit of the optic vesicle is
not sufficient to further lens development. The experiments in
which only partial regeneration occurred are likewise in accordance with
the conclusion drawn, that the optic vesicle is necessary for the subse-
quent differentiation of the developing lens.

In a still older stage (XII) the gill mass shows three divisions, and
the tentacles have begun to develop. The lens-vesicle is quite prominent.
It has pinched off from the inner layer of the ectoderm, but is still
tightly pressed against the same (Figs. 20-21). Twelve experiments
were made upon embryos of this age, and five of these show some regen-
eration of the eye.

An embryo (Experiment XII,,) of this series was killed two hours
after the operation. Within this time the skin had healed at the place
of incision. No special change had taken place in the’ right lens, and
it appears perfectly normal. Both lenses are still in contact with the
overlying ectoderm, and show well defined medial poles with the begin-
ning of lens-fiber formation (Figs. 22-23).

In another embryo (Experiment XII,,) which was allowed to live
two days, the lenses have just separated from the ectoderm, and show
no special difference in development (Figs. 24-25), although from the
appearance of the right lens the loss of influence of the optic cup is just
beginning to be felt. Its medial pole is very definite, but the shape and
arrangement of the nuclei are somewhat abnormal.

Instances like the above indicate further that the operation of turning
the skin flap forward does not interfere with the developing lens, which
always has a certain amount of independent self-differentiation even

254 Origin and Differentiation of the Lens

after removal of the optic cup. Hence, whatever retardation of the
lens rudiment may occur is not to be ar by the immediate results
of the operation.

In an embryo (Experiment XII,,) ‘alled six days after the operation,
the right lens (Fig. 28) is somewhat smaller than the left one (Fig. 27).
It is rather normally developed and contains some healthy lens-fibers
which have arisen from the medial pole. The pole, as it now appears,
is not as broad as normal, but is becoming obliterated by the overgrowth
of the layer of columnar epithelium, which surrounds the central mass
of lens-fibers. Had the embryo lived longer, no doubt the medial pole
would have entirely disappeared, and thus the lens would have been com-
jetely surrounded by this layer of epithelium, as in the following ex-
periments.

Great retardation is shown in four of the experiments (XII; 55 50 57)
of this series (XII), of which two of the embryos were killed 10, and
the other two 12 days after the operation. The right lenses are entirely
surrounded by a single layer of cuboidal epithelium, and the medial poles
from which the lens-fibers within had been formed are completely ob-
literated. The right lenses are rather small, and are not widely sep-
arated from the ectoderm. The lens-fibers adjacent to the epithelial
covering seem rather healthy, while the central mass appears degenerated,
but takes a similar protoplasmic stain. These lenses, then, differ con-
siderably in structure as well as in size from the normally developed
ones (Fig. 26, and compare Fig. 9).

In an experiment (XII,,) where the embryo was allowed to live 30
days, a bit of the optic cup had remained, but its influence, if any, did
not long continue, for the right leris developed until it was only 150 p
in diameter, and then came to a standstill, and the lens-fibers within
degenerated. A thin epithelial covering surrounds this vacuolated mass
of degenerated fibers which are in marked contrast to the healthy fibers
of the left lens. The normal lens measures about 250 » in diameter, and
thus shows considerable difference in size from the right one. In the
experiments which continued over long periods of time, especially for
30 days, one is impressed not only with the retarded development of
the right lenses, but also with the marked degeneration of the more
highly differentiated lens-fiber tissue.

In three other embryos (Experiments XIJ,, 4. 4)) of this same stage
(XII), the right lenses somehow came into contact with the nasal pits
on their respective sides. The lenses have developed rather normally.
It is possible that this peculiar arrangement was brought about by the
operation. The skin flap perhaps became somewhat twisted, thus throw-
ing the medial pole of the lens into contact with the nasal pit, which

Wilbur L. Le Cron 255

thus has the appearance of influencing lens-differentiation. However,
these few experiments prove nothing, but suggest the questions, whether
the nasal pit is capable of influencing lens development or not, and
whether the lens-fiber pole may be formed at different parts of the lens
circumference.

In the following stage (XIII) (Fig. 29), the lens vesicle is quite well
developed (Fig. 30), with a definite medial pole and the beginning of
the formation of lens-fibers. It is completely divided off from the ecto-
derm, but still in partial contact with the same. It is found that the
normal lens, after complete division from the inner layer of the ecto-
derm, still adheres or sticks to the same, and only after a certain time
really separates from the ectoderm,—the processes of dividing off and
separating not being simultaneous. Such a well advanced lens does not
show at once any particular changes in development when the optic cup
is removed. The lens-vesicle has evidently received something of a mo-
mentum from the continued influence of the optic cup, and now, after
removal of this influence, develops for a short time apparently inde-
pendently.

In an embryo (Experiment XIII,,) of this stage, killed four days
after the operation, no particular difference between the two lenses is
noticeable (Figs. 31-32). The medial poles are well defined and the
lens-fibers within have a healthy appearance. The right Jens is still in
contact with the overlying ectoderm, but otherwise appears like the
normal left lens. Both measure about 140 in diameter, and are per-
fectly developed. This experiment shows that the lenses of embryos of
such a late stage (XIII) have power of considerable self-development
after the removal of the optic cup. It requires some time before a
marked difference in growth and in differentiation can be observed, but
the difference invariably occurs, showing that the lack of influence of the
optic cup is ultimately felt.

In an embryo (Experiment XIII,,) killed seven days after the opera-
tion, the left lens measures 170» and the right one 140 in diameter.
The latter is thus somewhat smaller. It is well separated from the
ectoderm, and contains normal lens-fibers. The medial pole, however,
is rather obliterated by the epithelial covering of the lens (Fig. 33), and
is not nearly so definite as in the normal lens (compare Fig. 13).

In a still older embryo (Experiment XIII,,) which was killed nine
days after the operation, a greater difference is to be seen. The left
lens is about 200 » in diameter, while the right one measures only 150 p,
and is considerably smaller than the former (Fig. 34, and compare
Fig. 9). It lies rather close to the ectoderm, and contains some healthy

256 _ Origin and Differentiation of the Lens

lens-fibers. The medial pole is quite obliterated by the overgrowth of
the layer of cuboidal epithelium that surrounds the lens completely.

In the oldest stage (XIV) operated upon, the lens was in very slight
contact with the overlying ectoderm (Figs. 35-36).

The lenses of an embryo (Experiment XIV,,) allowed to live five
days show no difference in size, both measuring 150 », but the right one
has a complete covering of epithelium around the nuclear mass in the
center (Fig. 37), and is not widely separated from the ectoderm. A
small bit of the eye that had been left in contact with the lens, appears
te have had little or no influence upon it.

The right lenses of embryos (Experiments XIV, ,) killed eight and
ten days after the operation are smaller than the normal, one is 20, and
the other 30, less in diameter. Both right lenses (Figs. 38-40) are
surrounded with complete layers of columnar epithelium. However, in
the eight-day embryo there still remains an indication of the medial
pole, while in the older embryo the pole is quite obliterated. The lenses
are not widely separated from the overlying ectoderm, are smaller and
in marked contrast to the perfect ones (Fig. 39). The fibers within the
eight-day lens are fairly normal, but are beginning to show some degen-
eration, while those in the 10-day lens show considerably more.

An embryo (Experiment XIV,,) of this stage (XIV) that was al-
lowed to live 30 days affords a very striking instance of the lack of de-
velopment of the lens when the influencing optic cup is remoyed. ‘The
normal lens measures about 220, in diameter (compare Fig. 9), and
is only 20m larger than the right lens. However, the latter consists
only of a vacuolated mass of degenerated lens-fibers surrounded by a thin
layer of epithelium (Fig. 41). The lens has separated completely from
the ectoderm, but is still adjacent to same. It has grown considerably,
but in order to accomplish perfect differentiation, the influencing me-
dium of the optic cup was wanting, and the result was the extensive
degeneration of the lens fibers with the complete obliteration of the
medial pole.

Whether this continued influence of the optic cup upon the develop-
ing lens is a specific one or not, can be determined perhaps only by fur-
ther experimental work. It may be that the nasal-pit, otic vesicle, or
brain can exert such an influence, as to cause the lens to develop and
differentiate normally. Lens-plates, lens-buds, Jens-vesicles, and well
differentiated lenses with some of the surrounding ectoderm might be
transplanted into the brain itself, and thus determine whether there
also the lens can find the influences essential to its normal differentia-

Wilbur L. Le Cron 257

tion. Is the medial pole, destined for the formation of the lens-fibers,
predetermined for a definite part of the developing lens, or can the pole
be formed at any part of the circumference by modifying the surrounding
influences at an early stage? Many interesting questions suggest them-
selves, some of which can be determined by experiment, while for others
only hypotheses at present seem possible.

What the nature of this continued contact influence exerted upon the
lens may be, is purely hypothetical. Perhaps, substances chemically
formed in the protoplasm of the optic cup cells may be the important
factors, which in some way are able to change the chemical nature of
the lens cells, and thus promote their development into a normally dif-
ferentiated lens. In the early stages of lens formation, the optic vesicle
is in close contact with the developing lens, and there may be some
kind of a protoplasmic connection, a relation of the two tissues, such
that there may occur an interchange of protoplasm, or, what is more
hkely, of substances chemically formed in the protoplasm. This influ-
ence of the adjacent optic vesicle or optic cup, whatever may be the
manner of its production, being chemical in nature or otherwise, proba-
bly continues throughout lens development and differentiation, and per-
haps is even exerted to a certain degree upon the lens of the adult eye.
However, the conditions of the lens in the older embryos are such that
the effects of removal of the optic cup are only felt after some days.
It is evident, then, that the age of the embryo indicates somewhat the
extent of the effect produced by the removal of the optic vesicle. The
lens may continue to grow some, even after disturbing these normal re-
lations with the optic cup, but its growth is inevitably checked and is
especially abnormal with regard to differentiation, due, undoubtedly, to
the loss of influence, whatever its nature may be, of the optic cup.

CONCLUSIONS.

(1) A lens will not arise from the normal lens-forming area of the
ectoderm without the contact influence of the optic vesicle. The lens
is not self-originating.

(2) A lens will not develop from the lens-plate, lens-bud, or lens-
vesicle, when the optic cup is removed. The lens is not self-differ-
entiating, but is dependent upon the continued influence of the optic
cup for its normal development.

(3) The older the lens rudiment at the time of removal of the optic
cup, the greater the amount of independent differentiation the lens rudi-
ment possesses. -

(4) The lens rudiment ultimately ceases to develop after removal of
the optic cup, and finally degenerates.

PLATE I.

Fig 1. Outline of an early stage of amblystoma, just after closure of the
neural folds. First operative stage (VII). X 6% diameters.

Fig. 2. Section through eye region of an embryo of the same stage as
above (VII). Optic vesicle in contact with the ectoderm, which shows no
signs of lens formation. X-°45 diameters.

Fie. 3. Experiment VII,,.2 Section through small abortive lens-like struc-
ture in the ectoderm in normal position for the lens. The embryo was
killed 30 days after the complete extirpation of the optic vesicle. x 180
diameters.

Fig. 4. Outline drawing of an embryo of the second operative stage (VIII),
showing first appearance of tail-bud. Xx 6% diameters.

Fic. 5. Section through eye region of an embryo of the same stage (VIII)
as above. The ectoderm shows some changes, as the beginning of the thick-
ening of its inner layer for lens formation. X 45 diameters.

Fie. 6. Experiment VIII,,. Section through normal lens after 6 days.
xX 180 diameters.

Fic. 7. Experiment VIII,,. Section through right lens of above embryo
6 days after the complete extirpation of the optic vesicle, showing abnormal
development as a resultant effect. At the time of the operation the lens-
plate was visible as a mere thickening of the ectoderm. > 180 diameters.

Fie. 8. Experiment VIII, Section through the right lens of an embryo
killed 30 days after removal of the optic vesicle. The lens shows marked
retardation and lack of differentiation, and the lens-fibers are much degen-
erated. It measures about 110 u in diameter, being much smaller than the
normal lens. X 180 diameters.

Fie. 9. Experiment VIII.,,. Section through normal left lens of above
embryo after 30 days. 260 »in diameter. (Contrast Fig. 8.) X 180 diame-
ters.

The Roman numerals indicate the operative stage.

ORIGIN AND DIFFERENTIATION OF THE LENS PEATE!
WILBUR L. LE CRON

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AMERICAN JOURNAL OF ANATOMY--VOL., VI

PLATE Ii.

Fic. 10. Outline drawing of an embryo of the third operative stage (IX).
x 6% diameters.

Fig. 11. Section through eye region of an embryo of the same stage (IX)
as above, showing the lens-plate thickening of the inner layer of ectoderm.
x 45 diameters.

Fic. 12. Experiment IX,,. Section through right abortive lens of an
embryo 8 days after the removal of the optic vesicle. There is an indication
of the medial-pole, but no definite lens-fibers are present. The lens measures
140 » in diameter, being much smaller than the normal one below. X 180
diameters.

Fic. 13. Experiment IX,,. Section through left normal lens of above
embryo after 8 days. Developed from lens-plate thickening of ectoderm
(IX) to lens 170 in diameter. X 180 diameters.

Fic. 14. Experiment IX,. Section through right abortive lens, showing
degenerated ectodermal cells apparently, and no well defined medial-pole.
The embryo was killed 14 days after the removal of the optic vesicle. X 180
diameters.

Fic. 15. Outline drawing of fourth operative stage (X), showing no
division of the gill mass. X 614 diameters.

Fic. 16. Section through eye region of an embryo of the above stage (X),
showing a well-defined lens-bud. > 45 diameters.

Fie. 17. Experiment X,,. Section through right lens 14 days after the
complete extirpation of the optic cup. The medial pole has disappeared, and
only a few lens-fibers indicate its former position. The lens is small, meas-
uring only about 100 uw, while the normal one is about 160 yw in diameter.
x 180 diameters.

Fie. 18. Experiment X,,. Section through right lens of an embryo killed
7 days after the operation. The lack of influence of the optic cup -has re-
sulted in the retardation in growth and differentiation of the lens. x 180
diameters.

Fic. 19. Experiment X.,. Section through the left normal lens of the
above embryo. > 180 diameters.

ORIGIN AND DIFFERENTIATION OF THE LENS PLATE Il

WILBUR L. LE CRON

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AMERICAN JOURNAL OF ANATOMY--VOL. VI

PLATE III.

Fic. 20. Outline drawing of the fifth operative stage (XII). The gill mass
shows three divisions, and the tentacles are beginning to develop. xX 6%
diameters.

Fic. 21. Section through eye region of an embryo of above stage (XII).
The lens-vesicle is divided off from the ectoderm, but still in close contact
with same. X 45 diameters.

Fig. 22. Experiment XII;,. Section through right lens-vesicle of an em-
bryo killed 2 hours after the complete extirpation of the optic cup, showing
no special changes within this time. X 180 diameters.

Fig. 23. Experiment XII,;, Section through left normal lens of above
embryo, killed 2 hours after the operation. X 180 diameters.

Fig. 24. Experiment XII,,. Section through the right lens-vesicle of an
embryo killed 2 days after removal of the optic cup. The lens is similar to
the normal one in size, 150 in diameter, and in differentiation. x 180
diameters.

Fie. 25. Experiment XII,,. Section through left normal lens of above 2
day embryo. X 180 diameters.

Fie. 26. Experiment XII,. Section through right abortive lens 12 days
after removal of the influencing optic cup. A complete layer of cuboidal
epithelium surrounds the central mass of degenerating lens-fibers, and has
obliterated the medial pole entirely. > 180 diameters.

Fic. 27. Experiment XII;,. Section through left normal lens of a 6-day
embryo. XX 180 diameters.

Fic. 28. Experiment XII,,. Section through right lens of the above embryo
killed 6 days after the complete extirpation of the optic cup. It is separated
from the ectoderm by mesenchyme, and the medial pole is almost obliterated
by the overgrowth of the epithelial layer. X 180 diameters.

ORIGIN AND DIFFERENTIATION OF THE LENS PLATE Ill
WILBUR L, LE CRON

AMERICAN JOURNAL OF ANATOMY--VOL. VI

PLATE IV.
Fie. 29. Outline sketch of sixth operative stage (XIII). > 614 diameters.

Fig. 30. Section through eye region of an embryo of same stage (XIII)
as above. The lens-vesicle is well divided off from the ectoderm, but still
adherent to it. »X 45 diameters.

Fig. 31. Experiment XIII,,. Section through right lens of an embryo
killed 4 days after removal of the optic cup. The lens appears perfectly
normal, but is not widely separated from the ectoderm. X 180 diameters.

Fie. 32. Experiment XIII,,. Section through left normal lens of above
4-day embryo. The lenses both measure the same, being about 140 w in
diameter. X 180 diameters.

Fie. 33. Experiment XIII,,. Section through right lens, 7 days after the
complete extirpation of the optic cup. It measures only about 140 «in diam-
eter, being about 30 ~ smaller than the normal lens. The medial pole is
obliterated, and the entire lens is surrounded by a single layer of columnar
epithelium. Xx 180 diameters.

Fic. 34. Experiment XIII,. Section through right lens of an embryo
killed 9 days after the complete extirpation of the optic cup. The lens
measures about 150 in diameter, and is about 50. smaller than the normal
left lens. The medial pole is quite obliterated, and the mass of lens-fibers
within are beginning to degenerate. X 180 diameters.

ORIGIN AND DIFFERENTIATION OF THE LENS PLATE IV
WILBUR L. LE CRON

AMERICAN JOURNAL OF ANATOMY--VOL. VI

PLATE V.

Fig. 35. Outline sketch of seventh operative stage (XIV). XxX 6% diam-
eters.

Fia. 36. Section through eye region of an embryo of above stage (XIV).
The lens-vesicle is but slightly adherent to the ectoderm. X 45 diameters.

Fic. 37. Experiment XIV;,. Section through right lens, 5 days after the
almost complete extirpation of the optic cup. The lens measures only about
150. in diameter. The lens-fibers appear rather healthy, but the medial
pole has disappeared, owing to the complete layer of surrounding epithelium.
x 180 diameters.

Fig. 38. Experiment XIV,. Section through right lens of an embryo
killed 8 days after removal of the optic cup. The lens is completely sur-
rounded by a single layer of columnar epithelium, and measures about
160 « in diameter. The lens-fibers are degenerating, but an indication of the
medial pole still remains. (Contrast normal lens, Fig. 39). X 180 diam-
eters.

Fic. 39. Experiment XIV,. Section through left normal lens of above
8-day embryo. The lens measures about 180 in diameter. X 180 diameters.

Fic. 40. Experiment XIV,,. Section through right lens, 10 days after
removal of the influencing optic cup. It measures about 160 ~ in diameter,
being considerably smaller than the normal lens 190 «in diameter, and shows
no medial pole. X 180 diameters.

Fig. 41. Experiment XIV,,. Section through right lens of an embryo
killed 30 days after the complete extirpation of the optic cup. The lens
measures about 200 w in diameter, while the normal lens of the opposite
side is about 220 . in diameter. It shows marked retardation in growth and
differentiation, and is completely surrounded by a thin layer of epithelium,
containing within a degenerating and vacuolated mass of lens-fiber material.
(For contrast to left normal lens, see Fig. 9). X 180 diameters.

ORIGIN AND DIFFERENTIATION CF THE LENS PLATE V

WILBUR L. LE CRON

AMERICAN JOURNAL OF ANATOMY--VOL. VI

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DEVELOPMENT AND VARIATION OF THE NERVES AND
THE MUSCULATURE OF THE INFERIOR EXTREMITY
AND OF THE NEIGHBORING REGIONS OF THE TRUNK
IN MAN.

BY

CHARLES R. BARDEEN,
Professor of Anatomy, University of Wisconsin, Madison.

WiTH 10 PLATES AND 7 TEXT FIGURES.

TABLE OF CONTENTS.

A. OUTLINE OF THE DEVELOPMENT OF THE NERVES OF THE INFERIOR EXTREMI-

EDINGs yore Sal si nte, sane Vasilelce, alle ove ove ciley Grete: Risen chee LOMOR Ne ROeHS sualiekesaMleweh events eilsuclevers 263
MGeneraly Features auca.cve mores ele etic ee Se eer PT RLS rek ie oioiscons 263
lEVerimary, period of nerve development eesiciie eer aea ce 264

iii Muscle; iditierentiation: 4.0 2 eqns ont ene ne aeuen e Oeeke macialetore 267
IVeOuterowth (of the nerves. soc. reise ie oeine ae ieee echone ee rae 270

B. DEVELOPMENT AND VARIATION OF THE CUTANEOUS NERVES............... 270
PeAnternior border’ Nerves’ 2uis. scene ede cee ee oC re cis Ooi eore 271

APD CUCLOD INGE. = vrs ake Sass STOO OS OE neo ene 271

OPRVGTAGUON: 23k nats COOL Cee eee ene 274
ie Cutanecous nerves of the femoral eroupso.+ 420 eee ce ee 200

ER DECVCLODINEIUE Hvac. ators eee COE OT I tet od see os 200

OMAVOTIGTLOT os s(t dic) pF ARTA OO OC ee eas 278

i Naicutaneus: Lemorismlareaew sees 5 entero eos a.censi acts 278
2. Separate anterior cutaneous. nerve.................. 279
3. Anterior and medial cutaneous branches of the femo-
TANCE Vier scare tice ree cetera RIES eS eS Craver oes eis Sse 283
Ae Noe SAD EMUS Ms were aee TOR Ae oar eel hie iees ae a 284
Mie Cutancous branchyor. thejopunatorsnenves-no see) oe ee eee 285
IV APACCESSOLY, ODLUTALOL MCL Curie pre eree t tea) erate ee es Rees 285
VeaGutarecous nerves -orethessclaticaSroupaen sce coe ae cee 286
OPE MLUTY OTIC ACUCLO DICE AE ene ae eee eee 286
i Ne cutancussremonissposterions soe. sr. aaetae eee os 286
OPIN SULALISE At: eerie eta ee ree eine sw cieee a: wttondearele 287
3. o NG ASUS Slater alig meee Ney Tee hres a Sey. teh 287
As UN DORON ee ees a PRA ans eat As chee S cack bea OSE Gee 287
5. Cutaneous branches ofthe tibial nerve... .o.%.0c7.. 37. 287

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.
18

260 The Nerves and Muscles of the Leg

DENVIGTEGUON: Soxciw wvwilecors, © Siow ele yoreie le lean honsiea7 sears telenehelle (oletelete onetoncnere 288
ie Ne cutancusefemonrnisepostenlone ce seein 288
2 PerroratineacutanecouSmniehveraree ee eeniconeeee ete 292
3. Cutaneous branches of the peroneal nerve........... 292
a. Ni. cutaneus sure mlatenailistyray.cceeie cictecier is einerieie 292
be Ne cutancus speroneigeremonallism qian cere 293
ec. Nn; cutaneiadorsalesmpedisnnas ccc cero ere 293
4, Cutaneous branches of the tibial nerve.............. 296

VI. Cutaneous branches to the inferior extremity from the dorsal
divisions of therspinalemenves*. oe eerrieoecee reece eer 297
Ville Summany- and “conclusions aeirasrrieiereiret= teeiciaoeicicke erate cieksraienerenen 297

C. DEVELOPMENT OF THE MUSCULATURE AND THE DEVELOPMENT AND VARIA-
TION IN DISTRIBUTION OF THE NERVES TO THE MUSCLES OF THE

INFERIOR) EXTREMITY, Wicyrs:s:cveieuete ahels eter clears evekesol nist cvelonesete ter oneaereteners 300
Pe ahemoral 2S Ow cd ceteeeeisie cies eee seals Gein eee eiedehelehaiske Siewerexe neers 300
(Oy JOM RV OMHKE CALAN OVUGIE “SaobdoopapeosddeabsobeG6dso0c adc 300
1, General) TEAbUTES Vacisieinets crete wie ete eles ete) clausholeusestspenereeoints 300
2 MNGi Vid walsmiUSCles: Paks csoerctere ces oh orracee cert eee 301
TI OPSOAS esis Ferric Sten eke ere aie ae eee nae aca eereeeene 301
IPCCUIMSIIS Eri areratets take! ccoratovels era lotic tals pe tonttoastevonerons elonekeaes 302
SAMCOLIUS) lexcisrctave Sine evden te ce ete eiohians lepers erelden as Nolte er enone 303
QuadTrICEPSTEMONIS. Gee sesctecio sole ee ous eles oo eet ereereeiete 304
Os IN CTV EU OLVUALI ON isan AN ap cite a ees sc oncd el oxe eee NOL IE SR eer 306
1. Variation in origin of the femoral nerve............ 306

2. Relations of the branches springing from the femoral
nerve, totthe. nerve roost. ise 26s se cache ieee 306

3. Variation in the association of the terminal branches
arising from ythesremoralenerviere. srs ceo 308
PEVODEMRAtOLASTOUD: oi. Sirs. Sona o eloteneete meter ie tee Eanes niet de etone ee Rer oer Sula
GPE MOTVONICTLCDEClODINENT. aa ee eee aii!
General. L6atures: sinsaoecuscew cuice cchotersinckerers ete ener Buell
Awinrdividual MUSCLES eer teomcoeeiee eae eenoe 312
GLAGCIIS® 32 s.5.0504 reise deneleracleleisverstioue te ieeren Tole eae iG eee 312
Adductor Drevis® <3.cc wadremioe serosa aki eee 312
Adductor- longus” c..lekewieraccec eee oe aoe 312
Obturator’ externus) 242. c ose eee rain 312
Adductor- Magnus: 2. Sees ae Oe ELC eee 313
Comparative anatomy of the adductor group........ ule}
OMNCTUCLUGTMALIONIn. the AQULE =. suc Gee oe 314
1. Variation in the origin of the obturator nerve....... 314

2. Relations of the nerves springing from the obturator
menve Lowe spinal nerves... oe eee eee eee 314

3. Variation in the branches of distribution arising from
UReFODUUTALOL Nerve: jac. s accion oe eee CSneEee 316

Charles R. Bardeen

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Ch, LHTOPOKOOKO CAMO MME coasbcondando0bagqcocdoudnoGo0KGE

Do ACHE COMMONS. poo oooduanvoben ob noSbonocoboOoGU DUO OD OdO

1. Separate origin of the peroneal and tibial nerves.....

2. Frequency of variation in origin of the peroneal and

CIDTAIS MET VES: Late, sac ervele Succes eva cealebei ehetowemee tence mieten movers avai

3. Relations of the branches springing from the pero-

neal and tibial nerves to the nerve roots...........

LVAS SUDETIOMe 216A SOW DA. 5 eisnsccce wasestoners Geieieus sonar chester sete eucnerel ehoi/everte
Oh SUG EORV OOK KAN OMAN Godaacc buono onodaconaGudongooKgnE
General LEAtu res oc 5 Sesrcisrac sve eters ences ayo varctevarn s. sueker

Ae aACbhy CHEN, WAHIONES GacssoaccosscusopaccaoovoDGeDOUC

MensSOr: LASCIES Tate. 1s ssvandossara eee tatees ele er eusteleias Sr okerines evens: ©

Gluteus mMedius) and min Seeereeee ee reiereeteieie sree)

PitifOrMiS. <%.2). 3 eve otelaln esevscecetone Aimee Pelee ere ors ever e

1. Variation in origin of the superior gluteal nerve and

THE NEEVELLO) ChempIFMOLMN Sept acelin

2. Variation in the branches of distribution............

V. The gluteus maximus and the short head of the biceps........
a. Embryonic development of the gluteus maximus.........

On AHONSANE tN VETO TO LULCOUMNEi Caner icice:

c. Embryonic development of the short head of the biceps....

VI. The mm. obturator internus, gemelli, and quadratus femoris....
ORE ORUONUGIACUCLODINGN baat ere iia

i ObDtUraAlor ImternuwsPandiesemcliitrrn seer eeieiiereicre sion

2 QUAATATUS LLCMOLFIS ty. yar mutes ie Neriere sare io ciate. «

DEN CVC = VOTIGLIOM. arava tartenetel deters oi tere ra Seka e one eer one niece oS wicks &

fe Variations oriole mre ete oo cericm cicloneiee a a ere

7, Neheeynorn sh Chinen, ooccccoscsncocusoudeodoadn

Weiner hamstrings miSClesi. < ctaciersusienstersiayeteer aioe oetoiesucharene teens tele evens
Oy LT ROTVONC COVHOVMAHUE ocanoecasbceosoououddgusdnasuDoe

i, (General feauunesi miesctwere ane evcheee ote hteroie ncla renee: Societe ore cs

2, Individual “mus Clesitexereseiewea ers Cea a isisG. lo esihe fiscal

Adductor Mmaciismare: myers myn ertics cites wloisrowe cea: cosas
Semimembranosucwmesrrrresty see aera cke reo arena eres cla ares
Semiitendinosuseerrerenerserste ciao chet co selene uiere's sues eis

BICEPS Vite Seeker RARE Tee oi chece relish &. Sse sails bales
OeNCrVELSUDDLY) cuter eerie Pee eticarcienats Giese ars Reith cctes

1. Relations of the muscle nerves to the spinal nerves...

Ze ela biOns stOnuUneESClallGanenVve nner aemiciice seis cerlcicleicles

Willie eroneali mus Glos i) .piie treet rer Porro ere ee he aoe Busters ac avees arerees: &
OREN MUOTUONUCRACUCLODIN CLimmEr Te eioinieee see ict

Dei INORG CISA LUDORIOO!- coin: 0-060 bina oto) Ctis'o GDR ODO ROR er

Oh, SHV VOME WER AOQUIBENE co ocadoagsbadeneo been bol G50.50 60 6c

ike General: sieacuimesperpeieria cae access. ce eos) Seis sole we oetas eae 3

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The Nerves and Muscles of the Leg

2. Andividuall muUSclesP acre steak eis te siege ewes ses Peereneteaone rere
TLDIA] ES: ‘AMTELTOT SS Pareto siete uciess (es ue iedop shone ree iensaor vale] amore

HM xtensor iS 1voTuamM wom SWS ee cers cele ey etele Sa ferreve eens
Peroneus terkius!| eee ee ea ae eee
Extensor. Halll wersmloneuisketyeeicecetcienatisersioloie creeteneretere
Extensor Gisitorum goLeviSee > cena seks ce cierekene ieee

DO: NCYVE GQUStTIDULION, . weirs cle de on eerste ai aye Bichon oneal
X. Musculature of the plantar side of the crus and foot..........
A" GOneral PEAS mG rceopvmois ce analeqale re tere Bitte uso eieionchola dotnet uate
b. Embryonic development and variation in the adult in the
nerve supply of each of the chief groups of muscles.

1. Development of the Gastrocnemius-soleus group......
GAStrOCNEMIUS “cGy tian ian aioe wei ces e elieke Sere cenetoneenene
SOlGUS. 5 ds iceareterscwrs onus, coset olevelotenayer sie ais @.eeehe Geese nero
Plantaris! Wye cctesicrct Bist einen bin wise Selec sue ete eons

2. Development of the deep muscles of the back of the
CLUS BARR aC IES eae ECE oe

a. Popliteug oe tesa eee eee

b:. Deepricruro-pedalesroulpee. cece oe ree
Mexor shall Cis lOnSuSeere cca eee nie

HIexor Gieiconumy slOMeUIS sis ectie cictere ester nierenet
TAIDLIBIIS POSTETION js oesree: serois ie = tsraes erecy aorta ees

3. Nerve supply of the muscles of the back of the crus in
CO GLUE 5 Sree Bros faves o afer al ood Seyeneiey elements wieNer meres

a. Relation of muscle branches to the spinal nerves.

b. Order of origin of muscle branches from the
CiDIAal ‘MEL Ve Fas sied. Sicislavs ce Sys oas, ays ua oie CREM ee

c. Relation to one another of the nerves to the:
Plantaris: jnig 54 ciate cde rsteletaweneebones Lhe ve eer ee
Gastrocnemius: wate Macrae eieln Sere Ceo eeeaee

Caput, rmediale. saa nicteten oh oe ee eee

Capit: Jaterales mse. eae neat eee eee

Soleus; proximal branch ase ee eee eee
POpliteus’. sacs cect ears hc eee ee ee
Tibialis posterior Be. 2 sce e eee eere

Soleus; idistals branche sane ane eee

Mlexora digitorum lonsus eee cee

Hiexor hallucis. loneus- +s. eee eee eee

4. Development and innervation of the muscles supplied
bY Lhe Nateral planta nenvcare Eee eee
Quadratus: plants, ::....2cc sonata oe eee
Abductor Mi<itia quintie...sce eee eee ee
Mlexor dicitiy quintl breviseormeee seein ieee
Opponens, digit (quinti- + sna eee eee eee
IMGCTOSSEL sie. o\< acre a5. ticle omen ee ee notes
Adductor hallucis .. sthc0:..cnaneee ee eee
TAUMDEICAIES, (oo oie > «accowc- ties See eee

Charles R. Bardeen 263

5. Development and innervation of the muscles supplied

OOP MAA TRAE GOKU WEPRDEs oo bc op obo aoduonGeoos 366
INES Cre Ohl=AKormbtcl IRM; cdo caccadocacobopoupDocanc 366
NoHo HOO INNS Gao ecosocadgandoonccoomeazooasans 367
IMD Cove MOR VDIOKENEY loreeNAISA Sts ob ood ooo Undo ODDO Oboe dao oR 367
Comparative anatomy of the intrinsic plantar
boaHU ISTO) eit eee ote ciao Craig o ltd Gobo COCO occa ome 368
6. Muscle branches of the plantar nerves............... 369
ML Sinise hay Eail CeopKolhwispouiss asm ceoadacécaqdooeocooongdboduOdd 369
D. PERINEAL MUSCULATURE AND NERVES OF THE PUDIC GROUP...............; 374
Os IHR OV OME CLVOCMMEDE bacconusasccueoosaasboncbeuoOmoD 374
Os INGRRE CUPUHHOM Uh HIE CHWs occ aaoanenecuckacanenoooedDeE 380

In a previous article in this journal (Bardeen and Lewis, o1), an
outline was given of the early development of the limbs, body-wall and
back in the human embryo. Lewis subsequently, 01, gave a more detailed
account of the development of the arm, and I have recently, 05, described
at some length the development of the spine and of the skeleton of the
leg. The purpose of the following paper is a more detailed account of
the development of the nerves and musculature of the leg and of the
neighboring regions of the trunk and a consideration of the relation of
developmental conditions to variations found in the adult. The em-
bryological studies have been based chiefly on embryos belonging to the
collection of Professor Mall of the Johns Hopkins University, who kindly
placed them at my disposal. The statistical studies of nerve variation
are based upon charts drawn from specimens in the dissecting rooms of
the Johns Hopkins University and at the University of Wisconsin.

A. OUTLINE OF THE DEVELOPMENT OF THE MUSCLES
AND NERVES OF THE INFERIOR EXTREMITY.

I. GENERAL FEATURES.

For a description of the development of the external form of the limbs
and of the chief features which characterize the earlier stages in the
internal differentiation, reference may be made to the three papers men-
tioned above. The posterior limb-bud is first seen as a massing of the
mesenchyme at the posterior extremity of the Wolffian ridge, usually
opposite the 21st to the 26th spinal segments. This mesenchyme arises
in part from the axial mesenchyme, in part possibly from the somato-
pleure. There is no good evidence that in the mammals the myotomes
contribute directly to it. On the contrary the myotomes are sharply
marked off by a limiting membrane from the mesenchyme of the limb-

264 The Nerves and Muscles of the Leg

bud until this has become extensively developed. Afterwards this lim-
iting membrane disappears, but there is little likelihood that cells de-
rived from the myotomes then wander any considerable distance into
the limb-bud. See Bardeen, oo. A capillary network connected with
the umbilical artery and the cardinal vein is formed in the limb-bud at
an early period. Somewhat later nerves extend into the limb. At the
same time the mesenchyme begins to be differentiated into skeletal, mus-
cular and dermal regions. During the development of the limb it shifts
distally so that the distal margin of the limb-bud is brought opposite the
27th and 28th, and sometimes also the 29th, spinal segments. As this
occurs, bundles of nerve fibres from these more distal spinal segments
extend into the lmb-bud to contribute to the posterior nerves of the
limb. In the adult the most distal nerve to contribute to the nerves of
the limb varies from the 26th to the 29th, but is most commonly the
28th. (Bardeen and Elting, or). The number of spinal nerves contrib-
uting to the chief nerves of the limb varies from six to nine, but is usually
seven or eight (Op. cit.). These variations are in all probability asso-
ciated with variation in position of the limb-bud to the spinal axis during
embryonic development.

The development of the main nerve trunks of the limb may be called
the primary stage of nerve development and the associated variation in
origin of the nerves, primary variation. As opposed to this primary
development and primary variation we may call the growth which dis-
tributes the nerves within the limb the secondary stage of development
and the variation there found secondary variation. During the primary
period the spinal nerves send fibre bundles by direct paths to certain
cutaneous areas and muscular anlages. During the secondary period
the cutaneous nerves extend over the surface of the limb from the areas
to which they are first distributed and the muscle anlages become differ-
entiated into specific muscles to each of which nerve branches are given.

II. PRIMARY PERIOD OF NERVE DEVELOPMENT.

The general structural relations at the period when the nerves begin
to extend into the limb-bud are shown in Plate I, Figs. 1 and 2. In
Fig. 1 are shown the right limb and the distal half of the trunk from
the 17th (9th thoracic) to the 29th (4th sacral) spinal segments in Em-
bryo II (length 7 mm., age 26 days). The limb-bud lies opposite the
21st to the 26th spinal segments. The ccelom extends to a point opposite
the 26th segment, but in the region of the limb it does not extend so far
dorsally as in the thoracic region. In the figure several of the myotomes

Charles R. Bardeen 265

of the left side, the axial mesenchyme, the aorta, the left cardinal vein,
the intestines and the uro-genital organs are not shown. A portion of
the right cardinal vein and a portion of the right umbilical artery are
represented, reduced in size for the sake of clearness. The umbilical
artery curves about the distal extremity of the celom. From the um-
bilical artery a branch passes into the limb-bud. Veins pass from the
limb-bud into the cardinal vein. The blood-vessels of the limb exist at
this time in the form of an irregular plexus.

The second, third and fourth lumbar nerves may be seen sending
spreading bundles of nerve fibres into the dense tissue of the limb,
dorsal to the cardinal vein. They extend, however, for no considerable
distance into the limb-bud. The myotomes end abruptly near the base
of the hmb-bud.

Plate II, Fig. 1, represents the tissue differentiation in a section
through the posterior limb-buds of Embryo II. At the left the bud is
shown cut through an area near the distal extremity of the cclom. At
the right the cut is more dorsal and extends through the tips of the
lumbar spinal nerves.

In Plate I, Fig. 2, are shown the right limb and the posterior half
of the trunk from the 26th (8th thoracic) to the 30th (5th sacral)
spinal segments in a slightly older embryo (CLXIII, length 9 mm.).
Bundles of nerve fibres from the five lumbar and first two sacral nerves
have become anastomosed into a plexus from which in turn four nerves
have sprung. These represent the femoral, obturator, tibial and per-
oneal nerves. Within the limb the central mesenchyme, near the axis
of the embryo, has become condensed. This condensed mesenchyme
represents the femur and hip bone of the adult limb. In the drawing
the outline of this sclerogenous tissue is made diagrammatically sharp.
The femoral portion of the skeletal mass fades gradually into the un-
differentiated mesenchyme of the distal portion of the limb. It is this
skeletal mass which seems to divide the bundles of nerve fibres into the
four main divisions which constitute the origin of the four chief nerves
of the limb. The main artery and vein of the limb are represented at a
reduced scale. The border vein at this period is well developed (see
also Fig. C, Plate III of the article by Bardeen and Lewis, or).

The differentiation of the tissue of the limb-bud, first noticed in a
condensation of tissue in the region corresponding to where the femur
projects against the hip girdle, is quickly followed by further changes.
Externally there becomes visible a differentiation of the limb into foot-
plate, crus and thigh, while within the limb-bud the further development

266 The Nerves and Muscles of the Leg

of the skeleton is marked by condensation of tissue, scleroblastema, to
form the anlage of the skeleton of the foot, leg, thigh and hip girdle.
About the scleroblastema is a myogenous zone, the myoblastema, com-
posed of a slightly less dense tissue. In Embryo CIX, length 11 mm.,
this zone is best marked in the region of the hip (Plate II, Fig. 2). It is
not clearly defined in the foot region. Between the myoblastema and the
ectoderm lies a zone of less condensed tissue, the dermoblastema.

The chief nerves of the limb extend into the myoblastema. This is
not a homogeneous layer. On the contrary from the time of its forma-
tion regions which represent the anlages of muscles or groups of muscles
may be more or less clearly distinguished from regions which represent
intermuscular spaces. In Plate III, Figs. 1 and 2, an attempt has been
made to outline the muscle masses which represent the anlages of future
muscle groups in Embryo CIX, length 11 mm. It is impossible to do
this with exactness because the various regions are indefinitely bounded.

In this embryo the pelvic portion of the skeleton consists of a central
region continuous with the head of the femur. From this central ace-
tabular portion spring iliac, ischial and pubic processes, The femur is
short and thick. The tibia and fibula are fairly definitely outlined, the
foot-plate less definitely so.

The main nerve trunks have grown for a considerable distance into the
limb. From them several of the chief muscular and cutaneous branches
have sprung. The figures show these branches fairly well. In addition
to the intrinsic nerves of the limb the anterior and posterior border
nerves are also represented.

In Fig. 1 it may be seen that the myotomes in the region of the body
wall have fused to form the anlage of the abdominal musculature. The
lower margin of this extends distally about to the 21st spinal (1st lum-
bar) nerve. In Fig. E, Plate V of the article by Bardeen and Lewis,
o1, it is represented slightly too short. From the ventro-posterior ex-
tremity of the abdominal musculature a somewhat indefinitely differenti-
ated band of tissue may be followed to the pubic process of the pelvic
girdle.

A slight communicating branch connects the twelfth thoracic with
the first lumbar nerve. The main portion of this latter nerve extends
forward on the internal surface of the distal margin of the anlage of
the abdominal musculature and gives off a lateral, “iliac,” branch.
Ventrally the nerve divides into branches which represent the hypogastric
and inguinal nerves. The 1st lumbar nerve also gives off a branch which
passes to the lumbar plexus.

Charles R. Bardeen 267

The obturator nerve arises from the first four lumbar nerves, passes
through the obturator notch of the hip girdle and divides into two main
divisions. Each of these terminates in a differentiated mass of tissue,
the more anterior of which represents the adductor longus and brevis
and the gracilis muscles, the more posterior, the obturator portion of the
adductor magnus and possibly also the obturator externus muscle.

The tibial nerve arises from the fourth and fifth lumbar and first
three sacral nerves. From it branches pass to muscle masses represent-
ing the obturator internus, quadratus femoris, hamstring, and the super-
ficial and the deep posterior crural musculature. Distal to the tibial
nerve the posterior cutaneous nerve of the thigh and the pudendal and
caudal nerves may be seen.

In Fig. E, Plate V of the article by Bardeen. and Lewis, o1, the
urachus was represented much foreshortened in order to reveal the
muscle masses of the leg. In Fig. 1, Plate III, the urachus is outlined
in its true position as seen directly from the side.

In Plate III, Fig. 2, the genital and lumbo-inguinal nerves are seen
passing ventro-laterally from the junction of the 1st and 2d lumbar
nerves. The femoral nerve is seen passing outwards over the region of
the acetabulum. It is surrounded laterally by the ihopsoas muscle mass
and terminates in the quadriceps femoris muscle mass. From it arise
lateral and anterior cutaneous branches, a branch which passes to the
sartorius muscle mass, and the saphenous nerve.

The peroneal nerve arises from the 4th and 5th lumbar and first two
sacral nerves, gives off branches for the anlages cf the superior gluteal,
inferior gluteal, short head of the biceps and peroneal muscle masses and |
terminates in the anterior crural muscle mass.

An idea of the relations of the main nerves as they enter the limb in
Embryo CIX may likewise be gained from Plate III, Fig. 3. The pelvis,
the abdominal and dorsal musculature, the lining of the body cavity, the
border nerves and the main nerve trunks of the limb are here represented
as viewed from in front. The femur and the main nerve trunks are
shown cut in a plane somewhat distal to the head of the femur. The
division of the main nerve trunks into separate branches for individual
muscles is schematic.

III. MUSCLE DIFFERENTIATION.
At the period under consideration several possibilities of muscle differ-
entiation must be considered. ist——The tissue which represents the
muscle masses just mentioned may extend into the limb-bud with the

268 The Nerves and Muscles of the Leg

nerves and become differentiated as the muscle branches are given off.
The fact that Harrison, 04, has shown that in the tadpole muscle differ-
entiation may take place when no nerves are developed makes this pos-
sibility highly improbable. 2d.—The ingrowth of the nerves and the
development of muscle branches may cause a “ precipitation” of pre-
muscle tissue about these branches. This likewise is rendered improbable
by Harrison’s experiments. 3d.—Muscle differentiation begins in specific
regions. Under normal conditions this differentiation begins simulta-
neously with the ingrowth of the nerves into the limb. Muscle branches
extend into the differentiating musculature, owing perhaps to some
specific attraction exerted upon the growing nerves. This seems on the
whole to be the most probable course of development. The considerable
variation shown in the origin and distribution of the nerves to the muscles
renders it not improbable that their ingrowth is due in part to some
special attraction exerted by the developing musculature upon the grow-
ing nerves, and variously responded to by the latter.

The paths opened up for the growth of the nerves to the muscles are,
however, at first not as a rule in regions in which muscle tissue is to be
differentiated, but in intermuscular areas. Thus the chief nerve trunks
usually grow along paths which lie between main muscle groups. As
the muscles of these various groups become differentiated the main nerve
trunks of each muscle group are distributed in the septa which separate
the individual muscles and finally after a nerve has entered the muscle
for which it is destined it is usually distributed at first in the coarser
intramuscular septa. During the early stages of development, however,
the true muscle tissue cannot be sharply distinguished from the tissue
which is to make up the skeletal framework of the muscle. For this
reason it often appears as though the nerve to a muscle plunged at once
into the midst of muscular tissue.

At a slightly later stage of development than that of Embryo CIX the
differentiation of muscular tissue from the skeletal framework of the
musculature is much better marked than in that embryo. Thus in Em-
bryo CXLIV, length 14 mm., the individual muscles of the thigh may
many of them be clearly distinguished (Plate II, Fig. 3). It may be
seen in this embryo that although muscle differentiation in a given muscle
is most clearly marked in the region where the respective nerve has come
in contact with or has entered the muscle, the differentiation is not hm-
ited to this area but extends for a considerable distance toward the
skeletal areas to which the muscle is to be attached. It is probable, how-
ever, that the differentiation of a given muscle begins as a rule in a

Charles R. Bardeen 269

region which corresponds with the site of entrance of the chief nerve of
that muscle. In Plate IT, Fig. 3, several nerves and muscles are shown.
The nerve to the gracilis muscle shows especially clearly. From this
region the gracilis muscle may be traced in successive sections toward the
pelvis and toward the tibia. The entrance of the inferior gluteal nerve
into the gluteus maximus muscle also shows well in the figure. The two
parts of the adductor magnus muscle, the obturator and sciatic portions,
are shown near the site of entrance of the respective nerves. The semi-
tendinosus muscle and the two heads of the biceps are shown cut at some
distance from the site of entrance of nerves. About the two divisions of
the sciatic nerve there is some dense tissue which probably does not,
however, represent muscle tissue.

It is to be noted that during these earlier stages of muscle differentia-
tion the muscle anlages are often connected at one extremity, less fre-
quently at both extremities, with the skeletal anlages to which the muscle .
is subsequently attached. The tendons of the muscles are developed in
continuity with the anlages of the muscles. As a rule the differentiation
of the longer tendons begins in the vicinity of the muscle bellies and
gradually extends toward the skeletal attachments.

In a considerably older embryo, CXLYV, length 33 mm. (Plate II,
Fig. 4), differentiation of the muscles is much further advanced. Not
only the muscles but also the fasciculi are separated by a large amount
of connective tissue. This shows especially well in the gluteus maximus
muscle. The main branches of the nerves of the muscle may be followed
in the larger intramuscular septa, the smaller branches in the smaller
intramuscular septa. I have elsewhere described the intramuscular
growth of nerves in the mammals (Bardeen, 00 and 03). It is of
interest to note that after muscle differentiation is well under way there
is relatively a much greater amount of connective tissue in the muscu-
lature of the embryo than in that of the adult.

After the stage of development exhibited by Embryo CIX the condi-
tions within the limb become so complex that they can be better fol-
lowed by tracing through the development of specific groups of nerves
and muscles than by attempting to picture all the details of each suc-
cessive stage of differentiation of the whole limb. In order, however, that
the relations of specific groups of nerves and muscles to the general
structural condition of the limb may be followed we shall first briefly
describe the relations of the peripheral nervous system to the skeleton
at two important stages of development.

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The Nerves and Muscles of the Leg

IV. OUTGROWTH OF THE NERVES.

In Embryo CXLIV (length 14 mm.) the main nerve trunks are well
developed as far as the foot. The relations of the nerves to the spinal
column, abdominal musculature, skeleton of the limb and the surface
of the limb are represented in Plate IV, Figs. 1 and 2. The 12th
thoracic nerve sends a communicating branch to the first lumbar and
from this latter arise the hypogastric and inguinal branches.

From the first lumbar nerve a branch is also given off to the lumbar
plexus. From the 1st and 2d lumbar nerves arise genital and lumbo-
inguinal branches. ‘The femoral and obturator nerves arise from the
Ist, 2d, 3d, and 4th lumbar nerves and give off the branches shown in the
figures. The sciatic nerve, which arises from the 4th and 5th lumbar and
first three sacral nerves, is composed for the greater part of its course
of separate peroneal and tibial nerves. The various muscular and cutan-
eous branches are labeled in the drawing.

In Embryo XXII, length 20 mm. (Plate V, Figs. 1 and 2), the various
nerves mentioned are much more highly developed than in Embryo 144.
This difference of development is especially to be noticed in the feet. The
figures indicate sufficiently well the relations of the nerves to the skeletal
apparatus, the skin and the abdominal musculature.

A noteworthy fact brought out by these figures is that the. cutaneous
nerves are distributed at first to the anterior, distal and posterior margins
of the embryonic limb, while the dorsal and ventral regions of the hmb
are given up to the differentiation of musculature.

Having thus considered in brief outline the more general features in
the development of the muscles and nerves of the posterior limb we shall
take up in turn a more specific study, first, of the development of the
cutaneous nerves and then of that of the muscles.

B. DEVELOPMENT AND VARIATION OF THE CUTANEOUS
NERVES.

Grosser and Frohlich, 02, have given a good account of the development
of the cutaneous nerves of the trunk. I have been unable to find any
specific account of the embryonic development of the cutaneous nerves
of the limbs, although the work of Sherrington, Head, and others on the
segmental distribution of these nerves makes it of interest to inquire
whether or not embryonic conditions can help to explain the phenomena
these authors have described. In the following section the embryonic
development and the variations in distribution of specific groups of nerves
are first described and then the more general facts disclosed by this study
are briefly reviewed.

Charles R. Bardeen Dveat

I. ANTERIOR BORDER NERVHS.
a. Development.

When the nerves begin to enter the limb-bud this lies, as pointed out
above, usually opposite the five lumbar and first sacral nerves (Plate I,
Fig. 1). The posterior margin of the developing body-wall and the
anterior margin of the lmb-bud usually overlap opposite the 21st seg-
ment. The nerves arising from the 21st spinal (1st lumbar) nerve are
therefore true border nerves, being in part distributed to the abdominal
wall and in part to the limb. The 20th and 22d spinal nerves (12th
thoracic and 2d lumbar) also usually contribute to a greater or less ex-
tent to both regions, the 20th contributing to the cutaneous supply of the
leg, the 22d slightly to the extreme margin of the abdominal musculature.

In Embryo CIX, length 11 mm. (Plate III, Figs. 1, 2 and 3), the
border nerves are beginning to extend toward the skin. At this stage the
oblique and the rectus muscles of the abdomen are beginning to be differ-
entiated. The transversus muscle has not yet appeared. Between the
ventro-anterior margin of the pubis and the ventro-caudal angle of the
differentiating abdominal musculature a slight thickening of the
mesenchyme represents the beginning of the tendon of the rectus and of
the inguinal ligament. A considerable interval exists between the distal
margin of the abdominal musculature and the anlage of the iliac crest.
The musculature hes near the peritoneal cavity, while the crest is in the
mesenchyme lateral to this cavity. Between body cavity and crest lies
the femoral nerve with its branches (Fig. 3). From the first and second
lumbar nerves the iliohypogastric and inguinal and the genital branch *
of the genito-femoral extend ventrally between the ccelomic wall and the
distal margin of the developing abdominal musculature. From the com-
mon trunk of the ilLohypogastric and inguinal nerves a lateral branch,
the “iliac,” extends toward the skin in an area considerably anterior to
the ilium. The lumbo-inguinal nerve and the lateral and anterior cuta-
neous branches of the femoral extend toward the anterior margin of the
limb-bud.

In aslightly older embryo, CXLIV, length 14 mm. (Plate VI, Fig. 1)
differentiation of the abdominal musculature has proceeded much fur-
ther. The external oblique muscle is a thin sheet, somewhat wrinkled in
the specimen. In the figure merely its origin from the lower ribs is
shown. It extends distally into a sheet of mesenchyme which is thick-

1The term “ genital’ nerve is here used in preference to ‘“‘spermaticus
externus.”

272 The Nerves and Muscles of the Leg

ened at its distal border into an embryonic inguinal ligament (lig. ing.).
This latter extends from an anterior mesenchymatous process of the ilium
toward the pubis. Ventrally it becomes continuous with the blastema
of the pubic crest. Beneath the external oblique lies the internal oblique
muscle. Distally this is connected by a mesenchymatous membrane with
the inguinal ligament. In the figure merely the costal and inguinal por-
tions of the muscle are shown.

The transversus, muscle is differentiated immediately beneath the
peritoneal membrane. It is not clear whether the material of the trans-
versus musculature is derived from the ccelomic ning or from the
myotomes. If from the latter the tissue wanders along the peritoneum
from the region of the ribs.

At this early stage the anlage of the processus vaginalis may be seen
in the form of a thickened mass of tissue which is continued from the
plica gubernatrix through the internal oblique muscle and the aponeuro-
sis of the external oblique above the inguinal hgament to the junction of
the thigh with the trunk. Here it spreads out into processes which ex-
tend on the one side toward the mid line of the body, on the other toward
the femur.

Between the transversus musculature and the internal oblique run the
main trunks of the thoracico-abdominal nerves. The ilio-hypogastric and
inguinal nerves pierce the internal oblique muscle and the aponeurosis
of the external oblique much as in the adult. The iliac branch of the
iho-hypogastric, however, pierces the oblique muscles in a region anterior
to its relative adult position. This is also the case in Embryo XXII,
length 20 mm., Plate V, Fig. 1. Beyond the region of the inguinal nerve
the coelomic wall, backed by a thickened membrane representing the trans-
versalis fascia, curves medially while the oblique musculature takes a
somewhat lateral direction toward the inguinal ligament. Between the
two is a space in which lie the femoral nerve, its proximal branches and
the anlage of the ilo-psoas muscle. The genital branch of the genito-
femoral nerve follows along the ccelomic wall almost parallel with the
hypogastric and inguinal nerves but converging toward the latter. ‘The
point “ X ” in the figure represents a region where later the peritoneal
wall will be pushed laterally over the ilo-psoas muscle so as to cover
this and be brought in contact with the iliac crest. The lumbo-inguinal
nerve passes out beneath the inguinal ligament in the vicinity of the
femoral artery. It probably represents a lateral branch of the genito-
femoral considered as the ventral division of a typical spinal nerve.

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~
Ce

Charles R. Bardeen

Ventrally the genital nerve, usually after anastomosing with the in-
guinal, passes along the vaginal process through the aponeurosis of the
external oblique and over the inguinal ligament to the thigh. It is inter-
esting to note that this development considerably precedes the descent
of the testicle.

In Plate VI, Fig. 2, the border nerves of Embryo XXII, length 20
mm., are pictured. It is somewhat difficult to trace with certainty the
border nerves in this embryo, but the figure is believed to illustrate ap-
proximately the actual relations. While in Embryo CXLIV a consider-
able interval separates the anlage of the iliac crest from the distal margin
of the abdominal musculature, in Embryo XXII the crest is much fur-
ther developed and at the same time has been rotated toward the dorsal
portion of the distal margin of the oblique abdominal musculature. This
at the same time has extended distally and become attached to the iliac
erest. Meanwhile the peritoneal wall has bulged laterally so that the
fascial extension of the transversus muscle covers the ilio-psoas muscle
in the region of the pelvis and the transversus muscle has formed its
pelvic attachments. The main trunks of the border nerves have been
brought by these changes into relations which closely resemble those char-
acteristics of the adult. Adult conditions are reached by some further
relative shifting of parts and by the growth of the nerves within the
areas for which they are destined.

The segmental relations of the border nerves may be best understood
by comparing the position of the pelvie girdle when the nerves first ex-
tend toward the skin with the condition brought about by the shifting of
the girdle. See Plates III, 1V, V and VI. In Embryo CIX, the stage in
which the nerves first extend toward the skin, the border nerves arise
from the spinal nerves in the following order: iliohypogastric, inguinal,
genital and lumbo-inguinal. As these nerves grow forward there takes
place a rotation of the base of the hmb medially, ventrally and posteriorly.
At the same time the spinal column becomes straightened and the limb-
bud as a whole descends posteriorly. The pubis is carried from a point
opposite the 21st (12th thoracic) segment to a point opposite the 26th,
and at the same time the posterior margin of the ilium is usually brought
to lie opposite the 26th and 27th vertebra to which it becomes attached.
The two pubes are carried forward ventrally until they are united by the
symphysis pubis.

As the pubis rotates ventrally and posteriorly the inferior portion of
the abdominal wall is extended in a corresponding direction. The ven-

Ca)

74 The Nerves and Muscles of the Leg

tral margins of the distal portion of the rectus muscles are brought into
approximation when the symphysis pubis is formed. By the rotation of
the hip bone the crest of the ilium is brought up against the dorsal por-
tion of the distal margin of the abdominal musculature. The ventral
portion of this margin becomes converted into the inguinal ligament.
The courses of the abdominal nerves and the hypogastric and inguinal
nerves are determined by their positions in the abdominal musculature.
The genital nerve takes a more direct course towards its region of termi-
nation, although it too is usually bound up for some of the distal part
of its course with the distal margin of the abdominal wall.

The peripheral region to which the lumbo-inguinal nerve extends is
carried in a ventral, medial and posterior direction by the rotation of the
limb. The main trunk of the lateral cutaneous nerve is caught by the
rotating hip bone usually in the vicinity of the future anterior superior
iliac spine and is carried up against the inguinal ligament. Thus by this
rotation and shifting marked changes in the relative positions of the
more anterior nerves arising from the lumbar plexus are brought about.

b. Variation.

A study of variation in the distribution of the fibre bundles of the
spinal nerves to the various peripheral areas of the limb reveals the fact
that any two nerves shown in Plate III, Figs. 1, 2 and 3, may be com-
bined into a single trunk when they arise ordinarily in succession, but
not otherwise. Thus the 12th thoracic and the hypogastric, the hypo-
gastric and the inguinal, the inguinal and the genital, the lumbo-inguinal
and the lateral cutaneous, the lateral cutaneous and the femoral, may be
bound together for a greater or less part of their courses from the plexus
to the limb. On the other hand, two or more nerve trunks may serve
to convey fibres commonly carried in a single nerve. Separate iliac
branches, extra lumbo-inguinal and genital nerves belong to this category

as do also those “ ?

middle cutaneous ” nerves which arise directly from
spinal nerves, and the accessory obturator nerve. The frequency of varia-
tion of this sort in the border nerves I have previously described in this
journal, o2. In the same paper I have treated of the frequency of
variation in segmental origin of the various border nerves. This is most
marked. Thus the hypogastric nerve arose in 2% of instances from the
19th and 20th spinal nerves; in 32% from the 20th; 34%. from the 20th
and 21st; and 32% from the 21st. The iliac arose in 2.1% of instances
from the 19th and 20th spinal nerves; 27.4% from the 20th spinal nerve ;

cas)
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Or

Charles R. Bardeen

37.7% from the 20th and 21st spinal nerves; and 32.7% from the 21st
spinal nerve. The inguinal nerve arose from the 20th spinal nerve in
3.5% of instances; from the 20th and 21st in 38.3%.; from the 21st in
51.5% ; and was absent in 6.6%. The genito-femoral nerves arose
from the 21st spinal nerve in 19% of instances; from the 21st and 22d
in 79% of instances; and from the (21st) 22d and 23d in 2% of in-
stances. In 1.2% of instances no lumbo-inguinal (crural) branch was
found. It is probable that the variation in origin of the border nerves
is due in part to a variation in position of the base of the limb-bud with
respect to the spinal column, the more anterior spinal nerves serving to
supply the limb when the limb-bud has a more anterior position at the
time of the outgrowth of the spinal nerves. There is, however, no perfect
correspondence between variation in origin of individual border nerves
and that of the border nerves as a group.

In the same paper I showed that out of 133 instances, in 27 (20.30%)
the lumbo-inguinal (crural) nerve emerged from the pelvis into the thigh
in a lateral (external) region; in 81 instances (60.9%) in the middle
(anterior) region; and in 25 instances (18.8%) in a medial (internal)
region. After the nerve has passed into the thigh it may have a slight, a
moderate or an extensive distribution to the skin. While this distribution
usually corresponds to the region of exit, lateral, middle or medial, this
is not always the case. Jor instance, a nerve emerging laterally may
send a branch over to supply the fascia on the medial side of the leg. The
following table indicates the frequency and extent of distribution of the
lumbo-inguinal nerve to the skin of the lateral, anterior and medial por-
tions of the thigh. By “lateral” region is meant an area lying lateral
to a line drawn from the anterior inferior spine of the ilium to the
lateral edge of the patella; by “ medial,” an area lying medial to a line
drawn from the medial margin of the hip joint to the medial edge of the
patella; and by “anterior,” the intervening area. By “slight distribu-
tion” it is meant that by gross dissection the branches of the nerve could
be followed but a short distance below the inguinal hgament; by “ ex-
tensive distribution” it is meant that the branches could be followed
readily over half way down the thigh. By “ moderate distribution ” is
meant a distribution lying between these extremes. It will be understood,
of course, that no hard-and-fast lines can be drawn between the various
types of distribution tabulated. The table is intended merely to give an
idea of the approximate frequency of distribution of the lumbo-inguinal
nerve to approximate areas.

19

276 The Nerves and Muscles of the Leg

TABLE I.
Table Showing the Region and Extent of Distribution of the Lumbo-
inguinal Nerve.

EXTENT OF DISTRIBUTION.
—

Type of Distribution. Slight. Moderate. Extensive.
No. of No. of No. of
inst. inst. inst.
ILE AgseanogedbouoodeT 5 8 6 19 15.4%
ANMGPHOTE Gg id. d Soo ooo ado OOC 11 36 8 55 44.7%
IMS Gea ohato oes ee ee ela wate wae 6 15 10 31 25.2%
Lateral and Medial........ 18 18 14.6%
22 59 42 123

(17.9%) (48%) (34.1%)

From this table it will be seen that the type of distribution most com-
monly met with is that of a moderate anterior distribution (36 instances,
29.2%). This corresponds to the distribution commonly given as the
“normal” in the text-books and shown on the left side of the widely
borrowed Léveillé figure given on Plate LIV of the Hirschfeld-Leveillé
Neurologie.” The other types of distribution are, however, met with two
thirds of the time. A study of the association of the types of distribution
above given with race, sex and side of body, with various types of lumbo-
sacral plexus and with variations in the spinal column has brought to
light no intimate relations. The following table illustrates the relations
of origin to distribution of the lumbo-inguinal nerve.

TABLE II.

Frequency of Types of Distribution of the
N. Lumbo-inguinalis.

| | |
Spinal Nerves from which the | Lateral. Anterior. Medial. oS
Lumbo-inguinalis arises. | Ea
| 43 eee ih a8 a BS = | ao
Evan esl eS a og S rs S £ as
LO Muon ae z | 2 | S H 1B
7 = ica D = ica na = |
XX, XXI 2 5 1 3
»-O.Gt (rez! 1 ie 4 3 1 1
(XX), XXI, XXII 1 1 1 5 2 3 8
| |
0O-U5, BOC | 3 3 2 8 18 3 4 10 5 3
XXII leet lM 2| 8 1
(¢.6.4§fe-6 cites ©6001 ok (a seta leat 2 1

From this it will be seen that there is slight relationship between the
origin from the plexus and the distribution of this nerve. In case of

Paris, 1853.

wo
3
2

Charles R. Bardeen

origin from the 23d spinal nerve the distribution is extensive in most of
the instances studied.

The inguinal and genital nerves show relatively much less extensive
variation in distribution. My data concerning the variation in their dis-
tribution as well as that of the iliac nerves are less accurate than those
of the lumbo-inguinal so that the latter nerve may serve as an example
of variation in the distribution of the border nerves.

II. CUTANEOUS NERVES OF THE FEMORAL GROUP.
a. Development.

By “cutaneous nerves of the femoral group” may be designated the
lateral (external) cutaneous nerve of the thigh and the cutaneous nerves
which usually spring directly from the femoral nerve. These nerves are
all directed at first toward the anterior margin of the limb-bud. Figs. 1-3,
Plate III, show their situation in an embryo of 11 mm. length. In Plate
IV, Figs. 1 and 2, and Plate VI, Figs. 1, their position is shown in an
embryo of 14 mm.

In this latter embryo (CXLIV) the lateral cutaneous nerve arises from
the main trunk of the femoral, passes outwards through the anlage of
the psoas muscle and approaches the skin near the junction of the anterior

“margin of the limb with the thigh. Several anterior and medial cutaneous
nerves arise from the femoral nerve. The most proximal of these ap-
proaches the surface of the limb-bud somewhat more distally than the
lateral cutaneous. A branch may likewise be followed through the anlage
of the sartorius muscle and two through the septal tissue which divides
the sartorius from the adductor group of muscles. The saphenous nerve
passes between the anlages of the tendons of the sartorius and gracilis
muscles to reach the subcutaneous tissue near the knee (Plate VI,
Big. 1).

In an older embryo, XXII, length 20 mm. (Plate V, Figs. 1 and 2,
and Plate VI, Fig. 2), the further growth of the nerves just men-
tioned may be followed. The lateral cutaneous nerve has spread out in
several branches toward the lateral surface of the thigh. The anterior
and medial cutaneous branches have spread out over the antero-medial,
surface of the thigh, while the saphenous nerve has continued its growth
toward the ankle. During the ventro-posterior rotation of the hip the
lateral cutaneous nerve has been caught near the anterior superior spine
of the ium.

The further growth of these nerves to reach the conditions character-
istic of the adult may easily be deduced by comparing Plate VI, Fig. 2.

278 The Nerves and Muscles of the Leg

with Plate VII, Fig. 1. The fascia lata which covers the lateral, anterior
and medial cutaneous nerves for a considerable part of their course is
just beginning to be differentiated in Embryo XXII.

b. Variation.
1. N. Cutaneus Femoris Lateralis.

The lateral cutaneous nerve in the adult usually springs by one or more
roots from the lumbar plexus and takes a direct course through the psoas
muscle and beneath the iliac fascia to a region near the anterior superior
spine of the illum whence it passes for some distance beneath the fascia
lata and is finally distributed to the skin of the lateral region of the
thigh.

The nerve varies considerably in origin. Out of 287 instances I found
it arising in 39% from the 20th, 21st and 22d; in 48% from the 21st,
22d and 23d; and from the main trunk of the femoral in 18%. (Bardeen,
02).

The region where the nerve passes out into the thigh varies somewhat.
It may be over the crest of the ium just above the anterior superior
spine or some distance below the latter. In two instances out of 146 it
was found to emerge near the femoral nerve and then curve sharply out-
wards toward the lateral surface of the thigh. Rarely it is absent, its
place being supplied by branches which spring directly from the femoral
nerve below the inguinal ligament.

It varies considerably in extent of distribution. The distribution of
the chief branches was found to be lateral to a line drawn from the an-
terior inferior iliac spine to the outer edge of the patella in 92 out of
146 instances (63%), Plate VII, Fig. 1. The area of distribution cor-
responds here essentially with that given as the normal one in most text-
books. In 45 instances (30.8%) the branches of the lateral cutaneous
extended medially over the anterior portion of the thigh taking the place.
to a greater or less extent of the anterior cutaneous branches of the
femoral nerve. An instance of this sort is figured on the right side of
the Léveillé figure mentioned above (p. 276). In 9 instances out of
148 (6.2%) a “lumbo-inguinal” branch, given off by the lateral cuta-
neous nerve, was distributed to the skin of the upper antero-medial
region of the thigh. In two instances out of 148 the lateral cutaneous
nerve was missing, its place being supplied by a large nerve which in
origin, course through the psoas muscle and entrance into the fascie of
the thigh resembled a lumbo-inguinal nerve. In two of the instances in
which the “anterior or middle” distribution was extensive the lateral

cw
~2
ide)

Charles R. Bardeen

cutaneous nerve passed into the thigh near the femoral nerve and then
curved laterally towards the anterior superior spine. In one instance it
gave a large communicating branch to the lumbo-inguinal nerve.

No relationship: between race, sex or side of body and variation in the
distribution of the lateral cutaneous nerve is apparent in the charts.
An extensive distribution on the front of the thigh is somewhat more
often associated with anterior than with posterior forms of lumbo-sacral
plexus. This may be seen from the following table.

TVA Te Relele
Type of Plexus from which the N. Cut. Frequency of Typesof Distribution of
Fem. Lat. arose: the N. Cutaneus Femoris Lateralis:
Most Distal Lateral and | Lateral and
ype. re. inal Nery gly || ;
ave Furcal Nerve | poner News Lateral nee | Medial.
‘ XXIV XXVI 1 | |
| = é
Ant. B XXIV KOKOVALT 6 8
NORV Mehichy |. lust ‘ ‘i ’
C | to sacral plexus | XXVIII 22 15 1
Norm. | D Gina See XXVIII 37 | 16 6
| (XXIV) | Pine ? |
Post. F XXIV xX XIX 8 2 1
Col foe KOMIK 10

The extensive anterior type of distribution is also most frequently
associated with an “abnormal” type of vertebral column, especially a
short one, as may be seen in the following table.

TABLE IV.
Distribution of Lateral Cutaneous Nerve.
Vertebre of Spinal Column. Ry ft, a : ze,
| trees ec Dateral and
MO MONGE DIC SSC4GS © 5 acc-s crete vis, ee melee sare 4
(@ Wlte ll OS 71 Ga eae ee cececnorcieecne o oor eta Cac Gide 2 2
Gale eA MG Sen Giscs. Sia 2. sa: se val dyecls elowel ot aemeyepeene 2
FG ime Woe Me NS tes Csi evay ed, oraiveve ecerlv'ia (eves ebaper spelen i
Himmler MwA SAC cya aux dnct-ankl sheet) sLoisyeceusetonees if
IR UGhiaaKSs NH Ay AP Nas aaotom ono 6 ooe 19 9 2
IN(OIRITEN LS Saracio Cicec ICE EERO ooo cercone 29 ial 7
HICH Ate OSE Sly es cient corals ae wpa oreaeone 1 il
HiCmelter OURS SOC) a ce cee kenales «+s 5 ayareuehet ons atlets 2 3
Gm tE esc Css gis,e is c:6 cit clots Geta setae 2 2

280 The Nerves and Muscles-of the Leg

The extensive anterior type of distribution is also more frequently
associated with an anterior origin of the lateral cutaneous nerve than
with a posterior origin. This is indicated in the following table.

TABLE V.
Distribution of Lateral Cutaneous Nerve.
Spinal Nerves from which Lateral Lateral. Lateral and Lateral and
Cutaneous Nerve arose. ee : eee
No. of No. of No. of
inst. inst. inst.
((.<9;6) iad @.l FD, OG fl enaeartctraie. Geman acre cus cies 10 8 2
XOX) REND «a. 5 isin dosstettnsne eke sgevoronave Se 18 10 1
>, ©.4l AiG SRA ain B.tiets ctor 9 3
(ROK) WOME DONE Sloe 65 og c0.c.oac 25 13 5
EXO TTL “eXEX TAT eee epee cher cuctore 5 3 al
XX EXERC tepaeyeravateveters atte te 10 3
Trunk of femoral Nerves...) ses. 15 3

The relations existing between the various types of distribution of
the lateral cutaneous nerve and the various types of distribution of the
lumbo-inguinal nerve are shown in the following table.

TABLE VI.
Types of Distribution of the Lumbo-inguinal Nerve.
Types of Distribution of =
Lateral Cutaneous Latera). Anterior. Medial. Lat.
Nerve. oO, nil
SUt. Mod. Ext. SI’t. Mod. Ext. SI’t. Mod. Ext. Med.
Watherale ewe ne ckae %,- 2 5 1 4 21 4 3 4 0 9 1
Lateral and anterior 1 2 2 4 3 iL 7 i 4 2
Lateral and medial. 1 i 3 il 1 il 1
Wantine .... . 2 1

The most striking feature brought out by this table is the frequent
association of an extensive anterior distribution of the lateral cutaneous
nerve with a moderate or extensive medial distribution of the lumbo-
inguinal nerve. This is shown in the Léveillé plate referred to above.
This extensive distribution on the thigh of nerves derived directly from
the 21st and 22d spinal nerves is, as has been pointed out above, most
frequently associated with an anterior type of lumbo-sacral plexus and
this in turn probably with a somewhat anterior position of the limb-bud
at the time of the ingrowth of nerves. When the 21st and 22d spinal
nerves are called upon to furnish a greater supply than usual of nerve
fibres to the limb they are more apt to do so through direct paths (the
Jateral cutaneous and lumbo-inguinal nerve trunks) than through the
more indirect route of the femoral nerve and its branches. This feature

oe.

Charles R. Bardeen 281

is further brought out in the not infrequent association with the an-
terior forms of plexus of a direct anterior cutaneous branch from the
plexus to the front of the thigh.

2. Separate Anterior Cutaneous Nerves.

Nerves of this sort spring usually from the XXI and XXII spinal
nerves, but also sometimes from the XXIII and very rarely from the
XXIV as well. Henle considers them as varieties of the lumbo-inguinal.
In their course, however, they usually, at least, lie beneath or deep in

TABLE VII.
Type of Plexus from which the | Wagers bs 7 ee emma
N. Cut. Fem. Lat. arises: £
From From
M Distal Spinal (GXEXT) EXCX XE XOX
Type. Furcal Nerve. pane ae chi ae Spinal Nerves. | Spinal Nerves.
erve to Limb. | No. of instances. | No. of instances.
B XXIV le eXeXO VAT 1 |
C XXIV chiefly to XXVIII 10 1
sacral plexus |
D XXIV chiefly to XXVIII p
lumbar plexus
Ye 11lt 51 68s 2¢ 1
12th rib rudimentary 8 1
Type of
vertebral axis
Not recorded 2 1
|
Normal i

the psoas muscle and beneath the iliac fascia instead of lying above
the latter like the lumbo-inguinal nerve. A direct anterior (high middle)
cutaneous nerve of the thigh was found 14 times in 123 instances
(11.4%). It arose 11 times from the XXI and XXII spinal nerves
and 3 times from the XXII and XXIII (once from a region opposite
the XXIV spinal nerve). In the last instance entrance of fibres from
the XXIV nerve was possible but was not certain. In all instances ex-
cept two it arose in association with an anterior form of plexus. In all
instances recorded except one the spinal axis showed a tendency to re-
duction by the presence of a rudimentary 12th rib and in one instance
there were but eleven thoracic vertebre. These facts are illustrated in
the above table.

ww
OO
ras)

The Nerves and Muscles of the Leg

The following table illustrates the relation of a separate anterior
cutaneous nerve of the thigh to the lateral cutaneous and Jumbo-inguinal
nerves.

TABLE VIII.
Origin of Separate Anterior
Cutaneous Nerve.
Types of Distribution of Lateral pn ee
ire From the From the
Cutaneous Nerve XEXGTODNONGTIT XXL, GRexsaeh
Spinal Nerves. Spinal Nerves.
No. of instances. No. of instances.

MaterauGisStnibwwlOns ~ .ssercciieceucioeie ace 10 3
Viena anne 355 glows Dhyssray hes ara NOAAVMER RSI ROR CST One as i!
Types of Distribution of Lumbo-inguinal Nerve.
Slit plate malware <-c.c) ochcroredn eee elcek oe tene aes it
Moderatemlateral <5 sis ccccu.o gees ares eae alt
FSXLENSIVE MAELO 2.14 mske siamo aentac oor 1
SiiahigantertOr: 22. cserkeroeerioe ie eters 1
Moderates@anterior | ..6.0t.. chmaskereiters soten 2 2
FEXPCNSIVEMAMUCCLIOL) ici. slesscie cers oe c eneeceae oe fh
SUN ou nga Vero bts Re perce aeecie Beloit eaciciarcrcta a a aie if
Moderatemmedial. oxK!p.eaenecs eae ee jae 2 it
Maperale anda mediala. scp acst nas eee iL

From this table it will be seen that a separate anterior cutaneous
nerve may be associated with a moderately developed lateral cutaneous
nerve and with any form of distribution of the lumbo-inguinal nerve.

There is no indication that sex or race has influence on the frequency
of development of this nerve. It was found more frequently on the
left side than on the right, but this might not hold true were a greater
number of instances studied. In only one instance was the nerve found
on both sides of the same body. The following table indicates the race,
sex and side of body in which the fourteen instances here studied were
found.

TABLE IX.
White. Negro.
——E—E——_———— ———————e
Special Anterior Cutaneous Nerve from Male. Female. Male. Female.
> === ==> HSS
R L R L Rk L R L
>: OG AD, 4 Neely iG aiclo des aie AOA ae 2 3 iL 1 2 2
VOAIEP-O-GHub Bios aKa c dase oper 1 1 if

The separate anterior cutaneous nerve is distributed on the thigh in
company with branches derived directly from the femoral nerve. As a
rule it is distributed in a territory separating that of the lateral cuta-

ow
(oo)
(5%)

Charles R. Bardeen

neous nerve from that of the branches of the femoral nerve, but occa-
sionally it may have a more medial distribution. We may now pass to
a consideration of the cutaneous branches of the femoral nerve arising
beyond the inguinal ligament.

3. Anterior and Medial Cutaneous Branches of the Femoral.

The cutaneous branches of the femoral nerve to the thigh have been
commonly divided by English and other anatomists into two groups, the
“middle cutaneous” (nn. cutanei anteriores of Hénle) and the “in-
ternal cutaneous” (nn. cutanei medii of Henle). It is not possible
always to draw a sharp distinction between these two groups of nerves.”
In the most common form of distribution (Plate VII, Fig. 1) two
“anterior cutaneous” nerves, a lateral and a medial, arise in the upper
part of Scarpa’s triangle. These branches descend in Scarpa’s triangle,
pass to the medial side of or through the substance of the sartorius
muscle, pierce the fascia lata over the upper third of the sartorius muscle
and are distributed to the skin of the lower two-thirds of the front of
the thigh. The lateral branch pierces the sartorius muscle more fre-
quently than does the medial branch. In place of two branches there
may be three or only one. The “ medial cutaneous ” nerves arise as rami
from one or more branches of the femoral nerve. The rami usually pass
outwards in the septum between the sartorius muscle and the adductor
group of muscles. Sometimes one or more of the rami pass through the
substance of the sartorius muscle. The various rami supply the skin
of the medial surface of the thigh and the more distal usually extend
to the knee and join the saphenous and obturator nerves in supplying
the medial side of the knee and upper part of the medial side of the
back of the leg. There is great variation in the number and distribu-
tion of these rami of the “ medial cutaneous” nerves. In two instances
out of 80 a medial cutaneous nerve sent a branch as far as the ankle,
parallel with the saphenous nerve.

Frequently the most proximal ramus of the medial cutaneous nerves
on reaching the subcutaneous tissue, or even beneath the fascia lata turns
back to take a course toward the region of distribution of the inguinal
nerve (5 out of 80 instances).

The great variation in the number and territory of distribution of the
anterior and medial cutaneous nerves of the thigh makes their statistical
study both difficult and unsatisfactory. They vary in extent of distri-
bution inversely with the lumbo-inguinal, lateral cutaneous, saphenous

“In the B. N. A. but one set of nerves is recognized, the nn cutanet
anteriores. We shall here, however, adopt the Henle terms.

284 The Nerves and Muscles of the Leg

and obturator nerves, with branches of which anastomoses are usually
formed.

Considering as a group the nerves which supply the front of the thigh
it is found that the most common form of distribution is that of a mod-
erately extensive lateral cutaneous nerve associated with two anterior
and one or two medial cutaneous nerves the branches of which are dis-
tributed over the front and medial side of the thigh. As a rule the skin
beyond the knee is supphed mainly by branches from the saphenous.
This general mode of distribution was found in 64% of instances. In
about 33% of instances there was an extensive distribution of the lateral
cutaneous nerve with a more restricted distribution of the anterior and
medial cutaneous nerves.

In the following table an attempt has been made to show the number
of chief nerve branches distributed to the anterior surface of the thigh

TABLE X.
Cutaneous | Number of Main Nerve Branches Distributed to the
Nerves. | Anterior Surface of the Thigh.
| | | eal | boul
N. cut. fem. lat.../0}0/1)/1)/1)1/1}/1/1)/1)1}1)1)1)2)/2)2)2)2)2/2)3/3)3)3)4)4) 4
N. cut. fem. aa | | | | | | | |
from plexus....|..|..}1]/1]..| 1] --]1].-]--]e-]--]e-]ee | ee] ee] eel ee|cefee| ec} ee] ee} en} ee] eel ects.
| | hea
N. cut. fem. ant. en | |
from N. femor- fe | Ite] lee | ee
GMliStscectieee sen: 2 AND ee Oe) Py Qe eaei 2 ens B/1j)1) 2/2) 2/2) 3s) 1i1j1i2iri1 2
| | | | | | | | | |
N. cut. med....... jj) 1)1}1)1)/2)2)8)2)1 2)1/1 PAU See Eo Shea ab tk |) Pe) a
| | Hn |
N. saphenous R. | | | | el hel
infrapatellaris..|1|1]1)1)/1|)1)0/2)0/)1 L/OjLj1j1jiiij1)2)oj1 Oat yal aby) abi at |) a!
| | LEAN
Res weancens.\1 ilelelalela 1 slelalaliia 1/6lij2/si1lelil2}3 2) 56

13) 3
Ml] ea (antl | |

in 87 instances. The lateral cutaneous nerve has been counted as single
when numerous small rami are given off from the main trunk; as double
when the nerve divides into two main trunks before or soon after pass-
ing under the inguinal ligament; as triple and quadruple when it divides
into three or four main nerve trunks. By separate anterior cutaneous
is meant a branch arising directly from the plexus. The anterior cuta-
neous nerve is counted as single when one main trunk arises from the
femoral nerve; as double when two separate trunks arise; and as triple
when three such trunks arise. The same is true of the medial cutaneous
nerve.
4. N. Saphenous.

The saphenous nerve is fairly constant in its general mode of distribu-
tion. The greatest variation comes in the distal extent of its distribu-
tion. In three instances out of 75 it was found to extend to the great

Charles R. Bardeen 285

toe. Although in several instances students failed to trace the nerve
further than the knee, I do not feel that their work is sufficiently accurate
to give figures as to the frequency of extremely limited distribution of
the saphenous nerve. I have seen no instances of the passing of the
saphenous nerve to the back of the thigh through the adductor magnus
muscle as described by Hyrtl (Henle, Nervenlehre, s. 573). As a rule
the main trunk of the saphenous nerve passes skinwards between the
tendons of the sartorius and gracilis muscles. The patellar branch of the
saphenous usually passes through the substance of the sartorius muscle
but may pass over the anterior margin of the tendon. Below the knee
the saphenous nerve may be continued in one or two main trunks toward
the ankle.

III. CUTANEOUS BRANCHES OF THE OBTURATOR NERVE.

The superficial branch of the obturator nerve may terminate in a
cutaneous branch of variable size. In the embryos studied I have been
unable satisfactorily to trace the development of this nerve. In the adult
it usually passes distally between the gracilis and adductor longus and
becomes superficial between the gracilis and sartorius muscles in the
middle third of the thigh. It commonly anastomoses with branches either
from the medial cutaneous nerves of the thigh or from the saphenous
nerve or both, and helps to form the subsartorial plexus. The fibres of
the cutaneous branch of the obturator may join the medial cutaneous or
the saphenous nerve beneath the sartorius and be distributed in the
branches of these nerves without giving rise to any independent branches.
How constant the cutaneous branch of the obturator may be I have been
unable satisfactorily to determine. Students dissecting frequently
fail to find it. Owing to the fact that this may often be due to
its small size the negative records cannot safely be used in making up
statistics.

Out of 80 instances in which the nerves of the thigh were carefully
charted, in 12 a large cutaneous branch passed from the obturator to the
region of the knee and in 10 other instances one passed to or beyond the
middle third of the crus. A well developed obturator branch to the skin
is found more frequently associated with “ normal” and “ anterior”
than with posterior types of lumbo-sacral plexus and relatively more
frequently in white than in negro subjects.

IV. ACCESSORY OBTURATOR NERVE.
This nerve was not found in the embryos studied. Out of 250 plexuses
in the adult it was found in 21 (8.4%). It seems to be especially fre-
quently associated with the anteriorly situated types of plexuses. It was

Oo
o/2)
[oP

_ The Nerves and Muscles of the Leg

found relatively more frequently in males (9.3%) than in females
(5.4%) and in white (10.8%) than in negro subjects (6.4%). In most
instances an anastomotic branch could be traced to the cutaneous branch
arising from the obturator nerve. The following table shows the fre-
quency with which accessory obturator nerves of various types of origin
were associated with various types of lumbo-sacral plexuses.

TABLE XI.

|

Type of Plexus from which the

We Gut: Moninakoarisos: Origin of Accessory Obturator.

| “Most Distal | From(XXI) | From (XXII) | prom XXIV
Type.| Furcal Nerve. Spinal Nerve | (XXII) XXIII) XXIII XXIV) z
to Limb. Sp. Nerves. Sp. Nerves. Sp. Nerve.
Biy,|) <XOXSINW. XXVIT. 1 | 1
XXIV
C | chiefly to sac- XOX VITM 5 2 | 1
ral plexus.
XXIV | |
D | ecniefly to lum- DONA OIE 2 5 2
bar plexus.
F EXOXGD Vi XXIX. 1
—|— |
G (XXIV) sss |
XXV. XXIX. 1 |

V. CUTANEOUS NERVES OF THE SCIATIC GROUP

The cutaneous nerves originally extending toward the posterior and
distal margins of the embryonic limb are greater in number and have a
more extensive distribution than those of the anterior border. They
consist of the posterior cutaneous nerve of the thigh, n. cutaneus femoris
posterior (small sciatic), with its cluneal, perineal, hamstring and termi-
nal branches, and the cutaneous rami which arise from the peroneal and
tibial nerves. We may consider first the early embryonic development of
these nerves and then the variations found in the adult.

a. Embryonic Development.
1. N. Cutaneus Femoris Posterior.

In Embryo CXLIV, length 14 mm., Plate IV, Figs. 1 and 2, two
nerves may be seen extending out toward the posterior margin of the
base of the hmb. One of these nerves represents the posterior cutaneous
nerve, the other either the perineal ramus (inferior pudendal) of that
nerve or the perforating cutaneous nerve. The gluteus maximus muscle

©o
(o 6)
-~2

Charles R. Bardeen

does not at this period completely overlap the sciatic nerve and the two
cutaneous nerves have a free path for growth. In Embryo XXII, length
20 mm. (Plate V, Figs. 1 and 2) but one cutaneous nerve can: here be
distinguished. This is clearly the posterior cutaneous nerve. In subse-
quent development the nerve is shifted from the posterior margin over a
region corresponding to the original medial surface of the hmb-bud and
gives rise to extensive branches. In Embryo XXII perineal and cluneal
rami may be traced for a short distance from the main trunk.

2. N. Suralis.

The main trunk of the sural nerve (external saphenous) may be seen
arising, through the N. cut. sural medialis, from the tibial nerve in Em-
bryo CXLIV (Plate IV, Figs. 1 and 2). In Embryo XXII, Plate V,
Figs. 1 and 2, it is well developed and branches may be traced over the
dorsum of the foot. The main trunk of this nerve at this period occu-
pies a much more lateral position than subsequently. With the develop-
ment and shifting of the gastrocnemius muscle the trunk of the nerve
near its origin becomes shifted toward the middle of the calf and buried
between the two heads of the gastrocnemius muscle.

3. N. Surae Lateralis.

In Embryo CXLIV (Plate IV, Fig. 2) this may apparently be recog-
nized as short branch. In Embryo XXII (Plate V, Fig. 2) it is not
much more highly developed. Subsequently it too becomes shifted over
the back of the calf, but its branches are supphed to the original posterior
margin of the limb (the external surface of the leg) as well as to the
back of the leg. One of these branches finally anastomoses with the main
trunk of the sural nerve. Variations on the adult indicate that the latter
may at times arise from the N. sure lateralis.

4, Nn. Peronei.

The superficial and deep branches of the peroneal nerve-may readily be
distinguished in Embryo CXLIV (Plate IV, Fig. 2) but the terminal
cutaneous rami are not clearly developed. In Embryo XXII (Plate V,
Fig. 2) these cutaneous rami may be followed over the dorsum of the
limb-bud. They seem to have a simple direct growth toward the areas
they are subsequently to supply.

5. N. Tibialis.
The terminal cutaneous branches of this nerve likewise cannot be dis-
tinguished in Embryo CXLIV (Plate IV, Fig. 1) but are clearly to be
made out in Embryo XXII (Plate V, Fig.1). Like those of the peroneal

288 The Nerves and Muscles of the Leg

nerve they seem to have a fairly direct path of growth. From the dorsal
surface of the tibial nerve the calcaneal branch may be seen taking its
rise.
b. Variation.
1. N. Cutaneus Femoris Posterior (small sciatic).

This nerve shows considerable variation in origin and distribution.

a. ORIGIN FROM SACRAL PLEXUS.—As stated in most text-books, it com-
monly arises from the 26th, 27th, and 28th spinal nerves (1st, 2d, and 3d
sacral). It may, however, arise from the (25th) and 26th; (25th), 26th,
and 27th; 26th and 27th; (25th), 26th, 27th, and 28th; 27th and 28th,
or from the (27th), 28th, and 29th spinal nerves. In table XII the fre-
quency of these various modes of origin is shown. It is possible that in
some of the instances tabulated the origin of the posterior cutaneous
nerve was more extensive than the tabulation charts show, because in
tracing back a nerve to the spinal roots from which it springs, small
bundles of nerve fibres are sometimes torn. It is believed, however, that
the chief roots of the nerve are indicated in the charts from which the
table was made. It is to be noted that while an anterior position of the
roots of the posterior cutaneous nerve of the thigh usually corresponds
with an anterior position ef the lumbo-sacral plexus, this correspondence
is not perfect.

In these variations no special relations to race, sex, or side of body are
apparent in the charts tabulated.

According to A. Soulie (Poirier and Charpy, or), the branch from the
2d sacral nerve to the posterior cutaneous nerve is constant while branches
from the Ist and 3d are less constant and occasionally one may find a
branch from the 4th sacral.

There is great variation in the extent to which the roots and the trunk
of the posterior cutaneous nerve are bound up with neighboring nerve
trunks, such as the inferior gluteal, sciatic and pudic nerves. As a rule,
however, the union between these trunks is so slight that they may be
readily separated. It has not seemed, therefore, worth while to attempt
a tabulation of relations of this sort. Not infrequently (in about 25%
of instances) the perineal rami arise from a trunk which springs by
special roots from the plexus. See table XIII.

b. DistrrpuTion.—Gluteal branches (nn. clunium inferiores). These
most commonly arise from a single branch given off from the posterior
cutaneous nerve while this lies beneath the gluteus maximus muscle. This
branch may pass up over the lower margin of the muscle before dividing
into terminal rami (31 out of 77 instances) or it may divide into two or
more rami which pass out under and turn back over the muscle in several

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290 The Nerves and Muscles of the Leg
areas. In 33 out of 77 instances two such rami were found (Plate VII,
Fig. 2) ; in 9, three; in 2, four; and in 2, five.

Perineal branches.—As a rule the perineal rami arise from a single
trunk which branches from the posterior cutaneous nerve (70 out of 94
instances) or arises separately from the plexus and runs a parallel inde-

TABLE XIII.

#| ge | ge] ;

3 | a 35 | Gus Cree Origin of N. Perinealis. Verte
218 | 1B} Wily 1R|) 183 XERIV i. EXONS PXORGV iT XXVI, X XVII 11t, 51, 5s, 4e
269 | We Me eB XSROV TRE XG VL) XXVII 12th rib, short
395 B, ¥, R | B XX VI, XXVII | XX VIL 12t, 51, 4s, 4co
607; W,M,L; B ? ? 12th rib, short
649 | B,F,R| B XXVI, XXII XXVI, XXVII 12t, 41, 5s, 3co
243 | W,M, L| CO | GRAV ERG VAT ROS VALET XXVII, XXVIII 12th rib, short
S010 | Sa Hevlag |) aC) || eXOXeVaIe eXGXe Vala XoXo Val: XXVIII, XXVIII 12th rib, short
282 | W, M, L Gl || EXOXS VAL PROXOValI XeXO VID XXVII, XXVIII 12t, 51, 6s, 2co
218 | B, M, L | Cai XXVII, XXVIII XXVII, XXVIII lit, 51, 5s, 4co
476 | B,M,R| C XXVII, XXVIII XOXO VILL, ROXG VAL 12th rib, short
423 | B, M, R | Cc | XXXVI, XXVII XXVII, XXVIIT 11t, 51, 6t, 2co
583 | W, M, L]} D | XOXO T, XEXGV AT ROX! Vil XXVII, XXVIII Normal
405 | W, M, Bye) |XX Vil, XX VII, XXVIII XXVII, XXVIII ss
B03) sake aD) | XexValy ROX VDE ok VET XXVII, XXVIII Us
211 | B, M, | 1a) XOX Ve RXV IL, KON LEL XXVII, XXVIII te

152 | B, F, L | iD) XXVIII, XXVIII ON Vi eV es

547 B,M, L| E | XXVI, XXVII, XXVIII| XXVII, XXVIII ae

108 | [gh Set Deals. 6A Bo. @-4"a8ap-e-4 a 11 | XXVII, XXVIII o*

108 | B, F, R | B | XXVII, XXVITI | XXVII, XXVIII et

612 | B, F,R | E XXVII, XXVIII | XXVIII, XXVIII 12t, 61, 5s, 3co
282 | W,M,R F | XXVIII, XXIX XXVIII, X XIX 12t, 51, 6s, 2co
42 | B, M, R. an XXXVI, XXVII, XXVIII | XXVII, XXVIII 13t, 51, 4s, 3co
247| W,M, L| G | XXVIII, XXVIII, XXIX | XXVII, XXVIII, XXIX| 12t, 51, 6s,2co
418| B,M, L| G | XXVI, XXVII| XXVII, XXVIII, XXIX| Normal

pendent course (24 out of 94 instances).

In the latter

ease a small

anastomosing twig usually passes from the perineal branch to the main
trunk of the posterior cutaneous. A separate perineal branch occurs much
more frequently in the unusual forms of plexus than in the usual, as
shown by the above table.

Charles R. Bardeen 291

Frequently the perineal branch arises from the posterior cutaneous
nerve in common with a large branch which passes to supply the medial
surface of the leg (31 out of 110 instances). A branch of considerable
size may arise in common with the perineal nerve and then pass upwards
to be distributed over the medial margin of the gluteal muscle (15 out of
110 instances). Two separate perineal branches are given off from the
posterior cutaneous nerve infrequently (5 out of 110 instances). Rarely
the perineal branch gives off rami both for the buttock and for the medial
surface of the leg (2 out of 110 instances).

Occasionally a root containing fibres destined mainly for the perineal
nerve arises separately from the plexus, passes through the sacrotuberosal
ligament and then joins the posterior cutaneous nerve immediately before
this gives off the perineal branch (2 out of 94 instances).

Femoro-popliteal branches.—As a rule several branches arise from the
posterior cutaneous nerve as it passes down the back of the thigh. Those
on the medial side of the nerve are the better developed. In the most
common form of distribution (67 out of 94 instances) three or four
branches arise from the medial side of the nerve between where it emerges
from under the gluteus maximus muscle and the popliteal space. On
the lateral side two to three branches are commonly given off. Another
common type of distribution is one in which the upper half or two-thirds
of the medial posterior surface of the thigh is supplied by a branch
which arises in common with the perineal branch of the posterior cuta-
neous nerve (24 out of 94 instances). This condition was found most
frequently associated with an anterior type of plexus (type A, 1; type
B, 4; type C, 9; type D, 5; type F, 1; type G, 1).

In one instance soon after the posterior cutaneous nerve emerged from
under the gluteus maximus muscle a long medial branch arose to supply
the inner side of the leg as far as the knee, and a long lateral branch to
supply a corresponding lateral area. ‘This was found on the right side
of a subject with a normal, type D, plexus and a normal vertebral column.
In another instance the posterior cutaneous after it emerged divided into
two branches which extended to the knee and in addition a long medial
branch arose from the perineal division of the nerve. This was found
on the right side of a subject with a posterior, type G, form of plexus and
13 thoracic, 5 lumbar, 4 sacral and 3 coccygeal vertebrae. In one instance
the upper medial portion of the thigh was supplied by a nerve arising
from the perineal branch of the pudic nerve. This was found on the

20

292 The Nerves and Muscles of the Leg

right side of a subject with a normal, type D, plexus and a normal
skeleton.

Terminal branches.—The absence of a posterior cutaneous nerve has
been reported, but this condition I have not seen. In one instance
(308-R) the terminal branches could not be followed as far as the knee
by gross dissection. In the great majority of instances (81 out of 110)
the terminal branches could be readily followed into the upper third of
the back of the leg. But rarely was there found the branch described in
Poirier and Charpy’s anatomy as extending to anastomose with the sural
nerve. In 21 out of 110 instances the chief terminal branch extended on
the medial side of the leg well into the middle third of the back of the leg.
In 8 instances out of 110 the chief terminal branch could be followed
nearly to the medial malleolus. This extensive distribution of the pos-
terior cutaneous nerve of the thigh was found twice associated with the
B type of plexus, once with the C type, four times with the D type, and
once with an F type. No obvious relation therefore exists between the
extent of distribution of the posterior cutaneous nerve and the form of
the plexus from which it springs. This also is true of relations to the
origin of the nerve from the sacral plexus and to race, sex, and side of
body.

2. Perforating Cutaneous Nerve.

A distinct perforating cutaneous nerve arising from the 2d and 3d
sacral nerves and passing through the sacro-tuberosal ligament to supply
the skin over the medial margin of the buttock was found in but 8 in-
stances out of 94. To what extent this small percentage is to be attrib-
uted to lack of sufficient care in dissection cannot at present be stated.
Only the better charts have been used in this tabulation. Eisler found
the nerve in 22 out of 34 instances.

In one instance the perineal branch of the posterior cutaneous passed
beneath the sacro-tuberosal ligament on the way to its destination.

3. Cutaneous Branches of the Peroneal Nerve.

a. N. Cutaneus surae lateralis—This nerve arises in the popliteal
space and runs down over the lateral head of the gastrocnemius to supply
the lateral cutaneous area of the leg and usuaily sends a branch to
anastomose with the n. cutaneus sure medialis to form the sural (ex-
ternal saphenous) nerve. This form of distribution was found in 38 out
of %6 instances. In 30 out of 76 instances the communicating branch to

Charles R. Bardeen 29:

Oo

the n. surae medialis was not found. In 7 out of 76 instances two
branches arose from the peroneal nerve in the popliteal space. One of
these supphed the side of the leg below the knee; the other supplied the
back of the leg and sent a branch of communication to the n. surae
medialis. This mode of distribution is described as the normal by many
authors. In one instance the peroneal nerve gave rise to the sural (ex-
ternal saphenous) while the tibial furnished a cutaneous branch for the
skin over the calf. In five other instances the n. cutaneus surae medialis
gave rise to a cutaneous branch for the supply of the upper part of the
calf. The extent of distribution of the various branches mentioned is in-
versely proportional to the extent of distribution of the posterior cutan-
eous nerve of the thigh and the saphenous and obturator nerves. Great
individual variations are found.

b. N. Cutaneus peronei femoralis—In one instance in which the
tibial and peroneal nerves arose separately from the plexus the peroneal
nerve passed between two divisions of the pyriformis muscle,
then lateral to a fasciculus of the short head of the biceps
which arose from the proximal end of the gluteal tuberosity. It
crossed the antero-lateral surface of this fasciculus, and then between
it and the main portion of the muscle to its usual position in the thigh.
Near the middle of the shaft of the femur it gave off a branch which
passed through the short head of the biceps to the side of the thigh
where it divided into ascending and descending branches and supplied
a large area between the territories of the n. cutaneus femoris lateralis
and the n. cutaneus femoris posterior. This abnormal cutaneous nerve
I have not found previously described. It resembles somewhat a nerve
in the orang described by Klaatsch, 02.

c. Nn. Cutanei dorsales pedis——In the great majority of instances
the n. cutaneus peroneus superficialis divides into two main terminal
branches just above the ankle. One of these, the n. cutaneus dorsalis
medialis, passes directly to the outer side of the big toe, giving off on
its way a branch to supply the contiguous sides of the 2d and 3d toes
and small branches to anastomose with those branches of the n. peroneus
profundus which supply the contiguous sides of the first and second
toes. The other, the n. cutaneus dorsalis intermedialis, passes down
to supply the contiguous sides of the third and fourth, and fourth and
fifth toes. This general mode of distribution was found in 44 in-
stances out of 111 (about 40%). In two of the 44 cases above men-
tioned the sural nerve failed to extend to the little toe and a special

294 The Nerves and Muscles of the Leg

branch arose from the n. cutaneus dorsalis intermedialis to supply the
outer side of the little toe. Variations in the cutaneous nerve supply
of the dorsum of the foot occurred with the following frequency.

In 17 instances the n. cutaneus dorsalis lateralis (external saphenous)
supplied the place of the n. cutaneus dorsalis intermedius and sent
branches to the contiguous sides of the 4th and 5th, and 3d and 4th
toes. In two instances a branch from the n. cutaneus dorsalis lateralis
anastomosed with the n. cutaneus dorsalis intermedialis and the com-
bined nerve then divided into branches for the contiguous sides of the
4th and 5th, 3d and 4th, and 2d and 3d toes. With the branch to the
last a ramus from the n. cutaneus dorsalis medialis anastomosed. In
15 instances a branch from the n. cutaneus dorsalis lateralis anastomosed
with one from the n. cutaneus dorsalis intermedius and the branches
arising from the combined nerve suppled the contiguous sides of the
3d and 4th, and 4th and 5th toes. In two instances the n. cutaneus
dorsalis lateralis sent a branch to anastomose with one from the n.
cutaneus dorsalis intermedialis going to the 2d and 3d toes. In five
instances the n. cutaneus dorsalis lateralis supplied the outer side of
the little toe and the contiguous sides of the 4th and 5th toes. In 10

instances a branch from the n. cutaneus dorsalis lateralis anastomosed’

with the branch from the nm. cutaneus dorsalis intermedialis sent to
supply the contiguous sides of the 4th and 5th toes. In four instances
the n. cutaneus dorsalis intermedialis supplied the contiguous sides of
the 2d and 3d, 3d and 4th, and 4th and 5th toes while the n. cutaneus
dorsalis medialis supplied the outer side of the first toe and aided in
the supply of the contiguous sides of the Ist and 2d toes. In one of
these instances the n. cutaneus dorsalis medialis arose in the leg from
the n. peroneus profundus. In five other instances the nerve distribu-
tion was similar in nature but an anastomotic branch passed from the
n. cutaneus dorsalis medialis to the nerve going to supply the contigu-
ous sides of the 2d and 3d toes. In one of these instances the n. cuta-
neus dorsalis medialis arose from the n. peroneus profundus nerve and
emerged from between the peroneus tertius and the extensor digitorum
longus muscles. In two instances the n. cutaneus dorsalis medialis sup-
plied, in addition to its usual territory, the contiguous sides of the 3d
and 4th toes; the n. cutaneus dorsalis intermedialis was confined in
distribution to the 4th and 5th toes. In one instance the n. peroneus
profundus supplied in addition to its usual branches, the chief branches
to the medial side of the great toe and the lateral side of the 2d toe.

295

Charles R. Bardeen

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296 The Nerves and Muscles of the Leg

In another instance it sent a branch to aid in the supply of the con-
tiguous sides of the 2d and 3d toes and in three instances it formed the
chief source of supply for the contiguous sides of the 1st and 2d, and
2d and 3d toes.

In two instances, as we have seen above, the n. cutaneus dorsalis
medialis arose in the leg from the n. peroneus profundus. In five in-
stances the n. cutaneus dorsalis medialis arose from the peroneal nerve
soon after this passed beneath the head of the peroneus longus muscle.
The nerve then took a course somewhat independent of that of the n.
cutaneus dorsalis intermedialis.

The very great variation found in the distribution of the nerves of
the dorsum of the foot seems not to be associated with such factors as
age, sex, race, side of body or relative position of the lumbo-sacral
plexus. Rough estimation of these factors have given so little promise
of positive results that I omit here a detailed tabulation.

For the sake of comparison the following data obtained by the Commit-
tee of Collective Investigation of Great Britain and Ireland’ are appended.
Our results agree with regard to the condition most frequently observed,
although I found this condition in but 40% of the feet examined while
the committee found it in 55%. In those instances in which the n.
cutaneus dorsalis lateralis served wholly or in part to supply the con-
tiguous sides of the 3d and 4th, and 4th and 5th toes, although the total
frequency is approximately the same there is considerable difference in
frequency in variation in the nature of the relations between the n. cuta-
neus dorsalis intermedialis and lateralis.

4, Cutaneous Branches of the Tibial Nerve.

The nearly constant origin of the chief root of the n. suralis from the
tibial nerve and the account of it which I have given above in treating
of the cutaneous branches of the peroneal nerve render further descrip-
tion here unnecessary. ‘The rami calcanei mediales vary somewhat in
extent of distribution but offer no features of special interest. The
cutaneous supply of the toes is singularly constant, the main variation
being found in the extent of development of a branch from the n.
plantaris medialis to the nerve supplying the 4th and 5th toes, or from
the n. plantaris lateralis to the nerve supplying the 3d and 4th toes.

In 69 out of 87 instances no well developed branch of this nature

Journal of Anatomy and Physiology, Vo. 26, 1892, p. 89.

2

Charles R. Bardeen 29

was found. In eight instances a branch passed from the n. plantaris
medialis to the nerve to the 4th and 5th toes. In seven instances a
branch passed from the n. plantaris lateralis to the nerve supplying the
contiguous sides of the 3d and 4th toes. In two instances the n. plan-
taris lateralis furnished the chief supply of the contiguous sides of the
3d and 4th as well as of the 4th and 5th toes. In one instance the n.
plantaris medialis supplied the contiguous sides of all the toes.

VI. CUTANEOUS BRANCHES TO THE INFERIOR EXTREMITY FROM
THE DORSAL DIVISIONS OF THE SPINAL NERVES.

In Embryo CXLIV, length 14 mm. (Plate IV, Fig. 2), the lateral
branches of the dorsal divisions of the last five or six thoracic and the
first three lumbar nerves may be followed to the subcutaneous tissue.
They take at this period a somewhat simple course and have a distinctly
segmental arrangement. The dorsal divisions of the fourth and fifth
lumbar and of the sacral and coccygeal nerves are connected by anasto-
mosing branches. From these nerves rami may be followed toward, but
cannot be followed distinctly into the skin.

In Embryo XXII, length 20 mm. (Plate V, Fig. 1), the lateral
branches of the first three lumbar nerves may be followed distally to
the base of the limb where they terminate over the proximal margin of
the iliac crest. The branches of the second and third lumbar nerves
are connected at this period by anastomoses, although the plexiform
arrangement characteristic of the adult is not yet apparent. In sub-
sequent development these nerves extend over the postero-lateral surface
of the thigh, nn. clunium superiores. For the variations in origin of
these nerves in the adult, see Bardeen and Elting, ot.

The lateral branches which arise from the dorsal divisions of the first
three sacral nerves anastomose and from them in Embryo XXII two
delicate branches may be traced toward the skin, nn. clunium mediales.
There is considerable variation in these nerves in the adult, but I have
not sufficient data on which to base a statistical study of the subject.

VII. SUMMARY AND GENERAL CONCLUSIONS.

The cutaneous nerves of the posterior limb in the embryo first ap-
proach the anterior, posterior and distal margin of the limb-bud and
from these areas send branches of distribution over the medial (ventral)
and lateral (dorsal) surfaces of the developing limb. This method of
development may be recognized in the adult. The chief nerve
trunks approach the fascia in a line which corresponds fairly closely
with the primary margins of the hmb. The posterior cutaneous

298 The Nerves and Muscles of the Leg

nerve of the thigh and the sural nerve are both shifted over the dorsal
(primary medial) surface of the thigh during development, but none
the less distribute their cutaneous branches from a line which corre-
sponds to some extent to the posterior margin of the lmb-bud. The
line along which the anterior cutaneous nerves of the thigh reach the
fascia likewise corresponds with the original anterior margin of the
limb-bud. In Plate VII, Figs. 1 and 2, a schematic diagram is given
to illustrate the mode of distribution of the cutaneous nerves of the
adult limb. So far as possible each main nerve is represented approxi-
mately as it occurs with the greatest frequency.

This mode of distribution of the cutaneous nerves from the region
of the margin of the limb-bud is probably due to a differentiation of
function, the musculature of the limb-bud being differentiated on the
medial and lateral surfaces and the margins serving for the primary
development of the cutaneous areas. In sharks the cutaneous branches
suppled to the dorsal and ventral surface of the fin extend upwards in
numerous branches between the muscle bundles. In most higher forms
a distribution of cutaneous nerves from the margins of the limb is well
marked, although numerous exceptions occur.

It is of interest to inquire whether or not anything may be found
during embryonic development to account for the segmental distribu-
tion of the nerves of the limb described on the basis of physiological
and clinical evidence by Head, Sherrington, Bolk and a large number
of other investigators. It is quite certain that no evident dermatomes
associated with specific spinal nerves are to be found in the embryo. It
seems probable that the cutaneous nerve fibres contained in a given
spinal nerve find a path of least resistance toward the marginal area
lying most directly opposite and that to any given area one or two
nerves may thus serve to furnish the bulk of the fibres. In Figs. 2 and
3 I have shown diagrammatically the approximate marginal areas to
which each spinal nerve most directly contributes in the embryo. Sub-
sequently these areas become extended by the growth of branches from
the margins of the limb over the medial and lateral surfaces.

Thé great variation in the distribution of the nerves supplied to
different areas can be best accounted for, I think, by assuming that the
nerves grow as plants grow: in part they are guided in their course by
definite paths, as climbing plants may be guided by strings, but
where definite paths are not offered great variation in the distribution
of the cutaneous rami may be seen. Extensive development of one nerve
tends to retard its neighbors, lack of development tends to excite them
to more active growth.

Bese eG hypogastric

Niel Ss et Saf Se SS ee mei ee ee ee Inguinal
- Genital
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-Medial plantar

--Lateral plantar

ance Une
..Posterior cutaneous
= ====s4--=-Perineal branch

= = ranean SA Ga A aia ace Nec tetra i ele tg Dorsal a penis
See I tehverey

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=: --Tlio PyeCeeevte
te --Inguinal

Se ON al

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-------------] ateral culaneous
---- ----------Qnterior cutaneous
iors ieee oMediall (Cutaneous
“Saphenus

--Deep peroneal

“Superficial peroneal

-Lateral sural

--Posterior culanedous
Soc e oa enineal, pronen

Fig. 3.

/ Fic. 2. Diagram of the early course of growth of the cutaneous
ve nerves arising from the ventral side of the lumbo-sacral plexus.

f Fic. 3. Diagram of the early course of growth of the cutaneous
yy nerves arising from the dorsal side of the lumbo-sacral plexus.

300 The Nerves and Muscles of the Leg

C. DEVELOPMENT OF THE MUSCULATURE AND DEVELOP-
MENT AND VARIATION IN DISTRIBUTION OF THE
NERVES TO THE MUSCLES OF THE INFERIOR EX-
TREMITY. .

I. FEMORAL GROUP.
a. Embryonic Development.
1. General Features.

Soon after the femoral nerve begins to extend into the base of the
limb differentiation of the femoral musculature commences. At about
the center of the shaft of the femur a mass of tissue may be distin-
guished as the anlage of the quadriceps muscle (Plate III, Fig. 2).
Into a cleft in this tissue the main trunk of the femoral nerve extends
and gives off branches for each of the main divisions of the quadriceps.
Anterior to this an ill defined mass of tissue may be distinguished as
the anlage of the sartorius muscle. A special nerve is given to this.
About the main trunk of the nerve as it passes over the region of the
acetabulum a mass of tissue represents the anlage of the iliopsoas and
possibly also the pectineus muscles. In this region the lateral and an-
terior cutaneous nerves of the thigh pass toward the ectoderm. Distal-
ward the anlage of the saphenous nerve may be seen.

In a slightly older embryo (Plate VI, Fig. 1) the muscle differentia-
tion is much further advanced. The sartorius muscle extends well
toward the blastema of the ilium and toward the medial surface of the
proximal end of the tibia. Definite tendons are not, however, developed.
The nerve to the sartorius extends a short distance distally within the
substance of the muscle. The ilopsoas muscle is likewise further differ-
entiated and has extended more toward the vertebral column. The
pectineus muscle has become distinct and to it runs a branch from the
femoral nerve.

The quadriceps muscle begins to show definite differentiation. Ten-
dons of attachment are not, however, clearly differentiated. The vari-
ous branches of the nerve to this muscle, Fig. a, have extended further
into the body of the muscle. They follow in this course developing lines
of cleavage of the muscle into its constituted portions.

The various cutaneous nerves mentioned above have extended con-
siderably in length and in addition there are two medial cutaneous
branches. ‘These run toward the skin in the dense fascia which now
separates the obturator from the femoral group of musculature.

In an embryo of 20 mm. (Plate VI, Figs. 2 and b) the individual
muscles of this group are clearly demarkated. .The figures illustrate

Charles R. Bardeen 301

sufficiently clearly the position of the various nerves and muscles. The
muscles are all attached to the skeletal apparatus by distinct tendons.
The main nerve trunks run in the connective tissue which serves to
separate the various muscles from one another and to divide each muscle
into its constituent parts.

2. Individual Muscles.

Ihopsoas muscle (Plate VI, Figs. 1 and 2). The iliopsoas muscle
arises from a mass of tissue which embraces the femoral nerve as it
passes into the limb-bud. In subsequent development the iliacus muscle
spreads out over the ilium, the psoas major muscle extends up along
the course of the roots of the femoral nerve to form its attachments to
the vertebral column, and in close union the two muscles extend distally
to be attached to the lesser trochanter. The psoas minor seems to be
differentiated from the psoas major muscle anlage, but this is uncertain.

To the psoas major as it is developed toward the vertebral column
branches are given from the femoral nerve or its roots of origin as far
anterior as the 22d spinal (2d lumbar) and occasionally as far as the
21st spinal nerve. These branches extend in between the developing
bundles of the muscle and have a complex, extensive distribution.

The nerve to the psoas minor muscle frequently arises from the trunk
of the genito-femoral or from the lumbo-inguinal branch of this nerve.

To the ilacus muscle as it spreads out over the surface of the ilium
several branches, often united in a plexiform manner, are given. These
branches pass across or near the superficial surface of the muscle about
midway between the crest of the ilium and the combined iliopsoas ten-
don. Special nerve branches are likewise usually distributed from the
‘main trunk of the femoral nerve to the fleshy portion of the muscle as
it passes over the acetabulum and the head of the femur.

There is considerable variation in the exact mode of distribution of
the nerves mentioned. Frequently a special layer of the iliacus covers
the nerves distributed to this muscle. The trunk of the femoral nerve
may be divided by one or more bundles of the iliopsoas muscle. The
variations in the distribution of the nerves to the iliopsoas muscle do not
readily lend themselves to statistical treatment and hence this is here
omitted.

The iliopsoas is probably represented in the urodela and reptiles by the
posterior portion of the pubi-ischio-femoralis internus and the anterior margin
of the ilio-femoralis. The psoas muscle, which is phylogenetically younger

than the iliacus, is by many (see Pardi, 92) considered to be a prevertebral
muscle belonging primitively to the trunk musculature. Its ontogenetic de-

302 - The Nerves and Muscles of the Leg

velopment from an anlage common to it and the iliacus muscle indicates that
it should be placed with the intrinsic musculature of the limb.

The phylogenetic development of the psoas minor, on the other hand, is
somewhat uncertain. It may be derived from the trunk musculature. This
is perhaps indicated by its frequent innervation through a branch from the
genitofemoral nerve, while the psoas major is innervated by branches which
arise from the femoral nerve or its roots. Its embryonic origin should be
studied in some of those forms in which the adult muscle is highly developed.

In mammals with an ilium triangular in cross-section the iliacus lies ex-
ternally, a position which corresponds with the situation in which its anlage
appears in the human embryo (Lubsen). In those forms in which the
iliac blade is developed the muscle comes to have an internal position.

The variations of the iliopsoas in man are chiefly those of a greater or less
independence of the two muscles composing it and a greater or less special-
ization of fasciculi in either. There are also slight variations in the origin
and attachment of the muscles. The very inconstant psoas minor varies
chiefly in the extent of its development. The inferior insertion may take
place into the iliac fascia, the inguinal ligament, the femur between the
small trochanter and the head, or together with the iliopsoas into the small
trochanter (Le Double). Fasciculi may unite the psoas major and the psoas
minor. These variations may indicate a common origin of the two muscles.

Pectineus (Plate VI, Figs. 1 and 2). In an embryo 11-mm. long the
anlage of the pectineus is not distinct. it may be represented in those
portions of the iliopsoas and the obturator muscle anlages which he
nearest the region in which the pectineus will be developed. Grafenberg,
04, describes in the region immediately distal to the superior pubic
ramus a union of a branch of the obturator nerve with a branch of the
femoral before the muscle anlage of the pectineus appears. This I have
not found in the embryos of a corresponding stage which I have ex-
amined. Grafenberg describes the pectineus anlage when it first appears
as fused proximally with the iliopsoas anlage. It is probable that the
superficial portion of the pectineus is thus at one stage usually fused
with the iliopsoas anlage. In the youngest embryo in which I have
found it distinct it is, however, separated by a small interval from the
iliopsoas muscle mass (Plate II, Fig. 3) and seems more closely associ-
ated with the anlage of the adductor longus.

In this 14 mm. embryo (Plate VI, Fig. 1) the anlage of the muscle is
closely applied to the pubic blastema and can be followed from the body
of the pubis to the blastema of the femur. Into it a nerve branch may
be traced from the femoral nerve. Dorsal to the obturator nerve in the
obturator foramen is a mass of tissue closely associated with the anlage
of the obturator externus on the one side and with that of the pectineus
on the other. No definite nerve branch can be traced into it from the

Charles R. Bardeen 303

obturator nerve, but it seems not improbable that this represents the
anlage of that portion of the pectineus which is supplied by the obturator
nerve in many individuals.

In an embryo of 20 mm. (Plate VI, Fig. 2), the pectineus occupies a
position which corresponds with that of the adult muscle. The femoral
nerve gives to it a large branch which passes at first across its outer
surface about midway between its tendons of origin and insertion. I
have been unable to trace a branch to it from the obturator nerve.

In the adult, as is well known, the nerve supply of the pectineus is usually
through a branch from the femoral but not infrequently also from the obtura-
tor or the accessory obturator nerve. Paterson (91, 95) has shown that
in man the muscle is often divisible into a superficial portion supplied by the
femoral nerve and a deep portion supplied by the obturator nerve. Similar
conditions are normal in some of the lower mammals, while in others the
muscle may be supplied by the femoral nerve only or the obturator nerve
only. The dorsal or femoral portion is probably derived from an anlage
intimately associated with the iliopsoas anlage, the ventral portion from
the anlage of the obturator externus. W. Leche has shown that in many
mammals there is separated from the obturator externus a muscle which he
calls the obturator intermedius and that in those forms in which the pectineus
is supplied by the obturator nerve it is probable that this obturator inter-
medius has entered into the formation of the anlage of the pectineus.

The pectineus of the mammals is, together with the iliopsoas, probably
represented in the urodeles and reptiles by the pubo-ischio-femoralis internus.
This, like the pectineus, may be supplied wholly by the femoral, or in part
also by branches from the obturator. The accessory obturator nerve, which
in about 10% of bodies innervates, or helps to innervate, the pectineus muscle
in man, indicates, perhaps, that the division of the limb musculature in man
into dorsal and ventral portions is not strictly to be traced in the respective
territories of the femoral and obturator nerves. Nerve elements belonging
to the ventral territory may be normally bound up in the femoral nerve in
those branches which supply the pectineus muscle. When those branches
become isolated we have the accessory obturator nerve (see Hisler, g2).

There is considerable variation in the extent of separation of the pectineus
into two portions in the human body. The muscle is very frequently fused
with the adductor longus. Occasionally a fasciculus passes from the iliacus
to the pectineus or between the pectineus and the obturator externus.

Sartorius——The sartorius develops from an anlage not directly fused
with that of the quadriceps. Grifenberg has described a fusion near the
ilium of the proximal ends of the aniages of the rectus muscle and the
sartorius with that of the iliacus. In those embryos I have studied in
which these anlages are beginning to appear the quadriceps anlage is
quite distinct from that of both the sartorius and iliacus. The upper
limit of the sartorius anlage approaches closely, however, the iliacus

304 The Nerves and Muscles of the Leg

anlage. The first well marked differentiation of the sartorius anlage
takes place in a region corresponding with that in which the nerves enter
the muscle in the adult, Plate VI, Fig. 1. From here the differentia-
tion of the muscle extends towards its iliac and tibial attachments. The
embryonic muscle is proportionately larger than the adult muscle and
forms more extensive tibial attachment. (Plate VI, Fig. 2, Plate VIII,
Big. 2).

Simultaneously with the differentiation of the muscle, branches extend
into it from the femoral nerve. These branches are more or less inti-
mately bound up with the anterior (middle) cutaneous nerve. As a rule
there are two main branches, one of which serves to supply chiefly the
lateral and proximal, the other the distal and medial, portion of the
muscle. For the distribution of branches in the adult, see Frohse, 98.

In the urodeles the sartorius does not seem to be represented. In reptiles
it is probably represented by the ambiens or the pubi-tibialis or both. The
ambiens arises either from the ilium, as in the crocodile, or from the pubis, as
in most forms. Its tendon passes to the front of the leg. It is an extensor
of the knee. The tendon of the pubi-tibialis passes to the back of the leg.
It is a flexor of the knee (Gadow, 82). It is probably not homologous with
the pubi-tibialis of urodeles, which is innervated by the sciatic nerve and is
a differentiated portion of the pubi-ischio-tibialis.

In the monotremes and insectivora the proximal attachment of the sartorius
is in the neighborhood of the ilio-pectineal eminence. In the marsupials,
prosimians, and primates it takes place into the ventral margin of the ilium.
In other mammals it may take place in either place or from an intermediate
region, from the ilio-pectineal fascia, the tendon of the psoas minor, the in-
guinal ligament, etc. (W. Leche). The insertion takes place into the medial
side of the tibia or into the crural fascia. It may be fused with the gracilis
at its insertion. It may be double (dog—Ellenberger and Baum). The
partial longitudinal splitting of the muscle found in the dog and other carniv-
ora and as a variation in man may possibly indicate a primitive relation-
ship to two muscles, the ambiens and the pubi-tibialis of reptiles. There
seems to be nothing either in the phylogenetic or the ontogenetic history of
the muscle to account for the transverse tendinous inscription or tendon
which occasionally is found dividing the muscle into two parts.

Quadriceps Femoris Muscle, Plate VI. Rectus Femoris.—This muscle
is developed from the quadriceps muscle mass by gradual differentiation.
Its tendon of attachment to the anterior inferior iliac spine is developed
later than that to the supra-acetabular groove, Roger Williams, 78, and
is a consequence of the development of the iliac blade, Le, Double, 97-
As a rule the nerve to the muscle divides into two main branches, one
of which goes chiefly to the medial half, the other chiefly to the lateral
half of the muscle. The main trunk of the former, which enters about a

C1

Charles R. Bardeen 30

third of the distance from the anterior extremity of the muscle, may
be followed for a considerable distance toward the distal extremity of
the muscle; the main trunk of the latter, which enters more anteriorly,
extends a less distance distally and has a recurrent branch which ex-
tends toward the proximal extremity of the muscle. This branch has
been followed to the iliac insertion of the muscle and has been reported
as extending to the M tensor fascie late. This last condition I have
never seen.

Vastus Lateralis——This muscle is differentiated from the quadriceps
muscle mass by the development of septa between it and the vastus inter-
medius. The muscle is usually composed of two distinct layers. an outer
and an inner, separated by fascia containing nerves and blood vessels.
Often the inner layer is further partially subdivided into two sheets by
fascia in which nerves and blood vessels run. Commonly the nerve to the
vastus lateralis divides into three branches of which one runs on the
inner surface of the outer layer of the muscle, the second betwen the
two sheets of the inner layer, and the third passes through the inner
sheet of the inner layer to be distributed to the most lateral part of the
vastus intermedius muscle. The larger intrinsic nerve trunks cross the
fasciculi of the muscle sheets and about midway between the extremities
of the fasciculi.

Vastus Intermedius.—This is differentiated from the quadriceps
muscle mass at the time of the ingrowth of the main nerves and blood
vessels of the muscle. The muscle is composed of muscle lamelle con-
centrically arranged about the diaphysis of the femur. The lowest, most
distal, and most completely separated of these lamellw is the subcrureus
muscle. Several nerves are distributed to the muscle. To the lateral
region a branch from the nerve to the vastus lateralis usually extends.
A special ramus from the femoral nerve generally passes to the middle
portion, and from the nerve to the vastus medialis several branches are
often given to the medial side of the muscle.

Vastus Medialis—This muscle is differentiated from the quadriceps
muscle mass by the formation of a connective tissue sheet between it and
the vastus intermedius. Its nerve of supply runs along on the medial
surface of the muscle sending branches from time to time into its sub-
stance and finally near Hunter’s canal the terminal twigs of the nerve
enter the muscle. The nerve is often more or less bound up with the
saphenous nerve. The rami which enter the muscle extend at first across
the fibre-bundles of the muscle sheet about midway between the extremi-
ties of the fibre-bundles.

306 The Nerves and Muscles of the Leg

The rectus is phylogenetically the oldest part of the quadriceps. In the
urodela, where it is represented by that part of the ilio-tibialis supplied by
the femoral nerve, there seem to be no muscles which correspond with the
vasti but these are differentiated in the reptiles and the higher forms. In
some of the mammals the three vasti of the muscle are more or less fused.
In man the chief variations found in the quadriceps result from a greater or
less division of the primitive extensor mass into individualized parts.

b. Nerve Variation.
1. Variation in Origin of the Femoral Nerve.

The femoral nerve in the great majority of instances arises in the main
from the 22d, 23d, and 24th spinal (2d, 3d, and 4th lumbar) nerves.
There is, however, considerable variation in the size of the nerve bundles
derived from the 22d and 24th spinal nerves. The 21st spinal nerve
usually contributes some fibres, and the 20th and 25th spinal nerves occa-
sionally do so. In Table XIV there is shown the frequency with which
certain root origins of the femoral nerve were found and the relation of
these various modes of origin to various types of plexuses. A study of
these variations in relation to race, sex, and side of body has revealed no
marked associations, and therefore the tables embracing these data are
omitted.

2. Relations of the Branches Springing from the Femoral Nerve to the
Nerve Roots.

In considering the cutaneous nerves we have seen reason to believe
that the peripheral segmental distribution of the spinal nerves disclosed
by physiological experiments, is due to a directness of growth which a
given spinal nerve has toward a given peripheral area so that the nerve
can send more fibres into this area than its neighbors can and hence serves
in the main to innervate the area. This is also probably true of muscle
innervation. In Plate III, Fig. 3, is shown the position .of the femoral
nerve as it enters the femoral muscle mass in a young embryo. The fol-
lowing diagram shows a cross section of the adult femoral nerve as it
passes under the inguinal ligament. The regions occupied by the fibres
of the chief motor and sensory branches are outlined, while the approxi-
mate areas occupied by the main bulk of the fibres of each spinal nerve
are shown by stippling the position occupied by the fibres of the 23d
spinal (3d lumbar) nerve. While the diagram is schematic it may serve
to illustrate the relation of peripheral to spinal nerves of the femoral

307

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21

308 The Nerves and Muscles of the Leg

group. A truly accurate determination could be made only by section-
ing each of the various roots and each of the various nerves of distri-
bution in various plexuses and then following the paths of degeneration.

3. Variation in the Association of the Terminal Branches Arising from
the Femoral Nerve.

Soon after the femoral nerve passes under the inguinal hgament it

gives rise to the various branches which serve to innervate the skin and

muscles of the front of the thigh and the arteries and joints. There

UN. cut. ant. 1

_-N. cul. ant.2

<. _N.cul. med.

N. Vast.lat
&intermed.

/
N. vast. med.

Fic. 4. Diagram to illustrate the approximate regions of the femoral nerve
trunk occupied by the fibers which pass out into its muscular and cutaneous
branches. The stippled area indicates the position of the main bulk of the
fibers of the third lumbar nerve.

is great variation in the association of the nerves going to these various
structures. Now one set of nerves may be bound in a common trunk for
a part of their course, now another. The association of the various nerves
into common trunks is limited, however, to the association of contiguous
regions shown in the diagram, Fig. 4. Two or more of these regions
may continue for a time to be associated after the femoral trunk has
split into branches. Thus the nerves of the rectus femoris muscle are
often bound for a part of their course with those of the vastus lateralis
or with those of the vastus lateralis and intermedius before the final
division takes place. But the nerve to the rectus femoris muscle is never
found bound up in a common trunk with the nerve to the vastus medialis
or the saphenous nerve unless the nerves to the vastus lateralis and

Charles R. Bardeen 309

vastus intermedius are also included in the trunk. In the following table
the association of nerve branches found in 77 instances is indicated, and
at the left is shown the number of times each condition was found.
Minor differences are not recognized and no attempt has been made to
include the nerves for the arteries and joints. In one instance the
femoral nerve was found to furnish a branch to the adductor longus
muscle. This is said by Poirier to be a normal condition.

TABLE XV.

ASSOCIATION OF THE BRANCHES OF DISTRIBUTION OF THE FEMORAL NERVE IN

ComMMOoN NERVE TRUNKS.
A. No distinct division into two parts.

PEI RCSIS 2S) Sas Ac) Clie CEE 32) es Gene 2 se 3) (Cle C2)) ea Gis)
(Sph) ]

PETRA CAz ae CSE S 2) i CRD) (CR 2) Cl 12? 13) (Cle C2
[(M) (Sph)]

MEN) (LE) (GO) ISH, TSP) (NA) IEG Ral a4) (Gbaly be be) (Onl, (OR 3) TI
[(M) (Sph)]

Tse CAL) SS CAZ) EN GS2) ihe ae [KCRG U2) es Clee ate See (Cilla 2)) ale EVE]
[Sph]

EX *]’ [(A2;, S1) (S82) (11) (2)] [CR1, R2) (Li, L2, £3) (Ci, €2)] [M]
[Sph]

ET PAE PAZ Stee SO Soil Claro) Glala2)) (CI)i (C2) is EME a CSph)
(1) ]

Ee CATES S2) i CS25553) li Cl) Gh2)) ee CES) Me EE 2s ele ese Sih Ci,
C2] [M1, M2, M3] [Sph]

EES RASS S2 is hG@Rae sb) G2 a2) es Cl C2 Val [sph]

(OP (Si, Se, ANT) [Ral Re all) {iCal Oe, i) [| {eyo

eases Soe PANE VAL sai (Rea) ea Gialela2)) ia eo iCis «C2 “PV [Sphy ia

[2] [CAL) (Si, S2) -(A2, S3)] TCR, R2) Cal, b2)] [C1, C2] [Mil, M2]
[Sph, I] ‘

[(P) (branch to adductor longus)] [I] [S1, S82, Al, A2] [(R1, R2)
(Ci C2) (Gas) eke) CSph) Gb)

Pei P2i ECS), CA, (82,53) sD), ECR 2) (aS. 2, 3) [C] ECM)
(Sph) ]

PET CAI Si rAg S2)) Gb) i ERAGE Chas 23) CC) P6e2)) Gb
[Sph]

G2, 1A) UD), ISH, S25 AMI ZV2]) (RL, IR) | Gbalp Py wey) (@) Al TG)
(Sph) ]

IDs) Sais S245 VSP YO) [Bale se Isai [Gal IDPS heyy (aly (era) (Gi)
(Sph) ]

[X] [P] [S1, S2, Al, A2] [R1, R2] [(ii, L2) (C)} [M] ([(Sph) (1)]

Re GS1a5 SZ) CAM) 1) RAS Re alee es Cie C21 ihe (Sph) Ci

*From the 23d spinal nerve.

310 The Nerves and Muscles of the Leg

1 [a] [P] [S1, 82] [A] [R1, R2] [(u1, L2, L8) (Cl, C2)] [(M) (Sph)

(1) ]

2 TP] [1] FAL] [S1, 382; A2i TR R2: Reis Pal, Les 3 C1) Pa aee)
(Sph) ]

A PPL P2] [1] (Si, S22) All AZ] [REAR], Pie 2) 18] [Cie ez rae
(Sph) J

2° fal (P) ‘{t)], (¢S% 82). (A2)1. TRI Res al 2), 3) (Ck C2 hem
(Sph) ]

EX] fal [¢St. S2)oCA2) Tia 27 ERE Re] als 2, 3 eee
[M] [Sph]

1 [P] [1] 181,782; All [Ra R23R3) [Ea 2,3) TO), M2) (Gree2n
[Sph]

[P] [(S1, $2) (A)] [R1, R2] [L1, L2] [C1, C2] [M] [(Sph) (1)]
[P] [1] [(S1, 82) (Al, A2) (R1, R2)] [L1, L2] [(C) (M)] [Sph]
TEI LIES S25 Ade Aa PRS eri ae eS eis C2 sve iSphi
Pal [PJ [Si, $2; AZ] EN) PRi, R21) Gal; 2; 3] ie, C21 MP isphi
[a] TX] [TP] [S1, S2; 421 [Ty (RI, 2) [ha 22; 3) (C1, C21 1 Misa
ESS CSiS2) 1¢Si5 Ad) ERG R21) al 2 Cie C2] si hula i Sphy

4
Re bo wre

B. Distinct division of femoral nerve into two parts.
Ce): GH) ESI 4S2) CAI eA?) es Ree Si eels Crue Zh
II. [C3] [M] [(Sph) (1)]
iL ils (HEA) (RSH, Se ANE N21) (ISB) Hs CORA) |) [Sra (G0)
Die ER R27 ew, 12)
ifm | ea |) [eAv PA2 I a2 eS EN Sp hi
1G, \USals Sep seals ise] bal, 4]
eee bal ed hh CSL) S82) CA2))
II. [S82] [R1, R2] [Li, L2] [C1, C2] [ (M1, M2, M3) seus)
le Pee iSie S21 (EA) S(AZ) I is3si Gb) (2a
II. [(R1) (R2)] [(L1) (L2)] [M1, M2] [Sph]

P—Nerve to pectineus muscle.
S—Nerve to sartorius muscle.
R—Nerve to rectus femoris muscle.
L—Nerve to vastus lateralis muscle.
C—Nerve to vastus intermedius muscle.
M—Nerve to vastus medialis muscle.
A—Anterior cutaneous nerve.
a—Anterior cutaneous nerve arising directly from plexus.
I—Medial cutaneous nerve.
S—Saphenous nerve.

X—Accessory obturator nerve.

In this table the various muscular and cutaneous nerves are represented
by letters. The numerals placed after a letter indicate a first, second,
or third nerve. The nerve trunks arising directly from the femoral are
enclosed in brackets. The component nerves which are bound up in a
common trunk for but a short distance are enclosed in parentheses. Those

Charles R. Bardeen Sula

which are united for a longer distance are separated by commas. In
several instances the femoral nerve divided into two main divisions before
separating into branches. These are shown in part B of the table.

II. OBTURATOR GROUP.
a. Embryon¢ Development.
1. General Features.

In an embryo 11 mm. long (Plate III, Fig. 1), the obturator nerve
passes about the pelvic blastema between the pubic and ischial process
and terminates some distance beyond in several branches. Differentia-
tion of musculature is beginning in the region about the terminus of
the nerve. One branch of the nerve terminates in a mass of tissue which
represents the anlage of the obturator portion of the M. adductor mag-
nus, and possibly also the M. obturator externus. The main nerve trunk
then breaks up into several short branches about which hes a mass of
tissue representing the anlage of the adductor longus and brevis and
the gracilis muscles

In an embryo of 14 mm. (Plate VIII, Fig. 1) the individual muscles
may be clearly recognized. None of them have well developed tendons.
The figure represents sufficiently well the relations of the adductor
muscles at the period under consideration. The gracilis muscle is merely
outlined in order to show the short and long abductors. Each muscle is
separated from its neighbors by a loose connective tissue. In this tissue
the nerves take their course to the muscles. The nerve to each muscle
strikes it about the center of greatest development, and may extend into
the muscle substance for some distance. The paths for this intramuscu-
lar nerve growth are not in all cases clearly marked. In older embryos
they are much plainer. The obturator and sciatic portions of the ad-
ductor magnus muscle are distinctly separate. See also Plate II, Fig. 3.

In an embryo of 20 mm. (Plate VIII, Fig. 2) the muscles have all
become attached by tendons to the skeleton. Merely the origins and
attachments of the adductor brevis and gracilis muscles are shown in
Fig. 2. Figs. a and b represent the gracilis and adductor brevis muscles
seen from the deep surface. The obturator and sciatic portions of the
adductor magnus muscle have become fused.

In slightly older embryos the muscles become much more separated by
relatively great development of intermuscular connective tissue than is
found in late foetal life and after birth. Compare Figs. 2, 3, and 4,
Plate II, with figures of frozen sections of the adult limb.

312 The Nerves and Muscles of the Leg

The cutaneous branch of the obturator nerve is not clearly distinguish-
able in the embryos studied. In embryo XXII the articular branch of
the nerve to the adductor magnus muscle is well marked.

2. Individual Muscles.

Gracilis—The anlage of this muscle becomes distinctly differentiated
at an early stage (Plate II, Fig. 3; Plate VIII, Figs. 1,2, anda). It
first appears in a region which corresponds with that in which the nerve
enters the deep surface of the muscle in the adult, near the junction of
the proximal with the middle thirds. From this region the muscle ex-
tends toward its pubic and tibial attachments.

In urodeles and reptiles (Saurians) the place of the gracilis is taken by
the pubi-ischio-tibialis, which is innervated by the tibial portion of the
sciatic nerve in the former and by the sciatic and obturator nerves in the
latter (Gadow, 82). In all the mammals it is innervated by the obturator
nerve. In several mammals the origin takes place from the abdominal wall
anterior to the pubis. The insertion is usually in considerable part into the
crural fascia and in some forms (edentates) extends to the foot (W. Leche).
In many mammals the gracilis near its insertion is fused with the sartorius.
Origin by two heads and fusion near the insertion with the sartorius have
been found as variations in man. On the whole, however, the muscle is
singularly independent.

Adductor brevis. Plate II, Fig. 3; Plate VIII, Figs. 1, 2, and b.
This muscle is differentiated at first in somewhat close association with
the obturator externus muscle and with the obturator portion of the ad-
ductor magnus. From its anlage processes of attachment are sent toward
the pubis and femur (Plate VIII, Fig. 2). In the adult the muscle is
usually innervated by a nerve which enters its middle third near the
proximal border.

Adductor longus. Plate II, Fig. 3; Plate VIII, Figs. 1 and 2. This
is differentiated from a muscle mass at first not perfectly distinct from
that of the adductor brevis and in a region corresponding with that where
the nerves enter the muscle. From here the muscle extends to its at-
tachments. In the adult the nerve usually enters the deep surface of the
muscle in several branches about midway between its tendons of origin
and insertion.

Obturator externus. Plate II, Fig. 3; Plate VIII, Figs. 1 and 2.
This muscle is differentiated from dense tissue lying beneath the obtu-
rator nerve in the obturator foramen and close to the embryonic hip joint.
From here it extends to its attachment to the femur. The relations
of the obturator externus to the pectineus muscle have been described

Charles R. Bardeen Hil

Oo

above, p. 302. The nerve for the obturator externus usually arises before
the obturator nerve enters the obturator foramen. The nerve generally
divides into two branches, one of which enters the superior border of the
muscle, and the other passes to its external surface.

Some of the superior fasciculi of the obturator externus muscle may
be separated from the main belly by the obturator nerve or its deep
branch.

Adductor magnus. Plate II, Fig. 3; Plate VIII, Figs. 1 and 2. This
muscle is developed from two distinct anlages, to one of which a branch
from the obturator nerve is given, and to the other a branch from the
sciatic nerve. These anlages are distinct in an embryo of 14 mm., but in
one of 20 mm. (Plate VIII, Fig. 3) they have fused and a rearrangement
of tissue has begun so that the three divisions of the muscle described
by Poirier‘ are beginning to be distinct. The exact steps in this
rearrangement of tissues I have been unable clearly to follow in the
material at my disposal. In the adult muscle the obturator nerve usually
gives off one or more branches which enter the main body of the superior
division of the muscle, the adductor minimus, on its obturator surface
about midway between the tendons of origin and insertion, and several
branches which pass in between the larger fasciculi of the middle and
inferior divisions of the muscle about midway between their tendons of
origin and insertion. The branch from the sciatic nerve likewise enters
between the main muscle bundles on the posterior surface of the middle
and inferior divisions of the muscle about midway between their tendons
of origin and insertion and usually sends a recurrent branch into the
lower border of the superior division of the muscle. Not infrequently
the nerve to the quadratus femoris muscle sends a branch into the upper
margin of the superior division of the adductor magnus. In one instance
a special branch of the sciatic was given to this portion of the muscle.

There is nothing to indicate that either the obturator or the sciatic
branch is confined in its distribution to the tissue of the muscle mass to
which it is originally sent. On the contrary it is exceedingly probable
that the sciatic branch helps to innervate a portion of the obturator
muscle mass and the obturator branch a portion of the sciatic muscle
mass.

CoMPARATIVE ANATOMY AND VARIATION IN THE ADDUCTOR GROUP.
In this group are included the obturator externus and the three adductor
muscles. The pectineus also belongs anatomically and physiologically and
probably in part also morphologically with this group (see p. 303). The

*Traité d’Anatomie, Tome 2, p. 229.

314 The Nerves and Muscles of the Leg

adductor magnus in most mammals is innervated merely by the obturator
nerve and there is a special presemimembranosus muscle (W. Leche) which
extends usually from the tuber ischii to the medial side of the distal end of
the femur parallel with the semimembranosus. In the gorilla, orang, and
gibbon the presemimembranosus is combined with the adductor magnus, as
it is in man (‘W. Leche). In many forms the presemimembranosus is more
or less fused with the semimembranosus. It may be looked upon as derived
phylogenetically from the semimembranosus or medial flexor of the thigh
(A. Biihler, 03). In echidna the adductor magnus is innervated both by
the obturator and sciatic nerves (W. Leche).

In urodeles the elements of the adductor group are probably contained in
the pubi-ischio-femoralis externus and possibly also in part in the pubi-
ischio-femoralis internus. The pubi-tibialis may represent the mammalian
presemimembranosus and the sciatic portion of the adductor magnus in man.
In reptiles the adductor elements are contained in the pubi-ischio-femoralis
externus (and the ischio-femoralis, Gadow). In the different groups of mam-
mals there is considerable variation in the number of individual muscles into
which the adductor musculature is divisible; from one to six according to
Le Double (97). In man the chief variations noted have to do with the
greater or less fusion of the different muscles into which the group is
divided. The adductor magnus may be united by fasciculi or fused not only
with the neighboring long adductor but also, owing to the origin of its
posterior portion from the hamstring group, with the semimembranosus
muscle. The adductor minimus portion of the adductor magnus is frequently
fused with the quadratus femoris and may be supplied by the same nerve
although this portion of the muscle belongs normally chiefly to the territory
of the obturator nerve. The short adductor is frequently fused with the
obturator externus.

b. Nerve Variation in the Adult.
1. Variation in the Origin of the Obturator Nerve.

In the great majority of instances the obturator, lke the femoral,
nerve arises chiefly from the 22d, 23d, and 24th spinal (2d, 3d, and 4th
lumbar) nerves. ‘Table XVI indicates the spinal roots from which the
nerve arose in 246 instances and the various types of lumbo-sacral plexus
with which the various modes of origin were associated.

2. Relations of the Nerves Springing from the Obturator Nerve to the
Spinal Nerves.

In the case of the obturator nerve it is even more difficult than in case
of the femoral nerve to trace with accuracy the relations of the nerves
of distribution to the nerve roots. Examination of several nerves leads
me to the belief that the bulk of the nerve fibres distributed to each of
the nerves of distribution occupy in the obturator nerve as it approaches

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316 The Nerves and Muscles of the Leg

the obturator foramen a position approximately shown in the following
diagram. The position occupied by the chief bulk of the fibres of each
of the main nerves of distribution is shown in outline while the approxi-
mate area of the 23d spinal (3d lumbar) nerve is indicated by stippling,
in this diagram.

3. Variation in the Branches of Distribution Arising from the
Obturator Nerve.

In the great majority of instances the obturator nerve divides at the
proximal border of the adductor brevis muscle in such a way that the
nerves of supply to the gracilis muscle and to the adductor longus and
brevis muscles pass in the anterior division of the nerve external to the

R. cutaneous

-R. gracilis

R.add_ brevis

R. obt. externus :
R.add. maenus

Fic. 5. Diagram to illustrate the position occupied in the obturator nerve
. by the nerve fibers going to the muscular and cutaneous branches.

adductor brevis muscles, while the nerves to the adductor magnus muscle
and to the obturator externus muscles arise from the posterior division
of the nerve. Very often the nerve to the obturator externus muscle is
given off before the obturator nerve passes through the obturator fora-
men or above the place of division of the obturator nerve into anterior
and posterior divisions. The nerves of distribution, the fibres of which
occupy contiguous areas in the cross section of the main trunk shown
above, may be associated for some distance in a common trunk before
ultimately becoming independent.

In the following table are indicated the relations of the chief muscular
and cutaneous branches of the obturator nerve in 88 instances. In all
instances the branch to the obturator externus is included with the in-
ternal division of the obturator nerve, although in the majority of in-
stances this branch was given off from the main trunk of the nerve before
it split into anterior and posterior divisions. The articular and arterial

Charles R. Bardeen il

branches are not included in the table because the data concerning them
are too incomplete. It is possible that in several instances the small
cutaneous twig which anastomoses with the medial cutaneous nerve was
lost in dissection. This table therefore indicates a minimum number
of instances in which the obturator furnished a cutaneous branch. The
same is true of the nerve to the pectineus muscle. ;

The parentheses in the table indicate the simultaneous division of a
nerve trunk into the branches included within them. The commas indi-
cate that the division of the nerves thus separated took place later than
that of the combined nerve from the parent trunk.

TABLE XVII.

B—Nerve to adductor brevis muscle.
C—Cutaneous branch.

E—Nerve to obturator externus muscle.
G—Nerve to gracilis muscle.

L—Nerve to adductor longus muscle.
M—WNerve to adductor magnus muscle.

a. Branch to adductor brevis from anterior division.

No. of inst. Anterior division. Posterior division.
26 (E) (M) (BY Ga)” (Gy?
9 (E) (M) (B)) CE) (G3 Ge)

In all instances extensive distribution of cutaneous branch; in
one half way down back of leg (481), in another nearly to
ankle (693).

12 (E) (M) (B) (G.C) (G)*

In five instances extensive distribution of cutaneous branch.
18 (E) (M) CB) ECGs) =
6 (E) (M) (B), (GG; ©)

In three instances the distribution of cutaneous branch was
fairly extensive.

5 (BE) (M) (By) (G)>

2 (E) (M) (B, L, C) (G)
1 (E) (M) (B, L) (C) (G)
1 (E) (M) (BAG) ds)

1 (EZ) (M) (B, G) (L, C)

®In one instance a branch to adductor minimus from nerve to quadratus
femoris.

Tn one instance a branch to the pectineus muscle was found.
4JIn two instances a branch to the pectineus muscle.

318 The Nerves and Muscles of the Leg

b. Branches to adductor brevis from anterior and posterior divisions.

2 (E) (B) (M) (B) (L) (G)

ec. Branch to adductor brevis from posterior division.
4 (E) (B) (M) (G) (L)
1 (E) (B) (M) (G) (L, C)

Ill. THE SCIATIC NERVE.

In early embryonic life the separation between the tibial and peroneal
nerves is well marked nearly to their origin from the sacral plexus. Near
the plexus there intervenes between them a considerable amount of dense
tissue (Plate II, Fig. 3) and more distally they are separated by the
anlage of the fibula (Plates III, IV, and V).

a. Embryonic Development.

The peroneal nerve in avn embryo 11 mm. long (Plate III, Fig. 2)
may be traced as far as the middle of the dorsal side of the limb-bud.
Four fairly distinct muscle anlages are visible along its course. The
first of these, the gluteus medius mass, represents the anlage of the
gluteus medius and minimus, the piriformis and the tensor fascie late,
and toward it special branches: are proceeding from the plexus. The
second muscle mass represents the anlage of the gluteus maximus and
the third that of the short head of the biceps. These two anlages adjoin
one another. The fourth represents the anlage of the extensors of the
ankle and peroneal muscles. Some differentiation is apparent between
the anlages of the last two groups of muscles. In an embryo of 14 mm.
(Plate IV, Fig. 2; Plate VIII, Fig. 4; Plate IX, Fig. 1) muscle differ-
entiation has taken place in each of the anlages mentioned above and
the anlage of the short extensor of the toes has appeared. To each muscle
rudiment a nerve branch is given. In an embryo of 20 mm. (Plate V,
Fig. 2; Plate VIII, Fig. 5; Plate IX, Fig. 2) muscle differentiation is
more marked and the branches to each muscle resemble somewhat those
of the adult.

The tibial nerve in an embryo of 11 mm. (Plate III, Fig. 1) extends
to the middle of the plantar side of the leg. Along its course several
muscle anlages may be seen. Of these the first is that of the obturator
internus, the second that of the quadratus femoris, the third that of the
hamstring muscles, the fourth that of the gastrocnemius—soleus group,
and the fifth that of the deep muscles of the back of the leg. In an
embryo of 14 mm. (Plate IV, Fig. 1; Plate VIII, Fig. 1;, Plate IX,
Figs. 3 and 4) individual muscles have appeared in each of the anlages
mentioned and a muscle mass has appeared in the foot. In the leg the

Charles R. Bardeen 319

lateral and medial plantar nerves are separated from one another.
Nerves are given to the various muscle anlages. In an embryo 20 mm.
long (Plate V, Fig. 1; Plate VIII, Figs. 2 and 3; Plate IX, Figs. 5 and
6) the muscles of the foot are beginning to be differentiated, the medial
and lateral plantar nerves on the back of the leg have become fused and
the branches to the various muscles somewhat resemble those of the
adult.

b. Adult Conditions..
1. Separate Origin of the Peroneal and Tibial Nerves.

During early embryonic development, as mentioned above, the per-
oneal and tibial nerves arise separately from the plexus. In about 10%
of instances studied at the Johns Hopkins University this condition was
found present in the adult, the two nerves being separated by a portion
of the piriformis muscle or more rarely by the whole muscle (see Bar-
deen and Elting, o1). Eisler, 92, found the condition in 18.1%. of 123
plexuses and Paterson, 94, in 13% of 23 plexuses. The nerves arise
separately from “normal,” proximal or distal types of plexuses with
about equal frequency.

2. Frequency of Variation in Origin of the Peroneal and Tibial Nerves.

In Tables XVIII and XIX are shown the frequency of various
modes in origin of the tibial and peroneal nerves from the sacral plexus,
and the types of plexus with which these various modes of origin were
associated. No detailed explanation of these tables seems requisite.
Tabulation of the relation of the various types of origin in relation to
race, sex, and side of the body has revealed no facts of special interest,
and hence tables covering these points are here omitted.

3. Relations of the Branches Springing from the Peroneal and Tibial
Nerves to the Nerve Roots.

It is not often possible to trace back with certainty to their origin
from the plexus the various branches springing from the peroneal and
tibial nerves. It can be done only under special conditions and cannot
be well carried out by students in the dissecting room. For this reason
no attempt has been made to collect statistical data on this subject. The
following diagram based on special dissections, indicates roughly the
regions occupied by the chief nerve branches in the peroneal and tibial
trunks, and their relations to the spinal roots of these nerves. Although
the peroneal and tibial nerves are usually bound up on the back of the

The Nerves and Muscles of the Leg

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Charles R. Bardeen

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322 The Nerves and Muscles of the Leg

thigh into a common trunk there seems normally to be no crossing of
nerve fibres from one nerve to the other. Branches arising from each
nerve may be bound for a certain distance into a common trunk provided
that they occupy contiguous positions in the parent nerve, as indicated
in the diagram Fig. 6.

N. peron. prof, N. popliteus.
‘ ~ N. plant.med.

N. peron. superf.

N.semit.
-distal. branch

‘\ N. gastroen.
fs ao - Sapmed.
N.cut.surae. lat. cag roa
N.semit
-t--prox. branch

. N. biceps.
cap. breve.- _N. gastroen.
c&p.lat
: N. biceps
/ Cap. long.
N.tib. posl.- ay “ff
N. Flex. dig. long: 3 eee

N. soleus (R.profy ae \
N.flex. hal. long’

N.cut.Sur.ae. medi
N. pla nt. lat:

rGaos
See text above.

1
! \

R.M.plant’

IV. SUPERIOR GLUTEAL GROUP.
a. Embryonic Development. —

1. General Features.

This group consists of the gluteus medius and minimus muscles, the
piriformis, and the tensor fasciz late. The last becomes distinct from
the general muscle mass at a very early stage, the others are closely bound
together during the earlier stages of differentiation in the anlage. Graf-
enberg, 04, has described the development of these muscles in man. In
an embryo of the fifth week he describes a cone-shaped mass of dense

Oo
“
(SX)

Charles R. Bardeen

tissue the point of which extends toward the upper end of the femur.
This he considers the anlage of the gluteus maximus, the piriformis, the
gluteus medius and minimus, the tensor fascie late, quadratus femoris,
and obturator internus. In the embryos of this period which I have
studied I have not found an intimate union between the anlages of the
gluteus medius group, the gluteus maximus, the obturator internus and
the quadratus femoris groups. When differentiation of the muscles in
this region begins the four anlages, though none of them sharply out-
lined, seem to me fairly distinct from one another as I have attempted
to show in Plate III, Figs. 1 and 2.

In an embryo of 14 mm. (Plate VIII, Fig. 4) the m. tensor fasciz
late is quite distinct from the rest of the group. Grifenberg states that
at first it is closely connected with the anlage of the gluteus minimus.
There is no connection between the anlages of the tensor fascie late and |
that of the gluteus maximus. The separation of the gluteus medius from
the gluteus minimus is marked best in the region through which the
superior branch of the superior gluteal nerve passes out to end in the
tensor fasciz late (Plate II, Fig. 3). The piriformis is still closely
bound to the anlage of the two gluteals. I can find no connection be-
tween it and the gluteus maximus such as that described by Grafenberg.
The anlages of the two gluteal muscles and the piriformis pass distally
into the proximal part of the back of the femur in the region where later
the great trochanter will be developed. The gluteal anlages, closely ap-
pled to the anlage of the acetabulum, extend to the femoral margin of
the embryonic ilium. The piriformis extends over the peroneal nerve
toward but does not reach the pelvis. It is to be presumed that in those
instances in which the peroneal nerve passes through the piriformis the
course of development of the muscle toward the sacrum takes place on
each side of the nerve. The dense tissue between the peroneal and tibial
nerves in this region may represent an interneural process of this kind.
It is continuous with the piriformis anlage.

Two distinct branches of the superior gluteal nerve may be seen. One
of these extends to the tensor fasciz late, the other ends in the anlage
of the gluteus medius. The nerve to the piriformis is likewise beginning
to grow toward this muscle.

In an embryo of 20 mm. (Plate VIII, Fig. 5) the great trochanter is
becoming well marked, Bardeen, 05, and the attachments of the two
deeper gluteal muscles and the piriformis begin to resemble those of the
adult. The gluteal muscles have extended a considerable distance over

the ilium the ala of which is much better developed than in the 14 mm.
22

324 The Nerves and Muscles of the Leg

embryo. The two muscles are clearly differentiated from one another,
but the gluteus medius is partly fused to the piriformis and in this em-
bryo the piriformis has not extended to the sacrum. In other embryos
of about this stage the piriformis has, however, become attached to the
sacrum. he distribution of nerves to these muscles is very similar to
that found in the adult. The tensor fasciz late extends distally over the
thigh into the anlage of the tractus ilio-tibialis which at this period is
but slightly marked.

In subsequent development the iliac ala imcreases in size and the
muscles extend over it to their adult attachments. With the develop-
ment of the anterior superior spine of the ilium the iliac attachment of
the tensor fascizw late is carried far from its original position near the
back of the head of the femur.

2. Individual Muscles.

Tensor fascie late.—This is differentiated near the lateral edge of the
anlage of the gluteus medius and minimus. According to Grafenberg,
04, it is at first closely fused with this anlage and extends from the
“ Beckenschaufel” to the anlage of the great trochanter. In the speci-
mens which I have studied the anlage of the muscle when it first becomes
distinct has no skeletal attachment but lies near the gluteal anlage (Plate
VIII, Fig. 4). From here it shifts laterally and its proximal extremity
soon becomes attached to the ilium somewhat distal to the crest and be-
hind the anlage of the anterior superior ihac spine. Distally it extends
toward the lateral] side of the knee (Plate VIII, Fig. 5) and is continued
into the tractus iliotibialis which toward the end of the second month
begins to be distinct.

In the adult the nerve usually enters the muscle about midway between
its origin and insertion. This area corresponds to that first differentiated
in the embryo.

It seems probable that that portion of the m. ilio-tibialis of urodeles and
reptiles innervated by the sciatic nerve (the m. gluteo-rectus) represents the
tensor fascia late of mammals. In different mammals the tensor fascie#
late varies greatly in development. It is said not to be present in mono-
tremes and marsupials (W. Leche). It is large in all anthropoids except
the orang (Le Double).

Gluteus medius and minimus.—These two muscles are differentiated
in close association with one another and remain closely associated in the
adult. The myoblastema from which they are derived lies close to the
back of the embryonic skeleton near the junction of the femur with the
pelvis (Plate VIII, Fig. 4). The anlage of the two muscles seems from

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Or

Charles R. Bardeen

the first to extend distally into the anlage of the great trochanter, but
proximally it extends only to the acetabular portion of the ilium. From
here the muscles extend over the lateral surface of the iliac. ala and finally
reach the iliac attachments characteristic of the adult. The ascending
branch of the superior gluteal nerve takes a course at first distal to the
transverse branch, but as the gluteus medius grows toward the iliac crest
the ascending branch is carried proximally across the transverse.

The gluteus medius and minimus muscles correspond with the ilio-femoralis
of urodeles and reptiles, a muscle supplied by branches from both the femoral
and sciatic nerves. In the monotremes the “ gluteus medius”’ is a thin muscle
which arises from the sacro-caudal vertebre and is supplied by a branch of
the peroneal nerve while the gluteus minimus and scansorius are represented
by a mass of muscle which arises from the fascia lumbo-dorsalis, the lumbar
and sacral vertebre and the ilium and is innervated by branches of both the
femoral and peroneal nerves. In all higher forms the gluteus medius-
minimus musculature is innervated by branches which arise directly or indi-
rectly from the peroneal portion of the sacral plexus (Westling, cited by
Leche). It seems not unlikely that in the urodeles, reptiles and mono-
tremes elements of the ilio-psoas musculature of higher forms are included
in the ilio-femoral musculature. The more superficial and posterior part of
the sciatic portion of the ilio-femoral anlage has given rise in the higher
mammals to the gluteus medius and piriformis, the deeper and more anterior
portion to the gluteus minimus and scansorius. The degree of separation of
these various elements varies greatly in different mammals.

The variations of the two muscles which have been found in man are
chiefly those of a greater differentiation than usual of individual muscles
from the common anlage (i. e., M. scansorius) or a partial or complete fusion
of the muscles with one another or with the piriformis.

The piriformis.—This is differentiated from tissue at first closely as-
sociated with the gluteus medius and minimus (Plate VIII, Fig. 4).
According to Grifenberg the muscle anlage can from its first appearance
be traced to the sacrum. While it is true that a dense mass of cells sur-
rounding the sciatic nerve and its roots of origin can be followed back
to the sacrum this condensed tissue is not, I believe, to be looked upon
as the anlage of the piriformis, although the two are not at first sharply
to be distinguished. Differentiation of the muscle is first clearly marked
in the region between the sacral plexus and the anlage of the great
trochanter. From here the developing muscle may be followed in older
embryos toward its sacral attachment. In embryo XXII (Plate VIII,
Fig. 5) the sacral attachment has not yet been reached. The region in
which the nerve enters the adult muscle corresponds with the area in
which muscle differentiation is first seen. As pointed out above, the
differentiation of the muscle at a period preceding the fusion of the

The Nerves and Muscles of the Leg

326

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The Nerves and Muscles of the Leg

peroneal and tibial nerves into a common trunk may account for the
variation in the relations of those nerves to the muscle in the adult.

Although Gegenbaur and others have considered the piriformis to be de-
rived from the caudo-femoral muscle of urodeles and reptiles, both com-
parative anatomical and embryological studies speak against this view. The
caudo-femoral muscle of these lower forms is represented in many of the
mammals by a caudo-femoralis (W. Leche), which typically extends from
the caudal vertebre to the lateral side of the distal half of the femur and
runs parallel with the presemimembranosus. As in the reptiles and urodeles
so here the muscle toward its femoral insertion lies in front of the sciatic
nerve while the piriformis normally runs dorsal to this nerve.

The piriformis is to be looked upon as an especially differentiated portion
of the ilio-femoral muscle of urodeles and reptiles. In a considerable number
of mammals it is not differentiated (some ungulates, etc.).*

In man the piriformis is frequently fused with the gluteus medius. Its
origin may take place from the great sciatic notch instead of from the sacrum.

b. Nerve Variation in the Adult.

1. Variation in the Relations of the Superior Gluteal Nerve and of the
Nerve of the Piriformis to the Nerve Roots.

The preceding tables, XX, XXI, indicate the frequency of certain
modes of origin from the sacral plexus of the superior gluteal nerve and
the nerve to the piriformis muscle and the relation of these modes of
origin to certain types of Jumbo-sacral plexuses. While there is some
correspondence between an anterior or a posterior form of plexus and a
“high” or “low” mode of origin of the nerves this correspondence is
by no means perfect.

2. Variation in the Branches of Distribution.

Supertor gluteal nerve-—Most frequently this nerve arises by two roots,
one from the lumbo-sacral cord (4th-5th lumbar) and the other from
the first sacral nerve. The trunk usually soon divides into two branches.
The ascending branch is distributed mainly to the more dorsal part of
the gluteus medius muscle in the middle third between its tendens. Ac-
cording to some authors it also sends fibres to the gluteus minimus
muscle. J have found it much more frequently confined in distribution
to the gluteus medius muscle. The transverse branch passes across the
external surface of the gluteus minimus muscle about midway between
its tendons and near the lateral border of the muscle passes beneath a

*“ According to Kohlbrugge, g7, the piriformis has a double origin, on the
one side from the gluteal musculature, on the other from the metameric
caudal muscles.

Charles R. Bardeen 329

special slip of the muscle and terminates in the proximal portion of the
middle third of the m. tensor fascize late. It gives branches of innerva-
tion to the gluteus minimus muscle, to the lateral portion of the gluteus
medius muscle and to the tensor fascie late. The fibres of the ascend-
ing branch always arise from the plexus lower down than those of the
transverse branch. Sometimes it arises as a separate branch from the
first sacral nerve. It may then pass through the substance of the piri-
formis muscle and be associated with the nerve to the piriformis muscle.

Nerve to the piriformis muscle-——Very commonly the nerve to this
muscle may arise from a loop connecting the first and second sacral
nerves, but more often the branches arise directly from the first or second
sacral nerve and pass into the substance of the muscle in the middle third
between the tendons. The ascending branch of the superior gluteal nerve
may send a ramus to the piriformis muscle. J have never seen a branch
from the third sacral nerve to the piriformis such as those described by
Weber, Hildebrandt, Valentine and Henle.

V. THE GLUTEUS MAXIMUS AND THE SHORT HEAD OF THE BICEPS.

The studies in comparative anatomy of Ranke, 97, Klaatsch, 02, and
others have gone to prove the close morphological association of the
gluteus maximus and the short head of the biceps.

It seems probable that both the short head of the biceps and the gluteus
maximus are represented in the urodeles by the ilio-(femoro)-fibularis and
in reptiles by the ilio-fibularis muscle which is supplied by the peroneal
portion of the sciatic. In the mammals the proximal attachment of this
musculature has extended well into the caudal region from the ilium. In
the monotremes it is represented by a muscle which extends from the caudal
region to the foot and in Echidna lies posterior to and does not cover the
other glutei (Westling). In most of the higher forms it is divisible into
three muscles, the superficial gluteus, or gluteus maximus, the femoro-
coccygeus (Leche), and the gluteo-crural (Klaatsch). The superficial glu-
teus is inserted into the femur or into the fascia of the thigh. The femoro-
coccygeus is inserted into the shaft of the femur, and the gluteo-crural into
the fascia of the leg or into the fibula. The superficial gluteus and the
femoro-coccygeus are not infrequently fused to form the gluteus maximus.
The gluteo-crural is absent in some forms. In most mammals it extends as
the tenuissimus from the caudal vertebre or the gluteal fascia to the leg.
In man and a few of the higher primates it arises from the femur and be-
comes applied to the tendon of the long head of the biceps to form the short
head of this muscle. Klaatsch, o2, has given an especially valuable account
of the gluteo-crural muscle. See also Windle and Parsons, oo.

In man the gluteus is not infrequently found divided into two portions, a
condition normal in the embryo. The short head of the biceps not infre-

330 The Nerves and Muscles of the Leg

quently has a tendon of insertion more or less distinct from that of the long
head. Its tendon of origin may be attached to the tuber ischii, the fascia
covering the gluteus maximus, or the sacrotuberosal ligament.

a. Embryonic Development of Gluteus Maximus.

The gluteus maximus arises from an anlage which lies dorso-lateral to
the anlage of the great trochanter (Plate III, Fig. 2). Its proximal
edge overlaps and lies near but does not seem to be fused with the gluteus
medius anlage. Distally it is slightly fused with the anlage of the short
head of the biceps. Into the gluteus maximus anlage two nerves extend
from the back of the sacral plexus.

In an embryo of 14 mm. (Plate II, Fig. 3; Plate VIII, Fig. 4) the
gluteus maximus is quite distinct from the neighboring muscles.” It is
beginning to show a division into two portions each of which is supplied
by a separate nerve. The more distal of the two portions is continuous
with the blastema of the femur. Proximally the muscle is extending over
the gluteus medius and obturator internus anlages toward the ilium and
sacrum. I find no primitive intrapelvic extension of the gluteus maximus
such as that described by Grafenberg, but the fascial extension which he
describes from the dorsal muscles over the gluteal muscles is quite evi-
dent (see Plate II, Fig. 3).

In an embryo 20 mm. long (Plate VIII, Fig. 5) the gluteus maximus
has extended from the trochanteric region where it first appears to the
ium, sacrum and coccyx. It is at this period very distinctly separated
into two portions the more distal of which is inserted into the femur
distal to the great trochanter while the more proximal is inserted into the
fascia over the attachment of the distal portion. In the adult the two
portions are only rarely thus distinct. The distal portion represents the
femoro-coccygeus muscle so common in the-lower mammals. In the
younger embryos two nerves pass from the plexus to the muscle. In this
embryo a special nerve is given to each portion of the muscle, but the
two nerves arise by a common trunk from the plexus. The nerve to the
superficial portion of the muscle curves toward the ilium and passes up-
wards on the deep surface of the muscle along a line about midway be-
tween the origin and insertion of the muscle. The nerve to the distal
portion passes distally and enters its proximal margin (Plate VIII,
Fig. 5).

& The early union with the piriformis described by Grafenberg I have not
found in any of the embryos I have examined, although I find, as he describes,
an early transitory union between the anlages of the short head of the biceps
and the gluteus maximus.

Charles R. Bardeen 331

b. Variations in the Inferior Gluteal Nerve.

This nerve arises in the main from the first sacral nerve, but in part
usually also from the lumbo-sacral cord; often from the 2d sacral, and
rarely from the 3d sacral. Its roots may be superficially bound up with
the trunks of origin of the posterior cutaneous nerve and not infrequently
with the main sciatic trunk. In the great majority of instances the main
trunk of the nerve divides into an ascending and a descending branch.

TABLE XXII.

Type of Plexus from which the N. Gluteus Frequency of Origin of N. Gluteus
Inf. arises. Inf. from:
Wn. Sp. Nn. Sp. Nn. Sp.
Most Distal (XXIV) [XXIV] | Total
Type. Furcal Nerve. | Spinal Nerveto | XX I ERONGV, (exexe Walaa
Tafa XXVI | xxvI_ | xxvi_ | Number.
| ; | OXEXGV/ IAI) | eeXoXGValiT |
| | (XXVIID
A | SXOXGLV, XO 1 | 1
Ant. | B SONY; XXVII 12 6 ee
SXOXCNV, £ ‘

C chiefly to XXVIII 24 27 3 54
sacral plexug

XXIV | =

Norm. | D chiefly to XXVIII 29 40 5 v4
ry lumbar plexus

XEXTAVESXONAVE a55
E or XXV SROXOVALIT 9 | 1 10
Post. | F XXIV KXCBX 1 Gee yl 3 10
| XXIV-XXvV, ae |
G Bey BXOXCIDNs 10 | 1 ul
MO Sail N UID OT: a nah a ct See neh oe parked agwee eee 67 | 98 | 192). eleaeae

The ascending branch curves upwards on the under surface of the gluteus
maximus muscle midway between the tendons of origin and insertion.
The descending branch is distributed in the middle third of the deep
distal portion of the muscle. The fibres of the descending branch have a
more distal origin than those of the ascending branch. In the adult the
two branches often arise separately from the plexus. The table above
shows the frequency of origin of the nerve from various groups of spinal
nerves and the frequency with which each is associated with a given type
of plexus. .

332 The Nerves and Muscles of the Leg

c. Embryonic Development of the Short Head of the Biceps.

In an embryo of 11 mm. (Plate 3, Fig. 2) the anlage of the short head
of the biceps extends along the distal half of the fibular margin of the
femur dorso-lateral to the peroneal nerve. Proximally it is continued to
the anlage of the gluteus maximus.

In an embryo of 14 mm. (Plate IJ, Fig. 3; Plate VIII, Fig. 4) it
does not extend proximally quite to the femoral insertion of the gluteus
maximus. Distally it is beginning to be attached to the tendon of the
long head of the biceps. The nerve to the muscle which at the former
stage was not evident may at this stage be seen entering the fibular margin
of the muscle.

G. D. Thane mentions an instance in which the nerve to the short head of
the biceps arose in connection with the inferior gluteal nerve from the sacral
plexus.”

In an embryo 20 mm. long (Plate VIII, Fig. 5) both the femoral and
distal attachments of the muscle are well marked.

VI. THE MM. OBTURATOR INTERNUS, GEMELLI AND QUADRATUS
FEMORIS.

These constitute a distinct group of muscles which are differentiated
on the ischial side of the anlage of the hip joint. Although closely as-
sociated, the anlage of the obturator internus and gemelli seems to be
from its earlier stages of differentiation distinct from that of the quad-
ratus femoris. I do not find the anlages of these muscles fused at an
early stage with the gluteal anlages as described by Grafenberg, 04.
When they first appear (Plate III, Fig. 1) the anlage of the quadratus
femoris has a somewhat more anterior position than that of the obturator
internus. This may account for its nerve supply in the adult from a
more proximal set of spinal nerves.

a. Embryonic Development.

Obturator internus and gemelli.mAn indistinct region of tissue differ-
entiation near the ischium in Embryo CIX, length 11 mm. (Plate III,
Fig. 1) I take to be the anlage of the obturator internus and the gemelli.
To it a nerve is given from the sacral plexus. The anlage of these muscles
is much more distinct in an embryo 14 mm. long (Plate VIII, Figs. 1
and 4). Here it may be seen extending from the anlage of the great

1% Quain’s Anatomy, 10th ed.

Charles R. Bardeen Bae

trochanter across and then upwards for a short distance on the pelvic
surface of the ischium toward the obturator foramen. No distinction
can at this time be made between the obturator internus and the two
gemelli. From the sacral plexus a nerve branch may be seen extending
across the outer surface of the muscle. Beneath the muscle another nerve
may be traced to the anlage of the quadratus femoris.

In an embryo 20 mm. long (Plate VIII, Fig. 5) the obturator internus
has extended well over the obturator foramen and in its growth into the
pelvis has carried its nerve in the same direction. The gemelli cannot
yet be clearly distinguished from the obturator internus. A good descrip-
tion of the architecture of these muscles in the adult and of the distribu-
tion of nerves to them is given in the J’raité @anatomie humain of Poirier
and Charpy. In the adult the chief variations in structure are those of
a greater or less independence of the gemelli and a greater or less extent
of the pelvic attachments of the obturator internus.

Quadratus femoris ——This is differentiated comparatively early in a re-
gion lying between the anlage of the great trochanter and that of the
tuber ischii (Plate III, Fig. 1; Plate VIII, Fig. 4). It soon forms at-
tachments which correspond well with those of the adult muscle (Plate
VIII, Fig. 5). In the embryo, as in the adult, the nerve enters the deep
surface of the muscle near the junction of the middle and ischial thirds.
In the adult the muscle is frequently fused either with the inferior
gemellus or with the adductor minimus. Its nerve of supply may extend
into the adductor minimus.

The quadratus femoris, gemelli, and obturator internus muscles of mam-
mals are apparently related to the ischio-femoral musculature of urodeles
and the pubi-ischio-femoralis posterior (Gadow) of reptiles. Among the
mammals the obturator internus is said not to be found in the monotremes
(W. Leche) but it occurs in most, although not all, of the higher forms. The
degree of isolation of the gemelli and the mode of attachment of the obturator
internus vary considerably in different mammals. The quadratus femoris
seems to be a fairly constant muscle in the mammalian series. In a con-
siderable number of mammals, however, it is innervated by the obturator
nerve instead of by a special branch from the sacral plexus (see W. Leche).
I do not know of an instance of this kind being reported as a variation in
man. The innervation of the adductor minimus portion of the adductor
magnus by the nerve to the quadratus femoris is, however, frequent and
rarely this nerve may send a branch to the M. obturator externus. The
adductor minimus is normally supplied chiefly by the obturator nerve. In
Talpa the quadratus femoris and obturator externus are fused and the com-
bined muscle is supplied both from the obturator nerve and from the sacral
plexus (W. Leche).

The Nerves and Muscles of the Leg

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Charles R. Bardeen Bis i)

b. Nerve Variation.
1. Variation in the Origin of the Nerves to the Obturator Internus and
Quadratus Femoris Muscles.

In Table XXIII the frequency of variation in the origin of the nerves
to these muscles is shown. The nerve to the quadratus femoris muscle
arises usually from the lumbo-sacral cord and the 1st sacral nerve (24th,
25th, and 26th spinal nerves.) Not infrequently the 25th spinal nerve
is the most distal nerve to furnish fibres to this nerve. This condition
occurs usually in the more proximal forms of plexus. In the more distal
forms of plexus the 25th and 26th spinal nerves furnish the fibres for
this muscle.

TABLE XXIV.
Number of Instances = i
Association of the Branches Distributed to the Obturator iecenivon at | Ze
Internus, Gemelli, and Quadratus Femoris Muscles. the Bie
BoC (Deal seaieG
Branch 1, to Mm. obturator int. and gemellus sup. | |
Branch 2, to Mm. quadratus femoris and gemellus inf. !® per hee eos ak | | 13
ss |
|
Branch 1, to M. obturator int. | | |
Branch 2, to M. gemellus sup. 1 | | | 1
Branch 3, to Mm. quadratus femoris and gemellus inf. |b oa |
| | I>
Branch 1, to M. obturator int.
Branch 2, to Mm. quadratus femoris and both gemelli. | 3/ 3
Branch 1, to Mm. obturator int. and gemellus sup.
Branch 2, to M. quadratus fem., gemellus inf., and adductor 2 2
magnus.
|
Total Number. ' 1} 8] 4] 2 | Wea. 18
| { |

“ Hor types of plexus see preceding table.
47 Tn one instance a branch was traced to the M. obturator externus.

The nerve to the obturator internus muscle arises usually from the
(24th) 25th, 26th, and 27th spinal nerves. Rarely the 26th spinal nerve
is the most distal nerve to furnish fibres to it and occasionally in distal
forms of plexus the 28th spinal nerve may do so.

It is difficult to trace these nerves back to their roots of origin. The
charts on which these tables are based are those recording the most ac-
curate dissections of these nerves. They are, however, of positive rather
than negative value and it is possible that a more extensive origin than
here indicated was present in some of the plexuses here recorded.

iss)
Oo
oO

The Nerves and Muscles of the Leg

2. Variation in the Nerves of Distribution.

The frequency of this variation is indicated in Table XXIV. Only
those charts are used for tabulation which were based on the more
accurate dissections of the distribution of the nerves to the
muscles. Most frequently the nerve to the obturator internus
muscle furnishes a branch to the superior gemellus muscle
while that to the quadratus femoris muscle furnishes a branch to
the inferior gemellus muscle. Occasionally a separate branch passes
from the sacral plexus to the superior gemellus muscle. In distal forms
of plexuses the nerve to the quadratus femoris muscle may furnish
branches to both gemelli muscles. Not infrequently the branch to the
quadratus femoris muscle is continued into the proximal portion of the
adductor magnus muscle. This condition has been described by Wilson,
89. In one instance I have followed a branch to the M. obturator
externus.

VII. THE HAMSTRING MUSCLES.
a. Hmbryonic Development.
1. General Features.

In an embryo 11 mm. long (Plate III, Fig. 1) two branches from the
tibial portion of the sciatic nerve represent nerves to the hamstring
muscles. They terminate in a mass of tissue on the plantar side of the
femur. The more proximal of the two nerves represents the proximal
branches to the long head of the biceps and the semitendinosus ; the more
distal nerve, that to the distal part of: the semitendinosus and the long
head of the biceps and to the semimembranosus and adductor magnus
muscles.

In an embryo 14 mm. long (Plate VIII, Fig. 1) the various muscles
mentioned are distinctly differentiated. But a single nerve branch is
given to the sciatic portion of the adductor magnus (at this period a
distinct muscle not closely fused with the obturator portion) and to the
semimembranosus. ‘To the semitendinosus and to the long head of the
biceps proximal and distal branches are given. About the terminus of
each motor nerve the muscle differentiation is best marked. The tendi-
nous attachments at each extremity of the muscles are indefinite. Proxi-
mally they fuse with the ischial blastema.

In an embryo 20 mm. long (Plate VIII, Fig. 3) the muscles of this
group are attached by tendons to the skeleton. The obturator and sciatic
portions of the adductor magnus have become fused.

Charles R. Bardeen Bon

2. Individual Muscles.

Adductor magnus.—See p. 313. That portion of this muscle which
is attached to the distal end of the femur represents the praesemimem-
branosus of the lower mammals and belongs primitively to the hamstring
group.

Semimembranosus.—This muscle arises from a special anlage in close
association with that of the sciatic portion of the adductor magnus (Plate
II, Fig. 3; Plate VIII, Fig. 1). The belly of the muscle becomes dis-
tinct before the tendons. In an embryo of 20 mm. (Plate VIII, Fig. 3)
there is a flat tendon of origin which is closely applied to the adductor
magnus and which arises from the ischium. The tendon of insertion
fuses with the tibial blastema near the back of the knee joint. The nerve
enters near the center of the muscle belly. In the adult the nerve enters
by several branches into the substance of the muscle about midway be-
tween the tendinous attachments of the muscle bundles composing it.
The superior branches curve upwards either on the surface or within
the substance of the muscle. There is much individual variation in the
exact mode of distribution of the branches of the nerve to this muscle.

The semimembranosus is probably represented in urodeles by a part of the
(caudali)-pubi-ischio-tibialis and in reptiles by a portion of the flexor tibialis
internus. In most mammals it arises from the ischium or pubis, runs paral-
lel with, and may be incompletely differentiated from the presemimembra-
nosus, mentioned above in connection with the adductor magnus, and is
inserted into the _ tibia. It may be fused with the semitendinosus.
A. Forster, 03, has shown that although in the lower mammals the semi-
membranosus is a flexor and may send a tendinous expansion to the plantar
aponeurosis, in apes and monieys it is chiefly an internal rotator of the leg.
In many mammals it is associated with a caudo-femoral (W. Leche) muscle
which extends from the caudal vertebre to the distal end of the femur.

In man it may be longitudinally doubled, may be partially fused with the

adductor magnus or the semitendinosus and may arise from the ischial spine
or the sacro-tuberosal ligament as well as from the tuber ischii.

Semitendinosus.—This muscle is formed from two anlages, one of
which is differentiated in close conjunction with the anlage of the ischial
tuberosity, the other more distally. These anlages correspond with the
two parts of the muscle found in the adult and to each a separate nerve
is given (Plate VIII, Fig. 1). The anlages are visible in an embryo of
14 mm. and the muscle is well differentiated in one of 20 mm. (Plate
VIII, Fig. 3). In the latter the tendinous inscription which subdivides
the muscle is as distinctly marked as in later life. The tendon of in-
sertion is inserted relatively more distally in the 20 mm. embryo than in
the adult.

The Nerves and Muscles of the Leg

©
(Se)
CO

In the embryo as in the adult a special nerve is given to each portion
of the muscle. The nerve to the more proximal part arises from a more
distal set of spinal nerves than that to the more distal part. It gives rise
to branches which enter between the bundles of the proximal portion of
the muscle about midway between the tendon of origin and transverse
tendinous inscription. The more distal nerve enters the distal portion of
the muscle by branches which have a similar distribution with respect to
that portion.

The semitendinosus is probably represented in urodeles in the (caudali)-
pubi-ischio-tibialis and in reptiles by a portion of the flexor tibialis internus.
In monotremes it arises with the semimembranosus from the tuber ischii,
is inserted into the tibia, and is supplied both by the obturator and sciatic
nerves. In the higher forms it is either single as in man, double as in
several insectivores, or has two heads of origin, one of which usually springs
from the tuber ischii, the other from the caudal vertebre. This last, accord-
ing to W. Leche, is probably the most primitive condition. The tendinous
inscription of the semitendinosus marks the region where the two heads join
the common belly in this type of muscle. Humphrey believed the tendinous
inscription to mark the place where in the lower vertebrates the caudo-
crural joins somewhat perpendicularly the flexor musculature of the thigh.
In most of the lower mammals and in all the apes the tendon of insertion
sends fibrous expansions far down in the crural fascia and together with
similar expansions from the biceps and gracilis helps to form a sheath for
the tendon of Achilles (Parsons, 04).

In man the semitendinosus and long head of the biceps sometimes arise
independently from the ischium, a variation which is supposed by Le Double,
97, to be a reversion to a primitive condition in which the two muscles were
quite independent. Klaatsch, 02, on the other hand, states that in the
lowest mammals the muscles are more closely united than in the higher. In
the human embryo the two muscles are closely united from their earliest
differentiation and the union extends relatively more distal than in the adult.
The semitendinosus may be more or less fused in the adult with the semi-
membranosus or connected by fasciculi with the long head of the biceps.

The semitendinosus in the embryo extends more distally in the crus than
is normal in the adult. The fascial extension of the tendon in the adult is,
however, frequently well marked and may be muscular (Gruber, 86).
Proximally the semitendinosus in man may be reinforced by fasciculi from
the pelvis or coccyx. These fasciculi may join the muscle at its tendinous
inscription (Le Double). In the normal development I have found nothing
that seems to represent a “latent” caudal head of the muscle. It is note-
worthy that the proximal segment of the semitendinosus is innervated by
a more distal set of spinal nerves than the distal segment (see above). The
proximal end of the biceps is likewise innervated by a more distal set of
spinal nerves than the distal end of that muscle. The proximal ends of
these two muscles may therefore represent a caudo-femoral anlage shifted
distally into the thigh.

Charles R. Bardeen 339

Biceps, caput longum.—The long head of the biceps is differentiated
from a special anlage which, near the ischial tuberosity, is closely fused
with. that of the semitendinosus. This anlage is well marked in
an embryo of 14 mm. (Plate VIII, Fig. 1) and the muscle is differen-
tiated in one of 20 mm. (Plate VIII, Fig. 3). To the anlage in the -14
mm. embryo two nerves are given each of which is associated at its origin
with corresponding nerves to the semitendinosus. In the 20 mm. embryo
two nerves are likewise given to the muscle, but in this instance the
nerves arise nearly in conjunction with one another from the tibial
portion of the sciatic nerve.

In the adult two nerves are commonly distributed to the muscle. One
of these enters the proximal portion of the muscle, the other in the distal
third. ‘The terminal branches of these nerves are distributed across the
muscle bundles of the biceps about midway between their tendons of
origin and insertion, but nearer the proximal than the distal tendon. The
more distal nerve sends back recurrent branches across the muscle bundles
when the more proximal nerve is absent or ill developed.

Biceps, caput breve. See p. 332.

The long head of the biceps or lateral crural flexor is probably repre-
sented in the urodeles by the ischio-flexorius and in reptiles by the flexor
tibialis externus. In the mammals it usually arises from the tuber ischii and
is inserted into the tibia or into the fascia of the leg, often as far as the
foot. In marsupials it arises from the tuber ischii and the caudal vertebre.
In the carnivora and some of the other mammals it has occasionally a double
origin. As in the case of the semitendinosus the caudal origin of this muscle
is looked upon, however, by many investigators as a caudo-femoral muscle in-
‘serted into the lateral flexor rather than as a true head of the muscle. Ac-
cording to Testut the long head of the biceps represents a muscle which
primitively arose from the ilium and the coccyx. The sacrotuberosal liga-
ment represents a transformation of that portion of the muscle which origi-
nally extended between the ilio-sacro-caudal region and the present origin of
the muscle from the tuber so that the ligament may be looked upon as the
tendon of insertion of the muscle. In the human embryo the ligament de-
velops after the anlages of the ischial tuberosity and the long head of the
biceps: have appeared. It apparently is differentiated from the tuber ischii
toward the ilium, sacrum, and coccyx. In the human adult fasciculi from
the coccyx, sacrum, or sacrotuberosal ligament to the head of the biceps are
frequent.

The distal insertion of the muscle in most of the lower mammals takes
place further down the leg than in man. In most of the lower mammals,
according to Parsons, 04, aS mentioned above, extensions from the tendons
of the semitendinosus, gracilis, and biceps into the crural fascia serve to
form a sheath for the tendon of Achilles. According to A. Forster, 03, in
foetuses the insertion of the biceps takes place into the sural fascia and even

23

The Nerves and Muscles of the Leg

340

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342 The Nerves and Muscles of the Leg

in young children the attachment to the head of the fibula is weak. In
embryos of the third month the tendon of insertion of the biceps can be
followed for some distance down the fibular side of the leg but there seems
to be some attachment to the fibula.

b. Nerve Supply.

1. Relations of the Nerves of the Muscles of the Hamstring Group to the
Spinal Nerves.

In the adult it is difficult to trace back with certainty to the spinal
nerves the nerves distributed to these muscles. In general the special
dissections which I have made have revealed conditions which correspond
well with the data given by G. D. Thane in Quain’s Anatomy, Vol. III,
Part II, p. 331, which in turn are based on data derived from Pater-
son and Hisler. According to the description there given the nerve to
the adductor magnus arises from the 4th and 5th lumbar nerves, that of
the semimembranosus from the 4th and 5th lumbar and 1st sacral nerves.
The two nerves of the semitendinosus arise from the 5th lumbar and Ist
and 2d sacral nerves. I have found the inferior nerve arising from the
4th and 5th lumbar and 1st sacral, the superior from the 1st and 2d
sacral nerves. The nerves for the long head of the biceps arise from the
1st, 2d, and 3d sacral nerves ; that of the short head of the biceps from the
5th lumbar and Ist, or Ist and 2d, sacral nerves. In Text Fig. 6 the re-
lation of these nerves to the sciatic nerve is diagrammatically shown. A
study of Plate III, Fig. 3, and Plate VIII, Figs. 1 and 3, will show that
a distribution of spinal root fibres corresponding with this scheme would
follow from the more direct paths to the muscle anlages open to fibres
growing out from the sacral spinal nerves when the muscle anlages first
appear.

2. Relation of the Nerves of the Hamstring Muscles to the Sciatic Nerve.

To the semitendinosus and to the long head of the biceps, as a rule,
two separate nerves are given, one going to the proximal the other to
the middle or distal third of each muscle. Occasionally each of these
nerves may be doubled and not infrequently the nerves to each muscle
are combined for a part of their course in a common trunk. For the
semimembranosus and adductor magnus as a rule a single branch springs ”
from the sciatic nerve. This branch soon divides into separate branches
for each muscle. The various nerves mentioned spring at varying heights
from the sciatic nerve and are variously combined in the branches which
spring from this nerve. In Table XXV the relative origins of the

Oo

Charles R. Bardeen Su!

branches of the sciatic nerve are tabulated and there is shown the fre-
quency with which the different combinations occurred in 34 plexuses.
From this table it may be seen that the nerve to the proximal segment
of the semitendinosus muscle is most frequently the first of these branches
to arise. Very frequently this branch is associated with one or both of
the branches distributed to the long head of the biceps. Often the latter
branches are the first to arise from the sciatic nerve. Rarely the branch
to the distal segment of the semitendinosus arises in common with that
to the proximal segment. When the most proximal branch given off is
that to the proximal segment of the semitendinosus the next branch is
usually that to the long head of the biceps. The more distal of the
branches to the long head of the biceps may arise low down from the
sciatic nerve. ‘The nerve to the distal segment of the semitendinosus
arises about on the level and often in common with the nerve to the
adductor magnus and semimembranosus.

VIII. PERONEAL MUSCLES.
a. Embryonic Development.

During the sixth week the anlage of the peroneal muscles becomes sep-
arated from that. of the long extensors of the toes and the tibialis an-
terior (Plate IX, Fig. 1). Between the two anlages runs the n. peroneus
superficialis. The anlage of each peroneal muscle begins at the same
time to become distinct. Schomburg, oo, has described a connection in
early embryonic development between the peroneus brevis and the ex-
tensor digitorum brevis. In the embryos of corresponding stages which
I have examined the two muscles are distinctly separated as shown in
Plate IX, Fig. 1.

The m. peroneus longus occupies the more proximal and superficial
position. It lies dorso-lateral to the upper end of the fibula. Its proxi-
mal extremity is some distance from the tibia. The distal extremity is
continued into a tendon which can be followed to the neighborhood of the
base of the fifth metatarsal where it is lost in tissue not yet distinctly
differentiated.

In somewhat older embryos the tendon of the muscle may be followed
as it develops across the sole of the foot toward the base of the first
metatarsal. In an embryo 20 mm. long the tendon is intimately fused
with the scleroblastema of the foot and can be distinctly followed only
partially across the sole. In an embryo 30 mm. long the tendon can be
followed to the first metatarsal, but it is considerably later than this when
the tendon becomes free in its sheath. In an embryo 14 mm. long,

344 The Nerves and Muscles of the Leg

Plate IX, Fig. 1, the tendon passes lateral to the anlage of the lateral mal-
leolus. In one of 20 mm. it passes behind this anlage (Plate IX, Fig. 2).

Proximally in an embryo 20 mm. long (Plate IX, Fig. 2) the origin
of the muscle extends to the lateral condyle of the tibia next to the
attachment of the m. flexor digitorum longus. At this period the two
heads characteristic of the adult muscle may be distinguished.

A nerve may be seen entering the anlage of the peroneus longus in
embryo CXLIV (Plate IX, Fig. 1) and in embryo XXII (Plate IX,
Fig. 2) two nerves to the muscle may be seen. One of these enters the
deep surface of the anterior head, the other passes distally into the pos-
terior head.

From the peroneal nerve as it passes beneath the muscle two branches
usually arise in the adult. One of these passes to the central third of the
anterior portion of the muscle, the other extends down across the middle
third of the deeper muscle bundles which run obliquely from the fibula
to the tendon of the muscle. The latter branch may arise from the n.
peroneus superficialis and it may extend to supply the m. peroneus brevis.

The M. peroneus brevis, (Plate IX, Figs. 1 and 2) arises proximally
under cover of the peroneus longus and relatively higher up on the fibula
than in the adult. It lies a httle more on the flexor side of the leg than
the peroneus longus. When first developed the tendon of insertion of the
muscle is closely associated distally with that of the m. peroneus longus.
It hes somewhat near the m. extensor digitorum brevis, but, as mentioned
above, I can find no such intimate union with this muscle as that which
Schomburg describes as lasting till the third month of development.
It is attached to the base of the fifth matacarpal in an embryo 20 mm.
long (Plate IX, Fig. 2).

In an embryo 14 mm. long (Plate IX, Fig. 1) a branch of the peroneal
may be seen entering the muscle. In one 20 mm. long (Plate IX, Fig.
2) this branch may be readily followed beneath the peroneus longus to
the peroneus brevis which here occupies a more distal position than in
the 14 mm. embryo. In the adult the nerve may arise either from the
distal nerve to the peroneus longus or from the n. peroneus superficialis.
The nerve enters the proximal margin of the muscle and extends distally
about midway between the origin and insertion of the constituent fibre
bundles.

In amphibians the femoro-fibularis, which extends from the lateral epicon-
dyle of the femur to the fibula probably represents the peroneal musculature
of the higher vertebrates. In the reptiles two peroneal muscles are recog:

Charles R. Bardeen 345

nized, the peroneus anterior and peroneus posterior (Gadow, 82). The m.
peroneus anterior extends in most forms from the proximal extremity of the
fibula to the base of the fifth metatarsal. The peroneus posterior in croco-
diles is more or less fused with the gastrocnemius and extends from the
extensor musculature of the thigh to the calcaneus. In Hatteria and many
saurians it is more or less fused with the peroneus anterior and extends
from the lateral condyle of the femur to the outer side of the fifth metatarsal
(Gadow, 82). In the mammals the peroneal group consists in most forms
of three muscles, a peroneus longus, peroneus brevis, and peroneus
extensorius.

The peroneus longus may be inserted into the base of the fifth metatarsal
or into various structures in the sole of the foot, as far as the base of the
first metatarsal. In Ornithorhynchus the tendon of this muscle may be fol-
lowed to this last insertion.

The peroneus brevis and peroneus extensorius in Ornithorhynchus consti-
tute a muscle, one part of which sends tendons to the extensor surface of
the terminal phalanges of the first four toes, the other to that of the fifth
toe. In marsupials the peroneus brevis is distinct from the peroneus ex-
tensorius. The latter arises from the lateral condyle of the femur and from
the fibula and sends tendons to the second and fifth toes. In rodents the
peroneus extensorius sends tendons to the fourth and fifth toes. In carniv-
ora it sends a tendon to the fifth toe. In some apes the peroneus exten-
sorius is differentiated and sends a tendon to the fifth toe. In others it is
not isolated from the peroneus brevis. In man a peroneus extensorius
(peroneus quartus of Le Double) is not infrequently found as a variation
under most diverse forms. Most frequently the tendon only is isolated and
is inserted into the fifth metatarsal, cuboid, calcaneus, etc. The tendon of
the peroneus brevis frequently sends expansions to the tendon of the fifth toe,
that of the fourth toe, the metatarsal of the fourth toe, ete. In normal
embryonic development, however, the peroneal musculature does not seem
to become connected with the extensor tendon plate.

b. Nerve Distribution.

The nerves to the peroneal muscles probably arise from the more distal
spinal nerves which go to form the peroneal nerve, but this cannot be
satisfactorily determined by dissection.

The nerves to the peroneal muscles (brevis and longus) may arise from
the main trunk of the n. peroneus, from the n. peroneus superficialis, or
from both. In 15 out of 20 instances a single branch passed from the
n. peroneus superficialis to the peroneus brevis, in one instance two such
nerves were given, in four instances the nerve arose from the more distal
branch to the peroneus longus.

In three instances out of 20 a single nerve branch ran from the n.
peroneus to the peroneus longus muscle; in 8 instances two such branches.
In four of these cases the second branch sent a nerve of supply to the

346 The Nerves and Muscles of the Leg

peroneus brevis. In four instances the nerves of supply of the peroneus
longus arose from the n. peroneus superficialis (by one branch in one
instance). In five instances a proximal branch (in one instance, two)
arose from the n. peroneus and a more distal branch from the n. peroneus
superficialis.

In some of the instances above cited the nerves of supply subdivided
before entering the muscle.

IX. MUSCULATURE OF THE EXTENSOR SIDE OF THE CRUS AND FOOT.
a. Embryonic Development.
1. General Features.

Tn an embryo of 11 mm. (Plate III, Fig. 2) the peroneal nerve ex-
tends over the dorsal surface of the limb-bud and ends in a mass of
shghtly differentiated myogenous tissue, the anlage of the extensor
muscles of the leg and foot. This anlage is more or less fused with the
anlage of the peroneal muscles.

In an embryo 14 mm. long (Plate IX, Fig. 1) the peroneal nerve has
given rise to the nn. peroneus superficialis and profundus. The n. per-
oneus profundus may be traced to the region between the bases of the
first two metatarsals. Above and on each side of it may be distinguished
muscle anlages representing the extensor muscles of the leg and foot.
To these muscle anlages nerves are given as shown in the figure. The
tendons of the extensor digitorum and extensor hallucis proprius are rep-
resented by a sheet of tissue in which the segmentation is just beginning.
The conditions here described correspond well with those pictured by
Schomburg, 00, except in a few minor details to which attention is called
in considering the development of the individual muscles.

In an embryo of 20 mm. (Plate IX, Fig. 2) the individual muscles
and their tendons, as well as the nerves distributed to them, indicate
clearly relations corresponding in many features with those characteristic
of the adult.

2. Individual Muscles.

Tibialis anterior—The anlage of this muscle becomes distinct from
the general dorsal myogenous sheet of the limb-bud during the sixth
week. In an embryo of 14 mm. (Plate IX, Fig. 1) the muscle anlage
is most distinctly differentiated in the region where the two nerves are
extending into it. From here it may be followed distally into a broad
tendon which fades out over the region of the first cuneiform and the
base of the first metatarsal. In an embryo of 20 mm. (Plate IX, Fig. 2)

Charles R. Bardeen 347

the muscle has made tendinous attachments which correspond with those
of the adult and the chief nerve branches have extended for a considerable
distance into the muscle.

In the adult muscle as a rule several small branches extend into the
upper extremity of the muscle and one or two large branches enter the
middle third of the muscle. Within the muscle these branches run in
intramuscular septa and are distributed chiefly across the middle third
of the component muscle bundles. These run obliquely from their origin
from the tibia and surrounding aponeurotic sheets to the tendon of inser-
tion which arises high in the muscle.

The anterior tibial muscle is probably represented in the urodeles by the
femoro-tibial muscle which extends from the lateral epicondyle of the femur
to the tibia and os tarsale tibiale and is more or less fused with the femoro-
digital or long extensor muscle. In reptiles and mammals the anterior tibial
is fairly constant in general relations. It arises in most mammals from
the proximal end of the tibia and is inserted into the lateral side of the
tarsus or into the first metatarsal. In many mammals, including monkeys
and apes, the muscle is partially divided into two portions from one of which
a tendon goes to the metatarsal of the big toe (abductor hallucis longus),
the other to the first cuneiform. This division may affect merely the tendon
of insertion or extend into the belly of the muscle. In man there is not in-
frequently (25% of bodies, Le Double) a similar division of the terminal
tendon but this rarely extends to the belly of the muscle. Schomburg, oo,
describes a distinct division of the anlage of the tibialis anterior in the
embryo into two parts, that toward the tibial side representing an abductor
hallucis longus. This division does not appear in the embryos I have
examined.

Extensor digitorum longus.—From the central portion of the dorsal
myogenous sheet the extensor digitorum longus and the extensor hailucis
longus are differentiated simultaneously (Plate IX, Fig. 1). The ex-
tensor digitorum occupies a position relatively more fibularwards
than in the adult. It is broad and ends below in a broad flattened process,
or tendon plate, at the center of the dorsum of the foot. There is no
very distinct division into special tendons. Two nerve branches extend
to the deep surface of the muscle where this overlies the n. peroneus
profundus. Schomburg, oo, has described conditions in a six weeks
embryo which do not differ very essentially from those here given.

In an embryo of 20 mm. (Plate IX, Fig. 2) we find that tendinous
attachments have extended to the digits from the tendon plate and that
proximally the muscle has extended more toward the tibia. The nerve
supply corresponds with that of the adult.

In the adult as a rule two chief nerve branches arise from the n.

348 The Nerves and Muscles of the Leg

peroneus profundus. One of these runs to the muscle near its upper
extremity and passes distally across the central third of the obliquely
placed fibre bundles of the proximal portion of the muscle. The other
branch leaves the n. peroneus profundus more distally, extends to the
middle or lower third of the muscle and then distally across the middle
third of the obliquely placed fibre bundles of the lower portion of the
muscle and across the corresponding fibre bundles of the m. peroneus
tertius. 'The two nerves may be bound up in one trunk or their place
may be taken by a considerable number of branches, but in 9 cases out
of 10 essentially the arrangement described may be found.

The extensor digitorum longus and extensor hallucis longus are represented
in urodeles by the femoro-fibule-digiti I-V which extends from the lateral
epicondyle of the femur and from the fibula to the foot and thence by means
of tendinous processes to the phalanges (Hoffmann). In reptiles the two
muscles are probably also represented by the extensor digitorum longus
which in most reptiles arises from the lateral epi-condyle of the femur and
is inserted by tendinous slips into the bases of some of the metatarsals. In
chelonians it is inserted by tendons into the phalanges (Gadow, 82). In
the mammals the extensor digitorum and extensor hallucis are distinct in
most forms. The extensor digitorum arises chiefly from the proximal part
of the tibia and is united to the back of the toes by tendinous process which
vary considerably in different forms (Ruge, 78). In man doubling of the
digital tendons and aberrant tendon slips are very frequent (Le Double).
Early in embryonic development, as we have seen above. the tendons are
represented by a tendon plate. In the adult the tendons may be connected
by an uninterrupted aponeurotic lamella or by tendinous slips, conditions
normal in many of the lower mammals. Occasionally in man slips from the
tendons of the long digital extensor pass to the first, fourth, or fifth meta-
tarsals (Testut). This corresponds to the attachment of the extensor ten-
dons to the metacarpals found in reptiles. In the human embryo the
extensor tendon plate is at first connected with the metatarsal scleroblastema
but is gradually separated from this as development proceeds.

M. peroneus tertius.—Schomburg, oo, finds this muscle distinct from
the extensor digitorum pedis longus even in the sixth week. In the two
embryos in which I have made the most careful study of these muscles
(144, length 14 mm., Plate IX, Fig. 1; and 22, length 20. mm., Plate
IX, Fig. 2) I have been unable to find a sharp distinction between the
two muscles, although the tendon of the peroneus tertius in embryo 22
is quite distinct from that of the extensor digitorum longus. Schomburg
finds the tendon of the m. peroneus tertius runs at first toward the third

“It is possible that the m. extensor hallucis proprius of reptiles is homo-
logous with the extensor hallucis longus of mammals. It seems more likely
that it should be classed with the dorsal pedal muscles.

Charles R. Bardeen 349

metatarsal, but I have found no such condition in the embryos studied.
The tendon when differentiated runs toward the fifth digit. The nerve
supply of this muscle, described above in connection with the extensor
digitorum longus, serves to support the contention of Gegenbaur that
the m. peroneus tertius is but a differentiated portion of the extensor
digitorum pedis longus. It varies greatly in size and is frequently fused
with the m. extensor digitorum longus. Its tendon may terminate on
the fourth metatarsal. Rarely a tendon shp is given to the extensor
tendon of the little toe (Le Double).

M. extensor hallucis longus. Even at an early stage this muscle may
be distinguished from that of the m. extensor digitorum longus as Schom-
burg, 00, has pointed out. Its tendon at first is fused with the tendon
plate of the extensor digitorum longus (Plate IX, Fig. 1), but soon
begins to acquire some independence (Plate IX, Fig. 2).

The nerve of supply in embryos CXLIV and XXII enters the muscle
near the center of its oblique tibial border. As a rule in the adult the
nerve approaches the tibial border and passes distally across the oblique
muscle bundles midway between their origin and insertion. This single
trunk may, however, be replaced by two or more branches arising inde-
pendently from the n. peroneus profundus.

Variations in the muscle are most frequently found with respect to
its tendon of insertion. The tendon may divide into two or more parts.
In one instance it has been found sending a slip to the second toe. The
body of the muscle may be more or less fused with that of the m. extensor
digitorum longus.

The extensor hallucis longus is to be looked upon as an especially differ-
entiated deep portion of the extensor digitorum longus. Occasionally in
man there is found arising from the fibula a special long extensor of the
second toe (Gruber, 75). This is homologous with the extensor indicis
proprius of the forearm. Chudzinski, 74, has described a deep extensor
sending tendons to the first metatarsal, to the second and third, and to the
fourth and fifth toes, an arrangement corresponding somewhat to one normal
in several mammals (marmot, porcupine, beaver, Le Double). Both the
extensor hallucis longus and the extensor digitorum longus are connected
with the dorsal tendon plate in the embryo at an early stage. Normally a
tendon for the first toe develops from the deep surface of this plate in con-
nection with the extensor hallucis longus muscle, but the variations found
in the adult show that the primitive tendon plate may be variously subdivided
during embryonic development. The tendon of the extensor hallucis may
send a tendon to the first metatarsal or to the second toe, ete. (W. Gruber,
75)-

Mm. extensor digitorum brevis—This muscle becomes differentiated
beneath the extensor tendon plate and is best developed on the fibular side

390 The Nerves and Muscles of the Leg

of the dorsum of the foot. At the end of the sixth week, as pointed out
by Schomburg, the muscle mass is not differentiated into special parts
(Plate IX, Fig. 1), but toward the end of the second month the bellies
of which it is composed and their tendons begin to stand out distinctly
(Plate IX, Fig. 2). The differentiation of the terminal tendons begins
on the fibular side and extends toward the tibial. The nerve to this
muscle mass arises at an early stage from the n. peroneus profundus and
extends across its deep surface. In the adult this nerve extends across
the component muscle bundles about midway between their tendons of
origin and insertion.

_ Extreme variability is shown in the form of this muscle in the adult.

It may be absent or be reduced to two or three bundles or there may be
an unusual development of the muscle and the differentiation into bellies
corresponding to the digital tendons. The m. eat. hallucts brevis is the
most frequently isolated of these bellies.

The extensor digitorum brevis doubtless represents the remains of an
intrinsic dorsal pedal musculature relatively better developed in urodeles
and reptiles than in most mammals. In most urodeles (Ribbing, 06) and
reptiles (Gadow, 82) the extensor tendons of the toes arise from these pedal
muscles and the “extensor digitorum longus” tendons are inserted into the
bases of certain of the metatarsals. The great variation in the development

of the extensor digitorum brevis in man is well known. It seems to be rela-
tively better developed in the embryo than in the adult.

b. Nerve Distribution.

The relations of the nerves supplying the muscles under consideration
to the spinal nerves cannot be clearly made out by dissection. It is prob-
able, however, that the nerves suppled to the more tibially situated
muscles contain the greater number of the fibres springing from the 4th
lumbar nerve, and the nerves passing to muscles situated most to the
fibular side contain the greater number of fibres from the 1st sacral
nerve.

Variation in the Branches of Distribution Arising from the N.
Peroneus Profundus.

The nerve to the extensor digitorum brevis seems to be constant in
its general relations, although the height at which it springs from the
main trunk varies greatly.

The nerves to the remaining muscles show considerable variation
owing to the fact that the nerves to a given muscle may arise as suc-
cessive branches from the main nerve trunk or they may be combined

Charles R. Bardeen B51

into a single nerve of distribution which has a proximal origin-and as
it passes distally gives off successive branches which pass to the middle
third of the obliquely placed fibre bundles comprising the muscle. Usually
the nerve to the peroneus tertius arises in common with the nerve that
is distributed to the more distal portion of the m. extensor digitorum
longus. The following table, XX VI, shows the number of branches which
passed in 20 instances from the main trunk to each of the muscles under
consideration. Often a branch subdivides before entering the muscle. In
the part treating of the individual muscles the most frequent form of
nerve distribution for each muscle is described.

TABLE XXVI.
Tibialis anterior.

No. of
instances.
ISDE ALOLpLroxamealy portion: a br atocenter Of Muscles. a.eccaecietes a
2 OES. .to proximal portion. li br. to) center of muscles... .aaseeee 6
iD EamcORDLOxXTmall pOLtIOnN= 172, DES-sco) Center ot musclerei..-..aaece ne 3
2 DES) to) proximal portion. 2° brs. to center of muscle.....2.2.-.-.- 4
20
The branches to the proximal portion are closely associated

with an articular branch to the knee and in almost all instances

arise from the peroneal nerve trunk before the n. peroneus pro-

fundus separates from the n. peroneus superficialis.

Extensor digitorum longus (e. d. 1.) and peroneus tertius (p. t.).

1 br. to proximal portion e. d. 1. 1 br. to central area e. d. 1. and

Depa espero she avae tees < lenrcwey on onave tevscucuche ei cyavelsreh Gre oo Otebis: ais so erelic sherlon eimai te are Bhaterse 8
HMO Teele @ eusCl Sele tare se Wen egiwa yous poi eictevcievahosnttte caus. Gie sols tarieauasehehcaval voc elena Malsfiersvoneke 5
1 br. to proximal] portion of the e. d. 1. 1 br. to central area e. d. 1.

LE [OWE Rio). Oat a ese ler Gtr are AON SEN Aa eas tee 5
DT etOMpPLrO xd ale pO OMe Gales raisers elararie ciate cise sncvevereligoearel sot ave il
1 br. to proximal portion e. d. 1. 1 br. to central area e. d. 1., to

EXTON SOLM MAUI CIS ATE Op satin, ter ericnetier cseicoh tote ial oveey or cifsi oviete tel eicriewecclerle colti one aft

20

Extensor hallucis.
BLP ED TseT AG Ma Saeey aie steel oh sotay choee, aeaiechevtcl ste chtohen see ake vert oiolic alos al ote ales etre@\ eis''s: snes Peouaboys 11
MMT REL CIOS tesiere ir stay ern ro oko loysiceen ee teteiaie le teranes sleteie rouciel ovo) oto meole ladle dre cste sual siatala 4
MOT ATCC Sime pepcric eecce en ll cc ueaehey eotaeanenete kaka enertc rote onos cialis retlep deel seepal aude, stlanaietusis 4
Branch arose: wath) distal branch! to ext; dic Jomeusi..2..:..2......6. i
20

*1TIn one instance two separate branches to p. t.; in one, one branch to p. t.;
in three instances p. t. not present.

(SW)
Or
ww

The Nerves and Muscles of the Leg

X. MUSCULATURE OF THE PLANTAR SIDE OF THE CRUS AND FOOT.
a. General Features.

In an embryo 11 mm. long (Plate ITI, Fig. 1) the tibial nerve divides
below the knee inte two branches. Of these that on the tibial side rep-
resents the medial plantar, that on the fibular side the lateral plantar
nerve. The lateral plantar branch descends to the tarsus, the medial
plantar nerve not so far distally. Near the knee a mass of slightly
differentiated tissue lying superficial to the nerve represents the
gastrocnemius-soleus group of muscles. Beneath the nerves beyond this
region a mass of slightly differentiated tissue represents probably the
anlage of the deep muscles of the calf and possibly of some of the muscu-
lature of the sole of the foot.

In an embryo 14 mm. long (Plate IX, Figs. 3 and 4) the muscles of
the plantar side of the leg are so far differentiated that the individual
muscles can be fairly clearly made out. In the drawing for the sake
of definiteness the outlines of these muscles are made diagrammatically
sharp but the main relations shown are true to the conditions found in
the embryo. Two groups of muscles may be distinguished, a superficial
lateral group composed of the gastrocnemius, soleus, and plantaris; and
a deep more medially placed group consisting of the flexor hallucis
longus, flexor digitorum longus, the popliteus, and the tibialis posterior.
The gastrocnemius group is connected by a mass of tissue with the
blastema of the calcaneus. The two long flexor muscles are connected
by condensed tissue with a flat aponeurotic “ foot-plate” from which
tendinous processes extend to the blastema of the metatarsals and toes.
The medial and lateral plantar nerves extend independently from the
region of the knee to the foot. Near where they arise there is a plexi-
form arrangement of the fibres of the tibial nerve and from the back of
this plexus arise the nerves to the deep muscles of the back of the leg
and to the deep surface of the soleus muscle. The nerves to the gastroc-
nemius-soleus group, with the exception just mentioned, arise from the
plantar surface of the tibial nerve proximal to where this changes its
course from the thigh into the leg. In the foot the medial plantar nerve
spreads out superficial to the pedal aponeurosis while the lateral plantar
nerve crosses medially beneath it. Along the course of the medial plantar

“In the article by Bardeen and Lewis, o1, the two divisions of the tibial
nerve are represented combined into a single trunk too far distally.

Charles R. Bardeen BoD

nerve in the foot a mass of slightly differentiated tissue represents prob-
ably the anlage of the musculature subsequently innervated by this
nerve.

The general relations of the plantar nerves at this period are strikingly
similar in many ways to the plantar nerves in the crus of the lower mammals
as recently pictured by McMurrich in this journal (04) and offer analogies
with types there pictured by him for the lower vertebrates. The chief differ-
ence between the nerves of the plantar side of the crus of mammals and that
of the reptiles and amphibia is, as McMurrich has pointed out, the path for
the fibers going to the medial side of the foot. In the mammals the nerve
fibers take a course superficial to the deep muscles of the crus; in the inferior
vertebrates they take a course in part beneath the deep muscles. In the
amphibia and reptiles the nerve fibers for the medial side of the foot are
more or less bound up with the nerves to the deepest muscles of the crus;
in the mammals they are more or less bound up with nerves to the more
superficial muscles. The nerve for the lateral side of the foot runs in most
forms between the superficial and the deep musculature of the crus.

In an embryo 20 mm. long (Plate IX, Figs. 5 and 6) the various
muscles of the plantar side of the leg are much more highly differentiated
than in the 14 mm. embryo. The soleus and gastrocnemius muscles have
begun to extend tibialwards over the tibial nerve. The tendon of
Achilles is well differentiated. The long flexor muscles are attached to
an aponeurosis from which tendons extend to the digits. The poplteus
muscle is clearly marked. The tibialis posterior is inserted into the side
of the skeleton of the foot near the base of the first digit. In the foot
the anlages of most of the intrinsic muscles can be distinguished but here
the muscles are but incompletely differentiated. A group of muscles
innervated by the lateral plantar nerve is to be distinguished from one
innervated by the medial plantar nerve.

The lateral and medial plantar nerves in this embryo are fused into
a common trunk as far as the ankle. The nerves to the gastrocnemius-
soleus group arise from the plantar surface of the tibial nerve in the
thigh. To the deep surface of the soleus, however, a branch is given
which arises from the deep surface of the tibial nerve in the leg. From
this surface arise the nerves for the deep muscles of this region. In the
foot the distribution of nerve branches to the muscles corresponds with
that found in the adult.

b. Embryonic Development and Variation in the Nerve Supply in the
Adult of Each of the Chief Groups of Muscles.

The development of the individual muscles of the back of the leg and

foot can best be followed by taking them up according to the groups

304 The Nerves and Muscles of the Leg

which develop from common anlages. We shall therefore first take up
the gastrocnemius-soleus group, then the deep musculature of the back
of the leg, then the musculature innervated by the lateral plantar, and
finally that innervated by the medial plantar nerve. The nerve supply
of the muscles of the back of the crus is taken up after treating the em-
bryonic development of the two sets of muscles in this region; the nerve
supply of the plantar musculature is taken up after considering the
differentiation ot the plantar muscles.

1. Development of the Gastrocnemius-soleus Group.

M. gastrocnemius.—As pointed out by Schomburg, oo, the lateral por-
tion of the flexor plate of the leg gives rise to the gastrocnemius and
soleus muscles. The anlage of the gastrocnemius is the more lateral and
superficial of the two muscles and shows two incompletely separated
heads (Plate IX, Fig. 3). These heads are connected by a fairly dense
tissue with the anlage of the calcaneus but do not extend across the tibial
nerve to the femur. During the latter half of the second month the heads
of the gastrocnemius develop rapidly. In an embryo of 20 mm. (Plate
IX, Fig. 5) the lateral head of the gastrocnemius has formed a tendinous
attachment above the lateral condyle of the femur while the medial head
has not yet quite reached its final destination. The nerves to the gastroc-
nemius enter each head of the muscle soon after the anlages appear. The
nerves may be readily distinguished in an embryo of 20 mm. (Plate IX,
Fig. 5).

In the adult the fibre bundles of each head of the gastrocnemius take
an oblique and nearly parallel, though somewhat diverging, course from
the tendons of origin to the tendon of insertion. The nerve to each head
enters about the middle third of the superior margin of the muscle and
its main branches take a course distally across the obliquely running fibre
bundles, a course corresponding to the course of the nerve in the embryo.

M. soleus—The anlage of this muscle is closely associated with that
of the gastrocnemius. It hes beneath and projects beyond the tibial
margin of the gastrocnemius (Plate IX, Fig. 4). It arises on the upper
end of the fibula and distally extends into an anlage of the tendon of
Achilles. At first it is as large as the gastrocnemius. During subsequent
development it extends over the posterior tibial nerve to be attached to
the tibia. This attachment is not completed in an embryo of 20 mm.
(Plate IX, Fig. 6). The nerves for the muscle arise at an early stage
as shown in Plate IX, Fig. 4. Their distribution in an embryo of 20
mm. is shown in Plate IX, Fig. 6.

Charles R. Bardeen

In the adult the superior nerve te the soleus enters the superficial sur-
face near the superior border and divides into two main branches, one
for the tibial and one for the fibular side. The inferior nerve to the
soleus divides, usually before it enters the muscle, into two branches, one
for the distal portion of the fibular, the other for the distal portion of
the tibial side of the muscle. From both nerves branches may usually
be followed both to the main body of the muscle and to the specialized
bi-pennate portion visible on its deep surface. .

M. plantaris—According to Schomburg, oo, the anlage of this muscle
arises proximal to the soleus and on the tibial side of the gastrocnemius.
In embryo CXLIV, length 14 mm., the muscle mass is not clearly differ-
entiated from the anlages of the soleus and gastrocnemius but what I

TABLE XXVII.

Amphibia. Lacertilia. Opossum. Higher Mammals.

Plantaris sup. med. Gastrocnemius med. | Gastrocnemius med.
{Gastrocnemius, cap.

int., Gadow.]

Plantaris sup. med.
[Ischio-flexorius,
Hotffmann.]

Plantaris sup. lat.
[Gastrocnemius, cap.
ext., Gadow.]

Gastrocnemius lat. | Gastrocnemius lat.

Plantaris sup. lat. Plantaris sup. access.

{Femoral head sup. [Flex. long. dig., cap. | Plantaris. Plantaris.
flexor.] fem., Gadow. |
Plantaris sup. tenuis. Popliteus.

{[Flex. long. dig., cap. (Sup. portion)

access., Gadow.]

Plantaris prof. ITI.
[Flex. subl. dig.,
Hoffmann. ]

Gastrocnemius lat. Soleus.

Plantaris prof. IT.
[Fem. fib. metatars.

Plantaris profundus
III-II

[Flex. long. dig., cap.

int., Gadow.]

(Soleus portion.)
Flexor fibularis.

Flexor fibularis.

I-III. Hoffmann.]

take to be the anlage of the plantaris is a small mass of tissue situated
anterior to the main soleus mass and partly covered by the gastrocnemius
(Plate IX, Fig. 4). Even in embryo XXII, length 20 mm., the muscle
cannot be made out distinctly. I have represented in Plate LX, Fig. 6,
what I take here to be the anlage of the plantaris muscle. It is closely
associated with the lateral head of the gastrocnemius. No traces of the
tendon were found in the early embryos I have studied, nor did Schom-
burg find any in the leg reconstructed by him.

Comparative anatomy of the gastrocnemius-soleus growp.—MecMurrich in
this journal has recently (04) given an important account of the compara-
tive anatomy of the crural flexors from the standpoint of muscle layers as

seen in cross-section. He tabulates the relationships of the gastrocnemius-
soleus group as shown in Table XXVII.

24

356 The Nerves and Muscles of the Leg

In the development of the human embryo it has been shown that two fairly
distinct chief myogenous regions are to be distinguished on the plantar side
of the crus and that one of these gives rise to the gastrocnemius-soleus group,
the other to the deeper muscles of the back of the leg. Both BHisler (1895)
and MecMurrich have performed a distinct service in again emphasizing that
in the vertebrate series a superficial plantar musculature of the crus is
to be distinguished from a deep plantar musculature. In many mammals, at:
least, including man, the two layers of musculature are separated by a fascial
septum which passes from the fibular to the tibial side of the leg and in which
run the main nerves and blood-vessels of the back of the crus. In the reptiles
also there appears to be a similar fairly distinct division between the super-
ficial and the deep plantar muscles of the crus. In them, however, tne
muscles called plantaris superficialis tenuis and plantaris superficialis accesso-
rius by McMurrich seem to be related proximally to the superficial muscuia-

TABLE XXVIII.

Urodela. Reptilia. | Marsupalia. Man.

: t | Crural Tendon of Fascial Insertions eee SEE SIS
Ischio Flexorius. | , = of Biceps, Semiten-
flex. tib. ext. of Flexors of Knee. 3 me
| dinosus and Gracilis.
Plantaris sup. med. Gastrocnemius med.} Gastrocnemius med.
Plantaris sup. |
minor, (Hisler.) | |
[Plant. prof. ITI,
minor, Mc.M.]
| Plantaris sup. ten- Plantaris. Plantaris.
J uis.
Plantaris sup. |
major, (Hisler).
{Plant. prof. III,
Mc. M.j |
|| Plantaris sup. lat. Gastrocnemius lat. Gastrocnemius lat.
Plant. sup. lat.
(Me. M.) J|| Plantaris sup. ac- Soleus (all but deep
cess. portion.)

ture while distally they are inserted into the deep musculature. The plan-
taris superficialis tenuis lies chiefly superficial to, the plantaris superficialis
accessorius, chiefly deeper than the nerve trunks which correspond with the
nn. plantaris medialis and lateralis of the mammals (rr. superficiales medialis
and lateralis of McMurrich). In the amphibia there seems to be a distinct
division between the deeper musculature and a superficial group of muscles
composed of the muscles called by McMurrich the plantaris superficialis
medialis, the plantaris superficialis lateralis and the plantaris profundus I1J1.
Comparing the conditions found during embryonic development of the human
crus with those present in the legs of the lower mammals and inferior verte-
brates I should prefer to rearrange McMurrich’s table as shown in Table
XEXCVTT,

It seems probable that the muscles into which the superficial musculature
of the plantar surface of the crus becomes divided are not perfectly homo-
logous in the amphibia, reptiles, and mammals, although there are some
obvious similarities.

Charles R. Bardeen 3a

In the mammals the homologies seem more certain. McMurrich considers
the medial head of the gastrocnemius to be a muscle primitively distinct
from the lateral head. He bases this conclusion on the fact that in many of
the lower mammals each head forms a distinct muscle. The ontogeny of the
muscle in man indicates that the two heads are derived from an anlage situ-
ated on the fibular side of the leg. The twisting of the tendon of Achilles
may be explained by the shifting which the muscle undergoes during onto-
geny. Embryological development in man supports the idea advocated by
McMurrich that the plantaris is a derivative of the deeper portion of the
lateral head of the gastrocnemius. When absent it is likely that this separa-
tion has failed to take place during ontogeny. In many mammals it is not
differentiated (several edentates, carnivora, etc.) ; in others, especially in some
rodents (Leche), it is highly developed. The soleus is considered by McMur-
rich to be derived from the profundus musculature of the crus. It seems to
me likely that the deep portion of the soleus, innervated by the distal nerve
to that muscle may be thus derived from the profundus musculature although
I have been able to distinguish no such special anlage in the development of
the muscle in man. In the monotremes the soleus is bound up with the
lateral head of the gastrocnemius. This arises from the epiphysial process
of the fibula. It forms a part of the lateral head of the gastrocnemius in
marsupials, in most edentates, in the chiroptera and galeopithecide, several
carnivora, ungulates, and prosimians (Leche). The great number of mam-
mals in which it is thus undifferentiated as a distinct muscle indicates
strongly that its phylogenetic as well as its ontogenetic origin is, in the main
at least, from an anlage common to it and the gastrocnemius. LHisler, g5,
regards it as derived from the gastrocnemius lateralis.

The variations in the muscles of the soleus-gastrocnemius group in man
seem to be essentially due to a greater or less separation of the original an-
lage into independent muscles. The fascial extension of the biceps, semi-
tendinosus and gracilis, which I take to represent the plantaris superficialis
medialis (McMurrich) of the amphibian crus, may be muscular instead of
tendinous and may be somewhat fused to the gastrocnemius.

2. Development of the Deep Muscles of the Back of the Crus.

a. M. Popliteus—In an embryo 14 mm. long (Plate IX, Fig. 4) I
have been unable to distinguish clearly a popliteus muscle. The anlage
of the muscle doubtless lies in the dense tissue posterior to the tibial
nerve and proximal to the anlage of the m. tibialis posterior. There is
a differentiation of tissue there which indicates this. This tissue is out-
lined in the drawing. In an embryo of 20 mm. (Plate IX, Fig. 6) the
muscle is well defined, has the skeletal relations characteristic of the adult
and at its distal border there enters a well marked nerve of supply.
Schomburg does not mention this muscle in his article.

In the adult the nerve usually enters the muscle near the center of its
distal edge. Often some of the branches of this nerve extend into the

358 The Nerves and Muscles of the Leg

posterior surface of the muscle. Rarely a slender second branch enters
the superior margin of the muscle (2 in 25 instances).

The place of the popliteus is taken in the lower mammals, the amphibia
and reptiles by an interosseous muscle, pronator tibie, which passes obliquely
from the fibula to the tibia. A popliteus muscle corresponding essentially
to that of man is found in nearly all mammals except the monotremes and
marsupials. A popliteus in addition to a pronator tibie is likewise described
for Myrmecobius (Leche). The popliteus is said to be absent in most chirop-
tera (Leche). In the dog in addition to the popliteus there is a small fibulo-
tibial muscle in the proximal part of the interosseous space. A similar muscle
(the peroneo-tibialis, Gruber) has been found in a number of mammals and
not infrequently as a variation in man (128 times out 860 instances, Gruber).
It seems probable that the popliteus is an especially differentiated portion of
the fibulo-tibial muscle of the lower vertebrates, and that its origin has ex-
tended from the fibula to the lateral condyle of the femur. LHisler, g5,
considers it homologous with the brachialis anterior of the arm. According
to McMurrich the muscle in the mouse receives two nerve branches, one asso-
ciated with that for the soleus from the “internal popliteal stem,” the other
from the deep muscle nerve of the crus. The former is supplied to the more
tibial oblique-fibered portion of the muscle, the latter to the more vertical
fibular portion. From these facts McMurrich concludes that the popliteus is
a compound muscle consisting of a portion derived from the “ plantaris super-
ficialis’”’ and a portion which represents a part of the pronator tibie of the
marsupials and the interosseous of the lower vertebrates. That it is there-
fore similar to the pronator teres of the arm. While this may be true
of the muscle in some of the mammals it does not seem to be true for the
muscle as it is found in man. A double innervation is infrequent in man.
During embryonic development the muscle appears to come from a single
anlage which lies deeper than the tibial nerve. Gordon Taylor and Victor
Bonney, o5, conclude that the popliteus is homologous with the deep por-
tion of the pronator teres while the superficial portion of the pronator teres
is homologous with the gastrocnemius. Occasionally in man a second head
may arise medially from above the lateral condyle. This may possibly be
equivalent to the superficial portion of the pronator teres. According to Le
Double the m. popliteus biceps coincides frequently with the absence of the
plantaris.

b. Deep cruro-pedal group. M. flexor hallucis longus.—The anlage
of this muscle is distinct from those of the other muscles of the calf in
an embryo of 14 mm. (Plate IX, Figs. 3 and 4). Lateral to the anlage
lies the caleaneus, the tendon of Achilles and the distal end of the soleus.
On the tibial side it slightly overlaps the anlage of the tibialis posterior.
Proximally it extends nearly to the head of the fibula. It lies beneath the
n. plantaris lateralis which in this embryo separates high up from the
n. plantaris medialis. Distally it terminates in an aponeurosis common

Charles R. Bardeen 359

to it and the m. flexor digitorum longus. The nerve enters the proximal
extremity of the muscle anlage.

In an embryo 20 mm. long (Plate IX, Figs. 5 and 6) the muscle occu-
pies a relatively somewhat more proximal position and is somewhat more
under cover of the soleus. It is attached to the blastema of the shaft
of the fibula and distally is inserted into the deep surface of the plantar
aponeurosis. The nerve runs along and enters the tibial margin of the
muscle.

In the adult the nerve or nerves to the muscle run along its tibial
margin or deep surface and send twigs into its substance.

M. flexor digitorum lonyus—This is differentiated from an anlage
medial to that of the m. flexor hallucis longus. In the 14 mm. embryo
(Plate IX, Fig. 3) it lies beneath the n. plantaris medialis which gives
two branches to the upper extremity of the anlage. The muscle extends
into a somewhat irregular plantar aponeurosis of which mention has
been made in connection with the m. flexor hallucis longus. The tendons
are partially differentiated. ‘The anlage of the muscle nearly covers that
of the m. tibialis posterior. Schomburg found in the leg he reconstructed
that the tibial side of the muscle had not reached the tibia. In embryo
CXLIV this is also true. The tibial attachment has begun to take place
in embryo XXII, length 20 mm. (Plate 1X, Fig. 5). In this embryo
also the pedal aponeurosis has become still further differentiated into
tendons, but it is not yet possible to distinguish clearly the tendons be-
longing to the fibular flexor (flexor hallucis longus) from those belong-
ing to the tibial flexor (flexor digitorum longus). ‘Two nerves enter the
muscle on its superficial surface. One of these extends to the fibular
side of the muscle, the other to the tibial side. A similar arrangement is
usually found in the adult.

M. tibialis posterior—This muscle is formed from the deeper layer of
the tibial portion of the flexor anlage near the lateral portion of the lower
half of the tibia (Plate IX, Fig. 4). Its tendon is differentiated early
and may be followed to the anlage of the navicular. In subsequent de-
velopment, as pointed out by Schomburg, it develops in a proximal and
lateral direction (Plate IX, Fig. 6). Its nerve enters near the tibial
border of the anlage. In the adult the nerve enters the posterior surface
of the muscle in its proximal third and gives off one or two branches for
the tibial fasciculus. The main trunk descends across the centers of the
fasciculi arising from the fibula.

Comparative anatomy of the deep plantar muscles of the crus.—Hisler,

95, and MecMurrich, 04, consider that the flexor fibularis (hallucis) is
derived from a layer primarily superficial to the layer from which the flexor

360 The Nerves and Muscles of the Leg

tibialis (digitorum) and tibialis posterior are derived. McMurrich* bases
this idea chiefly on the supposition that the flexor fibularis is supplied by the
equivalent of the ramus superficialis medialis, while the flexor tibialis is sup-
plied from the ramus profundus. In man, at least, the nerves passing to the
two muscles are very frequently bound up for some distance in a common
trunk. The flexor tibialis and tibialis posterior seem, however, to be more
intimately connected during ontogeny than is either of these muscles with
the flexor fibularis. In many mammals the tibialis posterior is absent
(Leche). In these it may be undifferentiated from the flexor tibialis. On
the other hand in several mammals the tibialis posterior is doubled, the
deeper portion sending a tendon to various structures in the tarsus, or even
to the base of the first phalanx of the big toe (Le Double, 97). The inti-
mate relations between the tibial and fibular flexors are revealed by the
fasciculi which so frequently have been found passing from one to the other
as well as by their tendons (see Le Double, g7).

The tibial and fibular flexors are inserted primarily into a deep plantar
aponeurosis in which tendons are developed in accordance with varied func-
tions of the foot and digits (Keith, 94). The arrangement of the tendons
varies greatly in different forms. In many forms the flexor tibialis is rudi-
mentary. In the chiroptera it is highly developed. For the variation of the
tendons in the anthropoids and man see Le Double, 97.

3. Nerve Supply of the Muscles of the Back of the Crus in the Adult.
a. Relation of Muscle Branches to the Spinal Nerves.

The difficulty of tracing these nerves back to their sources from the
spinal nerves is so great that no statistical study of the subject has been
attempted. It is evident, however, that the main bulk of the nerve fibres
distributed to the gastrocnemius-soleus group has in general a somewhat
more distal origin than those going to the deep muscles of the calf. The
special dissections which I have made serve in the main to support the
spinal nerve origins given in Quain’s Anatomy. These are as follows:
popliteus, 4th and 5th lumbar, 1st sacral; soleus, 5th lumbar, 1st and 2d
sacral; gastrocnemius, lst and 2d sacral; deep musculature of the calf,
5th lumbar, 1st and 2d sacral. The nerve to the plantaris is given as
arising from 4th and 5th lumbar and 1st sacral, but the 5th lumbar,
1st and 2d sacral nerves seem to be the more probable sources of supply.

* According to McMurrich the flexor tibialis and tibialis posterior of the
mammals are represented in the reptiles (Lacertilia) and amphibia (uro-
deles) by the plantaris profundus I (tibialis posterior of Gadow). The flexor
fibularis is according to this author derived from a portion of the plantaris
profundis III-II of reptiles (flexor longus digitorum, caput internum, Gadow)
and the plantaris profundus II of the urodeles (femoro-fibule-metatarsales
I-III, Hoffmann).

Charles R. Bardeen 361

b. Order of Origin from the Tibial Nerve.

The most proximal branches given off are those to the gastrocnemius,
the proximal branch to the soleus and the nerve to the plantaris muscle.
Out of 19 instances the nerve to the plantaris was the most proximal
branch in 9, the nerves to the gastrocnemius in 9, and in one instance
the nerve to the plantaris, in conjunction with the branch to the lateral
head of the gastrocnemius and the proximal nerve to the soleus. Usually
the nerve to the medial head of the gastrocnemius arises slightly proximal
to that to the lateral head. The latter arises near or in conjunction with
the proximal nerve to the soleus muscle.

Next distal to the nerves to the plantaris and gastrocnemius muscles
and the proximal nerve to the soleus arise the nerves to the popliteus and
posterior tibial muscles. These nerves often arise from a common branch.
When they arise separately the nerve to the poplitus is the more proximal
in the great majority of instances.

Next distal usually comes the distal nerve to the soleus, although this
nerve may arise proximal to the nerve to the tibialis posterior or in con-
junction with this. Then follow the nerves to the flexor digitorum longus
and to the flexor hallucis Jongus. The two latter frequently arise from
a common trunk which may also be combined with the distal nerve to the
soleus. The nerve to the flexor hallucis is almost always the most distal
in origin of the nerves under consideration, but occasionally a distal
branch to the flexor digitornm longus has a more distal origin (in two
instances out of 34).

c. Relation to One Another of the Nerves to the Muscles.

Nerve to plantaris.—In all but one out of 21 instances the nerve to the
plantaris muscle arose independently from the tibial nerve. In this in-
stance it arose in conjunction with the nerve to the lateral head of the
gastrocnemius and the proximal nerve to the soleus muscle. In one
instance two nerves could be traced to the plantaris.

Nerve to medial head of gastrocnemius.—In one instance out of 35 two
separate parallel branches passed into this head. Occasionally near its
origin from the tibial nerve the nerve to the medial head of the gastroc-
nemius is bound up for a short distance with that to the lateral head.

Nerve to the lateral head of the gastrocnemius.—Out of 35 instances
in 20 this nerve arose independently or in conjunction with that to the
medial head from the posterior tibial; in 14, in conjunction with the
proximal nerve to the soleus and in one in conjunction with the proximal
nerve to the soleus and the nerve to the plantaris.

362 The Nerves and Muscles of the Leg

Proximal branch to soleus—Out of 35 instances in 20 this branch
arose independently, in 14 it arose in conjunction with the nerve to the
lateral head of the gastrocnemius, and in one in conjunction with the
nerve to the lateral head of the gastrocnemius and the nerve to the
plantaris.

The above nerves form a group, the trunks of which may be more or
Jess united with one another, but not with those of the following set.

Nerve to the popliteus.—This nerve arose independently in 15 out of
26 instances. In 10 instances it arose in conjunction with the nerve to
the tibialis posterior and in one, with the distal nerve to the soleus and
the nerve to the tibialis posterior. In two instances a secondary branch
entered the superior edge of the muscle.

Halbertsma, 47, described a nerve which arises in the popliteal space,
gives rami to the popliteus and posterior tibial muscles, and is continued
distally, partly in the substance of the interosseous membrane, to the inferior
tibio-fibular articulation. It gives branches to the superior tibio-fibular
articulation, to the tibia and the interosseous membrane. When the nerves
to the popliteus and tibialis posterior arise separately this nerve is continued
distally either from nerve to the popliteus or, more rarely, from that to the
tibialis posterior. McMurrich, o4, considers this branch to represent the
important ramus profundus of amphibia and reptiles. This supplies the deep
muscles of the plantar surface of the crus and is continued into the foot as
the internal plantar nerve. In the lower mammals it ends at the ankle. In
man another nerve arises from the nerve to the deep muscles and passes

distally along the course of the peroneal vessels to the ankle. It gives
branches to the shaft of the fibula and the medullary artery.”

Nerve to the tibialis posterior—Out of 38 instances in 20 the nerve
arose independently, in 5 it arose in conjunction with the nerve to the
popliteus. In 5 instances it arose in two branches, one of which in each
instance was associated with the nerve to the popliteus while the other
in one instance was independent, in one was associated with the nerve’
to the flexor digitorum longus and in three with the distal nerve to the
soleus muscle. In 4 instances the nerve to the tibialis posterior was
associated with the distal branch to the soleus, in 3 with the nerves to
the flexor digitorum longus and flexor hallucis longus muscles, and in
one instance with the distal nerve to the soleus and with the nerve to
the flexor digitorum and flexor hallucis.

Distal nerve to the soleus—Out of 37 instances, in 20 this nerve arose
independently. In 7 it arose in conjunction with a nerve to the posterior
tibial muscle; in 4, in conjunction with one to the flexor digitorum longus

** Rauber, cited by G. D. Thane, Quain’s Anatomy, 10th ed.

Charles R. Bardeen 363

and flexor hallucis longus muscles; in 3, in conjunction with the nerve
to the flexor hallucis muscle; in one in conjunction with that to the flexor
digitorum ; in one, in conjunction with that to the popliteus and posterior
tibial muscles; and in one in conjunction with a nerve to the posterior
tibial, flexor digitorum and flexor hallucis muscles.

Nerve to the flexor digitorum longus. Out of 36 instances, in 20 the
nerve arose independently, in 6 of these by two separate branches; in
6 instances it arose in conjunction with the nerve to the flexor hallucis
muscle; in 4 others, in conjunction with this and the distal nerve to the
soleus. In three instances it arose in conjunction with the nerve to the
flexor hallucis and that to the tibialis posterior muscle; in one, in con-
junction with that to the tibialis posterior; in one, in conjunction with
the distal nerve to the soleus; and in one, with the nerve of the tibialis
posterior and flexor hallucis and the distal nerve to the soleus.

Nerve to the flexor hallucis muscle—Out of 85 instances, in 18 the
nerve arose independently, in two of these by two separate branches. In
six instances the nerve arose in conjunction with the nerve to the flexor
digitorum longus muscle; and in 4 other instances, in conjunction with
this and the distal nerve to the soleus. In three instances it arose in
conjunction with the distal nerve to the soleus; in three, in conjunction
with the nerve to the flexor digitorum and tibialis posterior; and in one,
in conjunction with the distal nerve to the soleus and the nerves to the
tibialis posterior and flexor digitorum longus.

4. Development and Innervation of the Muscles Supplied by the Lateral
Plantar Nerve.

To this group belong the quadratus planta, the abductor, flexor brevis,
and opponens digiti quinti, the interossei, and the three lateral lumbrical
muscles.

M. quadratus plante.—The anlage of this muscle appears in a 14 mm.
embryo (Plate IX, Fig. 4) medial to the lateral plantar nerve as this
curves about the tuber calcanei. Schomburg, oo, describes it in about
the same position, but fused with the flexor hallucis longus at a nearly
corresponding stage. In the 14 mm. embryo the nerve to the muscle is
not distinct but in a 20 mm. embryo (Plate IX, Fig. 6) a well marked
nerve enters its superficial surface from the deep surface of the lateral
plantar nerve as this crosses the muscle. The muscle can readily be
traced from the calcaneus to the deep surface of the plantar aponeurosis.

In the adult the nerve to the quadratus plente arises from the lateral
plantar nerve near the medial margin of the muscle and crosses on or

364 The Nerves and Muscles of the Leg

near the superficial surface of the muscle about midway between its origin
and insertion and parallel with the tendon of the flexor digiti quinti
longus. I have never seen the nerve for this muscle arise from the medial
plantar nerve as described in the anatomy of Poirier and Charpy.

In the adult this muscle is frequently reinforced by a fasciculus which
may arise from either of the bones of the crus, from one of the deeper
muscles of the crus, from the deep muscle fasciew, or from the calcaneus,
Le Double, 97. The muscle may be inserted into any of the digital ten-
dons, but most frequently into the 2d, 3d, and 4th; into that to the 5th
toe less frequently ; into that to the great toe rarely.

MeMurrich, 04, on phylogenetic grounds thinks that the quadratus plante
is differentiated from the distal end of the same deep layer of crural muscles
from which are derived the tibialis posterior and the flexor digitorum (tibial
flexor). Schomburg, on the other hand, considers it more intimately related
to the flexor fibularis, a point of view strengthened by the fusion which he
found between the quadratus plante and the flexor hallucis longus in a young
embryo. As mentioned above, I did not find this connection in the 14 mm.
embryo. Nor does the nerve supply of the muscle indicate a close union be-
tween it and the tibialis posterior or either the tibial or the fibular flexor.

The quadratus plante ‘is clearly represented in the lacertilia where it is
supplied by a branch of the ramus profundus.” (McMurrich, o4).

In monotremes it arises from the calcaneus (Leche). In the majority of
marsupials it is probable that it exists in a rudimentary condition (Mc-
Murrich). In edentates it is absent in some forms, well marked in others.
In some insectivora it is fused with the abductor metacarpi digiti minimi. In
the higher mammals it is absent in some forms and well developed in others
(i. e., dog and cat). In some apes it is fused with the flexor digitorum
tibialis (Leche).

M. abductor digiti quinti.—In the 14 mm. embryo (Plate IX, Fig. 4)
the anlage of this muscle may be seen immediately distal to the tuber
calcanei and lateral to the n. plantaris lateralis. In an embryo of a cor-
responding age Schomburg, oo, pictures the muscle as extending to the
4th metatarsal, but I have found no corresponding condition in the
embryos I have studied. In embryo XXII, length 20 mm. (Plate IX,
Fig. 6) the muscle extends to the base of the 5th metatarsal and has a
more lateral position than in the 14 mm. embryo. At this stage a nerve
may be seen extending into the medial margin of the muscle from the
deep surface of the lateral plantar nerve.

Jn the adult the muscle is developed medially so as partially to overlap
the lateral head of the m. quadratus plante. It varies greatly in struct-
ure. The main bulk of the fibre bundles usually extends somewhat
obliquely from the caleaneus, the plantar fascia and the tendinous
aponeurosis on the lateral side of the muscle near its origin to a tendon

so
for)
Or

Charles R. Bardeen

which extends high on the medial side of the deep surface of the muscle.
Fibre bundles may also run from the calcaneus to the tuberosity of the
fifth matatarsal and from this to the tendon of insertion. ‘The more
lateral and distal fibre bundles are those least frequently developed.

The nerve may be distributed either near the deep or near the super-
ficial surface of the muscle. The former appears to be the case when the
muscle is slightly developed. The chief muscle branches then extend
across the middle third of the constituent muscle bundles near the deep
surface. In case the caleaneo-metatarsal bundles are well developed a
special branch may be sent to these. When the muscle is well developed
the nerve enters the proximal margin of the muscle and its chief branches
extend across the middle third of the more superficial muscle bundles
finally terminating in those most distal bundles which lie on the lateral
side of the fifth metatarsal. Other modes of distribution are also found
but they agree in general features with those described.

Flexor brevis and opponens digiti quinti.—Beyond the anlage of the
abductor digiti quinti the lateral plantar nerve in the 14 mm. embryo
(Plate IX, Fig. 4) lies superficial to an ill-defined mass of tissue in which
no segmentation into muscles can be made out. In the 20 mm. embryo
(Plate IX, Fig. 6) a nerve branch extends from the lateral plantar nerve
to the base of the 5th metatarsal and near the tip of this two slightly
defined areas of partially differentiated tissue probably represent the
anlages of the two muscles under consideration. According to Schomburg
the anlage of these muscles lies at first in the area between the 4th and
5th metatarsals but for this statement I find no support in the embryos
studied. According to Ruge, 78, and Schomburg, oo, the flexor brevis
and opponens muscles arise from a common anlage which becomes later
differentiated into the two muscles.

In the adult a single nerve is commonly distributed across the middle
third of the bellies of each muscle.

Mm. interossei.mRuge, 78, called attention to the fact that the inter-
osseous muscles with the possible exception of the first dorsal have a
plantar origin and that later the dorsal interossei wander between the
metatarsals to the dorsal surface. Schomburg, oo, has confirmed this
observation and has also shown that the dorsal interosseous I is originally
plantar in position. In later embryonic stages Schomburg describes the
dorsal interosseous II as extending on the plantar surface somewhat like
the plantar interossei while the plantar interosseus I shows a tendency
to wander dorsally like a dorsal interosseous.

The first signs of the interessei muscles which I have seen are ill-de-

366 The Nerves and Muscles of the Leg

fined anlages in an embryo of 20 mm. (Plate IX, Fig. 6). To the prox-
imal extremity of each anlage branches are given from the lateral plantar
nerve. The later stages of development I have not followed out carefully.

M. adductor hallucis—This arises, as pointed out by Ruge, from an
anlage at the base of the 2d metatarsal and from here wanders into its
adult position. The anlage of the muscle is shown in Plate IX, Fig. 6.
The later development of the muscle I have not followed. According to
Ruge, 78, the transverse head of the adductor comes from the same anlage
as the oblique, while Schomburg, oo, considers that the latter muscle
arises from a separate anlage. According to Poirier the nerves of the
two portions of the adductor arise from a common trunk which would
be in favor of Ruge’s view. I have found the nerves arising usually from
quite distinct branches of the lateral plantar nerve. One nerve enters the
caput obliquum near the proximal end of the middle third; and the other,
the caput transversum near its centre.

Lumbricales.

In neither embryo CXLIV, length 14 mm., nor in
embryo XXII, length 20 mm., are the lumbricales clearly differentiated.
In the latter embryo, Plate IX, Fig. 5, however, the anlage of the lum-
brical muscle of the 2d toe is just beginning to appear and to it a slight
nerve twig may be traced from the medial plantar nerve. As pointed out
by Schomburg the lumbrical muscles appear during the second half of
the second month as separate anlages near the distal extremity of the
metatarsal bones and from here wander toward their attachments to the
tendons of the flexor digitorum longus. The three lateral lumbrical
muscles were found supplied by the lateral plantar nerve and the medial
by the medial plantar in 9 out of 10 instances by Brooks, 87, while the
two medial muscles were supplied both by the medial and lateral plantar
nerves in one instance. He considers that the lumbrical muscles belong
primitively to the medial plantar territory.

5. Development and Innervation of the Muscles Supplied by the Medial
Plantar Nerve.

To this group belong the flexor digitorum brevis, abductor hallucis, the
flexor hallucis brevis and the medial lumbrical muscle. This last has
been considered in connection with the muscles of the preceding group.

M. flexor digitorum brevis—This muscle develops comparatively late.
In the 14 mm. embryo I have been able to determine no distinct signs of
the muscle. In a 20 mm. embryo (Plate IX, Fig. 5) the anlage of the

Charles R. Bardeen 367

muscle may be made out on the surface of the aponeurosis of the long
flexor muscles above the region of the middle cuneiform bone. Differ-
entiation is just beginning so that no distinct muscle fibres may be made
out. A small nerve may be traced into its medial margin. The tendons
have not begun to develop. Soon after this stage the muscle undergoes
rapid development. Proximally it extends to the tuber calcanei, distally
it sends forth tendons to the toes.

In the adult the chief variations are those marked by a reduction of
the muscle, especially that portion belonging to the fifth toe. The muscle
is supplied by a nerve which enters the medial margin.

M. abductor hallucis—This muscle is not distinctly visible in embryo
CXLIV (length 14 mm.). It can be distinguished in embryo XXII
(length 20 mm.), although differentiation is not here well marked.
(Plate IX, Fig. 6). The muscle arises on the medial edge of the plantar
surface of the foot over the navicular, first cuneiform, and the base
of the 1st inetatarsal bones and at a considerable distance from the tuber

mA

calcanei. It arises in close association with the m. flexor hallucis brevis.
With the torsion of the foot which carries the caleaneus in a medial di-
rection the anlage of the abductor extends proximally to be attached to
the tuber calcanei.

In the adult a branch from the medial plantar nerve usually enters
near the middle of the lateral border of the muscle. The relation of the
nerve to the muscle anlage in embryo XXII is shown in the figure.

Flexor hallucis brevis—Like the other muscles of this group this
muscle is not distinguishable in embryo CXLIV, length 14 mm. Even
in embryo XXII, length 20 mm. (Plate IX, Fig. 6), it is only beginning
to appear. The cells of the anlage are closely packed together. To the
anlage a nerve branch is given. I find the anlage somewhat more medially
placed on the base of the first metatarsal than that shown by Schom-
burg, oo. The anlage is incompletely divisible into two portions, a
medial and a lateral. During further development the lateral belly ap-
proaches the adductor hallucis. The medial belly in embryo XXII is
associated with the abductor hallucis, although according to Schomburg
it is brought into association with this muscle later than the lateral head
is brought into association with the adductor hallucis.

In the adult the nerve enters between the two bellies of the muscle and
spreads out into branches which pass between the constituent muscle
bundles. It is only rarely that the lateral head of the muscle is supplied
by the lateral plantar nerve.

368 The Nerves and Muscles of the Leg

Comparative anatomy of the intrinsic plantar muscles.—According to Mc-
Murrich, 04, the muscles of the crus terminate primarily at the ankle either
on the plantar aponeurosis or the tarsus. The tendons whereby the long
flexors of the toes are attached to the digits he looks upon as a differentiation
of a deep plantar aponeurosis. According to this view the foot, in which
but one set of crural muscles is attached through tendons to the digits, is to
be looked upon as more primitive than the hand, in which superficial and deep
forearm flexors are thus attached. In the foot there are to be distinguished
several layers of intrinsic muscles, the more superficial of which, the
flexor digitorum brevis and the lumbricales, arise in man from or in con-
nection with the plantar aponeurosis or its derivatives, while the deeper lay-
ers arise from the tarsus and metatarsus.

The deeper intrinsic muscles of the hand and foot are considered by Cun-
ningham, 82, and Brooks, 87, to have been derived from three primary
layers, a superficial layer of four muscles primarily adductors, an interme-
diate layer of bicipital short flexors, one for each digit, and a deep layer of
six abductors. The lateral plantar nerve crosses between the superficial and
the intermediate layer.”

MeMurrich, 03, differs greatly from Cunningham in the layers to which
he would ascribe the muscles of the hand. Thus he recognizes the following
layers:

Flexor brevis superficialis: Palmaris brevis, abductor digiti quinti, oppo-
nens digiti quinti, flexor brevis digiti quinti, abductor pollicis, opponens pol-
licis, flexor pollicis brevis.

Flexor brevis medius, stratum superficiale: The lumbricales.

Flexor brevis medius, stratum profundum: The adductor pollicis.

Flexor brevis profundus: The interossei volares, interossei dorsales (in
part).

Intermetacarpals: The interossei dorsales (in part).

McMurrich has not yet published his paper on the phylogeny of the muscles
of the foot, so that his views as to the origin of these muscles cannot be
given, but doubtless the layers there, from his point of view, resemble those
of the hand.

The subject of the comparative anatomy of the plantar muscles is too intri-
cate to be entered upon here at length. Leche gives a brief summary of the
conditions found in the mammalian series.

From the standpoint of embryological development the division of the deep
plantar muscles adopted by Ruge, 78, is of the greatest value. He recog-
nizes a medial group consisting of the abductor and the flexor brevis hallucis,
innervated by the medial plantar nerve, and two groups innervated by the
lateral plantar nerve, a more superficial group of ‘“ contrahentes ’”’ which lie
plantarwards from the deep branch of the nerve and a group of “ interossei ”’
which lie deeper than this nerve. He also points out that in many of the
mammals the interossei have permanently a plantar position which corre-
sponds with the early embryonic condition in man.

* See Quain’s Anatomy 10th ed., Vol. II, Pt. II, p. 276.

Charles R. Bardeen 369

6. The Muscle Branches of the Plantar Nerves.

While it is certain that the set of spinal nerves supplying the lateral
plantar nerve as a group are more distally situated than those supplying
the medial plantar nerve, the difficulties of tracing the nerves supplied
to the muscles of the sole back to the sacral plexus make it impossible at
present to give the spinal nerve supply of these muscles.

Compared with the other nerves of the leg the plantar nerves seem to
be unusually constant in their mode of distribution of branches. The
difficulties of accurate dissection of the nerves of the intrinsic muscles
of the sole of the foot, however, make it more difficult than in other
regions to utilize the work of students in getting reliable charts of this
nerve supply. In general the descriptions given in the various anatomies
agree pretty well. In the anatomy of Poirier and Charpy the supply
of the quadratus plante is given as coming from the medial plantar. In
a large number of dissections which I have followed this branch arose in
every case from the lateral plantar, usually proximal but sometimes distal
to the branch to the abductor of the fifth toe. This is the situation
usually described for it in the text-books. The dissections which I have
followed also serve to substantiate the statement given in Quain’s
Anatomy (10th edition) that the lateral plantar nerve only occasionally
gives a branch to the lateral head of the m. flexor brevis hallucis and to
substantiate the statement cf Brooks, 87, that in about one in ten in-
stances the medial as well as the lateral plantar nerves supply both the
first and second lumbrical muscles. There is, however, considerable
variation in the way in which the different nerves to the interosseous and
lumbrical muscles and the transversales pedis are bound for a distance
in common trunks.

XI. SUMMARY AND CONCLUSIONS.

The intrinsic musculature of the inferior extremity in man is differ-
entiated from the blastema of the limb-bud. No processes from the
myotomes are sent into the limb from the lumbar or sacral myotomes.
After the differentiation of the myotomes from the somites the myotomes
are bounded on the external surface, the sides and ends by a clearly
marked membrane which is retained until after the lumbo-sacral nerves
have extended well into the limb-bud.

Soon after the lumbo-sacral spinal nerves begin to extend into the
limb-bud tissue differentiation takes place in the blastema of the bud.

370 The Nerves and Muscles of the Leg

At the center a core of scleroblastema, on each side of this a thick layer
of myoblastema, at the periphery of the limb-bud a thinner layer of
dermoblastema are differentiated. This differentiation begins near the
anlage of the hip joint and extends proximally and distally.

The myoblastema represents the anlage of the muscles and of the
skeletal framework of the musculature, including the fascie and the
tendons.

The spinal nerves which grow into the limb-bud fuse to form a plexus
and from this the nerves of the limb arise. At the time these nerves ex-
tend distally and give off branches the myoblastema becomes differentiated
into anlages for specific grcups of muscles and each of these anlages be-
comes further differentiated into the anlages of the specific muscles which
compose the group. The main nerve trunks grow as a rule in regions
which lie between the anlages of muscle groups, the main branches to
each of the groups between the anlages of the muscles which constitute
the group, and the intramuscular branches in the intramuscular septa
which appear between the differentiating bundles of muscle fibres. Finally
the terminal branches for the individual muscle fibres are given off. The
site of entry of a nerve marks the region of earliest differentiation in the
muscle. In many instances, at least, the distribution of the nerve in
an adult muscle indicates the course of development of that muscle.
(Nussbaum, 94).

The development of muscles from the muscle anlages consists es-
sentially of a differentiation of the, at first, apparently nearly homogene-
ous tissue of the anlage, into muscle cells and into the connective tissue
framework which serves to hold these in place and harness them to the
structures on which they are to act. The adult architecture of a muscle
must be understood before its development can be intelligently followed.

In the simplest muscles in the adult the muscle fibres are bound by the
endomysium into bundles which are inserted at each end of the muscle
into a tendon. Asa rule neither the muscle-fibres nor individual bundles
of fibres extend the entire distance from tendon to tendon.” 'The fibre
bundles anastomose in such a way that they form a long-meshed net-
work, such as that diagrammatically represented in Fig. 7 b.

The muscle-fibres either take a nearly parallel course from one tendon
to the other, Fig. 7 c, or they diverge from one tendon toward the other,
Fig. 7 d. In the majority of simple muscles the distance from tendon

°° In some short mammalian muscles, like the segments of the rectus ab-
dominis of the mouse, the muscle-fibers run from tendon to tendon. On the
segmental musculature of elasmobranches and urodeles, see Bardeen, 03.

Charles R. Bardeen 371

to tendon along lines parallel with the muscle fibres is approximately the
same in all parts. There are, however, numerous exceptions, the most
marked of which are found in larger sheet-like muscles such as the oblique
and transverse muscles of the abdomen. Frequently in case of exceptions
of this nature, as for instance in case of the abdominal muscles, the adult

h ]

Fig. 7. Diagrams to illustrate nerve-muscle development. a. Embryonic
muscle anlage. 6. Anastomosing bundles of muscle fibers. c. Band-like muscle
developed transverse to course of main nerve trunk. d. Triangular muscle
developed transverse to course of main nerve trunk. e. Pennate muscle devel-
oped parallel with course of main nerve trunk. ff. Bipennate muscle devel-
oped parallel with course of main nerve trunk. g. Fusiform muscle. h. Band-
like muscle developed parallel with course of main nerve trunk. 7, Triangular
muscle developed in direction with course of main nerve trunk.

human muscle represents a combination of several simpler muscles in
each of which the general rule holds good in the embryo or in some of
the lower mammals. For the architecture of the abdominal muscles in
the mammals, see Bardeen, 03.

25

372 The Nerves and Muscles of the Leg

Since the more complex muscles are usually capable of being analyzed
into parts the structure of which resembles that of the simpler muscles,
we shall consider here the development merely of several simpler types
of muscle. The structural units of the more complex muscles develop in
a similar manner.

The nerve usually enters the anlage of a muscle near the center of the
side toward the main trunk from which its special nerve arises, Fig. 7 a.
The relation of the chief branch or branches of the nerve of the muscle
to the fibre-bundles depends on whether the course of the muscle fibres
is transverse to or parallel with the main trunk from which the nerve
arises.

Tf the fibre bundles of the muscle take a direction directly or obliquely
transverse to the course of the main nerve trunk the nerve to the muscle,
or its chief branches, usually passes for some distance across the fibre
bundles about midway between the tendons and give off rami on each
side from which in turn an intramuscular nerve plexus arises, Fig. 7 c-g.
The direction of the course of the main nerve branches on or in an adult
muscle of this nature indicates the course of growth of the muscle from
the anlage in a direction transverse to the long axis of the muscle fibres.
This growth is relatively slight in case of ribbon-like muscles, Fig. 7 ¢,
somewhat greater in case of triangular muscles, Fig. 7 d, and extensive
in case of muscles like the intercostal muscles and pennate or bipennate
iuscles, Fig. 7, e, f. Two or more branches may enter muscles of these
latter types at different levels from the main nerve trunk, dotted lines
Fig. 7 e.

It is at first difficult to recognize that in most fusiform muscles the
distance from tendon to tendon along the course of the muscle-fibres is
approximately equal. The course of the muscle-fibres in such muscles is
diagrammatically represented in Fig. 7, g. It will be noted that the
course of the chief branches of the nerve to the muscle is approximated
midway between the tendons to which the fibre-bundles are attached.

If the long axes of the fibre-bundles of a muscle are developed in a
direction somewhat parallel with the course of the main trunk from
which the chief branches to the muscle arise, the branches usually enter
the proximal third of the belly of the muscle and extend distally parallel
with the muscle fibres, at the same time giving off rami from which an
extensive intramuscular plexus is formed. The course of the chief rami
within a muscle of this type indicates the course of growth of the muscle
in a direction parallel with the muscle fibres. See Fig. 7, h and i.

Metameric segmentation in the innervation of the limb muscles is due

ee

Charles R. Bardeen 3

not to the ingrowth into the limb of myotomes accompanied by nerves,
but to the fact that a given region in the developing musculature is in
the more direct path of fibres extending into the limb from one or two
specific spinal nerves. ‘The number of spinal nerves contributing to the
innervation of the inferior extremity in man varies from six to nine, the
number contributing to the innervation of the musculature probably
varies from five to eight. The number as well as the position of the
spinal nerves serving to innervate a given muscle varies greatly in differ-
ent individuals.

With a few exceptions it is difficult or impossible to trace back to their
origin from the plexus the fibres composing the nerve of supply of a given
muscle in the inferior extremity in man. The path of fibre bundles in
a nerve is quite different from that of the nerve fibres composing the
nerve. The connective tissue which serves to hold together the nerve
fibres and to distribute blood vessels to the nerve does not form continu-
ous sheets about continuous bundles of nerve fibres. On the contrary it
forms enveloping layers which are continued for but a short distance
about a given group of fibres and then breaks up and becomes fused with
similar enveloping sheets about other groups of fibres. The nerve fibres
take a much more direct course in a nerve than any bundles that can be
dissected from the nerve. A study of the origin of the branches of a
nerve and the variation in the relation of these branches to one another
makes it possible to construct a schematic cross section of a nerve trunk
in which the relations of the nerve fibres in the trunk are more accurately
revealed than in mere dissection of the branches back into the component
fibre bundles of the nerve. On pages 308, 316, and 322 I have shown such
schematic diagrams of the femoral, obturator and sciatic nerves. The
nerve fibres of contiguous areas may branch off in a common trunk, but
nerve fibres in discontinuous areas never do. On Plate III I have shown
schematically the probable regions occupied by the fibres destined for the
chief branches of the main nerves of the inferior extermity in the nerve
trunks near the pelvis at the period when the segmental relations of the
spinal nerves to the limb are becoming established.

In the adult nerves variation is frequent and extensive. The main
nerve trunks are fairly constant in position, the greatest variation being
found in the course of the peroneal nerve in the thigh. This nerve is
frequently separated from the tibial nerve by a part or the whole of the
piriformis muscle. In one instance I have seen it separated by a part of
the short head of the biceps, p. 293. In the embryo the peroneal and

374 The Nerves and Muscles of the Leg

tibial nerves in the thigh are separated by a considerable amount of dense
tissue.

There is much variation in the number and position of the spinal
nerves which supply the main nerve trunks of the limb as well as in those
which supply the smaller branches which pass directly from the plexus
to the gluteal muscles and the piriformis. There is also great variation
in the number, course and distribution of the branches which pass from
the main nerve trunks to the muscles and the skin. No correlation has
been discovered between variation in the source of supply and variation
in peripheral distribution of the intrinsic nerves of the limb, with the
exception of the cutaneous border nerves. No marked correlations have
been discovered between either sort of variation-and race, sex, or side of
body.

While the development of the musculature is fairly direct, there is
probably as much correlation between the ontogeny and phylogeny of the
muscles of the leg as between the ontogeny and phylogeny of the skeleton.

D. PERINEAL MUSCULATURE AND THE NERVES OF THE
PUDIC GROUP.

a. Embryonic Development.

In an embryo of 11 mm. (Plate III, Fig. 1) the sacral plexus is fully
formed and several branches may be seen extending out toward the cloaca
and viscera. These branches indicate the developing pudic and visceral
nerves, but differentiation has not proceeded sufficiently far to make it
possible to determine with certainty what each of the branches represents.
The myotomes of the sacro-coccygeal region are distinct. No specific
differentiation of the perineo-caudal musculature is apparent.. The rela-
tions of the pubic nerves to the nerves of the leg are shown schematically
in Plate ITI, Fig. 3.

In a slightly older embryo (Plate X, Fig. 1) the main branches of the
pudic and visceral nerves have appeared. The dorsal nerve of the penis
arises in the main from the 3d sacral nerve. The perineal nerves arise
from the (2d), 3d, and 4th sacral nerves. The hemorrhoidal nerve arises
from the 3d and 4th sacral nerves. About the region of the cloaca there
is some condensation of tissue, but there is no distinct differentiation of
muscle. From the 3d and 4th sacral nerves branches are given to a highly
developed visceral plexus in which a large amount of chromophile tissue
is apparent. This tissue mass lies lateral to the intestine and extends

Charles R. Bardeen BW és)

nearly to the urachus. Anteriorly it is continued into a similar mass
extending down from the region of the suprarenal gland.

In company with the visceral branch from the 4th sacral nerve there
arises a nerve which extends out into a differentiating mass of tissue
which probably represents the levator ani muscle. There is no good evi-
dence to show that this muscle arises from the myotomes. The coccygeal
musculature which lies dorsal and lateral to the levator ani seems, how-
ever, evidently to arise from the ventral tips of the caudal myotomes.
Into it extend nerves from the 4th and 5th sacral and possibly from the
caudal nerve. This, as also in embryo CIX, is relatively at this stage
very large.

In embryo XXII, length 20 mm. (Plate X, Fig. 2) conditions similar
to those just described may be found. The direction in which the sec-
tions are cut makes a reconstruction of the region somewhat imperfect.
The results have been controlled by study of another somewhat older
embryo, CXLV, length 33 mm. The plexus is of a more anterior type
than that of 144. The dorsal nerve of the penis and perineal nerves ap-
parently arise largely from the 2d sacral nerve and the 4th sacral nerve
seems not to enter into the pudic plexus. The perineal musculature is
undergoing specific differentiation, but no attempt has been made to de-
termine definitely the boundaries of the various muscles. The levator
ani muscle is well differentiated. The visceral plexus is even more exten-
sive than in the preceding stage.

For comparison of embryonic conditions with the distribution of the
pudic nerves in the adult male, the well-known illustration of Hirschfeld
and Léveillé may be used. It is to be noted that previous to the out-
growth of the pudic nerves the cloaca and urachus occupy a more distal
position relative to the spinal column (Fig. CIX) than they do at the
period when these nerves are developed (Plate X, Figs. 1 and 2). Later,
the external genitalia shift again distally.

The paths taken by the growing nerves are fairly direct. That of the
dorsal nerve of the penis is most so. The perineal nerves bend more in a
distal direction. The nerve of the levator ani muscle takes a course at
right angles to the path of the main trunk from which it arises. It may
readily be seen that the most anterior root fibres of the pudendal nerve
enter the dorsal nerve of the penis, the most posterior the hemorrhoidal
nerve. Cutaneous branches also arise from the caudal nerve.

According to Popowsky, 99, at a period when the cloaca is still present
a sheet of muscle forms a sphincter around its opening. Later, when the
rectal becomes separated from the urogenital portion of the cloaca the
sphincter is divided, the posterior portion becoming the sphincter ani

Oo

Types of Origin of Pudic Nerves from Spinal Nerves.

The Nerves and Muscles of the Leg

TABLE

Frequency of

®1]n one instance: B. M. R.

A B C
Separate Separate SXSXEDV: 3) SEXOXETEV GG | NEXNGSTVG
N. Pudendus. N. Heemorr- N. Dorsalis
hoidalis. Penis.
XOXO || XSXSVAE XEXOValeTe
XXVI, XX VII 1 1
XXVII 2
XXVI, XXVII XXVIII 1
XXVIT XXVIII 1
XXXVI, XXVII, 9 4
XXVIII
XX VII, XXVIII XXVII7" ipl 37
XXVIT, XXVIII XXVIII 3 il
XXVIII 1
XXXVI. XXVIT, XXVII, XXVIII, 1
XXVIII XXIX
XOXCValils
XX VII, XXVIII XXVIII, XXIX XXVIES 1 2
XXVII, XXVIII, 2 7
XXTX XX VII? 1 7
XXVIII, XX1IX XXVIII, XXITX XX VITI2
XXVIII, XXIX XXVIII? 1 8
XXVIII, XXIX xxx
XXVIII, XXTIX,
XXX
NiumMbervoO TNStanCes® <i. ics ol. ecisteisivsisiese wines cleste(oiw celdincladen cee 1 24 61
27 In three instances: W.F.R; W. W.M.R
*2In one instance: W.M. R.
*2In two instances: W.M.L; B.M.
3° In two instances: B.F.R; B.M.

XXIX.

Charles R. Bardeen

ot7

Association with Various Types of Plexuses.

Race, Sex, and Side of Body.

D & 8 e White. Negro.
XXIV gent XXIV oe Furcal N.
Male. | Female.| Male. | Female.
Most Distal | Las Sag Seed
XXVILI|XXVIII| XXIX | XXIX Bene to | elena lear | eal peel tena well ae
_No. of Inst. |
2 1 1
2 1 San ie
to fa) ee ee eae
1 1 oa rear ae
i 7 2 1 ise 2 lh a
34 3 85 13 10. 1 SE: 20 18 Tn re
‘ 1. Se |e ee aaa ee ae
3 ree.) jk 1) ee ae
ee sles
1 1 5 cia 2 1 1
22 1 5 3 39 fan 7 1 ores 8 6 a 8
5 1 1 7 Too \ee eel
37 8 6 15 75 7 6 2 DL ake | ea0 i, 10
7 | ae eee
1 ee one ree ea ie ae |e |
108 19 13 14 235 al | 28 6 5 | 53 | 51 | 381 | 30

The Nerves and Muscles of the Leg

378

IUV 1OJBAD'T ‘Pf 0O} BAAON JO UTSIIG Jo Aouonbeay

Ig | I | Fg or | QI | IT eae? 1equinN [BIOL
AXX 10
8 I 9 I xXIXX ‘AXX-AIXX 13)
[ I XIXX AIXX A “ys0d
AXX 10
if
L g T ITIAX xX ‘AXX-AIXX a
snxotd ag geen
¥8 66 b I TITAXX ivquin, 0} Ayerqo | q | ‘wu0N
AIXX
snxoid
96 tL g 6 ITIAXX jB1oRs 03 AYOryo 9
AIXX
i] F T ITIAXX AIXX a uy
IAXX AIXX V
“edatie xXIxXxX XIXX XIXX “quiry
Bote HIAXX | IIAXX Tee 0} OAION [Buldg ‘IAMON [BOING ‘od A,
z [BySIq ISOM
‘dg ‘uN ‘dg ‘uN ‘dg ‘uN ‘dg ‘uN ‘dg ‘uN
: ULOA “SOSTIB IUY 10}BAOT "FW oq} 0

VALON JOGO 94} YOIYM WOT sNXe[q Jo odAT,

“XXX WIAVL

379

ral 6 I é a
= “O1.99N
ol i I I a
7 ol tl
q ¥ T g T
5 :
3 a[BUle,y
I 5 eo g U
o
wal
' 2 “OPN M.
Ps ol h T ¥ at
ch
@ = : — ‘OTR
H
& or q & I : a
Oo
: XIXX XIXX XIXXK
ToquinN TITAXX | INIAXX | INIAXx
aan WAXX
‘dg uN | ‘dg ‘un | ‘dg'un | ‘dg‘un | ‘dg ‘un

; WOIy
JUY 109BAO'T "TT 0} OAION JO ULSZIAG Jo Aouonbeary

“Apog JO apig puw ‘xog ‘eouyy

“‘panuyjUuogo— XXX WIAVL

380 The Nerves and Muscles of the Leg

while the anterior portion becomes differentiated into the various perineal
muscles. The details of this process I have not followed.

For the comparative anatomy of the perineal muscles, see H. Eggeling,
96, M. Holl, 96.

b. Nerve Variation in the Adult.

There is considerable variation in the origin and distribution of the
nerves of the perineal region in man. Commonly the hemorrhoidal and
the perineal nerves and the dorsal nerve to the penis (clitoris) are
branches of a common trunk, the pudie nerve, which arises from the
27th and 28th spinal (2d and 3d sacral) or the 28th and 29th spinal
(3d and 4th sacral) nerves. The origin may, however, be from the 26th
and 27th; the 27th; the 26th, 27th, and 28th; the 28th; the 27th, 28th,
and 29th; or from the 28th, 29th, and 30th spinal nerves. In the accom-
panying table the frequency of these various modes of origin is shown.

Not infrequently (in 20 out of 235 instances, 8.5%) the hemorrhoidal
branch has a separate origin from the plexus, usually from the 28th, or
28th and 29th spinal nerves.

Less frequently the dorsal nerve of the penis (clitoris) has an inde-
pendent origin (in 9 out of 235 instances, 3.9%). In such instances
the nerve arises from the 27th; 27th and 28th; or 28th spinal nerves
(see Table XXIX).

The chief nerve to the levator ani muscle arises usually in conjunction
with viscereal branches from the 29th spinal (4th sacral) nerve. It may
arise from the 27th and 28th; the 28th; the 28th and 29th; or the 29th
and 30th spinal nerves (see Table XXX). Other small branches are
also frequently given to this muscle.

The nerves to the coccygeus muscle arise from the last spinal nerve
contributing to the pudic plexus and also usually from the next more
distal spinal nerve.

The visceral branches usually arise from the last two spinal nerves
entering the pudic plexus and also often from the next most distal spinal
nerve.

The charts which I have show great variation in the peripheral course
and distribution of all of these nerves. The difficulties of making thor-
oughly accurate dissections and charts of the nerves of the perineal region
make it seem inadvisable to try to use these charts for statistical pur-
poses. In general the distribution corresponds with that given in the
anatomies of Poirier and Charpy and Quain, and with those pictured in
the atlases of Toldt and Spalteholz.

a _—

—— ee ee

Charles R. Bardeen 381

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382 The Nerves and Muscles of the Leg

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Charles R. Bardeen 383

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Vol. 38, p- 138, 1904.

Paterson, A. M.—The pectineus muscle and its nerve supply. Journal of

Anatomy and Physiology, Vol. 26, 1891, and Vol. 28, 1895.

The origin and distribution of the nerves to the lower limb.

Journal of Anatomy and Physiology, Vol. 38, pp. 84 and 169, 1894.

PorRieR, P., and Cuarpy, A.—Traité d’anatomie humaine, T. 2, Paris, 1901.

Popowsky.—Zur Entwickelungsgeschichte der Dammmuskulatur beim Men-
schen. Anat. Hefte, Vol. 12, 1899.

QuaiIn, I.—Elements of Anatomy, 10th ed., Vol. 2, Pt. 2, edited by G. D.
Thane, London, 1892.

Rigpine.—Anat. Anzeiger, Bd. 28, p. 355, 1906.

Ruce, G.—Untersuchungen tiber die Extensorengruppe am Unterschenkel und
Fuss der Satigethiere. Morpholog. Jahrbuch, Bd. IV, Suppl., 1878.
Zur vergleich. Anatomie der tiefen Muskeln der Fusssohle. Morpholog.

Jahrbuch, Bd. 4, 1878.

Entwickelungsvorgange an der Muskulatur des menschl. Fusses. Mor-

pholog. Jahrbuch, Bd. 4, Suppl., 1878.

SaLv1, G.—La filogenese ed i resti nell uomo dei moscoli pronatori personzo-
tibiales. Monitore zodlg. italiano, Anno. 11, 1905.

ScHompBure, HAns.—Entwickelung der Muskeln und Knochen des mensch-
lichen Fusses. Gekronte Preisschrift, Gottingen, 1900.

SCHWALBE, G.—Ueber das Gesetz des Nervemuskeleintritts. Archiy Anatomie
und Physiologie, Anat. Abth., pp. 167-174, 1879.

SEVEREANO, G.—Du plexus lombaire. Bibliot. Anat., T. 13, 1905.

TAyLor, G., and BonnrEy, V.—On the homology and morphology of the popli-
teus muscle. Journal of Anatomy and Physiology, Vol. 40, p. 34,
1905.

TertuT, L.—Les anamolies musculaires chez l’homme, expliquées par l’anato-
mie comparée. Paris, 1884.

VALENTI, G.—Sopra le prime fasi di sviluppo della muscolatura degli arti.
Archivo Ital. di Anatomie e Embryologie, Vol. 2, pp. 272-280, 1903.

WILLIAMS, RogEeR.—Journal of Anatomy and Physiology, Vol. 13, p. 204, 1878-9.

384 The Nerves and Muscles of the Leg

Witson, J. F.—Abnormal distribution of nerve to quadratus femoris in man.
Journal of Anatomy and Physiology, Vol. 23, p. 354, 1889.

WINDLE, B. C. A., and Parsons, F. G.—Morphology of the femoral head of the
biceps flexor cruris. Journal of Anatomy and Physiology, Vol. 34,
1900.

DESCRIPTION OF PLATES.

PLATE I,

Two figures, repeated from this journal (Vol. I, Plate II), to illustrate
early stages in the differentiation of the inferior extremity.

Fic. 1. Embryo II, length 7 mm., age about four weeks. -

Fie. 2. Embryo CLXIII, length 9 mm., age about four and a half weeks.

For description of figures see text, p. 264.

PLATE II.

Four sections through the base of the posterior limb to illustrate different
stages in the development of the nerves and musculature.

Fic. 1. Section passing through the right limb-bud in embryo II, length
7 mm., age 26 days. The tips of the neighboring myotomes do not extend
into the mass of tissue of which the limb-buds are composed and in which
as yet no specific differentiation is visible. 33 diam.

Fig. 2. Secton passing transversely through the base of the right limb-
bud of embryo CIX, length 11 mm., age about five weeks. At the center of
the limb-bud the acetabular region of the skeleton appears as a condensed
mass of tissue. About this the femoral, obturator, and sciatic nerves may be
seen extending into the limb bud. Myogenous tissue is fairly well marked
near the femoral and sciatic nerves. 25 diam.

Fic. 3. Transverse section passing through the acetabular region of left
leg of embryo CXLIV, length 14 mm., age about five and one-half weeks. The
femoral, obturator, gluteal, and sciatic nerves may be seen extending into
the limb bud, and in the vicinity of these nerves the anlages of the iliopsoas,
pectineus, adductor, hamstring, and gluteal muscles. 25 diam.

Fic. 4. Transverse section passing through the acetabular region of em-
bryo CXLYV, length 33 mm., age about two months. The femoral, obturator,
inferior gluteal, and sciatic nerves may be seen entering the limb. The
chief fasciculi of the iliopsoas, pectineus, adductor, and gluteus maximus
muscles are separated by an amount of connective tissue relatively greater
than in the adult. 10 diam.

PLATE III.

Three figures to illustrate the skeletal, muscular, and nervous apparatus
of the right posterior extremity of embryo CIX, length 11 mm., age about five
weeks. The nerves are represented black; the muscle anlages by stippling;
the skeletal structures, light grey; the skin of the leg, transparent. About
17 diam.

Fie. 1. Median view. The urachus is shown in outline.

Fig. 2. Lateral view.

Charles R. Bardeen 385

Fic. 3. Ventral view showing the relation of the pelvis to the boay wall
and the main nerve trunks. At the right the outline of the peritoneal mem-
brane is shown. The division of the main nerve trunks into branches is
diagrammatic.

PLATE IV.

+ Medial and lateral views showing the relations of the nerves of the abdomi-
nal wall and posterior limb to the abdominal musculature and the skeleton
of embryo CXLIV, length 14 mm., age about five and one-half weeks. The
skin of the thigh and leg is represented transparent. About 17 diam.

Fie. 1. Medial view.

Fic. 2. Lateral view.

PLATE V.

Medial and lateral views showing the relations of the nerves of the abdomi-
nal wall and posterior limb to the abdominal musculature and the skeleton of
embryo XXII, length 20 mm., age about seven weeks. The skin of the leg
is represented transparent. About 13 diam.

Fic. 1. Medial view.

Fic. 2. Lateral view.

PLATE VI.

Figures showing the nerves of the abdomen and the nerves and muscles
of the extensor side of the thigh.

Fic. 1. Embryo CXLIV, length 14 mm., age about five and one-half weeks.
The abdominal musculature has been partially removed to show the course
of the main nerve trunks. About 17 diam.

Fie. 2. Embryo XXII, length 20 mm., age about seven weeks. The ventral
portion of the abdominal wall has been removed. About 13 diam.

Fics. a@ and b. Branches of the femoral nerve to the quadriceps femoris
muscle in embryo CXLIV and in embryo XXII. The muscle in each instance
is represented semitransparent. About 15 diam.

PLATE VII.

Two diagrammatic outline sketches to illustrate the distribution of
the cutaneous nerves of the inferior extremity in the adult.

Fie. 1. Front view of the left and medial view of the right leg. The
dotted line on the right leg represents approximately the proximal margin of
the embryonic limb-bud.

Fic. 2. Back of the left leg and lateral side of the right leg. The dotted
line represents approximately the distal margin of the embryonic limb-bud.

PuLatTe VIIL.

Figures to illustrate the early differentiation of the adductor, hamstring,
tensor fascie late, gluteal, obturator internus, and quadratus femoris muscles
and the short head of the biceps and the nerves supplied to these. 25 diam.

Fic. 1. Adductor and hamstring muscles in embryo CXLIV, length 14 mm.,
age about five and a half weeks. In order that the adductor longus and
brevis may be seen, merely the outline of the gracilis muscle is shown.

386 The Nerves and Muscles of the Leg

Fie. 2. Adductor group in embryo XXII, length 20 mm., age about seven
weeks. The adductor brevis and the gracilis muscles are shown cut out a
short distance from their attachments. In ‘“‘a” the relation of the nerve to
the belly of the gracilis muscle is shown; in “b” that to the belly of the
adductor brevis. The muscles are represented as semitransparent so that the
intramuscular course of the main nerve trunks may be followed.

Fic. 3. The hamstring group in embryo XXII. The belly of the semi-
tendinosus has been cut out so as to show the deeper structures.

Fic. 4. The gluteal and obturator internus muscle groups in embryo
CXLIV. The gluteus maximus muscle, except at its attachment to the femur,
is represented merely by an outline. The central portion of the belly of the
gluteus medius has been cut out to reveal the gluteus minimus. The tibial
nerve is cut off near the plexus, the peroneal nerve more distally.

Fig. 5: The gluteal and obturator internus muscle groups in embryo XXII.
The m. gluteus maximus and the lig. sacrotuberosum are shown in outline.
The central part of the belly of the gluteus medius is cut out. The tibial
nerve is cut off near the plexus.

PLATE IX.

Six figures to illustrate the early differentiation of the musculature of the
crus and pes. 25 diam.

Fic. 1. Peroneal and extensor muscles and nerves of the crus and pes of
embryo CXLIV, length 14 mm., age about five and a half weeks. The pero-
neal muscles and the m. extensor digitorum longus are made semitransparent
in order to reveal the deeper muscles and nerves.

Fic. 2. Peroneal and extensor muscles and nerves of the crus and pes of
embryo XXII, length 20 mm., age about seven weeks. The peroneal muscles
and the m. extensor digitorum longus are represented semitransparent.

Fic. 3. Superficial plantar musculature and nerves of the crus and pes of
embryo CXLIV.

Fic. 4. Deep plantar musculature and nerves of embryo CXLIV. The
gastrocnemius and flexor digitorum longus and the main trunk of the medial
plantar and a part of that of the lateral plantar nerves have been removed.

Fie. 5. The superficial plantar muscles and nerves of the crus and pes of
embryo XXII.

Fic. 6. The deep plantar muscles and nerves of the crus and pes of em-
bryo XXII. The gastrocnemius and flexor digitorum longus muscles and the
greater part of the tibial nerve have been removed.

PEATE) XX.

Two figures to illustrate the early stages in the development of the pudic
nerves and the distal portion of the sympathetic system.

Fic. 1. Medial view of the right half of the distal portion of embryo
CXLIV, length 14 mm., age about five and a half weeks. Enough of the sur-
rounding undifferentiated mesenchyme has been removed to reveal the course
of the nerves and the neighboring blood-vessels. The intestines are not
represented. 25 diam.

Fic. 2. Similar view of the right half of the distal portion of embryo
XXII, length 20 mm., age about seven weeks. 20 diam.

Charles R. Bardeen

iss)
ios)
~z

ABBREVIATIONS USED IN LETTERING FIGURES.

QODSIVUSCE akirte ciatetere retard wleraee abdominal musculature.
LCC Ga areretaoreus eyo) oceans) Soke sie landea acetabulum.
CO Pike mrsveraicustis sis chels ss shar alched % aorta
PAs CodsWataes Leuaiayis taveuetel ¢-o nie apat erases: artery.
1 KDE RDO ROTORS rT OS .... femoralis.
US LRG Co HOO IO CTR ae iliaca externa.
Oh GNHOUR iiend oO. On Din. oer dorsalis penis.
USGIDS SOB reece ond DIDO rae sciatic.
ODL efisitersves <iowe sieve sseterels soeoe vers poplitea.
CUO LIVES erates cus (areisy acd waere e hle tibialis anterior.
GUD S DOS Gra tetra csoke auaiots tibialis posterior.
UENU DEN Tevetortieds o iafe eins, wiste ess 3 umbilical.
COLCA Feist anie se <6 e tieeia 6 ale ee eset calcaneus.
TUS Lapa atten colors, erate) chon cien's: oe sSus xe chorda dorsalis.
(GOT Gch OIC LCE TAI CES TORS celom
GOSCOE tauetaicca Sieve: tance, cinle erences rib.
CRM ta vests area: che ascent aneteyes femur.
UMP oc ets okcousl sesh sid aha tee reo sllae's fibula.
(fo TOUs- Chaco SL OIRO ROR ONC ERG. foot plate.
QUASIUDU GUIS vale sucis te. Gie a areal ashe glandula suprarenalis.
USM ep cesie eee PNe: 6 ois aiiollal a ne Viehe, sidane ilium.
US CTU ME a seo sinieevenle arene nireccus toners ischium.
iG), SOK; Ue aoe pO DOS ODEO ligamentum sacrotuberosum.
LUG ATUO SER Rer cn eee ere ere Ore eS ligamentum inguinale.
HNESONS. 1G. OS OOOO OIG ree mesonephros.
HIKING 5 OIRO OO metanephros.
HUNGER, BS BESO ODOC DIOL metatarsus.
INS Meche OC AIOE EOC One musculus (li).
COOLS Wh SOR RRE REICROC RRO ECC abductor digiti quinti
(GHOST GH os SCE MROIRCR CED Orc ROIEE OC abductor hallucis
CHOI CNR RE ORS PRRCRCR ER MRR CNIS adductor group.
GOGO Nore cies cue occa aes adductor brevis.
(OHOES LG Is. See CR RCE FAIS Core adductor hallucis.
CEO UU 5 ac. 4) ore ausiataycte a sieietilerwe adductor longus.
QLISIUAGUS is ices so rocks eres adductor magnus
GOGEITLIVAS Wesley jerare otehaleraers adductor minimus.
(5) SES ROR er RT Ro ROS biceps femoris.
CODNOGS Serscd weave ys wieyaretarsts short head.
COD a eicrs:sectre.osa srs cheers long head.
COCCH Gin mete orci shone cvejoys tenets coccygeus.
CREO area nies @ ane jae. ole Clare crurales anteriores.
CRE DOSESDTOfial vais sss sels sito crurales posteriores profundi.
CIEDOSTASUD Es. cies z.cusele seston crurales posteriores superficiales.
CLLUUOMES dias. cc lenis evel eaves extensor digitorum longus.
CUEROAGE OTE “i iccsevnmo eo sisters extensor digitorum brevis.
CRESTUOUS Van fo Sie ier erie S ope-y.0 spcutters extensor hallucis longus.
CLUS OLOMIVE Cheloneysysisia seis eterene extensor digiti quinti brevis.

26

388

fem. post.
flex. dig. br.

flex. dig. V br.

flex. dig. 1.
EL. NALD |.
flex. Rai.
gastroc.

eeee
DO Oo
seer
ener
seer

ee

WE Soe GeOIEe
il. cost.
il. ps.

interos. dors.

interos. plant.

obl. abd. ext.
obl. abd. int.
obt. ext.
obt. int.
opp. V
pect.

peron.
peron. br.
peron. l.

pirif.

ps. mj.
quadr. fem.

quadr. lumb.
quadr. pl.
quadri. fem.

PALO Cette
rect. fem.
sart.

e-
eels, eo «ele

eeeee

semit.
sol.

0) 0) 0.8 6) o00

tens. fase. lat.

tib. ant.
1005 DOSE aoe
trans.abd. .
vastus lat. .

eeee

VOSTUSANTENM. 22s s2cse ance

vastus med.

The Nerves and Muscles of the Leg

hamstring group of muscles.
flexor digitorum brevis.
flexor digiti quinti brevis.
flexor digitorum longus.
flexor hallucis brevis.
flexor hallucis longus.
gastrocnemius.

gemelli.

gluteus maximus.
gluteus medius.

gluteus minimus.
gracilis.

iliacus.

iliocostales.

iliopsoas.

interossei dorsales.
interossei plantares.
obliquus abdominis externus.
obliquus abdominis internus.
obturator externus.
obturator internus.
opponens digiti quinti.
pectineus.

peroneal.

peroneus brevis.
peroneus longus.
piriformis.

plantaris.

popliteus.

psoas major.

quadratus femoris.
quadratus lumborum.
quadratus plante.
quadriceps femoris.
rectus abdominis.
rectus femoris.
sartorius.
semimembranosus.
semitendinosus.

soleus.

tensor fascie late.
tibialis anterior.
tibialis posterior.
transversus abdominis.
vastus lateralis.

vastus intermedius.

- vastus medialis.

eevee ereeeces

Ce)

eee eer eee ee eee

eee ewer eee reece

eoceeeoeeee rece

ee

Ce

eee er eer eee ese

ee) ble 050 \e%n)\abe (e.ele

ee)

|

ee

ee

je) oi 0}010].0 (ee) 6.6 (= (0 ere

ee eee nee eee eee

Ble es =) so 0 = «6010

er

er ?

eee ew ese eee cees

OCC Soro Cinco ocd

Ce

a\.| evel ep le 0) 0.6 6 s)1ele.

ee

a J0' 0 \6),0) 0 /\6)(0/ 010 10)0)

Charles R. Bardeen 389

(WD be a seein Oe OOD Obs OOOO myotome.
Ub). Corcier OO GIG CO CISIO Oa eer lumbar.
85 (ou Oe cic OOS DIIGO ORES BICC sacral.
(i Bi GO Ot CO ROE Oe thoracic.

NCS MP peRs Cet Nene clay siaicbss «61/0 vei slic nervus (vi).
COUMDS Par rocistacls sree ss syansteety ss caudalis.
GUUTURUN Fag fareracia wiske eieis cys sie es clunium inferiores.
CHENEINVEO asters nie stoic. s sislel <8 > clunium mediales.
CLUNF SUDAN ol feiss \aia5.5 88 see ee clunium superiores.
CUTSONUGS, Me ieverarctevetsrs avopsreeie sale cutaneus femoris anterior.
COL SOUTHS SOI NAO ODIO digitales plantares propr.
CULRIGIN Moke citscote clase eraisie cutaneus femoris lateralis.
CULRITE Oe Leon eee cutaneus femoris anterior, medial branch.
CULTADOSTR. Sstace ea eee ee selene cutaneus femoris posterior.

ARN CTA 205, Seer at anaiersince ore perineal branch.
CUCASUREAUGUS war etaretaley<tetolsseres cutaneus sure lateralis.
CULASU EMME errere arstersietnelcle cutaneus sure medialis.
EDEN a oui Sis tata ara were tele dorsalis penis.
Aik OO DEO ACR ROO O DOr femoralis.
Gy) 5 Oip OEE Ie OI eon genital.
GOR Mec chete: ccbinwllelerScssaistersceheus ec genitofemoral.
GUS ea oo) ei oo) 5 cid sve! eva levels ests - gluteus superior.
Gs Co Sos BAe oR ee ER ROIOOE gluteus inferior.
WEMOGIIUDINEWs \ vteiel-\ae)s «she hemorrhoidalis medialis.
MUEANOGEIUSAILI A estes seve tere ae hemorrhoidalis inferior.
TEAL OO sta ee eve sists sts'e) siclie7s) cloths hypogastricus.
fMRI ay a ars) oieie, fiestas) auersnaters iliacus.
HOOPS GAC ONS EIOOCIOC ROTTER OOS inguinalis.
UXO Sis ot DO OOD ischiadicus (sciatic).
DMM ere ahere fatal s (ehiaios) a tereusners reuse lumbalis.
LE HAGE. SCE FOO EOS CO lumbo-inguinalis.
DD is Ae RS OIRO OOO OOOO obturatorius.
(ATU: BOAO POD OOS eRe OOO perinei.
(NEOs. BOAO EO DEO EAAGO DOS peroneus.
CT ONUASUD tame aietsioticte siaceraeriele peroneus superficialis.
(NARDOS ORs Gagoood JOO00 00 peroneus profundus.
iS Ac oSSOODOOROD OO UO GO plantaris.
(TOs Up padito do beooopoxcos plantaris lateralis.
VLIVESOTU A Siaia avn eis keiths plantaris medialis.
DUSSUMUD Socios ee aia etere aun eisions sympathetic plexus.
MOL Ora aRecthn, siassh oteleisrsy sile.e%e\ she, less pudendus.
OMLOUSEM A ckete, = Sree vacesensrscorene lateral cutaneus branch of the dorsal (pos-
terior) division of a spinal nerve.

IMO ORL AOI OO Ooi . medial cutaneus branch.
SUM yel ete nene ors: 55/502 evens SiGe ayerens sacralis.

2 Wor abbreviations of terms applied to nerves going to muscles, see under
muscle, M. .

390 The Nerves and Muscles of the Leg

Qi ls or osdsaadonoarauGonooc saphenus.
SP PPR esau ch alias. sueye Srotcietas spinalis.
SQUIRTS > Bioiah obocD OO BOC Ones suralis.
RNS odeehe ash coves Fiaderoueds, (ei casichars thoracicus.
EVO Mets ene eye ops foieie carers Geahero Od tibialis.
HS MDS —coiscobereoogoccoc truncus sympathicus.
AIS Gs eesare ek esaXor les) 0) wile los Tooke visceral.
(Nii MAD asta kooormpo neo Da ooo8 c patella.
HG be CGO OOO CORE CUOIOG 6 OOD peritoneum.
(WKS Ih Ghpaoaeohootdoeoue processus vaginalis.
WHS. ding docdaooopotodoaddac pubis.
Si OMB SUH, soocuouacoo0pDe > spina anterior superior.
SDs POStASUDs Sterile He Ae spina posterior superior.
SOLD Lae © RAO CIO DOIG. O ISCO OO C sympathetic nervous system.
UO eGo as Socios ature a ooo tarsus.
SHE BG. OG Gia FO. IO ORO ORT ODO Ota tendon.
COST divas al enseorau ni tsl oral ohencns teers testicle.
(GU OO CORIO ORE ICED ROC tibia
AL ierMaraned shot anions iatiane caiahoueue ns aretenenees ureter.
VHACKG Ve. NS GeO aI ea HD ODIO COOL urachus.
WANA cio HG OID DD OREO Dano Ot vertebra.
Vahey arora avon Sea aanc le ak hortraiac helen shee vena
COLDS ae Soc elie eee cardinalis.
GSD CNAs sie che otis seen anette dorsalis penis.
TCM reveleus:ieuecsiore tele cxckevetonerahe femoralis.
WUD OGM i sias,ciriaate a suction eee oee hypogastrica.
UUNCD EG. Mer enc sas classe siuat eee hen iliaca externa.
XEL ON mechcosae OPORTO ONO Og OCOD sciatic.

WA GIe tan 8 SOR RRR RO Dike orc Wolffian duct.

THE NERVES AND MUSCLES OF THE LEG PLATE |
CHARLES R. BARDEEN

AMERICAN JOURNAL OF ANATOMY--VOL. VI

THE NERVES AND MUSCLES OF THE LEG PLATE Il

CHARLES R. BARDEEN

ap,-A. fem.
VN eut lat
oN

Fae
2 oe N _M ilps

T reet. abd ae ay

a ss

M. ada. 1 ee. _N fem.
M. gr. ul.
adds |
M. add.m.—£ : 3
M. se mi m: ue eee Tihs)
M-semt L: Ses :
> —M. el. min.
icon! Tig ——N. el. S-
aap M gastroe M. gl. med.

Cap.m——

Myo. t.12.

M.bi. cap]. cap br.
Nt. 1

Tages

M.rabd: s | meson.

N.obt

NI SGHE
—————V, {SCh:

acet

/ M.obt int.

N.isch.

N.caud:

AMERICAN JOURNAL OF ANATOMY--VOL. vi

THE NERVES AND MUSCLES OF THE LEG PLATE Ill
CHARLES R. BARDEEN (Fig. D

M.T. abd.
N. Coat,

N hypog
NB 8

Mm.add. Nl ine
Mm.er post sup \\M quadri fem Neut.lat.
Ft.pl. Mex post prof Nn fem post N.saph\_N. Neut ant,
N ureenig \ N
Pu.

N.biesemi t-
Prox. port
isch?

N. caud?

frel a eavgcess aN
N pud M qawar.Fem, M obt.int.

N. cul, post.

AMERICAN JOURNAL OF ANATOMY=-VOL. VI

THE NERVES AND MUSCLES OF THE LEG PLATE lll
CHARLES R. BARDEEN (Figs. 2, 3)

Mm. er. art -

Mm. peron.
di M.bi cap br.
N.caud.

N oii.

N Sa

aspera a

T. post: In pud 'N.haemorrh.

N ecu

L762

N cut. lat.

Nn cut. ant
med 8 Sart.

N. rectus..

N vasl.lat. N.

i add: br

. add.
:iseh meet
N.quadrafem.
N. ‘obt’ inr.

N. haemorrh,
N. coud

FIGS

N. cut. post.|

AMERICAN JOURNAL OF ANATOMY--VOL. VI

THE NERVES AND MUSCLES OF THE LEG
CHARLES R. BARDEEN

nol

\

\

N plant I.

AMERICAN JOURNAL OF ANATOMY--VOL. VI

N.cut post.

| nese
NSemit & br. -perin
prox. port

Nn Semit.Semim,br
& add magm

Sural.\ Nn. gastroc & Sol

fitG6.1

N obtint.

PLATE IV
(Fig. 1)

THE NERVES AND MUSCLES OF THE LEG PLATE IV
CHARLES R. BARDEEN (Fig. 2)

N. quadri . Fem.

Be ——_ N. saph.

N. tib. ant.

N. ext. dig.1.

N. ext. hal. J.

N. peron. prof.

N.bi. cap.br!
N.cut. post.

AMERICAN JOURNAL OF ANATOMY--VOL. VI

THE NERVES AND MUSCLES OF THE LEG : PLATE V
CHARLES R. BARDEEN (Fig. D

Nuil. Ps=

N.ihy
Nn. Heee

N.cuk ani:

N. cut med:
Mayan fem.

saph
an

N
Neeser

N. flex.hal.]

plen : ; s p Pl Shere : n. VIS!
N lex a : ,

N.abd.hal.
=~ > 7 N.castroe.
Sa f Nn. sol
8 f yi. quadr. si

Nnsemif

& Diprox. por

Nn.addm. semi
semita bi. Cop:
IN. bi.cap-br.

<a

Need

~ N, sural. N.cut.post — N.obr.int

N. dig. plant. Of \

Wien:

R prof.

AMERICAN JOURNAL OF ANATOMY--VOL. VI

THE NERVES AND MUSCLES OF THE LEG PLATE V
, CHARLES R. BARDEEN (Fig. 2)

Nil
N.hypoo:

Ning of
.cut.ant.é
N.cut_lat

N. quedri.fem.

N. saph.

N tib.ant.
N.ext.dig L
Newt hatb
— N peron.prof

Mery

Nol. i

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THE NERVES AND MUSCLES OF THE LEG
CHARLES R. BARDEEN

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AMERICAN JOURNAL OF ANATOMY--VOL. VI

THE ARTERIOLA RECTA OF THE MAMMALIAN KIDNEY,’

BY
G. CARL HUBER,
From the Laboratory of Histology and Embryology of the
University of Michigan.

WitH 4 TrExtT FIGURES.

In a comprehensive and relatively recent contribution on the blood
supply of the mammahan kidney, Golubew * calls attention to the differ-
ences of views still existing concerning the minute anatomy of this organ
and states that of these controversial questions special mention may be
made of the “ vasa recta of Henle and Donders or of the arteriole rectee
of authors.” A study of the literature which is fully reviewed by Golu-
bew leads him to state that at the time of his communication three views
were current pertaining to the origin of the arteriole recte. According
to one view, these vessels arise from the vasa efferentia of the glomeruli
which lie nearest to the pyramid of the kidney, a view early expressed by
Bowman who was followed by Gerlach, Kolliker, and Ludwig in their
earlier writings. According to another view, recognition and prominence
are given to the arterial branches forming straight medullary vessels
which arise directly from the renal vessels and their branches without the
interposition of glomeruli and known as the arteriole recte vere. Ac-
cording to a further view maintained by Huschke and otier observers, the
origin of the arteriole rect was traced to the capillary plexuses sur-
rounding the tubules of the cortex of the kidney. Steinach, who denies
the existence of arterial straight medullary branches, presents a view
which cannot be included in the above classification and may be disre-
garded as his observations have not met with acceptance. Virchow and
many other observers who have followed him have to some extent har-
monized these conflicting views by assuming what may be regarded as a
middle position in that they recognize the arteriole rect vere, vessels

1Golubew: Ueber die Blutgefasse der Niere der Saugetiere und des
Menschen. International monatsschr. f. Anat. u. Physiol., Bd. X, 1898. Gives
references to literature appearing before the date of his publication.

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

31

o

392 The Arteriole Recte of the Mammalian Kidney

which arise directly from the renal vessels, but concede the presence of
straight medullary vessels which have their origin in the efferent
branches of the glomeruli situated in the deeper layers of the cortex and
known as the arteriole recte spurize. Golubew, whose very careful work
has justly received merited consideration, describes and figures both
arteriole recte vere and spuriw. A study of the diagrams of the renal
circulation as found in the recent text-books of Anatomy and Histology
warrants the conclusion that the majority of the present day writers be-
lieve in the double origin of the arteriole recte, namely in part directly
from the renal vessels, for the remainder, from the efferent branches of
glomeruli.

Of the various methods that have been used in the study of the arteriolie
rect, the injection methods in one form or another are given preference.
Golubew used colored gelatin masses injected through either the renal
artery or vein, but more particularly a solution of silver nitrate which
was injected into the vessels after these had been thoroughly washed out
with distilled water. The silver nitrate was injected under low pressure
and the injection interrupted as soon as reduction of silver was evident
in the capsule of the kidney, after which the vessels were again washed
out with distilled water, the organ divided into pieces, placed in alcohol
and exposed to light. Free hand sections, dehydrated and cleared in 9il
of cloves and mounted in damar were used for study. Corrosion prepara-
tions of the renal vessels obtained mainly with the celloidin method have
enabled Brédel * and others who have confirmed him to extend our knowl-
edge of the genera! distribution, relations, and manner of termination of
the renal vessels. The results thus obtained have also been confirmed in
Roentgen photographs taken after suitable injection of the renal vessels.
Particular attention was, however, not given to the arteriole recte bv
these observers.

The observations here briefly to be recorded were made on a series of cor-
rosion preparations of the renal vessels of the dog, cat, rabbit, rat, and
guinea pig made after a method which is a modification of one suggested by
Krassuskaja.®

This observer recommended an injection mass consisting of photoxylin
(or celloidin), camphor and acetone, colored by the addition of pigments

rubbed up in acetone. (For a red color, cinnabar is suggested; for a blue,
Berlin blue; a yellow, chrome yellow; a black, asphalt.) The mass may be

*Max Brodel: The Intrinsic Blood-Vessels of the Kidney and Their Sig-
nificance in the Nephrotomy. Proc. Ass. Amer. Anat., 1901.

°* A. Krassuskaja, as reviewed by Stieda in Ergeb. Anat. u. Entwickl., Bd.
XE 1903; pl 521,

G. Carl Huber

©)
we)
Oo

filtered through flannel or muslin. Injection may be made by means of an
ordinary syringe; it is only necessary to fill the cannula with acetone. The
tissue or organ is placed into hydrochloric acid 12 to 24 hours after the in-
jection and is removed from the acid after two or three days and washed in
flowing water. The pigments suggested by Krassuskaja are not soluble in
acetone and are, therefore, only held in suspension. The mass did not seem
to me suitable for injecting capillaries and other exceedingly fine tubular
structures. This led to a modification of it and the mass now used in this
laboratory is as follows:

To obtain a stock solution, 30 grms. of photoxylin are dissolved in 550 ec.
of acetone, which requires about 24 hours. Twenty grms. of camphor are
dissolved in 50 cc. of acetone. The two solutions are then thoroughly mixed

Fie. 1. Injection apparatus: A, pipe carrying water to tank; B, conveying
water from tank and connected with waste-pipe. For further details see text
(page 394).

in a bottle with a well-fitting glass stopper. This stock solution may be kept
for a long time. After experimenting for a long time, it was found that
Alkanin* answered very well for the purpose of a red color. It is readily
soluble in acetone, is not washed out of the preparation with water, alcohol,
or xylol, and is not decolored in the hydrochloric acid. It is the only sub-
stance soluble in acetone and meeting the other requirements which I have
thus far been able to find. The preparation of alkanin first used was one
that had been in the laboratory a long time and had become hard and brittle.
The alkanin, as obtained from Gruebler, is in the form of a thick paste.
Later observations have shown that the dried form answers the purpose
better than a fresher preparation. The injection mass now used is made by

*Fettlosliches Roth, Gruebler, also written Alcanin.

394 The Arteriole Recte of the Mammalian Kidney

adding 0.3 to 0.5 grms. of the alkanin rubbed up in 20 ce. of acetone to 80 ce.
of the stock solution, thoroughly mixed by stirring in a mortar and then
filtered through absorbent cotton with the aid of a Chapman suction pump.
For purposes of injection, I have made. use of compressed air, obtained by
connecting a water tank with the laboratory water pipe. The tank is pro-
vided with a pressure gauge, from which the pressure obtained is read. The
pressure is conveyed to the table by means of a gas-pipe, provided with a stop-
cock. The injection mass is placed in a large glass tube with 3 cm. lumen and
about 20 em. long, held upright by clamping the same to a support. The upper
end of the tube is provided with a perforated rubber cork, which can be
clamped in tightly. A rubber tube leads from the end of the pipe bringing the
compressed air to the table to a short glass tube fastened in the rubber cork,
by means of which the pressure is conveyed to the injection mass. To the
lower end of the glass tube, which tapers, is attached a rubber tube, provided
with a clamp, by means of which connection may be made with the cannuia.
The simplicity of this apparatus commends itself. It is shown in Fig. 1.
As is usual in injections, better results are obtained by injecting a limited
area, that is directly through the blood-vessel supplying the organ to be
studied. It has not been found necessary to wash out the blood-vessels be-
fore injecting. The animal is bled as freely as possible by severing the neck-
vessels before death. The administration of amyl nitrite does not appear to
influence materially the completeness of the injection. The injection is to
be made soon after the death of the animal. The cannula is first filled with
normal salt, and the salt solution renewed by means of a pipette until it re-
mains clear in the cannula. It is then replaced with acetone, which is like-
wise renewed several times to dehydrate the interior of the cannula. If the
area to be injected is very small, a portion of the acetone is withdrawn from
the cannula and this is filled with the injection mass. Better results are ob-
tained by using relatively high pressure. In injecting the renal arteries of
the dog, cat, and rabbit, a pressure of 20 to 25 pounds, as registered by the
gauge connected with the tank, gave the best injection; for smaller animals,
12 to 15 pounds. Better results are also obtained if the full pressure to be
used is thrown onto the mass as quickly as possible. The pressure is main:
tained for five to ten minutes. Before removing the cannula from the vessel,
the vessel should be tied distal to the cannula, so as to avoid a back flow. A
75 per cent solution of hydrochloric acid (sp. gr. 1.20) is used for macerating
the parts to be removed. The entire organ may be placed in this macerating
fluid, or, as it is often desirable to study a corrosion in small pieces, the or-
gan may be cut into segments as desired and these placed into the macerat-
ing fluid. The injected tissue may be placed into the macerating fluid 10 to
20 minutes after the completion of the injection; there is no advantage in
‘waiting 12 to 24 hours, as recommended by Krassuskaja. Pieces with one
diameter not more than 1 cm. are thoroughly macerated in 18 to 24 hours.
The macerated pieces are then transferred to a large dish of water and the
softened tissues removed by playing water against them with a dropper pro-
vided with a rubber bulb. It is not advisaple to use a stream of water with
considerable force, as the delicate parts of the corrosion are likely to be
injured. After the corrosion has been thoroughly cleaned, it may be allowed

G. Carl Huber 395

to dry or it may be studied in water. I have found it advantageous to mount
in balsam the parts to be studied particularly. If this is desired, the thor-
oughly cleansed corrosions are placed in distilled water for several hours,
are then dehydrated in absolute alcohol, transferred to xylol and mounted in
balsam, the cover glass being supported by fragments of glass of the re-
quired thickness. These preparations should be viewed with a binocular mi-
croscope giving stereoscopic vision.

This method has proven very satisfactory in the study of the renal
vessels, as it has often been possible to obtain corrosions in which the
course of the vessels could be readily followed through their several di-
visions until the capillaries are reached. In such preparations, a con-
fusion of arterial and venous branches is not possible. The method en-
ables a definite solution of the course and divisions of the major branches
of the renal artery, of the radiate cortical branches (arteriw interlobu-
lares), of the origin of the afferent branches to the glomeruli and of the
fate of the efferent glomerular branches. It is the purpose at this time
to consider primarily the arteriole recte and other efferent glomerular
branches; a fuller consideration of the renal vessels, both arterial and
venous, is reserved for further contribution.

As is well known, the renal artery, on entering the hilus of the kidney,
divides into branches which, after division, course in the peripheral part
of the pyramid near the junction of the medullary and cortical portions.
(This statement has reference to the kidneys particularly studied, namely,
those with a single pyramid.) These major branches, which in their
course undergo several subdivisions, have a direction which is in the main
parallel to the surface of the kidney; they describe, therefore, ares with
convexity outward, and constitute the arterial branches designated as
arcuate arteries (arterie arciformes). From the convex side of these
arcuate arteries, there arise at intervals of 2 to 5 mm. branches which very
generally form an acute angle with the arcuate artery and pass with
slight inclination toward the cortex. he length of these branches varies,
and from their outer side, that toward the cortex, there arise short
branches at relatively close intervals which pass toward the periphery
of the kidney and very generally subdivide into several branches from
which arise the so-called interlobular arteries (arterixe interlobulares).
The arcuate arteries ultimately terminate in smaller branches which also
give origin to interlobular arteries. Golubew simply states that from the
convex side of these arches (arteriz arcuate), as also from the terminal
divisions, arise the interlobular arteries. Von Ebner® after mentioning

>In Kelliker’s Handbuch, Vol. III, Pt. 1, page 369.

396 The Arteriole Recte of the Mammalian Kidney

the arterie arciformes, states that ‘* from the cortical side of these there
arise with great regularity and mostly at right angles small arteries which,
after several or more repeated divisions, end in fine branches of 135 to
220 mw caliber, which, with a straight course, pass outward between the
cortical fasciculi (Rindenfascikeln) or lobules and are most appropriately
termed the arterix interlobulares.” If the designation arteria arciformes
is retained for arterial branches having an arched course, relatively large
branches arismg from these and passing through two to three further
subdivisions need to be recognized before arterial branches known as the
arterie interlobulares are reached. The usual description of the inter-
lobular arteries is also open to question. Arterial branches passing quite
regularly with radial course through the cortex as generally diagrammed
are seldom met with. This must be evident to one who has had oppor-
tunity to observe numerous kidney sections of material injected with a
colored gelatine mass, as usually given to classes, and to note the relative
infrequency with which sections are met showing interlobular arteries
which may be traced from the deeper portion of the cortex to the per-
iphery. Branching of the interlobular vessels at various levels of their
course is frequently met with; certain ones pass only through a portion of
the cortex, others again break up in the deeper portions of the cortex into
clusters of smaller branches (afferent glomerular branches). These de-
tails are shown in the figures presented. If the interlobular arteries are
to be regarded as associated with vascular units, it must be conceded that
such units must vary greatly in shape and in relative position and recog-
nition must be given to the fact that of the probable functional activity of
each uriniferous tubule structurally associated with an interlobular artery,
only a portion of this functional activity is associated with the portion of
the uriniferous tubule which falls within the vascular area of an inter-
lobular artery, as the loops of Henle of such tubules are generally situ-
ated outside of such a vascular area.

It seemed desirable to discuss thus briefly certain points in the arterial
vascular system of the mammalian kidney in order that emphasis may
be given to the statement that afferent glomerular branches arise from
all the branches of the renal artery, beginning with the arterie arci-
formes, and that all the branches with the few exceptions to be mentioned
terminate in glomeruli. ‘The main exceptions are found in the A.
nutricize pelvis renalis and within the sinus renalis the arterie recur-
rentes. These branches, especially the latter, are readily recognized in
corrosions, although their relations to the structures which they supply
are not evident in such preparations. The arrangement of their terminal

G. Carl Huber 397

branches is, however, such that they are not to be confused with the
arteriole recte. They generally arise from primary branches of the renal
arteries. Other exceptions will be noted later. In corrosions of very
fully injected material, there are often observed small branches arising
generally from the concave side of the arcuate arteries, beginning with
about the third division of these, which can be traced to glomeruli; they
are, therefore, afferent glomerular vessels. ‘hese small branches are not
numerous. ‘Their length varies from 1 mm. to much less than that.
They generally end in only one glomerulus, though now and then such
an afferent glomerular branch divides to supply two glomeruli. On the
branches which arise from the convex side of the arcuate arteries and
through their second and third divisions, afferent glomerular branches be-
come more numerous, the number increasing with each successive division
of these arterial branches. These afferent glomerular branches generally
arise from the under surface (toward medulla) or the sides of these
larger arterial stems, though now and then from the upper surface, in
which event the branch bends downward to reach the respective glomeru-
lus. Such afferent glomerular branches vary in length and arrangement.
Branches ending in a single glomerulus are met with; clusters of two,
three, four, or even more afferent branches, each ending in a glomerulus
are also seen. Numerous afferent glomerular branches arise from the
arterial branches which divide to form the interlobular arteries. Here
also they may arise singly or in small groups or a small arterial twig may
divide into four, six to eight branches, each ending in a glomerulus.
From the interlobular arteries, as is generally stated, arise at all levels
through the cortex and from all sides numerous afferent glomerular
branches. Attention may, however, be drawn to the fact that the ar-
rangement of afferent glomerular branches arising from the interlobular
arteries is not a regular one, single afferent branches or clusters consist-
ing of two to five or even more such branches resulting from a division of
small lateral twigs of the interlobular arteries are met with. The termi-
nal portions of such interlobular branches as reach the periphery of the
cortex, ultimately divide into afferent glomerular branches. The num-
ber of such terminal afferent glomerwlar branches thus formed in the
periphery of the cortex varies with different interlobular arteries. The
figures presented will serve to elucidate this statement. It should, how-
ever, be stated that Figs. 2, 3, and 4 are drawn from actual preparations
and are not composite pictures, and it will be readily understood that not
all of the preparations present all of the details with equal clearness and
perfection. Even when care is taken to cut and tease out certain portions

398 The Arteriole Recte of the Mammalian Kidney

of a corrosion, selected for mounting and special study, which may
readily be done under the binocular stereoscopic microscope, there are
broken off during the manipulation small portions which thus become
detached from the preparation. In Figs. 3 and 4, for imstance, the
peripheral portions of the interlobular arteries are not in every instance
fully injected, giving the impression that certain of the interlobular
arteries present peripheral branches which do not end in glomeruli. Other
corrosions in which the peripheral branches of the interlobular arteries
were more fully injected, but in which the injection in other details was
not wholly successful will serve to show that the interlobular arteries
end at the periphery of the cortex in branches which are recognized as
afferent glomerular branches.

As is well understood, each glomerulus constitutes a rete mirabile, its
branches uniting to form a single efferent vessel, the vas efferens, which
is regarded as an arterial and not a venous structure. The efferent glom-
erular vessels, soon after leaving the glomeruli, divide to form capillaries,
the disposition of which differs in the different portions of the kidney.
The efferent branches of the glomeruli, the afferent branches of which
arise from the arcuate arteries and from the successive branches of these
until the interlobular arteries are reached, as also the efferent branches
of a varying number of the glomeruli the afferent branches of which
spring from the lowermost portions of the interlobular arteries, divide
into bundles of long, slender branches and capillaries which pass into the
medulla of the kidney, constituting the arteriole recte of writers, more
specifically stated the arteriole recte spurizw. The efferent branches of
the remaining glomeruli divide to form capillary plexuses which sur-
round the segments of the renal tubules found in the cortex, the efferent
branches of the glomeruli situated in the outermost portion of the cortex
passing into the peripheral cortical region free from glomeruli, before
forming capillary plexuses. It may here be emphasized that there is
not a difference of kind in the capillary plexuses formed from the effer-
ent glomerular branches in the different parts of the kidney, but one of
plexus arrangement determined by the character and arrangement of the
tubular structures found in the different regions of the kidney.

As has been previously stated, the majority of recent writers recognize
the existence of terminal arterial branches which end in capillaries in
the kidney substance, with which glomeruli are not associated, such
branches conveying arterial blood to the kidney tubule or portions thereof,
which has not passed through a glomerulus. Such branches are recog-

G. Carl Huber 399

nized in the boundary zone and medulla as arteriole rectee vere and in
the peripheral portion of the cortex as end branches of the interlobular
arteries. The criticism may be made that the corrosion method em-
ployed is not suitable for determining the existence or non-existence of
such branches, as the possibility of their being present without being
injected must be considered. It would seem, however, reasonable to sup-
pose that arteriole recte vere should be more readily injected than
arteriole rect spurie, since in injecting the former it would not be
necessary for the injection mass to pass through the glomerular capillaries
before reaching the branches and capillaries constituting the arteriole
recta. The ven recte are very readily injected through the veins. In
the rat, guinea pig, and rabbit and practically without exception in the
cat, the arteriole rectee observed in my corrosions could readily be traced
to the efferent glomerular vessels. In these forms then, the existence of
arteriole rectee vere may be denied with the possibility of very rare ex-
ceptions in the cat. A similar conclusion is reached by Petraroja, whose
account I have, however, seen only in review, as his original publi-
cation was inaccessible to me.” In the dog, I have now and then observed
arterial twigs which terminate directly in arteriole rectee—arteriole rect
veree—these constitute, however, a very small per cent, the great majority
resulting from a division of efferent glomerular branches. In the dog
there may be further observed what may be designated as very small glom-
eruli, which appear fully injected as a capillary network may generally
be made out in the corrosion, the efferent branch ending in typical
arteriole rectee. These very small glomeruli (?) are also not numerous.
Golubew has described and figured for the dog and the cat what he has
termed “ retia mirabilia renum nova” situated in the deeper portion of
the cortical substance and the boundary zone. Being aware of these ob-

servations of Golubew, I sought for confirmation of them in the corrosion —
preparations at my disposal, as it seemed likely that they should be in-
jected as readily as the glomerular vessels. Such retia mirabilia have

not been found, unless, as seems to me probable, what has been spoken of
as very small glomeruli may constitute the structure described by Golubew
as new renal retia mirabilia. From the fact that the efferent vessels of
such structures always end, so far as I have been able to determine, in
arteriole recta, I have been led to conclude that they represent the re-
mains of normal glomeruli associated in their development with urinifer-

®°Petraroja: Sulle arteriole recte del rene. Monit. Zool. ital., Bd. 15, 1904.
Reviewed in Jahresberichte tiber die Fortschritte der Anatomie und Ent-
wickelungsgeschichte. Neue Folge, Bd. X, 3 Abth. 1. Teil, 1905.

400 The Arteriole Recte of the Mammalian Kidney

ous tubules, which tubules have later disappeared, a portion of the glom-
erular plexus with afferent and efferent vessels remaining intact as a
rete mirabile, the degree of retrogressive change varying with different
glomeruli and in some instances going on to a complete obliteration of
the glomerular plexus, the afferent and efferent vessels alone remaining
as a continuous structure. As has been expressed to me—* The mill-
dam remaining after the mill has disappeared and in some few instances
the dam itself disappearing.” According to this hypothesis (for the sub-

Fic. 2. Corrosion preparation of terminal arterial branches from kidney
of dog.

stantiation of which it is difficult to obtain definite data, as my attempts
to obtain corrosion preparations of the arterial system of foetal kidneys
and of kidneys of new-born dogs have not been successful), the arterial
twigs which end in arteriole recte without the interposition of glomeruli
—arteriole rectee vere—and the arterial twigs in the course of which are
found retia mirabilia prior to ending in arteriole rectw are to be regarded
as derived from glomeruli with afferent and efferent vessels, the glom-
erulus in each instance degenerating in whole or in part consequent to
the disappearance of the uriniferous tubule with which said glomerulus

G. Carl Huber 401

was structurally associated. If this interpretation be correct, the arterial
vascular supply of the dog forms only an apparent exception to the gen-
eral statement that all the arteriole recte are formed by a division of
efferent glomerular branches, the few arteriole recte vere noted being
regarded as developed from arteriole rect spurie on the disappearance
of the uriniferous tubule structurally associated with the glomerulus
which thus degenerates. Golubew has further described arterial twigs,
which after division present one branch, which may be very short, and
which forms an afferent glomerular vessel, the other branch passing by
but in close contiguity to the glomerulus and dividing to form arteriole

Fic. 3. Corrosion preparation of terminal arterial branches from kidney
of cat.

recte—thus arteriole recte vere. This I have not observed and must
regard it as an error of observation for which the method used by him
(silver nitrate injection) is responsible, as is clearly the case in the fol-
lowing observation, shown in his Fig. 2, Plate XXIV. Here is shown
an arterial branch having a horizontal course, from the under side of
which there arise several branches (six) which divide to form arteriole
recte vere. ‘The preparation is taken from the base of the renal pyramid
of the dog’s kidney injected with silver nitrate. The structure figured
was undoubtedly a small vein receiving several groups of venule recte,
as in corrosion preparations of the venous system of the dog’s kidney,
such small vessels having a horizontal course and receiving on their under
side small branches formed by the union of straight capillaries are fre-
quently met with, but are traceable to larger venous stems. In corrosion

402 The Arteriolae Recte of the Mammalian Kidney

preparations of the rat’s kidney, I am able to confirm the observation
made by Golubew that the efferent glomerular branches which form the
arteriole rect give off side branches which form capillary networks at
the level of the base of the renal pyramid. Such side branches of the
efferent glomerular vessels destined to form arteriole rectee I have ob-
served also in the kidneys of the other animals studied, though they are
not nearly so numerous as in the white rat. So far as may be determined
in corrosion preparations in which the peripheral portions of the inter-
lobular arteries appeared completely injected, these end in afferent
glomerular branches and do not present terminal branches which end
directly in capillaries in the peripheral portion of the renal cortex. Now
and then, and more particularly in the dog, have I found an interlobular
branch which did not completely break up into branches within the renal
cortex, but could be traced beyond the outer border of the cortex
anastomosing, as would appear, with capsular branches. Afferent glom-
erular branches arise from such interlobular branches to near the per-
ipheral part of the cortex. The question may be asked whether the
“arteria capsularis glomerulifera” described by Golubew may not pe
interlobular arteries of the above type imperfectly injected from the out-
side. As I have not attempted corrosion injection through the aorta after
tying the renal arteries, I am not able to decide this point.

In this account, no mention has been made of observations on the
human kidney with reference to the points more particularly under dis-
cussion. The limited human material at my disposal has not been fresh
enough to enable capillary injection with subsequent corrosion by the
method used. In the attempts made the injection mass could readily be
forced into the glomeruli, but only to a limited extent into the efferent
glomerular vessels, as a rupture of the glomerular vessels in many places
allowed its escape into the space enclosed by Bowman’s capsule and then
into the uriniferous tubules in which it would pass to about the begin-
ning of the descending limb of the loop of Henle, giving excellent and
instructive corrosions of the proximal convoluted portions of the urinifer-
ous tubules. Similar observations were made on injecting through the
renal vessels of the kidneys of animals several hours after death. It is
hoped that this method may prove useful in the hands of others with
access to fresh human material in determining the origin of the arteriole
recte and the existence or non-existence of terminal arterial branches not
directly associated with glomeruli.

From observations made on corrosion preparations of the dog, cat,
rabbit, guinea pig, and rat, in which it is possible to trace the renal

G. Carl Huber 403

arteries through their severaji branchings to their termination, including
the branches which go to the glomeruli, the glomeruli themselves, the
branches leaving the glomeruli, and often the capillary plexuses formed
by these, the conclusion seems warranted that practically all of the blood
found in the capillaries surrounding the different portions of the urinifer-
ous tubules is blood that has first passed through the glomerular vessels.
This was so clearly stated by Bowman‘ many years ago that it seems but
just to use his own words to give further emphasis to this point. In
Bowman’s classical contribution to the anatomy of the kidney is found

Fic. 4. Corrosion preparation of terminal arterial branches of kidney of
rabbit.

the statement: ‘ According to my own _ observations, the circulation
through the kidney may be stated te be as follows:—AII the blood of the
renal artery (with the exception of a small quantity distributed to the
capsule, surrounding fat, and the coats of the larger vessels) enters the
capillary tufts of the Malpighian bodies; thence it passes into the capillary
plexus surrounding the uriniferous tubes and it finally leaves the organ
through the branches of the renal vein.” With this clear and correct
statement of facts, dating back to 15842, it is somewhat surprising that
even at the present time, there should be a question as to the existence or
non-existence of terminal branches of the renal artery which end in

“Bowman: On the Structure and Use of the Malpighian Bodies of the
Kidney, with Observations on the Circulation through that Gland. Philosoph.
Trans. of the Royal Society of London, 1842, p. 57.

404 The Arteriole Recte of the Mammalian Kidney

capillaries about uriniferous tubules without being directly associated
with glomeruli.

There are, as is well Known, two leading and opposing theories on the
nature of urinary secretion tersely stated by Hans Meyer as follows in a
recent summary of observations on renal function: “According to one of
these theories, which was developed most fully by Heidenhain, we have to
deal with a true secretory process by which water and perhaps the salts pass
through the glomerulus, whereas the specific constituents of the urine are
liberated from the tubules, so that the sum of both secretions is represented
by the outflowing urine. According to the other hypothesis, which was first
proposed by Ludwig and subsequently modified by his successors (in a bio-
logical sense), there goes on in the kidney, side by side with the glomerular
activity, dependent essentially on the mechanical conditions of the circulation,
and independently also on the secretion of certain urinary constituents, a
process of resorption in the urinary tubules. Through this resorption the
slightly concentrated secretion of the glomerulus, corresponding to the water
of the blood, undergoes concentration to a point characteristic of the urine.’

It is not my purpose here to discuss either of these theories. It may, how-
ever, be permitted to call brief attention to certain points in the structure of
the uriniferous tubules in connection with an account of the relations of ter-
minal branches of the renal arteries, points which, it seems to me, shoul:
be considered by the followers of either of the leading theories on the nature
of urinary secretion.

In each uriniferous tubule, including the glomerular capsule, there may
be recognized four types of epithelium with distinct regional distribution.
(1) The flattened epithelial cells lining the glomerular capsule continuous
with the flattened epithelium, probably syncitial in character covering the
glomerulus; (2) the epithelium of the proximal convoluted portion, colum-
nar in shape with striated protoplasm and striated inner border; (3) the
peculiar flattened epithelium of the descending limb of Henle’s loop; (4) the
short columnar epithelium of the ascending limb, the distal convoluted por-
tion and a portion of the junctional tubule, an epithelium with indistinct
cell outline, with basal striation, but differing in structural detail and in re-
action to stains from the epithelium of the proximal convoluted portion.
These four types of epithelium are found, not only in the uriniferous tubules
ot the mammalian kidney, as determined by reconstruction in this laboratory,
but also in the tubules of the simpler reptilian kidney (recently reconstructed
in this laboratory and to be described in another conimunication) as also in
the tubules of the amphibian kidney (mesonephros). The epithelium of the
neck of the uriniferous tubules is here not especially considered; it differs
from the other epithelia described, though it probably has little functional
significance. If it is true, as stated by Starling in introducing the section on
“The Mechanism of the Secretion of Urine” in Schaefer’s Text-book of
Physiology, tuat “a difference of function is invariably associated with a
difference of structure, so that the interdependence of function and structure
has become an axiom,’ we should be justified in postulating a difference of
function to the different parts of the uriniferous tubules lined by the differ-
ent types of epithelium, and the extent to which this may be done is, as it

G. Carl Huber 405

appears to me, briefly as follows: The weight of evidence appears to sub-
stantiate the statement that the water, the sodium chloride and urea and
probably other substances existing in a free state in the blood are secreted
(pass out by filtration or transudation) by the glomerular epithelium. It is
estimated that 1-12 to 1-14 of the volume of blood entering through the af-
ferent glomerular vessels is abstracted during the course of the blood through
the glomerular vessels, so that the blood leaving the glomeruli is thus pro-
portionately concentrated. To this fact attention may be especially drawn,
since, as has been shown, practically all the blood found in the capillaries
surrounding the different parts of the uriniferous tubules is blood which has
passed through the glomeruli. Uric acid and phosphoric acid appear to be
specifically secreted, as their quantity cannot be increased by any of the
known diuretics—Hans Meyer. The evidence is in favor of connecting this
specific secretion with the epithelium of the proximal convoluted tubules. In
confirmation of this may be cited the sodium sulphindigotate experiments
of Heidenhain and Ribbert, the carmine injection experiments of Schmidt
and Ribbert, the detection of uric acid granules in this epithelium and the
presence of what has been regarded as secretory granules in the same
epithelium. To what extent the presence of concentrated blood found in
the capillaries surrounding the proximal convoluted tubules, having,
as may be assumed, a larger per cent of uric acid, since this is
apparently not secreted by the glomerular epithelial, favors the se-
cretion of this substance by the epithelium of the proximal convoluted
tubules, cannot be stated. The possibility of its doing so may, how-
ever, be considered. The experimental evidence appears to favor the
view that there is a compensatory resorption of water (probably also certain
salts in proportion to their diffusibility or permeability of the renal cells—
Cushny) during the passage of the renal secretion through the tubules. That
this resorption of water takes place in the loops of Henle is probable from
the experiments of Ribbert, and Hausman and Hans Meyer who obtained an
increased flow of urine of less concentration after removing the medullary
portion of one kidney following extirpation of the other. The suggestion is
here made that this resorption, more especially of the water, takes place in
the descending limb of Henle’s loop, largely owing to the peculiar flattened
epithelium possessed by it. That the loops of Henle are longer than gen-
erally thought is shown by reconstruction, the larger per cent extending
through or nearly through th entire medulla. These segments of the urinif-
erous tubules are in relation with capillaries conveying concentrated blood,
favoring a resorption, Since, as has been shown, the arteriole recte are
formed almost without exception by a division of certain of the efferent
glomerular vessels. The blood passing to the medulla through the arteriole
recte is returned by the venule recte#, which are, if one may judge by corro-
sion preparations, much more numerous than the arteriole. The loops of
Henle are, therefore, in relation with numerous capillaries. Whether special
function may be ascribed to the ascending limb of Henle’s loop and the distal
convoluted portion, which again has a special epithelium, is difficult to
state. Heidenhain believed these tubular segments to possess a secretory
function similar to that possessed by the proximal convoluted portions, bas-

406 The Arteriole Recta of the Mammalan Kidney

ing his conclusions on observations made after injecting sodium sulphindigo-
tate. It would seem, however, that absorption of the dye by the epithelium
of the ascending limb of Henle’s loop and distal convoluted portions, after a
concentration as a result of absorption of water in the descending limb is not
excluded. Ribbert states distinctly that ‘‘a secretion of specific substances
takes place only in the convoluted tubules of the first order, while in the
loop of Henle, the distal convoluted portion and the collecting tubules, there
takes place exclusively or for the greater part a resorption of water.” He
further draws attention to the fact that in normal kidneys of older individ-
uals there are often found pale yellow granules, contained exclusively in the
epithelium of the distal convoluted portions and parts of the loops, and,
further, that toxic agents secreted by the kidney affect first the glomeruli
and then the distal convoluted portions and in part the loops. It would ap-
pear, therefore, that a resorption takes place from these tubular segments
perhaps of more specific substances than from the descending limb of Henle’s
loop.

(Excellent reviews of the literature bearing on renal secretion may be
found in a number of recent publications—Ribbert, Untersuchungen tiber dis
Normale und Pathologische Physiologie und Anatomie der Niere, Bibliotheka
Medica, 1896; Hans Meyer, Herter Lectures, Bull. Johns Hopkins Hospital,
Noy. and Dec., 1905; R. Metzner, Die Absonderung und Herausbeforderung des
Harnes, Nagel’s Handbuch der Physiologie des Menschen, Bd. II, Erste Halfte,
1906,—to which the interested reader is referred.)

THE PHYLOGENY OF THE PLANTAR MUSCULATURE.

BY
J. PLAYFAIR McMURRICH.
From the Anatomical Laboratory of the University of Michigan.

WitH 9 TExT FIGURES.

In three papers which have appeared in this JourNAt I have given the
results of a comparative study of the flexor muscles of the antibrachium,
hand and crus, and have shown that in each of these parts there is an
arrangement of the musculature in definite layers, which can be identified
in the amphibia, reptilia and mammalia. And, further, it was shown that
there is a close correspondence in the arrangement of the musculature of
the antibrachium and crus in the lower forms. There remain to be deter-
mined the existence of an arrangement in primary layers in the plantar
musculature and the correspondence of these layers with those occurring
in the palm. In the present paper I shall consider especially the question
of the primary layers of the plantar musculature and their differentiation.

The material which has served for this study consisted of series of
transverse sections of the same forms that were employed in my study of
the crural flexors, 04, except that, through the courtesy of Dr. M. J.
Greenman, Director of the Wistar Institute of Anatomy, I have been able
to add to the mammalian series a representative of the Insectivora, Scap-
hanus sp.?, which, unfortunately, however, proved to be of only subordi-
nate value for my purpose, owing to the extensive transformation of the
plantar musculature into tendinous structures. I have also had oppor-
tunity for studying the plantar muscles of Jguana tuberculata, through
the courtesy of my colleague, Dr. J. E. Reighard.

I. THE PLanTar MUSCLES OF THE URODELE AMPHIBIA.

The plantar muscles of Amblystoma are arranged in four primary lay-
ers, which correspond, layer for layer, with those occurring in the palm.
In a transverse section through the foot a little distal to the bases of the:
metatarsal bones, the arrangement represented in Fig. 1 is seen. Super-
ficially, immediately beneath the integument, is the strong plantar
aponeurosis (pa), beneath which lies a continuous layer of muscle tissue,

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.
32

408 The Phylogeny of the Plantar Musculature

the flexor brevis superficialis (fbs). Dorsal to this is a layer, consisting
at this level of four distinct portions, which is the flexor brevis medius
(fbm) ; resting directly upon the metatarsals is the third layer, showing
indications of division into a number of subordinate portions, and form-
ing the flexor brevis profundus (fbp) ; and, finally, extending between the
adjacent surfaces of contiguous metatarsals, are the representatives of the
fourth layer, the intermetatarsales (vm).

The plantar aponeurosis and flexor brevis superficialis. The plantar
aponeurosis is the direct continuation of the strong aponeurosis which
covers the muscles of the crus, and over the metatarsals it divides into
five slips, which pass to the various digits; the slips to the hallux and

Fig. 1. Transverse section through the foot of Amblystoma. fbm = flexor
brevis medius; fobp—flexor brevis profundus; fbs=— flexor brevis super-
ficialis; im Zintermetatarsales; 7p—lJateral plantar nerve; mp —medial
plantar nerve; pa—plantar aponeurosis; J/-V —=metatarsal bones.

minimus had already separated at the level of the section shown in Fig. 1.
More proximally, over the tarsals, the aponeurosis receives upon its dorsal
surface the insertion of the majority of fibers of the plantares profundi
of the crus, these muscles acting on the phalanges through the aponeuro-
sis. In tracing a series of sections from the crus downwards into the foot
one finds the plantares gradually diminishing in size as their fibers insert
into the aponeurosis, until they are represented only by a few slips which
are prolonged further distally than the main masses of the muscles. But
just as one begins to expect these slips to completely disappear, they begin
to enlarge and more distally form the continuous sheet of muscle which
is represented in Fig. 1 as the flexor brevis superficialis, this muscle, ac-
cordingly, appearing to be the direct continuation of the plantares pro-

J. Playfair MeMurrich 409

fundi. The continuity is, however, probably merely an apparent one, the
fibers of the flexor brevis superficialis beginning to arise from the plantar
aponeurosis before those of the plantares profundi have completed their
insertion, so.that there is a confusion of the two groups of muscles. The
fact that one finds, first the continuous sheet of the plantares, then for a
short distance three slender slips separated by portions of the plantar
aponeurosis, and then again a continuous sheet of flexor brevis superfi-
cialis, seems to indicate that one has to do with two distinct muscles, es-
pecially when comparison is made with the arrangement in the hand, and
when it is noted that the portions of the superficial flexor which pass to
the marginal digits arise from the aponeurosis independently of the
plantares, the portions continuous with these muscles passing only to the
three central digits.

Fic. 2. Transverse section through the metatarsals of the third and fourth
digits of Amblystoma near their heads. fm and fm’—=central and lateral
slips of flexor brevis medius; fp—slips of flexor brevis profundus; fs and
fs’ central and lateral slips of flexor brevis superficialis; im = intermeta-
tarsalis; pa— plantar aponeurosis; JJJ and /V = metatarsal bones.

If the plantar aponeurosis and the flexor brevis superficialis be traced
distally they will be found to split into as many slips as there are digits,
the prolongations of the aponeurosis inserting into the terminal phalanges.
In the muscle slips destined for the third and fourth digits the marginal
portions (Fig. 2, fs’) separate and pass to an insertion into the sides of
the heads of their metatarsals, these insertions being closely associated
with those of the flexores breves profundi. A little more distally the
central portion of each slip (fs) begins to undergo a transformation into
connective tissue and gives rise to a tendon which applies itself to the
dorsal surface of the slip derived from the plantar aponeurosis and fuses
with it over the base of the first phalanx, the muscle fibers on either side
of this central tendon inserting into the sides of the fibro-cartilages over
the metatarso-phalangeal joint.

410 The Phylogeny of the Plantar Musculature

In the ship to the second digit there is a similar transformation of the
median portion into tendon and an insertion of the muscle fibers adjacent
to this tendon into the metatarso-phalangeal fibro-cartilages, but there is ”
only one slip passing to the head of the metatarsal, namely, that to the
fibular side. The slip to the fifth digit behaves essentially lke that to
the fourth or third, the only striking difference being the large size of the
fibular metatarsal slip; but that to the first digit differs from the rest in
that it fails to separate into subordinate slips, but inserts entirely into the
metatarso-phalangeal fibro-eartilages.

In addition to the portions of the flexor brevis superficialis described
above, another portion is probably represented by the abductor quintt
digiti, or, as it may be more accurately termed, the abductor ossis meta-
tarsi V., which arises from the fibular border of the tarsus and inserts
into the base of the fifth metatarsal, a sesamoid cartilage being developed
at its insertion.

The flexor brevis medius takes its origin from the aponeurotic layer
which lies immediately dorsal to the plantares profundi. It appears as
four distinct slips, one of which (Fig. 1, fom **) later divides, so that
there is a slip for each digit. Toward the distal ends of the metatarsals
the slips which pass to the third, fourth, and fifth digits divide into two
portions, one of which (Fig. 2, fm), much smaller than the other, hes
upon the plantar surface of the mediam slip of the corresponding flexor
brevis profundus, while the other portion (fm’) rests upon the fibular
slip of the same muscle. This latter portion inserts into the side of the
head of its metatarsal, in more or less close association with the fibular
slip of the flexor brevis profundus, and the smaller portion inserts into
the metatarso-phalangeal fibro-cartilage. The slips to the second and
first digits do not divide in this manner, but insert entirely into the
articular fibro-cartilages.

The flezor brevis profundus is composed of three slips for each digit, a
median and two lateral (Figs. 1 and 2). The lateral slips arise from the
tarsal bones, and, in the cases of the marginal digits, partly from the
plantar aponeurosis. The median slip, on the other hand, arises from the
plantar surface of its metatarsal, and in the central digits separates the
lateral slips, which, up to the level of its appearance, form a single mass.
The lateral slips insert into the heads of the metatarsals, the fibular slips
of the four tibial digits being intimately associated with the intermeta-
tarsals, and the same slips of the third, fourth and fifth digits with the
fibular slips of the flexor brevis medius for those digits. The median slips,
which are the metatarso-phalangei of Humphry, 72, extend further dis-

J. Playfair McMurrich 411

tally and insert in all five digits into the metatarso-phalangeal fibro-
cartilages.

In the third and fourth digits inter-phalangeal muscles, the phalangei
of Humphry, also occur, passing from the plantar surface of the proximal
phalanx to the base of the second one.

The intermetatarsales (Fig. 1, im), extend obliquely across the inter-
metatarsal spaces from the fibular to the tibial side. They are four in
number, arising from the tibial sides of the bases of the second, third,
fourth and fifth metatarsals, and inserting into the fibular sides of the
heads of the first, second, third and fourth metatarsals in association
with the fibular slips of the flexor brevis profundus of those digits.

The lateral plantar nerve is, as I have shown elsewhere, 04, the con-
tinuation into the foot of the ramus superficialis fibularis of the crus,
while the medial plantar is the continuation of the ramus profundus.

In the proximal tarsal region the lateral plantar nerve lies immediately
upon the fibular border of the fibulare and the medial plantar upon the
centrale. When the flexores brevi profundi appear they lie between the
nerves and the bones, and still more distally, after the flexores breves
medii have appeared, the nerves are situated between these muscles and
the flexores breves profundi, the medial plantar over the interspace be-
tween the second and third metatarsals and the lateral plantar over that
between the fourth and fifth (Fig. 1, mp and Ip). The medial nerve
gives off branches both medially and laterally, the lateral one meeting a
medially directed branch from the lateral plantar opposite the inter-space
between the third and fourth metatarsals, so that it becomes difficult to
determine from which of the two nerves the branches to the muscles arise.
It would seem, however, that the lateral plantar supphes all the muscles
of the fifth digit and those inserting into the fibular side of the fourth,
while the remaining plantar muscles are supplied by the medial nerve.
Certain it is that the terminal cutaneous branches of the two nerves are
distributed in such a way that the contiguous surfaces of the four tibial
digits are supplied by the medial plantar and those of the fourth and
fifth digits by the lateral plantar, the lateral surface of the minimus and
the medial surface of the hallux being supplied by branches which descend
from the crus.

This distribution differs materially from that described by Humphry,
72, for Cryptobranchus. In that form the lateral plantar was found con-
tributing to the supply of the third and second digits. In amblystoma it
does not extend tibially beyond the fourth digit, the intermetatarsal be-
tween the fourth and third digits, for instance, being supplied by the
medial plantar.

412 The Phylogeny of the Plantar Musculature

II. THe PLANTAR MUSCLES OF THE LACERTILIA.

The manus of the lacertila compared with that of the urodeles showed
a considerable increase in the number of muscle layers, the four urodelan
layers being represented by seven. In the pes a similar increase occurs,
but it is not carried to quite the same extent as in the manus, the flexor
brevis medius layer being divided into only two layers instead of three. *

In a previous paper, 04, I showed that the aponeurosis of the crural
flexors is, in the lacertilia, divided into a superficial and a deeper layer.
The superficial layer is continued into the planta as a well marked aponeu-
rosis (Fig. 3, pas) intervening between the integument and the flexor
brevis superficialis, and contains several thickened bands which pass to

ia. 3. Transverse section through the foot of Scincus, near the bases of
the metatarsals. ab*—abductor quinti digiti; fom, =flexor brevis medius
str. profundum; fom, — flexor brevis medius str. superficiale; fbp, and fbp =
fibular and tibial slips of flexor brevis profundus; fbs = flexor brevis super-
ficialis str. superficiale; im — intermetatarsal ligaments; Jp = lateral plantar
nerve; mp —medial plantar nerve; pa; and pa, —superficial and deep layers
of the plantar aponeurosis.

the digits and insert with the tendons of the flexor brevis superficialis.
The layer is especially developed towards the fibular side of the foot, pass-
ing in Scincus to all the digits except the first, but in Iguana being lim-
ited to the third, fourth and fifth, only an exceedingly thin layer of fascia
covering the muscles passing to the first and second digits. The slip to
the minimus is a strong triangular sheet which easily separates from the
rest of the aponeurosis.

The flexor brevis superficialis (Fig. 3, fobs) les immediately beneath
the superficial plantar aponeurosis and consists of a stratum superficiale

=u

J. Playfair McMurrich 413

and a stratum profunduf. The stratum superficiale (Fig. 4, fbs), takes
its origin from the sesamoid bone developed in the tendon of the crural
plantaris profundus II-III, and, therefore, from a portion of the super-
ficial aponeurosis. In Scincus it forms a continuous sheet, lying at first
to the medial side, of the terminal portion of the plantaris superficialis
lateralis and resting directly on the continuation of the tendon of the
plantaris profundus II-III. As this tendon divides into slips for the five
digits, the flexor superficialis divides into corresponding portions, each of
these, as a rule, again dividing into two slips, which insert into either side
of the base of the proximal phalanx, the tendon of the plantaris profundus
II-III passing between them. The slip to the hallux could not be traced

se

Fic. 4. Transverse section through the foot of Scincus near the heads of
the second metatarsal. fbm, —flexor brevis medius str. profundum; fom, =
flexor brevis medius str. superficiale; fbp, and fop,—fibular and tibial slips
of flexor brevis profundus; fbs, — flexor brevis superficialis str. profundum;
fbss = flexor brevis superficialis str. superficiale; im—<intermetatarsal liga-
ments; pa, and pa, —deep and superficial layers of the plantar aponeurosis;
II-IV = metatarsals.

to the phalanx, but faded out over the tendon of the plantaris before
reaching the metatarso-phalangeal joint.

In Iguana the muscle, though well developed, is limited in its insertion
to the three tibial digits, the slip for the hallux early separating from the
rest of the muscle and no slips passing to either the fourth or the fifth
digit.

In addition to the flexor brevis superficialis the abductor quinti digiti
(Fig. 3, ab°) is probably to be assigned to the superficial plantar layer.
In Scincus it is a small muscle which arises from the surface of a strong
ligament extending from the fibular surface of the proximal tarsal bone to
the base of the fifth metatarsal. As was the case with the corresponding
muscle in the urodeles, its assignment to the superficial plantar Jayer is
not beyond question, although it is indicated by the position of the muscle

414 The Phylogeny of the Plantar Musculature

and by the fact that it is supplied by a branch given off from the lateral
plantar nerve before it bends dorsally to reach its final position between
the middle and deep layers of flexors.

The stratum profundum (Fig. 4, fbsp) of the flexor superficialis is
represented by two muscles which take their origin from the plantar sur-
face of the tendon of the plantaris profundus II-III before it separates
into its terminal slips. The muscle lies in the intervals between the second
and third and third and fourth of these terminal slips, and some of their
fibers arise from the slips passing to the third and fourth digits. The
more tibial muscle is directed fibularly in its distal course, and, passing
over into a tendon, is inserted into the tibial side of the base of the proxi-
mal phalanx of the third digit, in close proximity to the slip of the flexor
brevis medius str. superficiale to that digit. The more fibular muscle has
almost the same relations, except that it fuses with the large slip of the
flexor brevis medius str. superficiale to the fourth digit, forming a mus-
cular mass which completely invests the plantaris profundus III-IT ten-
don to the digit. The relations of the muscles are practically identical in
both Scincus and Iguana; they seem to correspond to the muscles 7 and €
of Gadow’s, 82, second layer and to those numbered 17 and 18 by Perrin,
93.

The flexor brevis medius is represented by two distinct muscle layers.
The stratum superficiale (Figs. 3 and 4, foms), lies immediately dorsal to
the tendons of the plantaris profundus III-IT, and in Scincus consists at
its origin of three portions. The tibial and middle portions arise from
the plantar surface of the base of the second and fourth metatarsals and
from the connective tissue covering those bones, while the fibular portion,
much the strongest of the three, has an extensive origin from the fibular
border of the fifth metatarsal. The tibial and fibular portions retain their
individuality throughout, the former passing distally and tibially to be
inserted into the first metatarso-phalangeal fibro-cartilage, while the latter
passes to the corresponding structure of the fourth digit. The middle
portion divides into two slips, which pass respectively to the metatarso-
phalangeal fibro-cartilages of the second and third digits (Fig. 4,
Usa ae

In Iguana the arrangement of the layer is essentially the same as in
Scincus, although there are some differences in detail. The muscles
take origin in part from the tarsal bones, instead of from the meta-
tarsals, and in part receive numerous fibers from the dorsal surfaces of
the tendons of the plantaris profundus III-II. The tibial portion is
strong and quite independent of the others; the median portion, which

J. Playfair MeMurrich 415

has a considerable origin from the dorsal surface of the plantaris tendon
as well as from the tarsus, passes to the second and third digits; while
the fibular portion, which is strong, as it approaches the fourth meta-
tarso-phalangeal joint, invests the fourth plantaris tendon and comes
into such intimate connection with the fibular slip of the flexor brevis
superficialis str. profundum as to be unseparable from it.

This layer seems to correspond to Gadow’s, 82, second plantar layer
(less the slips 7 and ¢ already referred to the flexor brevis superficialis
str. profundum) together with slip a of his third layer. His second
layer contains no slip to the hallux, while his third layer possesses two
slips to that digit, one of which presents an appearance and arrangement
similar to the hallucal slip of the flexor brevis medius str. superficiale
of Scincus and Iguana. The same slip is described by Hoffmann, go,
as the tarso-digitalis primus and by Perrin, 93, as No. 30 flexor of the
first phalanx. The latter autnor.describes the middle slip as 91, tarso-
flexor of the digits, and the fibular slip as the external portion of
18, flexor of the fourth phalanx, the internal portion of that muscle
being the slip of the flexor brevis superficialis str. profundum to the
fourth digit.

The flexor brevis medius stratum profundum (Figs. 3 and 4, fbmp)
is a thin sheet which hes immediately dorsal to the str. superficiale and
is separated from the flexores breves profundi by the deep branches of
the plantar nerves. It arises from the bases of the fifth and fourth
metatarsals and, to a certain extent, from that of the third, and its
fibers are directed obliquely distally and tibially. It divides over the
shafts of the metatarsals into four slips which pass to the fibular side
of the metatarso-phalangeal fibro-cartilage of the first, second, third,
and fourth digits. This muscle corresponds to the third plantar layer
of Gadow, 82, with the omission of slip a, and to the deductors of Perrin,
93.

The flexores breves profundi (Figs. 3 and 4, fbp). These muscles
form a layer resting directly upon the metatarsals and separated from
the flexor brevis str. profundum by the deep branches of the plantar
nerves. They form in Scincus ten slips, which do not, however, corre-
spond by pairs to the five digits. So far as the three tibial digits are
concerned a paired arrangement is clear, although the muscles for each
digit arise from the next adjacent metatarsal. The fourth digit, how-
ever, has three slips attached to it, and the fifth only one, an arrange-
ment which may indicate a transference of one of a pair corresponding
to the fifth digit to the fourth.

416 The Phylogeny of the Plantar Musculature

Of the slips which pass to the metatarso-phalangeal fibro-cartilages of
the first, second, and third digits, one (fbp:) arises from the tibial sur-
face and the other (fbp;) from the plantar surface of the next adjacent
metacarpal on the fibular side. In the case of the fourth digit one slip
(Fig. 4, fbp’:) arises from a strong ligament which extends distally
from the cuboid; a second slip (fbp*;) arises partly from this same liga-
ment, which, much reduced in size, accompanies it throughout its course,
and partly from the tibial surface of the fifth metatarsal; while the
third (fbp’:) takes its origin from the head of the fifth metatarsal in
close association with the intermetatarsal ligament. All three slips
insert into the metatarso-phalangeal fibro-cartilages of the fourth digit.

The third slip from its position might readily be interpreted as a
portion of the flexor brevis medius, stratum profundum. The fourth
digit, however, has another slip which is plainly a part of that layer,
and, furthermore, the deep branch of the lateral plantar nerve passes
to its deep position between the slip under discussion and the flexor
brevis medius, the slip, therefore, lying practically dorsal to the nerve
layer and having the same relation to it as the other deep flexors. Its
identification as one of these makes it seem probable that it really rep-
resents one of a fifth pair, its fellow being a slip (Fig. 3, fbp’ ¢), which
arises from a ligament extending between the talo-calcaneus and the
base of the fifth metatarsal. It passes distally upon the fibular surface
of the metatarsal, parallel to the lower part of the plantaris superficialis
lateralis, from which it is separated by the lateral plantar nerve, and
inserts into the fifth metatarsal near its distal extremity.

In Iguana I find essentially the same arrangement of the flexores
breves profundi, although there are slight differences in detail. The
first and second digits each receive two slips, but the slip to the third
. digit could not be divided into two portions. It arose, however, partly
from the tibial surface and partly from the plantar surface of the
fourth metatarsal and consequently agreed with the two slips found in
Scincus. To the fourth digit three slips can be distinguished, of which
that corresponding to fbp*; is much the most prominent and completely
conceals fbp':, which is represented by a narrow and thin band of fibers,
inseparable at its origin from fbp*;, although diverging from it distally.
The third slip, which arises from the base of the fifth metatarsal, is quite
small and after a short course unites with the fourth intermetatarsal.
The single slip to the fifth digit is much stronger than in Scincus, cov-
ering the whole plantar surface of the metatarsal and having upon it
in the median line the tendon of the flexor plantaris profundus.

J. Playfair McMurrich 417

The intermetatarsales. In Scincus and Iguana these muscles have
the same structure and relations as the corresponding muscles of the
hand. They are represented by four slender tendons (Figs. 3 and 4 im)
which pass to the bases of the proximal phalanges of certain digits from
certain metatarsals. The first tendon passes from the first metatarsal
to the phalanx of the second digit; the second from the second meta-
tarsal to the phalanx of the third digit; the third from the third meta-
tarsal to the phalanx of the fourth digit; and the fourth from the fifth
metatarsal to the phalanx of the fourth digit. Consequently the fourth
digit receives the insertion of two of the tendons and the second and
third digits each receive one.

The nerve supply of the plantar region presents some interesting
differences from what obtains in the urodeles. At the level of the ankle
joint the medial plantar nerve or ramus profundus is situated deeply,
resting upon the talo-calcaneus dorsal to the plantaris profundus I,
while the lateral plantar or ramus superficialis fibularis hes upon the
dorsal surface of the plantaris superficialis lateralis and has, therefore,
a plantar position with reference to the plantares profundi (see Fig. 5
of my paper on the crural flexors,04). The medial plantar retains
its deep position as it is traced onwards into the foot, the flexores breves
profundi, however, appearing between the nerve and the metatarsals so
that the nerve comes to lie between these muscles and the flexor brevis
medius str. profundum over the line of the second metatarsal (Fig. 3,
mp). Over the proximal half of that bone it gives off a branch which is
supplied to the various slips of the flexor medius and profundus sets
of muscles which are inserted into the hallux, apparently also to the slip
of the flexor superficialis which passes to that digit, and is finally dis-
tributed to the adjacent sides of the first and second digits. The re-
mainder of the nerve continues its distal course, bending slightly towards
the fibular border of the foot so that it comes to lie at first over the
second intermetatarsal space and then over the fibular border of the third
metatarsal. It gives off a branch to the fibular side of the second digit
and toward the head of the third metatarsal it divides into two terminal
branches which supply the sides of the third digit. The peculiar condi-
tion is thus produced that the muscular distribution of the nerve is con-
fined to the muscles inserting into the hallux, while its cutaneous dis-
tribution extends over the three tibial digits.

The lateral plantar behaves quite differently. Beyond the ankle joint
it continues its course lying over the fifth metatarsal between the termi-
nal portion of the plantaris superficialis lateralis and the abductor quinti

418 The Phylogeny of the Plantar Musculature

digiti; but just opposite the insertion of the latter muscle it bends dor-
sally (Fig. 3, /p), passing between the terminal portion of the abductor
on the one side and the hallucal long flexor tendon and slip of the flexor
brevis superficialis in the other, giving off at the same time two branches.
One of these remains superficial and is continued along the medial border
of the fifth digit, while the other passes dorsally with the main stem of
the nerve and then bends medially to be supplied to all the slips of the
flexor brevis medius str. superficiale except that which passes to the
hallux. The main stem when it reaches the interval between the flexor
brevis medius str. profundum and the flexores breves profundi (Fig. 4)
makes an abrupt bend and passes medially as far as the line of the second
metatarsal, passing dorsal to the main stem of the medial plantar.
Without going into details regarding the various branches given off by
the nerve in this deep portion of its course, it may be said that it sup-
plies all the portions of the flexor brevis medius and flexor profundus
layers except those which pass to the first digit, and that its cutaneous
distribution is limited to the fourth and fifth digits.

This condition is very different from what cccurs in Amblystoma, in
which the medial plantar has the major supply of the pes. It would
seem that there has been a shifting of fibers from the profundus to the
fibular superficial stem, so that muscles originally supplied by the former
are, in the lacertilia, supplied by the latter, and it is interesting to note
that the transference has taken place to a much greater extent in con-
nection with the motor fibers than with the cutaneous ones, so that the
sensory supply of the medial plantar extends to digits where muscles are
entirely supplied by the lateral plantar.

In making a comparison between the muscles of the urodele and
lacertilian it is evident that the nerve supply fails to give any criterion
for homology and there is left only the evidence from topographic rela-
tions. This, however, yields results which seem conclusive.

The homologies may be briefly stated in the form of a table, no dis-
cussion seeming to be necessary except in regard to the flexor brevis
medius. In this muscle, as has been noted, there are two layers in the
lacertilia, while only one was recognized in the amphibia. There seems
little room for doubt, however, that indications of the double layering
exist in the urodeles, each slip of the medius in these forms dividing into
a more tibial and smaller portion and a larger fibular portion. These lie
practically side by side and therefore do not represent exactly the con-
dition in the lacertilia, but nevertheless it seems probable that the tibial

J. Playfair McMurrich 419

slips are the urodele equivalents of the lacertilian stratum superficiale.
If this view be adopted the homologies of the muscles in the two groups
may be tabulated as follows:

Urodeles. Lacertilia.
Flexor brevis superficialis stratum
: Soper superficiale.
eee Cray Supericialis Flexor brevis superficialis stratum
profundum.

Flexor brevis medius fasciculus Flexor brevis medius stratum super-
tibialis. ficiale. ‘

Flexor brevis medius fasciculus Flexor brevis medius stratum pro-

fibularis. fundum.
Flexores breves profundi. Flexores breves profundi.
Intermetatarsales, Ligg. intermetatarsalia.

III. THe PLantar MUSCLES oF THE MAMMALIA.

In 1878 Ruge published two important papers dealing with the plantar
muscles of the mammalia, one, 78, being a consideration of the
muscies of the human foot from the embryological standpoint, and the
second, 78a, a comparative study of the deeper plantar muscles. In
the first paper two important results were recorded, namely, (1) the
plantar nature of both the dorsal and plantar interossei, and (2) the
primary unity of the adductor hallucis and the transversus pedis. In the
second paper, disregarding the superficial muscles and relying upon the
doctrine of the immutability of the nerve supply of muscles, the author
separates these muscles which are supplied by the medial plantar nerve
from those innervated by the lateral plantar, and divides the latter into
two groups, one of which the contrahentes, hes to the plantar side of
the deep branch of the lateral plantar nerve, while the other, formed by
the interossei, lies dorsal to the nerve.

In the same year that Ruge’s papers appeared Cunningham, 78a, pub-
lished the results of his extensive comparative studies of the plantar
muscles, furnishing later, 82, a more detailed account of his observa-
tions. Like Ruge, he disregarded the superficial muscles, but, on the
other hand, he declined to accept nerve supply as an absolute criterion
for muscle homology, and found in what he termed the “ intrinsic”
muscles, representatives of the same three layers he had already demon-
strated, 78, in the hand. ‘The most superficial to these layers lies dor-
sal to the tendons of the long flexor and plantar to the deep branch
of the lateral plantar nerve; it is termed the plantar layer of adductors

420 The Phylogeny of the Plantar Musculature

and corresponds to Ruge’s contrahentes. The second and third layers he
dorsal to the nerve and together correspond to Ruge’s layer of interossei ;
the more superficial muscles Cunningham termed the intermediate layer
of flexores breves and the deeper one the dorsal layer of abductors.

In the urodeles and lacertilia I have shown that three (or four) layers
are distinctly recognizable dorsal to the long flexor tendons or their
homologue, and for this reason Cunningham’s arrangement of the deep
muscles is preferable to that of Ruge. His dorsal layer of abductors is
in part equivalent to what have been described in the preceding pages of
this paper as the intermetatarsales; his intermediate layer is similarly
equivalent in general to my flexores breves profundi; while his plantar
layer of adductors corresponds to my flexor brevis medius str. profundum.
But by the exclusion of the superficial layers—indeed of everything su-
perficial to and connected with the long flexor tendons, both Cunningham
and Ruge fall into error in the assignment to their “ intrinsic” or deep
muscles of certain structures which are derivatives of the superficial
layer. There are in the foot two superficial layers of muscles which are
just as properly termed intrinsic as are the adductors and interossei, and
in what follows I shall recognize the same layers as have been described
in the foot of the lower forms, giving special names to certain of the
marginal muscles when this seems desirable.

The flexor brevis superficialis. In the mammalia studied the flexor
brevis superficialis was represented by both the strata described in the
lacertila. In the superficial stratum of the lacertilia there was a distinct
tendency for the muscle slip for the hallux to separate from the rest of
the layer; this becomes more pronounced in the mammals, in which the
slip becomes practically an independent muscle. Furthermore, we must
include in this layer certain marginal muscles of the foot, which may be
termed abductors.

Considering first the main portion of the muscle, it arises in the
opossum from the plantar surface of the strong tendon of the flexor
fibularis cruris, a short distance above the ankle joint, and partly also
from the tendon of the flexor tibialis at about opposite the ankle joint,
and is distributed by four slips (Figs. 5 and 6, fbss), to the second, third,
fourth, and fifth digits, each slip dividing into two tendons, between
which passes a tendon of the long flexor. In the cat and mouse the fibers
arise from the aponeurosis into which the long flexors insert. The dif-
ference, however, is more apparent than real, since, as has already been
pointed out, MeMurrich, 04, both the aponeurosis and the tendons of the
long flexors represent portions of an original plantar aponeurosis.

J. Playfair McMurrich 421

The well developed slip of the flexor brevis superficialis which passed
to the hallux in the lacertilia is represented in the opossum by two closely
associated muscles. One of these, which may be termed the abductor
hallucis (Fig. 5, ah), arises from the dorsal surface of the tarsal spur,
over the base of the first metatarsal, and is inserted into the first phalanx
in close proximity to the medial metatarso-phalangeal sesamoid cartilage ;
the other, which may be designated the flexor brevis superficialts hallucis
(Fig. 5, fbs’), arises in part from the dorsal surface of the tarsal spur,
but more extensively from the sheath which encloses the tendon of the
long flexor, the main portion of the flexor brevis superficialis and the
medial plantar nerve, and is inserted into the medial metatarso-phalan-
geal sesamoid cartilage.

Fic. 5. Transverse section through the junction of tarsus and metatarsus
in the opossum. abV and abV’=abductor quinti digiti; aa —=abductor hal-
lucis; amV — abductor ossis metatarsi quinti digiti; cn. and cn,—second and
third cuneiforms; cw—cuboid; fbs’—hallucal slip of flexor brevis super-
ficialis; fds, — flexor brevis superficialis str. profundum; /f—tendon of flexor
fibularis; ff—tendon of flexor tibialis; ts tarsal spur; 7, JJ, IV, and V=
metatarsals.

These two muscles, although closely associated, are separated by a dis-
tinct sheet of connective tissue, and their fibers have a markedly different
direction, so that in sections they are readily distinguishable. Consider-
able differences of opinion have been expressed as to their significance.
Coues, 72, failed to recognize the abductor as distinct from the more
lateral slip, which he describes as a portion of the flexor brevis hallucis;
Ruge, 78a, who fails to recognize two muscles, terming the combination
the abductor hallucis; while Cunningham, 82, who does recognize the
abductor, refers it to his dorsal layer (7. ¢., the intermetatarsal layer),
while the flexor superficialis hallucis he assigns as part of the flexor

422 The Phylogeny of the Plantar Musculature

brevis hallucis to his intermediate layer (1. e., the flexores breves pro-
fundi layer). The fact that both muscles are supplied by the medial
plantar nerve and their apparent equivalency to the strong hallucal shp
of the lacertilian flexor brevis superficialis lead me to regard Cunning-
ham’s assignments as incorrect; they are due to the erroneous conception
of the intrinsic muscles adopted by Cunningham.

In the mouse, and to a greater extent in the cat, there is a reduction
of the hallucal muscles owing to the reduction of the digit. In the mouse
a muscle, supphed by the medial plantar nerve, arises from the navicular
bone and is directed medially and distally to be inserted into the tibial
side of the first phalanx of the hallux. It represents the hallucal slip
of the flexor brevis superficialis, and in two out of three individuals
studied was a single muscle. In a third individual, however, it had two
heads, one from the navicular and the other, much smaller, from the
tibial sesamoid bone which Baur, 85, has identified with the tibiale. The
two heads eventually fuse, but it seems not improbable that the smaller
one represents the abductor hallucis.

In the cat the only representative of a superficial hallucal flexor seems
to be the muscle named scapho-cuneiformis by Reighard and Jennings,
or. It arises from the navicular bone and from the calcaneo-cuneiform
lhgament and, in the individual studied, could be distinctly traced to the
base of the rudimentary metatarsal. Reighard and Jennings describe it
as inserting into the lateral surface of the first cuneiform; the difference
may be due to my preparations having been made from an advanced
fetus. The muscle is supplied by a branch from the medial plantar
HETVes) ©

The fifth digit, in addition to a slip from the main mass of the flexor
brevis superficialis, receives certain other superficial muscles which must
be assigned to that layer. In the opossum there are three such muscles.
One, for which Cunningham’s, 82, name abductor ossis metatarsi quintt
digiti (Fig. 5, amV), may be adopted, has its origin from the lateral
surface of the tuberosity of the caleaneus and passes distally, over the
quadratus plant to be inserted into the lateral surface of the base of
the fifth metatarsal. A second muscle (Fig. 5,abV), arises from the cal-
caneus in close proximity to the preceding and over the base of the fifth
metatarsal is continued into a slender tendon, which inserts into the lat-
eral surface of the base of the proximal phalanx of the digit; while the
third muscle (Figs. 5 and 6, abV’), takes its origin from the plantar
surface of the sheath enclosing the long flexor tendons and passes distally
and laterally to unite with the tendon of the second muscle near its

\

J. Playfair McMurrich 423

insertion. These last two muscles, Cunningham, 82, describes as a two-
headed abductor quinti digiti. AJl three muscles are supplied by branches
from the lateral plantar nerve.

Ruge, 78a, describes the same three muscles, but Coues, 72, failed to
find the abductor ossis metatarsi, the arrangement described by him being
similar to that observed by Ruge in Didelphys cancrivorus. Further-
more, Coues terms the oblique head of the abductor the flexor brevis
minimi digiti, an objectionable term for it on account of its leading to a
confusion both with the slip which passes to the minimus from the main
mass of the flexor brevis superficialis and with the homologue of the
muscle known by the same name in human anatomy (see p. 430). While
Cunningham’s terminology for the muscles is acceptable, it may be noted
that here, as in the case of the abductor hallucis, he is in error in referring
them to his dorsal layer.

In the lists which Cunningham, 82, gives of the muscles of his dorsal
layer as they occur in the large number of mammals he studied, it will be
noticed that while the three muscles occur in several marsupials, in other
forms they are reduced to two and in others to one. When two exist they
are a single-headed abductor and an abductor ossis metatarsi; and when
but one occurs it may be either an abductor or an abductor ossis metatarsi.
The cat belongs to that group of forms in which there are two muscles,
an abductor ossis metatarsi (the caleaneo-metatarsalis of Reighard and
Jennings, or), passing from the side of the tuberosity of the calcaneus
to the base of the fifth metatarsal, and an abductor (the abductor medius
quinti digiti of Reighard and Jennings), arising from the plantar
aponeurosis over the abductor ossis metatarsi and inserting into the
lateral metatarso-phalangeal sesamoid bone. 'The mouse, on the other
hand, possesses only an abductor ossis metatarsi. The muscles of both
forms are supplied by branches from the lateral plantar nerve.

The flexor brevis superficialis stratum profundum is represented in all
the mammalia studied by muscle fibers which are distinctly separated
from those of the stratum superficiale and take their origin from the
plantar surface of the long flexor tendon shortly before it divides into its
slips for the digits. In the opossum the muscle forms a practically con-
tinuous sheet, covering the entire width of the tendon, and divides dis-
tally into four slips (Figs. 5 and 6, fbsp), one of which passes to each
of the tendons of the main mass of the stratum superficiale. In the cat
(Fig. 7, fbs,) and mouse the stratum is represented by only two slips
which unite with the superficial tendons for the third and fourth digits.
Those occurring in the cat have been described by Reighard and Jen-

99
Bes)

424 The Phylogeny of the Plantar Musculature

nings, or, as lumbricales, but they cannot properly be regarded as belong-
ing to that group of muscles.

The flexor brevis medius stratum superficiale. This layer is represented
in the mammals in part by the lumbricales. In the opossum these are
four in number (Fig. 6, fbms), arising from the tendon of the flexor
fibularis just as it divides into the tendons for the four lateral digits.
Three of the muscles consequently arise in the angle between the four
diverging tendons and the fourth from the tibial side of the tendon to
the index. The muscles pass to the tibial side of the base of the proxi-
mal phalanx of the second, third, fourth, and fifth digits. In the mouse
there are also four lumbricals arising in the angles formed by the split-

Fic. 6. Transverse section through the foot of the opossum. abV’ =ab-
ductor quinti digiti; fom, —flexor brevis medius str. profundum; fom, =
flexor brevis medius str. superficiale; fop—flexor brevis profundus; fbs,=
flexor brevis superficialis str. profundum; fbs;— flexor brevis superficialis
str. superficiale; fid—tendon of flexor longus digitorum; flm—=tendon of
flexor longus hallucis; 7p =lateral plantar nerve; mp — medial plantar nerve;
Ss = metatarso-phalangeal sesamoid cartilage of hallux; J-V = metatarsals.

ting of the long flexor tendon and passing to the same digits as in the
opossum, while in the cat there are only three muscles in the group, that
for the second digit being lacking.

In the opossum and mouse the muscle to the second digit is supphed
by a branch of the medial plantar nerve, while the other three are sup-
plied by the lateral plantar. In the cat all the muscles of the set are
supplied by the lateral plantar, the muscle to the second digit being, as
stated, wanting.

In the Jacertilia a large muscle belonging to the superficial layer of
the flexor brevis medius passes to the hallux, and in the opossum what I
believe to be the same muscle (Fig. 6, fbm’s) occurs. It arises in close
association with the hallucal portion of the flexor brevis superficialis,

J. Playfair McMurrich 425

from the medial surface of the sheath enclosing the long flexor tendons,
and also from the base of the second metatarsal. It passes distally par-
allel with the hallucal slip of the flexor brevis superficialis, the long flexor
tendon for the hallux lying between the two muscles, and is inserted into
the lateral surface of the base of the first phalanx, a sesamoid cartilage
being developed in its tendon. It is supplied by the deep branch of the
lateral plantar nerve. In the cat and the mouse the muscle does not
occur, unless it be represented in the mouse by a few scattered muscle
fibers which occur in new born animals between the hallucal slips of
the flexor brevis superficialis and the flexor brevis medius str. profundum.

This is the muscle which Coues, 72, and Cunningham, 82, describe as
the lateral head of the flexor brevis hallucis associating it with the
hallucal slip of the flexor brevis superficialis, the latter author referring
both slps to his intermediate layer (7. e., the flexor brevis profundus).
Ruge, 78a, on the other hand, regards the muscle as distinct from the
flexor brevis superficialis slip and refers it to his layer of contrahentes
(1. e., to the flexor brevis medius str. profundum). I shall have occasion
to consider this muscle or rather its human equivalent in connection with
the human flexor brevis hallucis and shall remark concerning it here
only that it occupies a plane ventral (7. e., plantar) to that of the hallueal
portion of the flexor brevis medius str. profundum (Fig. 6) and that this
fact, together with its innervation from the lateral plantar nerve and the
relations of what is apparently the corresponding muscle in the lacertilia,
lead me to consider it a portion of the superficial layer of the flexor brevis
medius.

The flecor brevis medius stratum profundum is formed by what are
usually known as the adductors or, as they have been termed by Ruge, 78a,
following Bischoff, 70, the contrahentes. In the opossum they are four
in number. The two muscles which pass respectively to the hallux and
minimus (Fig. 6, fom, ’) are large fan-shaped structures which arise
from a median tendinous raphe extending from the base of the third
metatarsal to the base of the proximal phalanx of the third digit, the
fibers converging from this raphe on the one side to the base of the proxi-
mal phalanx of the hallux and on the other to the base of the proximal
phalanx of the minimus. The other two muscles are almost concealed by
those just described, beneath which they lie, taking their origin from the
dorsal surface of the tendinous raphe and passing distally to be inserted, -
the one into the fibular side of the base of the second digit and the other
into the tibial side of the base of the fourth digit. All four muscles are
supplied by branches from the deep branch of the lateral plantar nerve.

426 The Phylogeny of the Plantar Musculature

In the mouse the layer is represented by only three muscles, that to the
fourth digit being wanting. They arise from the strong fibrous sheath
which invests the peroneus longus and insert into the fibular side of the
bases of the proximal phalanges of the first and second digits and into
the tibial side of the corresponding phalanx of the minimus. In the cat
the reduction in number-is carried one step farther, in that the muscles
pass to only two digits, namely the index and minimus (Fig. 7, fbm,'’),
but the latter digit receives two slips, one of which is inserted into the
tibial side of the base of the proximal phalanx, while the other inserts
into the tibial side of the metatarsal near its head and forms what has
been termed the opponens minimi digiti. In both forms all the muscles
are supplied from the lateral plantar nerve.

Fic. 7. Transverse section through the foot of the cat. abV —abductor
quinti digiti; fbm,—flexor brevis medius str. profundum; fbm, = flexor
brevis medius str. superficiale; fbp— flexor brevis profundus; /fbs,=— flexor
brevis superficialis str. profundum; fbs— flexor brevis superficialis, str. super-
ficiale; im —intermetatarsalis; JJ-V = metatarsal bones.

The flexores breves profundi and the intermetatarsales are so intimately
associated in all the forms studied that they may be considered together.
Compared with the lacertilia it is noticeable that the intermetatarsales
are represented by muscles, instead of ligaments, and that there is a very
different arrangement of the flexores profundi with reference to the vari-
ous digits. Instead of a general inclination of the slips from the fibular
to the tibial side one finds that in the mammala they are almost directly
longitudinal in their course and that certain of them are so fused with
the intermetatarsales as to be distinguishable from them only by inter-
vening fibrous bands, in some cases by a more or less pronounced differ-
ence in the direction of their fibers and by their more plantar position.

)

)

J. Playfair McMurrich 427

In the opossum there occurs in the interval between the first and second
metatarsals a muscle (Fig. 6) which arises from the base of the first
metatarsal and passes distally to be inserted into the tibial metatarso-
phalangeal sesamoid cartilage of the second digit. In cross sections near
its origin the muscle is distinctly seen to be composed of two portions,
separated from one another by a tendinous partition, and the fibers on
either side of the partition have a distinctly different direction. How far
the presence of the partition and the difference in direction may be relied
upon as an indication of a fusion of two primarily distinct muscles is un-
certain, especially since towards their insertion the two portions fuse so
as to be indistinguishable. J am inclined to believe, however, on the basis
of comparison with lower.forms, that in this case the peculiarities do indi-
cate a fusion and that the muscle really represents a flexor brevis pro-
fundus hallucis and an intermetatarsalis I.

Passing fibularly one finds over the second metatarsal two muscles
again separated by a tendinous partition and also showing a very different
direction of their fibers. These are evidently the flexores breves profundi
II and they insert into the two sesamoid bones over the metatarso-phalan-
geal joint of their digit, the more tibial one coming into relation with
the combined flexor profundus I and intermetatarsalis I. Over the third
metatarsal the arrangement is more complicated. Proximally one finds
two muscle bundles lying side by side and separated by a tendinous parti-
tion, but as one passes distally additional fibers, with a slightly different
direction, become added upon either side, so that eventually four muscle
bundles may be recognized over the bone (Fig. 6). The two lateral ones
extend dorsally between the third and the adjacent metatarsals, while the
two median ones, which are considerably smaller than the others, are con-
fined to the plantar surface of the metatarsal. Eventually the plantar and
lateral muscles of one side of the median line separate from the corre-
sponding bundles of the other side, so that two pairs of muscles become
recognizable which insert into the tibial and fibular metatarso-phalangeal
sesamoid cartilages of the third digit. The two median bundles I take to
be the flexores breves profundi III, while the lateral bundles are the inter-
metatarsales IT and III.

In the case of the fourth digit a determination of the arrangement is
somewhat difficult, but just as muscles pass from the hallux to the second
digit, so muscles from the minimus pass to the fourth digit, and it will
therefore be convenient to consider the two digits together. In sections
taken near the bases of the metatarsals two indistinctly separated bundles
are to be seen over the third intermetatarsal space and a third one lies

428 The Phylogeny of the Plantar Musculature

partly over the fourth space and partly over the tibial border of the fifth
metatarsal. The two more tibial bundles, which probably represent but a
single muscle, take their origin from the sheath of the peroneus longus
tendon, while the fibular one seems rather to come from the base of the
fifth metatarsal. Additional fibers are added to the fibular muscle from
an origin on the tibial surface of the fifth metatarsal. These fibers are
easily recognizable from their decidedly oblique direction, and it seems
probable that they represent a distinct muscle, namely, the intermetatar-
salis IV. Eventually the two more tibial bundles insert into the tibial
metatarso-phalangeal sesamoid, while the fibular muscle, together with
the intermetatarsalis 1V terminates on the fibular sesamoid. :

Over the plantar surface of the fifth metatarsal two muscles, which
arise from the sheath of the peroneus longus tendon, occur. They are
separated by a distinct tendinous partition, and, as they are traced distally,
separate to be inserted into the two sesamoid cartilages of the metatarso-
phalangeal joint.

Finally, I find beneath the hallucal slip of the flexor brevis medius str.
superficiale and between that muscle and the conjoined flexor brevis pro-
fundus I and intermetatarsalis I a small bundle of muscle fibers, which
may possibly represent an additional slip of the flexor brevis profundus
layer. It is so rudimentary, however, that it is impossible to assign to it
an origin or an insertion; it is distinctly separate from the combined in-
termetatarsal and flexor profundus I and its fibers have a direction almost
at right angles to those of the flexor brevis medius str. superficiale above
it. No equivalent of the muscle occurs either in the mouse or the cat, in
both of which the hallux is considerably reduced as compared with that
of the opossum.

In the mouse the arrangement of the flexor brevis profundus is very
similar to that of the opossum, except that the various slips are much
more intimately fused and consequently much more difficult to recognize.
Over the base of the first metatarsal there is a mass of muscular tissue
in which no indications of a composite nature €ould be detected. It passes
to the tibial metatarso-phalangeal sesamoid of the second digit and seems
therefore to correspond to the combined intermetatarsal and flexor pro-
fundus I of the opossum. Over the second metatarsal are two muscle
slips separated only by a tendinous partition. They both insert into the
fibular metatarso-phalangeal sesamoid of the digit, the arrangement
differing in this respect from that of the opossum. ‘The muscles of the
third digit are arranged as in the opossum, though much more exten-
sively fused. The mass which they form curves dorsally around each
surface of the metatarsal and the lateral parts of the crescent so formed

J. Playfair McMurrich 429

are separated from the central portion by a tendinous partition. The
entire mass eventually separates into two portions which insert
respectively into the tibial and fibular metatarso-phalangeal sesamoids
of the digit and seem to represent the two flexores breves profundi III
together with the-intermetatarsales Il and III. In the fourth digit
also there is much fusion of slips. A relatively large bundle passes
from the fifth metatarsal to the fibular sesamoid of the digit, repre-
senting the fourth intermetatarsal, but whether it also includes a por-
tion of the flexor profundus IV remains uncertain. The main mass
of the flexor profundus V passes to the fibular sesamoid of its digit,
but it gives off a slp which unites with the adductor slip for the digit
and appears to represent the tibial portion of the flexor profundus V.

In the cat, owing to the reduction of the haliux, no representatives
of the flexor brevis profundus I nor of the intermetatarsalis I exist.
Otherwise the arrangement resembles closely that of the mouse. In the
second digit only the two slips of the flexor brevis profundus (Fig. 7,
fbp*), occur, and these pass to the two metatarso-phalangeal sesamoids
of the digit. Both slips of the flexor profundus V are well developed.

A comparison of the mammalian plantar muscles as described above
with those of the lacertilia may be made as follows:

Lacertilia. Mammalia.
3 AN Flexor brevis superficialis str. super-
Flexor brevis superficialis str. super- fisiate
ficiale. Abductor hallucis.
Abductor ossis metatarsi V.
Abductor V. Abductor V,
Flexor brevis superficialis str. pro- Flexor brevis superficialis str. pro-
fundum., fundum.
Lumbricales.

Flexor brevis medius str. superficiale. + Hallucal slip of flexor brevis medius
str. superficiale.

Flexor brevis medius str. profundum. /dductors.

Flexor brevis profundus intermeta- :
Interossei.

tarsales.

IV. THE PLANTAR MUSCLES IN MAN.

It remains now to consider the plantar muscles of the human foot in
the light of the conclusions reached in the preceding pages, and in
doing so the nomenclature employed in human anatomy may be fol-
lowed.

430 The Phylogeny of the Plantar Musculature

The flexor brevis digitorum presents little difficulty. It is clearly
homologous with the main mass of the flexor brevis superficialis str.
superficiale, and certain of the peculiarities it presents are explicable
on the basis of the phylogenetic history of that muscle. Thus the fre-
quent origin of the slp for the minimus from the plantar surface of
the tendon of the long flexor is clearly a reminiscence of the signifi-
cance of the tendon as a portion of the plantar aponeurosis, and it
merely obscures the true relationship to regard the slip having such an
origin as something distinct from what is usually regarded as the
normal slip.

Fic. 8. Transverse section through the foot of a human fetus of 9 cm.
abh =abductor hallucis; abV abductor quinti digiti; ak —adductor hal-
lucis; fbh and fbh’ = medial and lateral heads of flexor brevis hallucis; foV
= flexor brevis quinti digiti; fdb—flexor brevis digitorum; flm—=tendon of
flexor longus hallucis; ZJd@—=dorsal interosseus; Ip—plantar interosseus;
7=lumbricalis; J-V = metatarsal bones.

The stratum profundum of the flexor brevis superficialis is so evident
a constituent of the plantar musculature both in the lacertilia and
mammalia, that its occurrence in the human foot seemed more than
likely. An examination of sections of a foot from a fetus of 9 em.
revealed what I take to be its representative. Before the flexor brevis
begins to divide into its terminal slip, tendons appear imbedded in the
center of the muscle mass, and of these, in the foot in question, there
were four, notwithstanding the fact that the tendon for the fifth digit,
as is so often the case, was derived from the tendon of the long flexor.
The muscle fibers surrounding the most medial tendon gradually sep-
arated from the rest to form the slip to the second digit, and those sur-

J. Playfair McMurrich 431

rounding the most lateral tendon similarly separate to form the slip
for the fourth digit. The fibers surrounding the two remaining ten-
dons arrange themselves in two bundles, one lying immediately above
the other, which, with their tendons, eventually unite to form the slip
for the third digit (Fig. 8, fdb*). I take the more dorsal of the two
bundles to represent the flexor brevis superficialis str. profundum; the
slip to the third digit being the only portion of it persisting.

The flexor brevis digitorum has been taken to be the exact equiva-
lent of the flexor sublimis digitorum of the arm, and on that basis has
been regarded as primarily a crural muscle which has secondarily de-
scended into the foot. If the homologies traced in the preceding pages
be correct, there are no grounds for assuming a descent of the muscle
from the crural region. It is from the beginning an intrinsic muscle
of the foot.

The abductor hallucis (Fig. 8, abh), and abductor quinti digiti (abV)
likewise require but brief consideration. There seems no doubt but
that they are equivalents of the correspondingly named muscles in the
lower mammals and are therefore derivatives of the primary super-
ficial plantar layer.

In the case of the flexor brevis hallucts, however, the matter is more
complicated. The conception of it as a single muscle, so constantly
found in text-books, probably dates back to Albinus, 34, by whom it
was thus described, and is sufficiently satisfactory to the anatomist who
studies muscles through physiological spectacles. So soon as a morpho-
logical basis is sought for the classification of muscles, this one loses
its simplicity. Cruveilhier, 77, reserves the name flexor hallucis brevis
for the more medial portion of the muscle, regarding the lateral por-
tion as a part of the adductor, and in this he has been followed by
Flemming, 87, and Gegenbaur, 92, as well as by Ruge, 78a, in his com-
parative studies. Cunningham, 82, however, regards the two heads as
representing the haJlucal portions of his intermediate laver, the layer
to which the plantar interossei belong and which, in typical cases con-
sists of a pair of muscles for each digit.

From my own studies I am compelled to dissent from the interpreta-
tion which Cunningham gives to the muscle and to side with Cruveilhier
and Flemming in regarding it as composed of two distinct portions
which belong to different layers. My reasons for this belief are based
largely upon the results obtained from the studies recorded in the
preceding pages, which seem to point to the morphological independ-
ence of the two heads of the muscle. But confirmation of this view is

432 The Phylogeny of the Plantar Musculature

afforded by the frequency with which, throughout the mammalia, as
described by Cunningham, 82, the lateral head disappears, leaving the
medial head as the sole representative of the muscle, and also by the
fact pointed out by Ruge, 78, and which I can confirm, that the tendon
of the flexor longus hallucis throughout the greater part of its rela-
tionship with the muscle rests wpon and not to the medial side of the
lateral head (Fig. 8). For if the medial head, as there is every reason
to believe, is a portion of the flexor brevis superficialis, then a muscle
lying dorsal to the long flexor tendon cannot be regarded as belonging
to the same layer that it does.

In accepting this view I differ, however, from Cruveilhier and Flem-
ming as to the morphological significance of the lateral head. The
medial head (Fig. 8, fbh), as just stated, is undoubtedly a portion of
the flexor brevis superficialis, but instead of referring the lateral head
to the adductor layer, 7%. e., to the flexor brevis medius str. profundum
I would assign it to the flexor brevis medius str. superficiale, 7. ¢., to
the layer from which the lumbricals are derived. This conclusion is
based upon the comparative series shown by the lacertilia, the opossum
and man, in all of which the same muscle is recognizable and in the
first named group is evidently a portion of the flexor brevis medius str.
superficiale. The muscle, then, may be regarded as the lumbrical of
the hallux.’

Flemming, 87, attaches considerable importance to the nerve supply
of the lateral head being from the lateral plantar, while that of the
medial head is from the medial plantar. In so describing the supply
he follows the account given in most of the continental anatomies. Cun-
ningham, 87, however, takes exception to this, stating that not only
in the human foot but in those of all the mammals he studied, with one
exception, the supply of both heads was from the medial plantar and
my own observations on one adult and two fetal human feet give the
same result. The nerve supply, however, cannot be taken as a criterion
in this matter, especially if the lateral head of the muscle be regarded
as belonging to the lumbrical layer; for the lumbrical of the second
digit is normally supplied by the medial plantar and, furthermore,

1Tt seems probable that the corresponding medial head of the flexor brevis
pollicis has the same significance. In an earlier paper, 03, I referred it to
the adductor set, but the arrangement in the foot throws new light upon the
question. Young, 79, has identified in the Rock kangaroo a muscle, distinct
from the flexor brevis pollicis, as a pollical lumbrical, but its isolated occur-
rence makes it questionable whether it can properly be regarded as such,
rather than as an anomaly.

J. Playfair McMurrich 433

Brooks’, 87, observations on the innervation of the lumbricals show that a
confusion of the constituents of the two plantar nerves may occur, similar
to that which may obtain between the ulnar and median in the hand.

A word concerning the origin of the flexor brevis hallucis may not be
out of place. In certain English text-books, Morris, 3d Ed., and Cun-
ningham for example, it is stated to arise in part from the cuboid bone.
This may possibly represent its physiological origin, but it certainly
gives a very incorrect idea of its morphological relations. It is very
clear from the study of fetal preparations that the muscle has its origin
primarily from the first cuneiform and secondarily from a dense lamella
of connective tissue which forms a sheath for the tendon of the flexor
longus hallucis and proximally is continuous with a dense fascia cov-
ering the tendon of the peroneus longus and also with a strong liga-
ment which extends from the navicular to the third cuneiform. It is
through its connection with the peroneal sheath that the muscle reaches

Fic. 9. Diagram to show the constitution of the interossei in the human
foot. fbp=—flexor brevis profundus; im =intermetatarsal; /-V = metatarsal
bones.

the cuboid, but it is a mistake to suppose that this bone forms part of
its true morphological origin.

The lumbricales are, as in the hand, clearly representatives of the
flexor brevis medius stratum superficiale of the lower forms. And,
similarly, both portions of the adductor hallucis are portions of the
flexor brevis medius str. profundum, Ruge’s, 78, observations on the
development of the caput transversum, apart from comparative studies,
showing its embryological relations to the caput obliquum. Meckel’s,
32, identification of the caput transversum as a lumbrical is unsup-
ported by either embryological or comparative evidence.

The interossei resemble closely those of the hand, whose phylogeny I
have already, 03, considered. They represent a combination of the
flexores breves profundi and the intermetatarsales, and their constitu-
tion may be understood from the accompanying diagram (Fig. 9).
Considering first the three central digits; each possesses two slips of
the flexor brevis profundus. Both slips of the second digit unite with

434 The Phylogeny of the Plantar Musculature

the first and second intermetatarsals forming the first and second dorsal
interossel, which insert into the proximal phalanx of that digit. The
medial slip of the third digit forms the first plantar interosseous, while
the lateral slip unites with the third intermetatarsal to form the third
dorsal interosseous; the medial slip of the fourth digit forms the second
plantar interosseous, while its lateral slip unites with the fourth inter-
metatarsal to form the fourth dorsal interosseous.

In the fifth digit both slips of the flexor brevis profundus retain their
separate individualities, the medial one forming the third plantar in-
terosseous, while the lateral one is the muscle known as the flexor
brevis quinty digit.

I have found nothing in my preparations that I could regard as a
hallucal flexor profundus, although on comparative grounds such
muscles should be expected. There is a probability, judging from what
occurs in the opossum, that the first dorsal interosseous contains an
element representing a hallucal flexor profundus, just as is the case in
the hand, but of this I have not been able to obtain definite evidence.
The so-called interosseous primus volaris of Wood, 67, does not appear
to belong to the interosseous set of muscles; its position towards the
medial surface of the hallux and its relations with the abductor are
opposed to such an assignment of it. It seems rather to be a slip of
the flexor brevis hallucis which has retained its primary origin from
the first cuneiform, instead of shifting to the plantar aponeuroses with
the rest of the muscle. So too the portion of the oblique head of the
adductor hallucis, which Henle, 71, regards as the equivalent of the
interosseus primus volaris of the hand, is rather to be regarded as a
portion of the flexor brevis medius str. profundum.

In the preparations I have studied there is no distinct opponens
hallucis, nor is the opponens quintt digiti represented as a distinct
muscle. I am unable to determine the significance of the former
muscle, whether it be a derivative of the adductor or the flexor brevis
hallucis, but viewing the possibilities as they appear in my prepara-
tions I am inclined to look to the adductor for its origin (cf. Brooks,
87). In a fetus of 9 cm. I find that a portion of the flexor brevis
quinti digiti inserts upon the upper part of the fifth metatarsal, a fact
which seems to point to the derivation of the opponens quinti digiti from
the flexor brevis. In this opinion I am in accord with Ruge, 78. It
is certainly a very different structure from the so-called opponens quinti
digiti of the cat (see p. 426).

Many difficulties are encountered in the working out of the detailed

a

J. Playfair MeMurrich 435

homologies of the human plantar muscles, and the final determina-
tion of some of them requires more extensive material than has been
at my disposal. But one point is, I believe, conclusively settled, and
that is the arrangement of the human plantar muscles in a series of
layers which are homologous with those found in both the reptila and
the amphibia. This point established gives a broader basis for com-
parison than the study of individual muscles can afford, and, it is to
be hoped, will lead to a full understanding of the morphology of the
plantar muscles. The identifications described above may be repre-
sented in tabular form as follows:

Lacertilia. Man.

Flexor brevis digitorum.
Flexor brevis superficialis str. super- | Medial head of flexor brevis hallucis.
ficiale. Abductor hallucis.
Abductor quinti digiti.

Flexor brevis superficialis str. pro- (Portion of slip of flexor brevis digi-
fundum. torum to 3d digit.

Lateral head of flexor brevis hallucis.

Flexor brevis medius str. superficiale. { Traribeicalon:

Adductor hallucis.

Flexor brevis medius str. profundum. { Opponens hallucis?

Flexor brevis quinti digiti.
Opponens quinti digiti.
Interossei.

Flexor brevis profundus.
Intermetatarsales.

SUMMARY.

1. The plantar muscles in the urodele amphibia are arranged in four
layers and are all intrinsic to the foot, arising either from the plantar
aponeurosis or from the bones of the foot.

2. In the lacertilia the number of layers becomes increased to six by
the division of the superficial and middle layers so that in each of them
there is a stratum superficiale and a stratum profundum.

3. The plantar aponeurosis has also differentiated into two layers,
the deeper of which forms the plantar portion of the tendons of the
long flexors.

4. In the mammalia the six layers found in the lacertilia persist.

5. The marginal portions of the flexor brevis superficialis early sep-
arate from the main mass of the muscle and form the abductors of the
hallux and minimus.

4

436 The Phylogeny of the Plantar Musculature

6. The mammalian flexor brevis hallucis is a compound muscle; its
medial head is derived from the flexor brevis superficialis and its lat-
eral head from the flexor brevis medius stratum superficiale.

?. The flexor brevis superficialis stratum profundum is represented
in the human foot by a muscle bundle and tendon which unites with
the slip of the flexor brevis digitorum passing to the third digit.

8. The flexor brevis digitorum is not a crural muscle which has sec-
ondarily descended into the planta. It is from the beginning an in-
trinsic muscle of the foot.

9. The flexor brevis quinti digiti is not equivalent to any portion of
the flexor brevis hallucis, but is a portion of the flexor brevis profundus
layer.

10. The oblique and transverse heads of the adductor pollicis are
primarily parts of a single muscle and are portions of the flexor brevis
medius stratum profundum.

11. The opponens hallucis is probably a derivative of the oblique
head of the adductor hallucis; the opponens quinti digiti is a portion
of the flexor brevis quinti digiti and therefore a portion of the flexor
brevis profundus.

12. The dorsal interossei are formed by the fusion of portions of the
flexor brevis profundus with the intermetatarsales, the plantar interossei
being formed by the remaining portions of the flexor brevis profundus.

ANATOMICAL LABORATORY, UNIVERSITY OF MICHIGAN,
January 11, 1907.

REFERENCES.

ALBINUS, B. S., 34.—Historia Musculorum Hominis. Leide Batavorum. 1734.

Bavr, G., 85.—Zur Morphologie des Tarsus der Saugethiere. Morph. Jahrb.,
X, 1885. :

Biscuorr, T. L. W., 70.—Beitrage zur Anatomie des Hylobates leuciscus. Abh.
math.-phys. Classe konigl. Bayerischen Akad. Wiss., X, 1870.

Brooks, H. Sr. J., 87.—Variations in the nerve supply of the lumbrical mus-
cles in the hand and foot, with some observations on the innervation
of the perforating flexors. Journ. Anat. and Phys., XXI, 1887.

Brooks, H. Sr. J., 88.—On the short muscles of the pollex and hallux of the
anthropoid apes with special reference to the opponens hallucis.
Journ. Anat. and Phys., XXII, 1888.

Cougs, E., 72—The osteology and myology of Didelphys virginiana. Mem.
Boston Soc. Nat. Hisc., II, 1871-1878.

CRUVEILHIER, J., 77—Traité de l’Anatomie. 3me Edn, Tome I, 1877.

CunnINGHAM, D. J., 78.—The intrinsic muscles of the hand of the Thylacine
(Thylacinus cynocephalus), Cuscus (Phalangista maculata) and
Phascogale (Phascogale calura). Journ. Anat. and Phys., XII, 1878.

J. Playfair McMurrich 437

CUNNINGHAM, D. J., 78a.—The intrinsic muscles of the mammalian foot.
Journ. Anat. and Phys., XIII, 1878.

CUNNINGHAM, D. J., 82.—Report on some points in the anatomy of the Thyla-
cine (Thylacinus cynocephalus) Cuscus (Phalangista maculata) and
Phascogale (Phascogale calura), etc. Scient. Results Voyage of H. M.
S. Challenger. Zool., V, 1882.

CUNNINGHAM, D. J., 87,—The flexor brevis pollicis and the flexor brevis hal-
lucis in man. Anat. Anzeiger, II, 1887.

FLEMMING, W., 87.—Ueber den flexor brevis pollicis und hallucis des Menschen.
Anat. Anzeiger, II, 1887.

Gapow, H., 82.—Beitrage zur Morphologie der hinteren Extremitat der Rep-
tilien. Morph. Jahrb., VII, 1882.

GEGENBAUR, C., 92.—Lehrbuch der Anatomie des Menschen. 5te Aufl. Bd. I,
Leipzig, 1892.

HENLE, J., 71.—Handbuch der systematischen Anatomie des Menschen. 2te
Aufl. Bd. I, 1871.

HOFFMANN, C. K., 90.—Reptilien in Bronn’s Klassen und Ordnungen des Thier-
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HUMPHRY, 72.—The muscles and nerves of Cryptobranchus Japonicus. Journ.
Anat. and Phys., VI, 1872.

McMorricyH, J. P., 03——The phylogeny of the palmar musculature. Amer.
Journ. Anat., II, 1903.

McMurricu, J. P., o4.—The phylogeny of the crural flexors. Amer. Journ.
Anat., IV, 1904.

MECKEL, J. F., 32,—Manual of general, descriptive and pathological anatomy.
Translated from the French by H. Sidney Doane. Philadelphia, 1832.

PERRIN, M. A., 93.—Contribution a l’étude de la myologie comparée: Membre
postérieur chez un certain nombre de Batraciens et de Sauriens.
Bull. Scient., XXIV, 1893.

REIGHARD, J., and JENNINGS, H. S., 01—Anatomy of the Cat. New York, 1901.

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Ruece, G., 78a.—Zur vergleichenden Anatomie der tiefen Muskeln in der Fuss-
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Woop, J., 67.—Variations in human myology observed during the winter ses-
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Youne, A. H., 79.—The intrinsic muscles of the marsupial hand. Journ. Anat.
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ON THE GROSS DEVELOPMENT AND VASCULARIZATION
OF THEY TLESLIS:

BY

EBEN C. HIUL.
From the Anatomical Laboratory of the Johns Hopkins University.

WitnH 14 Text FIGURES.

While studying the development of the blood supply to the Wolffian
bodies, my attention was called to the interesting manner in which the
testis becomes vascularized. Accordingly, following a suggestion from
Professor Mall, I injected a series of embryo pigs and later the testes
of adult pigs, hoping that a thorough knowledge of the blood supply
of this gland in the pig would facilitate the study of the vascularization
of the human testis. In this I was mistaken, for the information gained
from corrosions, injections, and cleared preparations of the testis of the
pig was of comparatively little value in unravelling the interesting
though complex blood supply of the sex gland in man. Hence, in this
paper I shall confine myself to the gross development and blood supply
of the pig testis, and will later publish the results of studies of the
human gland.

The blood supply to the human male sex gland, to use a rather unique
term for anatomical literature, is rational. It is much as one might
expect, knowing the lobular arrangement. The vascularity of the pig
testis, on the other hand, is quite unusual and it is difficult to imagine
what causes could have produced such an unique arrangement.

Interature.—The literature on the vascularization of the testis is sur-
prisingly meagre, considering the enormous bibliography which has
accumulated on spermatogenesis and the descent of the testis. Kollker,
Mihalkovics, Bardeleben, Pfltiger, Waldeyer, and many others have added
much to our knowledge of these two subjects, but as yet comparatively
little has been accomplished toward unravelling the blood supply.
Kolliker traced the spermatic artery as it branched to supply the cord,
epididymis and testis. He states that the blood vessels follow the
trabecule of the sex gland after penetrating the albuginea near the

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

34

440 Development and Vascularization of the Testis

epididymis. He also describes a capillary network around the tubules.
Astley Cooper has investigated a capillary plexus covering the internal
surface of the tunica albuginea which he has termed the tunica vasculosa.
But beyond this there is little to be found in the literature of the subject.

The vascular units which Professor Mall and his students have shown to
be present in several organs and which they assume are to be found in
all organs, have not as yet been demonstrated in the testis, and, indeed,
aside from what has been cited above, nothing is known of the arterial
or venous supply of the male sex gland. Dr. Mall has frequently dem-
onstrated the presence of certain units of the blood system which may
or may not be peculiar to the organ in which they are found and which
correspond to the histological or structural wnit of the organ. These
units are composed of small branching blood vessels which pass into
capillaries and the blood from which is collected into small veins. This
theory of vascular units may be briefly summed up in the statement
“similar blood supply to similar histological units.” These vascular
units have been proved to be present in the liver,’ spleen,’ and adrenal,”
but in the testis of the pig I can make out no definite units. In man,
however, the lobular arrangement is less complex, and results have been
so encouraging that probably these units will be shortly discovered.

Methods and Material—The necessity of clearly understanding the
development of the circulatory system in the earliest embryonic stages
in order to properly interpret the course of the blood stream in the adult
organs has been frequently emphasized. Accordingly, in attempting
this research frequent use has been made of embryonic material. Em-
bryo pigs, many of which were alive when delivered at the laboratory,
were injected and cleared, and, after tracing the development of the
circulation in these stages, the investigations were completed with adult
material. For adult human testes I am indebted to Professor MacCal-
lum of the pathological laboratory. To Professor Brédel I also wish
to express my appreciation for several valuable specimens of human
embryonic testes as well as for his helpful suggestions in making the
illustrations. For the courtesies of their laboratory I wish to. thank
Professors Wiedersheim and Keibel of Freiburg.

All injections of embryonic material were made with India ink. In

1 Mall, F. P.: A Study of the Structural Unit of the Liver. Am. Jour. Anat.,
WoOlsaV suNOe.

?Mall, F. P.: The Structure of the Spleen. Johns Hop. Hosp. Rep.

’ Flint, J. M.: The Blood Vessels, Angiogenesis, Organogenesis, Reticulum
and Histology of the Adrenal. Johns Hop. Hosp. Rept., Vol. IX.

a

——————a—”-S—ti—

eS

Eben C. Hill 441

the youngest stages, measuring from eighteen min. to seventy-five mm., a
hypodermic syringe with a fine needle forced the injection fluid into one
of the umbilical arteries, and by watching the hind legs and head excellent
results devoid of extravasation could be obtained. This mode of in-
jection is particularly desirable in these early stages for it is not nec-
essary to rupture the surrounding membranes and thus the embryo is
protected against injury in handling. In larger embryos injections were
made directly into the aorta by puncturing the left ventricle. Consid-
erable pressure was necessary to overcome in the earlier stages the re-
sistance resulting from the small lumen of the spermatic artery, and in
larger embryos because of its remarkable tortuosity. On account of this
pressure the Wolffian bodies were frequently doubly injected, the in-
jection mass passing through the sinusoids and capillaries described by
Minot into the veins. In nearly all such specimens the testes showed
only an arterial and capillary injection. In this connection it may not
be amiss to emphasize the advantages of India ink in all cases where
a fluid is desired which will flow wherever the blood stream goes, and
yet is resistant to the ordinary laboratory acids and to concentrated
solutions of potassium or sodium hydroxide.

After the injection of each embryo, the right testis together with the
Wolffian body, kidney, and aorta were removed and placed in ninety-five
per cent alcohol for clearing, while the left sex gland with its appendages
was prepared for sectioning. Of the various clearing procedures, the
modified Schultze method* was found to give the most satisfactory
results. This method is as follows:

After the injections have been completed all unnecessary tissues sur-
rounding the parts under investigation are removed and the specimens
are placed in ninety-five per cent alcohol. The removal of adventitial
tissue is most important, though entire embryos may be successfully
cleared if openings are made into the abdomen, thorax and cranium. In
embryos ranging above one hundred and fifty mm. in size, it is still
better to make sagittal sections of the hardened specimens and to clear
in halves. The alcohol should be frequently changed and large quanti-
ties should be used. In order to obtain transparent specimens the
tissues must be completely shrivelled before removal from the alcohol,
and the length of time necessary to accomplish this result depends, of
course, upon the size of the objects. For very small specimens at least

4Hill, E. C.: On the Schultze Clearing Method as Used in the Anatomical
Laboratory of the Johns Hopkins University. Johns Hop. Hosp. Bull., Vol.
XVII.

442 Development and Vascularization of the Testis

three days should be allowed, while for large objects the time should not
be less than a week. Experiments with absolute alcohol in place of
ninety-five per cent alcohol gave no better results, and its use is an
unnecessary expense. The coagulation of the proteid occurs almost as
quickly in one percentage as in the other.

After the specimens have been sufficiently shriveled they should be
placed in one per cent potassium hydroxide. When a higher percentage
is resorted to, so rapid is the action that the safety of the specimen is
endangered, and it was the use of the strong solutions recommended by
Schultze which caused the loss of much valuable material. In this
weaker solution the tissues become transparent in from four to forty-
eight hours, depending upon the size of the specimens. After sufficient
clearing in this medium they should be transferred to twenty per cent
glycerine, in which clearing continues and a certain amount of harden-
ing occurs, rendering the tissues firm enough to permit of dissection.
Should the specimens be as transparent as is desired, they may be re-
moved from time to time to higher percentages of glycerine till at last
they are permanently stored in pure glycerine. <A certain amount of
shrinkage is noted in some organs after an immersion in this fluid for
a year or more, but when the specimens are studied immediately after
being cleared the measurements are practically the same as in the fresh
tissue. The shrinkage which some observers have noticed is probably
due to transferring the specimens too rapidly to higher percentages of
glycerine. After some experimenting we have found that embryos hard-
ened in formalin can be cleared also, though in this case 10 per cent
potassium hydroxide is essential and the specimens must remain in this
solution for several weeks or months.

Blood pigment in some instances will not be entirely removed by this
process alone and in organs, such as the kidney, transparency can some-
times only be obtained by a secondary treatment. After passing through
the one per cent potassium hydroxide as outlined above and being placed
in twenty per cent glycerine, the specimens containing the objectionable
pigment are treated with equal parts of fifty per cent ammonium
hydroxide and one per cent potassium hydroxide. In this solution there
is comparatively little danger to the specimen on account of the harden-
ing produced by the twenty per cent glycerine. Indeed, in cases where
it is deemed advisable for any reason to stop the clearing action, or it
is found to be more convenient to continue the process at some future
time, the objects may be removed to this twenty per cent glycerine and
retained in this medium until a more fitting time when much higher

OO

Hibens@s Halll 443

percentages of the caustic solution may be resorted to without danger
to the specimens. The specimens may be studied in glycerine or by a
method devised by Bardeen may be mounted upon glass slides and placed
in any desired position in jars of glycerine. The objects are removed
from pure glycerine, wiped and quickly washed. They are then placed
in a little thick gelatin solution and are laid upon a warm glass slide.
As soon as the gelatin is hardened the specimens are returned to the
pure glycerine without any danger of becoming loosened from the slide.
The purity of the glycerine should be assured, as the presence of foreign
substances such as water may tend to soften the gelatin.

The clearing reagents advocated by Van Wijhe* and Lundvall* did
not give the same degree of transparency as was gained by following
the above method, though in clearing the capsule of the adult testis
beautiful specimens were obtained by using absolute alcohol and xylol
as outlined by Van Wijhe. In following the distribution of the blood
vessels in the capsule, quite satisfactory results were obtained by the
very practical and simple method devised by Flint in his work on the
adrenal.’ “ After carefully dissecting away all of the paricapsular con-
nective tissue from the injected and hardened gland, it is cut in half,
longitudinally, with a sharp razor and the parenchyma is scraped out
with a scalpel. The remaining fibrous capsule is then treated exactly
like a section and, after dehydration, is cleared and mounted in a cell.”

Although the modified Schultze method is particularly applicable to
embryonic tissues, yet sections of the adult testis 3-4 mm. thick, the
arteries and veins of which had been injected with India ink were
speedily rendered transparent by a clearing treatment similar to that
for the less resistant embryonic tissue.

In preparing injections for corrosion specimens of larger embryonic
and adult testes, great difficulty was experienced until it was discovered
that seven per cent celloidin would flow quite readily through a medinm-
sized hypodermic needle. ‘Thus in injecting the adult sex gland it was
only necessary to find the point at which the spermatic artery reached
the gland in order to avoid the difficulty met with in forcing the in-
jection mass through the many coils of artery which lie in the cord of
the pig near its attachment to the epididymis. 'The corrosion was ac-
complished with hydrochloric acid and pepsin, after which the specimens

5 Van Wijhe, J. W.: A New Method for Demonstrating Cartilaginous Mikro-
skeletons. Kononklijke Akademie van Wetenschappen Te Amsterdam, 1902.

®Lundvall, H.: Ueber Demonstration Embryonaler Knorpelskelette. Anat.
Anz., pp. 219-223, Band XXV.

444 Development and Vascularization of the Testis

were washed and placed permanently in glycerine. Because of the
thick and very resistant albuginea a rapid corrosion was more easily
obtained when the fresh gland was placed for an hour in concentrated
hydrochloric acid, after which it was treated in the usual way with a
ten per cent aquous solution of this acid for twenty-four hours, followed
by the ordinary peptic digestion in the thermostat.

Gross Development of the Testis——TKeibel in his Normentafel for the
pig gives the anlages and traces histologically the development of the
organs, but gives no measurements of these organs.

TABLE SHOWING IN MILLIMETERS THE LENGTH OF THE Bopy AS COMPARED TO
THAT OF THE KIDNEY, WOLFFIAN Bopy, AND SEX GLAND OF PIG EMBRYOS.

The measurements were made from vertex to breech, and include all of the
embryos in each uterus. In case of asymmetric development of these glands
in any embryo averages were made of the lengths of both organs.

ea Kidney?) (0.9 "ae |) eae
20 2 7.3 1.5
ray 1 73 1.5
20 eal 7.4 1.4
IVETAUIS ls Ata ees cites eet 2D 1.2 7.2 1.5
22 163 “(eal 1.4
21 1.0 fies 1.5
20 1-2 T2 1.5
(28 2.5 9.0 1.7
28 2.6 8.0 1.6
(WiPERUSUZ' 5 Rel aee ee mete eer: 4 29 2.5 8.5 1.7
27 2.4 9.2 ing,
28 3.0 7.0 ihe 7/
bop PAM 87 1.4
FSi 3.9 9.2 2
33 4.0 9.1 2
WUC Gk oe ¢-culo eas ~ 33 3.8 9.0 2
3 Ba 9.1 1.9
33 4.1 8.9 2
40 5.8 10.0 Sen
41 5.9 10.0 3.2
40 5.9 11.0 Sif
42 5.9 11.0 3.1
UG erUSR ee Rak ene ees 41 5.8 10.0 3.1
| 43 6.0 11.2 a2,
41 5.8 10.5 3e2
42 5.9 113 3.2
| 39 5.6 11.5 3.1

L41 5.9 10.0 3.0

Uterus 5

Uterus 6

Uterus 8

Uterus »

Uterus 10

Uterus 11

ppieiisice ie! .w)ie}vells'..e, 5; ie yiets

Eben C. Hill

Vertex-
Breech.

49
‘
yo 49
48
50

( 68
67
68
69
67
68
67

Kidney.

7.3
7.8
6.8
8.0
7.8

Ilr
11.5
11.5
11.4
11.2
11.5
11.6

14.5
1b Ti
14.3

SC
15.9
15.6
15.5
15:7
15.6

18.0
19.0
19.0
18.5
18.2

23.5
22.8
23.0
24.0

31.0
30.0
35.0

Woiffian
Body.
11.0

10.5
12.0
10.0
10.5

11.8
12.0
11.4
11.5
11.2
Lat
11.4

10.2
10.0
10.5

9.2
9.0
9.0
9.8
8.7
8.9

7.0

7.2

7.5

7.0

7.0
Epididymis

7.0
7.0
7.3

8.2
8.0

445

Testis or
Ovary.
3.5
3.2
3.2
3.5

3.5

4.0
4.0
4.0
3.7
3.8
4.0
3.8

4.6
4.4
4.6

4.8
5.0
5.1
5.0
5.0
4.7

5.0
5.3
5.0
5.5
5.2

Testis

5.8
5.6
5.5

6.0
6.3
6.5

A study of the foregoing table shows little variation in the body

lengths of the embryos in each uterus.

In the cases of abnormal de-

velopment of the kidney, it is interesting to note the corresponding size

of the Wolffian body.

That a balancing of function exists between these

two glands is suggested by the fact that an embryo having an unusually
large kidney development has correspondingly small Wolffian bodies.

446 Development and Vascularization of the Testis

The measurements were made regardless of whether the sex gland
was male or female, although in embryos beyond thirty-three mm. in
length this sex distinction is observable.

In the adult pig testis the measurements vary considerably. The
average might be placed at sixty-five mm. long, forty-two mm. deep and
thirty-seven mm. wide with an average weight of sixty-eight grammes.
These results are of interest when compared with those obtained by
Krause‘ for the human adult sex gland. His averages were thirty-seven
mm. long, twenty-eight mm. deep and twenty-four mm. wide, with the
weight falling fifteen and twenty-four and a half grammes.

Development of the Blood Supply.—Concerning the embryonic devel-
opment of the testis much has been written and it seems useless to enter
into a discussion of the histogenesis and descent of the gland. Among
the more recent studies of these subjects the article by Allen® on the
ovary and testis of mammals will be found to contain a comprehensive
survey of the literature and in this monograph the author outlines
minutely the growth of the testis of the pig. He. however, makes no
mention of the blood supply.

The first macroscopic indication of the vascularization of the testis
is found when the embryo pig is thirty-three mm. in length. At this
time the sex gland is situated relatively lower on the mesial surface of
the Wolffian body, which may to some extent account for the low level
at which the spermatic artery arises from the aorta. Concerning the
origin of this artery there has been some discussion as to whether at
times it may arise from one of the lower Wolffian arteries. Of the
seventy-five or more specimens ranging in length from twenty-five mm.
to two hundred and twenty mm., only one was found in which the artery
arose otherwise than from the aorta. In this exception the spermatic
artery came from the most caudal Wolffian artery close to its origin from
the aorta.

In the thirty-three mm. stage seen in Fig. 1 no convolution is ap-
parent in the spermatic artery which courses ventral to the Wolffian
arteries. In this figure, as well as in the two following ones, it was
found advisable to lay back the Wolffian body from its normal position
in order to more clearly demonstrate the vascular supply to these glands.
The renal artery which penetrates the kidney when the embryo is twenty-
eight mm. in length is quite prominent and a few glomeruli are seen

7Krause, W.: Zum Spiralsaum der Samenfaden. Biol. Cent., 1881.
8 Allen, B. M.: Embryonic Development of Ovary and Testis of Mammals.
Am. Jour. Anat., Vol. III, No. 2.

Eben C. Hill 444

in the cleared specimen. As in the human embryo, rotation of the kid-
ney occurs before the entrance of the blood supply, as has been shown
by Pohlman.’ My study of sections of pig embryos places the rotation
of the kidneys in this genus between twelve and fifteen mm.” In the
human embryo Pohlman has shown that the vascularization occurs be-
tween twenty-five and thirty mm., while in the pig I find this vasculariza-
tion of the kidney at twenty-eight mm. The adrenal, which is depicted

EDTGeale IG 22

Fic. 1. Cleared specimen of the testis, kidney, and Wolffian body of an
embryo pig 33 mm. in length, showing the first appearance of vascular supply
to the testis. x 6. W. B., right Wolffian body, 8.6 mm. in length; Ad., ad-
renal; A.,-aorta; 7., testis measuring 2 mm. in length; S. A., spermatic ar-
tery; K., kidney, 3.8 mm. in length; W. D., Wolffian and Miillerian ducts;
U. A., umbilical artery.

Fig. 2. Cleared specimen of the right testis, kidney, and Wolffian body of
an embryo pig 48 mm. in length, showing the commencement of convolutions
in the spermatic artery and the increased blood supply. xX 6. K., right
kidney, 6.5 mm. in length; Ad., adrenal; W. B., Wolffian body, 10 mm. in
length; A., dorsal aorta; R. A., renal artery; 7., testis, 3.5 mm. in length;
S. A., spermatic artery; W. D., Wolffian and Miillerian ducts; U. A., umbilical
artery.

Medicine, Vol. VII, No. 25.

Hill, E. C.: On the First Appearance of the Renal Artery and the Rela-
tive Development of the Kidneys and Wolffian Bodies in Pig Embryos. Johns
Hop. Hosp. bull., Vol. XVI.

448 Development and Vascularization of the Testis

in the illustration merely as a land mark, is densely injected but no
attempt has been made to show its blood supply and in this and the fol-
lowing series it appears as if uninjected.

The Wolffian body at this time receives from ten to twelve arteries
which richly supply the gland. In many cases a pressure sufficient to
insure a perfect injection of the sex gland resulted in a double injection
of the Wolffian bodies and kidneys. In the Wolffian bodies the sinusoids
described by Minot are beautifully demonstrated in sections of five to
twenty p in thickness.

Allen* has noted a great activity in the formation of the primitive
sex cells of the seminiferous tubules and of the cords of Pfliger and
rete cords about this time, and it is posible that this increased activity
is due to the presence of the blood stream. Allen has also shown a sex
differentiation in the embryo of twenty-five mm. which he bases upon
histological observations. Through a study of the vascularization, this
sex distinction is clearly marked at 33 mm., for as Clark” has shown
“upon the peculiarities of each circulation the differential signs of sex
are based, a visible dorsal vessel always indicating a male; an alabaster-
like non-vascular white cortex a female embryo.” This distinction, how-
ever, is true more particularly of the pig and is of doubtful value in
differentiating the human sex glands.

In the embryo of forty-eight mm: (Fig. 2) the spermatic artery is
found to have encircled a greater portion of the capsule of the sex gland
and a certain amount of convolution is evident in the artery just before
it reaches the testis. These convolutions are more marked as descent of
the gland occurs, and this may be due in part to an attempt to shorten
the artery. Thoma, however, in his studies of the development of the
vascular system gives no such method of shortening. Nor, indeed, could
this explanation account for the subsequent convolutions which occur
after the testis has begun its descent from below the lower pole of the
kidney. In this latter case there is a most decided lengthening accom-
panied by more marked convolutions. A similar condition is not found
in the human embryo, nor to such marked extent in the mouse of this
stage.

Microscopic sections demonstrate the capsular artery branching with
a certain definite regularity on the surface of the gland, and sending
minute arteries into the substance of the testis. A thick section shows
these vessels entering perpendicularly and giving off branches which
form capillary anastomoses around the medullary cords.

“Clark, J. G.: The Origin, Development, and Degeneration of the Blood
Vessels of the Human Ovary. Johns Hop. Hosp. Rept., Vol. IX.

Eben C. Hill 449

When: the embryo has attained a length of eighty-seven mm. (Fig. 3)
several of the anterior Wolffian arteries have disappeared and there is
a decided atrophy of the organ itself. The capsular artery, a name which
may be applied to that portion of the spermatic artery which supplies
the albuginea and glandular substance proper, is.seen to give off many

Fic. 3. Cleared specimen of the right testis, Wolffian body and kidney of
an embryo pig 87 mm. long, showing descent of testis and atrophy of the an-
terior Wolffian arteries. X 6. A., dorsal aorta; K., right kidney, 14.7 mm.
in length; S. A., spermatic artery; W. B., Wolffian body, 9.5 mm. in length;
W. D., Wolffian and Miillerian ducts; J. A., iliac artery; U. A., umbilical
artery.

small branches, some of which are growing over the surface of the organ
while others are penetrating deeply into the substance of the gland. In
several specimens of this and later stages the capsular artery is found
to divide into two main branches immediately after reaching the gland.
The attachment of the sex gland to the Wolffian body is quite firm at
this time.

450 Development and Vascularization of the Testis

Fig. 4 (128 mm.) shows a marked increase in the diameter of the
spermatic artery, and also greater tortuosity of this vessel. The sex
gland is seen to have assumed a different position in relation to the re-
mains of the Wolffian body. This semi-rotation is, perhaps, caused by
the convexity of the lower pole of the kidney as the testis in descending

Fic. 4. Abdominal cavity of an embryo pig 128 mm. in length, cleared
specimen of a right testis taken from an embryo or the same size and substi-
tuted in order to show the relative positions of the organs. X 6. This figure
also shows the great tortuosity of the spermatic artery and by a comparison
with Fig. 3, illustrates the occurrence of semi-rotation; K., right kidney, 18.2
mm. in length; S. A., spermatic artery; A., dorsal aorta; 7., testis, 5.3 mm.
in length; U., ureter; R., rectum; W. M., Wolffian and Miillerian ducts; U. A.,
umbilical artery; B., bladder.

assumes a more dorsal position. Frequent anastomoses are noticed upon
the capsule, and a small twig at the anterior end of the gland anasto-
moses with the branch from the spermatic artery which supplies the
future globus major. Since the blood supply to the epididymis is not
shown in any of the drawings, this branch has not been indicated.

Eben C. Hill 451

The entrance of the testis into the internal ring occurs between the
sizes of one hundred and ninety and two hundred and twenty mm. The
left gland usually enters the internal ring first, and in Fig. 5 (210 mm.)

Fic. 5. A transparent specimen of the right testis of an embryo pig 210
mm. in length. The relation of this gland to the other organs was obtained
from a fresh tissue specimen of an embryo of the same size. xX 6. In this
figure the left testis is seen to have nearly passed the internal ring, while
the right sex gland has just entered. K., right kidney; A., dorsal aorta; Z£.,
epididymis; U., ureter; R., rectum; M. D., W. D., Miillerian and Wolffian
ducts; U. A., umbilical artery; B., bladder; 7., testis, 6 mm. in length.

only the globus major of the epididymis is apparent. The capsular
artery has sent out branches which nearly encircle the sex gland, and

452 Development and Vascularization of the Testis

these encircling arteries have almost completed their growth around the
testis. A certain limited portion lying close to the epididymis is never
encroached upon by these branching arteries. From the spermatic
artery before it reaches the testis several branches arise, from five to
seven in number, which supply the cord and globus major and minor.
Frequent anastomoses are seen on the albiginea, and small branches
which encircle the anterior portion of the gland form anastomoses with

Ee. 6. HIG. 1.

Fic. 6. The macroscopic appearance of an arterially injected adult pig
testis, showing peculiar tortuous arrangement of the branches of the cap-
sular artery in the tunica albuginea. xX %.

Fic. 7. A macroscopic drawing of the left injected testis of a human fetus
of seven months. X 6%. The dotted lines show the position of the cord and
epididymis.

arteries supplying the globus major: thus allowing blood to penetrate
the testis should the posterior portion of the capsular artery become oc-
cluded. The Mullerian ducts are atrophied and appear as ridges upon
the Wolffian ducts which have increased in diameter of lumen and in
thickness of wall.

The relations of the superficial blood supply in the testes of the adult
pig and mouse and of the human fcetus of seven months together with

Eben C. Hill 453

the comparative positions of the spermatic cords is shown in figures
6, 7% and 8. In the illustration showing the testis of the human
embryo, the arteries are found to encircle the gland, being distributed
to the under surface of the albuginea. Frequent anastomoses are formed
similar in many ways to the adult superficial supply in the mouse, but
differing materially from the arterial distribution in the capsule of the
pig testis. The relative position of the spermatic cord to the epididymis
is quite noticeable and may be due to the differences in the manner of
suspension of the gland with reference to the horizontal plane of the
body. The comparative size of the globus
major and minor is also markedly differ-
ent. In the pig the globus minor is at
times as large and not infrequently larger
than the globus major. Few convolutions
are apparent in the human embryonic
gland, while in the testis of the embryo
mouse as well as in the adult a certain
amount of tortuosity is met with, not, how-
ever, anywhere near as marked as in the
pig.

Arterial and Venous Supply of the
Adult Testis of the Pig—rThe capsular
artery gives off on the internal surface of
the Tunica albuginea at rather regular in-
tervals tortuous rib-like branches which +
nearly encircle the gland. These branches Fic. 8. The capsular dis-
penetrate the substance of the gland follow- eee ee oF Lou poe
ing the septa and entering perpendicularly adult mouse. xX 3%. This
till they reach the mediastinum. Except in eeoianal eee ips peers
a very few instances no branches are tive positions of the cords

given off from these perpendicular arteries ieee HOES NET 1a

until after the abrupt retro-flexion occurs
near the center of the gland. After this sudden backward bending,
many branches are given off which, coursing toward the surface of
the testis, send off smaller twigs which in turn divide into capillaries
around the seminiferous tubules and supply the stroma of the gland.
The veins collect from these capillaries and merging into larger vessels
follow the septa directly toward the albuginea where passing under the
arteries on the internal surface of the tunica albuginea, they encircle
the gland and passing toward the epididymis form the pampiniform

454 Development and Vascularization of the Testis

plexus. These veins are about twice the size of the branches from the
capsular artery and show an intricate anastomosis. Upon cross section
of a doubly injected gland some seven or eight perpendicular descending
arteries will appear and perhaps eight to twelve collecting veins.

The extreme vascularity of the gland is shown in Fig. 10, which is
an arterial and capillary injection made with India ink and cleared by

Fic. 9. A semi-diagrammatic representation of the circulation in the left
testis of the adult pig. xX 1%. H., globus minor of the epididymis. The
arrows indicate the course of the blood stream. The arteries and capillaries
are red; the veins, blue.

the modified Schultze method. The anastomoses around the tubules are
so profuse that they give the section an appearance of ancient mail
armor.

A microscopic section of the injected testis, stained and cleared by
the Van Wijhe method shows beautifully under low power the manner in

See as

Eben C. Hill 455

Fic. 10. A thick cleared cross section of the testis of an adult pig. x 1%.
This shows the arteries and rich capillary network. C. A., tortuous branch
of the capsular artery which can be seen penetrating to the mediastinum of
the gland and there forming the typical loop before giving off any branches.
A., spermatic artery in its course along the epididymis. The section was
taken about midway between the globus major and minor, and as most of
the epididymis was dissected away an atypically shaped piece of tissue re-
mains.

Fic. 11. A microscopic section of an injected testis of an adult pig cut
504, showing the capillary supply around the tubules. X about 40.

35

456 Development and Vascularization of the Testis

which the capillaries encircle the tubules. In Fig. 11 are seen the
capillaries, some larger arteries and a portion of one of the large as-
cending perpendicular branches given off shortly after the looping of
the descending perpendicular branch near the center of the gland. The
tubules of the pig testis show this capillary arrangement somewhat better
than do those of the human adult male sex gland, for as was shown by
Krause the tubules of the human testis measure two-tenths of a mm. in
diameter, while I find that the tubules of the sex gland of the pig are
between two-tenths and three-tenths mm. in diameter.

Fig. 12. Corrosion specimen of the testis of an adult pig, showing the
typical arterial loops. X 2.

The various types of arterial loops comprising the descending perpen-
dicular artery and its ascending branches are shown in Fig, 12. These
loops were taken from corrosion specimens of the adult testis of the pig.
In the figure they are arranged in order of frequency, the last having
a recurrent branch before the abrupt looping, being very rare. No simi-
lar arrangement was found from studies of corrosions of the human
gland.

What the causes are which produce this peculiar arrangement it is
difficult to say. In the first five figures representing early embryonic
stages the arteries were found to penetrate the gland perpendicularly
but to give off branches as they descended.

This is depicted in Fig. 13.

Eben ¢.Eall 457

No typical looping occurs till after the gland is in the scrotum and
the subsequent rapid development has begun. This leads one to sur-
mise that the sudden growth, which changes the embryonic organ from
one measuring, 6 mm. x 3 mm. x 2.8 mm. unto the adult gland measuring
65mm. x 42 mm. x 37 mm., is accountable for this peculiarity of blood
supply. This seems to be especially plausible when the relative positions
of the mediastina of the human and pig testes are compared. Probably
the development is so rapid that the arteries which enter perpendicularly

Fic. 13. Fig. 13a shows the left testis of an embryo pig 48 mm. in length.
Then entrance of the perpendicular branches of the capsular artery is shown.
x< BE

Fig. 13b illustrates the depth of penetration of these same arteries in the
testis of an embryo pig of 87 mm. X 9.

Fie. 13c shows the entrance and distribution of these same arteries in the
left testis of an embryo pig of 210 mm. X 9.

in order to penetrate to the center are of necessity twisted back upon
themselves in supplying the rapidly growing tubules whose development
must be toward the circumference. The mediastinum is in the center
of the gland and hence the tubules in developing radiate from this as a
center.

The Blood Supply to the Albuginea—The vascular supply to the albu-
ginea is shown in Fig. 14.

Above the large capsular arterial branches and veins which supply the

458 Development and Vascularization of the Testis

glandular tissue of the testis and which le on the inner side of. the
tunica albuginea, is found a delicate plexus of small arteries, capillaries
and veins grouped in irregular polyhedral forms, mostly of four or five
sides, having an area of from nine to sixteen square millimeters. The
arteries enclosing these polyhedrons spring from the encircling capsular
branches and lie external to them. ‘These small vessels are usually ac-

Fic. 14. A cleared specimen of the tunica albuginea of the testis of an
adult pig, showing the arrangement of the vessels supplying this tunic. xX 9.
This specimen was studied in glycerine with a magnification of 80 diameters
(Leitz) for the capillaries of the individual lobules, and with a magnification
of 8 diameters for the arrangement of the lobules. Drawn with camera lucida.
A., one of the tortuous capsular arteries given off from the main capsular
artery. These vessels lie below the small arteries supplying the albuginea.
B, an encirciing capsular vein which ultimately empties into the pampiniform
plexus. These veins also lie beneath the arteries and veins supplying the
albuginea and also pass under the large capsular arteries. OC, a small
artery arising from a capsular artery. D, a small vein emptying into a cap-
sular vein. The arrows indicate the course of the blood stream.

companied by vene comites which carry back the blood to the encircling
veins. Each lobule is filled with a network of capillaries. In corrosion

Eben C. Hill 459

specimens this superficial albugineal blood supply appears as a fine mesh
over the larger encircling arteries and veins. A few of the veins collect-
ing from these superficial vascular lobules empty directly into the pam-
piniform plexus, but as a rule the course is as described.

SUMMARY.

1. The measurements of the embryonic testis, Wolffian body and kid-
ney from the time that the embryo is 20 mm. in length till the sex gland
enters the internal ring are given.

2. A comparison of the size and weight of the human testis with that
of the adult testis of the pig is made. The comparative sizes of the
semniferous tubules of the adult pig testis and human testis is noted.

3. The testis of the pig receives its first blood supply when the embryo
is 33 mm. in length, the kidney having received its blood supply when
the embryo has attained a length of 28 mm.

4, Out of seventy-five specimens only one exception was found to the
usual source of the spermatic artery, and in this case the artery instead
of coming directly from the aorta arose as a branch from the most
caudal Wolffian artery.

5. Marked convolutions in the spermatic artery are first evident when
the embryo is 48 mm. in length.

6. A change in the position of the testis relative to the remains of the
Wolffian body is noted between 110 and 130 mm. This change is almost
a semi-rotation; the testis assuming a more lateral position and having
the future epididymis between it and the aorta.

?. The entrance of the testes into the internal rings occurs when the
embryo has attained a size of 190-220 mm. Generally the left testis
enters first.

8. The differences between the superficial blood supply in the human
embryonic testis and the testes of the adult pig and mouse are indicated,
and the relative positions of the spermatic cords to the epididymes are
shown.

9. The vascularization of the testis of the adult pig is diagrammat-
ically represented, and a theory to explain the peculiarities of the ar-
rangement of the vessels is advanced. This hypothesis is based upon a
suggestion from Dr. Mall that the sudden growth of the testis brings
about a backward looping of the arteries in order to supply the rapidly
developing semi-inferous tubules.

10. The vascularization of the tunica albuginea is illustrated by a
drawing made with the aid of a camera lucida.

EXPERIMENTAL EVIDENCE IN SUPPORT OF THE THEORY
OF OUTGROWTH OF THE AXIS CYLINDER-

BY

WARREN HARMON LEWIS.
From the Anatomical Laboratory, Johns Hopkins University.

WITH 21 FIGURES.

INTRODUCTION.

The controversy over the origin and development of the peripheral
nerves has of late years assumed considerable importance, in ‘that
vigorous attacks have been made on the neurone doctrine as formulated
by His, who considered the axis cylinder to be an outgrowth of the nerve
cell.

Harrison~ has most successfully controverted the cell chain theory,
as advocated by Dohrn, and the Hensen theory of an early invisible
connection between center and periphery, by a brilliant series of experi-
ments on the larve of frogs, where he proves the axis cylinder to be an
outgrowth of the nerve cell.

My experiments given here were not primarily directed towards the
solution of the outgrowth theory, but I think offer, nevertheless, im-
portant evidence in support of this theory.

In transplanting the optic vesicle for the purposes of studying the
origin of the lens and the development of the eye it often happened
that a portion of the brain-tube adjoining the optic vesicle was trans-
planted with it. In experiment Pc,, for example, the optic vesicle and
an adjoining portion of the neural tube of an amblystoma embryo at
a stage shortly after fusion of the neural folds was transplanted into the
region between the optic vesicle and the medulla of another and slightly
older amblystoma. The transplanted eye and brain tissue have con-

1The substance of this work was presented to the Association of American
Anatomists December 28, 1905, and a brief summary is to be found in the
Proceedings of the Association. Am. Jour. of Anat., Vol. V, No. 2.

2Wurther experiments on the development of peripheral nerves. Am. Jour.

of Anat., Vol. V.

AMERICAN JOURNAL OF ANATOMY.—VoOL. VI.

462 Evidence of the Outgrowth of the Axis Cylinder

tinued their growth and differentiation in this strange location. From
the small piece of brain there extend into the mesenchyme large num-
bers of nerve fibers (Figs. 1 and 2). hese fibers extend in various
directions into the mesenchyme without reaching end organs of any
kind. For the most part the fibers are free from sheath cells. At
first, of course, no protoplasmic bridges existed between the nerve cells in
the transplanted brain, and any of the end organs of the host, and it is
very difficult to imagine how they could have subsequently been formed.
It is, of course, equally difficult to form a rational conception of how
such bridges are subsequently transformed into nerves which take such
an erratic course as those shown in Figs. 1 and 2. What possible end
organs are there to account for this enormously greater number of
nerves than are normally present in this region. Then again, these
nerves are ones which, if the piece of brain had remained in its original
position as a part of the brain, would, for the most part at least, have
remained within the brain, and therefore the cells of the transplanted
piece are ones which normally never connect in a direct way with
peripheral end organs. I see no possibility of explaining these fibers
by the Hensen doctrine. The absence of sheath cells makes it likewise
impossible to explain their origin by the cell chain theory. It is evident
also from this experiment that it is not necessary to have a predetermined
path laid down before a nerve can grow out from its center towards
the periphery. That there are such paths, and that they are necessary
under normal conditions for the proper connection of the central nervous
system with its end organs is in no way controverted by this experiment.

In experiment (IV,) (Fig. 3) there is a somewhat similar piece of
transplanted brain tissue connected with a transplanted optic vesicle of
rana palustris. The section shows the differentiated piece of brain
tissue ventral to the otic capsule and from it a large nerve has grown
to the myotome. ‘The piece was transplanted from the eye region into
the otic region of the same embryo, whose neural folds had just closed,
and so long before the nerves were present, and this nerve, as in the
preceding experiment, occupies a strange path in the mesenchyme, and
its presence can scarcely be explained in any other manner than as an
outgrowth from the piece of transplanted brain. Whether the myotome
has acted as a chemotactic agent or not can scarcely be determined from
this one experiment alone, as it might be that the nerve has merely
followed the path of least resistance which happened to be towards the
myotome. Nor am I able to determine as to whether the nerve ends
bear the same functional and anatomical relation to the muscle fibers
that a normal motor nerve would.

Warren Harmon Lewis 463

In experiment DL,, (Fig. +) there is another such piece of brain
tissue connected with the transplanted eye, and between this piece of
transplanted brain tissue and the irregular medulla is a large, strange
nerve in a strange path. Its origin is certainly most satisfactorily ex-
plained by the outgrowth theory.

In another experiment (t;) a small piece of the ectoderm from the
region, destined to form medullar plate in a gastrula of rana palustris,
was transplanted into the region ventral to the otic vesicle of another
and older embryo of rana palustris. This piece of transplanted ecto-
derm has differentiated into nervous tissue and has sent out a long, large
nerve, which passes through the mesenchyme into the region between
the roof of the pharynx and the base of the cranial cartilage, where
it ends abruptly in the mesenchyme (Fig. 5). This experiment illus-
trates at once the great power of self-differentiation various portions
of the central nervous system possess, even when they are isolated at
such very early stages. It is impossible to determine whether this
nerve is an extra one or represents a nerve, which, if the piece of central
nervous system had remained in its normal position, would have arisen
from it and taken a normal course as a cranial or spinal nerve. It
occupies a path which is in no sense predetermined and can only be
explained by its having grown out from this piece of transplanted
tissue probably along the path of least resistance or in a direction accord-
ing to the orientations of the nerve cells from which it arose.

In experiment t, (Figs. 6 and 7) a piece of tissue just anterior to
the dorsal lip of the blastopore of an embryo of rana palustris younger
than that used in the preceding experiment was transplanted into an
older embryo of rana palustris, whose neural folds had just closed.
The piece of tissue hes in intimate contact with the dorsal wall of the
pharynx in the region ventral to the otic capsule. It has differentiated
into nervous system, and from it arises a nerve which runs in among
the epithelial cells of the pharynx wall, divides into several bundles
and can be traced for about 100 micro mm. and then appears to end
in among these cells. This nerve clearly occupies a path in no sense
predetermined for any known nerve, and the utter inadequacy any
such explanation of its origin as would be given by a supporter of the
Hensen doctrine is almost self-evident.

In another similar experiment (t,,) almost exactly the same con-
ditions are to be found, where nerve fibers extend, from a piece of
brain tissue in contact with the pharynx wall, into the wall itself among
the epithelial cells.

Figs. 8, 9, and 10 are from embryos where the brain anterior to

464 Evidence of the Outgrowth of the Axis Cylinder

the place of attachment of the eye was injured at the time of extirpation
of the optic vesicle. These injuries have in some way resulted in
irregularities in the brain in this region, that have given rise to extra
nerves. ‘These nerves extend from a region of the brain which, under
normal conditions, never gives rise to peripheral nerves, or at least to
nerves that leave the brain in the region to run into the mesenchyme.
These nerves are, of course, unilateral and follow strange paths in the
mesenchyme. The injury in the brain in these experiments was done
long before the nerves normally appear. The nerves were not dragged
out from the brain by the needle. The injury in some way made a
path perhaps and may even have altered the orientation of some of
the nerve cells.

In experiment bx, (Fig. 11) the fore part of the brain was removed
before there were any traces of nerve fibers, without injury to the eyes
or nasal pits. From the latter have arisen the olfactory nerves, which
extend in various directions in the mesenchyme without reaching the
brain. ‘The nerve fibers are evidently outgrowths of the cells in the
olfactory organ and are in no way connected with a central organ. It
would seem impossible to explain their presence by the Hensen doctrine.
In like manner the nerve fibers arise from a transplanted nasal pit.
Fig. 12 is from a nasal pit which was transplanted beneath the ectoderm
dorsal to the eye. After transplantation differentiation of the organ
continued and nerve fibers were sent off into the region between it and
the ectoderm, Fig. 13, but extend only a short distance.

The extraordinary interesting behavior of the optic nerve in the
transplanted eyes afford very striking evidence of the outgrowth of the
optic nerve from the eye.

The optic vesicle in these experiments was cut off and transplanted
into the otic region of the same embryo, shortly after closure of the
neural folds and long before the optic nerve forms.

In the majority of transplanted eyes the optic nerve pierces the retina
as far as the outer or pigment layer, it then takes a course in among
the pigment cells, often running for long distances as a compact bun-
dle between these cells, and finally ending among them without enter-
ing the mesenchyme. Such nerves do not seem to be provided with
sheath cells. Occasionally the optic nerve passes through the pigment
layer into the mesenchyme and after running in it a short distance ends
there.

In one embryo the optic nerve was evidently turned from its course
in the outer molecular layer, in which layer it can be followed for many
sections, and ends there without reaching even the pigment layer. Some-

Warren Harmon Lewis 465

times, in the somewhat irregular transplanted eyes, the optic nerve
may divide as it passes through the retina, one branch passing out into
the mesenchyme and the other entering the pigment layer.

Fig. 14, from experiment DF,, shows a very remarkable condition, in
which the transplanted eye touches the medulla in the region of
the choroidal fissure. The optic nerve passes into the medulla. Fig.
15 shows an even more remarkable example of the course of an optic
nerve from the eye to the medulla. The position of the choroidal fissure
is on the ventral side of the transplanted eye and some distance from
where the eye nearly touches the medulla. The nerve leaves the eye
at the fissure, passes at first in among the cells of the outer layer (Fig.
16) then comes to lie close against the outer layer (Figs. 17 and 18),
and finally jumps across a thin layer of mesenchyme into the medulla
(Fig. 19). In some instances, the optic nerve after entering the medulla,
can be followed as a fairly compact bundle of fibers for a considerable
distance towards the anterior end of the brain, but in most of the
specimens it soon disappears after entering the brain.

In a few of these somewhat irregular transplanted eyes the optic
nerve takes a very curious course passing across the cup cavity from
the ganghonic layer through the pupil and then into the mesenchyme
(see Figs. 20 and 21), ending there. In both of these experiments a
small bundle of optic nerve fibers pierces the retina as far as the pig-
ment layer. In transplanting these eyes the ganglionic layer was
probably injured in such a way as to interfere with the normal path
of the nerve fibers, and so they have probably followed the path of
least resistance through the pupil and out into the mesenchyme.

It would seem to me impossible to explain these various conditions
of the optic nerve on any other basis than that they are outgrowths of
nerve cells of the ganglonic layer of the retina.

These experiments furnish important evidence in favor of the out-
growth theory of the nerve fiber, and also that the fiber is a process
of a nerve cell and not from a chain of cells.

Nerves do not necessarily need predetermined paths, but may grow
into the mesenchyme in various directions, probably along the path of
least resistance. That under normal conditions normal nerves do follow
predetermined paths is in no way disproyed by the experiments.

The hap-hazard manner in which these various nerves grow into the
mesenchyme without reaching any end organ or even apparently going
towards any especial end organ, would indicate nerve fibers can grow out
in the embryonic mesenchyme without being attracted there by a special
chemotactic agent.

466 Evidence of the Outgrowth of the Axis Cylinder

Fic. 1. Experiment Pe,. Embryo amblystoma killed 21 days after opera-
tion. The optic vesicle and piece of brain tissue adjoining it were trans-
planted from an amblystoma embryo, of a stage shortly after fusion of the
neural folds, before the nerves appear, into the region between the otic
vesicle and medulla of an older embryo. Section through transplanted eye
and brain showing large groups of nerve-fibers running out into the mesen-
chyme in various directions; most of them are without sheath cells. Portions
of these fibers from the neighboring sections were projected into the figure.
x 90 diameters.

Fic. 2. Experiment Pe, Section through another portion of this trans-
planted brain showing other groups of nerve-fibers leaving it. These various
nerve-fibers have been projected into the figure from the neighboring sections.
They do not include those in Fig. 1. X 90 diameters.

Fic. 3. Experiment IV,. Embryo rana palustris killed 5 days after trans-
plantation of right optic vesicle and part of adjoining brain wall to the
region ventral and posterior to the otic capsule. Section through portion of
transplanted brain, posterior end of otic vesicle and medulla. From the
transplanted brain a large nerve passes directly through the mesenchyme to
the myotome. The distal 144 of the nerve is projected into the figure from
the adjoining sections. xX 90 diameters.

Fic. 4. Experiment DL,,. Embryo rana sylvatica killed 5 days after trans-
plantation of optic vesicle caudal to otic capsule. The ectoderm and medulla
were injured during the operation in the region into which the eye was trans-
planted. Section through this injured region caudal to otic capsule and
anterior to transplanted eye. There is an increase in the size of the irregular
medulla at the place of injury, and from here a large extra nerve passes to
the ganglionic mass connected with the injured place on the ectoderm. Ven-
tral to this extra nerve is seen a portion of the 10th cranial nerve which
joins the medulla here. x 90 diameters.

Fig. 5. Experiment t;. Embryo rana palustris killed 12 days after having
had a small piece of the medullary plate region from the gastrula of another
rana palustris embryo, transplanted into the region ventral to the otic cap-
sule. Section through this transplanted piece which has developed into brain
tissue and sent off a long nerve which passes medianwards between the base
of the cranial cartilage and the roof of the pharynx. This nerve ends in the
mesenchyme. The ventral part of the otic vesicle is seen in the section
x 90 diameters.

Warren Harmon Lewis 467

468 Evidence of the Outgrowth of the Axis Cylinder

Fic. 6. Experiment t,. Embryo rana palustris killed 12 days after having
had a small piece of the ectoderm anterior to the dorsal lip of blastopore of
an embryo of the same species transplanted into the otic region. The
embryo at the time the piece was transplanted into it had completed the
closure of the neural folds. An examination of the sections show that the
transplanted piece has differentiated into nervous tissue like that of the
central nervous system. The figure is from a section through this trans-
planted piece and shows its location ventral to the otic vesicle and in inti-
mate contact with the dorsal wall of the pharynx. From this piece of brain
tissue nerve-fibers pass into the epithelial wall of the pharynx and can
then be traced in among the cells for about 100 u. XX 90 diameters.

Fie. 7. Experiment t,. Section through pharynx wall showing position
of nerve-fibers between the epithelial cells. » 360 diameters.

Fig. 8. Experiment DL,,. Embryo rana sylvatica killed 5 days after
partial extirpation of the right optic vesicle. The brain anterior to the
attachment of the optic stalk was injured. Section through injured region
showing extra nerve which runs out into mesenchyme, it divides part, forms
a loop, and enters the brain again dorsal to its origin; the main branch
turns abruptly caudalwards and can be traced through many sections until
it appears to join a portion of the Gasserian ganglion. A few sections caudal
to the origin of this nerve another extra nerve arises from a similar position
on the brain; it can only be traced a few sections out into the mesenchyme.
The nerve shown in the figure from a composite of the 3 or 4 neighboring
sections. X 90 diameters.

Fic. 9. Experiment DL,,. Embryo rana sylvatica killed 5 days after
partial extirpation of the right eye. Section through anterior end of regen-
erated right eye and brain showing a small extra nerve running from the
brain to a small ganglionic mass. This section is 80 ~ anterior to the optic
nerve, and on the normal side there is no trace of such a nerve between the
olfactory and optic nerves. X 90 diameters.

Fic. 10. Experiment DL. Embryo rana sylvatica killed 3 days after
partial extirpation of the right eye. Section through fore-brain 100 ~ anterior
to optic nerve showing large extra nerve leaving brain and running out into
mesenchyme. This nerve splits into several branches, the longest and largest
runs caudally over the dorsal surface of the regenerated eye and is lost in
the mesenchyme caudal to the eye. There is no corresponding nerve on the
normal side. The brain on the right side is irregular and evidently was
injured during the operation. X 90 diameters.

Fic. 11. Experiment bx,. Embryo of amblystoma killed 9 days after
removal of anterior portion of brain. The brain was removed through a
dorsal incision before the first appearance of the cranial nerves. Section
through anterior end of head showing anterior ends of eyes and nasal pits
with nerve-fibers passing out into the mesenchyme from the latter. The
ectoderm in the mid dorsal region dips down into the mesenchyme in the
region from which the brain was removed. X 45 diameters.

Fig. 12. Experiment Sp,. Embryo of amblystoma killed 16 days after
having had the nasal pit of another embryo transplanted beneath the ecto-
derm in the region dorsal to the right eye. Section through right eye and
transplanted nasal pit. Where the nasal pit is in contact with the outer
layer of the optic cup this portion of the outer layer is free from pigment
and in places of greater thickness than normal. Between the nasal pit and
ectoderm are olfactory nerve-fibers. %* 45 diameters.

Fic. 13. Experiment Sp,. Section through edge of nasal pit and ecto-
derm showing the position of the olfactory nerve-fibers. Sections anterior
to this one show olfactory nerve-fibers passing from the nasal pit a short
distance ventrally close to the median surface of the eye. X 180 diameters.

Warren Harmon [Lewis 469

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470 Evidence of the Outgrowth of the Axis Cylinder

Fic. 14. Experiment DF;. Embryo rana palustris killed 4 days after
transplantation of optic vesicle into position caudal to normal eye region.
Section through transplanted eye showing formation of optic cup and course
of optic nerve from transplanted eye into medulla in the region of the V
nerve. The optic nerve passes through the marginal veil into the grey matter
where the fibers become scattered and can only be followed for a few sec-
tions anteriorly. The transplanted eye is separated from the ectoderm by
mesenchyme and is without a lens. It is in contact with the medulla.
x 90 diameters.

Fic. 15. Experiment DF,,. Embryo of rana palustris killed 5 days after
transplantation of the optic vesicle into the region caudo-central to the otic
capsule. Section through anterior end of transplanted eye showing position
of optic nerve-fibers. The optic nerve is projected into this section in part
from the adjoining section. x 90 diameters.

Fic. 16. Experiment DF,,. Section 30 micro mm. caudal to one in figure,
showing optic nerve-fibers leaving retina and running among the cells of the
outer layer. X 360 diameters.

Fic. 17. Experiment DF,,. Section 50 micro mm. caudal to above, show-
ing optic nerve-fibers close against the outer layer. > 360 diameters.

Fic. 18. Experiment DF,,. Section 30 micro mm. caudal to above, show-
ing similar position of optic nerve. X 360 diameters.

Fic. 19. Experiment DF,,. Section 40 micro mm. caudal to above, show-
ing optic nerve running from outer layer into medulla caudal to otic vesicle.
The optic nerve-bundle can only be followed a few sections in the medulla.
x 360 diameters.

Fic. 20. Experiment DF,,,. -Embryo rana palustris killed 19 days after
transplantation of the optic vesicle into the region between the eye and the
otic vesicle. Section through middle of transplanted eye and side of brain.
The transplanted eye shows invagination and differentiation of the layers of
the retina, the cup cavity and pupil are much smaller than normal, and
there is no trace of a lens. The ganglionic layer bordering the cavity has in
places only a few scattered cells as most of the cells form a heap projecting
into the cavity and from this heap of cells a nerve arises which passes across
the cavity through the narrow pupil out into the mesenchyme to the sub-
ectodermal pigment band. There is also a small optic nerve passing through
the retina into the outer layer, the figure shows its position here as pro-
jected from the two neighboring sections. It can be traced for a short
distance only in the outer layer. X 90 diameters.

Fic. 21. Experiment DF,,;. Embryo rana palustris killed 18 days after
transplantation of the optic vesicle into the region between the otic capsule
and medulla. Section through transplanted eye, otic capsule, and medulla.
The eye, owing to irregular invagination, has only a very narrow pupil and
small, irregular cup cavity. Most of the optic nerve-fibers pass from the
irregular ganglionic layer through the narrow pupil into the mesenchyme
ventral to the otic capsule, where it splits into two divisions, the ventral
one of these runs a short distance and ends abruptly in the mesenchyme,
the other runs to the cartilage ventral to the otic vesicle and can be traced
for a short distance along it. The peripheral portion of the nerve, as shown
in the figure, is projected into the section from the neighboring sections.
Another portion of the optic nerve, smaller than the first one, runs through
the retina and seems to end abruptly at the pigment layer. This portion of
the nerve has also been projected into the figure from a neighboring sec-
tion. > 90 diameters.

Warren Harmon Lewis

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EXPERIMENTAL STUDIES ON THE DEVELOPMENT OF
THE EYE IN AMPHIBIA.
III. ON THE ORIGIN AND DIFFERENTIATION OF THE LENS.

BY

WARREN HARMON LEWIS.
Associate Professor of Anatomy, Johns Hopkins University.

WITH 83 FIGURES.

INTRODUCTION.

Since the publication of my paper on the origin of the lens in rana
palustris," I have made many new experiments on lens-formation with
regenerating and transplanted eyes, not only in rana palustris but in rana
sylvatica and amblystoma punctatum. My pupil, Mr. Le Cron, has
also made experiments in the same field on amblystoma.* These new
experiments confirm the conclusions given in my previous paper for
rana palustris and throw additional light on the origin and early de-
velopment of the lens. They leave no doubt, I believe, but that a lens,
arising from the ectoderm in the amphibian embryo, is dependent for
its origin on the contact infiuence of the optic vesicle on the ectoderm,
in other words, the lens is not a self-originating structure. These
experiments indicate that actual contact between optic vesicle and ecto-
derm is essential, and that the optic vesicle has not the power of acting
at a distance to stimulate lens-formation. The size of the lens-plate, the
lens-bud, the lens vesicle, and the early stages of the lens are shown
to be dependent in part upon the actual area of this contact between
optic vesicle and ectoderm. These experiments indicates also that not
only is the lens dependent on the influence of the optic vesicle for its
initial origin, but that its subsequent growth and differentiation is
dependent on the continued influence, probably contact influence, for a
time at least, of the optic vesicle. Le Cron’s experiments were directed
more especially towards this point and show that in amblystoma the
influence of the optic vesicle must be exerted for a considerable period

1Am. Jour. of Anat., Vol. III, 1904.
2Am. Jour. of Anat., Vol. VI, 1907.

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

474 The Origin and Differentiation of the Lens

of time in order that a perfect lens may form. My experiments throw
some light on the nature of the earlest influence of the optic vesicle
on the ectoderm; lens-like structures of the ectoderm can be produced
by mechanical injuries of the ectoderm, as with a needle or other
instrument. ‘These lens-lke structures consist merely of a proliferation
of cells of the inner layer of the ectoderm into small buds, small solid
bodies or small vesicles, but they do not show signs of differentiation
into lens fibers, ete. Their great similarity to some of the earlier
stages of abortive lens-formation suggests the idea that the initial stimulus
of the optic vesicle is such as ‘to cause at first only an increase in the
rate of cell division in the area of contact of the ectoderm, and it may
be that the earhest stimulus of the optic vesicle is purely mechanical.
It was shown in my previous paper that probably any portion of the
inner layer of the ectoderm is capable of giving rise to a lens when
properly stimulated. There is, then, no especial predetermined group of
cells which must be stimulated, in order that a lens may arise. There
cannot be then in the ovum or fertilized egg, substances, either proto-
plasmic or chromatic, or otherwise, which represent the lens in the
sense in which Conklin has found certain definite kinds of protoplasm
that differentiate into the central nervous system, the muscular system,
ete. Such tissues or organs as the central nervous system, the muscular
system, the ectoderm, etc., which are in a way represented by substances
in the egg, possessing more or less power of self-differentiation, might
be designated as fundamental or primary tissues and the others as the
lens and cornea as secondary tissues. The latter would be dependent
for their origin on reactions between the primary tissues during the
course of development, or if the reaction takes place very early, between
substances which represent them in the ovum. How large this group
of secondary tissues is can only be determined by experimentation. That
the cornea belongs to this class has been clearly shown by my experi-
ments.”

The present paper is concerned for the most part with the effects
on lens-formation after total or partial extirpation of the optic vesicle
with total absence or varying degrees of regeneration of these eyes.

Anatomy of the Eye Region at the Operating Stage.

In embryos of rana palustris and rana sylvatica at about the time
of, or shortly after, the closure and beginning fusion of the neural folds,
the optic vesicle projects from the sides of the brain and produces a

° Jour. of Expt. Zo6l., Vol. II.

Warren Harmon Lewis 475

bulging of the ectoderm on the surface of the embryo (Figs. 1, 2, and 3).
The cavity of the optic vesicle communicates with the brain cavity by
a wide opening and there is very little indication of the formation of
the optic stalk. The ganglionic mass of the fifth nerve lies in direct
contact with the posterior surface of the optic vesicle and partially
covers it. The walls of the optic vesicle are of about the same thickness
as the ventral wall of the brain. There is at this time no indication
of any differentiation between those cells which are destined to form
the eye, or the optic stalk and the brain. That there are differences
not brought out by the ordinary histological methods is evident from
the results of my experiments. Among the embryos allowed to live
about the same length of time after partial or total extirpation of the
optic vesicle, there are regenerated eyes of all sizes, ranging from those
of nearly normal size, to complete absence. In some, only the optic
stalk has regenerated, and in others the brain wall may be defective
from loss of tissue, which was cut away with the optic vesicle. Correlated
with these differences in the regenerated eyes are differences in the
transplanted eyes. In most of the embryos the optic vesicle, after having
been cut away, was transplanted into the otic region of the same or
another embryo, and among these transplanted eyes there are small
ones and large ones, and some with bits of brain tissue attached. Such
results are most readily explained if we assumed that those cells which
go to form the eye are already determined and that the line of separation
between brain and eye cells is sharp and can be indicated by a line, as
cd (Fig. 3°). Cuts separating the eye from the brain lateral to ed would
leave varying numbers of optic vesicle cells attached to the brain and so
give rise to regenerated eyes of various sizes, cuts along the plane cd
would leave no cells for regeneration, while cuts median to the plane cd
would include, with the transplanted eye, various amounts of brain
tissue. As all these conditions are found, in the embryos experimented
upon, it is but natural to conclude that the eye cells are already pre-
determined, although on microscopical examination no line of demar-
cation can be seen between brain and eye cells at the time of closure
of the neural folds. This question will be more fully discussed in
another paper on the origin and differentiation of the optic vesicle.
The lateral surface of the optic vesicle at this stage is in direct con-
tact with the inner layer of the ectoderm over a considerable area (see
Fig. 3). There is very little mesenchyme at this side of the brain, but
ventral to the brain in this region there is a considerable layer of mesen-
chyme. ‘There are no indications of any changes in the ectoderm

476 The Origin and Differentiation of the Lens

leading to lens-formation, nor do these changes appear for some little
time. That is, no indications are to be seen by the ordinary histological
and embryological methods. The cells of the inner layer of the ectoderm
are apparently all alike in this region. That there may be invisible
differences in the cytoplasm or in the nuclei, or differences which have
not as yet been recognized, is, of course, possible, but the following ex-
periments would seem to indicate otherwise.

Thus the optic vesicle will continue its differentiation independently
of any especial environment, as when transplanted into various regions
of the embryo. Not, so, however, does the normal lens-forming ecto-
derm behave when its special environment is altered by removal of the
optie vesicle, for without the influence of the optic vesicle no traces
of the lens appear.

If the lens-forming ectodermal cells are different—the difference only
appearing as development progresses—we should expect this to appear
independently of their special environment (the presence of the optic
vesicle) unless this environment is a necessary factor in bringing about
their progressive differentiation, and as such environment is necessary
the lens cannot be considered as a self-originating structure. It is
only by the experimental method that we can so alter the normal
environment as to afford a means of solving such questions. The
elimination of the possible influence of the optic vesicle is naturally
the first factor to be considered. |

Method of Operation.

The embryos were operated upon under the binocular microscope.
They were placed in small glass dishes either in ordinary tap water
or in a 0.2 per cent salt solution. The latter solution does not offer
any especial advantages, however. The embryo was held with a small
pair of forceps and a semi-circular incision made, through the ectoderm
caudal in the bulge produced by the optic vesicle, with a fine pair of
scissors or a sharp needle. The skin flap thus formed was turned for-
ward and left attached anterior to the eye. The optic vesicle and
surrounding structures were thus exposed without injury either to them
or the overlying ectoderm. In Fig. 2 a much larger skin flap than
was used in the operation is shown turned forward from over the optic
vesicle and ganglionic masses. At a later stage the operation of turning
forward the skin flap becomes quite difficult, owing to adhesion which
takes place between optic vesicle and ectoderm preceding lens-formation.

Warren Harmon Lewis ANT

With the point of a fine needle or a small pair of scissors the optic
vesicle was cut off close to the brain, thus leaving a large opening into
the ventricle.

After removal of the optic vesicle the skin flap was returned into its
original position and held in place for a few minutes either by the
pressure of short pieces of silver wire, or better, by turning the embryo
over, with the skin flap against the bottom of the dish, the weight of
the embryo above being sufficient to hold the flap in place. There is
often more or less contraction of the skin flap, so that it does not always
cover the entire denuded ana. Healing takes place quite rapidly and
in an hour or two the process is usually complete. The embryos were
kept in small glass dishes and the water changed every day or two.
The embryos were killed in Zenker’s fiuid at periods varying from 2
to 20 days after the operation, embedded in paraffine, cut into serial
sections 5 to 10 micro mm. in thickness and stained in hematoxylin and
congo red.

As great care was taken not to injure the skin flap, and especially
that portion of the inner layer of the ectoderm which would, under
normal conditions, have given rise to the lens, it seemed probable that
the lens would arise unless it were in some way dependent for its origin
directly or indirectly on the presence of the optic vesicle. It is easily
shown that the mere turning forward of the skin flap from over the
optic vesicle and then replacing it does not interfere with lens-formation,
and even if a portion of the optic vesicle is cut away the remainder will
regenerate an eye, and a lens will form from the ectoderm, provided,
however, that the regenerated eye comes into contact with the ectoderm
(see Figs. 67, 69, 71, 72, 73, 74, and 76).

RESULTS FROM EXPERIMENTS.

Absence of Lens-formation After Total Extirpation of the Optic Vesicle.

In 50 of the embryos of rana sylvatica thus experimented upon and
killed at from 3 to 16 days after complete extirpation of the optic
vesicle, there was no regeneration of the eye and no indication of a
lens or of lens-formation in the normal lens region from which the eye
was taken. Fig. 4, from an embryo killed 3 days after complete extirpa-
tion of the eye, shows complete absence of the eye and lens. Remnants
of the optic stalk are imbedded in the ventral side of the brain. The
normal eye on the opposite side of the head shows a large optic cup and
lens (Fig. 5). Fig. 6 is from a section through the eye region of

478 The Origin and Differentiation of the Lens

another embryo killed 5 days after complete extirpation of the optic
vesicle. A portion of the optic stalk has regenerated, but there is no
trace of a lens in this region.

This resulting absence of lens-formation after complete extirpation of
the optic vesicle in rana sylvatica is in entire accord with my previous
work on rana palustris." Some new experiments on rana palustris give
me now 21 examples of complete extirpation of the optic vesicle by
the above operation, in which there was failure of lens-formation, asso-
ciated with complete absence of the eye.

Similar operations on amblystoma punctatum by Le Cron’ likewise
demonstrate the lack of lens-formation after complete extirpation of the
optic vesicle at this early stage.

Spemann’s experiments” on rana fusca, made on embryos younger
than those used by me, also show that if the eye spot is killed on the
open medullar plate, the lens fails to appear. In none of my experi-
ments are there to be found lenses or lens-like structures in the normal
lens region when the optic vesicle fails to regenerate.

As the lens often does arise when the eye regenerates its absence is
not due to the operation itself but to the lack of influence of the
extirpated optic vesicle. These experiments indicate very clearly that
the lens is not a self-originating structure.

Absence of Lens-formation After Partial Extirpation of the Optic Vesicle.

In 51 of the embryos of rana sylvatica there was more or less regen-
eration of the eye without any indications, however, of lenses or be-
ginning lens-formation. Figs. 7, 8, 9, and 10 are from sections through
such regenerating eyes of embryos killed from 3 to 5 days after the
operation. They are among the larger of the regenerating eyes without
lenses, but are much smaller, however, than normal eyes of the same
age (see Figs. 8, 75, and 77). These regenerating eyes are separated
from the ectoderm by mesenchyme and were probably never in contact or
not in contact with the ectoderm for a sufficient length of time to stimu-
late lens-formation. The majority of regenerating eyes without lenses
are separated from the ectoderm by mesenchyme; a few exceptional
ones, however, were found to be in contact with the ectoderm (Figs.
11, 12, and 13). The embryos from which these figures were taken

* See figure 6, p. 510, and figure 9, p. 512, Am. Jour. of Anat., Vol. III, 1904.
5Am. Jour. of Anat., Vol. VI, 1907.
®°Verhandl. der Anat. Gesell., 1901.

Warren Harmon Lewis 479

were killed 3 days after the operation, and on the normal side in each
is a well-formed lens (Fig. 59). The regenerating eyes in these embryos
are in contact with the ectoderm by the outer layer, which does not
seem to possess the power of stimulating lens-formation.

In 34 embryos of rana palustris killed at varying ages after partial
extirpation of the optic vesicle there are regenerating eyes of various
sizes without lenses or traces of lens-formation.

Regenerating eyes in contact with the ectoderm for a sufficient length
of time can stimulate lens-formation, but in the above experiments the
absence of lens-formation is to be explained through want of contact
between eye and ectoderm, or to the contact not having been of sufficient
duration; or to the contact having been over too small an area, or to
contact by the outer layer of the optic vesicle, which does not seem
to possess the power of stimulating lens-formation. The adhesion which
ordinarily takes place between optic vesicle and ectoderm before lens-
formation is probably an important factor, and if this is interfered with
even though contact may exist, it is possible that a lens would not arise.

These experiments indicate that the lens is not self-originating, and
that the regenerating eye cannot stimulate lens-formation when separated
from the ectoderm by mesenchyme.

Lens-formation Associated with Regenerating Eyes.

In 25 embryos of rana sylvatica killed from 3 to 16 days after partial
extirpation of the optic vesicle there are regenerating eyes of various
sizes associated with lenses or lens-like structures of various stages‘ and
sizes (Figs. 17, 27, 30, 34, 35, 36, 37, 64, and 65).

Small and imperfect Jens structures are often associated with some
of these smaller regenerated eyes. The larger regenerated eyes, how-
ever, give rise to normal lenses, indicating thereby that the operation
itself, unless a considerable portion of the optic vesicle is cut away, does
not interfere with lens-formation from the skin flap. These large re-

“It has seemed convenient to divide the development of the lens into
several stages: (1) the lens-plate, or the thickening of the inner layer of the
ectoderm; (2) the lens-bud (Fig. 67), projection of this thickened area until
its separation from the ectoderm; (3) the lens-vesicle (Figs. 43, 59, 66, 68, and
69), the vesicle-like structure from the time of its separation from the ecto-
derm until the differentiation of the anterior epithelial layer and lens-fibers;
and (4) the lens (proper), the earlier stages of which are seen in Figs. 61
and 70, and the later stages in Figs. 38, 74, 75, 76, 77, and 78.

480 The Origin and Differentiation of the Lens

generated eyes with lenses in rana sylvatica are very much the same as
in rana palustris (see Figs. 67, 69, 73, 74, and 76).

The sizes of the regenerating eyes during these early stages seem
to be dependent much more upon the amount of eye tissue left attached to
the brain, during the operation, than upon the length of time the embryo
is allowed to live.

In 76 embryos of rana palustris lenses or abortive lenses of various
sizes and stages are associated with.regenerating eyes. In some of
the smaller regenerating eyes the lens-bud or vesicle is very small and
does not show much differentiation (see Figs. 18, 19, 20, 21, 23, 24, 25,
26, 31, 32, 33, 40, 41, 60, 61, 62, 63, 64, and 65. In the larger regen-
erated eyes the lens-buds, or vesicles, or lenses approach more the normal,
as in Figs. 67, 69, 71, 72, 73, 74, and 76. These experiments indicate,
as in rana sylvatica, that the failure of lens-formation, when the eye
fails to regenerate or only regenerates a little, is not due to the mere
reflecting of the skin flap and replacing the latter, but to the lack of
the proper stimulus to the ectoderm. There is every reason to believe that
in these experiments the regenerating eyes were in contact with the ecto-
derm and then stimulated lenses to form, their sizes depending upon
the area of contact. between optic vesicle and ectoderm and upon the
duration of this contact. And the fact that the eye, after the lens
has been stimulated to arise, is separated from the ectoderm by mesen-
chyme is no indication that they were not at one time in contact. I
see no other way of explaining why some regenerated eyes are without
lenses and others have them, except in this manner, as the location of
two eyes may be almost exactly the same at the time of killing the
embryo, yet one may have a lens and the other not.

Abortive Lenses with Regenerating Eyes.

In both rana palustris and sylvatica there are a number of very
curious small lens-buds associated with the small regenerating eyes
(Figs. 18, 19, 20, 21, 23, 24, 25, 26, 27, and 30). Such abortive lens-
buds are merely solid outgrowths of the inner layer of the ectoderm, and
are very much smaller than the normal lenses; they show no especial
indication of differentiation into lens-like structures, yet they were
undoubtedly caused by the small regenerating eyes. Figs. 18, 20, 28,
36, and 37 show only small and imperfect areas of contact between
optic vesicle and ectoderm, and to this is due, in part, the small size
and imperfect development of the lens-buds. The ingrowth of mesen-

Warren Harmon Lewis 481

chyme between the small lens-buds and eye, as seen in Figs. 25, 27, 30,
34, 35, 57, and 58 is an additional factor in causing these abortive
lenses. Some of these lens-buds would probably have remained attached
to the ectoderm for many days, while others, such as shown in Figs.
21, 26, 27, 31, 32, 33, 36, and 37 might ultimately have separated, to form
small solid spherical masses, as shown in Figs. 40, 41, and 42. Figs. 14,
15, and 17 show how the mesenchyme may grow in between the early
lens-plate and the optic vesicle. These lens-plates would probably have
soon ceased to develop and small lens-buds have formed from them.

Somewhat larger regenerating eyes give rise to larger lens-buds and
vesicles, as in Figs. 60, 61, 62, 63, 64, 65, and 67. When such lens-
structures retain more normal relations with the optic cup they will
develop into small but fairly normal lenses, as in Figs. 71 and 72.

The larger regenerating eyes give rise to more normal lenses, as in
Figs. 69, 73, 74, and 76.

These experiments indicate that the lens is neither self-originating
nor self-differentiating, but is dependent for its origin, its size, its differ-
entiation, and its growth on the influence of the eye.

Abortive Lens-formation with Degenerating Eyes.

Among these experiments there are four rather fortunate examples of
degeneration and partial disintegration of the brain and the eye on
the unoperated left side of the head. Three of these embryos were
operated upon for extirpation of the right eye in succession and were
killed 4 days after the operation. The fourth (DL,) was operated upon
at an earlier time. In each the supposedly normal eye is much smaller
than a normal one of the same age, and shows only shallow invagination,
no differentiation of the layers of the retina except the outer pigment
layer. Compare Figs. 79, 80, 81, and 82 with a normal eye (Fig. 77),
from an embryo killed 4 days after the operating stage and of about
the same age. The important point in connection with these degener-
ating eyes is in the size and differentiation of the lenses. Instead of a
large lens, as in Fig. 77, with long lens-fibers and a well developed
epithelial layer we have only small lens vesicles. In experiment, DL,,
there are two such lens vesicles (Figs. 81 and 82) associated with the
same eye. The lens vesicles, although small and retarded in differen-
tiation, show no signs of degeneration. ‘They are very similar to some
of the small lens vesicles associated with small regenerated eyes (com-
pare with Figs. 60, 61, 62, 63, 64, and 65). What I imagine has taken

482 The Origin and Differentiation of the Lens

place is: that something connected with the operation started progres-
sive degenerative changes in the optic vesicle, but before they had
become much advanced a small lens-plate and lens-bud were stimulated.
Owing, however, to the increase in the degeneration the eye lost for the
most part its usual influence on the developing lens, consequently the
great retardation and apparent stoppage of the growth and differentiation
of the later in the vesicle stage. These eyes seem to be rapidly dis-
appearing and it is possible if one of the embryos had been killed a few
days later the eye would have completely degenerated and disappeared,
leaving a lens vesicle of unknown origin, and thus might have been mis-
taken for a self-differentiating and self-originating structure.

In another experiment (t,,) in which tissue from another embryo
was transplanted into the otic region without disturbing in any way
mechanically the optic region both eyes and the brain were found to be
degenerating, and associated with each eye is a small abortive lens
vesicle similar to the ones shown in Figs. 77, 80, 81, and 82. The
embryo was killed 6 days after the operation. ‘The embryo, at the time
of the operation, was of the same age as in the other experiments, con-
sequently there is even more difference between the size and degree of
differentiation of these two abortive lenses and normal ones of the same
age than between those shown in Figs. 79, or 80, and 77.

Lens-like Structures Due to Mechanical Injury of the Ectoderm.

In many of the embryos experimented upon, especially in rana syl-
vatica, but also in rana palustris and amblystoma, ectodermal buds pro-
ject into the mesenchyme from places on the ectoderm liable to have
been injured during the operation (Figs. 45, 46, 47, 48, 49, and 50).
Such buds are found anterior to the regenerating eye, or posterior to
it, more often, however, in the region of the otic capsule. The injury
bud in Fig. 48 is very similar to the lens-buds of Figs. 19 and 58, and
to the ectodermal bud in Fig. 44. The latter was probably formed at
the place where the normal lens was pinched off from the ectoderm.

In making the pockets beneath the ectoderm for the transplanted
eyes, the overlying ectoderm was often injured with the needle. Also
in transplanting the eyes small pieces of ectoderm, either from such
wounds or from the edge of the incision, were often pushed into the
mesenchyme. The smaller ectodermal bodies take on a solid spherical
form, as in Fig. 51 and resemble very much the small lens-like bodies
of Figs. 40, 41, and 42, caused by the influence of small optic vesicles

Warren Harmon Lewis 483

on the ectoderm. The larger ectodermal bodies form yesicle-like struc-
tures, as in Figs. 52, 53, 54, 55, and 56. Some of these bear a-very
close resemblance to small lens vesicles, associated with small regenerating
eyes.

This marked similarity between some of the early abnormal lens-buds
or vesicles and those caused by injury suggest, of course, the idea that
the initial stimulus of the optic vesicle on the ectoderm is one which
merely causes those cells of the inner layer to multiply more rapidly
than normal. The cells of the inner layer of the ectoderm seem to
have the power of responding to the stimulus of the optic vesicle, or
to the mechanical stimulus of the point of a needle, by increased rate
of cell division. There is also a tendency for these groups of cells to
hold together in the form of buds, or spherical masses, the latter being
hollow when large, forming vesicles. In many of the embryos, especially
in rana sylvatica, these injury processes are often very long and large
and seem to result partly from accidental transplanting an attached
piece of ectoderm into the mesenchyme; they may extend for ‘long
distances into the mesenchyme, and when caudal to the otic capsule often
unite with the pharyngeal epithelium. When they occur in the region
of a regenerated or transplanted eye, the entire process, or only a portion
of the process, may undergo transformation into a lens, provided it
comes into contact with the eye, but not otherwise. These will be con-
sidered later in connection with lens-formation from transplanted eyes.

DIscussION OF RESULTS FROM THE EXPERIMENTS.
Is the Lens Self-originating ?

These experiments point very clearly to the lens not being a self-
originating structure. Its entire absence after total extirpation of the
optic vesicle without regeneration of the eye, and its absence in many
of the embryos with small regenerating eyes are very conclusive evidence
in favor of this view. The small abortive lenses with small, irregular,
regenerating eyes and the evident dependence of the lens for its growth
and differentiation on the continued influence of the optic cup, and
Le Cron’s experiments on amblystoma showing that the lens is dependent
on the continued influence of the optic cup for its growth and differen-
tiation even after it has separated from the ectoderm, also support this
view.

These results are not in accord with the conclusions of King.” On

’ Experimental Studies on the Eye of the Frog. Arch. f. Entwirklungs-
merk. d. Organ., XIX, 1905.

484 The Origin and Differentiation of the Lens

page 97 we find the following statement relating to her results: ‘“ These
results apparently furnish evidence in favor of the view that in a
definite region of the ectoderm the cells are destined to form a lens, and .
therefore they will produce such a structure even if an optic cup is
lacking;” and on page 99: “It appears that the power of self-differen-
tiation must be granted lens-forming cells of the ectoderm in the embryo
of rana palustris.” King bases this view on the appearance of “ lens-
like structures” attached to the inner layer of the ectoderm directly
opposite the place where the lens is forming for the normal eye. On
the side of the head where these “lens-like structures” are forming
there is absolutely no trace of an optic cup, the latter having been killed
by puncturing with a hot needle. The very nature of King’s operation
makes us at once suspicious of these “ lens-like structures.” The punc-
turing the side of the head with a hot needle is a very uncertain
and crude mode of operation and as King says (p. 93): “ Presumably
the operation destroyed that portion of the ectoderm that would nor-
mally produce a lens.” In these operations, which were done on em-
bryos somewhat younger than those used by me, it would seem to me also
quite impossible to destroy the optic vesicle completely by puncturing
the side of the brain with a hot needle without destroying completely
those ectodermal cells which King supposed are predestined to differ-
entiate into a lens. If these cells are killed then by operation, King’s
whole argument falls to the ground. Operating on these embryos with
a hot needle to such an extent as to completely or almost completely kill
the optic vesicle and part of the brain is a very severe mode of procedure,
and the after effects of such operations as King’s must be quite extensive,
as about one-half of the embryos died during the first day. It is pos-
sible that King may not have destroyed, in some instances, at the time
of the operation, completely, the optic vesicle, and that the remnant
left may have started the lens-like bud before the complete degeneration,
disintegration, and disappearance of the eye resulted from the after
effects of the operation. I have already called attention to abortive lens-
formation, associated with degenerating eyes, and with a complete de-
generation of the eye, such a condition as King found, might readily
occur, especially when we consider what Le Cron found in amblystoma,
namely, that the early stages of lens-development and differentiation
are dependent on the continued influence of the optic vesicle or optic
cup, for if the eye is removed without injury to the lens-plate or lens-
bud, these structures soon cease to grow and differentiate; the earlier
the eye is removed the less power the lens rudiment has of progressive

Warren Harmon Lewis : 485

self-differentiation.” Again among my experiments on rana palustris
and rana sylvatica there are numerous instances in which the lens-bud
has apparently ceased to develop, owing, I believe, in part, at least, to
a shifting away from the bud of the small regenerated eye, or irregular
transplanted eye, and to an ingrowth of more or less mesenchyme —
between the two; in others irregular changes in the form of the eye which
shifts the contact of the lens from the retinal to the outer layer leads
to similar abortion in lens growth (see Figs. 58, 57, 18, 20, 25, 27, 30,
34, and 40.

Another possible explanation of King’s “ lens-like structures” is that
they are injury buds due to injury of the ectoderm during the operation,
and are so, perhaps, similar in origin to some of those I have already
called attention to in a preceding section.

My experiments on the extirpation, partial or entire, of the optic
vesicle, were performed on embryos a few hours older than those used by
King. The possibility or probability that the lens-forming ectodermal
cells possess at the stage King used any unusual powers of self-differ-
entiation into a lens is scarcely worth serious consideration, as one
would naturally expect this power of self-origination to show itself at
the stage I used more readily than at the stage King used. Spemann’s
experiments were on embryos younger than those used by King, yet
he had no indications of self-origination of the lens. In view of these
various facts of abortive lens-formation it seems to me much better
to explain King’s lens-like structures in one of these ways rather than
concluding that the lens is self-originating.

Can the Optic Vesicle Stimulate Lens-formation from a Distance or is
Direct Contact of the Optic Vesicle with the Ectoderm Necessary?

We have noted that in 85 embryos, 51 in rana sylvatica and 34 in
rana palustris, there were regenerating eyes of various sizes without
lens-formation. Such eyes are usually separated by mesenchyme from the
ectoderm, yet eyes may come into contact with the ectoderm by the
outer layer without stimulating lens-formation. ‘The regenerating eyes
without lenses are usually smaller than those with lenses. Owing to
the small size the mesenchyme is much more likely to grow in between
the eye and ectoderm and then to prevent contact between the two. Tiss

°Proc. Ass. of Am. Anatomists, December, 1906. Jour. of Anat., Vol. V,
p. XI; also Am. Jour. of Anat., Vol. VI, No. 2.

486 The Origin and Differentiation of the Lens

some embryos an irregularity in the skin flap may have prevented its
contact with the stump of the optic vesicle.

From a study of the abortive lenses and lenses of various sizes with
regenerating eyes it becomes apparent that not only is contact between
eye and ectoderm necessary for the initial origin of the lens, but that
the size of the lens-plate and lens-bud is dependent upon the area of
contact, also the development of the lens is dependent on the continued
influence of the eye. The fact that a regenerating eye is often separated
from the ectoderm by mesenchyme is no indication that it was never in
contact with it. Mesenchyme always grows in between the normal eye
and ectoderm (Figs. 77 and 78), yet one would not hesitate to state that
at the time of lens-formation the normal eye was in contact with the
ectoderm. The operations do not interfere very much with the growth
of mesenchyme in the region of the regenerating eye, and as the regen-
erating eyes are smaller than normal and are attached to the brain,
we should expect to find them more often separated from the ectoderm
by mesenchyme than the normal ones, and also at a greater distance from
the ectoderm than in normal eyes. This ingrowth of mesenchyme
often interferes with lens-formation, if the ingrowth takes place before
the lens has started the latter will fail to appear, but if the ingrowth
of mesenchyme occurs after the lens has begun to form, various degrees
of abortive lens-formation occurs, these depending, in part, on the
stage of development of the lens-bud at the time of the ingrowth of
the mesenchyme, separating the lens from the eye. In some instances
the lens-bud may be pulled out into a long process, owing to the adhesion
between eye and lens-bud. As the eye is attached to the brain the pres-
sure of the growing mesenchyme would tend to force the ectoderm away
from it and thus either stretch out the lens-bud or separate the eye
from the lens-bud or the lens-bud from the ectoderm. Figs. 31 and 33
are from experiments where I believe the optic vesicle was originally
in contact with the ectoderm, stimulated a small lens-plate and small
lens-bud, but with the growth of mesenchyme the small eye has been
pushed some distance from the ectoderm, and owing to the adhesion
between the lens-bud and eye the former was pulled out into an elongated
form. Following this has come more or less separation of the lens-bud
and optic-cup, and owing to the original small size of the lens-bud
and its later separation or partial separation from the optic-cup' re-
tardation in development has occurred. The embryos (DF,, and DF,,)
from which Figs. 31 and 33 are taken were killed 4 days after the
operation and are in contrast to the conditions found in another embryo ~

Warren Harmon Lewis é 487

(DL,,, Fig. 71) which was also killed 4 days after the operation. As
is seen in Fig. 71, the small regenerating eye and small lens are separated
from the ectoderm by a considerable layer of mesenchyme. Here there
was probably a larger area of contact between the regenerating eye and
ectoderm than in embryos DF,, or DF,,, and so a larger lens-plate and
lens-bud; there was also probably greater adhesion between optic vesicle
and lens-bud, so that when the mesenchyme expanded the side of the
head the lens remained with the eye. In Fig. 72 a similar condition
is shown, where the area of contact between the small regenerating eye
and ectoderm is very small and only a few cells enter into the lens-bud ;
the mesenchyme as it grows in between the eye and ectoderm tends. more
to separate the lens-bud from the eye than from the ectoderm, and such
conditions as seem in Figs. 27, 30, 34, 57, and 58 occur. Some varia-
tions occur, of course, and the lens-bud may form a small vesicle or
solid body which is separated from both ectoderm and eye, as in Fig. 40.

King has pictured a somewhat similar condition in rana palustris,
where the lens-bud is separated by mesenchyme from the optic-cup “
(Figs. 5 and 6). She, however, explains this condition by asserting
that the “lens can be formed from the ectoderm when the optic-cup is
some distance beneath the surface of the body,” and that “contact be-
tween the optic-cup and ectoderm is not necessarily the stimulus that
tends to the development of the lens.” King’s idea that the regenerating
eye (Fig. 6) was never in contact with the ectoderm (p. 95) is based on
a misconception of the conditions at the time of an immediately after
the operation. There is, of course, at this early stage very little mesen-
chyme in the eye region (see Fig. 3), and the ectoderm lies rather close
to the side of the brain and the developing eye, and I believe that in the
early stages of the regenerating eyes in King’s experiments they must
often have come into contact with the ectoderm and in some embryos
have stimulated lens-formation. The growth of the mesenchyme here,
as in my experiments, would tend either to separate the lens-bud and
optic-cup or to elongate the lens-buds, as in King’s (Figs. 5 and 6).

It would seem better to explain King’s lens-like structures to have
been formed in some such manner rather than to assume that the optic
vesicle can act at a distance, especially when we consider the fact that
these regenerated eyes of various sizes may remain at varying depths
beneath the normal lens-forming ectoderm for from 3 to 18 days without
any signs of lens-formation appearing, and so indicate quite clearly

1 Roux’s Archiv, Bd. 19, Taf. VI.
37

488 The Origin and Differentiation of the Lens

that action at a distance is a process not very likely to occur. ‘The larger
the regenerated eyes the greater the number that show lenses. This
is what one would expect if contact were necessary, for the larger the
regenerated eye the greater its chance for prolonged contact with the
ectoderm.

We are practically forced to conclude that actual contact between optic
vesicle and ectoderm is necessary for lens-formation, as this seems to
be the common factor lacking in the above 85 examples of regeneration
of the eye without lens-formation.

The lens is then dependent on direct contact of the retinal portion
of the optic-cup or vesicle on the inner layer of the ectoderm for its
origin. Abnormal relation between the early lens-plate or lense-bud
and eye is accompanied by more or less abnormal or abortive lens-
formation. The size of the lens, as will be shown further on, and even
the differentiation of the lens, are dependent on the continuance of the
normal relations between the optic-cup and developing lens.

The Lens is Not Self-differentiating.

An optic vesicle transplanted into the region of the otic vesicle, for
example, will continue its growth and differentiation independently of
any especial environment. Invagination, differentiation of the various
layers of the retina, and the formation of the optic nerve takes place
as readily as when the eye has its normal position and attachment to
the brain. It is a remarkable self-differentiating organ. The behavior
of the lens is in marked contrast to eye. Le Cron has shown that in
amblystoma removal of the optic-cup without injury to the developing
lens is followed by abortive lens-formation. The earlier the stage at
which the eye is removed the less power of growth and differentiation
the lens rudiment possesses. Even when the optic cup is removed after
the lens vesicle has separated from the ectoderm its progressive growth
and differentiation soon cease and ultimately degenerative changes occur
in the lens-fibers.

The numerous instances of abortive lens-formation in rana are con-
clusive evidence that here too the lens is not a self-differentiating struc-
ture, but is dependent, not only on the presence of the optic-cup, but
the maintenence of more or less normal relations between the two for
a considerable period of time. Disturbance of such relationship by
irregular invagination or lack of invagination of the optic vesicle and
by ingrowth of mesenchyme results, as in complete extirpation of the
optic cup, in abortive lens-formation.

Warren Harmon Lewis 489

The Size of the Early Stages of the Lens Dependent in Part Upon the
Area of Contact Between the Eye and Ectoderm.

Associated with the smallest of the regenerating eyes are the very
small lens-plates, as in Figs. 15 and 17, or small lens-buds, as in Figs.
18, 19, 20, 21, 23, 24, ete. Here the area of contact between optic
vesicle and ectoderm must have been much smaller than in the normal.
With the somewhat larger regenerating eyes we find somewhat larger
lens-buds and vesicles, as in Figs. 60, 61, 62, 63, 64, and 65. Still
larger eyes show larger lens-buds or vesicles, as in Figs. 67, 69, 71, 72,
and 73. Regenerating eyes which approach the normal eye in size
have lenses, nearly normal in size, as in Figs. 74 and 76. These differ-
ences in the sizes of the lenses, as well as in the sizes of the regenerating
eyes are not due to differences in ages of the embryos, for if cor-
responding stages in the differentiation of the lens are compared
it is found that the actual sizes of the lens-plates in small regen-
erating eyes is much smaller than a normal lens-plate, that the lens-
buds vary in size somewhat according to the size of the regenerating
eyes—the lens vesicles also and the early stages of the lens
proper likewise. In some instances the small sizes are to be accounted
for by loss of the continued influence of the optic vesicle, as when
the latter is separated from the lens structure by mesenchyme. These
differences in sizes are due in great part to the fact that the number
of cells infiuenced to take part in the formation of the lens-plate and
lens-bud is probably directly dependent upon the area of contact between
the ectoderm and the retinal, portion of the eye. In general the area
of contact will vary with the size of the eye. Owing, however, to
irregularities in the position and shape of the regenerating eyes it
might often happen that the area of contact would be either larger or
smaller than the usual relation of this area to the size of the optic vesicle.
Hence the size of the early lens is not always in proportion to the size
of the eye. The lens, for example, in Fig. 73 is larger than in 71 or
72, but the optic-cup is smaller. The adhesion which normally takes
place between optic vesicle and ectoderm preceding lens-formation and
accompanying it is probably an important factor and it is pos-
sible that without such adhesion lens-formation from the ectoderm
will not follow. The area of adhesion may not, perhaps, always be as
large as the area of contact. As, for example, the contact between
the outer layer of the eye and ectoderm is not followed by lens-forma-
tion. Whether the outer layer will adhere to the ectoderm or not
I am unable to say, but presume not, as this would be the most ready

490 The Origin and Differentiation of the Lens

explanation of contact without lens-formation, provided lens-forma-
tion is dependent upon adhesion. The size of the lens-plate and
lens-bud, that is the number of cells of the inner layer of the ecto-
derm which take part in their formation, is dependent then, upon the
area of contact of the retinal portion of the eye and probably upon the
area of adhesion between the retinal portion of the eye and the ectoderm.

The size of the lens-bud and lens vesicle is dependent also upon
the increase in the number of cells, and this is dependent upon the
continued influence of the optic-cup. For how long a period contact
is necessary or for how long a time the lens must maintain its normal
relations with the optic-cup in order that it may develop and grow in-
dependently I am unable to answer at present. Whether a very small
lens-bud would ever form a normal sized lens, even if its relations with
the small optic-cup were maintained as in a normal eye for a long period,
I am also unable at present to determine from my specimens.

Still other factors may play a role in regulating the size of these lenses
as retarded stimulation of the ectoderm by the regenerating eye. It is
possible, of course, that as the embryo gets older the ectoderm responds
less and less readily to the stimulation of the optic vesicle, or that older
optic vesicles stimulate less readily the ectoderm to form lenses. It is
not possible, however, to determine this from my experiments.

The Nature of the Initial Stimulus of the Optic Vesicle Leading to
Lens-formation.

Under normal conditions that portion of the optic vesicle which forms
the retina first comes into contact with the inner layer of the ectoderm,
the two soon become adherent and they can only be separated with
difficulty. Then follows a thickening of this inner layer, which is
greatest nearer the center of this area of contact. Accompanying this
thickening and probably in part, at least, the cause of it, is an increase
in the number of cells in this area. Jn order at first to accommodate
the increase in the number of cells they, through mutual lateral pressure
on each other, are compressed in the axis parallel to the ectoderm and
elongated in the perpendicular axis. ‘This gives the formation of the
lens-plate. Accompanying this thickening there takes place the in-
vagination of the optic vesicle. The invagination of the optic vesicle
into the form of the optic-cup is an active process on the part of the
optic vesicle, as I have already pointed out.” As the cells of the lens-plate

1 Am. Jour. of Anat., Vol. III, Proc. of the Ass. of Am. Anat., p. XIII.

eee eee

Warren Harmon Lewis 491

increase in number they project more and more into the optic-cup cavity
and form the lens-bud. It is possible, owing to the adhesion of the
lens-plate and lens-bud to the retinal layer of the actively invaginating
optic-cup that the latter exerts a pull on the lens-plate and helps in
the evagination of the lens-bud. This same pull may also be the stimu-
lus which causes the cells to multiply as they are elongated. These are,
of course, very difficult points to prove.

The first and most important early influence then which the optic
vesicle exerts on the inner layer of the ectoderm is a stimulus causing the
cells over the area of contact, and especially over the center of this area,
to multiply faster than those in the region about. This is clearly indi-
cated by several facts. There is at first no apparent alteration in
the constitution of lens-plate or lens-bud cells. The cells of some of
the small abortive lens-buds even several days after their formation
are similar in staining reactions to the cells of the inner layer of the
ectoderm. There is a difference in shape, but this is probably due to
mechanical relations. Again the cells of the abortive lens-buds or
vesicles are similar to the lens-buds or vesicles arising from mechanical
injuries of the ectoderm. The fact also that these early lens-buds are
not self-differentiating, points to the cells not being essentially different
in structure from the cells of the inner layer.

That the prolonged influence of the optic-cup does alter the structure
and chemical constitution of the lens-cells would seem self-evident.

CONCLUSIONS.

The lens will not arise from the normal lens-forming region of the
ectoderm without the contact stimulus of the optic vesicle on the inner
layer of the ectoderm. The lens is not a self-originating structure.

The lens will not develop, grow and differentiate, without the con-
tinued influence of the optic vesicle and optic-cup.

The lens is not a self-differentiating structure.

Probably only the retinal portion of the optic vesicle is capable of
stimulating lens-formation from the ectoderm.

The size of the early lens-structure is due in part to the area of
contact or adhesion between optic vesicle and ectoderm, and in part to
the length of time the optic vesicle or optic-cup remains in contact
by its retinal layer with the growing lens-structure.

The intitial stimulus of the optic vesicle on the skin is such as to
cause increase in the rate of cell-division at the place of contact, and
may be only mechanical.

492 The Origin and Differentiation of the Lens

The drawings were all made with the aid of a camera.

Fic. 1. Outline of rana sylvatica at operating stage. The neural folds
are partly, or in some embryos, completely fused. X 12 diameters.

Fic. 2. Same embryo with large skin flap turned forward, exposing the
brain with the optic vesicle, and the ganglionic masses of the cranial nerves.
The ganglion of the fifth nerve partly covers the optic vesicle and was often
partially removed. The skin flap is larger than the one usually used in the
operations, the caudal edge of the operating flap being between the ganglia
of the fifth and eighth nerves. X 12 diameters.

Fic. 3. Section through optic vesicle region of rana sylvatica at operating
stage. The fusion of the neural folds in this region is not complete. The
ectoderm as yet shows no signs of lens formation. x 90 diameters.

Fic. 3a. Outline of section through optic vesicles of rana sylvatica at
operating stage. cd, position of cut for extirpation of the optic vesicle.
xX 22 diameters.

Fic. 4. Experiment DL,,. Embryo rana sylvatica killed 3 days after
complete extirpation of the right optic vesicle. Transverse section through
the right eye region. No traces of right eye or lens are to be found in the
sections. Xx 90 diameters.

Fie. 5. Section through left normal eye and lens of above embryo (experi-
ment DL,,). X 90 diameters.

Fic. 6. Experiment DL.,. Embryo rana sylvatica killed 5 days after the
operation. Transverse section through the center of the left eye, right eye
entirely wanting, except for a bit of the optic stalk (e). There is no trace
of lens formation on the right side of the head. X 45 diameters.

Warren Harmon Lewis 493

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494 The Origin and Differentiation of the Lens

Fig. 7. Experiment DL;. Embryo rana sylvatica killed 3 days after
partial extirpation of the right optic vesicle. Transverse section through the
small regenerated eye. There is no trace of lens formation. The normal
left eye is like the one in figure 6. > 90 diameters.

Fic. 8. Experiment DL,,. Embryo rana sylvatica killed 5 days after
partial extirpation of the right optic vesicle. Transverse section through
small regenerated eye and through the normal left eye. No trace of a lens
is to be found on the right side. X 45 diameters.

Fie. 9. Experiment DL... Embryo rana sylvatica killed 4 days after
partial extirpation of the right optic vesicle. Transverse section through
small regenerated eye. No traces of lens formation are to be found in the
sections. Two sections caudal to this one the regenerated eye shows its
attachment by a long optic stalk to the brain. The normal left eye is like
the one in Fig. 77, experiment DF,,. > 90 diameters.

Fie. 10. Experiment DF,,. Embryo rana palustris killed 5 days after
partial extirpation of the right optic vesicle. Transverse section through
small regenerated right eye. No traces of lens formation are to be found in
the sections. It is attached to the brain by a long optic stalk. The normal
left eye is like the one in Fig. 75, experiment DF,,. > 90 diameters.

Fic. 11. Experiment dx,. Embryo rana sylvatica killed 3 days after
partial extirpation of the right eye. Transverse section through the regen-
erated right eye which is in contact by its outer layer with the ectoderm.
(There is an artificial separation in the sections.) No traces of lens
formation are to be found in the sections. The normal eye is like the one
in Fig. 59, experiment DF,,. > 90 diameters.

Fie. 12. Experiment DL... Embryo rana sylvatica killed 3 days after
partial extirpation of the right optic vesicle. Transverse section through the
small regenerated right eye which is in contact with the ectoderm by its
outer layer. No traces of lens formation are to be found in the sections.
The normal left eye is like the one in Fig. 6, experiment DL,,. xX 90
diameters.

Fic. 13. Experiment DF,;,. Embryo rana palustris killed 3 days after
partial extirpation of the optic vesicle. Transverse section through small
regenerated right eye which is in contact by its outer layer with the ectoderm.
No traces of lens formation are to be found in the sections. The normal left
eye is similar to that in Fig. 59, experiment DF, or DF,. X 90 diameters.

Fic. 14. Experiment DF,. Embryo rana palustris killed 3 days after par-
tial extirpation of the right optic vesicle. Transverse section through the
small regenerated right eye which is in contact by a portion of its retinal
layer with the ectoderm. There is a slight thickening of the inner layer of
the ectoderm opposite the eye but separated from it by a thin layer of
mesenchyme. The normal left eye is similar to that in Fig. 59, experiment
DF,,. X 90 diameters.

Fic. 15. Experiment DF,. Section through the lens-plate more highly
magnified than in Fig. 14. >X 3860 diameters.

Fie. 16. Experiment DL,. Embryo rana sylvatica killed 4 days after
partial extirpation of the right optic vesicle. Transverse section through the
caudal part of the small regenerated eye showing its connection with the
brain. The normal left eye is similar to the one in Fig. 70, experiment DF,,.
x 90 diameters.

Fie. 17. Section through part of anterior end of the eye in experiment
DL, showing its contact with the ectoderm by the retinal layer and the
formation at the place of contact of a small lens-plate. (The separation of
the eye and ectoderm in the figure is an artefact.) x 180 diameters.

495

Warren Harmon Lewis

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496 The Origin and Differentiation of the Lens

Fic. 18. Experiment DF,;. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through the small
regenerated eye showing where outer layer and small corner of the retinal
layer were in contact with the ectoderm. A small lens-bud is attached here
to the inner layer of the ectoderm. The normal left eye is similar to the
one in Fig. 77, experiment DF,,;. > 90 diameters.

Fic. 19. Lens-bud in the above experiment DF,, more highly magnified.
< 360 diameters.

Fig. 20. Experiment DF,,. Embryo rana palustris killed 3 days after
partial extirpation of the right optic vesicle. Section through anterior por-
tion of regenerated eye showing contact of edge of the cup with the ectoderm
and the formation of a small lens-bud from the inner layer of the ectoderm.
The normal right eye is similar to that in Fig. 42 or 68, experiment DF,,; or
DF,. X 90 diameters.

Fig. 21. Lens-bud in above experiment, DF,, more highly magnified.
x 360 diameters.

Fic. 22. Section through caudal portion of same eye, experiment DF,,,
show small cup separated from the ectoderm by mesenchyme. X 90
diameters.

Fic. 23. Experiment DF, . Embryo rana palustris killed 3 days after
partial extirpation of the right optic vesicle. Section through caudal end
of regenerated eye, it is separated from the brain by the anterior end of the
transplanted eye and apparently pushed against the ectoderm where there is
a small lens-bud attached to the inner layer of the ectoderm. The retinal
portion of the regenerated eye is in contact with the lens-bud. The normal
left eye is similar to the one in Fig. 42, experiment DF,;. X 90 diameters.

Fic. 24. Experiment DF,. Lens-bud more highly magnified. x 360
diameters.

Fig. 25. Experiment DF,. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through the small
regenerated right eye. It is separated from the ectoderm by a thin layer of
mesenchyme. There is a small lens bud attached in the inner layer of the
ectoderm. The normal left eye is similar to that in Fig. 61, experiment DF, .
x 90 diameters.

Fig. 26. Experiment DF,. Lens-bud more highly magnified. x 360
diameters.

Fic. 27. Experiment dx,. Embryo rana sylvatica killed 3 days after par-
tial extirpation of the right optic vesicle. Section through small regenerated
right eye and small lens-bud, the latter is still attached to the ectoderm but
is separated from the optic cup by mesenchyme. X 90 diameters.

Fic. 28. Experiment dx,. Caudal end of regenerated eye in contact with
the ectoderm by the outer layer. No signs of lens formation here. X 90
diameters.

Fic. 29. Experiment dx,. Section through normal left eye. X 90
diameters.

Fic. 30. Experiment dx,. Embryo rana sylvatica killed 3 days after par-
tial extirpation of the right eye.. The regenerated eye is about the size and
shape of the one in Fig. 27. Its caudal end does not show the outer layer
cells. Section through caudal end of the eye and lens-bud. A thin layer
of mesenchyme separates them. The normal left eye is similar to the one in
Fig. 29. X 360 diameters.

Warren Harmon Lewis 497

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498 The Origin and Differentiation of the Lens

Fic. 31. Experiment DF,,. Embryo rana palustris killed 4 days after the
partial extirpation of the right eye. Section through regenerated right eye
and lens-bud. The eye is separated from the ectoderm by mesenchyme. The
normal left eye is in outline on the opposite side. »X 45 diameters.

Fic. 32. Experiment DF,,. Lens-bud more highly magnified. x 360
diameters.

Fic. 33. Experiment DF,,. Embryo rana palustris killed 4 days after par-
tial extirpation of the right eye. Section through regenerated eye and lens-
bud. The eye is separated from the ectoderm by mesenchyme. The normal
eye is similar to the one in Fig. 31. The lens-bud is very similar to that in
Fig. 32. x 45 diameters.

Fic. 34. Experiment dx,. Embryo rana sylvatica killed 4 days after the
operation. Section through caudal portion of regenerated eye. Here its
outer layer is towards the ectoderm and separated from it by mesenchyme.
There is a small lens-bud opposite this portion of the eye. The normal left
eye is similar to that of Fig. 29. X 90 diameters.

Fic. 35. Experiment dx,. Lens-bud more highly magnified. x 360
diameters.

Fic. 36. Experiment DL,. Embryo rana sylvatica killed 5 days after
partial extirpation of the right eye. Section through regenerated eye and
lens-bud. The regenerated eye is separated from the ectoderm by mesenchyme.
There is an artificial separation of the lens-bud from the inner layer of the
ectoderm. X 90 diameters.

Fic. 37. Experiment DL,,. Lens-bud more highly magnified. xX 360
diameters.

Fic. 38. Experiment DL,. Section through normal left eye and lens.
x 90 diameters.

Fig. 39. Experiment DF,. Embryo rana palustris killed 5 days after
partial extirpation of the right optic vesicle. Section through regenerated
right eye showing attachment to brain and separation from ectoderm by
mesenchyme. The normal left eye is similar to the one in Fig. 75, experi-
ment DF,,. X 90 diameters.

‘Fic. 40. Experiment DF,,. Section through anterior end regenerated eye
and small solid lens-sphere which is separate from the ectoderm. The outer
layer of the eye is nearest the lens-sphere but separated from it by mesen-
chyme. X 90 diameters.

Fic. 41. Experiment DF,,. Lens-sphere more highly magnified. xX 360
diameters.

499

Warren Harmon Lewis

a> xe
=e

°

oes
SioyeitaisioleOn

500 The Origin and Differentiation of the Lens

Fie. 42. Experiment DF,;. Embryo rana palustris killed 3 days after
partial extirpation of the right optic vesicle. Section through regenerated
eye and solid lens-sphere. The latter is separate from the ectoderm. xX 90
diameters.

Fic. 43. Experiment DF,;. Section through normal left eye and lens
vesicle. > 90 diameters.

Fig. 44. Experiment DF,,. Embryo rana palustris killed 3 days after the
operation (see Figs. 20 and 21). Section through edge of normal left eye,
edge of lens, lower edge of optic cup and ectoderm showing small lens-bud-
like structure attached to the inner layer of the ectoderm. Probably place
where lens was pinched off. X 360 diameters.

Fie. 45. Experiment DL.,. Embryo rana sylvatica killed 4 days after
extirpation and transplantation of the right optic vesicle. Section through
lens-like bud of the inner layer of the ectoderm. It is near the otic vesicle
but not near either the normal lens area or near the transplanted eye. It
was probably caused by injury to the ectoderm during the transplantation of
the eye. X 180 diameters.

Fic. 46. Experiment DL,,. Embryo rana sylvatica killed 3 days after
partial extirpation of right eye. Section through lens-like bud of inner
layer of ectoderm, anterior to otic vesicle, and is probably from wound
caused in making skin flap. x 180 diameters.

Fig. 47. Experiment DF,,. Embryo of rana palustris killed 5 days after
partial extirpation of right eye. Regenerated eye has two lenses. Section
through lens-like bud of the inner layer, anterior to regenerated eye, and is
probably from an injury to the skin flap. XX 180 diameters.

Fie. 48. Experiment DL... Embryo of rana sylvatica killed 4 days after
the operation. Section through “lens-like bud’ which has arisen between
the normal lens-forming region and the otic vesicle about in the region of
the wound of incision. X 360 diameters.

Fie. 49. Experiment DL,,. Embryo rana sylvatica killed 5 days after the
operation. Section through long lens-like bud near otic vesicle. It is prob-
ably from injury to the ectoderm made during the transplantation of the
eye. xX 90 diameters.

Fic. 50. Experiment DL,,. Distal end of lens-like bud. X 360 diameters.

Fic. 51. Experiment DL,,. Embryo of rana sylvatica killed 3 days after
the operation. Section through solid ectodermal spherical mass which has
probably arisen from the ectoderm. It is located caudal to the normal lens-
forming area about in the region of the wound of incision. x 360 diameters.

Fic. 52. Experiment DL,. Embryo of rana sylvatica killed 5 days after
the operation. Section through ectoderm and small lens-like vesicle, located
caudal to the normal lens-forming area about in the region of the wound of
incision. The ectoderm over the vesicle shows still evidences of the origin
of the vesicle and dips down into the mesenchyme. X 360 diameters.

Fic. 53. Experiment DL,,. Embryo rana sylvatica killed 5 days after
transplantation of optic vesicle into otic region. Section through otic vesicle,
transplanted eye, and ventral to it a small lens-like vesicle, probably from
injury to the ectoderm. X 90 diameters.

Fig. 54. Experiment DL,,. Lens-like vesicle more highly magnified.
xX 360 diameters.

Warren Harmon Lewis

"0 Ore
"a phleste #55 [ey4
nel eee’

WS pala

Or
SS
oo

The Origin and Differentiation of the Lens

Fic. 55. Experiment DL;,. Embryo rana sylvatica killed 3 days after
transplantation of the optic vesicle into the otic region. Section through
edge of outer layer of transplanted eye, ectoderm, and small lens-like vesicle.
The transplanted eye has a large lens. X 360 diameters.

Fie. 56. Experiment DL,,. Embryo rana sylvatica killed 4 days after
transplantation of the optic vesicle into the otic region. Section through
ectoderm, anterior end transplanted eye, and lens-like vesicle. The latter
is near an injured place in the ectoderm. X 90 diameters.

Fic. 57. Experiment DL,. Embryo rana sylvatica killed 3 days after
transplantation of the optic vesicle into the otic region. Section through
caudal edge of eye and ectoderm showing small lens-like bud. The trans-
planted eye has a large lens, and this bud is probably an injury process.
x 360 diameters.

Fic. 58. Experiment DF,,. Embryo rana palustris killed 4 days after
transplantation of the optic vesicle into the otic region. Section through
one edge of the transplanted eye and small lens-bud. The transplanted eye
has also another and larger lens. The normal left lens is similar to the one
in Fig. 61, experiment DF,,. x 360 diameters.

Fic. 59. Experiment DF,,. Embryo rana palustris killed 3 days after
partial extirpation of the right optic vesicle. Transverse section through
regenerated and normal eyes, and normal lens vesicle. Xx 90 diameters.

Fic. 60. Experiment DF,,. Section caudal to above, through regenerated
eye, and end of transplanted eye and small lens-bud. X 90 diameters.

503

wis

Warren Harmon Le

.
e
°
1)
>
a
°
°
°

504 The Origin and Differentiation of the Lens

Fic. 61. Experiment DF,. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through regenerated
and normal eyes. Both regenerated eye and its small lens vesicle are sepa-
rated from the ectoderm by mesenchyme. The normal side shows the early
“lens” stage. X 90 diameters.

Fic. 62. Experiment DF,,. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through regenerated
eye and small lens vesicle. Normal eye is similar to the one in Fig. 61.
x 90 diameters.

Fic. 63. Experiment DF,,. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through regenerated
eye and small lens vesicle, both are separated from the ectoderm by a layer
of mesenchyme. Normal eye as in Fig. 61, experiment DF,. XxX 90
diameters.

Fic. 64. Experiment DL,. Embryo rana sylvatica killed 3 days after par-
tial extirpation of the right optic vesicle. Section through regenerated and
normal eyes. X 45 diameters.

Fie. 65. Experiment DL,. Section through small lens vesicle of regener-
ated eye. X 180 diameters.

Fic. 66. Experiment DL,. Section through normal lens vesicle. X 180
diameters.

505

Warren Harmon Lewis

°= <<
2*o*c"a 0 OO

=

eo SS
OSes
GEO

506 The Origin and Differentiation of the Lens

Fic. 67. Experiment DF,. Embryo rana palustris killed 3 days after
partial extirpation of the right optic vesicle. Section through large regen-
erated eye and lens-bud. X 90 diameters.

Fic. 68. Experiment DF,. Section through normal eye and lens vesicle.
x 90 diameters. ;

Fic. 69. Experiment DF,. Embryo rana palustris killed 3% days after
partial extirpation of the right optic vesicle. Section through large regen-
erated eye and lens vesicle. X 90 diameters.

Fic. 70. Experiment DF, . Section through normal eye and lens. xX 90
diameters.

Fic. 71. Experiment DF,,. Embryo rana palustris killed 4 days after par-
tial extirpation of the right optic vesicle. Section through regenerated eye
and small lens. Normal eye, as in Fig. 77.

Fic. 72. Experiment DF, . Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through the regener-
ated right eye and small lens, both are separated by a considerable layer of
mesenchyme from the ectoderm. Normal eye, as in Fig. 77. X 90 diameters.

Warren Harmon

3%0%? A

200
00

°
2
°

07

508 The Origin and Differentiation of the Lens

Fic. 73. Experiment DF,;. Embryo rana palustris killed 5 days after
partial extirpation of the right optic vesicle. Section through regenerated
eye and lens. Normal eye, as in Fig. 75. X 90 diameters.

Fic. 74. Experiment DF,,. Embryo rana palustris killed 5 days after
partial extirpation of the right optic vesicle. Section through large regen-
erated eye and lens. X 90 diameters.

Fic. 75. Experiment DF,,. Section through normal eye and lens. xX 90
diameters.

Fic. 76. Experiment DF,,. Embryo rana palustris killed 4 days after
partial extirpation of the right optic vesicle. Section through large regener-
ated eye and lens. X 90 diameters.

Fic. 77. Experiment DF,,. Section through normal left eye and lens.
< 90 diameters.

Fic. 78. ExperimentDF,,. Section through normal eye of rana palustris
killed 41% days after the operating stage. X 90 diameters.

Fic. 79. Experiment DL,,. Embryo rana sylvatica killed 4 days after
operation on the right optic vesicle. Section through degenerating left eye
and small lens vesicle. A normal eye at this age is as in Fig. 77. x 90
diameters.

Fic. 80. Experiment DL... Embryo rana sylvatica killed 4 days after
operation on the right optic vesicle. Section through degenerating left eye
and small lens vesicle, compare with a normal eye at this stage, Fig. 77.
x 90 diameters.

Fic. 81. Experiment DL,. Embryo rana sylvatica killed 4 days after
operation on the right optic vesicle. Section through degenerating left eye
and small lens vesicle. Normal eye of an embryo at this age, as in Fig. 77.
x 90 diameters.

Fig. 82. Experiment DL,,. Section through more caudal part of same eye
as in Fig. 81, showing second small lens vesicle which is entirely separate
from the first. X 90 diameters.

509

8

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Warren Harmon Lew

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2°,

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ae
XX)

BON

a

THE EMBRYONIC HISTORY OF THE LENS IN BDELLOS-
TOMA STOUTI IN RELATION TO RECENT EXPERIMENTS.

BY

CHARLES R. STOCKARD.
Pathological Laboratory, Cornell University Medical College, New York City.

WitTH 3 TEXT-FIGURES.

Spemann, Lewis, and others, have shown by experiments on amphibian
embryos that there is no localization of lens-forming material in any
given area of the ectoderm, and that the formation of a lens depends
directly upon the stimulation of the ectoderm by a contact with the
optic-cup. Spemann* has since discussed the question of the self-differ-
entiating power of the lens and concluded from a consideration of
Schaper’s * experiments on the frog that the lens is not self-differentiating,
but that a durable influence or contact of the optic-cup is necessary
to cause the lens-plate or lens-bud to develop into a typical lens. Le Cron °
has lately shown by a series of convincing experiments that the lens
in Amblystoma is not self-differentiating. He found when the optic-cup
was artificially removed from below the lens-plate, lens-bud, or lens-
vesicle that the lens structure soon ceased to further differentiate and
commenced to undergo degeneration. In most of these experiments
the authors have considered the possibility that the injury caused by
the operation might be responsible for the failure of the lens to form,
although their methods and care have been sufficient to convince one
that such was not the case.

1Spemann, H., Ueber Correlationen in der Entwickelung des Auges.

Verhandl. der Anat. Gesellsch., 1901.
2Lewis, W. H., Experimental Studies on the Development of the Eye in

Amphibia. I. On the Origin of the Lens. Rana palustris. Am. Jour. Anat.,
III, 1904.

3 Spemann, H., Ueber Linsenbildung nach experimenteller Entfernung der
primadren Linsenbildungzellen. Ausftihrlich: Zool. Anz., 28, 1905.

*Schaper, A., Ueber einige Falle atypischer Linsenentwickelung unter
abnormen Bedingungen. Anat. Anz., XXIV, 1904.

5 Le Cron, W. L., Experiments on the Origin and Differentiation of the Lens
in Amblystoma. Am. Jour. Anat., VI, 1907.

AMERICAN JOURNAL OF ANATOMY.—VOL. VI.

39

512 Embryonic History of the Lens in Bdellostoma

In the blind fishes, however, we have normal cases of degeneration
of the eye structures which one might expect to at least partially analyze
with the aid of the experimental results. Eigenmann” has shown in
Amblyopsis, a blind cave fish, that a lens structure is present in young
embryos and soon disappears, being entirely absent in old embryos. No
connection was noted between the appearance and disappearance of the
lens and the contact of the optic-cup with the ectoderm, although the
significance of such a relation is only made clear by the experiments.
In the adult eye of the burrowing lizard, Rhineura of Florida, Eigen-
mann recorded that the lens was absent in one-half of the eyes studied,
while the organ was extremely variable in those eyes in which it was
found.

Miiller,” in his early description of the Myxinoids, and lately Allen,
in studying the eye of the adult Bdellostoma noted the absence of a
lens. Miss Worthington,’ observing the living animals, records them to be
totally blind.

Price,” the first to study the embryos of Bdellostoma, found in a
young stage that a projection of cells from the inner layer of the ectoderm
extended toward the optic-cup. In older embryos this structure, which
Price interpreted correctly to be the lens-bud, had disappeared.
Kupffer” shows a slight thickening of ectoderm in one of his figures
and designates it a lens-placode; this he states disappears in older
embryos. In a former paper I” mentioned the disappearance of the
lens in the embryos of Bdellostoma. It is thus seen that the development
of this lens has received only passing notice, while in the light of
experiments the case seems to have gained sufficient importance to war-
rant a fuller description.

While studying the development of the brain and special sense organs

6 Higenmann, C. H., The History of the Eye in Amblyopsis. Proc. Indiana
Acad, Sci: 1901.

7 Miller, J., Vergleichende Anatomie der Myxinoiden, der Cyclostomen mit
durchbohrtem Gaumen. Berlin, 1839.

8’ Allen, B. M., The Eye of Bdellostoma Stouti. Anat. Anz., XXVI, 1905.

° Worthington, J., Contributions to Our Knowledge of the Myxinoids. Am.
Nat., XX XIX, 1905.

” Price, G. C., Some Points in the Development of a Myxinoid. Verhandl.
der Anat. Gesellsch., 1896.

4 Kupffer, C., Zur Kopfentwicklung von Bdellostoma. manor d.
Gesellsch. f. Morph. u. Physl., Mtinchen, 1900.

? Stockard, C. R., The Development of the Mouth and Gills in Bdellostoma
Stouti. Am. Jour. Anat., V, 1906.

Charles R. Stockard 513

in Bdellostoma I have been impressed with the manner in which the
history of the lens in these embryos seems to corroborate the conclusions
drawn by the experimenters mentioned above. In a brief way I wish to
‘present these points, which are readily interpreted in the hght of the

Fic. 1. A section through the eye of a 15 mm. embryo of Bdellostoma. L,
the lens at the height of its development, the contact between the optic-cup
and the ectoderm has just been lost.

Fic. 2. The eye of an older embryo, the lens, L, degenerating and the
optic-cup well removed from the outer wali.

Fic. 3. The eye of an old embryo in which the lens has entirely disap-
peared. All camera drawings to the same scale.

experiments, while they in turn also lend support to the experimental
conclusions by showing that many of the conditions artificially pro-
duced may occur in a normal embryo.

514 Embryonic History of the Lens in Bdellostoma

Very early embryos of Bdellostoma in which the nose is still a single
tube, and in which six or seven gill slits are present on the laterally
outspread plates, will show the lens in the following condition: A small
antero-dorsal portion of the irregularly shaped optic-cup comes in
contact with the ectodermal head-wall, and from this ectoderm a pro-
jection of cells extends inward toward the cavity of the optic-cup. The
lens-bud is thus to an extent conical in form and results from a contact
of only a portion of the optic-cup with the ectoderm. This structure
continues to develop for a time until in an embryo considerably more
advanced and measuring 15 mm. in length one sees the lens-bud with
a shght indication of a constriction about the periphery of its area
of union with the ectoderm, as if it were preparing to pinch off (Fig. 1).
Here the progressive development of the lens ceases and degeneration
begins. At this stage also the contact of the optic-cup with the ectoderm
or lens-bud is just being lost.

An older embryo in which all of the gill clefts have appeared, but
are still on the outspread lateral plates, and in which the nose exists
as two parallel tubes, shows the lens much reduced in extent. The optic-
cup is now well separated from the ectodermal wall and a considerable
layer of mesenchymous tissue is seen between the two (Fig. 2). The
lens, L, here is indicated only by a slightly thicker area of ectoderm
over the deeply buried optic-cup. In all embryos older than this one
no indication whatever of a lens-like thickening could be found, the ecto-
derm over the eye region being of the same thickness as that of adjacent
areas (Fig. 3). This figure also shows that the optic-cup has continued
to differentiate its parts, and is probably not so degenerate as to be
unable to influence the ectoderm should it remain in contact with it.

It is thus shown that the lens is not normally self-differentiating but
begins to degenerate when contact with the optic-cup is lost.

The embryos of Bdellostoma illustrate, therefore, by the changes
which their lenses undergo many of the points sought in the above-
mentioned experiments. They clearly show that the lens formation 1s
directly dependent upon a contact of the optic-cup with the ectoderm.
Secondly, contact with only a portion of the optic-cup is necessary to
cause the ectoderm to begin lens formation. Thirdly, to produce a lens
the contact of the optic-cup with the ectoderm must be durable; and
fourthly, the optic-vesicle may change into an optic-cup without the
aid of the mechanical pressure of the lens. This series of events would
be difficult to interpret without the facts demonstrated by the experi-
ments, while on the other hand it adds strength to the conclusions drawn

Charles R. Stockard 515

from the experiments by proving that such results may occur under
normal conditions and are, therefore, in no way attributable to the
injury caused by the operations.

The question of the localization of lens-forming material in a given
ectodermal area is not answered by Bdellostoma, but I have shown in
artificially produced cyclopean monsters in the Teleost that a lens may
form from a region out of the usual lens-forming area, and that the
size of the optic-cup regulates the size of the lens. So, in other fishes,
just as in amphibians, we would not expect to find localized lens-forming
regions in the ectoderm.

I wish to express my indebtedness to Professor Bashford Dean for
kindly placing at my disposal his complete series of Bdellostoma embryos.

18 Stockard, C. R., The Artificial Production of a Single Median Cyclopean
Eye in the Fish Embryo by Means of Sea-Water Solutions of Magnesium
Chlorid. Arch. Entw.-Mech., XXIII, 1907.

40

INDEX POny Ovi

(See also Contents of this Volume for Full Titles and Authors’ Names. )

References to articles appearing in THB ANATOMICAL RECORD will be indexed
separately on the completion of every 300 pages.

PAGE
ASSOCIATION OF AMERICAN ANATO-
MISTS, PROCEEDINGS OF TWENTY-
URS HS SION ve eleicuelereneneieien chatter

Ao oe OO Anatomical Record

ADDRESS OF PRESIDENT.......

ies .6 Cae Anatomical Record
TWENTY-SECOND SESSION

EP edcteis sss Anatomical Record

CONSTITUTION, OFFICERS,

LIst OF MEMBERS

Relenas Anatomical Record 93-108

DISTRIBUTION OF

Anatomical Record 107
Abdominal nerves, development and
variations of anterior border

ERO oan masnodamoodadecoot 271-77
Acoustic, ganglion, and nerve, devel-

opment of

Amblystoma, development of lens....

Amphibia, amblystoma, axis cylinder,

development of lens, frog, plantar

71-93
AND

eis] ,0.s))0 wee

139
245

TIGA, ooonoonoacdncdcccaoGe
Anatomical Record, references will be
indexed separ-

ately at the

end of every

300 pages.
plan of and pro-
spectus ; see An-
atomical Rec-
ord No. 1, page
ie Vile Wile eNO:
al
Anatomists, Association of, see Ana-
tomical Record.
Arteries, see pulmonary.
arteriole rect of kidney...
Association, see anatomists.
Auditory, see acoustic and ear.
Axis cylinder, experiments on the de-
velopment of

391

PO TOO HPO 461

BDELLOSTOMA, see lens.

Bird, see chick.

Blood vessels, see bronchial, pulmon-
ary, technique, testis.

Border nerves, development and vari-
ATLONWOLE che. eee ei prec eee tones 270-274

Bronchi, development of

Bronchial tree, development of..... 60-77

PAGE
CAT, see kidney.
DLAntaLe MUSCLE Sw erererenctcicteneneorser=
Chick, see heart muscle.
Cochlea, development of ........... ila
Cochlear nerye, and ganglion...... 154

Corrosion, see technique.
Crural muscles, development, and var-

LACLOMM re totekoteuclercucwerotercusnave: stetate 259-391
Cutaneous nerves, development and
VALIAtOMN OLLI LO agsreratsl creas 259-391
Doc, see gastric glands, kidney.
Ear, development of ............ 139-165
COCWMTCR Wiapcneisieverchorsueveue eienetenevenenere 152
PATELLA LON GHUSietelerelerted-tcnelet eter 113

Extremities, see development of leg.
Eye, experiments on development and

TESENELA LOM Oly mepereialelonelsleneneys 473
see lens.
FACIAL NERVE, development of ..... 139
Femoral, muscles, development and
variations of nerves, development
chavel SVEURENAKON SdooocoodoGaK0K 259-391
Fibril bundles, in heart muscle ..... 191
Fish, see Bdellostoma.
Foot, development and variation of
MOEVES: Of Sieve s 5.2 shel epeleners 346-370
see plantar.
Frog, see axis cylinder, eye.
GANGLION, see acoustic.
(GMs Gl sodgosdocconsonboGad 207
Glands, see gastric, submaxillary.
HPART MUSCLE, histogenetic develop-
MAINE Ol og ooadc 191-205
adult structure ..1938-195
ENISTOSENESIS] Of. MMS ieee -)elereletel ier 85-102
and see heart, leg.
Human, see ear.
acoustic nerve.
facial nerve.
development of nerves
and muscles of leg, of
foot, of perineum, of
trunk.
plantar muscles.
testis.
Eby parteniali bronchi sje sree) olei-nete 113

: » Index

PAGE
INGUINAL REGION, development and
variation of muscles and nerves. 259
Injection, apparatus for injecting kid-
TE Vik scenedece! ecaced si ahevsueyene 392-395
see technique.
Intermedius nerve, development of...159

SLTNSVee AE CCRICS) (OP Aroccrerscotcue, ster esesshss 391

LABYRINTH, development of, in ear.. 139
Leg, development of nerves and
IMUIS CES! SIM rer. iuc goons terisiensy is 259-390
Lens, development of, in Bdellostoma 511
origin and differentiation of .245-258

experiments on development
OL) Nee Ors. she, 2 sachs enesenerel Neier ene 245-258
Limbs, see leg.
Hobe; formation in Wun'y, <0... s.scce sss Tf

Lumbo-sacral plexus, see Bardeen on
development of nerves in leg.

Lungs, development of ............ 1-137

LV MUP WATS Of UNE peor sireveste chee 91-102
MEMBRANOUS (EAR) LABYRINTH, de-

VeElLOPMeEnE OL 2 isyspeiehe cree eyeierecsneits 139

Muscles, early differentiation ...... 267
development of muscles of

Leo P i) ahercvotetendis escrecon cers O O=O40)
development of muscles of

POOL UW sapexeleie sy s¥onoiel« sere + O= OEE
development of muscles of

PELINE WMI... setae te 374-380

embryology, comparative an-
atomy and phylogeny of
the muscles of the
LO CE rere sets wel sists rte DOLOOO,
see heart.

NERVES, see acoustic, facial, vestibu-
lar of the leg.

development and variation of

nerves and muscles of the

MCPs eke slsicis sods sue sisheiee Dosa OO
outgrowth Of) a... 2. + - 270 and 461
primary period of develop-
NCIC Wevecenlysvelehe’s aie cheteoorers 264
see axis cylinder.
OPOSSUM, see plantar muscles.
Optic vesicle, experiments on develop-
INO Acie ielerirehe ee ANG sae

PAGE
PERINEUM, development and variation
of muscles and nerves of....374-3891
Pig, see lung.

see testis.
Plantar, development of muscles and
NELVES od. cisleh clesense ie wiars SetehetOO EOE
Plexus, see lumbo-sacral.
Phylogeny, Of -miISCles yer: oie iene teenene 407
Pudie merves! iscclas cceccrantel stavetor oteteneuene 374
Pulmonary artery and vein, develop-
IMMOME “OLS ay ares Grebetepoytnene eteueieis ccna 60-77
Pylorus;, occlusion of -...... Were o. 232

RABBIT, see submaxillary gland.
Rat, see vessels of kidney.

Record, anatomical, see anatomical.
Renal, see kidney vessels.

Reptile, see plantar muscles.

SACCULE! OF VATS s1.1c ccs eucletelenorctsredoeine 151
Salivary glands, see submaxillary.
Semi-circular eanals, development
(0) aay CARON Borchok GICO LO TCG o 139-165
Skin, see cutaneous.
Stomach, see gastric.
Submaxillary gland, structure of ... 167
stages of activ-
LGV? o rorssororetenete 180

TECHNIQUE, corrosion
development of ear .... 140

development of heart
MUSCLE ty cus easter hertenonetens 191
SASTRIGs olan Super. erenaiche 207

injection of kidney .... 892
injecting vessels of de-

veloping testis ....... 440

structure of submaxillary
glands! (ier crac sien 170

Testis, development of blood vessels
Of PILTSMTCSULS var. cae seeene 446-453

blood supply of, in adult
No dab ono boob odes cs 453-460
gross development of pig’s... 444
Trachea, Geyelopmenity se. . «iclete ereteie 30
WIRICLE Of (GM sn cnc: eueoreinne 151

Veins, see lymphatics.
oN PMKOMENAT GeoagpwasooDoCDO DS
Vestibular nerve ganglion ......... 154

ZYMOGEN, see gastric.

64

at

up

<

—
ne See

Seererie

Aa beh?