HUMAN EMBRYOLOGY AND MORPHOLOGY.

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HUMAN EMBRYOLOGY
AND MORPHOLOGY

BY

ARTHUR KEITH, M.D.(Aberdn.), F.E.C.S.(Eng.)

LECTURER ON ANATOMY, LONDON HOSPITAL MEDICAL COLLEGE
FORMERLY HUNTERIAN PROFESSOR, ROYAL COLLEGE OF SURGEONS,
EXAMINER IN ANATOMY, UNIVERSITY OF ABERDEEN

ILLUSTRATED

LONDON

EDWARD ARNOLD
1902

GLASGOW : PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO.

The Students of the London Hospital.

PREFACE.

Fifty years ago it was possible for a teacher in a Winter
course of Lectures to lay all the essential facts of Embryology
and Comparative Anatomy before his pupils ; to-day fifty courses
were not sufficient, so boundless have these subjects grown. Yet,
in spite of their rapid growth, they have been retained in Higher
Examinations in Human Anatomy by our Universities and
Colleges without any principle being laid down to guide teachers
and taught as to the scope required. Ho book dealing with
these subjects exists to afford a precedent. The criterion which
the Author applied in determining the scope of this work he
believes will be accepted by pupils, teachers, and examiners.
The course of Demonstrations, of which this book is the
substance, was given under the walls of a great hospital to
students preparing to work within its wards. Hence, each fact
taught the student was necessarily one which was capable of
application in his life’s work or by the possession of which
he became a better workman. The extent to which each
subject was dealt with was determined by its practical im-
portance. In brief, clinical utility was the criterion employed.
The way to the wards is the road to the examination room,

Vlll

PREFACE.

and the right preparation for the one is the best qualification
the student can take with him for the other.

The Author hopes he has prepared a work which will prove
useful not only to students proceeding for Higher Examinations
in Anatomy and Surgery but also to men actively engaged in
practice. Every day conditions come under their notice which
can be explained only by a reference to Embryology. He
has sought to sketch as briefly and clearly as possible the
history of the developing human body. History is the best
key to an understanding of the present conditions of a country ;
the Embryologist and Comparative Anatomist are the dual
historians of the human body.

The Author is deeply indebted to Mr. F. G. Parsons for re-
vision of his proofs and numerous suggestions and corrections.

December , 1901.

CONTENTS.

CHAPTER I.

PAGE

Development of the Face, - -- -- -- - 1

CHAPTER II.

The Nasal Cavities and Olfactory Structures, 20

CHAPTER III.

Development of the Pharynx and Neck, ----- 28

CHAPTER IY.

Development of the Organ of Hearing, ----- 49

CHAPTER V.

Development and Morphology of the Teeth, 63

CHAPTER YI.

The Skin and its Appendages, 70

CHAPTER VII.

The Development of the Ovum of the Foetus from the Ovum

of the Mother, 80

CHAPTER VIII.

The Manner in which a Connection is Established between the

Foetus and Uterus, - 95

CHAPTER IX.

The Uro-genital System, -

102

X

CONTENTS.

CHAPTER X.

Page-

Formation OF THE PuBO-FEMORAL REGION, PELVIC FLOOR AND

Fascia, - 133

CHAPTER XI.

The Spinal Column and Back, 142

CHAPTER XII.

The Segmentation of the Body, 155

CHAPTER XIII.

The Cranium, - 161

CHAPTER XIY.

Development of the Structures concerned in the Sense of

Sight, - - - 175

CHAPTER XY.

The Brain and Spinal Cord, - 192

CHAPTER XVI.

Development of the Circulatory System, - 223-

CHAPTER XVII.

The Respiratory System, 249

CHAPTER XVIII.

The Organs of Digestion, 261

CHAPTER XIX.

The Body Wall, Ribs, and Sternum, ------ 282

CHAPTER XX.

The Limbs, - 289

HUMAN EMBRYOLOGY AND MORPHOLOGY.

HUMAN EMBRYOLOGY AND
MORPHOLOGY.

CHAPTER I.

DEVELOPMENT OF THE FACE.

Processes which form the Face. — About the middle of the
first month of foetal life, five processes begin to spring from the
base of the primitive cerebral capsule, which, by the end of the
second month, have completely united together to form the facial
part of the head. In figure 1, a diagrammatic representation is
given of the condition of these five processes about the end of
the first month. Of the five, one, the nasal or fronto-nasal,
composed of symmetrical right and left halves, is median, and pro-
jects beneath the fore-brain; the others are lateral, two on each side,
the mandibular and maxillary. The cavity which these five
processes surround is the stomodaeum (Fig. 1). It ultimately
forms the nasal and part of the buccal cavities. The part of
the adult face formed by each process is shown in figure 2.

Malformations of the Face. — These processes may fail to
unite in the second month and in this manner malformations of
the face are produced. The most common anomaly is a partial
failure of the nasal and maxillary processes to fuse, various
degrees of hare lip and cleft palate being thus caused. In hare
lip, the cleft appears in the upper lip between the middle part
formed by the middle nasal processes and the lateral parts formed
by the maxillary processes (Fig. 2). In cleft palate, the failure
of union occurs between the deep parts of the nasal and maxillary

A

o

HUMAN EMBRYOLOGY AND MORPHOLOGY.

piocesses (Hg. 8). There is a mesial cleft in the upper lip of
hares and labbits, but it occurs between the two maxillary

mid brain

cerebral

nasal field
lat. nas. proc.
mes. nas. proc.
maxillary proc.
mandib. proc.

hyoid arch,
cardiac emin.

Fia. 1. --Showing the formation of the face by the Nasal, Maxillary, and Mandibular
processes in an embryo of the 4th week. (After His.)

cerebrum

maxillary process
nasal field
lat. nas. proc.

mes. nas. proc. (proc. glob.)

mandib. proc.
hyoid arch

Fig. 2. — Showing the parts of the face formed from the Nasal, Maxillary, and

Mandibular processes.

processes, the labial part of the middle nasal process being
undeveloped. Macrostoma is due to a partial failure of the

DEVELOPMENT OF THE FACE.

3

mandibular to unite with the maxillary element. Any of these
processes may be under or over developed ; over-development ol
the nasal and under-development of the mandibular (micro-
gnathia) are of common occurrence.

THE NASAL PROCESSES.

The nasal process at a very early stage is seen to be
divided into two lateral processes and two mesial, the latter
having globular enlargements as tips (Fig. 1). It must be re-
membered that the lateral and mesial nasal processes are really

Fig. 3. — Showing the structures formed in the Mesial Nasal Processes.

vertical septa springing from the basis of the primitive capsule
of the fore-brain, and the parts seen on the face are the anterior
extremities of these septa (see Fig. 8).

What become of the Mesial Nasal Processes. — From the
mesial nasal processes, which fuse together, and may enclose
epithelial remnants between them, are formed the whole septum
of the nose (Fig. 3), the premaxillary part of the upper jaw
and the middle third of the upper lip (Fig. 2). It contains a
skeletal basis of cartilage, formed by the trabeculae cranii (Figs.
135, 136, p. 168).

Structures formed in the Mesial Nasal Processes. — The

4

HUMAN EMBRYOLOGY AND MORPHOLOGY.

mesial nasal processes fuse together; in their anterior inferior
angles are formed the premaxillae. The remainder forms the
septum of the nose.

In the mesial nasal processes a laminar plate of cartilage is
developed, which is continuous with, and forms part of, the
tiabeeulae cranii (Tig. 136). Part of this cartilage remains as
the septal cartilage of the nose (Pig. 3). From the septal carti-
lage, just over the naso-palatine foramina, a small scroll-like
or turbinate process is thrown out on each side to form a hood
for an isolated piece of olfactory epithelium — the organ of Jacobson.
They form the cartilages of Jacobson. The cartilages and organ
are vestigial in man. In the mesial nasal processes are developed,
also, the mesial or septal limbs of the alar cartilages of the nose
(Fig. 3).

The Vomer is developed in the membrane (perichondrium) which
covers the primitive septal cartilage. A centre of ossification ap-

trough for cartilaginous

Fig. 4.— Showing the trough-shaped Vomer of the newly born.

pears in the 3rd month at each side of the cartilage ; they fuse
together under the palatal margin of the cartilage. Thus the
vomer forms at first a shallow trough in which the cartilage of
the septum appears to be implanted (Fig. 4).

The Vertical Plate of the Ethmoid is formed by a direct
ossification of the primitive cartilage of the septum. Ossification
begins in the 4th month. The crista galli, the intra-cranial part
of the septum, is formed in part by the ossification proceeding
into the attachment of the falx cerebri.

Premaxillary Bones. — The two premaxillary bones form the
sockets of the upper incisor teeth. In the human foetus at birth
the suture between the pre-maxilla and maxilla can be seen on
the hard palate ; it runs on each side from the naso-palatine
foramen to the alveolus between the lateral incisor and canine
(Fig. 9). On the facial aspect, the premaxilla fuses with the

DEVELOPMENT OF TIIE FACE.

5

superior maxilla in the 3rd month of foetal life, the maxillae
overlapping and almost completely excluding them from the face.
The nasal spine is formed by the premaxillae.

In mammals generally the premaxillae are highly developed
and form the snout part of the face. In the higher Primates the
face becomes less elongated, less prognathous and the premaxillae
less developed. In the orang, for instance, the premaxillae are
distinctly seen on the face at birth (Fig. 5), but as the permanent
canines begin to develop they fuse with the maxillae.

Fig. 5. — Showing the suture on the face between the premaxilla and maxilla in the

skull of a young orang.

In man each premaxilla is usually ossified by two centres,
placed side by side. Hence it sometimes happens in cleft palate
that the fissure appears, not between the canine and lateral
incisor, but between the lateral and middle incisor. In such
cases the two centres of the premaxilla have failed to unite and
the cleft occurs between them. The two premaxillae unite together
in the first year after birth. Their vestigial character in man is
due to the small size of his masticatory apparatus and consequent
retrogression in the development of the facial part of his skull.

Naso-palatine Foramen. — The naso-palatine foramina are
formed where the mesial nasal and two maxillary processes unite
to form the palate (Fig. 9). In animals with well-developed pre-
maxillae the two naso-palatine (anterior palatine) foramina are
large, and through each passes the naso-palatine duct, which
allows a communication between the buccal and nasal cavities.
The odour of the food within the mouth thus reaches the organ

HUMAN EMBRYOLOGY AND MORPHOLOGY.

ot Jacobson. In man the upper ends of the ducts remain open ;
they terminate blindly below, behind the mesial incisor teeth, in
the naso-palatine or incisive papilla (see Figs. 9 and 19).

Nasal Duct. — The lachrymal sac and nasal duct, through
which tears pass from the eye to the inferior meatus of the nasal
cavity, are formed between the lateral nasal and maxillary pro-
cesses (Jigs. 2 and 7). The epithelium of the skin (epiblast)
enclosed between the processes, forms at first a solid cord ; it
afterwards becomes caniculised to form the duct.

Structures Formed in the Lateral Nasal Process. — In each

lateral nasal process a laminar plate of cartilage is developed ; it
is continuous with, and forms part of, the trabeculae cranii (Fig.
136, p. 168). Its inner or attached margin is continuous with the
septal cartilage of the mesial nasal process ; it forms on each side
the roof and lateral wall of the nasal cavities (Fig. 7).

What becomes of the Cartilage of the Lateral Nasal
Process (Fig. 6). — It forms on each side:

(1) The cribriform plate, around the olfactory nerves as they
issue from the olfactory bulb ;

DEVELOPMENT OF THE FACE.

I

(2) The lateral mass of the ethmoid, at first merely a plate
of cartilage ; the superior and middle turbinate processes are
developed from the plate (Fig. 7); ossific centres appear in the
cartilage of the lateral mass and turbinate piocesses during the
fourth month ot foetal life ;

(3) The inferior turbinate bone (Fig. 7) (maxillo-turbinal).
The body of the superior maxilla is developed on its outer side
in the maxillary process (Fig. 7);

sup. turb.
lat. nas. proc.

mid. turb
inf. turb

f -frontal.

orbit pi. front.

uomer

/ septum
fines, nas. proc. )

sup. max. (max. proc.)
dental sac
palate (max. proc.)

Fig. 7.— Coronal section of the skull of a 7th month human foetus to show the
cartilages of the Lateral and Mesial Nasal Processes and the bones formed
round them.

(4) The lateral and part of the alar cartilages of the nose ;

(5) In the membrane over the cartilage, between the ethmoid
behind and the cartilages of the nares in front, are developed the
lachrymal and nasal bones, and the ascending process of the
superior maxilla. The cartilage beneath these bones disappears
after birth (Fig. 6).

Arteries and Nerves of the Nasal Processes. — 1. Mesial
Nasal Process. The chief artery and nerve of this process are
the naso-palatine, but branches also come from the nasal nerve
and its accompanying artery, the anterior ethmoidal.

2. Lateral Nasal Process. The lateral nasal nerves are derived
from Meckel’s ganglion and the descending palatine nerve. Vessels

8

HUMAN EMBRYOLOGY AND MORPHOLOGY.

accompany the nerves from the descending palatine. The nasal
nerve and anterior ethmoidal artery supply the process in front.

It will thus be seen that the chief nerves and arteries of both
processes are derived from structures in the spheno-max illary
fossa.

MAXILLARY PROCESSES.

The Parts formed from each Maxillary Process. — The max-
illary process springs from the base of the mandibular arch and
sweeping forwards below the eye separates that structure from

mid brain

cerebral uesicle

, anterior nares

-1 — upper Up. mes. nas. proc.
eye

-premax. mes. nas. proc.
upper lip. max. proc.
alueolus max. proc.

pa/at. proc.

max. proc. (section)

inner recess 1st cleft.
(Eustach. tube )

posterior nares
'roof of pharynx
septal part, mes. nas. proc.

Fig. 8. — Showing the ingrowth of the palatal plates of the two maxillary processes
early in the 2nd month. (After Kollmann.)

the mouth (Figs. 1 and 2). In front it comes in contact and
fuses with the lateral nasal process which forms the outer wall of
the nasal cavity and with the globular process of the mesial nasal
which forms the premaxillary part of the palate and the middle
part of the upper lip. The part of the face formed by the max-
illary process is shown in figure 2. The hard palate (with the
exception of the premaxillary part), the soft palate and its muscles,
with the uvula, are formed by a horizontal plate which grows
inwards from the maxillary process and fuses with the plate of
the opposite side beneath the septum of the nose, with which
the horizontal plates also unite (Figs. 8 and 9). The palatal

DEVELOPMENT OF THE FACE.

9

processes separate the buccal from the nasal cavities, forming the
roof of the one and the floor of the other (Fig. 7). The horizontal
palatal plates meet first in front; the process of fusion spreads
backwards, and by the end of the second month it is complete.

premaxilla

Fig. 9. — Showing the Hard Palate at birth. The premaxillary part is formed from
the Mesial Nasal Processes ; the remainder by the Palatal Plates of the Maxillary
Processes.

The condition of cleft palate is due to a partial or sometimes a
complete failure of the process of fusion.

Bones formed in each Maxillary Process. — The zygomatic
process of the temporal, the malar, and the greater part of
the superior maxillary are formed directly from the connective
tissue within the process. They are membrane-formed bones.

Ptery go-palatine Bar. — Two other bones formed in this
process have quite a different history. The internal pterygoid
plate, which is originally a separate bone, and the palate, are
developed over cartilage. When the maxillary process grows
forwards from the base of the mandibular arch, it carries with it
a prolongation of the cartilaginous bar which forms the skeletal
basis of that arch. The cartilaginous bar is known as the
pterygo-palatine, and in the membrane over this bar the pterygoid
(internal pterygoid process) and palatal bones are developed (Fig.
10 A, B, C.). From the posterior end of this bar is developed
the incus in mammals and the quadrate bone in birds and reptiles.
In birds and reptiles the lower jaw articulates with the quadrate
bone (Fig. 10 B), and on the quadrate the superior maxilla is
supported by the pterygoid and palate bones. In amphibians
the quadrate, pterygoid and palate form a continuous bar of

10

HUMAN EMBRYOLOGY AND MORPHOLOGY.

cartilage (palato-quadrate). In fishes the Palato-quadrate bar
forms part of the mandibular arch (Fig. 10 A). In mammals the
quadrate is completely separated from the pterygoid and, instead

palato-quadrate bar

incus ( quadrate )

tympanic ( quad. -jug. )
malleus ( os articu

Eus. cart.

"-internal pterygoid
int. lat. Hg.
obliterated

symphysis

Fig. 10, a, b, c. — Showing what become of the skeletons of the Mandibular Arch
(Meckel’s Cartilage) and Maxillary Process (Palato-quadrate Cartilage). The
numerals indicate corresponding parts.

A. In Fishes and Amphibians. B. In Reptiles and Birds. C. In Mammals.

of acting as a supporting bone for the lower jaw, as in birds,
is subservient to hearing, and known as the incus.

It is difficult to understand, as Dr. Hans Gadow has pointed
out, how a bone such as the quadrate, constantly engaged with

DEVELOPMENT OF THE FACE.

11

the mandible, could have become subservient to hearing, and he
has produced good evidence to show that the quadrate does not
correspond to the incus but to the tympanic plate, with which
the mammalian jaw is still in contact. The incus he believes to

incus
stapes

malleus

hyoid arch'^%

upper part hyoid arch

tympanic-quadrate
Jnt. pteryg.

-palate

- Meckel s cartil.

use. ramus

Meckels cartil.

Y* .-i

r

articulare

Fig. 10 D. — Illustrating Gadow’s view of the origin of the Auditory Ossicles and

Tympanic Plate.

be derived from the upper segment of the hyoid arch (see Fig.

10 D).

Nerves and Arteries of the Maxillary Process. — A know-
ledge of the manner in which the maxillary process is developed
explains the distribution and course of its arteries and nerves.
The second division of the 5th, represented by the infra-orbital,
descending palatine, pterygo-palatine, and Vidian nerves, forms its
nerve supply. Its main artery is the internal maxillary. The
muscles of the palate are developed in the horizontal palatal
processes.

Formation of Foramina and Canals in Bone. — The develop-
ment of canals and foramina in the bones of the maxillary process
illustrates the manner in which these are formed in the skull
generally. Many foramina and canals occur originally between
separate elements (see page 170). The Vidian nerve lies between
the internal pterygoid plate (a separate bone) and the external
pterygoid, a plate which grows into the maxillary process as a
cartilaginous prolongation of the great wing of the sphenoid. The
pterygo-palatine canal is situated between the pterygoid and
palate part of the pterygo-palatine bar. The descending palatine

12

HUMAN EMBRYOLOGY AND MORPHOLOGY.

nerves lie between the palate bone and superior maxilla. These
are canals formed between different elements. The infra-orbital
nerve at first passes forwards in a groove on the orbital aspect of
the superior maxilla, but in the later months of foetal life,
upgrowths from the malar and nasal centres of ossification of the
maxilla meet over the nerve and convert the groove into a canal.

The foramen rotundum and foramen ovale are at first notches
on the edge of the great wing of the sphenoid, but in the course
of foetal growth the notches are converted into foramina. Hence
wherever a nerve foramen or canal is found one may conclude
that it marks the junction of two elements, originally distinct, oi-
ls originally a groove or notch on the edge of the bone (Bland
Sutton). The malar nerves issue between the two centres of
ossification of the malar. The two malar centres may fail to
unite ; the bone is then divided by a suture passing from the
orbit to the temporal fossa. It occurs rather more frequently
in Japanese and Mongolian skulls, hence the name of Os
Japonicum.

Palatal Rugae.— In all classes of mammals the mucous mem-
brane on the hard palate is ridged transversely ; three or four
of these tranverse ridges are seen on each side of the palate of
the newly born child ; they tend to disappear in the adult. Food
is triturated between them and the rough papillae on the palatal
aspect of the tongue. Their disappearance in man is probably
due to the soft nature of his food.

The Antrum of Highmore. — It will be seen from figure 7
that the maxillary process is at first a thin plate, lying between
the orbit and mouth, containing the tooth buds. It rests on the
outer aspect and covers the cartilaginous basis of the lateral nasal
process which forms the outer wall of the nasal cavity (Fig. 7).
About the third month of foetal life the mucous membrane in the
middle meatus begins to bud outwards, presses before it and
bursts through the lateral nasal plate of cartilage and begins to
distend the maxillary process. At birth the antrum is only a
shallow recess on the outer wall of the middle meatus. It
•continues to grow until the 25 th year, and is the only one of
the air sinuses developed from the nasal cavity, which is present
at the time of birth. In the years of adolescence the antrum
reaches out until it inflates the maxillary part of the malar and

DEVELOPMENT OF TIIE FACE.

ia

as it grows backwards, presses downward the posterior border
of the maxilla and thus brings the permanent molar teeth into

opening of antrum
milk teeth at birth

post, border of max. at birth

antrum at birth

antrum of adult

posterior border
of max. in adult

Fig. 11.— Showing the manner in which the development of the Maxillary Antrum
affects the size of the palate and position of the molar teeth.

position (Fig. 11). If the process of growth is arrested, the last
molar (wisdom) tooth is left on the posterior border of the maxilla,
where it may ultimately be the cause of an abscess.

MANDIBULAR PROCESSES AND ARCH.

The two mandibular processes unite in the middle line and
form the mandibular or first visceral arch. The arch forms the
lower or hinder boundary of the stomodaeum (Fig 1).

Parts formed from the Mandibular Arch. — Besides the lower
jaw, there are formed from this arch the soft parts over and
under the jaw, the lower lip, the muscles of mastication, the
internal lateral ligament and the malleus. The anterior two-
thirds of the tongue, the sublingual and submaxillary glands are
formed from the floor of the primitive pharynx between the man-
dibular and the second or hyoid arch. These parts are supplied
from the nerve of the mandibular arch, and are therefore probably
derived, in part at least, from the substance of the arch.

The Mandibular Arch bounds the stomodaeum behind, and is the
foremost of the five visceral arches which encircle and form the
walls of the primitive pharynx. Meckel’s cartilage forms its
skeletal basis (Figs. 10(7 and 12). The 3rd division of the
5th is its nerve, and its artery is the first aortic arch from which
the inferior dental, facial and lingual arteries afterwards arise.

14

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The structures formed from Meckel’s cartilage are shown in
Figs. 10 C, 10 D, and 12.

Development and Ossification of the Lower Jaw. — In some
animals, such as the kangaroo, the two halves of the lower jaw,
each developed in its own mandibular process, never unite. In
man ossific union takes place early in the second year. In figure
12 are shown the manner of formation and ossification of the
lower jaw, with the changes that take place with age.

The part of the lower jaw {A, Fig. 12) is developed directly
out of Meckel’s cartilage. The dentary centre ( B ) appears in the
membrane on the outer side of Meckel’s cartilage, and forms the
body of the jaw from the mental foramen almost to the angle ;
the splenial centre ( C ) appears on the inner aspect of the cartilage
and between it and the dentary, the canal for the dental nerves
and vessels is formed. The ascending ramus, developed in
mammals only, is formed from the coronoid and condylar centres
which appear in membrane. By the condylar process the lower
jaw comes to articulate directly with the skull (squamosal bone).

The growth of the antrum of Highmore, by pushing
downwards the body of the lower jaw, leads to an elongation
of the ascending ramus, and to its assuming a more vertical
position to the body of the jaw (Figs. 11 and 12). In old age,
when the teeth drop out and the alveolar margins are absorbed,

(relatiue length and angle of /\
asc. ramus in adult ) ~f— \

(for splenial on inner aspect
of Meckel’s cart.)

Fig. 12. — Showing the Centres of Ossification and age changes in the Lower Jaw.

DEVELOPMENT OF THE FACE.

15

the ascending ramus again becomes oblique, to allow the lower
jaw to come in contact with the upper during mastication. The
mental eminence is produced after birth, and is a human charac-
teristic. It gives attachment to the depressores labii inferioris.

The ascending ramus is peculiar to mammals. In other verte-
brates Meckel’s cartilage forms the skeletal basis of the lower jaw
and articulates with the quadrate bone (incus) by its upper
articular extremity, the os articulare (malleus) (Fig. 10 G ), The
part of Meckel’s cartilage between the malleus and body of the
lower jaw forms the internal lateral ligament, and possibly also
the interarticular cartilage of the temporo-maxillary articulation.

post-glenoid sp.

ext. ctud.
meat

mastoid

cartil. twmnic

condyle

ext. aud meat.

' paramastoid
^post-glenoid sp.
cartilage

condyle

Fig. 13.— The chief types of the Temporo-Maxillary Articulation.

A. Carnivorous Type. B. Omnivorous Type. C. Herbivorous Type.

Gadow regards the angle of the jaw as the representative of
the os articulare.

The Temporo-maxillary Articulation. — Two types of this joint

16

HUMAN EMBRYOLOGY AND MORPHOLOGY.

are found in mammals, one (see figure 13.4), exemplified in the
carnivora, in which only a hinge action is permitted, and hence the
jaws act like scissor blades; the second (see figure 13(7), in which

Fig. 14. — Showing the Chief Changes after birth in the form of the Temporo-

Maxillary Articulation.

A. At Birth. B. At Two Years. C. In the Adult.

a gliding movement is allowed, the teeth being thus able to act
as grinders. The second type occurs in all vegetable feeders. The
human articulation combines the characters of both types (Fig.
13 B), the gliding action taking place between the interarticular
cartilage and the skull, the hinge action between the cartilage and

DEVELOPMENT OF THE FACE.

17

the condyle. In rodents the glenoid cavity is a narrow gutter in
which the plate-like condyloid process glides backwards and
forwards. The interarticular cartilage is developed in all the
Mammalia except the monotremes, and one or two marsupials
(Parsons).1 It is probably a derivative of Meckel’s cartilage (see
Fig. 10Z)).

Development of the Tympanic Plate and Articular Emin-
ence.— if the chin be depressed the condyle of the jaw moves
on to the articular eminence (Fig. 13 B)\ if over-depressed
it springs over the eminence, and a dislocation is produced.
This is impossible in the early years of life, for at birth
there is no eminence and no glenoid cavity (see Fig. 14: A).
At birth the membrana tympani lies exposed on the surface
of the skull behind the condyle, supported in a fine osseous
hoop, the tympanic ring. The ring is imperfect above, and
there the flaccid part of the membrane occurs. By the second
year the ring has grown into a plate by sending out two
processes, which, as they grow out, unite and leave a gap between
(Fig. 14I>). This, as a rule, is soon filled up. By the 20th year
the tympanic plate is three-quarters of an inch long, forming the
bony floor of the external meatus and the posterior wall of the
glenoid fossa, which in man is remarkably deep. It protects
the meatus from the condyle ; every year until the 20th the
bony meatus gets longer, while the fibro-cartilaginous part
becomes relatively shorter. In the adult the bony part forms
two-thirds of the meatus. As the tympanic plate grows outwards,
the membrana becomes less easily accessible to the surgeon
(Fig. 14(7). The plate also grows inwards to form the floor of
the bony part of the Eustachian tube and downwards to form
the vaginal process, to which the upper end of the carotid sheath
is attached (Fig. 40, p. 54). Gadow regards the tympanic plate
as the representative of the quadrate bone of birds and reptiles.

THE STOMODAEUM.

The stomodaeum or primitive buccal cavity is the depression
or narrow pocket formed between the fore-brain above and
the mandibular arch below. It is bounded laterally by the

1 “Joints of Mammals,” Journ. of Anat. and Physio ., Yol. XXXIY.

B

18

HUMAN EMBRYOLOGY AND MORPHOLOGY.

maxillary processes (Jig. 15^4) and lined by the covering
epithelium of the skin— epiblast. It is blind at first, the floor

stomodaeum
<rmax. proc.

somatopleure
mandibular arch

A

Fig. 15 A.— Sagittal Section showing the Stomodaeum and position of the Oral Plate

in the 3rd week. (Schematic.)

Fig. 15 B. — Showing the parts of the Buccal and Nasal Cavities formed from the
Stomodaeum. The relative position of the Oral Plate is indicated.

or fundus being formed by the oral plate, which separates it
from the primitive pharynx (Fig. 15 A). The mesial nasal
and palatal plates of the maxillary processes divide it into an upper
part — the nasal cavities — and a lower, which forms part

DEVELOPMENT OF THE FACE.

19

of the permanent buccal cavity (see Fig. 15 B). The tongue is
developed in the door of the pharynx and the tonsils in the
pharyngeal wall, but the lips, teeth, and gums are formed in the
walls of the stomodaeum. In the 3rd week of foetal life the
oral plate breaks down and the stomodaeum then communicates
with the pharynx.

The Origin of the Pituitary Body. — The lining epithelium
(epiblast) of the stomodaeum becomes pouched out against the
door of the fore-brain and forms the buccal element of the
pituitary (Fig. Id A and Fig. 22, p. 30). A process from the
door of the hinder part of the fore-brain (thalamencephalon)
meets it and forms the neural part of the pituitary. The buccal
evagination is sometimes called Kathke’s pocket. With the
development of the base of the skull, the stalk of the buccal
evagination disappears. A canal may occasionally be seen
passing upwards between the basi- and pre-sphenoid, and opening
at the olivary eminence, marking the position occupied by the
pocket in the foetus (canalis cranio-pharyngeus, Fig. 3). Gaskell,
who regards the neural or cerebro-spinal canal as the homologue
of the invertebrate alimentary canal, homologises the pituitary
evagination of the buccal epiblast with the invertebrate mouth
and gullet.

CHAPTER II.

THE NASAL CAVITIES AND OLFACTORY STRUCTURES.

In tracing the development of structures subservient to the sense
of smell, the following elements have to be dealt with :

(1) The olfactory sense epithelium and olfactory nerves;

(2) The parts of the brain concerned with the sense of smell,
so far as we know them ;

(3) The capsule which contains the olfactory epithelium ;

(4) The respiratory tract of the nasal cavities.

lat. nets. proc.
mes. nas. proc.

olfactory pit ancl plate

for Jacobson’s organ
stomodaeum

Fig. 16.— The Olfactory Pit and Nasal Processes in a 4th week human embryo

(After Kollmann.)

(1) Origin of the Olfactory Sense Epithelium. — A small
area of the epiblastic cells, lying under the fore-brain becomes
demarcated on each side, to form the olfactory plates. Aiound
these two plates the lateral and mesial nasal processes grow up

TIIE NASAL CAVITIES AND OLFACTORY STRUCTURES.

21

(Fig. 16), the plates being depressed to form the olfactory pits.
With the growth of the nasal processes the olfactory plates and
pits are thrust into the roof of the stomodaeum, where they form
the epithelial lining of the upper third or olfactory area of the nasal
cavities. A small island is detached from each plate to form the
basis of Jacobson’s organ (Fig. 16). The sense epithelia send out
nerve processes which form arborescences round the neural cells of
the outgrowing olfactory lobe (Fig. 17). The olfactory nerves are
thus formed. At first the olfactory plate is directly in contact with
the neural tube, and it is probable that neuroblasts may migrate
then to the olfactory plate and form the olfactory sense epithelium.

The condition of olfactory pits, a developmental phase in the
human embryo, is the permanent form in fishes (Fig. 21-5); in
amphibians and all higher vertebrates the fundus of the pit
breaks down and thus the olfactory pits come into communica-
tion through the posterior nares with the stomodaeum (F’ig. 8).

(2) The Olfactory Lobe. — As the olfactory plates are being
thrust downwards, the anterior part of the floor of the fore-brain
grows out on each side as a hollow protrusion to form the

foramen of Monro

( rudiment
of corp. callosum)

caudate nucleus

olfact. lobe, ant. part (A)

olfact. lobe, post pent (B)

hypopophysis cerebri

pineal body
mid brain

cerebellum

Fig. 17. — The Mesial aspect of the Brain of a human foetus, 3£ months old, showing

the Olfactory Lobe.

olfactory lobes (Fig. 145, p. 178). At the end of the 3rd month
the olfactory lobe has assumed the form shown in Fig. 17. Its
cavity is at first continuous with that of the cerebral vesicle
(lateral ventricle), but this connection is soon lost; it becomes

oo

HUMAN EMBRYOLOGY AND MORPHOLOGY.

solid and divided into anterior and posterior parts by a transverse
fissure (Fig. 17).

The anterior part, as is shown in figure 18, becomes (a) the
olfactory bulb, (b) the olfactory peduncle or tract, (c) the trigonum
olfactorium, lying between the lateral and mesial roots into which
the tract divides, and (cl) the area of Broca. The posterior part
of the olfactory lobe (B in Fig. 17) becomes (a) the grey matter
of the anterior perforated space, and (b) the gyrus subcallosus
or peduncles of the corpus callosum (Fig. 18).

Termination of the Olfactory Tract. — As is shown in figure
18, the mesial root terminates in the supra-callosal gyrus and
fornix, while the lateral ends in the uncus of the hippocampal
convolution. Olfactory nerve fibres also terminate in the trigone
and area of Broca. To the parts derived from the olfactory

■callosal

temp, incis.

Fig. IS. — Showing the parts formed out of the Olfactory Lobe in the brain of an Adult,
and the termination of the Olfactory Roots in the Sub-callosal and Uncinate Gyri.

(After Elliot Smith.)

lobe together with the uncus, the fascia dentata, the supra-callosal
gyrus, the fimbria, the fornix and septum lucidum (Figs. 18 and
172) the term Rhinencephalon is given because these parts are
concerned with the sense of smell, and represent the parts first
and most highly developed in the brains of lower vertebrates

gyrus

sept, lucid,.

striae longitud.
gyms subcallos

area of Broca (D)
trigone {C)\ l
lateral root

bulb (A)_

tract ( B)

mesial root

anterior perforated space

supra
1 gyrus

gyrus dentatus

t 4 ' ’ olfactory centre
I'l'-.y % hippocampal gyrus^

'uncus ^collateral fissure

band of Giacomini
uncus

THE NASAL CAVITIES AND OLFACTORY STRUCTURES.

23

(Elliot Smith). In man many of these parts are merely vestigial.
The higher olfactory centre has been located in the hippocampal
gyrus, but no evidence has been given showing any connection
between the callosal gyrus and this sense. In animals with a
highly developed olfactory sense (carnivora, etc.) these parts of the
brain which form the rhinencephalon are well developed. The
fornix in its greater part, and the longitudinal striae as association
tracts, connect the brain areas which are subservient to the sense
of smell (see Fig. 18).

It is important, from a clinical point of view, to remember that
the olfactory nerves are surrounded by prolongations of the arach-
noid membrane and subarachnoid spaces, and through these spaces
infections may spread from the nasal cavities to the meninges.
Further, the olfactory tracts rest on the edges of the small wings
of the sphenoid, and may be injured in falls on the forehead.

The Nasal Cavities. — The separation of the nasal cavities from
the stomodaeum by the downgrowth of the mesial and lateral
nasal processes, and the ingrowth of the horizontal plates of the
maxillary processes, has been already described (p. 3). These
processes also form and bound the anterior and posterior nares.

Development of the Nasal Air Sinuses. — The manner in
which the nasal mucous membrane pushes its way through the
lateral nasal cartilage into the maxillary process to form the
Antrum of Highmore has been already described (page 12).
The other air sinuses — the frontal, lachrymo-ethmoidal, anterior,
middle and posterior ethmoidal, and sphenoidal sinuses — six in
all, arise in the same way as the antrum but at a much later date.
Although they begin to bud out about the 3rd year, they assume
their active growth in the earlier years of puberty, and reach
their full size before the 30 th year.

At birth, the lateral mass of the ethmoid is a thin plate,
carrying the superior and middle turbinate processes, which
almost fill the nasal cavity (Fig. 7). The entire ethmoid is narrow,
and hence the proximity of the eyes in children. Beneath the
middle turbinate is a thumbnail-like impression — the hiatus semi-
lunaris (Fig. 19). The antrum buds out near its posterior end,
and the point at which the bud starts becomes its opening. The
uncinate process of the lateral mass of the ethmoid forms the
prominent lower margin of the hiatus.

24

HUMAN EMBRYOLOGY AND MORPHOLOGY.

ironi the upper end ot the hiatus a bud of mucous membrane
grows upwards to lorm the frontal sinus, gradually works through
the ethmoid, and pushes its way into the frontal bone, separating

inferior turbinate

tp. antrum

lachrymo-ethmoid
naso-turbinal.
hiatus semilunaris.

infundibulum
ant. ethm. sinus
mid. ethm. sinus

vestigia! turbinates
post. ethm. sinus
sphen. sinus
sphen. turb.

Eustachian tube

lateral recess
of pharynx

Fiu. 19.— A diagram of the Lateral Wall of the Nasal Cavity, showing the position of
the Air Sinus. The parts beneath the turbinate processes are indicated by
stippled lines.

the outer from the inner lamellae. As a rule, by the 25th year
it reaches outwards over the inner two-thirds of the orbital roof,
and is an inch or more both in height and depth at its inner part.
It is smaller in women than in men, but it may be, and often
is, arrested at an early stage of development, or it may be absent
altogether. The size of the glabellar prominence is no index to
its development.

The stalk of the frontal bud forms the infundibulum, which is
narrow, half an inch long, and difficult of catheterization from the
nose. Into it open (or sometimes into the hiatus) the lachrymo-
ethmoidal and anterior ethmoidal cells which surround the infundi-
bulum. They are developed as outgrowths from the infundibulum
(Fig. 19). Occasionally the antrum of Highmore, as is frequently
the case in the gorilla, sends a process to form part of the frontal
sinus, and hence there may be a communication between the sinus
and the antrum.

The development of the frontal sinuses and supra-orbital ridges
lead to a marked change in the face at puberty. By the forma-
tion of the frontal sinuses the basal area of the skull, to which
the face is attached, is largely increased in extent. Such an

THE NASAL CAVITIES AND OLFACTORY STRUCTURES.

25

increase is necessary to support the palate, which grows rapidly
in size at puberty. Up to the fifth year the upper jaw has to
carry only ten milk teeth; in the adult it has to carry sixteen
permanent teeth. To support these the face and palate grow
rapidly in size. The formation of the frontal sinus gives the
necessary increase in the area of the base of the skull for their
support. It should be remembered that the growth of the brain
and of the cranial cavity is comparatively slight after the fifth
year.

Only the gorilla and chimpanzee show an arrangement of
frontal and ethmoidal sinuses comparable to that of man.

Above the hiatus lies the bulla ethmoidalis, which is inflated
by, and commonly carries the opening of, the middle ethmoidal
cell (Fig. 19). The posterior ethmoidal sinus opens beneath the
superior turbinate process, and is developed from the superior
meatus. The ethmoidal sinuses are produced in the cartilage
of the ethmoidal or lateral nasal plate (Fig. 7). They inflate
the ossifying cartilaginous plate until it becomes a cellular mass,
thus increasing the breadth of the intra-orbital septum. The
sphenoidal sinus (Fig. 19) is formed by the mucous membrane
growing into and expanding the sphenoidal turbinate bone, which
is a small, slightly ossified cartilage lying beneath the pre-
sphenoid at birth, and forming the uppermost (sixth) of the nasal
turbinate processes. Latterly the sinus grows into and expands
the pre-sphenoid and part of the basi-sphenoid, the sphenoidal
turbinate remaining as its anterior wall (Fig 19). The
sphenoidal turbinate is a detached part of the ethmoidal (lateral
nasal) cartilage.

It will thus be seen that all the nasal air sinuses are produced
primarily by a budding outwards of the nasal mucous membrane
into the cartilaginous basis of the lateral nasal processes. Disease
may readily spread to these sinuses from the nasal cavities.
By means of the sinuses the area of the face is increased to
support the adult palate bearing the permanent teeth. Most of
them open on the respiratory tract of the nasal cavity. They
are ventilated with every breath. They act also as resonance
chambers.

Vestigial Turbinates. — There is frequently to be seen in the
adult one, or even two, secondary meatuses above the superior ;

26

HUMAN EMBRYOLOGY AND MORPHOLOGY.

these are constantly present in the chimpanzee and in animals
with a keen sense of smell. In the human foetus of four months
six turbinates are usually present, besides secondary processes in
the meatuses beneath them. The uppermost of these, the sixth,
becomes the sphenoidal turbinate ; the fifth disappears ; the third
and fourth may remain separate or become united ; the first and
second form the inferior and middle turbinate processes. The agger
nasi (naso-turbinal, Fig. 19), in front of the attachment of the
middle turbinate process, is a vestige of the naso-turbinal, a
process well developed in most carnivora and animals with a
strong scent. The uncinate process, which forms the lower
border of the hiatus semilunaris, is continuous at its base
with the naso-turbinal. Through the hiatus semilunaris acting
as a gutter, the antrum may become a cesspool for a suppurating
frontal sinus.

Nasal Duct. — Although in no way connected with the sense
of smell, the nasal duct is closely related to the nasal cavities.

Fig. 20.— Showing on the inner wall of the Orbit (1) the position of the Infundibulum,

(2) the pars facialis lachrymalis.

It is formed between the lateral nasal and maxillary processes
(Figs. 1 and 7). Three bones bound it: the superior maxilla on
the outer side, formed in the maxillary process ; the inferior
turbinate, formed in the cartilage of the lateral nasal process,
and the lachrymal, formed over the lateral nasal cartilage, bound
it on the inner side. The formation of the palate cuts the duct
off from the mouth. The hamulus of the lachrymal varies much

THE NASAL CAVITIES AND OLFACTORY STRUCTURES.

27

in size, and is the vestige of a larger process, which in lower
primates enters into the formation of the inferior margin of the
orbit. This pars facialis sometimes occurs in man (Fig. 2 0). The
position of the infundibulum to the lachrymal is shown in Fig. 20 ;
it is seen to lie entirely in the lateral mass of the ethmoid behind
the lachrymal. Occasionally the frontal and superior maxillary
bones may articulate in an interval between the lachrymal in
front and lateral mass of the ethmoid behind.

CHAPTER III.

DEVELOPMENT OF THE PHARYNX AND NECK.

Pharynx of the Embryo. — There is very little resemblance
between the pharynx and neck of a human foetus in the third
week and that of the adult (Figs. 15 A and B, p. 18). Indeed,
at the third week the human pharynx resembles closely that
of a fish (Figs. 21 A and B). In both the human foetus and
fish the pharynx is bounded by visceral or branchial arches, which
are separated by depressions (human embryoes) or clefts (fishes) ;

Fig. 21 A. Showing the Visceral Arches and Cleft-depressions in the Pharyngeal

Wall of a 4th week human Embryo. Each Visceral Arch contains an Aortic Arch.
(After His.)

in both the heart is situated under the pharynx, and from the ventral
aorta aortic arches pass up on each side, one in each visceral arch,
to terminate in the dorsal aortae*. In fishes the aortic arches give

DEVELOPMENT OF THE PHARYNX AND NECK.

29

off vessels to the gills, in which the blood is arterialized. In
the human embryo the blood passes directly through the aortic
arches.

All that part of the human neck lying in front of the vertebral
column and between the mouth above and the thorax and
clavicles below, with the bounding walls of the adult pharynx,
is formed from the foetal visceral arches. A knowledge of the

Fig. 21 B. — Showing the position of the Heart, Visceral and Aortic Arches in a fish.

(Diagrammatic — after Gegenbaur. )

transformation of the foetal to the adult pharynx is of the
utmost practical importance: it explains the occurrence of fis-
tulae and cysts found in the neck ; it accounts for the peculiar
courses taken by nerves, such as the recurrent laryngeal and
phrenic ; it explains the peculiar distribution of nerves to the
pharynx ; and throws light on the nature and anomalies of the
thymus, thyroid and tonsil.

The Branchial or Visceral Arches. — The visceral arches
bound and form the whole thickness of the wall of the
primitive pharynx. Four arches, each bounded behind by a
depression, are to be recognised superficially on each side of
the pharynx of the fourth week human embryo (Fig. 2 1 ^4 ),
but behind the 4th cleft is a fifth arch, perhaps also a sixth
which, however, never become raised or superficially differentiated

30

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Irom the body wall behind. Sagittal and coronal sections of the
primitive pharynx (Figs. 22 and 23) give a better idea of the

arrangement and constitution of the visceral arches than can be
had from a surface view. They are developed round the most
anterior part of the fore-gut which forms the lining membrane of
the primitive pharynx. The pharyngeal lining membrane, there-
fore, is the same as that of the alimentary canal from which spring
all the organs and glands of digestion and assimilation.

The Visceral Clefts. — The epithelium or hypoblast, which
lines the primitive pharynx, covers the inner aspects of the
arches and passes outwards in the recesses between them and
there comes in contact with the epithelial covering of the
body (epiblast) which dips in to meet it. The membrane thus
formed by the union of the epiblast and hypoblast in the
recesses between the arches, may be named the “ cleft mem-
brane.” It is never ruptured nor disappears in the development

DEVELOPMENT OF THE PHARYNX AND NECK.

of mammals ; in fishes it disappears and real clefts are formed
between the arches. On the outer side of the membrane is the

Fjq. 23. Showing the Floor of the Pharynx of a 4th week human embryo. (After His.)

“ external cleft depression ” : on the inner side, the “ internal
cleft recess” (Fig. 23). From the hypoblastic lining of these cleft
recesses we shall see that the tonsil, thyroid and thymus arise ;
from 'the external depressions are formed the various branchial

cysts and fistulae, which occasionally occur in the neck of the
adult. Each arch contains, as may be seen from figure 24 :

(a) A skeletal basis of cartilage ;

(b) An aortic arch ;

(c) Vein;

(d) A larger nerve along its anterior border and a smaller

along its posterior ;

(e) A muscle segment.

mandib. arch

Jst deft membrane
— median lobe thyroid
—2nd or hyoid arch
-(basis of pharyngeal
| part of tongue
ceruical sinus

Fig. 24. — Schematic Section of a Visceral Arch.

32

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Formation of the Cervical Sinus. — The first arch especially,
and also the second, grow and increase at a much greater rate
than the third and fourth. It is observed that the second arch
(hyoid) which in fishes forms the operculum for the gills, grows
over and buries the third and fourth. As it covers them over and
comes in contact with the body wall behind the fifth arch (see A
and B, Fig. 23), the epiblast covering the third and fourth arches
and clefts is buried. The epiblast so enclosed forms the lining of
the cervical sinus. It usually disappears, but may remain and form
a cyst in the neck, which opens some distance above the sterno-
clavicular joint. If the outer cleft depression in front of or
behind the third arch persist, it must open in the cervical sinus.

What becomes of the Visceral Clefts. — By the end of the
second month the clefts, or, to be more exact, the representatives

of clefts in the human embryo, have disappeared, except the upper
part of the first which forms :

DEVELOPMENT OF THE PHARYNX AND NECK.

33

(1) The external auditory meatus from the external cleft
depression ;

(2) The Eustachian tube from the internal cleft recess. These
two parts of the first cleft are separated by the cleft membrane
which becomes the membrana tympani.

If traces of the other clefts remain as fistulae or cysts they
will occur in the positions shown in figure 25. Part of the
second cleft is marked in the goat by an opening and auricular
appendage.

Within the pharynx traces of inner cleft recesses are to be
seen besides the Eustachian opening (Fig. 33, p. 43). The tonsil
is developed in the second cleft ; the paiato-glossus in the
anterior pillar of the fauces represents the second arch. The
lateral recess of the pharynx (fossa of Eosenmiiller), behind the
Eustachian tube, represents the upper end of the second cleft.
The pyriform fossa, at each side of the laryngeal aperture, re-
presents the fourth cleft (See Fig. 33).

The Cartilages of the Arches. — The history of the skeletal
basis of the first arch (Meckel’s cartilage) has been already traced
(p. 14 and Figs. 10(7 and 12).

The cartilage of the 2nd or hyoid arch forms (Fig. 26) :

(1) The tympano-hyal, which is imbedded in the petro-mastoid.

(2) The stylo-hyal (Fig. 2 6), which ossifies in the early years
of life and becomes joined to the tympano-hyal to form the
styloid process.

(3) The segment below, the epi-hyal, which becomes ligamen-
tous, and forms the stylo-hyoid ligament, but it also may
become ossified.

(4) The lowest segment, the cerato-hyal, forming the small horn
of the hyoid.

The epi-hyal lies behind and outside the tonsil, and when
ossified has been excised under the belief that it was a foreign
body. The body of the hyoid (basi-hyal) probably represents the
fused ventral parts (copulae) of the 2nd and 3rd arches.

Gadow regards the auditory ossicles as derivatives of the upper
or hyomandibular segment of the 2nd arch (Fig. 10 21).

It will be seen later that the tongue arises from the floor of
the pharynx in the field between the 2nd and 3rd arches.
The skeletal bases of the ventral parts of the 2nd and 3rd

c

34

HUMAN EMBRYOLOGY AND MORPHOLOGY.

aiches come to form the bone of the tongue. The skeletal part
of the hyoid arch suspends the tongue.

The great horn of the hyoid represents the cartilage of the
3rd arch (Fig. 26). The formation of the larynx and lungs from

Fig. 26. — Showing what become of the Cartilages of the Visceral Arches.

the ventral part (floor) of the pharynx renders it difficult to say
what becomes of the cartilage of the 4th arch, but it probably
forms the whole or part of the thyroid cartilage. The thyroid in
Marsupials is composed of an upper and lower segment, hence it
is supposed that it may represent both the cartilages of the
4th and 5th arches. A perforation for vessels near the middle
of the thyroid cartilage, on each side, sometimes occurs in
man.

The Nerves of the Visceral Arches. — The 3rd division of the
Vth nerve is, as has been already seen, the principal nerve of the
first or mandibular arch. The nerve for the second or hyoid arch is
represented by the 7th and 8th (facial and auditory, Fig. 27).
The nerve of the 3rd arch is the glosso-pharyngeal, that for
the 4th is the superior laryngeal branch of the vagus, and for
the 5th the inferior laryngeal (Fig. 27).

Each nerve of a visceral arch, however, sends a branch to the

DEVELOPMENT OF THE PHARYNX AND NECK.

35

posterior border of the arch in front of it. It would be better,
perhaps, to describe them as distributed to the clefts rather than
to the arches. The anterior branch of the facial (nerve of the

Fig. 27. — Showing what become of the Nerves of the Visceral Arches.

hyoid arch) is represented by the chorda tympani and great super-
ficial petrosal ; that of the glosso-pharyngeal by its tympanic
branch (Fig. 27).

AORTIC ARCHES.

What become of the Aortic Arches — the Arteries of the
Visceral Arches. — In figure 21 is given the foetal arrangement
of the aortic arches, and in figure 28 the vessels in the adult
which are formed from them. The primitive aorta in the embryo
divides into two trunks, which run forwards along the floor of the
pharynx, one on each side, lying between the ventral ends of the
visceral arches. These may be termed the right and left ventral

36

HUMAN EMBRYOLOGY AND MORPHOLOGY.

aortic stems. From these stems five arteries (aortic arches) pass
upwards, one in each visceral arch, to terminate in the right and
left dorsal aortae, which run backwards to join together and form

lingual from 2nd
uperior thyroid
(from 3rd)

ventral aorta
(right trunk)

ventral aorta
(left trunk)

left subc/au.
th arch, left
L^5th arch, left

left dorsal aorta

Fig. 28. — Showing what become of the Aortic Arches in the adult. Only the shaded

parts persist.

one vessel at the 4th dorsal vertebra — the descending thoracic
aorta.

As may be seen from figure 28, the first and second arches
disappear ; the third remains as the first part of the internal
carotid, the fourth forms the 1st and 2nd stages of the right
subclavian. On the left side the 4th aortic arch forms that
part of the arch of the aorta between the origin of the left carotid

DEVELOPMENT OF THE PHARYNX AND NECK.

37

and entrance of the ductus arteriosus. The fifth arch on the left
side is represented in its entirety by the pulmonary artery and
ductus arteriosus. The fifth arch on the right side disappears,
in the greater part of its extent at least. Probably the
right pulmonary artery is formed from the inner part of this
arch.

All the aortic arches are not present at the same time ; some
have only a brief period of existence. One of these transient
arches is said to appear between what are usually described as
the 4th and 5th arches. If this is always the case then the
pulmonary arteries should be described as derived not from the
5th but from the 6th aortic arches.

Subclavian Arteries. — The visceral arches with their arteries
are well developed before the limb buds appear. When, at the
end of the third week, the buds grow out to form the upper
extremities, the artery which supplies each bud grows out from

innominate
carotid

uert.
subclau.

constriction
right pul. art.-

5th right arch • —
right dorsal aorta

uert.
ubclau.
•onstriction
5th arch, left side

left pulmonarg art

'eft dorsal aorta

Fig. 29.— The condition of the Right and Left Dorsal Aortae in a 6th week human
foetus. (After His.) The right arch disappears beyond the origin of the right
subclavian ; a constriction may appear at the corresponding point on the left side.

the dorsal aortae opposite the 4th arches (Fig. 29). This artery
forms the entire subclavian on the left side, but only its third
stage on the right.

The Arch of the Aorta on the Right Side. — In birds it
is the 4th right arch which forms the aortic arch, and
this occasionally happens in man. In amphibians both the
right and left 4tli arches persist as aortic arches. The two

38

HUMAN EMBRYOLOGY AND MORPHOLOGY.

dorsal aortae in which they end, unite together, as they do in
the human embryo, to form the descending thoracic aorta.
Probably a communicating arterial twig which runs in the human
body from the origin of the intercostal artery of the 3rd or 4th
space to join the superior intercostal of the right side represents
the right dorsal aorta between the 4th arch and the descending
aorta (see Fig. 28).

Cases are found in which the permanent aorta is very much
constricted at or near the point of entrance of the ductus arteriosus
(5th arch) (see Fig. 29). It will be noticed that the correspond-
ing part of the right dorsal aorta is obliterated. Such a con-
striction on the left side is to be regarded as corresponding to
that on the right side and indicates a partial attempt to produce
a right aortic arch ; it may give rise to clinical symptoms.

Dorsal Aortae. — It will be noticed that the parts of the dorsal
aortae between the 3rd and 4th arches disappear (Fig. 28). The
ventral aortae persist as the innominate, the common carotid and
external carotid arteries. It will be observed that, while the 1st,
2nd and 3rd vascular arches have almost retained their foetal
position, the 4th and 5th arches have been pulled downwards by
the descent of the heart. The 4th, which should lie opposite the
upper part of the thyroid cartilage, comes to lie on the 1st rib on
the right side and within the thorax on the left, while the 5th
dragging the nerve of its arch in front of it (the recurrent
laryngeal) is pulled right within the thorax from its foetal position
at the thyroid cartilage. With the descent of the heart the ventral
aortae between the 3rd and 4th arches are drawn out to form the
innominate and common carotid arteries on the right side, and
the left common carotid on the left.

Muscles of the Visceral Arches. — All the muscles supplied
by the facial nerve — the platysma, muscles of expression, the
stapedius, etc. — are derived from the muscle plate of the 2nd
or hyoid arch. The muscles of mastication, with the tensors of
the palate and tympanum, are derived from the muscle segment
of the mandibular arch. The stylo-pharyngeus is derived from
the 3rd arch.

The Platysma and Muscles of the Face and Scalp. — The

platysma myoides, the muscles of the face, scalp and external ear,
are derived from the muscle plate of the second or hyoid arch.

DEVELOPMENT OF THE PHARYNX AND NECK.

39

They are supplied by the facial, the nerve of this arch. The
muscle bud, from which the whole platysma sheet is developed, is
still confined to the area of the hyoid arch at the end of the
second month. During the third month the bud spreads out and
forms a continuous muscular hood over the head and neck. To
this hood or sheet, which is composed of two layers, a deep and
superficial, the name of platysma sheet may be given. It is
developed in the superficial fascia.

In man, the platysma sheet has undergone marked retrograde
changes in the neck, scalp and external ear, but over the face it
has become more highly specialized and differentiated than in any
other animal. From this sheet are derived the epicranial
aponeurosis, the occipitalis and frontalis. On the face the
platysma sheet forms the muscles round the orbit, nose and
mouth. The buccinator and levator anguli oris represent parts of
the deeper layer of the sheet. The transversus nuchae, fibres
occasionally seen in man passing from the middle line of the neck
behind, towards the ear and cheek, represent fibres constantly
developed in lower primates, and better still in rodents and car-
nivores as the sphincter colli and sterno-facialis.

The muscles supplied by the facial nerve are peculiar in that
they are the physical basis into which many mental states are
reflected and in which they are realized. Through them mental
conditions are manifested. It is found that the differentiation of
this sheet into well-marked and separate muscles proceeds pari
passu with the development of the brain. The more highly
convoluted the brain of any primate, the more highly specialized
are its facial muscles.

STRUCTURES DEVELOPED FROM THE WALLS OF THE

PRIMITIVE PHARYNX.

The Tongue and its Development. — The tongue is developed
in the floor of the primitive pharynx between the ventral parts of
the 1st, 2nd and 3rd visceral arches (Fig. 31). Two parts are to
be recognised in the tongue. The buccal part (Fig. 30) is situated
in front of the foramen caecum and the Y-shaped groove. It is
covered by papillae, concerned in mastication and liable to cancer.
The second or pharyngeal part, bounding the buccal part of the

HUMAN EMBRYOLOGY AND MORPHOLOGY.

pharynx in front (Fig. 30), is covered by glandular and lymphoid
tissue and concerned with swallowing.

The buccal part arises during the 3rd week from the mandibular
or 1st arch and the 1st interbranchial space by the upgrowth of a

Fjg. 30. — Showing the Buccal and Pharyngeal parts of the Tongue.

tubercle, the tuberculum impar (Fig. 31). Being mandibular in
origin, it is supplied by the mandibular nerve (3rd div. of Vth),
and its main attachment is to the mandible. Although its
bilateral origin is not apparent during development, the lingual
septum, the occasional occurrence of cysts in the middle line and
its bifid condition in lower vertebrates and occasionally in man,
make it extremely probable that it derives a half from each side
of the mandibular arch.

The pharyngeal part of the tongue is derived from the fused
ventral ends of the 2nd and 3rd arches in which, as we have
already seen, the body of the hyoid is developed. The glosso-
pharyngeal, the nerve of the 3rd arch, or more strictly of the 2nd
cleft, supplies it. The Y-shaped groove (sulcus terminalis) marks
the union of the tuberculum impar with the basal or pharyngeal
part. From the hypoblast, which lines the depression between
those two parts, arises, by a process of outbudding, the middle
lobe or isthmus of the thyroid gland (Fig. 34).

DEVELOPMENT OF TIIE PHARYNX AND NECK.

41

The Musculature of the Tongue— The muscles of the tongue
do not arise within the visceral arches, but are of extraneous
origin. It will be shown subsequently that the head is probably
composed of nine segments, and it is from the muscle plates of the

Fig. 31.— Showing the origin of the tongue in the floor of the primitive pharynx.
The condition represented is from an embryo in the 6th week. (After His.)

posterior four or five of these segments that the tongue muscles
are derived. Processes from the muscle plates of these segments
grow downwards and forwards until they reach the basis of the
tongue derived from the three visceral arches, carrying their nerves
with them — the hypoglossal or 12th cranial nerve, which contains
the motor nerve fibres of the posterior segments of the head.
Hence, while the sensory nerves of the tongue come from the
nerves of the 1st, 2nd and 3rd visceral arches, its motor fibres
are derived from the posterior cephalic segments.

Lingual Papillae. — The filiform papillae are the first to appear,
then the fungiform, a few of which, along the posterior border of
the buccal part, become enlarged and sink to form circumvallate
papillae, round the bases of which taste buds are developed. The
papillae tire confined to the buccal or masticatory part of the
tongue. It will be observed that the taste papillae are situated
at the brink of the pharnyx (Fig. 30), at which the food is seized
and carried away by the involuntary muscles. At the lateral
margins of the buccal part of the tongue, just in front of the
anterior pillars of the fauces, the fungiform papillae are arranged

42

HUMAN EMBRYOLOGY AND MORPHOLOGY.

in a series of laminae, recalling and corresponding to the papillae
foliatae of low primates and of rodents. Between the papillae
foliatae occur taste buds. On the under surface of the tongue at
birth, on each side of the sublingual papillae and over the position
of the ranine artery, are two fimbriated folds of mucous membrane,
the plicae fimbriatae, vestiges of the tongue-like processes of
Lemurs and possibly the entire tongue of lowly Vertebrates.
Their function and meaning are unknown, but they may be
connected with the sense of taste. A remnant of the plicae
fimbriatae can commonly be seen in the adult.

The Lymphatics of the Tongue, the highways for the spread of
lingual cancer, run for the greater part with the lingual vein and
terminate in the uppermost of the deep cervical glands, over the
jugular vein at the angle of the jaw. Many also terminate in the
submaxillary lymphatic glands which lie in and around the
salivary glands of the same name. The tongue rarely shows any
malformation.

The Epiglottis. — The origin of the larynx, trachea, bronchi
and lungs as a depression and bud from the floor of the pharynx,
will be dealt with later (page 256); but the origin at the 3rd
week of the furcula (Fig. 34), a process from which the epiglottis
is derived, may be noted here. It arises from the 4th visceral
arch. The upper part of the thyroid cartilage also arises from
the 4th. The superior laryngeal is the nerve of the 4th arch,
hence it supplies the epiglottis and upper part of the larynx.

Origin of the Salivary Glands.— Between the tongue and gum
of the foetus there are two furrows (Fig. 32). From the hypoblast

DEVELOPMENT OF THE PHARYNX AND NECK.

43

of the inner, by a process of budding, arises the submaxillary
gland ; the sublingual arises by a number of buds from the outer
groove (Fig. 32). The parotid gland springs as a bud from the
angle between the mandibular and maxillary processes. It is
probably hypoblastic in nature, but it may arise from the epiblast
of the stomodaeum, for as yet its exact origin has not been deter-
mined. It grows backwards in the connective tissue over the
masseter, and at birth is comparatively superficial in position, but
as the mandible and external auditory process grow, it sinks
inwards to surround the styloid process, pushing the deep cervical
fascia beneath it. In this way the stylo-maxillary ligament is
formed from the fascia pushed in front of it. Its nerve is derived
from the 3rd division of the Fifth (auriculo-temporal).

Seessel’s Pocket. — In the middle line of the roof of the
pharynx (Fig. 33), just under the basi-occipital, there is a

depression or recess of mucous membrane which gets this name.
It is of no practical importance, and its embryological significance
is doubtful. Lymphoid tissue is developed in its walls and in
the mucous membrane round it. It may be a remnant of the
pharyngeal opening of the notochord (see page 146). It is
developed behind the oral plate.

Fig. 33.— Showing the position of the Visceral Clefts in the Adult.

44

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The Tonsil. — The tonsil arises comparatively late in foetal
life. In the 4th month eight or ten isolated buds of hypoblast
push out from the second cleft (Fig. 34) into the mesoblastic
tissue in the wall of the pharynx above the basal part of the
tongue. The buds form the crypts and glandular tissue of the
tonsil. Lymphoid tissue — for the tonsil must be regarded as a
lymphoid structure — collects round these glandular buds.

Concerning the origin of the lymphoid cells, both of the tonsil
and the thymus, there are two quite distinct theories. The more
recent (Gulland’s) is that the epithelial (hypoblastic) cells, which
form the glandular buds of the tonsil, give rise to broods of
lymphoid cells ; the older, that these lymphoid cells arise from
the blood or surrounding connective tissue, creep in and form
follicles round the glandular hypoblastic buds. The tonsil rests
on the superior constrictor and the pharyngeal fascia, or inner
sheath of the constrictor muscles, which surrounds it and forms
its capsule.

Over the tonsil and between the pillars of the fauces is the
supra-tonsillar recess (Fig. 33), a remnant of the second cleft.
The Plica triangularis is a fold of mucous membrane which is
continued from the anterior pillar of the fauces to the under
surface of the soft palate, overhanging the supra-tonsillar recess
(Fig. 33). It is well marked in the foetus, but commonly dis-
appears before adult life.

The tonsil is part of a great lymphoid system stationed along
the alimentary canal. It reaches its fullest growth in youth, as
is the case with the lymphoid system generally ; when active
growth of the system is over, and especially in the years of decay,
it becomes markedly reduced in size.

The Pharyngeal Recess and Pharyngeal Tonsil.— At each
side, the roof of the pharynx is produced outwards, behind the
Eustachian tube and levator muscles of the palate, to form the
lateral recesses of the pharynx. They represent the upper ends
of the second cleft, the palate (from the maxillary processes)
having: grown backwards inside the first and second arches and
separated the tonsillar part ol the second cleft from the pharyngeal
recess. In the recess, and especially on the posterior wall of the
pharynx between the recesses and round Seessel’s pocket, there is
developed much lymphoid tissue, the pharyngeal tonsil, which may

DEVELOPMENT OF TIIE PHARYNX AND NECK.

45

become hypertrophoid. The lymphoid tissue of the naso-pharynx,
when hypertrophied, may press on and obstruct the Eustachian
tube and respiratory space (see Figure 33).

The Lingual Tonsil.— That part of the tongue (pharyngeal)
produced between the 2nd and 3rd arches is covered by
mucous glands which are surrounded by nodules of lymphoid
tissue — the collective glandular mass receiving the name ot
lingual tonsil. It will thus be seen that from the 2nd cleft is

O #

produced a circum-pharyngeal ring of lymphoid tissue of great
physiological and pathological importance.

The Thymus. — The thymus arises in the same manner as the
tonsil, only from the 3rd instead of the 2nd cleft (ligs o3
and 34). The 3rd cleft is represented in the adult by the

tubercalum impar.

/

1st recess (scdiuary glands)
2nd recess ( tonsil)
median thyroid bud.

-fare li I a (epiglottis)

3rd recess (thymus)

4th recess (tat. thyroid bud)
aryteno. epi. fold
coelom ( pericardium)

- pulmonary grooue

stomach

Fig. 34. — Showing the origin of the Tonsil, Thymus, and Thyroid from the Internal
Cleft Recesses during the 4th week. (After His.)

space in front, and on each side of the epiglottis. The hypoblast
in the 3rd cleft recess thickens, becomes pushed out as a minute
pouch, shaped like a Florence flask, between the 3rd (internal
carotid) and 4th (arch of aorta) vascular arches. The neck of
the glandular hypoblastic pocket disappears. By a species of
secondary budding it becomes broken up into islands or separated
acini. Either by a production of broods of lymphoid cells within
the hypoblastic epithelium or by an invasion of lymphoid cells

46

HUMAN EMBRYOLOGY AND MORPHOLOGY.

from the surrounding mesoblast, the thymus becomes an adenoid
structure, the epithelial parts becoming compressed into the
corpuscles ot Hassall. The surrounding mesoblast supplies its
connective tissue stroma and capsule. The lateral lobes come
together under the ventral aortae, and in the retrogression of the
heart are carried backwards to lie in the superior mediastinum.
The pointed upper extremity of each lateral lobe can be traced up-
wards in the fully developed foetus, under the lateral lobes of the
thyroid towards the thyro-hyoid membrane. These apical strands
represent the stalk of the thymic buds. In its growth back-
wards it crosses dorsal to the lateral thyroid buds which arise
from the 4th cleft.

The thymus reaches its fullest growth in early childhood (3rd
or 4th year), and continues large as long as the body is in a
state of active growth. It shrivels up when maturity is reached,
and only a remnant is left as a rule, less remaining in men than
in woman. It receives its blood supply from the 4th aortic
arches through the internal mammary. In manner of origin it
resembles the tonsil ; indeed it may be regarded as a buried tonsil.

The Thyroid. — At a very early period (4th week), while
the buccal and pharyngeal parts of the tongue are appearing as
elevations on the door of the primitive pharynx, the hypoblast in
the mesial part of the furrow between those two parts of the
tongue thickens. The bud thus formed grows downwards and
backwards and soon bifurcates (Fig. 34). The bifurcated
extremity, after redivision to form a network of acini, becomes
the isthmus or median lobe of the thyroid. The stalk of
the bud becomes the thyro-glossal duct, the lingual opening
of the duct remaining as the foramen caecum. It seems
probable that this part of the thyroid, at least, was originally a
gland which poured its secretion into the mouth. The connective
tissue and vessels of the thyroid are derived from the surrounding
mesoblast ; only the glandular elements arise from the hypoblast
of the pharynx.

Thyro-glossal Duct. — In the great majority of subjects the
thyro-glossal duct completely disappears ; the foramen caecum
marks one extremity, while a pyramid of thyroid tissue prolonging
the isthmus towards the hyoid bone often marks the other
extremity. The pyramid of the isthmus may carry on it a

DEVELOPMENT OF THE PHARYNX AND NECK. 47

detached part of the thyroid-hyoid muscle — the levator glandulae
thyroideae. The body of the hyoid bone is developed in the track
of the thyro-glossal duct (Fig. 33) and splits it up. Eemnants
of the duct or of secondary detached acini of the thyroid
may persist and form cysts in the base of the tongue above
the hyoid, and commonly between the genio-glossus muscles.
They may also occur between the hyoid and thyro-hyoid mem-
brane. The supra-hyoid or infra-hyoid bursae may also become
cystic, and may be mistaken for thyro-glossal cysts.

The lateral lobes of the thyroid are developed from the inner
recess of the 4th cleft, the position of which is marked in the
adult by the pyriform fossae (see Figs. 33 and 34). These
pockets, like the thymic of the 3rd cleft, soon lose their connection
with the hypoblastic lining of the pharnyx, and become isolated
buds which divide and re-divide until a collection of isolated
acini is formed. The lateral lobes come in contact as they grow,
with the median (lingual) lobe under the laryngeal and tracheal
groove in the floor of the primitive pharynx. As they grow
outwards the thyroid buds come in contact with the cervical sinus
(see page 32), and at one time were supposed to spring from the
epiblastic lining of the sinus. They receive their blood supply
from the 4th arch (inferior thyroids), while the median lobe
is mainly supplied from the ventral aortae, between the 2nd and
3rd arches (superior thyroids). The nerve-supply comes on its
arteries from the superior and middle cervical ganglia of the
sympathetic. Its nerves appear to have their origin in the upper
dorsal segments of the spinal cord.

In the process of development minor buds of the thyroid may
become detached. These form accessory thyroid bodies.

In manner of origin and growth the thyroid resembles the ton-
sil and thymus, but unlike these it is not transformed into a
lymphoid structure.

Para-thyroids.— These vary in number from three to five on
each side (Welsh) and are small bodies of a brownish-red
colour, and measuring 6 to 8 mm. in diameter. One or two are
situated on ihe outer side of each lateral lobe amongst the
branches of the superior thyroid arteries. They arise with the
bud of the thymus from the third cleft. One or two occur
constantly on the tracheal aspect of each lateral lobe, amongst the

48

HUMAN EMBRYOLOGY AND MORPHOLOGY.

branches of the inferior thyroid arteries. These are derived from
the lateral thyroid buds. In structure they are made up of
reticulating columns of cells, with vessels arranged between the
columns, thus resembling in structure the carotid body, and
probably also in nature and origin the medullary part of the
supra-renal. Their presence is essential to the function of the
thyroid body.

Carotid. Bodies. — The carotid body occurs at the inner side of
the fork, between the internal and external carotid arteries. The
commencement of the internal carotid represents the artery of the
3rd arch ; that of the external carotid, the ventral aortic trunk.
The body therefore appears to be developed in the wall of the
pharynx at the ventral end of the 2nd cleft. It receives a large
supply of nerves from the superior cervical ganglion, and it con-
tains a rich network of vessels. Swale-Vincent regards it as
similar in nature and origin to the coccygeal body and medulla of
the supra-renal (see p. 259).

CHAPTER IV.

DEVELOPMENT OF THE ORGAN OF HEARING.

The Structures which form the Organ of Hearing. — In

figure 35 is shown diagrammatically the derivation of the five
elements which unite together to make up the organ of hearing.
The five elements are :

(1) The otocyst — an area of epiblast (epithelial covering of
embryo) above the first branchial cleft which becomes invaginated
in a saccular form, and forms the epithelial lining of the mem-
branous labyrinth. Some of its lining cells become differentiated
into the auditory epithelium.

(2) A ganglion derived from the “ neural crest ” of the hind
brain (Fig. 35). The nerve cells form the cochlear and vestibular
ganglia. Each cell sends out two processes, one to become
connected with the auditory epithelium of the otocyst, the other
to end in groups of nerve cells in the floor of the 4th ventricle,
their collective fibres forming the auditory nerve. The develop-
ment of the auditory nerve thus resembles that of the posterior
or sensory root of a spinal nerve.

(3) The otocyst (membranous labyrinth) becomes surrounded
by a capsule of cartilage — the periotic capsule. This ossifies from
several centres, and forms the bony labyrinth and petro-mastoid.

(4) The dorsal end of the first visceral cleft. — The inner
recess of the 1st cleft forms the Eustachian tube, the tympanum
and antrum of the mastoid ; the external cleft depression, the
external auditory meatus ; while out of the cleft membrane is
formed the membrana tympani.

(5) The malleus is derived from the upper end of Meckel’s
cartilage, the incus from the posterior end of the palato-quadrate

D

50

HUMAN EMBRYOLOGY AND MORPHOLOGY

bar (cartilaginous skeleton of maxillary process), while the stapes
is an independent formation developed round the stapedial

Fio. 35. -Diagrammatic Section through the Cephalic region of an embryo, showing
the origin of the Auditory System.

artery. It may be derived, in part at least, from the upper end
of the cartilage of the hyoid arch (see also page 10).

In lower fishes the auditory apparatus is composed of elements
1 and 2, and are in them of the simplest form. The other
elements are added and specialized in the evolution of the
higher vertebrates.

External Auditory Meatus. — The external auditory meatus
is derived from the upper part of the first cleft depression. In
the adult the meatus is 1^ in. long ; at birth it is j of an inch,
and is surrounded by fibro-cartilaginous and fibrous tissue. In
the adult the tympanic ring grows outwards in the fibrous tissue,
as we have already seen (page 17) to form the tympanic plate
and the inner of the meatal floor. The squamous part of the
temporal, which is developed over in its roof, also grows outwards
and forms a thick, horizontal plate in the inner two-thirds of

( point at which acoustic

periotic capsule
ganglia ^

tymp.

Eustach.

DEVELOPMENT OF THE ORGAN OF HEARING.

51

the meatal roof (Figs. 36 A and B). Over the roof lies the third
temporal convolution.

Fig. 36 A. — A Section of the External Auditory Meatus of the Adult.

Fig. 36 B. — A Section of the External Auditory Meatus at Birth. (After Symington.)

The meatus is supplied in front by the nerve of the mandibular
arch (auriculo- temporal branch). Why the vagus should supply
it with a branch (Arnold’s nerve) is obscure. The vagus is a
visceral nerve and supplies the 3rd and 4th clefts by its
superior and inferior laryngeal branches. In fishes a branch
of the vagus passes backwards beneath the skin on each side
and supplies the sense organs of the lateral line. Many regard
the auricular branch of the vagus as a vestige of such a branch.

In the newly-born child the membrana tympani is so obliquely
set that its outer surface is almost in contact with the meatal
floor. With the development in length of the meatus, it becomes
more vertical in position. The meatus may be only partly
developed or even absent, the upper part of the 1st cleft
becoming completely closed like the lower part. In such a
case there is commonly a corresponding absence of development
of the middle and internal ear.

The External Ear. — Six tubercles appear on the mandibular
and hyoid arches round the 1st cleft depression and form the
external ear (see Figs. 37 and 38). Two of these tubercles
grow from the mandibular arch and form the tragus and crus
of the helix ; three from the hyoid to form the lobule, antitragus

A

B

J- HUMAN EMBRYOLOGY AND MORPHOLOGY.

and antihelix ; one above the cleft to form the helix. The pos-
terior margin of the ear is continuous with the lobule, and grows

Pig. 37. — Showing the Tubercles which arise round the First Visceral Cleft to form

the External Ear.

helix

I

Fig. 38.— Showing the part of the Adult Ear formed by each Tubercle.

from the tissue behind the tubercles which form the antihelix
and antitragus. The auricular tubercles may not fuse com-
pletely and thus leave fistulae between them. Such fistulae are

DEVELOPMENT OF THE ORGAN OF HEARING.

53

commonly seen between the tragus and root of the helix, or
between the antihelix and helix. The outgrowth of the ear may
be arrested at any stage. The mandibular part is supplied, as
one would expect from its origin, by the third division of the 5th,
while the sensory fibres of the hyoid part come from the 2nd
cervical by the great auricular and small occipital nerves.

Darwin’s Tubercle. — The human ear appears to be derived
from a form in which the margin was pointed at the posterior
superior angle, such as is seen in many of the lower forms of
apes and mammals generally. With the retrogression of the
posterior border or descending helix and increased development of
the antihelix in the human ear, the posterior margin became
infolded ; hence the tip appears as a tubercle on the inturned
posterior margin or welt of the human ear (Fig. 38).

Muscles of the External Ear. — These are derived from the

Fig. 39. — Showing the condition of the Auditory Organs in a 6th week human foetus.

(After Siebenmann.)

platysma sheet and are supplied by the nerve of that sheet —
the Vllth or facial. The ear muscles are not so reduced in man
as in some other primates, such as the orang.

54

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The Eustachian Tube. — The Eustachian tube is derived from
the first inner cleft recess of the pharynx (Fig. 39) and retains
through life the ciliated epithelial lining of the primitive pharynx.
It is l£ inches long. Its inner -| is bounded behind by a
triangular plate of cartilage, which is attached at its inner or
pharyngeal end to the internal pterygoid plate, a derivative, with
the incus, of the pterygo-palatine bar (Fig. 10 C). The cartilage
is developed in the walls of the 1st visceral cleft in the 4th
month of foetal life. The tympanic plate grows inwards and
forms the floor of its outer third (Fig. 40), while the periotic
capsule (petro-mastoid) which is developed above and behind the
1st cleft, grows over and forms the roof of its outer third. The
part of the petro-mastoid which grows over it, is the tegmen
tympani ; it also forms the roof of the tympanum and antrum of
the mastoid. It forms the roof of all the cavities derived from
the inner recess of the first cleft (Fig. 40). The anterior edge
of the tegmen tympani appears in the Glaserian fissure. The
tensor tympani and tensor palati are developed on the mandibular
side of the first cleft and are supplied from the nerve of the
mandibular process through the otic ganglion.

tympani

antrum

int. carotid

tympan.

_ drum
meatus

tympanic plate

Eustach. cart'd .

squam.

Fig. 40.— Showing the Cavities derived from the Inner Recess of the First Cleft.

The Tympanum. — As may be seen from Fig. 39, the tympanum
can scarcely be said to exist at the sixth week of foetal life. The
inner cleft recess ends in the jelly-like tissue containing the
cartilaginous bases of the malleus and incus. It is directed out-
wards and backwards between the periotic capsule to its posterior

DEVELOPMENT OF THE OPGAN OF HEARING.

55

and inner side and the external cleft depression (meatus) and
developing squamosal to its outer (Fig. 39). As the internal
recess extends and widens outwards and backwards, the gelatinous
tissue is absorbed, so that the malleus and incus and developing
stapes, with the chorda tympani, become surrounded by the
hypoblastic lining of the inner cleft recess and appear to be
situated within the cavity thus formed — the tympanum. The
tympanic plate forms its door, the membrana tympani and
squamosal its outer wall, while the petro-mastoid forms its inner
wall and roof (Fig. 40). That part of the tympanum which lies
above the level of the membrana tympani is named the attic,
and contains the head of the malleus and body of the incus
(Fig. 36).

In carnivora and some other mammals the floor of the tym-
panum, formed by the tympanic plate, is inflated into a bulla,
the tympanic bulla. Its meaning is unknown, but when a bulla
is developed the antrum of the mastoid is small or absent.

The Antrum of the Mastoid. — The antrum of the mastoid
represents the extreme outer or posterior end of the first cleft
recess (Figs. 39 and 40). It is formed at the same time and in

squam.

petro-squam. fis. f for ■ °^le

Eustach. tube^

antrum

ost-meat. proc. /.
petro-mastoid

I/ll.
antrum

tegmen tympani

ext. semicirc. canal
antrum

petro-mast.

' tympanum
tympanic ring

Eustachian tube } VH

for. rotundum

Fig. 41 . Fig. 42. Fig. 43.

Fig. 41. — The temporal bone at birth showing the formation of the Antrum between
tho Squamosal and Petro-mastoid.

Fig. 42. — A transverse section showing how the Walls of the Antrum are formed.

Fig. 43. — Showing the outer aspect of the Petro-mastoid at birth after the Squamosal
is removed.

the same manner as the tympanum. Its use is unknown, but it
has frequently to be exposed by the surgeon to remove the effects
of chronic middle ear disease. At birth its outer wall is formed
by the thin post-auditory part of the squamosal (Figs. 41
and 42). The squamosal forming its outer wall is then only
2 mm. thick, but every year until the 20th, or later, this plate
increases nearly 1 mm. in thickness, so that by the 20th year

56

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the antrum is buried by a plate of bone about 20 mm. thick.
There is a great individual variation, however, in the thickness
of its outer wall. The antrum lies above and behind the level of
the external auditory meatus ; the post-auditory spine and supra-
meatal triangle formed by the post-auditory part of the squamosal
lie over it and serve as surface guides to it. The antrum opens
in front into the attic of the tympanum. The tegmen tympani
(Fig. 42) forms its roof and the petro-mastoid its floor and inner
wall. The aqueductus Fallopii runs down the inner wall of its
mouth (Fig. 43), and in its inner wall is situate the external semi-
circular canal. The petro-squamous suture in its roof (Fig. 42),
and the masto-squamous suture on its outer wall (Fig. 14 B),
become closed in the second year, and thus the escape of pus from
it is rendered more difficult.

The Primitive Jugular Vein. — In the petro-squamous suture
a vein or sinus, frequently of considerable size, runs forwards from
the lateral sinus, and commonly ends in a tributary of the middle
meningeal vein. It receives as it runs along venules from the
antrum and attic and may be the means of carrying infection from
the middle ear to the lateral sinus or to the meningeal veins
(Cheatle). The petro-squamous sinus represents the primitive
jugular vein, and may open in man, as it does in mammals
generally, at the post-glenoid foramen, situated at the outer end of
the Glaserian fissure, near the base of the zygoma. In the early
weeks of embryonic development the primitive jugular is the chief
vein from the skull, but very soon the internal jugular vein
enlarges and takes its place, and thus the blood of the lateral
sinus comes to pass out by the jugular foramen instead of by the
petro-squamous sinus and temporal canal at the base of the
zygoma.

The Membrana Tympani. — As may be seen from Fig. 39,
the membrana tympani is of very considerable thickness until the
third month. It has an inner covering of hypoblast and an outer
of epiblast. In the inesoblastic tissue between the coverings lie
parts of the malleus, incus and chorda tympani. As the tympanic
cavity expands the inesoblastic tissue is compressed and absorbed,
and thus the handle of the malleus and chorda tympani come to
appear as if they lie on the membrane, although really within it.
The mucous lining of the tympanum covers them. The mem-

DEVELOPMENT OF TIIE ORGAN OF HEARING.

57

brane is supported by the tympanic ring, the age changes of
which have already been dealt with (p. 17). The nerves and
arteries of the first cleft supply the drum.

The Membranous Labyrinth. Origin. — The cells of a cer-

tain area of epiblast, situated above the 1st cleft (ligs. 35 and
37), become ultimately sensitive to sound waves. This area is
invaginated during the third week, and forms a simple closed
pyriform sac, the otocyst, which lies above the first visceral cleft
in the mesoblastic tissue between the hind brain and the epiblast
(Figs. 37 and 39). The sac contains a fluid, the endolymph, and
also otoliths are found in it later. The otocyst receives a thin

Fig. 44. — Diagram of the Membranous Labyrinth.

coat of mesoblast. The epithelial cells lining it, all of which are
originally columnar, soon become flattened, except at the maculae
acousticae, where they retain the columnar form and develop
hair-like processes. These become connected with the hind brain
by the auditory nerve fibres of the cochlear and vestibular ganglia.

In the lower vertebrates, as in the earlier embryonic stages of
the higher mammals, the otocyst is of a saccular form with a
stalk above — the ductus endolymphaticus (Figs. 39 and 45). The
simplest form of vertebrate otocyst is seen in the lamprey ; the
superior and posterior semicircular canals are present, but, as in
the mammalian embryo, the primitive cyst is undivided into
utricle, saccule, and cochlear canal. The semicircular canals
grow out from the cyst as flat, hollow plates, but only the
circumferences of the plates persist, the centres disappearing.

The primitive utricle, which represents the main part of the

58

HUMAN EMBRYOLOGY AND MORPHOLOGY

otocyst, subdivides into the saccule and utricle (Fig. 44). The
division occurs at the entrance of the endolymphatic canal, which
thus comes to open into both saccule and utricle. The endo-
lymphatic canal, which is simply the stalk of the otocyst, is

Fig. 45. — The Otocyst in an Embryo of five weeks ; it shows a demarcation into the
various parts of tho Membranous Labyrinth.

enclosed in the petro-mastoid, its extremity appearing at the
hiatus vestibuli, where it ends beneath the dura mater in a
dilatation. It appears to be a vestige of the primitive mouth of
the otocyst, which opens on the surface of the head in many
fishes. Lastly, the scala media or canal of the cochlea (Fig. 44)
grows out from the saccule. There is merely a rudiment of the
cochlea in fishes and amphibians. In reptiles, birds, and mono-
tremes it is a straight canal — the Lagena. Only in mammals
is it arranged spirally. In it the organ of Corti is developed.

The Petro-mastoid. Origin.— The mesoblast surrounding the
membranous labyrinth becomes cartilaginous at the end of the
3rd month of foetal life, forming the periotic capsule (Figs. 35
and 39). The tissue which immediately surrounds the mem-
branous labyrinth does not undergo chondrification, but becomes
converted into an open meshwork of cells, the intercellular spaces
containing perilymph. The lymphatic space thus formed within
the petro-mastoid, containing the saccule and utricle, is the
vestibule. The scalae tympani and vestibuli of the cochlea are
of similar origin. An oval space on the outer wall of the
vestibule is not cliondrified ; it contains the footplate of the stapes
and forms the foramen ovale. The foramen rotundum also
remains unchondrified and separates the tympanum from the
scala tympani (Fig. 43).

59

DEVELOPMENT OF THE ORGAN OF HEARING.

It will be observed that the various structures which serve to
transmit the sound waves from the external auditory canal to the
brain are drawn from many and very different sources. The
membrana tympani is the membrane of the first cleft ; the
malleus is the end of the skeleton of the first visceral arch ;
the incus is part of the palato-quadrate bar ; the stapes is of
independent origin or drawn from the cartilage of the second
arch ; the epithelial lining of the membranous labyrinth is derived
from the skin ; the petro-mastoid and the perilymph spaces from
the mesoblast; the ganglia and nerves from the hindbrain.

Although the derivation of the auditory ossicles given here is
the usually accepted one, there are many reasons for supposing
that Gadow’s views of the nature of the malleus, incus and
stapes will turn out to be the right one. He regards all three
bones as derived from the upper part of the hyoid arch (see
page 11).

The aqueductus cochleae is the outlet of the perilymphatic
space. It passes from the scala vestibuli to the anterior wall
of the jugular fossa.

Ossification of the Petro-mastoid. — About the end of the

4th month, four ossific centres appear in the periotic capsule ;
one, the pterotic, gives rise to the tegmen tympani which forms
the roof of the antrum, tympanum, and Eustachian tube ; the
petro-squamous suture marks its outer edge ; the hiatus Fallopii
marks its junction with a second centre — the opisthotic. This
centre forms the posterior half of the petrous bone. The pro-otic
forms the anterior half ; the mastoid part, which appears on the
surface of the skull, is developed from the epiotic centre.

The Mastoid. — The mastoid part of the petro-mastoid is flat at
birth : about the 2nd year the mastoid process appears as a slight
knob, and it gradually grows downwards to form a cephalic lever
for the sterno-mastoid, splenius and trachelo-mastoid muscles.
The period of its most active growth is marked by the eruption of
the permanent teeth. In most mammals the mastoid grows out
as a flat wing-shaped process continuous with the occipital crest,
and thus increases the basal area of the skull on which the neck
muscles are inserted. The post-auditory process of the squamosal
forms a considerable part of the mastoid process; it reaches to the
apex and forms the anterior border (Fig. 14(7). As the mastoid

60

HUMAN EMBRYOLOGY AND MORPHOLOGY.

process grows the diploic spaces within it enlarge into air spaces.
Those round the antrum come to open into it, but the more
distal remain closed. These spaces occupy the whole of the
mastoid part of the temporal, but they also extend forwards in the
post-auditory process of the squamosal, and may spread backwards
to the occipital. Three varieties of mastoids are recognised :

(1) Dense processes in which the air cells are minute or
absent ;

(2) Containing numerous large spaces (pneumatic) ;

(3) An intermediate type with large cells round the antrum, and
a few small ones near the surface. The third type is the commonest.

The Floccular or Subarcuate Fossa. — At birth there is a fossa
situate on the posterior aspect of the petro-mastoid. It is filled
with a process of the dura mater, and over it, but not within it,
the flocculus is situated. The posterior semicircular canal sur-
rounds the fossa. This is the condition in most mammals
throughout life, but soon after birth the fossa becomes closed in
man, merely a remnant being seen above and internal to the
hiatus vestibuli in the bone of the adult. Its meaning is
unknown.

The Acoustic Ganglia. — It has already been shown (Fig. 35)
that the mass of nerve cells which come in contact with the
otocyst arise from the neural crest of the hind brain in the same
manner as the ganglion of a posterior nerve root. The mass of
nerve cells is divided into three groups ( A , B, C, see Fig. 46).

One becomes the geniculate ganglion (A) ; in the formation of
the petro-mastoid it is included in the aqueductus Fallopii. Its
cells give rise to the great superficial petrosal nerve and chorda
tympani in the same manner as the ganglion of a posterior root
produces the sensory fibres of a spinal nerve (Dixon). The pars
intermedia, in part at least, represents the afferent or ingrowing
root of the ganglion.

The second or cochlear group (B) gives rise to :

(1) The spiral ganglion situated in the lamina spiralis;

(2) To the cells in the floor of the internal auditory meatus
which become connected with the saccule and posterior semi-
circular canal ;

(3) Another part remains stranded on the restiform body as
the accessory and lateral root ganglia (Fig. 46).

DEVELOPMENT OF THE ORGAN OF HEARING.

61

Each cell of the ventral acoustic ganglion (group B) sends
efferent processes to the organ of Corti in the cochlea and the
acoustic cells of the saccule and posterior semicircular canal and
afferent fibres which form the dorsal or lateral root of the

great sup. pet.~. P£^/0
chorda tympani' A.

inf corp.quadrigem.
sup. ped.
to sup. oliue

striae acousticae

dorsal gang,
mes. root.

acces. gang.

inf. ped.
cerebellum

gang, of /at. root

sac.

s. s. canal

coch. ' utricle

Fig. 46. — Showing the Nerve Structures concerned in the Sense of Hearing.

auditory nerve and run to the nerve cells in the opposite side
of the medulla oblongata. Some of these fibres form the striae

O

acousticae.

The third group of cells (C, Fig. 46) forms the vestibular
ganglion in the fundus or floor of the internal auditory meatus.
Its cells send efferent fibres to the utricle, external and
superior semicircular canals, and afferent fibres which form the
mesial root of the auditory nerve. This root passes beneath the
restiform body (Fig. 46) to terminate in the nerve cells of the
acoustic tubercle and trigone in the floor of the 4th ventricle.
These nerve cells and fibres are in no sense auditory, but con-
cerned with the balancing of the body.

HUMAN EMBRYOLOGY AND MORPHOLOGY.

62

By uncertain tracts the central auditory terminations in the
medulla are connected with : —

(1) The cerebellum ;

(2) The superior olive ;

(3) The posterior corpora quadrigemina ;

(4) The middle third of the 1st temporal convolution in
which is situated the “word-hearing” centre (Fig. 46).

Internal Auditory Meatus. — The internal auditory meatus is
formed round the VUIth nerve, its ganglia, and the Vllth nerve.
The falciform crest separates the fibres of the lateral from the
fibres of the mesial root. The meatus also contains a prolonga-
tion of the arachnoid and subarchnoid space. Fractures of the
base of the skull frequently cross the petro-mastoid in the line of
the internal auditory meatus, vestibule and membrana tympani.
In such cases the cerebro-spinal fluid and perilymph may escape
by the external auditory meatus.

CHAPTER V.

DEVELOPMENT AND MORPHOLOGY OF THE TEETH.

The Structure of a Tooth. — A tooth may be considered as
made up of five parts (see Fig. 47):

(1) The pulp, situated within (2) a capsule of dentine; the
exposed part or crown of the dentine is coated by (3) the enamel ;
the imbedded part or root by a layer of bone — (4) the crusta
petrosa. The root is secured within its socket by (5) the
peridental membrane, which acts as a periosteum to both the

enamel

dentine

odonto-blasts
-pulp

crusta petrosa
epidermis of gum
■peridental membrane

alueolus

dental canal
and artery

Fig. 47. — Showing the parts of an incisor tooth.

crusta petrosa and bony wall of the tooth socket. An account of
the development of a tooth has to deal with the origin of each of
these five parts.

HUMAN EMBRYOLOGY AND MORPHOLOGY.

( 1 ) Origin of the Enamel. — The enamel is formed by the
epiblast of the stomodaeum. At the sixth week the epiblast
with m the labial margin grows downwards so that a narrow
semicircular invagination of epithelium is formed. To the plate
of epiblast thus infolded the name of dental shelf is given ; its
position is marked superficially by an epithelial crest — the dental

V/77 eso-blast)

Fig. 4S.— Section through the lip and mandible of a foetus in the third month
showing the down-growth of the Dental Shelf.

lower Up '0-denta! groove

position of oral membrane

/.

- epiblast
-tongue

", W 1 r " ■ ■ .... dental shelf (epiblast

■ '

r : \ enamel bud of
manclib arch. Perm- incisor

• c enamel bud of

/ y-f-. f f ■ ‘

incisor

4 dental papilla

ridge (Fig. 48). From its ingrowing or deep margin ten epi-
thelial buds arise, both in the upper and lower jaw. Each of
these twenty enamel buds or organs produces the enamel to cover
the crown of a milk tooth. Each bud grows downwards and
inwards from the surface and conies against a condensed formation
in the mesoblast of the jaw — the dental papilla. On the papilla
the enamel bud becomes partly invaginated, the papilla coming to
lie within the invagination (Figs. 48 and 49). The epithelium
covering the papilla becomes a layer of columnar enamel-pro-
ducing cells or ameloblasts. The basal part of the ameloblasts
are converted gradually into enamel, or to put it somewhat
differently, they form and deposit enamel in their bases and thus
produce a coating for the dental papilla. Each ameloblast is
gradually converted into an enamel fibre, but their more super-
ficial parts are never so converted but persists as the cuticular
membrane which covers the enamel at birth and is soon after-
wards worn off. The enamel of the milk teeth is completely

DEVELOPMENT AND MORPHOLOGY OF TIIE TEETH.

G5

formed before birth ; and that of the first permanent molar is
already partly deposited.

Fia. 49. — Showing the stage of development in an incisor tooth of a foetus of

six months.

(2) Origin of the Dentine. — The dental papilla, formed from
the mesoblast, corresponds to a depressed skin (dermal) papilla, the
enamel cells representing its covering of epithelium. The dental
papilla determines the shape of the tooth. In its superficial
layers it contains numerous cells, odonto-blasts, with branched
processes radiating towards the enamel epithelium. By the
agency of the odonto-blasts a substance is deposited which
becomes calcified into dentine or ivory. It is deposited round
the processes of the odonto-blasts. The cavities in which the
processes are enclosed form the tubules of the dentine. In
rodents especially, but also in all mammals, although only to a
slight extent in civilized races of mankind, the odonto-blasts
react to wear, add new layers of dentine to the wall of the
pulp cavity, and thus prevent the pulp from being exposed.
The dentine is deposited first in the crown of the tooth beneath
the enamel ; the neck is laid down next, and then the root,
the last point of all to be formed being the narrow canal at
the apex of the root by which the dental vessels and nerves
reach the pulp cavity. It is the formation of the root that
forces the crown of the tooth through the gum. The roots

p i

•ection of lower jaw

E

66

HUMAN EMBRYOLOGY AND MORPHOLOGY.

grow continuously in Rodents. If from any accident to their
teeth the normal wear does not take place, the incisors grow
into long tusks which may ultimately prevent mastication.

(3) The Pulp. — The pulp is the remnant of the dental papilla
enclosed by the dentine. It is made up of a matrix of branching
cells and is said to have no lymphatics. Thus, like the tissue
of the umbilical cord and vitreous humour of the eye, it retains
the embryonic form of the mesoblast (Berry Hart). It contains
the ramifications of the artery, vein and nerve of the tooth.

(4) The Dental Sac. — The foetal tooth, as may be seen from
Fig. 49, lies imbedded in the alveolus within the dental sac.
When the enamel bud is invaginated on the dental papilla, the
invaginated layer forms the enamel, while the invaginating or
parietal layer becomes surrounded by a dense layer of mesoblast
and forms the dental sac. Between the enamel (invaginated) and
parietal (invaginating) layers, filling the cavity of the sac, lies a
mass of epithelium corresponding to the corneous epithelium of
the skin.

(5) The Peridental Membrane. — The peridental membrane
(Fig. 47) is formed by that part of the dental sac which sur-
rounds the fang of the tooth. The part of the dental sac
which surrounds the crown is destroyed by the eruption of
the tooth (Fig. 49).

(6) The Crusta Petrosa. — The peridental membrane is of the
nature of periosteum, and contains osteoblasts which deposit the
crusta petrosa (bone) on that part of the dentine which forms the
fang and also on the inner wall of the alveolus. It may
inflame and give rise to an abscess or keep on discharging pus ;
the tooth then becomes loosened in its socket and drops out.

Origin of the Permanent Teeth. — From the dental shelf,
besides the buds for the milk teeth, there grow inwards, so as to
lie on the lingual aspect of the milk buds, processes of epiblast
which form later the enamel of the ten teeth which replace the
milk teeth (Figs. 48 and 49). The three permanent molars of
each side arise from a process which prolongs the dental shelf
backwards behind the part from which the enamel buds of the milk
teeth arise. The first molar is the earliest of all the permanent
teeth to undergo development. The permanent teeth are formed
in exactly the same manner as the milk set. They develop on

DEVELOPMENT AND MORPHOLOGY OF THE TEETH.

67

the lingual aspect of the roots of the milk teeth (Fig. 49), and if
the milk teeth be roughly extracted the permanent bud may also
be torn out.

Dentigerous and other Cysts of the Jaw. — Cysts with
epithelial walls, containing fluid, teeth or other dermal contents,
occasionally develop in the jaw. They are formed from epithelial
remnants of the dental shelf, which normally breaks up and dis-
appear completely, or from detached parts of the enamel buds
(see Figs. 48 and 49).

Nature of Teeth. — A tooth must be regarded as an ossified
dermal papilla which has received a coating of enamel from the
epidermis covering it. In nature they correspond to the placoid
scales of the shark’s skin. The placoid scales and teeth of the
shark are similar in structure, the one series becoming continuous
with the other at the margin of the mouth. The dental papilla
and enamel bud represent an invaginated or depressed part of
dermis and epidermis.

Number of Dentitions. — In many lower vertebrate forms, such
as sharks, the dental shelf gives off constantly a series of buds,
so that as soon as one tooth is lost another springs up from
behind in its place. In mammals generally, as in man, the dental
shelf gives off only two series of buds — one for the milk set and
another for the permanent set. In marsupials it gives off only
one series, so that the first set of teeth is never replaced by a
second.

Morphology of Human Teeth. — The crowns of all the human
teeth seem to be modifications of the same type, the tritubercular
a form undoubtedly evolved from the simple conical tooth found
in fishes and reptiles (see Fig. 50). The conical peg-like tooth is
to be regarded as the most primitive type, and in man vestigial
teeth of this type occasionally occur. In the incisor teeth the
two outer or labial cusps are represented by the cutting edge of
the crown ; the inner remains as the heel at the base of the
crown. Secondary divisions of the two outer cusps into two or
three cuspules may be seen in newly-erupted incisors. In the
canine the outer two cusps of the tritubercular type are fused
into one while the inner remains slightly marked as a rule,
but it may rise up and form a prominent cusp as in the
premolars (Farmer). In the premolars or bicuspids the outer

68

HUMAN EMBRYOLOGY AND MORPHOLOGY.

cusp, as may be seen in many of the lower primates, is really
double.

In the upper molar teeth, to the three primary cusps which
form a cup, a fourth has been added (see Fig. 50 E). The two
outer or buccal cusps are distinguished as the A.E. cusp (antero-
external), the I’.E. cusp (postero-external) ; the two inner as the

Fig. 50. — A. The tritubercular Type of Tooth. The corresponding cusps are shown
in the crowns of an Incisor (/>), Canine (C), Bicuspid (D), Upper Molar (/:'), and
a Lower Molar (F).

A. I. (antero-internal) and P.I. (postero-internal). In the upper
molars the cusps are situated alternately and the P.E. and A.I.
cusps are united by an oblique enamel ridge, which represents the
posterior margin of the crown of the primitive tritubercular tooth
(Fig. 50 E). In the molar teeth of civilized races, especially in
their wisdom teeth, the 4th or posterior internal cusp is often
absent, the primitive tritubercular tooth thus reappearing. In
the lower molars two cusps have been added to the three primary
ones, making five in all. The fifth cusp is situated at the
posterior border of the crown ; the others are arranged in opposite
pairs. The fifth cusp has become lost in the 2nd and 3rd lower
molars of civilized races.

The Roots. — The upper molar teeth have three roots, two outer
and one inner, but in the wisdom teeth, especially of civilized
races, the roots are usually fused. The lower molars have two
roots, but each root appears to be essentially double in nature.
In lower primates the upper biscusps have three roots, but in man
these are usually fused so as to form one or sometimes two roots.
The lower bicuspids have usually one root, but as in lower apes,
they may have two.

DEVELOPMENT AND MORPHOLOGY OF THE TEETH.

69

Eruption of the Teeth. — The eruption of the milk teeth
commonly covers a period of eighteen months, beginning in the
6th with the lower incisors and ending in the 24th or
30th with the 2nd milk molars. The eruption of the
permanent teeth occupies a period of about eighteen years,
beginning with the 1st permanent molar in the 6th year and
ending about the 24th with the 3rd molars. In civilized
races the third molars or wisdom teeth frequently remain im-
bedded in the alveolus and may give rise to an abscess. The
upper wisdom tooth is developed in the posterior border of the
superior maxilla, which bounds the spheno-maxillary fissure in
front. The growth backwards of the maxillary antrum converts
part of the posterior border of the superior maxilla into the
alveolar border, thus bringing the wisdom teeth into position
(see page 12 and Fig. 11). The inferior wisdom teeth are
developed in the alveolus on the inner aspect of the ascending
ramus.

A fourth molar sometimes appears behind the third. A super-
numerary incisor or premolar is very rare. The upper lateral
incisor may be very small or even absent. If the teeth are too
large for the jaw, a not uncommon condition in civilized races, they
appear in irregular positions.

CHAPTER VI.

THE SKIN AND ITS APPENDAGES.

The Skin. — Considerable assistance in the understanding of the
diseases to which the skin is liable and of the nature of the growths
which arise from the epidermis, such as corns, bunions and cancer,
is to be obtained by studying the manner in which the skin is
developed. At first (see Fig. 51) the human embryo is covered
by a single layer of epithelium (epiblast or ectoderm) as is the case
in the adult amphioxus. By the end of the 1st month there are
two layers, the lower representing the germinal layer ; the upper,
the corneous layer (Eig. 52). In the 4th month intermediate
layers appear, from which the stratum mucosum and the stratum
lucidum are differentiated (Fig. 53).

- epiblast
nesoblast

corneous layer ~
H> germinal layer
■mesoblast

dermis-

__corneouslayer
stratum'lucidum
germinal Jayer

Fig. 51. Fig. 52. Fig. 53.

Fig. 51. — The strata of the skin during the first month.

Fig. 52.— The strata of the skin during the second month.

Fig. 53.- — The strata of the skin from the sixth month onwards.

The epidermis rests at first on undifferentiated mesoblast or
mesoderm, consisting of small round cells closely imbedded in a
mucoid matrix. This is the normal structure of undifferentiated
mesoblast. The superficial mesoblastic cells are subsequently
condensed beneath the epidermis to form a corium. Hiey become
frbrillated and by the fifth month the mucoid substance has almost
disappeared, but even in adult life, when the thyroid is diseased
or removed, a mucoid substance may reappear, and a condition

THE SKIN AND ITS APPENDAGES.

71

resembling the foetal state be thus produced. In the mucous
membranes of the lips, anus and vulva the superficial layer of
epithelium does not become cornified.

Formation of Dermal Papillae. — About the fifth month, the
dermal papillae, which are grouped in lines and ridges as is well
seen in the palm, are formed in the following manner :

Long, linear furrows of epidermis grow down into the dermis
(corium) and divide its surface into narrow ridges. These ridges
are subsequently subdivided into papillae. The down-growing
nature of the epiblastic (epidermal) cells which is here exemplified,
is of the greatest clinical importance. The enamel organs, we have
seen, arose by a species of downgrowth of the epidermis ; so do
hairs, sweat glands and sebaceous follicles. Prolonged pressure
and friction welds the corneous cells into a solid plate, such as
the callosities seen on the palms of manual labourers. Normal
desquamation is arrested ; the cells produced in the deeper layers,
unable to grow to the surface, grow inwards and produce corns.
In cancer, the epithelial cells of the skin renew their youth
and invade the dermis and deeper tissues.

The papillary lines on the palms and fingers give security of
grasp (Hepburn). They are arranged in most variable patterns.

Fig. 54.— The more common patterns formed by the dermal papillae on the tips of

the lingers.

A. The Loop Pattern. B. The Triangle Pattern. C. The Whorl Pattern.

but the prevailing types in man are those arranged as loops,
spirals or whorls, Fig. 54. So variably are the types arranged on
the pulps of the digits, that probably no two people show them in
the same sequence counting from thumb to little finger in both
hands. Hence the impress of the ten finger tips has been success-
fully used in the identification of criminals.

The Hairs.— Hairs begin to develop in the 5th month.
Morphologically a hair may be regarded as a dermal papilla,
which has become sunk in the subcutaneous tissue, and capped by

HUMAN EMBRYOLOGY AND MORPHOLOGY.

<i piocess of epidermis. Hairs appear to have been primarily touch
oigans and are modifications of the touch bodies found in the skin
of Leptilia (Gegenbaur). These touch bodies are composed of

germinal

layer

hair sheath

sebaceous gland

corneous layer
stratum lucidum
rete mucosum
basal layer
corium

hair point

Fig. 55. — Diagram of a Developing Hair.

epithelial cells, having the same shape and arrangement as those
which form the taste buds round the circumvallate papillae of the
human tongue. The cells which cap the hair papilla evidently
represent the primary sensory cells of the touch bodies ; they are
situated in line, and continuous with the basal or germinal layer
of the skin (Fig. 55). They produce the cells of the medulla of the
hair which bursts through the epidermis (Fig. 55). The primary
function of the hairs as touch organs is seen in the vibrissae
round the mouths of carnivora, but the hair of man no longer
is subservient to the sense of touch.

The first stage in the development of a hair is the ingrowth of
epidermis as a solid bud, which pushes in front of it the dermis
to form the papilla on which the hair grows (Fig. 55). Only the
two deeper of the primary layers of the epidermis are carried
inwards to form the hair sheath and hair root.

The hairs produced at the fifth month are fine in texture
(lanugo), and by the 7th month the whole body is covered by it.

THE SKIN AND ITS APPENDAGES.

73

The production of hair buds goes on until birth, the later buds
and hairs being thicker and stronger. After birth, new hairs are
constantly reproduced within the sheaths to replace the old.
Probably the manner in which new hairs are produced resembles
that of teeth, viz. : from processes of the original bud. Hairs
appear first on the head and then on other parts of the body. The
comparatively hairless condition of man must be regarded as due
to an arrest of development ; the hair distribution of adult man
corresponds to a late foetal condition of the anthropoids. Certain
sexual hairgrowths appear on the face, pubes and axilla at
puberty. Morphologically the pubic region represents the sepa-
rated axillary regions, and probably the explanation of sexual
hairs in the axilla is due to this correspondence, for there is a
persistent tendency towards symmetry of development in the upper
and lower extremities. The primitive mammary ridges, also
sexual structures, end at the axilla and groin.

The Nails. — The nails are made up of the basal, stratum
mucosum, and stratum lucidum layers of the skin (Fig. 56), the

nail fold

termin. of corn, layer

.stratum lucidum
rete mucosum

nail fold

corneous layer

nail bed ' \ rete mucosum

Fig. 56. — Diagrammatic Section across a Nail.

corneous layer being lost after the 4th month of foetal life. They
appear first in the 3rd month as fields of thickened epidermis
on the tips of the digits, but are afterwards shifted dorsally,
carrying their palmar nerves with them, so that the terminal
phalanx is wholly supplied from the palmar digital branches.
The nail of the little toe, a digit in a retrograde phase of develop-
ment, is frequently shaped like a claw, probably a reversion to a
primitive form. The nail is produced on the scattered papillae
(the matrix) at its root. The area of production is marked by the
lunule. On the nail bed, in front of the lunule, the papillae are
arranged in longitudinal rows. If the nail be pressed, as by the
boot, the lateral papillae, under the nail fold (see Fig. 56) are

74

HUMAN EMBRYOLOGY AND MORPHOLOGY.

directed downwards, and their epithelial outgrowths follow the
same direction, thus causing ingrowing nail.

About the end of the 7th month the matrix of the nail root
becomes differentiated, active growth sets in and the terminal
margin of the nails become free ; it grows forwards over the
corneous layer which covers the terminal row of papillae of the
nail bed. The ridge of corneous epithelium under the nail-tip
represents the hoof of ungulates.

Sweat Glands. — In the 5th month solid processes of epidermis
grow into the dermis and produce sweat glands. They arise at
the same time and in the same manner as, and often in common
with, the buds of hair roots and sebaceous glands. They are pro-
duced within the papillary ridges, and hence the ducts of sweat
glands, as may be seen on the palms and fingers, open along the
summits of these. The sweat glands in the axillae are peculiar.
In section they resemble the acini of the mammary gland, also
believed to be highly modified sweat glands. The axillary glands
contain much epithelial debris. They appear to be sexual in
nature.

Sebaceous Glands. — The sebaceous glands are outgrowths
from the more superficial part of hair buds (Fig. 55). Their
epithelial lining is derived from the germinal layer. In sheaths
which have become occluded from the loss of the hair, or when
the mouth of the gland is blocked, the secretion is retained, and
a sebaceous cyst or wen, so frequently seen in the scalp, is
produced. Round the mouth, on the lips and nose, the sebaceous
glands, especially in disorders of the sexual organs, arc apt to
retain their secretions and become inflamed, small pustules being
thus produced. The Meibomian glands in the eyelids are modified
sebaceous glands. At birth the child is covered by the vernix
caseosa, which is composed of desquamated corneous epithelium
and the secretion of sebaceous glands.

THE MAMMAE.

The mamma is developed in the same manner in both sexes.
At puberty the female breast undergoes a great development,
while in the male it retains the infantile lorm.

THE SKIN AND ITS APPENDAGES.

75

The Female Breast. — The mamma of the female is of cutaneous
origin, and in its earlier stages of development resembles, and
probably corresponds to, a collection of sweat glands arising from
a small circular depressed area of skin (Fig. 57 G). The manner
of its development is the key to its anatomy. The adult female
breast is composed of two elements :

(a) Glandular tissue derived from the epiblast by a process of
inbud din" ;

(b) An intricate arrangement of connective tissue derived from
the mesoblastic subcutaneous tissue over the pectoralis major.

I. Origin of the Glandular Tissue.

(1) The Mammary Line is a slight ridge of epiblast which
stretches along the ventral aspect of the body on each side, from
the axilla to the groin, and is the first stage of mammary
development in mammalian embryoes. In the sow, for instance,
mammae are produced along the whole length of the mammary
line. Although this stage has not been seen in the human
embryo, it probably does occur, for in 5°/0 of bodies a more or less
distinct trace of a supernumerary mamma or nipple is to be
found, and these occur for the greater part in the inguino-axillary
line. Such as occasionally occur on the back or thigh are pro-
bably of the nature of dermoid tumours. Supernumerary nipples
occur much more frecpiently in men than in women. This one
may expect because the more vestigial the condition of an organ,
the greater is the tendency to the production of ancestral
(atavistic) forms.

Developmental Stages. — Seven stages may be recognised in
the developmental history of the glandular mammary tissue.

Four of these take place before birth :

1st (Fig. 57 A). The deeper layer of epiblast thickens over the
mammary area ; this thickening represents a part of the mam-
mary ridge or line. This stage is seen in the 2nd month.

2nd (Fig. 57 B). The thickening becomes depressed, thus giving
rise to a slight pit on the surface.

3rd (Fig. 57 C ). From the depression arises a number of buds,
exactly similar to those of sweat gland (5th month). The stalks
of these buds form the epithelial lining of the lactiferous ducts.

4th (Fig. 57 B). The lobular buds, for each bud develops into
a lobe, subdivide at their growing extremities. At first solid,

76

HUMAN EMBRYOLOGY AND MORPHOLOGY.

they begin to caniculize (/til to 9th months). At or about birth
the pit 01 depression, from which the lobular buds originated, is
laised, evaginated and forms the surface of the nipple (Fig. 57 J7).

C. D.

Fig. 57.— Showing the various stages in the development of the Mamma.

A. During the 2nd month. B. At the commencement of the 3rd month. C. At the

5th month. B. At birth.

A = Epiblast. b = Subcutaneous tissue (mesoblast). c=Pectoralis major.

Thus the ducts come to open on the apex of the nipple. An
ampulla is developed in each duct within the base of the nipple.
Stage 3 represents the marsupial — the lowest mammalian form of
mamma.

Stages after Birth. — Stage 5 occurs at puberty ; the latent
infantile lobular buds again undergo a rapid growth and give rise
to the minor lobules and acini. Stage 6 occurs towards the end
of pregnancy and consists of a renewed production of glandular
tissue. Stage 7 sets in with the menopause and is characterized
by an atrophy of the glandular tissue formed in the later stages of
development.

In the process of subdivision, minor buds of adjacent lobes
frequently unite together. Hence it is found difficult, during
dissection, to separate the gland into its primary lobes. In any

THE SKIN AND ITS APPENDAGES.

77

of the three later stages a localized and invading hypertrophy of
the cells of the glandular tissue may take place. In this manner
cancer is produced. The part played by the lymphatics, which
are situated in the mesoblastic tissue of the gland, in the spread
of this disease, makes their study important.

II. Origin of the Capsular or Mesoblastic Part of the
Gland. — As the glandular buds grow out into the subdermal meso-
blastic tissue, which reacts and hypertrophies around the invading
processes, they divide it (see Tig. 58) into (a) superficial, and (b)
deep layers, these being joined together by (c) interstitial septa.
The superficial and deep layers are fused in (d) the circum-
mammary tissue in which the final glandular buds terminate.
The processes as they grow outwards also take on (e) perilobular
and periductal sheaths. The deep and superficial layers are also
connected with the anterior sheath of the pectoral muscles and the
skin — for they are all parts of the same subdermal or sub-
cutaneous mesoblastic layer.

'lymphatics of nipple

subcutaneous
periductal

peri-lobular

superfic. mam.

retro-mammary

Fio. 58. — Diagrammatic Section of the Breast to show the arrangement of its Capsule
and Lymphatics. The lymphatic vessels are represented by thin wavy lines.

Lymphatics. — As each part of the capsule carries a network of
lymphatic vessels, into which the glandular lymph passes, it will
be seen that the arrangement of the parts of the capsule is
an important matter in both the physiology and surgery of the
gland. The periductal and perilobular lymphatics communicate

78

HUMAN EMBRYOLOGY AND MORPHOLOGY.

through the septal or interstitial vessels with the superficial
mammary and deep (retro-mammary) lymphatics (Fig. 58). The
superficial communicate with the subcutaneous ; the deep with those
in the pectoral sheath and thus it will be seen that mammary cancer
may spread to the skin or pectoralis major. The deep and super-
ficial join in the circum-mammary lymphatics, and from these pass
efferent vessels to the pectoral and central glands of the axilla.
The lymph passes from these to the deep axillary and inferior
deep cervical glands — all of which are involved in late stages
of cancer of the breast. Other efferent vessels pass from the
circum-mammary to the anterior intercostal glands of the upper
four spaces ; one or two vessels may go to the cephalic gland.

Peripheral Remnants. — Isolated or semi-isolated small masses
of glandular substance are found situated in the circum-mammary
tissue, beyond the body of the gland. Some may pierce the
sheath of the pectoralis major and become a source of recurrent
cancer. The presence of glandular remnants is explained by the
fact that, when the primary budding takes place, the subdermal
mesoblast is shallow and of small extent ; in the subsequent
growth of the thorax, the tissue in which the mamma is developed,
is widely spread out.

The position of the Mammary Gland is as a rule wrongly described.
Quite a third lies on the serratus magnus and beyond the anterior
border of the axilla. The axillary lobe reaches upwards in the
axilla to the upper border of the third rib, where it is in contact
with the central set of lymphatic glands (Stiles).

Fat begins to be deposited in the subcutaneous tissue during
the 5 th month of foetal life. It forms a large element of the
mammary gland after puberty. The subcutaneous tissue, out of
which the capsule of the gland is formed, normally contains much
fat. After lactation, when the glandular tissue atrophies to a
considerable extent, a growth of fat replaces it. If no fat is
deposited or if it be absorbed, then the breast loses its plump form
and hangs on the chest.

The mammary nerves (secretory) come from the 3rd, 4th, and
5th intercostals ; the nipple is supplied from the same nerves.
The nipple contains non-striated muscle and is covered with
touch papillae and surrounded by modified sweat and sebaceous
glands.

TIIE SKIN AND ITS APPENDAGES.

79

To render the glandular mammary tissue clearly recognisable
from the surrounding connective tissue, Stiles adopted the method
of immersing the mamma in a 5°/0 of TING., for two days. The
glandular tissue becomes of a dark yellow tint, and thus can be
detected even in minute quantities from the surrounding tissue
of mesoblastic origin.

CHAPTER VII.

THE DEVELOPMENT OF THE OVUM OF THE FOETUS
FROM THE OVUM OF THE MOTHER.

From the Ovum of one generation to the Ovum of the Next.

— The manner in which the face, neck, pharynx and cutaneous
structures of the body are produced having now been traced, it is
necessary, before further progress can be made, to turn to the
phenomena which mark the opening stages in the development of
the embryo. This may best be done by following the cycle of
changes which lead to the production of a new generation of
germinal cells from the fertilized ovum of a former generation.
Every ovum 'is the offspring of a fertilized ovum, and the fer-
tilized ovum is the intermediary whereby the characters and
properties of a race are handed from one generation to the next.

Descent of the Ovary.— In the female human foetus of the fifth
month the ovary has reached the iliac fossa (Fig. 59) in the course
of its descent from the lower dorsal region where it originated to
its permanent position on the lateral wall of the pelvis. The ovary
is then long and narrow, with an upper and lower pole ; it is
three-sided in section — the surfaces being inner, outer and inferior
or ventral (Fig. 60). The Fallopian tube, derived from the upper
part of the Mullerian duct, lies along the outer side of the ovary
in the iliac fossa ; its upper fimbriated end terminates at, and is
attached to, the upper or cephalic pole of the ovary (Fig. 59).
As the parts lie on the iliac fossa, the tube and the ovary are
supported each by its own mesentery, the meso salpinx and
meso-ovarium. The two mesenteries have, however, a common
origin or attachment to the posterior abdominal wall and to the
common attachment the name of common genital mesentery may

DEVELOPMENT OF TIIE OVUM.

81

be given. The upper end of the common mesentery — the plica
vascularis (Fig. 59), as it is reflected from the cephalic pole of the
ovary and fimbriated extremity of the tube, is continued up towards

Fig. 59. — The position of the Ovary and Fallopian Tube in the 5th month.

the diaphragm and in it the ovarian vessels and nerves pass to
the ovary and tube. The caudal pole of the ovary is joined to the
uterus by its round ligament. The round ligament of the uterus,
corresponding to the gubernaculum testis of the male, passes from
the brim of the pelvis, where it is attached to the horn of the
uterus, almost straight to the internal inguinal opening and assists
in the descent of the ovary and tube.

By full time the ovary lies at the brim of the pelvis or partly
within it ; after birth the ovary, uterus and rectum come gradu-
ally to occupy their adult positions within the pelvis. This is
due to a relatively greater growth in the pelvis itself than in its
contents. The ovary, as is more frequently the case with the
testicle, may be arrested in its descent.

In Fig. 60 an earlier stage is shown; it represents on section
the condition about the end of the second month. The ovary
and tube with the remnants of the Wolffian body and duct

F

82

HUMAN EMBRYOLOGY AND MORPHOLOGY.

occupy the position in which they are developed. Both are sus-
pended by mesenteries from the dorsal wall of the peritoneal cavity,
at the side of the mesentery of the gut.

Fig. CO. — Diagrammatic section of a foetus at the end of the ‘ind month, showing the
Attachments of the Ovary and Mullerian duct.

Normal Position of the adult Ovary. — When the ovary
descends within the pelvis it occupies a definite triangle — the

ovarian triangle — on the lateral wall of the pelvic cavity (Fig. 61).
The ovarian triangle is bounded above by the upper half of the

DEVELOPMENT OF THE OVUM.

83

external iliac artery, below and behind by the internal iliac
artery, with the ureter lying on the artery ; in front by the
reflection of the posterior layer of the broad ligament on the side
of the pelvis.

The long axis of the ovary is parallel to the ureter and is vertically
placed in the standing posture. The peritoneum covering the
triangle forms a depression, or occasionally a pouch, for the ovary.
It will be seen that, with the descent of the ovary, the mesosalpinx,
the mesovarium, and the common genital mesentery have come
to form the major part of the broad ligament. The reflection of
the common genital mesentery from the upper or cephalic pole of
the ovary now forms the ovario-pelvic ligament (Pigs. 59 and 61).
In the ovarian triangle is also situate the internal iliac group of
lymphatic glands, into which most of the pelvic lymphatics drain.
The ovarian lymphatics end in the glands of the upper lumbar
region near to where the ovary was developed. The ovary brings
down with it, too, the ovarian vessels and plexus of nerves. The
nerves come through the aortic plexus from the 10th and 11th
dorsal segments of the cord.

-mesouarium
Wolffian tubule

germinal epithelium

t ouum in unripe
^1 Graafian fol.

primitive ouum

tubular incursion of germ
epithelium

Fig. 62. — Diagrammatic Section of an Ovary to show the manner in which the
Primitive Ova are carried in by incursions of the Germinal Epithelium

An Ovum. — As the infantile ovary descends, it is laden with
thousands of ova. Each ovum is surrounded by a capsule com-

84

HUMAN EMBRYOLOGY AND MORPHOLOGY.

posed of a layer of columnar epithelium, which is imbedded in,
and surrounded by, the stroma of the ovary. The entire capsule
— epithelium and stroma — forms a Graafian follicle (Fig. 62).
The ovary is covered by a layer of columnar epithelium, which is
named the germinal epithelium Amongst the columnar cells of the
germ epithelium during foetal life and for sometime after birth
larger cells occur. These are the primordial ova from which brood
ova arise. The ova are carried within the ovary by tubular
ingrowths of germinal epithelium (Fig. 62). These tubular
invasions of the ovary become broken up, the isolated masses of
the germinal epithelium remaining to form the linings of the
Graafian follicles.

Discharge of the Ova. — At puberty, possibly also before it,

*' — germinal epithelium

'tunica albuginea
. stroma-capsule

ffigp liquor folliculi

" 1 wcf

epithelial lining

zona racliata
germinal uesicle
germinal spot

ovum

discus proligerus

Fig. 63.— Diagram of a ripe Graafian Follicle.

and for 30 years after it, one egg after another ripens; the ovum
enlarges; so does the Graafian follicle (big. 63). The cells ol
the epithelial lining proliferate and a cavity appears amongst the
cells within the follicle, due to a collection of fluid the licpioi
folliculi. The ovum remains attached to the wall of the follicle
by a group of epithelial cells, the discus proligerus (Fig. 63). As
the fluid collects, the follicle works its way to the surface of the

DEVELOPMENT OF THE OVUM.

85

ovary ; the tunica albuginea, which forms a capsule for the ovary,
and the germinal epithelium, gradually atrophy over it, and at
last it bursts and discharges the ovum.

Two opposite opinions exist among gynecologists as to the
period of rupture: (1) That it occurs at the onset of menstrua-
tion ; (2) that it has no relationship to the menstrual period. The
truth is, probably, that both are right. The majority of ova,
however, appear to be discharged at the menstrual period.
Whether ova are discharged from both ovaries at once, or from
only one, and whether one or more than one in a month, are points
not yet settled ; but the usual opinion is that one ovum is shed
each month, and only from one ovary.

The Graafian follicle, after rupture, fills up with blood ; a
cellular tissue is soon developed within its cavity. If pregnancy
occurs this tissue forms a true corpus luteum, a large yellow body
as big as a pigeon’s egg. If pregnancy does not occur, a false
corpus luteum is formed, a formation which begins to disappear
before the next menstrual period. Both forms lead to a cicatrix,
which is seen on the surface of the ovary. The ovary of an old
person is commonly covered with such scars. The Graafian
follicles may become cystic and give rise to enormous ovarian
tumours.

The Fallopian Tube. — When the ovum drops from the ovary
it cannot easily escape the ciliated fimbriae of the Fallopian tube
which surround and clutch the ovary. In Fig. 61 the relationship
of the Fallopian tube to the ovary is shown. The tube may be
demarcated into three parts : («) the isthmus or arm directed
outwards to the wall of the pelvis to 1 inch) ; ( b ) the forearm
or ampullary part, directed backwards on the lateral pelvic wall
above the ovary ; (c) the hand, infundibular, or fimbriated part,
folded backwards and grasping the free border and exposed
surface of the ovary. The tube is tethered by one of its fim-
briae to the cephalic pole of the ovary.

Course of the Ovum in the Tube. — The cilia on the fimbriae
work towards the ostium abdominale, the abdominal mouth of the
Fallopian tube, which is situated at the bases of the fimbriae, and
carry the discharged ovum through the ostium within the
tube. The ostium abdominale is shut when the tube is
examined after excision ; the closure is probably due to reflex

86

HUMAN EMBRYOLOGY AND MORPHOLOGY.

contraction of the tube muscle, caused by handling and cutting.
In the infundibular and ampullary segments of the tube, the
mucous membrane is thrown into long plicated folds shown in

section in Fig. 64. They are covered with ciliated epithelium,
which urge the ovum towards the uterus. Within the tube
impregnation usually takes place. If an obstruction arrests the
fertilized ovum in the tube, perhaps it may be a patch denuded
of epithelium and cilia by gonorrheal inflammation, a tubular
pregnancy is the result. The growing embryo, commonly about
the second month, bursts the tube and falls within the broad liga-
ment or into Douglas’s pouch (Fig. 64).

The History of the Ovum within the Fallopian Tube. —
When the ovum enters the Fallopian tube, it is a cell of very
considerable size (‘2 mm. = in.) with a cell wall — the zona
radiata (Fig. 63), a nucleus — the germinal vesicle, and a nucleolus
— the germinal spot. Then, or before then, the ovum prepares
for fertilization by the extrusion from its nucleus of first one,
then another polar body, and, with the extrusion, the germinal
vesicle becomes the female pronucleus. The polar bodies, which
lie outside the protoplasm of the ovum, but within the zona
radiata, are parts of the germinal vesicle, which are extruded
with all the display of karyokinesis (Fig. 65 A ). The meaning
of the process has been much guessed at ; the outstanding fact
is that two parts of the ovum are segregated and are not
involved in the subsequent developmental changes. What

rci. lig.

Fig. 64.— Diagrammatic section of the Broad Ligament and Fallopian Tube.

DEVELOPMENT OF THE OVUM.

87

become of the polar bodies in the course of development of the
fertilized ovum is not known.

Fig. 65. — Showing the production of the Morula from the Ovum.

A. The ovum after the first division. B. After the second. C. The Morula stage.

In the course of fecundation thousands of spermatozoa are
lodged in the genital passage ; many stem the adverse current of
the uterine cilia, reach and live for days within the interlaminar
grooves in the wider parts of the tube. In the course of its
descent within one of the grooves the egg may be fertilized. The
spermatozoon bursts through the zona radiata, loses its tail, its
head enlarges, and forms the male pronucleus. The male and
female pronuclei unite, and from their union springs the nucleus
of the fertilized ovum. This is the centre from which all future
developmental changes start. The ovum may be, but rarely is,
fertilized in the ovary, or between the ovary and ostium ab-
dominale, the result being a pelvic gestation. The length of
time the fertilized ovum takes to reach the uterus is not known
exactly, but probably it spends about ten or twelve days within
the Fallopian tube, and during that time it passes through the
following stages :

1. The Morula. — The ovum, with a full display of karyoki-
netic changes in the nucleus, divides, subdivides, and grows
within the zona radiata until a rounded mass of cells is formed
— the morula or mulberry mass. The cells are of unequal size
and divide at unequal rates (Fig. 65, A, B, C ).

2. The Blastodermic Vesicle. — This stage is produced from

88

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the morula stage by the collection of fluid within the mass of
cells. The cells are then seen to be arranged in two sets — (a) a
layer lining the zona radiata and forming the wall of the vesicle

Pio. 66. — Diagrammatic section of a Blastodermic Vesicle.

(Rauber’s layer) (Fig. 66), and ( b ) a group of granular cells
within the cavity attached to Rauber’s layer at the embryogenic
pole of the vesicle (Fig. 66). In Vertebrates with huge stores
of yolk in their ova, such as birds have, the vesicle is filled by
yolk-bearing cells, continuous with Rauber’s cells at the vegeta-
tive pole, opposite to the granular cells.

Fig. 67.— A diagrammatic section of a Bilaminar Blastoderm made across the

primitive streak.

3. The Bilaminar Blastoderm. — This stage is produced by
the growth of the group of granular germinal cells within Rauber’s
layer. At first they arrange themselves in two layers — an outer

DEVELOPMENT OF THE OVUM.

89

which spreads oil the inner aspect of Bauber’s layer, and absorbs
or mixes with it, to form one layer of more or less columnar
cells, the epiblast or ectoderm (Fig. 67); an inner layer also spreads
out from the germinal group, and forms a second lining to the
blastodermic vesicle, the hypoblast or endoderm (Fig. 67).

The Primitive Streak and Groove. — In the diagrammatic
section of the bilaminar blastoderm, given in figure 67, the hypo-
blast and epiblast are seen to be fused at one point, and at the
point of fusion the epiblast is thickened and somewhat depressed.
When the blastodermic vesicle is viewed from the surface, the line
along which the fusion of the two layers and thickening of the
epiblast take place is marked by the primitive streak and groove
(Fig. 68). At the anterior end of the groove is situated the
blastopore or neurenteric canal, an opening into the cavity which
the hypoblast encloses — the archenteron (see Figs. 67 and 75).
Bound the blastopore and along the primitive streak the epiblast
is continuous with the hypoblast.

The Mesoblast. — The mesoblast or mesoderm, the third of the
primitive blastodermic layers, is produced from the margins of
the primitive streak (Fig. 67). At the line of junction of the

Fig. 68. — Diagram of the Embryogenic area of a Bilaminar Blastoderm viewed from

above.

epiblast and hypoblast there is a free proliferation of cells which
spreads out between the two primitive layers, and gives rise to a
third or intermediate layer — the mesoblast (Fig. 69). In lower
vertebrates the mesoblast is entirely produced from the hypoblast.

Neural Canal. — It is at this early stage, while the mesoblastic
cells swarm in between the epiblast and hypoblast, and while the

HUMAN EMBRYOLOGY AND MORPHOLOGY.

vesicle has not yet reached the uterus, that a grooved strip of
epiblast, the medullary plate, in front of the primitive streak
(fig. 68) is set aside, by a process of infolding, to form the great

central nervous system — the brain, spinal cord, and nerves
(Fig. 69).

A medullary fold rises up on each side of what is to be the
middle, line of the back to enclose the medullary plate. The
folds rise until their crests meet, and a tube of epiblast is buried
by the fusion of their lips. The blastopore is included within
the posterior end of the medullary folds (Fig. 68). Thus, for a

short space, the cavity of the yolk sac communicates with the
neural canal.

The Notochord. — Beneath the neural tube a similar infolding
ol hypoblast takes place, and a tube of cells to form the noto-

notochord,
hypoblast ,

neural canal

somatopleare.

splanchno-pleure

'■piblast

mesoblast

-fold of amnion

r intermediate
l cell-mass.

outer layer of mesoblast

inner layer of
mesoblast

coelom

Fig. 60. — Diagrammatic section of a Blastodermic Vesicle showing (1) the origin of
the neural canal, (2) the origin of the notochord, (3) the ingrowth of the
mesoblast, and (4) the formation of the coelom.

chord is detached (Fig. 69). It forms the first basis of the
spinal column.

The Somatopleure and Splanchnopleure. — The mesoblast
surrounds both the notochord and neural canal as they are
formed — perhaps assists in their formation (Figs. 69 and 70).
The solid layer which surrounds these structures is known as the
paraxial mesoblast. As the inesoblastic cells spread out to cover
the outer aspect of the hypoblast and inner aspect of the
epiblast, they separate into two layers which enclose between

DEVELOPMENT OF TIIE OVUM.

91

them a cavity — the coelom (Fig. 69). The epiblast, with its
accompanying layer of mesoblast, makes up the somatopleure
which bounds the coelom without ; the hypoblast and overly-
ing layer of mesoblast form the splanchnopleure which bounds
the cavity within.

Fig. 70. — Diagram, of the Blastodermic Vesicle separating into Embryo and

Membranes.

Embryo and Membranes. — During the second week, while
the blastodermic vesicle is still within the Fallopian tube and
some time before it has reached the uterus, changes take place
whereby part of the blastodermic vesicle forms the embryo and
part is transformed into the enclosing membranes which con-
tain, protect, and nourish it (Fig. 70). Part also forms the
yolk sac.

The Decidua. — When the uterine cavity is reached, the uterine
mucous membrane has become hypertrophied, the decidua being
thus formed. In the formation of the decidua, the uterine glands
become elongated and enlarged ; the inter-glandular tissue hyper-
trophied and the mouths of the glands closed, so that the surface
layer of the decidua is comparatively solid. The hypertrophy
of the mucous membrane is unequal, depressions or pits being
formed on its surface. The deeper layer of the decidua is

cavernous, the spaces being formed by the distension of capillary
veins into venous spaces, and perhaps also by the enlargement of
the gland luinina.

When the blastodermic vesicle reaches the uterus, it commonly
adheres to the posterior wall near the fundus (Fig. 7l). The

92

HUMAN EMBRYOLOGY AND MORPHOLOGY.

decidua^forms first a nest for it, and then grows over and buries it,
thus forming the outer or decidual membrane of the embryo. The
layer which covers the vesicle (see Fig. 71) is the decidua reflexa;
the pait by which it adheres to the uterus, the decidua serotina;

the rest, dining the uterus, the decidua vera. As the vesicle
grows, it fills the cavity of the uterus, thus bringing the decidua
reflexa in contact with the decidua vera ; the reflexa disappears
when it conies in contact with the vera, which remains as the
outermost of the foetal membranes. The serotina takes part in
the formation of the placenta.

The Amnion and Chorion. — Only part of the somatopleure
of the blastodermic vesicle takes part in the formation of the
embryo ; the remaining parts of the somatopleure are converted
into the amnion and chorion in this manner :

As is shown in Fig. 70, the embryonic area appears to sink
within the vesicle ; the somatopleure rises up as a wave-like fold
right round it, at each side, at the head as well as at the tail end.
The folds meet over the embryo and unite, as is shown in
Fig. 72. Thus the embryo is wrapped up in a double fold of
its own somatopleure. The inner fold or membrane is the
Amnion, the outer is the Prechorion (Fig. 72). The prechorion,
in turn, is covered by the decidua reflexa, and at its attached

decidua reflexa

blastodermic vesicle
decidua uera

^-decidua uera

' eruix

uagina

Fig. 71. — Section of the Uterus showing the tln'ee parts of the Decidua.

DEVELOPMENT OF THE OVUM.

93

part by the decidua serotina. Villi containing vessels grow out
from the chorion into the decidua.

line of union

prechorion
embryo
amnion

somatopleure
coelom

splanchnopleure

Fig. 72.— Showing the folds of the somatopleure uniting over the embryo and becom-
ing demarcated into Amnion and Prechorion.

Origin of Ova and Spermatozoa. — The cells which give rise
to the ova and spermatozoa, according to sex, are derived from the
mesothelial lining of the coelom. They appear with the cleavage

Fig. 73. — Diagrammatic section of the abdominal region of the coelom, showing the
position of the Genital Ridges from which the Ovary or Testicle is formed.

of the mesoblast and formation of the coelom. Soon after the
coelom is formed (beginning of third week) a ridge — the genital
ridge — is seen on the roof of the coelom, situated on each side of
the attachment of the mesentery of the gut (Fig. 73). The ridge
lies over the intermediate cell mass (Fig. 85, p. 1 11), and is covered

94

HUMAN EMBRYOLOGY AND MORPHOLOGY.

by a layer of columnar meso-thelium, the germinal epithelium,
which is continuous with the mesothelial lining of the coelom.
Some of the columnar cells covering the genital ridge become
primordial ova, and are set aside to produce ova or spermatozoa,
according to the sex. Beard’s researches have led him to the
conclusion that primordial ova are produced at a very early stage
of development, and that they migrate towards the coelom and take
up their position amongst the germinal cells covering the genital
ridge when that ridge is formed. These cells may stray and give
rise in various parts of the body to dermoid tumours (Beard). The
manner in which the germ cells and primordial ova grow within
the genital ridge to form the ovary or testicle was described at
the beginning of this chapter. The cycle of changes which leads
to the production of the ova of one generation from the ovum of
a former generation have thus been traced.

CHAPTER VIII.

THE MANNER IN WHICH A CONNECTION IS ESTAB-
LISHED BETWEEN THE FOETUS AND UTERUS.

Implantation of the Ovum. — The blastodermic vesicle is
represented in Fig. 69. In its course down the Fallopian tube
the somatopleure, which forms its outer wall, becomes differen-
tiated into four areas, shown diagram matically in Eig. 74, by a

Fig. 74. — Showing wliat becomes of the Somatopleure of the Blastodermic Vesicle.

species of folding, already described and shown in transverse section
in Fig. 72 and in longitudinal section in Fig. 75. The embryo
becomes enclosed in a double capsule of somatopleure — the outer
the prechorion and the inner the amnion. But, as is shown in
Fig. 75, the amnion in man is peculiar in that it forms practically
no tail fold; the head fold grows backwards like a hood until it
comes in contact with the caudal pole of the embryo. When

96

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the uterus is reached a third or decidual coat is added by the
mucous membrane of the uterus (Fig. 71).

The serotinal part of the decidua marks the place where the
placenta will be formed. The implantation of the ovum in the
decidua is in the posterior wall of the uterus in over 60°/o
of cases ; hence the placenta is developed there. It sometimes
happens that implantation occurs near the internal os of the
uterus, and in such cases the placenta is developed over the os
(placenta praevia), a form liable to give rise to a sudden haemor-
rhage from the uterus during pregnancy.

The Body-Stalk. — When the amnion and prechorion are
formed from the human ovum that part of the somatopleure

lying behind the neural groove, and in which the primitive streak
appeared (see Figs. 68, 74 and 75), joins the embryo to the
chorion. His named it the body-stalk. Through it the allantois

a hollow protrusion from the hind gut (Fig. 75) — grows out

to the pre-chorion Afterwards the body-stalk, with the con-
taind stalk of the allantois, becomes the umbilical cord. The

THE FOETUS AND UTERUS.

97

developmental changes which occur in the body-stalk are of the
greatest practical importance (p. 99).

The Chorion and Allantois. — At this early stage a diverti-
culum has arisen from the hind gut below the caudal part of the
embryo (Fig. 75). This ventral evagination of the hind gut,
which grows along the body-stalk, carrying the splanchnopleure
with it, forms the allantois. The allantoic bud, which is hollow
only at its basal part, spreads out on the inner surface of the
prechorion, forming a lining to it. The prechorion or false

chorion, with the addition of the allantois, forms the chorion, or
true chorion. The posterior ends of the two primitive dorsal
aortae, which terminate until now on the yolk sac, are carried out
with the allantoic bud and distributed within the villi of the
chorion. The posterior ends of the two dorsal aortae thus become
the hypo-gastric and umbilical arteries (Young and Eobinson).
The veins which return the blood become the umbilical veins.
At first there are two of them, and they return the allantoic blood
direct to the- sinus venosus, afterwards to the ducts of Cuvier,
and thus to the heart (Fig. 190, p. 232). In this way the
foetal circulation is set up. The heart pumps the blood into the
chorionic villi ; these are imbedded in the maternal decidual
covering; the blood is returned by the umbilical veins (Fig. 76).
At this stage, and indeed until the end of the 6th week, the
membranes, with the embryo within them, can easily be detached
from the decidual nest in the uterus, and then appear as a villous
vesicle, the chorionic vesicle, about the size of a pigeon’s egg.
The allantois never forms a free vesicle in man nor in the higher
primates. It occurs as a vesicle in other mammals, birds and
reptiles, in which it has a double function : 1st, to form a
respiratory medium, as is also the case in man ; 2nd, to form a
receptaculum for the secretion of the kidney.

Formation of the Placenta. — The condition of the mem-
branes in the 3rd month (Fig. 76) differs from that of the 1st
month (Fig. 75) by the formation of the placenta. In the first
month the chorion is uniformly covered by shaggy villi, which
project into the decidua and draw sustenance for the embryo
therefrom. This is the permanent condition in low primates
(Lemurs). In man the chorionic villi which project within the
decidua serotina hypertrophy, while those within the decidua

a

98

HUMAN EMBRYOLOGY AND MORPHOLOGY.

refiexa atrophy, and in this way the discoidal placenta of man is
formed. In lower primates (Monkeys) there are two discs
(bi-discoidal), and this form occasionally occurs in man.

vein

diagrammatically shown in Fig. 77. They are :

1st. The decidua serotina, formed from the mucous membrane

of the uterus.

2nd. The prechorion, from the somatopleure.
3rd. The allantois from the splanchnopleure.
4th. The amnion from the somatopleure.

THE FOETUS AND UTERUS.

99

The amnion is a thin, transparent membrane easily stripped off
from the inner surface of the placenta. The serotinal area of the
true chorion has become hypertrophied ; each villus branches

muscular coat

decidua serotina

prechorion
(somatopleure)

allantois

(splanchnopieure)
amnion

decidua uera 'decidua reflex a

Fig. 77. — Diagrammatic section to show the Elements which enter into the formation

of the Placenta.

again and again until it resembles, in the complexity of its
ramifications, a miniature beech tree. In these villi the umbilical
arteries of the foetus break up into capillaries, which in turn end
in the venules of the foetal umbilical vein. The villi project
within great blood spaces formed in the decidua serotina (Fig.
7 6). The ovarian and uterine arteries end in these blood sinuses,
and the ovarian and uterine veins begin in them. The blood
sinuses are formed :

1st. By the distension of uterine venules in the decidua serotina.

2nd. Possibly by the dilatation of uterine glands.

At full time all the membranes of blastodermic origin come
away in the after-birth ; also the decidua, except a thin, deep
layer next the uterine muscle, which contains the deepest parts
of the uterine glands. From this layer the mucous membrane
of the uterus is regenerated (Fig. 77).

Formation of the Umbilical Cord and Umbilicus. — The
body-stalk, the basis of the cord, is that piece of the embryonic
somatopleure situated between the chorion and neural groove
(Figs. 74 and 75). The outgrowth of the allantois into the body-
stalk adds to it the elements of the splanchnopieure. A

100

HUMAN EMBRYOLOGY AND MORPHOLOGY.

transverse section of the body-stalk shows within it the same
elementary structures as are seen in a transverse section of the
embryo. The cord must be regarded as a real part of the
embryo. The umbilicus, which marks the point of attachment of
the cord, is situated in the adult on the ventral surface, but
before it was thrust into this position by the development of the
caudal and perineal regions of the body, it represented the
posterior termination of the embryo (Figs. 74 and 75). The
amniotic somatopleure rises from the sides of the body-stalk and
encloses it just as it rises from, and encloses, the embryo (Fig. 72),
and, as is shown in the next paragraph, the lateral folds of the
somatopleure unite in the ventral hue of the body-stalk as in
the ventral line of the belly (Fig. 78).

mmon

cauity of amnion

continuation of
1 medullary grooue

limb, ueins

somatopleure

umb. arterg

coelom

cauity of allantois

Vitelline duct.

Fig. VS.— Diagrammatic section showing the structures which go to form the

Umbilical Cord. (After His.)

Transverse Section of the Umbilical Cord (Fig. 78). — A
section of the cord shows :

(1) Two umbilical arteries (continuations of the primitive
dorsal aortae).

(2) One umbilical vein, formed by the fusion of the two
original veins.

(3) The cavity of the allantois formed from the hind gut.
Within the cord its lumen becomes obliterated early.

(4) The vitelline duct, the stalk of the yolk sac, communica-
tiim with the intestine and yolk sac. It becomes oblitciated in
the 3rd month.

THE FOETUS AND UTERUS.

101

(5) Wharton’s jelly, a primitive embryonic tissue composed of
branching cells in a mucoid matrix.

(6) A covering of epiblast. The amnion is attached round the
placental insertion of the cord.

Up to the end of the 4th week the embryo is closely united
to the chorion by the short body-stalk (Fig. 75), but in the
second month the cord elongates, and in the third month it
measures about 12 cm. and about 40 cm. (16 inches) at birth.

Formation of the Umbilicus. — In the adult the umbilicus
marks the point where the umbilical cord was attached. It is
the point at which the lateral somatopleuric plates fused and
thus shut off the intra-embryonic coelom (peritoneal, pleural and
pericardiac cavities) from the extra embryonic coelom which is
enclosed between the amnion and chorion (Figs. 70 and 72). If
these diagrams be examined the yolk sac will be seen to hang
free within the great primitive coelom through the umbilicus
which, at this stage, is nearly as extensive as the ventral surface
of the embryo (Fig. 75). The embryo during the 3rd week is only
from 3 to 5 mm. long (i-4 inch) ; while it grows the umbilicus
increases at a slower rate. It thus comes about that when the
embryo is an inch (25 mm.) long, the primitive umbilicus, not
having kept pace with the body growth, remains comparatively
small. The somatic layers gradually contract round the yolk sac
and allantoic stalk, the umbilicus and umbilical cord being thus
formed. At first the belly end of the umbilical cord is funnel-
shaped and a coil of intestine hangs within the umbilicus until
the 3rd month of foetal life. After the third month the coil of
intestine, probably owing to an increase in the capacity of the
abdomen, retreats within the umbilicus, which then contracts
round the umbilical vessels. It occasionally happens that the
process of development at the umbilicus is arrested at the second
month and the child is born with some of its abdominal contents
within the umbilicus and upper part of the cord. This condition
is known as congenital umbilical hernia. Sometimes, too, the stalk
of the yolk sac — the vitello-intestinal canal — persists, giving rise
to an umbilical faecal fistula. The cavity of the allantois may
also remain open, leading to an umbilical urinary fistula. Fistulae
at the umbilicus are, however, comparatively rare.

CHAPTER IX.

THE URO-GENITAL SYSTEM.

The Wolffian Body or Meso-nephros. — In lower vertebrates
(pishes and Amphibians) the Wolffian body is the functional
kidney ; in higher vertebrates (Reptile, Birds, and Mammals) it is

tubules of
pro-nephros

Wolffian duct-
Wolf genit. tubules

propelling into coelom
glomerulus

Wolf renal tubules \

genital glancl

glomerulus

meso-nephros

Wolffian duct.

allantois

Fig. 79. — Scheme of the Wolffian Body of the right side.

merely a temporary or embryonic structure, the renal function
being taken over by the permanent kidney. Nothing is known
of how or when the permanent kidney arose and supplanted the

TIIE UKO-GENITAL SYSTEM.

103

Wolffian Body in the evolution of the vertebrates. Its presence
in the human embryo and in the embryonic stages of the three
great classes of higher vertebrates, with the presence of many
curious stages in the development of their genito-urinary system,
can be explained only by the fact that these higher forms are
descended from ancestors of the lower.

In Fig. 79 the AVolffian body, such as occurs in the frog, is
represented diagram matically and it corresponds in structure to the
Wolffian body which appears in the human embryo. Each body is
made up of a main duct and a series of tubules. In the frog, as
in the human embryo, the hind gut ends in a dilatation, the
cloaca. In the cloaca open the rectum, allantois, and the two
Wolffian ducts — right and left. In the frog, the Wolffian bodies
lie on each side of the spine, their anterior ends reaching forwards
to the region of the heart. Each duct is joined by numerous
convoluted tubules — the Wolffian tubules. Each tubule is fur-
nished with a glomerulus at its blind extremity and in most
features agrees with a secretory tubule — such as are seen in the
permanent kidney. These tubules secrete the urine ; the Wolffian
duct conveys the urine from the tubules to the cloaca. The anterior
tubules, however, loose their secretory function and become associ-
ated with the genital gland. In the male frog they convey the
spermatozoa to the Wolffian duct, which thus carries both urine
and spermatozoa. In the female, the genital Wolffian tubules are
connected with the ovary but are quite functionless (Fig. 79).

The Wolffian Body in the Human Embryo. — At the
beginning of the second month of foetal life, the Wolffian body is
well developed : by the end of that month it has become vesti-
gial, the only parts remaining being those connected with the
genital organs. It projects as a ridge from the lumbar and
dorsal regions on each side of the mesentery, extending, on
each side of the spine, from the posterior cervical region, where
the diaphragm is developed, to the pelvis behind, where the
ridges become approximated (Fig. 80). To its inner side, in
the lower dorsal region, lies the genital ridge. The genital and
the Wolffian bodies have each its own mesentery but these
two mesenteries have a common attachment — the common
uro-genital mesentery (Fig. 80). On section the Wolffian ridge
is seen to be made up of convoluted tubules terminating at

104

HUMAN EMBRYOLOGY AND MORPHOLOGY.

their blind extremities in glomeruli. The tubules open into
the Wolffian duct just as in the frog ; the duct is situated
in the basal or attached part of the ridge. It runs back-

Fio. SO. — Diagrammatic section to .show the position of the Wolffian and Genital
Ridges on the dorsal wall of the abdomen.

wards in this ridge and turns into the pelvis to end with the
Mullerian duct (also situated in the Wolffian ridge) in the cloaca
of the hind gut. The whole arrangement is similar to that seen
in the frog. Further, as in the frog (Fig. 79), the anterior or
genital tubules are connected with the genital glands, and are not,
as the posterior are, secretory in nature. If the testis were
functional at this time — which it is not — the spermatozoa and
urine of the Wolffian body would pass to the cloaca by the
Wolffian duct.

Origin of the Wolffian Duct and Tubules. — The tubules
which compose the Wolffian body are developed in the inter-
mediate cell mass (Fig. 85). At first they are minute transverse
vesicles formed by mesoblastic cells in the intermediate mass ;
these vesicles become tubular; one end opens into the Wolffian
duct ; at the other a glomerulus is developed.

The origin of the duct has been traced by Kollmann from the
epiblast. It arises on the lateral surface of the body by an

THE UROGENITAL SYSTEM.

105

invagination of the epiblast and comes to lie subsequently in the
intermediate cell mass (Fig. 85). Thus it will be seen that the
lining of the Wolffian duct and of the structures formed from it,
are epiblastic in nature and liable to all the diseases to which
epiblastic structures are subject.

The Pro-nephros (Fig. 79). — Even before the meso-nephros
(Wolffian body) there appears to have been another kidney — the
pro-nephros. While only permanently functional in some of the
lowest fishes and even in them it is partly replaced by the meso-
nephros, it still appears transiently in the embryos of vertebrates
and is said to occur also in the human embryo. It is developed
in the cervical region at the anterior end of the Wolffian ridge.
Like the Wolffian body it is composed of a longitudinal duct and
tubules ; the duct appears to be an anterior prolongation of the
Wolffian duct but its tubules are different. They open into the
coelom (peritoneal cavity) by trumpet-shaped ciliated ends, and
are derived from invaginations of the mesothelial lining of the
coelom. They are coiled and terminate in the pro-nephric duct,
an anterior continuation of the Wolffian. A glomerulus is
developed in the course of each pro-nephric tubule (Fig. 79).
The pro-nephric tubules are probably representatives of the
segmental (nephridial) tubules of the Vermes.

The Fate of the Wolffian Body (meso-nephros) and Pro-
nephros.— (1) In the Female.

In Fig. 81 are shown the various remnants of the embryonic
renal formations which may persist in the adult female. The
Mullerian duct, the upper part of which becomes the Fallopian
tube, is situated in the Wolffian ridge (Fig. 80). Hence when
the ovary and tube migrate to the pelvis, the Wolffian mesentery,
which comes to form the meso-salpinx, is also drawn within the
pelvis and with it all the Wolffian remnants in the female. A
hydatid attached to the meso-salpinx (part of the broad ligament)
at the fimbriated extremity of the Fallopian tube (Fig. 81) is
situated at the anterior end of the Wolffian duct and represents
the most anterior (cephalic) of the Wolffian tubules or perhaps
the cephalic end of the Wolffian duct, or even the pro-nephros,
although it is improbable that this transient embryonic structure
should persist (J. H. Watson). It certainly corresponds to the
pro-nephric remnant found in the frog. It may become enlarged

106

HUMAN EMBRYOLOGY AND MORPHOLOGY.

or cystic but never to a great extent. The Wolffian duct (Fig.
81) runs towards the body of the uterus in the meso-salpinx ; it
reaches the side of the uterus and passing down in the superficial

tissue of the cervix and vagina, terminates in the vulval cleft at
the outer side of the opening of the vagina near the duct of
Bartholin. Only the upper part of the duct (meso-salpingeal
part) persists in women. The uterine and vaginal segments
disappear. Barts of these may remain ; they constantly do so
in the sow. The uterine and vaginal segments, if they persist,
get the name of duct of Gartner. The genital tubules, those
attached to or connected with the ovary, persist and form the
epoophoron, Organ of Bosenmuller, or parovarium (Fig. 81).
They frequently become cystic and give rise to large tumours.
The renal Wolffian tubules — those which acted as renal structures
in the embryo, also persist, sometimes unconnected with the
duct. They lie between the ovary and uterus and form the
paroophoron (Fig. 81). They too may form cysts.

2. In the Male.

In the male (Fig. 82) the Wolffian duct forms:

(1) The tube of the epididymis, which is coiled up in the
globus major, body and globus minor of the epididymis ;

Mullerian duct

opening at margin of
uaginal orifice

Fig. 81. — Remnants of the Wolffian Body in the Female.

THE URO-GENITAL SYSTEM.

107

(2) The vas deferens and common ejaculatory duct. The duct
opens, as in the female, at each side of the uterus masculinus in
the prostatic urethra ;

Epididymis

Mullerian
duct.

uasa aber..
oblit-

urethra

open. com.
ejac. duct

sessile hydatid
stalked hydatid
right testicle

paradidymis

uas. deferens
(Wolf, duct)

uesic. semin.
Mull. duct.

uterus masculinus

Fig. 82.— Remnant of the Wolffian Body in the Male.

(3) The vesiculae seminales arise from the Wolffian ducts as
acino-tubular diverticula.

The stalked hydatid frequently seen on the upper extremity
of the testicle corresponds to the hydatid at the fimbriated
extremity of the Fallopian tube in the female, and is of similar
origin (Figs. 81 and 82).

The genital tubules of the Wolffian body become the vasa
efferentia and coni vasculosi.

The renal tubules of the Wolffian body form :

(1) The vasa aberrantia found in the globus minor;

(2) The paradidymis or organ of Geraldes occasionally situ-
ated in the cord above the globus major. All these tubules,
both genital and renal of the Wolffian body, are situated origin-
ally in the mesentery of the Wolffian body (Fig. 80).

Thus it will be seen that while in the male the Wolffian
tubules and duct become part of the genital system, in the female

108

HUMAN EMBRYOLOGY AND MORPHOLOGY.

they become functionless and only of pathological importance.
1 heir presence in the female is due to their being inherited from
the male.

THE KIDNEY.

Origin of the Permanent Kidney. — In Fishes and Amphibians
the Wolffian body alone acts as a kidney. In Reptiles, Birds
and Mammals, the permanent or hind kidney appears and
supplants the Wolffian kidney. The kidney appears in the
human embryo at the beginning of the second month. It arises

Fio. 83. — The Origin of the Renal Bud (diagrammatic).

(see Fig. 83) as a bud from the dorsal side of the Wolffian duct,
near the termination of that duct in the cloaca. At first it is a
stalked bud with a narrow lumen ; it rapidly extends forwards to
the lumbar region behind the Wolffian body and behind the
peritoneum. The stalk of the bud forms the ureter. The
connection of the stalk with the Wolffian duct is lost ; the ter-
mination of the ureter migrates along the duct until it reaches
that part of the cloaca which afterwards forms the bladder (Fig.
84). The dilated cephalic end of the bud divides into several
secondary buds. The dilated terminal part forms the
pelvis of the kidney, its infundibula and calyces. The tubules

TI1E URO-GENITAL SYSTEM.

109

of the kidney are formed from groups of epithelial cells on
the convex margin of the dilated end of the renal bud.
They arise as tubular buds which grow out into the intermediate

niesoblast

Fig. S4.— The Termination of the Ureter in the Bladder and Sub-division of the

Renal Bud.

cell mass, subdividing as they grow out. The tubular buds
spring out in groups, each group forming a pyramid. The
tubules at first are straight, but as they grow into the inter-
mediate cell mass, each tubule becomes convoluted, and at its
blind end a glomerulus, formed in the intermediate cell mass,
is developed and comes to project within the tubule. The
glomeruli, and perhaps also the convoluted tubules, are produced
from the renal mesoblast beneath the capsule, batch after batch
being formed as the kidney grows. The loop of Henle is late in
appearing. Thus it will be seen that the ureter, the pelvis of the
kidney and at least the collecting tubules of the kidney are
derived from the epithelial bud which springs from the Wolffian
duct ; the capsule of the kidney, the intra-renal connective tissue,

110

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the vessels and glomeruli, and possibly the convoluted tubules, are
derived from the mesoblast of the intermediate cell mass. It will
be remembered that the epithelial lining of the Wolffian duct is
epiblastic in origin, and the renal bud which springs from it must
also be of a similar nature.

The kidney grows forward until it touches the supra-renal
bodies. At the same time it undergoes a rotation so that the
hilum, instead of pointing backwards to the pelvis, is directed
inwards and forwards. The secretory tubules are grouped in
lobules or pyramids. Up to the time of birth and for some
time afterwards the lobules remain distinct, and this is the per-
manent condition in mammals, such as the ox, bear, seal, etc.
In man, with the formation of new cortex beneath the capsule,
all marks of separation between the surface parts of the renal
lobules disappear, only their apices or pyramids remaining
distinct. As the renal buds of opposite sides grow forwards,
they may come in contact and fuse partially together at their
caudal extremities. In this way the condition known as horse-
shoe kidney is produced. The renal bud may subdivide at its
commencement, thus giving rise to two or even three ureters.
Although the ureters remain distinct, the renal parts commonly
fuse again to form one kidney.

THE MULLERIAN DUCTS.

The Mullerian Ducts or Oviducts are present in almost all verte-
brates, and convey the ova to the surface of the body. In fishes,
amphibians, reptiles, birds and lower mammals (Marsupials) the
ducts terminate in the cloaca. This is also the case in the
embryonic stages of man and all higher mammals. The develop-
ment of the duct in man is very simple (Fig. 85). It is developed
on the outer surface of the Wolffian ridge, below (ventral to) the
Wolffian duct, by a tubular invagination of the mesothelium of the
coelom. The anterior (cephalic) end remains connected with the
coelom and forms the ostium abdominale. As it passes back-
wards in the Wolffian ridge it lies below and internal to the
Wolffian duct and comes in contact with the Mullerian duct of
the opposite side in the pelvis (Fig. 87). rIhe Mullerian duct

THE UEO-GENITAL SYSTEM.

Ill

is formed in the embryo later than the Wolffian duct. In
fishes the Mullerian is derived from the Wolffian duct. They

Fig. 85. — A transverse section to show the manner in which the Wolffian and
Mullerian Ducts arise, and their position in the Wolffian Ridge. (After
Kollmann.)

Fig. 86. — Diagram of the Genital Ducts at the commencement of the 3rd month of

foetal life. Lateral view.

open in that part of the cloaca which forms the neck of the
bladder, between the openings of the Wolffian ducts (Fig. 86).
They are developed in the male as well as the female embryo.

112

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The Genital Cord. — During the 3rd month the Mullerian ducts
show two distinct stages in their course :

(1) Lumbar, which lies in the Wolffian ridge and is suspended
from the posterior abdominal wall by the Wolffian mesentery.
This stage afterwards forms the Fallopian tube (Fig. 87).

(2) A pelvic stage, where it lies in the genital cord. The pos-
terior ends of the Wolffian ridges, with their contents, the
Wolffian and Mullerian ducts fuse in the pelvis, and thus form
the genital cord. The parts of the Mullerian ducts within the
cord form the uterus and vagina.

The genital cord of the foetus at the second month shows the
two Miillerian and two Wolffian ducts — in the male as well as in
the female (Fig. 86).

The Round Ligament of the Uterus is attached to the Mullerian
duct on each side (Fig. 87). The point of attachment marks the

junction of the uterine and Fallopian segments of the Mullerian
ducts. The round ligament corresponds to the gubernaculum
testis in the male and its development is similar. Both are
developed in the following manner :

Part of the Wolffian ridge is continued backwards as a peri-
toneal fold to the groin, this part forming the inguinal fold
(Fig. 87). Into this peritoneal fold muscle cells grow from the
rudiments of the transversalis and internal oblique in the abdo-

THE UEO-GENITAL SYSTEM.

113

minal wall and form the round ligament of the uterus. The
point from which the ingrowth springs becomes the internal
abdominal ring. Others grow out into the labium majus, carry-
ing with them a process of peritoneum — the canal of Nuck. The
inguinal canal and external abdominal ring and extra-abdominal
part of the round ligament are thus formed.

Formation of Uterus and Vagina. — Within the genital cord
the Mullerian ducts fuse and form the uterus and vagina (Fig. 87).
In all the members of the vertebrate series below and including

Fig. SS. — Evolution of the Human Form of Uterus.

A. Form seen in lowest mammals, reptiles, amphibians, fishes, and in the 2nd
month human foetus. B. Form of Mullerian Ducts in rodents. C. Form in
Carnivora, etc., and in the 4th month human foetus. D. Form found in man
and higher primates.

the Monotremes, the Mullerian ducts remain separate and open
in the cloaca (Fig. 88 A). The process of fusion begins in the
3rd month. The septum formed by the fused mesial walls
(Fig. 89) disappears first in the region of the uterine cervix; the

H

114

HUMAN EMBRYOLOGY AND MORPHOLOGY.

process may be arrested at this stage — a stage shown by some
adult marsupials. Then the lower or vaginal part of the septum
disappears; the human uterus then (34 months) resembles that
of higher mammals (carnivora, etc., Fig. 88 G). It may be
arrested at this stage (uterus bicornis). Lastly the upper part
of the septum disappears (4£ months, Fig. 89). The fundus,
which is the last part to be developed and is only found in the
highest primates, is quite well marked in the child at birth.

Fig. S9. Showing the manner in which the Mullerian Ducts fuse to form the Uterus

and Vagina.

The lining epithelium of the lower third of the vagina is
derived, according to the researches of Berry Hart, from the
lower ends of the Wolffian ducts. A solid epithelial bud (epi-
blast) grows from the end of each duct, and fills the lowei part
of the united Mullerian ducts. The central cells of the Wolffian
buds disappear, while the peripheral form the lining of the lower
third of the vagina.

The Mullerian Ducts in the Male. — All that remains of the
Mullerian ducts in the adult male are their fused terminal seg-
ments forming the sinus pocularis or uterus masculinis in the
prostate (Figs. 82 and 90). Its depth is commonly about 3 or
4 mm., but occasionally such a form as is represented in big. 91

2nd part of septum to

disappear

THE URO-GENITAL SYSTEM.

115

occurs and shows the real nature of the sinus pocularis. The
vagina, uterus, and part of the Fallopian tubes can be recognised
(Primrose).

Fio. 90.— A section of the Prostate showing the Remnants of the lower ends of the

Mullerian Ducts in the male.

Fig. 91.— A section of a Prostate showing an unusually developed Uterus Masculinus.

(After Primrose.)

The fimbriated ends of the Mullerian ducts persist as the
•sessile hydatids on the testicle (Fig. 82). The intermediate part
of the tube becomes greatly stretched during the descent of the
testicle and disappears, but a remnant of its upper end can be
found in the sharp anterior border of the epididymis until quite a
late period in foetal life (J. H. Watson). The mesosalpinx
shrinks and completely disappears in the anterior border of the
epididymis.

The Uro-genital Sinus or Canal. — The Mullerian ducts open
into that part of the cloaca which becomes the neck of the bladder
(Fig. 86). That part of the cloaca which serves as a common
channel for bladder, Mullerian, and Wolffian ducts is the uro-
genital sinus (Figs. 95 A and B). In the female foetus at the
4th month it is still well marked (Fig. 92 A). In all mammals

bladder

triang. lig.

116

hitman embryology and morphology.

except man it retains this form. By the beginning of the 6th
month in the female foetus (Fig. 92 B) it will be seen that the

Fio. 92. — Section showing the Uro-genital Sinus.

A. In the 4th month female human foetus. B. In the 5th month female human foetus.

vesico-vaginal septum (ci in Fig. 92) has grown down towards the
pudenal cleft, and all that remains of the uro-genital sinus is
the small space of the female pudendal cleft from the fossa

Fio. 93. — Section showing the Uro-genital Sinus in the male foetus.

a indicates the part corresponding to the vesico-vaginal septum of the female. It is
occupied by the 3rd lobe of the prostate.

navicularis behind to the vestibule of the vulva in front.
The vagina (Mullerian ducts) thus comes to open in the
pudendal cleft in the female. In the male (Fig. 93) the early
foetal form is retained, and the uro-genital sinus becomes that
part of the male urethra between the sinus poeularis and the

THE UROGENITAL SYSTEM.

117

anterior layer of the triangular ligament. The female urethra
corresponds to the prostatic part of the male urethra above the
opening of the sinus pocularis (Figs. 92 and 93).

The Hymen is formed at the junction of the vagina with the
uro-genital sinus. The terminal parts of the Mullerian ducts,
which form the vagina, are at first solid epithelial cords, the
epithelial mass being derived from the bulbous terminations of the
Wolffian ducts (Berry Hart). After the Mullerian ducts fuse to form
the vagina they become hollowed out, all but the terminal piece,
which forms the hymen and it, as a rule, is perforated (Fig. 94).

Fig. 94. — A section to show the condition of the Vagina and Uterus at the 7th month

of foetal life.

Cases occur in which the hymen is a complete or even thick
septum, or occasionally the whole vagina may retain the foetal
solid form (atresia vaginae).

The Vagina is at first not only solid, but extremely short ; the
downgrowth of the vesico-vaginal septum adds greatly to its
length (Fig. 92 A and B). It is separated off from the uterine
segment of the Mullerian duct in the fifth month (Fig. 92 B) by
(1) the growth of the external os and formation of the labia; (2)
by its epithelium becoming stratified, while that of the uterus and
upper half of the cervix remains columnar.

The Uterus is formed by the fusion of the Mullerian ducts ; its
muscular walls and thickened mucous lining appear in the 5th

118

HUMAN EMBRYOLOGY AND MORPHOLOGY.

month. By the seventh month (Fig. 94) it is divided into two
parts, the cervix or lower segment and body or upper segment.
The lower segment or cervix forms then two-thirds of the uterus ;
its walls are thick and its upper part is lined by columnar non-
ciliated epithelium, containing mucous racemose glands. Its
mucous membrane is arranged in palmate folds. The upper or
uterine segment proper composes only a third of the uterus. It
is lined by columnar ciliated epithelium. Uterine glands are
developed in it after birth. At puberty the body of the uterus,
instead of being half the size of the foetal cervix, becomes larger
than it. The cervix takes no part in menstruation nor in con-
taining the foetus ; its true function is unknown. It is probably
glandular.

THE PERINEUM.

The Perineal Body (Figs. 94 and 95) is the triangular septum
of tissue which is developed in the perineum so as to separate the
rectum from the vagina. It contains the origin of the sphincters
of the anus and vagina, fat and fibrous tissue, and all the
structures between the recto-uterine fold of the peritoneum above
and the perineum below.

In the male the corresponding part lies between the urethra
and the rectum. In man the perineal body is extremely large.
By its development the rectal part of the cloaca is separated from
that part of it which forms the uro-genital sinus (Fig. 95). The
perineal body is formed at the end of the 1st month of foetal
life in the following manner :

The allantois grows out from the hind gut ; a prolongation of
the enteric cavity forms the canal of the allantois (Fig. 75, p. 96).
Hence in the 3rd week the hindermost part of the gut is common
to allantois and intestine, the common part forming the cloaca.
There is then no perineal body (Fig. 95 A). This is the permanent
condition in all vertebrates save the higher mammals — those higher
than the Monotremes. The Wolffian ducts open in the cloaca
near the neck of the allantois. A cleft appears on each side of
the cloaca, and the dorsal or rectal part is separated from the
ventral or uro-genital part. The ventral or uro-genital part of
the cloaca forms the bladder and uro-genital sinus, which are

THE URO-GENITAL SYSTEM.

119

thus separated from the rectum by the ingrowth of two lateral

cloacal septa.

Fig. 95. — The Division of the Cloaca into Rectal and Uro-genital Parts.

A. First appearance of the separation of tne Cloaca into Rectal and Uro-genital

Parts (3rd week).

B. Separation of the Cloaca into Rectal and Uro-genital Parts (5th week).

The Proctodaeum or Primitive Perineal Depression. —

Although the main part of the perineal body is formed in the
manner just described, a second element, which forms the surface
part of the perineal body is formed thus :

Between the caudal protuberance behind, and the genital tubercle
in front (Fig. 95 B), the epiblast dips in as a median depression
to form the proctodaeum or primitive perineal depression. The
depressed area comes in contact with the ventral surface of the
cloaca (Fig. 95^4). By the ingrowth of a perineal septum from
each lateral margin of the depression, the proctodaeum is sepa-
rated into a posterior or anal part, and an anterior or uro-genital part.

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The cloacal membrane or plate (Fig. 95 M), formed of the applied
layers of epiblast and hypoblast between the perineal depression
and the cloaca, breaks down before the end of the 1st month of
foetal life. The uro-genital sinus then comes to open into the
uro-genital part of the depression, while at a rather later date the
rectum opens into the anal part. The lateral perineal ingrowths,
which divide the proctodaeum into uro-genital and anal parts,
complete the perineal body (see Fig. 98).

Imperforate Anus. — This condition occurs not unfrequently,
and in many forms. It is due to a mal- development of the cloacal

Fig. 96.— A case of Imperforate Anus due to a persistence of tlie Anal Plate.

and perineal septa. In some cases it is merely the anal part of the
cloacal plate that fails to break down (Fig. 96). The white line,
situated at the lower ends of the columns of Morgagni, marks the
situation of the anal plate and the junction of the epiblast of the
anal depression with the hypoblast of the rectum.

In over 50°/o of cases of imperforate anus the rectum ends
blindly two inches or more above the anus. In such cases the
lateral septa which divide the cloaca appear to have been mal-
placed and included the whole of the cloacal part of the hind gut
in the uro-genital canal. On the other hand the cloacal septa
may be incomplete, and the uro-genital sinus only partially
separated from the rectal. In such cases there is a communica-
tion between the rectum and termination of the vagina in the
female (Fig. 97), and the part of the urethra formed from the
uro-genital sinus in the male (Fig. 93).

Post-anal Gut. — It will be observed that the anus is formed

THE URO-GENITAL SYSTEM.

121

on the ventral aspect of the hind gut (Fig. 108, p. 136). There
is a projection of the gut behind the proctodaeal depression.
This is the post-anal gut. It commonly disappears, no trace of

Fig. 97. —A case in which the Rectal part of the Anal Plate has persisted and the
Cloacal Septum has failed to fuse with the Perineal Septum.

it being found after the 1st month, but remnants of it are said
to give rise to ano-coccygeal tumours and cysts.

The Neurenteric Canal — Ano-coccygeal tumours are also
believed to arise from remnants of the neurenteric canal. The
neurenteric canal, or blastopore, it will be remembered (p. 8 9,
Figs. 68, 75 and 158) is a communication of the cavity of the
hypoblast with the surface of the epiblast. It probably represents
the point at which an area of cells was invaginated within the
epiblast to form the hypoblast. As it opens at the anterior end
of the primitive streak, which afterwards is included in the
posterior end of the neural groove, such a canal, if it persisted,
might be expected to perforate the coccyx and communicate
between the canal of the notochord and filum terminate. No
remnant of this canal has been found after the 1st month of
foetal life.

The Uro-genital Cleft or Depression is the anterior part of the
proctodaeal or primitive perineal depression, cut off from the
posterior or anal part by the development of the perineal septa.
With the absorption of the cloacal membrane, the uro-genital
sinus opens in the cleft (Fig. 95 B). The cleft remains alike in
both sexes until the third month. It forms the greater part of

cloacal sept, imperfect

to cloacal septa.

HUMAN EMBRYOLOGY AND MORPHOLOGY.

1 99

I — —

the pudendal cleft of the female. In its anterior angle is developed
a prominent tubercle — the genital tubercle, the apex of which
becomes the glans penis or glans clitoris, according to sex. It is
bounded by lateral folds, the genital folds (Fig. 98). The
genital folds divide in front ; the inner division of each runs to
the glans and forms the fraenum ; the lateral divisions meet over
the glans and form the prepuce.

External Genitals of the Female. — In the female the parts
retain closely the early foetal form just described. The genital
tubercle becomes the glans clitoris. In the genital eminence — of
which the tubercle is merely the summit, the corpora cavernosa
develop. The lateral genital folds form the labia minora, the prepuce
and fraenum. By the junction of the genital folds within the uro-
genital depression behind, the fourchette is formed. Within the
lateral folds or labia minora, are developed the bulbs of the
vestibule. The uro-genital depression becomes the pudendal cleft.
After the third month external aenital folds arise and form the
labia majora. By their anterior union they give rise to the mons
Veneris. Their posterior extremities unite to form the posterior
commissure.

genital tubercle
outer fold
genital fold (inner)
urogenit. dep.

raphe

ana! depression

98.— The Uro-genital Cleft or Depression and the Genital Tubercle and Folds
towards the end of the 2nd month.

External Genitals of the Male. — In the male, at the end of
the third month, the lateral genital folds begin to unite from
behind forwards, thus closing the uro-genital cleft and forming the
floor of the penile urethra. While the floor of the penile urethra
is formed thus, its roof, corresponding to the vestibule of the
female, is derived from an angular forward prolongation of the
uro-genital sinus. (See Fig. 95 B.) When the genital folds unite

THE URO-GENITAL SYSTEM.

123

to form the urethral floor, the erectile tissue contained in them,
corresponding to the bulbs of the vestibule of the female, also
fuses and thus the corpus spongiosum is formed. The part
of the urethra within the glans is the last part to be formed,
and its development is peculiar. It is formed by a solid rod-
like ingrowth of epiblast within the glans which burrows back-
wards until it reaches the part formed out of the uro-genital
cleft. The glans part of the urethra becomes canaliculized a
short time before birth. The fossa navicularis and lacuna magna
occur at the junction of the part of the urethra formed in the
glans and the part formed by the union of the inner genital folds.

The closure of the uro-genital lips to form the floor of
the urethra in the penis may be arrested at any stage,

,oblit. in cord
AV urachus(allantois)

bladder (cloaca)

ingrowth of epiblast

, /, - i upper prostatic
l from cloaca

ejac. duct

(from Wolffian duct)

{ lower prostatic from
l urogen. sinus

memb. from urogen sinus

>r from genital furrow and
l urogenit. sinus

Fig. 99.— A section of the male bladder and urethra at birth, showing the structures
derived from the intra-abdominal part of the Allantois and from the Cloaca.

giving rise to the condition of hypospadias, a condition
normal in the female. It is not very rare to find the terminal
half inch of the urethra in a hypospadiac condition. If the

124

HUMAN EMBRYOLOGY AND MORPHOLOGY.

hypospadias is complete then probably the internal sexual organs
have been arrested in their development, and the sex of the
individual can be determined only by the microscopic examina-
tion of the genital glands. The development of the prostatic
and membranous parts of the urethra from the uro-genital sinus
has been already dealt with (see Figs. 91, 93, and 99).

The Scrotum is formed during the fourth month by the union
of the external genital folds (labia majora in the female) between
the penis in front and the perineal body behind. The line of
union is marked by the raphe.

The Fate of the Allantois (Fig. 99). — The part of the
allantois outside the abdominal parietes forms (1) the inner
or vascular layer of the chorion (p. 97), and part of the umbilical
cord (Figs. 75 and 78). The cavity of the allantois within the
cord disappears by the end of the third month.

The intra-abdominal part of the allantois forms :

(1) The Urachus between the umbilicus and bladder (Fig. 99).
It is reduced to a fibrous cord before birth. It may remain
patent in the middle and closed at both ends and thus give rise
to a urachial cyst behind the abdominal wall. Or it may open at
the umbilicus, or at the bladder or at both, thus giving rise to a
urinary fistula at the umbilicus. Urachial cysts are rare.

(2) The apical part of the bladder.

PARTS FORMED FROM THE CLOACA.

(1) The Bladder. It was formerly customary to describe the
bladder as a derivative of the basal part of the allantois but more
recent researches show it to be part of the cloaca (Figs. 95
and 108).

At birth the neck of the bladder lies above and behind the
symphysis pubis (Fig. 99). The cavity of the bladder is then
fusiform in shape and mostly extra-pelvic in position. It differs
from the adult bladder in having a very small trigone and its cavity,
therefore, a very short posterior limb in the closed condition.

(2) The Female Urethra and the corresponding part in the
male, viz., the part between the neck of the bladder and the sinus
pocularis.

THE URO-GENITAL SYSTEM.

125

(3) The uro-genital sinus or canal, and the parts derived from
it which have been already described (Fig. 93 and page 115).

(4) The terminal part of the rectum.

Ectopia vesicae is as yet unexplained. The condition is shown
diagrammatically in Fig. 100^4. This condition appears to be
produced thus :

(1) The development of the tail in the embryo thrusts the
region of the primitive streak, situated on the body-stalk,
towards the ventral aspect of the body. The uro-genital part
of the perineal depression and the cleft into the uro-genital
sinus are formed in the line of the primitive streak (Fig. 158,
p. 192). The body-stalk which at first is a direct continua-
tion of the body (Fig. 75, p. 96), is arrested in its migration
towards the ventral surface of the body. It is arrested
between the rami of the pubes, and prevents the formation
of the symphysis.

Fig. 100. — A. A section to show the condition of parts in Ectopia Vesicae.

B. Section of the pelvis of an embryo (4th week) to show how the condition
is probably produced.

(2) The uro-genital cleft is formed in front of and above the
genital tubercle instead of below it. The cleft opens thus into
the anterior wall of the bladder instead, as it normally does, into
the uro-genital sinus (Fig. 100 B).

It is also possible that the condition of ectopia vesicae is due
to a dropsical condition of the allantois, with subsequent rupture,
in the embryonic condition.

126

HUMAN EMBRYOLOGY AND MORPHOLOGY.

THE PROSTATE.

I he Prostate is developed round the uro-genital sinus. It con-
sists of glandular tissue and stroma.

(1) The glandular tissue is composed of tubular glands which
open into the prostatic part of the urethra. They are developed in
the 4th month, as series of solid buds from the epithelium lining

Fig. 101.— A diagram to show the position at which the Prostatic Tubules arise.

the upper part of the uro-genital sinus (Fig. 101). The buds grow
out as a right and left lateral mass, and form the glandular tissue
of the lateral lobes. At first the two lateral lobes, as in mammals
generally, lie separately behind the urethra. Then they fuse
behind the urethra ; in man only do they meet to form a dorsal or
pubic commissure over it. The third lobe (Fig. 91) appears later;
the tubular buds which form it rise from the posterior part of the
stalk of the bladder (Fig. 101) above the opening of the sinus
pocularis. It is not unfrequently absent or very small.

Skene’s tubules, which may be found opening into the urethra
of the female, probably represent prostatic tubules.

(2) The Stroma of the Prostate. — While the glandular tubes
arise in three groups — two lateral and one posterior median —
from the epithelium lining the uro-genital sinus, and stalk of the
bladder — the muscular and fibrous elements arise from the meso-
blastic tissue surrounding the terminal parts of the Wolffian and
Mullerian ducts. The stroma surrounds the glandular tissue and
forms the peripheral part of the gland. It contains muscular
tissue which is especially developed in the pubic commissure.

THE URO-GENITAL SYSTEM.

127

Probably the stroma is similar in its nature and origin to the
uterine muscle, for at a corresponding time of life it is apt to give
rise to the same form of fibrous tumours as occurs in the uterus.

As regards the nature of the Prostate :

(1) It is purely genital, and develops only in the rutting season
in such mammals as manifest a periodical sexual life.

(2) It remains comparatively undeveloped until puberty. At
the age of seven it weighs only 30 grains; after sexual life is
established it weighs about 300 grains.

(3) It atrophies on castration, a fact which is utilised by the
surgeon in cases of prostatic hypertrophy. Castration performed
on old men frequently leads to insanity. Atrophy of the prostate
is also produced by section of the vas deferens on both sides. In
one man out of three over 55 years of age the prostrate hyper-
trophies, both the glandular and fibro-muscular elements
participating. Hypertrophy of the third lobe causes a valvular
elevation behind the vesical opening of the urethra.

The Glands of Cowper and Bartholin are produced as solid buds
from the hypoblast lining the mouth of the uro-genital sinus.
Hence in the female the ducts of Bartholin’s glands open outside
the hymen at each side of the vagina, for the hymen marks the
junction of the Mullerian ducts with the uro-genital sinus. In
the male the ducts of Cowper’s glands open at the commence-
ment of the bulbous part of the urethra. Their function is
unknown, but they are certainly sexual in nature. The numerous
glands of Littrd, like Cowper’s and Bartholin’s glands, are pro-
duced by tubular outgrowths during the fourth month. In the
male the glands of Littre are produced most numerously along
the dorsal aspect of the urethra.

THE TESTES.

Descent and Development of the Testicle. — The position of
the testicle in a foetus of the third month is shown in Tig. 102.
It is situated in the iliac fossa. The mesorchium, a fold of
peritoneum, binds its attached border to the iliac fossa. At its
outer side lies the genital part of the Wolffian body which forms
the epididymis. It, also, is suspended by a mesentery — the
Wolffian mesentery. The two mesenteries have a common base —

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the common uro-genital mesentery (see Eig. 80, p. 104). The
upper part of the uro-genital mesentery forms the diaphragmatic
fold or plica vascularis (Fig. 102). This in the female becomes the

rectum

Fig. 102.— The Position of the Testis in a foetus of 2£ months

ovario-pelvic ligament (Fig. 59, p. 81). A fold of peritoneum,
the inguinal fold or plica gubernatrix, continues the common
uro-genital mesentery to the groin (Fig. 102). The guber-
naculum testis is developed in the plica gubernatrix; in the
corresponding fold in the female the round ligament of the uterus
appears. The vas deferens (Wolffian duct) turns into the pelvis
from the lower end of the epididymis (Wolffian body), and
within the pelvis lies in the genital cord (Fig. 87, p. 112). A
remnant of the Mullerian duct lies along the outer and ventral
aspect of the epididymis.

The Development of the Testis. — Its blood supply comes
from the level of the 12th dorsal vertebra; its nerve supply from
the 10th dorsal segment of the spinal cord. The testis is there-
fore developed in the genital ridge between the 10th and 12th
dorsal segments. The development of the testis is similar to that
of the ovary (Fig. 62, p. 83). The columnar germinal epithelium
which covers the genital ridge contain between them larger genital
cells — the primordial ova. Tubular buds of germinal epithelium
grow into the tissue of the genital ridge and form the epithelial
lining of the seminiferous tubules instead of, as in the female, the
Graafian follicles. Primordial ova are carried down within the tubes

THE URO-GENITAL SYSTEM.

129

of enclosing cells and these produce the spermatoblasts. The
tunica albuginea is formed from the mesoblastic covering of
the genital ridge. The visceral layer of the tunica vaginalis on
the testicle is the covering of germinal epithelium which remains
after the ingrowth of the genital cells. The vasa efferentia and
coni vasculosi are formed from the genital Wolffian tubules.
The tubuli recti and rete testis are new formations. The
epididymis is the elongated upper segment of the Wolffian duct
(Figs. 82, 107). The Wolffian elements (see p. 102) are produced
within the Wolffian ridge.

Formation of the Gubernaculum Testis. — As shown in Fig.
102 there is no trace of the inguinal canal in the 3rd month;

Fig. 103. — Showing the Position of the Testis at the 6th month, and the Formation of

the Gubernaculum Testis.

the various layers of the abdominal wall are unbroken. In the
fourth month the deep muscular layer of the abdominal wall,
composed of the internal oblique and transversalis, buds inwards
and expands the plica gubernatrix with muscular and fibrous
tissue. The tissue does not stop short at the uro-genital ridge
and Mullerian duct as in the female, but grows up and seizes the
caudal pole of the testis (Fig. 103). At the same time the
tissues in the whole thickness of the abdominal wall bud obliquely
inwards towards the scrotum. They are probably carried away by

i

130

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the downgrowth of the gubernacular bud which pushes its way to
the scrotum (Figs. 103 and 104). The gubernaculum grows
downwards as a solid fibro-muscular mass, until it reaches the
subcutaneous tissue which at that time completely fills the
scrotum. Its attachment to the scrotum is slight and easily
broken. The gubernaculum, as it grows through the abdominal
wall, carries with it : —

(1) A process of peritoneum (the processus vaginalis);

(2) The transversalis fascia (the infundibuliform fascia) ;

(3) The internal oblique and transversalis muscles to form the
cremaster ;

(4) The spermatic fascia from the external oblique ;

(5) The deep layer (Scarpa’s) of the superficial fascia of the
groin. All these layers are added to the primitive coverings of
the scrotum, which until then is made up simply of skin and
superficial fascia (Fig. 104).

Fig. 104.— The manner in which the structures in the wall of the abdomen are carried
out so as to form the Inguinal Canal and Coverings of the Testis.

It will be thus seen that the gubernaculum testis is an actively
growing mass of fibro-muscular tissue, which starting from the
inner muscular layer of the abdominal parietes in the groin, in-
vades first the plica gubernatrix and then the abdominal wall
itself, every layer of which it carries as a prolongation within the
scrotum. It is an invading army of cells. It draws with it into
the scrotum the peritoneum in the iliac fossa, on which the testis
is dragged like a log on a sledge.

cremaster

■peritoneum

—penis

-processus, uag.

■scrotum

internal ob.

THE URO-GENITAL SYSTEM.

131

The testis spends the seventh month of foetal life in its
exodus through the abdominal wall. In the eighth month it
leaves the inguinal canal and lies at the external abdominal ring.
After birth it reaches the fundus of the scrotum. The atrophy of
the gubernaculum pulls it down. A remnant of the guber-
naculum can always be found in the adult behind the epididymis
and testicle, within the mesorchium (Fig. 105).

The Processus Vaginalis. — The processus vaginalis becomes
occluded at two points about the time of birth (Fig. 105). The
upper point of occlusion takes place at the internal abdominal

plica, uasc.

epidid.
testis

rint. ab. ring (upper point
i of occlus .)

■ext. ab. ring
funicular process

lower point of
occlusion

-tunica uagina/is
remnant of gubernac.

Fig. 105.- A diagram of the Processus Vaginalis.

ring ; the lower a short distance above the testicle. The part of
the processus vaginalis between the points of occlusion is known
as the funicular process ; the part surrounding the testicle becomes
the tunica vaginalis. In quite 30 J° of children the occlusion
takes place at the internal abdominal ring some considerable time
after birth or it fails to appear altogether. Occlusion may fail at
the upper point, at the lower point, or at both. Or it may close
at both points, but the funicular process, instead of disappearing,
may remain open and form a cyst (Fig. 105).

Descent of the testicle may be arrested at any stage ; often
m the inguinal canal ; more frequently at the external abdominal

132

HUMAN EMBRYOLOGY AND MORPHOLOGY.

ring. Arrest of descent is commonly a symptom of arrest of
testicular development. On the other hand, the testicle may
assume an ectopic position. The gubernaculum ends in the
scrotum principally, but bands of it pass to end on the root of
the penis, in the groin and in the perineum. These bands,
normally slight, may be big enough to influence the direction
of descent of the testicle ; hence cases occur in which the testis
is found in the groin, by the side of the penis or drawn back in
the perineum almost to the anus (Lockwood).

The Mesorchium. — The testis and epididymis were suspended
within the abdomen by the common uro-genital mesentery. In
the course of the descent of the testis this becomes shortened and
binds the testis and epididymis firmly by their posterior borders
to the tunica vaginalis. Occasionally the uro-genital mesentery
and mesorchium persist ; the testicle is then liable to become
twisted and strangulated on its mesentery. I have seen three
such cases recently. The digital fossa is situated between the
mesorchium and mesentery of the Wolffian body.

The meaning of the descent of the testes is unknown. In
many animals the testes descend only during the rutting season.
The inguinal canal, formed by the descent of the testis, is a source
of weakness in man (see p. 133).

CHAPTER X.

FORMATION OF THE PUBO-FEMORAL REGION,
PELVIC FLOOR AND FASCIA.

Inguinal and femoral hernia occur so rarely amongst mammals
generally that they may be considered human peculiarities. Their

Fig. 106. — A. The form of Pelvis and Inguinal Canal in Man.

B. The corresponding forms in the Lower Primates.

frequency in man is due to certain structural changes in his
pubo-femoral region, changes which have resulted mainly from his
adaptation to upright progression. His susceptibility to hernia
is due to : —

(1) The unique form of Foupart’s ligament in man. It is

134

HUMAN EMBRYOLOGY AND MORPHOLOGY.

scarcely developed in any other animal (Fig. 107). In the orang,
for instance, also an upright primate, the external oblique has no
attachment to the crest of the ilium, and takes no part in forming
the outer part of Poupart’s ligament (Fig. 107), but its tendon
terminates over and strengthens the region of the inguinal canal.
This is the usual termination in the mammalia.

(2) The internal oblique and transversalis (conjoined parts) in

the orang, and in all primates except man, arise from the firm
tubular sheath of the ilio-psoas, also from the extensive anterior
border of the ilium, and arching over the spermatic cord end in
a long insertion on the ileo-pectineal line. They act as a

powerful compressor or sphincter of the inguinal canal, and thus
prevent hernia (Fig. 107i>).

(3) The human manner of walking, and the great head of the
human child at birth require a wide pelvis. All mammals

mt. sup. sp.
-Pouparts lig

muse.)
comp. |

pubic, sp.

uascular comp^'-:S^f^^^r. |

( tendon of ext
l ob. cut

muscular
comp.

uascular

rectus

I conjoined
( muscle.

inguinal canal.

Fig. 107. — A. Poupart’s Ligament and the Crural Passage of Man.

B. Poupart’s Ligament, Crural Passage, and sphincter-like Conjoined
Muscle of the Orang.

adapted to the prone posture have a narrow pelvis, and hence a
narrow anterior abdominal wall (Figs. 106 A and B) through
which the inguinal canal passes very obliquely. The course of
the canal is more direct in man, and therefore offers a greater
facility to the escape of the abdominal contents.

(4) The size of the space between the edge of the pelvis and
Poupart’s ligament (the crural passage) is very much greater in

PUBO-FEMOIiAL REGION.

135

man than in any other animal (Figs. 107^4 and B). In him, the
most internal part of the passage is left unfilled, and this unfilled
space forms the femoral or crural canal through which femoral
hernia may escape. The crural passage is relatively larger in
women than in men, owing to the greater size of the female
pelvis, and hence femoral hernia are much more common in
women than in men. Some hint as to the method of treatment
of hernia in man may be obtained from a consideration of the
arrangement of structures which prevent them in other animals.

THE PELVIC FLOOR.

The Coccyx.— It is necessary to consider the coccyx here
because the changes which it has undergone in the evolution of
the human body are intimately connected with the formation of
the pelvic floor.

The coccyx in man is commonly composed of four vertebrae,
more or less vestigial in nature, which represent the basal caudal
vertebrae of tailed mammals. Evidence of their vestigial or
retrograde nature is to be found in :

(1) Only their centra are developed — with the exception of the
first, which shows partial formation of transverse processes and
neural arches (superior cornua) ;

(2) Delay in the appearance of the centres of ossification.
These, instead of beginning in the 7 th week as in a typical
vertebra, commence after birth. The centre for the 1st coccygeal
vertebra appears in the 1st year, that for the 4th vertebra about
the 25th year; the 2nd and 3rd at intermediate periods.

(3) Late in life, between the 40th and 60th year, the verte-
brae fuse together, and then unite with the sacrum.

The number of coccygeal vertebrae varies ; four is the normal
number, but there may be three or five. In the young foetus
(2nd month) there are commonly 5, 6 or 7 (Rosenberg). The
first coccygeal vertebra may join the sacrum, making 6 sacral
vertebrae.

The evidence of the former existence of a true tail in the
ancestral human stock consists of :

13G

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(1) In the second month the coccygeal region of the spine
protrudes (Fig. 108).

(2) Vestiges of the extensor and flexor muscles of the tail are
Irequently found (10°/o of bodies) on the dorsal and ventral
aspects of the sacrum and coccyx.

(3) True tails, consisting of external prolongations of the
coccyx, commonly fibrous, rarely containing vertebrae, occasionally
occur.

Fig. 103.- — The caudal end of the body in a human embryo of the 3rd week.

(4) The post-anal pit, always to be seen in the newly-born
child, marks the point at which the coccyx disappears below the
surface. In man the coccyx forms part of the perineal floor.
Instead of projecting far beyond the gut, as in tailed mammals,
it terminates 1\ inches above the commencement of the anal
canal.

The Pelvic Floor is peculiarly extensive in man, an adaptation
to his upright posture. The floor is formed by the following
structures :

(1) The levator ani and its sheath (recto-vesical and anal
fasciae) on each side.

(2) The coccyx and coccygeus muscles.

(3) The constrictor urethrae and its sheath (the triangular
ligament).

(4) The pyriformis and its sheath may also be included.

Development of the Pelvic Floor. — The pelvic floor has been

evolved in man by a transformation of the tail and the caudal

PUBO-FEMORAL REGION.

137

muscles. The arrangement of tail muscles in a four-footed

mammal, such as the monkey or dog, is shown in Fig. 109, and
the modification of this form in man in Fig. 110. In mammals

138

IIUMA.N EMBRYOLOGY AND MORPHOLOGY.

two muscles, the pubo-coccygeus and ischio-coccygeus (Fig. 109),
act as depressors of the tail, which in four-footed animals plays
the part of a perineal shutter. They are attached to the small
V-shaped chevron bones on the under surface of the basal caudal
vertebrae. Another muscle, the ischio- or spino-coccygeus, acts as
a lateral flexor of the tail (Fig. 109). It is attached to the trans-
verse processes of the caudal vertebrae, and rises from the dorsal
border of the ischium. In man the pubo-coccygeus and ilio-
coccygeus unite into one sheet and form the levator ani. The
shrinkage of the tail leaves the muscle partly stranded on the
ano-coccvgeal ligament. Other fibres of the pubo-coccygeus loose
their primary insertion to the coccyx and become attached to the
prostrate, central point of the perineum and to the anal canal.
Both muscles, especially the ilio-coccygeus, retain in part their
primitive attachment to the coccyx (cauda). The spino-coccygeus,
or coccygeus muscle, is partly fibrous in man, its outer laminae
forming the small sacro-sciatic ligament ; its inner laminae remain
muscular and form the coccygeus. In man, too, the origin
of the ilio-coccygeus has sunk from the pelvic brim of the
ischium on to the obturator fascia (P. Thompson) ; traces of the
primitive origin from the pelvic brim can often be detected. The
white line, a structure peculiar to man, marks the new point of
origin of the levator ani from the obturator fascia. Further
details of changes undergone by the pelvic muscles and fasciae
may be found in papers by Dr. P. Thompson in the Journal oj
Anatomy and Physiology , vol. xxxv.

On the dorsal and ventral aspects of the sacrum and coccyx,
fibrous or muscular vestiges of the anterior and posterior sacro-
coccygeal muscles (elevators and flexors of the tail) are commonly
to be found in man.

The Pelvic Fascia and Fasciae in General.— It has been cus-
tomary to regard fasciae as separate structures forming distinct
sheets with devious and complex courses. It is possible by
dissection to prepare and display them according to accepted
descriptions, but the structures so displayed are artificial and
not the true structures with which the surgeon or physician
has to deal with in actual practice. Embryology is the best
guide to their nature. Take the development of the fasciae seen
on making a section of the upper arm, for example. When the

PUBO-FEMOKAL REGION.

139

limb bud has appeared, which it begins to do about the end of
the 3rd week of development, a section through it reveals a
uniform composition of more or less rounded mesoblastic cells with
a covering of epiblast (Fig. Ill A). Very soon the central
cells near the axis of the bud are densely grouped and form the
basis of the humerus. Others arrange themselves to form the
biceps, triceps and muscles of the arm ; others form the walls ol
vessels and the sheaths of nerves.

B A

Fig. 111. — A. Diagrammatic section of the Arm Bud of an embryo at the com-
mencement of the 4th week.

B. Corresponding section of the Adult Arm.

After these various groups of cells have become differentiated,
there are numerous cells left over which form a basis in which
the specialized cells and groups of cells are packed and en-
sheathed. The undifferentiated mesoblast forms the connective
tissue or fascial system of the part. From the manner of its
origin it is evident that the connective tissue system — the fasciae
and septa — must form a continuous formation of sheaths, each
being in continuity with that of every surrounding structure.
The sheaths of the biceps, triceps and brachialis anticus, the
periosteum of the humerus, the deep fascia, internal and external
intermuscular septa, the sheaths of the vessels and nerves of
the arm, represent the mesoblastic tissue which was left over
after the structures which they enclose were differentiated, and
are, from the manner of their origin, necessarily in continuity.

epiblast

mesoblast

biceps

artery

nerue

septum

tendon

periosteum
humerus

deep fascia

IdO HUMAN EMBRYOLOGY AND MORPHOLOGY.

They can only be artificially separated from each other. It is
more accurate and easier to describe fasciae, then, not as separate
structures, but as adjuncts of the structures which they surround
or ensheath.

The Pelvic Fascia, which strengthens the pelvic floor, is com-
posed of the sheaths of four muscles :

(1) The Levator Ani ;

(2) The Obturator Interims;

(3) The Pyriformis ;

(4) The Constrictor Urethrae and deep Transversus Perinei.

The fibrous capsules of the following viscera also form part

of it :

(1) Prostate and Vesiculae Seminales in the male;

(2) Vagina and Uterus in the Female ;

(3) The Bladder;

(4) The Piectum. Under the title of pelvic fascia these eight
elements are combined.

I. The Obturator Fascia is the sheath on the inner or pelvic
aspect of the obturator interims ; the sheath on the outer side of
the muscle is formed by the periosteum and obturator membrane.
The obturator fascia is attached at the circumference of the
muscle. There it becomes continuous with the periosteum of the
os innominatum. The part above the white line (supra-linear) is
intra-pelvic ; the part below (infra-linear) is perineal and situated
on the outer wall of the ischio-rectal fossa.

II. The Recto-vesical and Anal Fasciae. — The levatores ani form
a muscular floor for the pelvis, stretching from the white line of
one side to the white line of the other. The sheath on their under
surface — on the inner wall of the ischio-rectal fossa — forms the
anal fascia. On the upper surface, their sheath forms the greater
part of the recto-vesical fascia. The pelvic viscera rest on the
upper surface of the levatores ani and the capsules of the viscera
are continuous with the sheath on the upper surface of the
muscles. The combined visceral capsules and upper sheath of
the levatores ani form the recto-vesical fascia.

III. The Triangular Ligament is the sheath of the constrictor
urethrae muscle (Fig. 112). The inferior transverse fibres of the
constrictor form really a separate muscle — the deep transverse
perineal. The apex of the prostate rests on the muscle, its

PUBO-FEMOKAL REGION.

141

capsule being continuous with the posterior layer of the muscle
sheath — the deep layer of the triangular ligament.

IV. The inner sheath of the pyriformis forms the pyriform
fascia. As the muscle arises between the sacral foramina, the
sacral plexus lies within the sheath, the iliac vessels on its inner
aspect. The coccygeus is continuous with the levator ani and its
sheath forms part of the recto- vesical fascia.

The Cervical Fascia. — From what has been said of the pelvic
fascia, the nature and arrangement of the cervical fascia will be
readily understood. It is composed of (1) the sheaths of the
cervical muscles (sterno-mastoid, etc.) ; (2) of the sheaths of
vessels (carotid sheath, etc.) ; (3) the sheaths of nerves (axillary
sheath, etc.) ; (4) the fascial capsules of viscera, such as the
thyroid body, salivary glands, and pharynx.

constrictor urethrae

Fig. 112. — The Constrictor Urethrae Muscle.

CHAPTER XI.

THE SPINAL COLUMN AND BACK.

The Pyramids of the Spine. — The spine, when viewed from
the front, is seen to be made up of four pyramids: (1) Cervical;
(2) upper dorsal ; (3) dorso-lumbar ; (4) sacro-coccygeal (Fig. 113).

The bases of the two upper pyramids meet at the disc
between the 7th cervical and 1st dorsal vertebrae; the bases

THE SPINAL COLUMN AND BACK.

143

of the lower two at the disc between the 5th lumbar and 1st
sacral vertebrae. The apices of the two middle pyramids meet
at the disc between the 4th and 5th dorsal vertebrae, which
have therefore the narrowest bodies of the vertebral series.
The narrowing in the upper dorsal region is due to the
fact that the weight of the upper half of the trunk is
partly borne by, and transmitted to, the lower dorsal region by
the sternum and ribs which thus relieve the spine to some
extent (Fig. 113). At the sacrum the weight is transferred to
the pelvis and lower limbs, hence the rapid diminution of the
sacrum and coccyx. A well-marked thickening or bar in each
ilium runs from the auricular surface to the acetabulum and
transmits the weight to the femora.

The Curves of the Spinal Column. — There is only one curve—
an anterior concavity — until the 3rd month (Fig. 114 A). About

. A

B

Fig. 114.— Diagram of the Curves of the Spinal Column.

A. At the 6th week of foetal life. B. At the 4th month of foetal life. C. Curves
present at Birth. B. Curves present in the Adult.

the beginning of the 4th month the sacro-vertebral angle forms
between the lumbar and sacral regions (114 A). At birth the
cervical and sacral curves have appeared, but the sacral not to a
pronounced extent (Fig. 114 C ). The lumbar curve appears as
the child learns to walk. It is produced to allow the body
to be brought vertically over the lower extremities. The sacral

144

HUMAN EMBRYOLOGY AND MORPHOLOGY.

and cervical curves also become then more marked (Fig. 114 D ).
The dorsal curvature and the sacro-vertebral angle are the primi-
tive curves and are present in all mammals. The others are
adaptations to the upright posture. The lumbar curve is most
pronounced in the highly civilized races.

Proportion of Cartilage and Bone. — The inter-vertebral
discs form one third of the total height of the spine ; the pro-
portion of cartilage is greater in the lumbar than in the dorsal
region and greater in the dorsal than in the cervical. The
curvatures are due chiefly to the shape of the discs. In the
lumbar region, which is convex forwards, only the lower three
vertebrae are deeper in front than behind. This is true only for
the higher races of mankind, for as Cunningham has shown, in
lower races, as in the gorilla, only the lowest lumbar vertebra is
deeper in front than behind, and thus helps to maintain the
lumbar curvature.

Unstable Regions of the Spine. — In about 90 °/0 of men

there are 7 cervical, 1 2 dorsal, 5 lumbar, 5 sacral, and 4 caudal
vertebrae, making 33 in all. In the remaining 10 °/0 there is
some departure from the normal arrangement and these departures
affect certain definite regions.

I. The sacro-lumbar.— The 25 th vertebra in 95 0/o of people
forms the 1st sacral; in 1 °/0 the 24th, and 3 °/0 the 26th.

24th vertebra
ilium

26th vertebra
27th

Fig 115.— A section of the Lumbo-sacral Region of the Spine in a Foetus at the end
of the 2nd month, showing the 26th vertebra forming the 1st Sacral. (After
Rosenberg.)

These percentages are drawn from the observations of Paterson,
Rosenberg, and others who have made researches on this subject.
The vertebral formula is not fixed. Rosenberg’s investigations
showed (Fig. 115) that it is the 26th vertebra that forms the

THE SPINAL COLUMN AND BACK.

145

first of the sacral series in the early embryo; later the 25th
throws out great lateral masses, and thus forms a connection with
the ilia. In the lower primates (monkeys) the 27th forms the
1st sacral; with the evolution of man the 26th, then the 25th
underwent sacral modifications, the trunk being correspondingly
shortened. It will be seen that the number is not yet definitely
fixed. The anterior point of attachment of the ilium fluctuates
from the 24th to the 26th vertebra in man. With the sacral
transformation of the 25tli and 26th (lumbar) vertebrae, there
was a corresponding movement forwards of the sacral plexus.

II. Sacro-coccygeal. — The 30 th vertebra forms the 1st coccy-
geal ; not uncommonly this vertebra is sacral in type and forms
part of the sacrum.

III. Dorso-lumbar region. — This region is less liable to variation;
the 20 th vertebra instead of forming the 1st lumbar, may
simulate the last dorsal in the type of its articular processes,
and may bear ribs, probably an atavistic form, or, on the other
hand, the 12th dorsal vertebra (19th) may not carry ribs.
About 2 °/Q of bodies show variations of this kind.

IV. Dorso-cervical. — The 7th vertebra may carry ribs; rarely
the 8th vertebra has no ribs attached to it and is cervical in
type.

Evolution and Development of the Spinal Column. — The

human spinal column in the process of development passes
through three distinct phases :

(1) It is membranous; (2) it becomes cartilaginous; (3) it
becomes bony. In the evolution of vertebrates the same three
stages are observed. In Amphioxus and Marsipobranchs (except-
ing the neural arches), the spinal column is membranous ; in
Elasmobranchs it is cartilaginous ; in other fishes and in all other
vertebrates it is ossified. In the human embryo, as in every other
vertebrate, the spinal column is developed from the mesoblast
which surrounds the notochord and neural canal.

The Notochord. — It has already been seen that the notochord
is formed at a very early stage (before the 3rd week, Fig. 69,
p. 90) by a tubular invagination of the hypoblast under the
neural canal. The notochord, with the mesoblastic tissue round
it, represents the most primitive type of spinal support. It is
hollow — the canal of the notochord runs from end to end, and

K

lib HUMAN EMBRYOLOGY AND MORPHOLOGY.

into its posterior end the neurenteric canal opens. Afterwards it
becomes a solid rod composed of cells of a peculiar tYpe. A
sheath is formed round the notochord by cells of the paraxial
mesoblast (Fig. 116), which grow inwards and surround it.

Fig. 116. — Diagrammatic transverse section of a human embryo at the end of the

3rd week.

These cells form the sclerotome and spring from the inner parts
of the primitive segments (Fig. 116). At the same time the
cells of the sclerotome also surround the neural tube. From
these cells which grow inwards and surround the notochord and
neural canal, the spinal column is formed and also the basi-
occipital and part of the basi-sphenoid bones of the skull (Fig.

117).

What becomes of the Notochord. — In the second month of
foetal life the notochord begins to disappear ; the bodies of the
vertebrae and parachordal cartilages form in its sheath and
constrict it. The parachordal cartilages form the basi-occipital
and basi-sphenoid — the basal part of the skull — behind the
pituitary fossa. As they form, the notochord is obliterated

between them. Eternod, however, found the anterior part of
the notochord on the dorsal wall of the pharynx in the human
embryo, so that the parachordal cartilages are evidently de-
veloped on its dorsal aspect. The odontoid process represents
the body of the atlas and the suspensory ligament the disc
between the occipital bone and atlas. A remnant of the noto-

TIIE SPINAL COLUMN AND BACK.

147

chord is enclosed in the suspensory ligament. The body of each
vertebra is formed round the notochord and at first each has
an hour-glass canal surrounding the notochord. Within each

pituitary
- basi-sphen .

basi-occip.
suspensory lig.
E3 """" — axis.

interuert. disc.

i

%

centre of disc

r

coccyx.

Fig. 117. — Whore Remnants of the Notochord may occur in the Adult.

body the notochord ultimately disappears but in the inter-
vertebral discs it swells out and forms much of the central
mucoid core which each disc contains. The notochord with its
membranous sheath is the earliest form of spinal column known.
The real vertebral column, formed out of its sheath, begins to
supplant it even in low vertebrates and in the human foetus of
the second month this change is also seen to take place.

Proto-vertebrae or Primitive Segments. — Proto-vertebrae
are not the forerunners of the vertebrae ; they are the primitive
segments into which the mass of mesoblast at each side of the
neural canal and notochord divides (Figs. 233, p.289 and 116).
The process of division or segmentation begins at the occipital
region towards the end of the second week and spreads backwards
until 35 or more body segments or somites are cut off. Eacli
segment thus separated forms its own muscles (from its muscle

148

HUMAN EMBRYOLOGY AND MORPHOLOGY.

plate or myotome), has its own nerve (spinal nerve), its own artery
(inter-costal), and the basis for its skeletal tissue (sclerotome). The
inter-segmental septum separates one proto-vertebra or segment
from another. Ribs, transverse and spinous processes, are formed
in the inter-segmental septa. Hence an intercostal space with
its muscles, vessels, and nerves, with the corresponding inter-
vertebral structures, represents a differentiated proto-vertebra. In
the ventral aspect of the neck and loins, the inter-segmental septa
disappear. In the head nine segments are recognised, but their
recognition rests on observations made, not on the human
embryo, but on the embryoes of lower vertebrates.

Recent work on the segmental arrangements of the nerves
gives a practical importance to the number and position of the
primitive segments and to the part of the body which each
forms. Although the epiblast, hypoblast, and walls of the coelom
never show any trace of segmentation in the embryo, yet clinically
there is evidence that each part belongs to, and was formed from,
one definite segment. The upper extremity grows out from the
5th cervical to the 2nd dorsal segments (7 segments in all), and
the lower from the 2nd lumbar to the 3rd sacral (7 segments in
all). In each limb traces of these seven segments are to be found
in the nerve distribution.

Development of a Typical Vertebra,— the 6th Dorsal. —
(1) Membranous Stage (3rd and 4th weeks). The vertebra then
consists of 1st a body surrounding the notochord, formed from
the sheath (Fig. 118 A), and 2nd a horse-shoe shaped vertebral
bow (Fig. 118 A and B). The bow consists of a hypo-chordal part,
and two lateral limbs, united by the hypo-chordal part ventral to
the body.

(2) Cartilaginous Stage (Fig. 119). It commences in the 4th
week. The fibrous basis is transformed into cartilage except the
hypochordal part of the bow. It should be noticed (Fig. 118 B)
that the vertebral bodies are formed round the notochord, opposite
each inter-segmental septum. Hence each vertebra belongs to two
segments. The inter-vertebral disc is situated opposite the middle
of a segment. The hypo-chordal part of the bow lies also in
the segment, and becomes included in the course of development
in the disc in front of the vertebra to which it belongs.
The lateral limbs of the cartilaginous bow meet behind (dorsal

THE SPINAL COLUMN AND BACK.

149

to) the neural canal in the 4th month, and thus the bow is
converted into a ring. The atlas is a permanent representative

notochord

liypoch. bow

body of uert.
myotome
septum
rib

myotome
hypochord. bow
body o f uert.
spirit-, — septum

rib

myotome

hypochord. bow
B notochord

Fig. 118. — The development of the Membianous Basis of a Vertebra.

A. In transverse section. B. In horizontal section showing the relation of the
vertebra to the Primitive Segments. The section is viewed from the dorsal
aspect.

of the ring thus formed. In the atlas only does the liypo-chordal
part of the bow become cartilaginous and subsequently ossified ; in

neural canal

bow

sheath
notoch.
hypoch. bow

epiph. centres
costa! facet

neur. arch

notoch.

centrum

s'

MM-

plate

centrum

neuro-cent. sut. £ plate

centre
for arch

euro-cent.

sut.

centres for body

cauity

centrum

Fig. 119.— Showing the Stages in the Development of a Vertebra.

A. In the Membranous Stage. B. In the Cartilaginous Stage. C. The appearance
of Ossific Points. D. The appearance of Secondary Ossific Centres, h. Ihe
epiphyseal plates of the centra. F. Section of an amphicoelous vertebra.

all the other vertebrae it remains as a fibrous strand in the inter-
vertebral disc.

(3) Bony Stage. — The body and bow parts of the cartilaginous

150

HUMAN EMBRYOLOGY AND MORPHOLOGY.

vertebra fuse and give rise to the condition shown in Fig. 119 C.
In the 7th week two centres appear in the body, but quickly fuse ;
one appears in each neural arch (8th week) ; at birth the ossific
centres of the body and neural arch have met. The central and
neural ossifications meet at the neuro-central suture and unite at
the 4th or 5th year. The neural ossifications fuse behind (where
the spinous process is produced) in the 1st year. The ribs are
entirely supported from the neural ossifications (Fig. 119 D ). The
spinous and transverse processes are formed by outgrowths of
cartilage into the septa between the proto-vertebrae or primitive
segments. The ribs are also formed in the septa, each at first
articulating with a vertebra, which is also inter-segmental in
origin. In all the ribs, except the 1st, 11th and 12th the head
is shifted to the inter-vertebral disc in front of its vertebra.
Epiphyseal centres for the ossification of the transverse and spinous
processes appear about puberty.

The Bodies of Mammalian Vertebrae are peculiar (1) in the
development of an upper and lower epiphyseal plate ; (2) in that
no trace of the notochord remains within them. In Fishes, as in
the early human or mammalian foetus, the bodies are hour-glass
shaped (amphicoelous, Fig. 119 F) ; in Amphibians they may
retain a concavity in front (procoelous) or behind (opistliocoelous),
but in mammals both ends are filled up.

F

h hi * 1 * * 1 *

U 1 * 1 5 1 g si* * * M * M * M •• 1 ■■ 1 • 1 1

Axis in 4th month

< — 7th week > 5th Sac. in 5th month

A.

1st cent, in Atlas.

E

: l : 1 : 1 : \ ; 1 : 1 :

\ : : i . m : i ; i i ~T~m zn

7 th week

B.

3rd Sac. in 7th month

Fia. 120. — The Order in which the Centres of Ossification appear in the Bodies (A)
and in the Neural Arches (B) of the Spinal Column.

It will be observed (Fig. 120 B) that the centres of ossification
for the neural arches appear first in the anterior end of the spine
(1st cervical), the date becoming later the more posterior the
vertebra. In the 1st sacral they appear about the 4th month ;

THE SPINAL COLUMN AND BACK.

151

iu the 2nd sacral in the 5th month or later; in the 3rd they
may not appear. In the 4th and 5th sacral and 1st coccygeal
vertebrae only vestiges of the neural arches are formed. These
vertebrae retain the early foetal type shown in figure 119 B.
In the remaining coccygeal vertebrae only the centres for the
bodies appear. The centres for ossification of the bodies of the
vertebrae appear first in the mid dorsal region (6th dorsal). From
that point they spread forwards and backwards, the centres for
the odontoid process appearing at the 4th month and that for
the 5th sacral at the 5th month, while the last coccygeal does
not appear until about the 20th or 25th year (Fig. 120 A).

The Atlas and Axis. — The atlas represents the completed
bow of the 1st cervical vertebra. The body of the vertebra
fuses with the body of the 2nd, and forms the odontoid process.
A remnant of the disc between the 1st and 2nd vertebrae can

sometimes be seen when the odontoid is split open. The sus-
pensory ligament is the representative of the disc between the
last occipital segment and the 1st cervical (Fig. 121).

Occipito-atlanto-axial Articulations. — In the intervertebral
discs of the cervical region there is at each side, between the
lateral lips of the vertebral bodies, a small articular cavity (Fig.
122). It is situated between the parts of the body formed from
the neural arches and in front of (ventral to) the issuing spinal
nerves. Between the axis and atlas this articulation is greatly
enlarged. At it the rotatory movements of the atlas on the axis

nuiuunuru

Fig. 121.— A diagrammatic section of the Foetal Axis, Atlas, and Basi-occipital.

152

HUMAN EMBRYOLOGY AND MORPHOLOGY.

take place. The atlanto-occipital joint, which separates the atlas
and the last occipital segment is of the same nature. The atlas
has neither the upper nor the lower articular processes of the other
vertebrae. Hence the 1st and 2nd cervical nerves appear to issue
behind the articular processes. At one time the single median

occipital condyle seen in birds and reptiles was regarded as very
different in nature from the double condyles of mammals.
Recently Symington has shown that in the lowest mammals
(monotremes), the occipital condyles are fused in the middle line,
and that foetal mammals show approximations to this condition.
In the human skull a remnant of this median fusion of the
condyles is frequently seen on the anterior margin of the foramen
magnum ; it is named the third or median occipital condyle.
Sometimes the atlas is partly fused with or imperfectly separated
from the occipital bone. This seems to be a further manifestation
of the process which ha!s led to four segments, which were at one
time free body segments, being fused together to form the occipital
part of the skull.

The Ribs are developed in the septa between the dorsal
primitive segments. At their vertebral ends they come in
contact with the vertebral bow (Fig. 118 B). In lower vertebrates
(birds, reptiles, etc.) each rib has two heads, a dorsal and ventral
(Fig. 123). The tuberosity of the human rib represents the
dorsal head ; the ventral head is well developed in man, as in
mammals generally. The rib articulates with the neural arches
only (Fig. 119 D ).

Vestigial Ribs. — Although the ribs are only fully developed
in the dorsal region, yet a representative — a costal element — is

occip.

artic.

1st neural arch

artic.

2nd neural arch
artic.

3rd neural arch
3rd centrum

Fig. 122. — The nature of the Atlanto-axio-occipital Articulations.

THE SPINAL COLUMN AND BACK.

153

present in every vertebra. In the cervical vertebrae the anterior
part of the transverse processes represents a costal process, but
only in the 6 tli (sometimes) and 7th is the costal process formed

Fig. 123. — The Bicipital Rib of a Lower Vertebrate (crocodile).

by a separate centre of ossification. The costal process of the 7 th
may develop into a rudiment or even a fully formed rib which
reaches the sternum. In the lumbar vertebrae only the first shows
a separate centre for the formation of the costal process ; it fuses
with the tip of the transverse process in the later months of foetal
life ; in the other lumbar vertebrae the tips or perhaps the whole
of the transverse processes represent costal processes. The 12th
dorsal rib varies widely in size ; it may be six inches or ten
long or reduced to a mere vestige.

In the 1st, 2nd and 3rd sacral vertebrae the costal processes are
large and have their own centres of ossification. Their cartila-
ginous bases fuse early to form the greater part of the lateral
masses of the sacrum (Fig. 115). The part of the lateral mass

neural arch
trans. proc.
cost proc.
epiphysis

centrum

Fig. 124.— A section to show the Nature of the Elements composing the Sacrum.

formed by the costal processes is shown in Fig. 124. The costal
processes are absent in the 4th and 5th sacral and all the
•coccygeal vertebrae. The two lateral epiphyseal plates on each
side of the sacrum are new and independent formations.

The Accessory Processes are found in the lumbar and lowest two

154

HUMAN EMBRYOLOGY AND MORPHOLOGY.

dorsal vertebrae. They are developed at the base of the trans-
verse processes and are for the attachment of slips of the
longissimns dorsi. The mammillary processes are developed on
the articular processes of the lower two or three dorsal and all
the lumbar vertebrae. They give attachment to tendons of
origin of the multifidus spinae.

The Transverse and Spinous Processes grow out from the
vertebral bow (Fig. 119 A) into the septa between the primitive
segments. Each transverse process is pierced, while still in the
fibrous condition, by a branch of the corresponding segmental
(intercostal) artery. In only the cervical region do those per-
forating arteries and their foramina persist. In that region the
perforating arteries anastomose and out of the chain thus formed
is developed the vertebral artery. Thus the foramina for the
vertebral artery are formed independently of the costal element
in each cervical transverse process. The spines are absent on
the 1st cervical, 4th and 5th sacral and coccygeal vertebrae.
They are slightly developed and united by ossification of the
interspinous ligament in the 2nd and 3rd sacral vertebrae. The
2nd, 3rd, 4th, 5th, and 6th cervical spines are bifid in Europeans;,
but in lower races, as in anthropoids, the 5th and 6th spines are
undivided.

CHAPTER XII.

THE SEGMENTATION OE THE BODY.

Segmentation of the Body. — The human body or trunk
consists of 33 or 34 segments. Each segment is fundamentally
of the same type, but the resemblance is obscured owing to ex-
tensive modifications which they undergo to form the cervical,
dorsal (thoracic), lumbar (abdominal), sacral (pelvic) and caudal
regions of the body. The outgrowth of the limbs also renders it
difficult to recognise in the adult the simple system of segments
which can be seen in the embryo at the end of the third week
(Fig. 233, p. 289).

Until lately the segmentation of the human body was a matter
of only speculative importance, but recent advances in our
knowledge of the distribution of nerves, has shown that it has a
direct bearing on diagnosis and treatment.

Constitution of a Typical Segment (11th Dorsal). — It is
better to study the development of one typical body segment and
from that the student will be able to note for himself the
modifications which have taken place in the more highly differ-
entiated segments of the body. By the end of the third week,
the process of segmentation, which began in the occipital
region a few days previously, has spread backwards and
separated the 18th body segment (11th dorsal) from the one in
front and behind. As already explained, the process of segmen-
tation affects only the paraxial block of mesoblast which lies on
each side of the neural canal and notochord. In Figs. 125 and
126 a segment is represented in the adult and in the embryonic
condition.

The following elements make up the 11th dorsal segment: (1)
Its skeletal basis ; (2) Muscular element ; (3) Renal element ;

156

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(4) Vessels ; (5) Nerves; (6) Neural segment. Although the
epiblast and hypoblast are never segmented, yet a definite area
of each is associated with every body segment. The origin of
each element will be taken separately.

lat. dorsi

quad. lumb.

lumb. aponeur.

Kidney
( interned,
cell-mass)

nt. ob.

somato-pleure

Fio. 125.

ilio-costalis

uertebra ( sclerotome y

neural canal

- post . root. gang.

,musde plate
-shin plate
- sclerotome
-notochord
■aorta

r interned .

\ ceil -mass,
splanchno-pleure

somato-pleure

coelom
gut

Fit:. 126.

Fio. 125.— A transverse section showing the Elements of the 1st Lumbar Segment in

the Adult.

Fig. 126.— A corresponding section of an Embryo about the end of the 3rd week

(diagrammatic).

T. The skeleton of the 11th dorsal segment is represented by
the adjacent halves of the 11-12 dorsal vertebrae and the disc
between them, for, as already pointed out, the vertebrae are inter-
segmental in origin (Fig. 126). The transverse processes, the
spinous process and 11th and 12th ribs are also formed in the
septa in front of and behind the lltli segment. The septum in
the rectus muscle a little below the umbilicus represents the
inter-segmental septum corresponding to the 11th rib. Sometimes
another septum occurs in the rectus, midway between the pubes
and umbilicus, marking the lower limit of the 11th segment.
The linea alba separates the segments of the two sides.

In the linea alba or ventral median line of the thoracic region,
the sternum is developed. The inter-segmental septa are well

tiip: segmentation oe the body.

157

marked in the thoracic region ; the ribs and their cartilages
are developed in them. In the neck the septa are almost lost ; the
intermediate tendon of the omo-hyoid and the septa occasionally
found in the sterno-hyoid and -thyroid, complexus and trachelo-
mastoid muscles are the only representatives of them in the cervical
region.

II. The Muscles of the 11th Dorsal Septum. — All the muscles of
this segment are developed from the muscle plate of the primitive
segment (see Figs. 125 and 126). There is a cavity, which pro-
bably arises as a diverticulum of the coelom, in each primitive
segment (Fig. 6 9, p. 90). The cells of the mesoblast on the inner
side of the segmental cavity become columnar and form the
muscle plate (Fig. 126). Each segment has its own muscle plate.
The cells of each plate increase rapidly in number ; they spread
into the somatopleure, and form the muscles of the body-wall
and limbs. Each cell becomes elongated and directed across its
segment from septum to septum. The intercostal muscles retain
this arrangement, but in the abdominal region the fibres fuse
with those of neighbouring segments to form muscular sheets —
the external oblique, internal oblique, transversalis and rectus.
In fishes the embryonic segmental arrangement of the muscu-
lature persists. The manner in which the final groups of
muscles are derived from the muscle plates is not known, but
in the typical segment with which we are at present dealing it
will be seen that the musculature falls into two groups (see
Fig. 125): (1) axial (acting on the spine — the erector spinae,
etc.), and (2) ventro-lateral or body-wall muscles (intercostals,
rectus, oblique muscles, etc.). The musculature of the limbs is
derived from the ventro-lateral group.

Many of the ventro-lateral muscles (trapezius, rhomboids,
and latissimus dorsi), migrate dorsalwards over the axial muscles
and take origin from the spines of the vertebrae (Fig. 125).

Each muscle fibre is a cell derived from the endothelial cells
which make up the muscle plate. The protoplasm of each cell
is converted into a living contractile substance (myosin), which
reacts to nerve stimuli.

III. The Arteries of the 11th Segment (Fig. 127). — The 11th
intercostal is the artery of the segment. It gives off a dorsal branch
to supply the axial muscles, the spinal column, spinal cord and

HUMAN EMBRYOLOGY AND MORPHOLOGY.

membranes and skin. The segmental artery joins at its termina-
tion with a ventral longitudinal vessel, the deep epigastric. The
primitive arrangement in vertebrates appears to have been a

Fig. 127. — The distribution of a typical Segmental Artery.

■dorsal and ventral longitudinal vessel, with the segmental artery
passing between them. The vertebral, ascending cervical, deep
cervical, ascending lumbar and lateral sacral arteries are examples
of the anastomoses that may arise between segmental arteries.

Segmental arteries also arise from the aorta to supply the
structures formed from the intermediate cell mass (the kidney,
testis, ovary, etc., Figs. 125 and 127). As a rule only one renal
segmental artery persists, but frequently accessory renals are seen.
These are persistent embryonic vessels of the several segments
from which the mesoblast of the kidney arose. The splanclmo-
pleure shows no trace of segmentation ; hence its vessels (coeliac
axis and mesenteric) are not segmental in origin.

IV. The Nerve Elements of the 11th Segment (Fig. 128).—
(1) Although the spinal cord of the human embryo shows no
certain sign during development of being definitely divided into
segments corresponding to those of the body, yet from whatwe know
■of its condition in embryoes of other animals and from clinical

TIIE SEGMENTATION OF THE BODY.

159

evidence there can be little doubt that such a segmentation does
take place, and that it possesses segments corresponding to those
of the body. A group of cells in each segment, afterwards those
of the anterior horn, sends out processes to all the muscles of the
primitive body segment in which it is situated. The anterior
root of a spinal nerve is thus formed. Other motor cells send out
processes which reach viscera through the white rami comiiinni-
cantes and sympathetic system (Fig. 128).

Fig. 12S. — Diagram of the Nerve System of the 11th Dorsal Segment.

Fig. 129. — A diagram showing the derivation of the Parts of the Nerve System of
the 11th Segment in the Embryo.

(2) Besides the anterior horn cells, two other nerve groups
become connected with each segment. At the point where the
medullary plates are cut off from the epiblast to form the neural
canal, a crest, the neural crest, grows out on each side (Fig. 129)
composed of the cells which formed the junctional line between
medullary plates and epiblast. A group of these neuroblasts grows
into each segment and forms the posterior root ganglion. Each
neuroblast within the ganglion sends out a process which bifur-
cates, one branch or fibre growing into the cord and ending in the
posterior column and cells of the posterior horn, the other passing
to the skin, muscles, etc., of the segment. The posterior nerve
root is thus formed by the ingrowing processes of the cells of the
posterior root ganglion, and thus the body segment in which the
outgrowing processes are distributed is brought into sensory com-
munication with the central nervous system. The anterior and
posterior roots unite to form a spinal or segmental nerve. Like
the artery it divides into a posterior division for the epaxial part

160

HUMAN EMBRYOLOGY AND MORPHOLOGY.

of the segment and an anterior for the ventro-lateral part (Fig. 128)

(3) A third group of cells, the sympathetic, is also connected with
each segment. The origin of these cells is not yet certain, but
most ot the evidence goes to show that the cells are derived, with
the posterior root ganglion, from the neural crest and that a group
enters each segment. Professor Paterson’s research on their origin
led him to the conclusion that the nerve cells of the sympathetic
arose from the mesoblast. The sympathetic group of nerve cells
(Figs. 128 and 129) is broken up into —

(a) The prevertebral ganglion situated on the vertebra (in the
gangliated chain), ventral to the exit of the spinal nerve ;

(b) A group to the intermediate cell mass (renal ganglion) ;

(c) Another to the splanchnopleure (in the semilunar ganglia) ;

(cl) To the viscera (cells of Auerbach’s plexus, etc.).

Groups (c) and (d) show no trace of segmentation in their
arrangement, but, clinically, evidence is to be found that every
viscus or part of a viscus is connected with certain segments of
the spinal cord. The cells of the sympathetic ganglia throw out
axis-cylinder processes, which pass to the spinal cord by a white
ramus communicans and posterior root, and act as sensory path-
ways from the viscera. The distal end of the axis-cylinder
process ends in a viscus. In this manner certain segments of the
spinal cord are brought into touch with the viscera. The vaso-
motor supply of each body segment passes to it from the sym-
pathetic ganglia by a grey ramus communicans.

It will thus be seen that all the parts of a segment — body
wall (somatopleure), kidney (intermediate cell mass), and a part
of the abdominal or thoracic viscera (splanchnopleure) are con-
nected by nerves to a corresponding segment of the spinal cord.
In diseased conditions of any part of the body segment the
corresponding spinal segment of the cord is disturbed, the dis-
turbance being reflected from the cord to the segment. The
nervous mechanism of the whole segment is affected. Thus, tor
instance, a stone in the pelvis of the kidney (which is supplied
from the 10 th, 11th, and 12 th dorsal segments) is frequently
accompanied by pain referred along the 11th and 12 th inter-
costal nerves. The skin supplied by these nerves may become
hyper-aesthetic.

CHAPTER XIII.

THE CRANIUM.

Development of the Skull. — The facial parts of the skull
have already been dealt with (Chap. I.). Only those bones which
enter into the formation of the cranial cavity and help to form
the brain chamber are dealt with here. These bones are the
frontal, parietal, occipital, temporal, ethmoid and sphenoid.

Is the Skull made up of Segments ? — We have just seen
that the body is made up of 33 or more segments. Is the skull
made up of a series of segments ? The theory supported by
Owen and many others that the cranium is really composed of 4
modified vertebrae is now no longer tenable. On the other hand
the arrangement of the nerves and muscles, the evidence of
development and comparative anatomy, indicate that it is com-
posed of a number of segments, probably nine in number.
The four posterior, which form the occipital region of the
skull, are recognisable at an early stage of development, but
at no period in the development of the embryo have the anterior
five segments been seen to be demarcated.

The Primitive Membranous Skull. — The brain is developed
in the same manner as the spinal cord from the medullary plates
of the neural groove (Fig. 69, p. 90). In the same manner
the mesoblast grows under and over the cephalic part of the
neural canal, and forms for it a membranous covering. The
covering of mesoblast thus formed is the primitive basis of the
skull in the embryo.

The Evolution of the Mammalian Cranium. — It is not

possible to understand the manner in which the bones of the
human cranial cavity are developed without some reference to

L

162

HUMAN EMBRYOLOGY AND MORPHOLOGY.

comparative anatomy. Only the base of the human skull is
developed in cartilage, the rest is developed in membrane. How
has that come to be ? The brain of amphioxus, if it can be said
to possess one, is wrapped in a membranous covering. In fishes
with cartilaginous skeletons this embryonic mesoblastic capsule
becomes chondrified — plates of cartilage develop in it. As in the
spinal column, the process of chondrification begins at the base
and spreads slowly round to the crown or dorsum of the head.
The cartilaginous cranium is an advance on the membranous
stage. In many fishes a further most important element is

added. The dermal bony plates, to which the placoid scales are

fixed, are applied to the cartilage over the sides and dorsum
of the skull. Thus to the cartilaginous element of the skull
is added a third element — bone formed in membrane. Now in
the mammalian skull, and especially in that of man, the cerebral
vesicles grow so quickly that long before the process of chondrifi-
cation has had time to spread in the membranous capsule from
the base to the crown, the dermal bones have formed, and thus
supplant the cartilage on the calvarium. Hence in the human
skull, while the process of chondrification occurs in the base, and
afterwards undergoes ossification, the whole calvarium and sides
of the skull are formed by bones which, historically, are dermal
bones, and hence are formed directly in membrane. The dermal
bones of the human skull are: (1) the frontal, (2) the parietal,
(3) the inter-parietal part of the occipital (the part above the
superior curved lines), (4) the squamous part of the temporal.

Thus the calvarial part of the skull passes directly from the
membranous to the bony stage, while the base of the skull, like
the spinal column, passes through three stages: (1) membranous,
(2) cartilaginous, (3) bony. It will be thus seen that the base
of the skull, developed in cartilage, is the most ancient part,
while the dermal bones, which form the calvarium, represent a
later addition.

Development of the Roof (membranous or dermal part)
of the Skull. — In the 7th week of foetal life there appear on
each side of the membranous cranial capsule four centres of
ossification :

(1) For the frontal bone at a point which becomes afterwards
the frontal eminence (Fig. 130);

THE CRANIUM.

163

(2) For the parietal, at the position of the parietal eminence ;

(3) For the squamosal — at the base of the zygoma (Fig. 130) ;

(4) For the membranous part of supra-occipital (part above
superior curved line).

Fig. 130.- -The Centres of Ossification for the Dermal Bones of the Skull. The Bones
which are formed in Cartilage are stippled.

The two occipital centres fuse early into one at the position of
the external occipital protuberance. The two frontal ossifications
fuse about the end of the first year ; the metopic suture which
separates them disappearing then. This suture occasionally per-
sists. The parietal bones fuse together, at the sagittal suture,
late in life, commonly between the 35th and 45th year. The
squamosal partly covers the petro-mastoid cartilaginous element
and fuses with it in the first year, the temporal bone being thus
formed.

The Manner in which these Bones are Developed. — In

Fig. 131 a vertical section of the skull of a foetus 44 months
old is represented. The coverings of the brain are seen to be then
(1) scalp, (2) a stout white fibrous capsule, (3) a fine membrane
lining it — the inner layer of the dura mater — (4) the arachnoid
covering the brain (not shown in figure). Spicules of bone which
form the parietal are seen developing within the fibrous capsule
and radiating out from the centre of ossification. Lower down

164

HUMAN EMBRYOLOGY AND MORPHOLOGY.

are seen the ossifying fibres of the squamosal. The base of the
skull is formed of cartilage which is covered, or ensheathed, by a
perichondrium continuous with the membranous capsule. In the
cartilage appear the centres of ossification for the sphenoid.

Fio. 131.— A coronal section of the Skull of a Foetus, 41 months old.

As the bony spicules of the parietal spread out, they divide
the primitive cranial capsule into an outer layer — the peri-
cranium— and an inner — the periosteal layer of the dura mater.
At the periphery of the bone and in the sutures the continuity of
these two layers persists. The growth of the spicules of bone
keeps time with the growing brain which expands the capsule, but
there is, at each corner of the parietal bone, until the end of the
first year, a part of the primitive cranial capsule left unossified.
These unossified parts of the membranous capsule are the
fontanelles.

The Fontanelles.— There are five fontanelles connected with
each parietal bone, one at each of its rounded angles, and one,
the sagittal (Fig. 130) which occurs between the radiating fibres
of the parietal near the posterior end of the sagittal suture. The
parietal foramen marks its position in the adult. In about 15°/0
of children this fontanelle is unclosed at birth ; a large parietal
foramen may permanently mark its situation. The posterior
inferior fontanelle, situated at the asterion (Fig. 130), the anterior
inferior at the pterion, and the posterior superior at the lambda,

THE CRANIUM.

165

close before or about the time of birth. Separate ossifications,
which become Wormian bones, are often developed in the primi-
tive capsule of the skull at those three fontanelles and thus close
them. The anterior superior fontanel le, at the bregma, cannot be
distinctly felt during life after the first year (Warner), but it is not
completely closed until the second year is nearly over. This
fontanelle is lozenge-shaped, being bounded by four bones, viz.,
the two parietals and two frontals. The bregmatic or anterior-
superior and lambdoid or posterior-superior fontanelles are median
and common to both parietals.

The membrane-formed bones consist at first of a thin lamella
of osseous fibres radiating out from the point at which ossification
commenced. The osteoblasts beneath the pericranium on the
outer surface of the lamella and the dura mater on the inner
surface deposit bone, and by the 5th year an outer and inner
table, with deploic tissue between, are developed. Into the
diploe of the frontal bone protrude the growing buds of the two
frontal sinuses. As the brain expands new bone is formed at
the sutures to increase the capacity of the skull, but the opera-
tion of craniotomy to allow the expansion of a confined brain,
by the formation of a new suture, is founded on a wrong
principle. Expansion of the cranial cavity takes place princi-
pally by a deposit on the outer table and an absorption from
the inner ; for this manner of growth, sutures are unnecessary.
The synostosis of the sutures does not necessarily prevent growth ;
synostosis of the skull bones occurs only when the brain has
ceased to expand. If the brain of the infant is arrested in its
growth, premature ossification of the sutures occurs, the condition
of microcephaly resulting therefrom. In hydrocephalus, when the
ventricles become enormously dilated, the membranous capsule of
the cranium expands so quickly that the process of ossification
cannot keep up with its rapid growth. Hence in hydrocephalus
the fontanelles are enormous. The cartilaginous part of the skull
is scarcely affected in this disease. The membrane-formed part
of the skull is liable to diseases which do not affect the cartilage-
formed part. The dura mater is very adherent to the bones
formed in cartilage.

Development of the Cartilaginous part of the Skull.

(1) The Occipital Bone. — The occipital bone is developed from

166

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the parachordal cartilages, two cartilaginous bars which partly
surround the cranial part of the notochord (Figs. 133 and 121).
The parachordal cartilages represent in the skull the cartilaginous
sheath of the notochord out of which, in the spinal column, the
bodies of the vertebrae are developed. Each cartilage throws
out a wing (Fig. 134); these meet over the hind brain and
form the exoccipitals and cartilaginous part of the supra-occipital,

and thus enclose the foramen magnum. In Fig. 132 the con-
dition of the occipital region is shown in a 5th -month foetus.
The supra-occipital parts of the parachordal cartilages have fused.
A suture between the membranous and cartilaginous parts is
clearly visible — especially near the fontanelle at the asterion.
The membranous and cartilaginous parts of the supra-occipital
become completely fused soon after birth. It will be observed
that the process of fusion between the lateral parts of the cartila-
ginous supra-occipital is not complete at the 5th month (Fig. 132).
The occipital fontanelle (Sutton) projects upwards between them
from the foramen magnum (Fig. 132). This fontanelle is filled
by a continuation of the posterior atlanto-occipital ligament ; and
becomes closed soon after birth. It is the most common site of a
cerebral meningocele — a saccular protrusion of the membranes of
the brain which contains cerebro-spinal fluid, and possibly also
a part of the brain.

font, at
asterion

uA/a

Fig. 132. — The Occipital Region in a Foetus of 5 months.

THE CRANIUM.

167

§£;

Separate centres of ossification appear in the parachordal
cartilages to form (1) the basi-occipital, (2) the two exocci-
pitals, and (3) the supra-occipital. The occipital consists of

Fig. 133. — The Parachordal Cartilages out of which the Cartilaginous Parts of the
Occipital Bone are formed.

Fig. 134. — The expansion backwards of the Parachordal Cartilages to enclose the
Foramen Magnum and form the Supra-occipital.

these four pieces until the fourth year, when synostosis occurs.
The occipital condyles are formed from the exoccipitals and basi-
occipital, the exoccipital element constituting by far the larger part.
The anterior condylar foramen is formed between these two parts.
The occipital protuberance is formed by both membranous and
cartilaginous parts of the supra-occipital.

(2) The Petro-mastoid forms part of the base of the skull. We
have already seen that the petro-mastoid part of the temporal
bone is developed out of the cartilage which forms the periotic
capsule (Figs. 35, p. 50, and 135). The periotic cartilages fuse
at points with the parachordal, which form the basis of the
occipital bone. Even to a late stage (30 th year or later)
remnants of these cartilages may be found between the petro-
mastoid and occipital bones, especially between the jugular
process of the occipital and the mastoid (Fig. 136). The fibro-
cartilage in the foramen lacerum medium is a remnant of the
periotic cartilaginous capsule. (See also p. 58.)

168

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(3) Trabeculae Cranii (Figs. 135 and 136). — The notochord
terminates behind the pituitary body and sella Turicae ; the para-
chordal cartilages develop above and at each side of it (Fig. 135).
Two bars of cartilage — the trabeculae cranii — develop in the

Fir;. 135. — Diagram of the Trabeculae Cranii, Parachordal Cartilages, and Periotic

Capsules.

trabecula

periotic
( petro-mast .) (if:

septum of nose
lat. nas. cart,
tat. mass, ethmoid

- cribriform pi.

r small wing
l sphen.
optic, for.
pituitary

great wing
sphen.

petro-mast

parach. cartil.

Flo. 136.- Diagram of the structures formed from the Trabeculae Cranii.

membranous basis of the embryonic brain capsule in lront of the
notochord and on each side of the pituitary body. In Fig. 136 is
shown what become of these two cartilaginous bars. Their posterior

THE CRANIUM.

169

extremities fuse round the anterior termination of the notochord
with the parachordal cartilages. The buccal part of the pituitary
grows into the cranial cavity in front of the notochord and
keeps the two cartilages apart ; but in front of the pituitary
the two bars fuse in the middle line. The mesial fused
parts of the trabeculae grow into the mesial nasal processes
of which they form the skeletal basis and become transformed
into the primitive cartilaginous septum of the nasal cavities
(Figs. 136 and 3, p. 3). The posterior segment of the median
fused bars forms the cartilaginous basis of the pre-sphenoid and
basi-sphenoid (Fig. 136). From the trabeculae four lateral
processes or wings grow out on each side (Fig. 136). The pos-
terior, which is small at first, forms the great wing of the
sphenoid and external pterygoid plate; the second is originally large,
and forms the small wings (orbito-sphenoids) ; the third and fourth
outgrowths are closely joined, — they form the lateral masses of
the ethmoid and alar cartilages of the nose (Fig. 6, page 6). The
nasal bones, the lachrymal and ascending nasal processes of the
superior maxilla, develop in the membrane over the lateral nasal
wings of the trabeculae, in the same way as the vomer develops
over the cartilage of the septum.

Development of the Sphenoid (Fig. 137). — At birth the
sphenoid bone, which is developed by ossification of the posterior

Fig. 137. — The Sphenoid in a foetus of 4 months. The Centres of Ossification are

deeply shaded. (After Sappey.)

parts of the trabeculae cranii, consists of three parts, the
great wings being separated from the rest of the bone. The
sphenoidal turbinate bones, afterwards inflated by the development

jorbito-sph,

for. rot.

for. ouale

intpter. bas'-sPh-

170

HUMAN EMBRYOLOGY AND MORPHOLOGY.

of the sphenoidal air sinuses, are then nodules of bone, surrounded
by cartilage. They also are separate and form part of the lateral
ethmoidal cartilaginous plates. The internal pterygoid plates,
formed from the ptery go-palatine bar, become adherent to the
external pterygoid plates, which are developed as outgrowths from
the ali-sphenoids or great wings. The pre-sphenoid unites with the
basi-sphenoid in the 7th month; the great wings unite with the
basi-sphenoid soon after birth. The lingula which bounds the
outer side of the carotid groove (Fig. 137) is ossified from a
centre which appears during the 4th month of foetal life. The
orbito-sphenoids unite over the pre-sphenoid and cover its cranial
aspect.

The Pituitary Body is developed between the trabeculae cranii ;
the pre-sphenoid is formed in front of it and the basi-sphenoid
behind it (p. 19). A canal may remain in the foetal or even
adult hone to mark the point of ingress of the buccal part of the
Pituitary, Fig. 167, p. 203. The wings of the vomer cover the
opening of the pituitary canal on the pharyngeal aspect of the
skull, if it be present. On the cerebral aspect it opens at the
olivary eminence which also marks the union of the pre- and the
basi-sphenoids. The pre-sphenoid and afterwards the basi-sphenoid
are much altered by the growth of the sphenoidal sinuses which
commence about the 7th year. The great wings support the
temporal poles of the brain, their size depending on the develop-
ment of that part of the brain. They are much larger in man
than in any other mammal. The small wings project within the
vallecula Sylvii. In the early foetus the dorsum sellae is
enormously developed, and fills the deep and sharp angle
between the mid-brain and fore-brain (Fig. 167).

Formation of Foramina in Bone. — The foramina of the skull
are formed in one of three ways (Sutton) :

(1) By the union of two bones; examples of this form are
the jugular foramen, sphenoidal fissure, Glaserian fissure, etc.

(2) By the union of two elements of one bone ; the anterior
condyloid foramina, optic foramina, the foramen magnum, aque-
ductus Fallopii, etc.

(3) By the enclosure of a notch on the edge of a bone of
which the foramen ovale is the best example. This foramen
is at first a notch in the posterior border of the great wing of

THE CRANIUM.

171

the sphenoid (Fig. 137); it remains in this condition in all
mammals except man. In him the margins of the bone on each
side grow out and fuse and thus convert the notch into a foramen.
Other examples are the foramen spinosum, the foramen rotundum,
parietal foramen, mastoid, etc.

Wormian Bones. — In the six fontanelles which occur at the
parietal angles separate ossific centres frequently appear and close
them. The fontanelle ossifications form Wormian bones. They
occur most frequently at the posterior angles of the parietal
(Lambda and Asterion) but they are also common at the Pterion
(epipteric Wormian) and rare at the Bregma. The wormian at
the last-mentioned point receives the name of os anti-epilepticum.
Much confusion has been caused by naming a large wormian,
which may occur in the lambdoidal (posterior-superior) fontanelle,
the inter-parietal bone.

The Inter-parietal Bone. — It has already been shown that
the part of the supra-occipital above the superior curved lines is
developed from membrane by two centres of ossification and is at
first and almost until birth nearly separated from the lower part
developed from cartilage (Fig. 132). The membranous part of the
supra-occipital represents the inter-parietal bone. In marsupials,
ruminants, and ungulates, the inter - parietals fuse with the
parietals and not with the occipital. In rodents they fuse
with both occipitals and parietals. In primates and carnivora,
as in man, they fuse with the occipital. It is extremely rare
to find the whole inter-parietal separate in man, but a large
wormian, partly replacing the inter-parietal, is very frequent.
Such a wormian bone, if large, is named variously, os epactal,
os Incae, os triquetrum, or pre-interparietal.

The Post-frontal does not occur in mammals as a separate
bone ; in them it has fused with the frontal, and forms that part
of the bone which articulates with the great wing of the sphenoid
and malar. A wormian bone — the epipteric — which is occasion-
ally developed in the fontanelle at the pterion, may be mistaken for
it. Traces of a true post-frontal, partly separated from the frontal,
rarely occur in man. The suture round a wormian bone ina}'' be
mistaken for a fissure or fracture when exposed by the trephine.

The Cephalic Index. — Anthropologists have employed the
shape of the head as a character in classifying the races of man-

L -1 IIUMAN EMBRYOLOGY AND MORPHOLOGY.

kind. The cephalic index is used to express the shape of the head,
t states the proportion that the breadth bears to the length of the
skull (Figs. 138 and 139). The length or long diameter of the
skull is measured from the glabella to the inion (external occipital
protuberance); the breadth or widest diameter is measured

Fio. 138. — Diagram of a Long-head (Dolichocephalic).
Fig. 130. — Diagram of a Short-head (Brachycephalic).

between the widest points — the parietal eminences. If the length
of a skull is 100 mm. and the breadth 75, the cephalic index of
that skull is 75, i.e. the breadth is 75°/0 of the length. Human
races, on an average, are either Dolichocephalic (long-headed), the
breadth being 75°/0 or less of the length; Brachycephalic, in
which the breadth is 80°/o or more of length; or Meso-cephalic,
in which the breadth is between 7 5°/0 and 80°/o of the length.

The English people have an average index of 75, the South
Germans 83, but it must be remembered the individuals vary
widely. It will be seen that the topography of the brain, worked
out by German surgeons, cannot be applied to the longer English
heads without modification.

The Facial Angle is the angle at which the face projects from
the base of the skull (Fig. 140). The skull consists in man, as
in all mammals, of two parts — the facial part, which carries the
teeth and is developed according to their size, and the brain
capsule, which depends on the size of the brain. The smaller the

THE CRANIUM.

173

brain and the larger the face, the more does the face project in
front of the skull and therefore the greater is the facial angle,

and vice versa.

Fig. 140. — The Facial Angle of a [European contrasted with that of an Anthropoid.

It will thus be seen that the facial angle is a good index of
brain development. It is smallest in the most highly developed
races of man ; it is larger in the lower races, and larger still in
the anthropoids ; it increases in size with the advent of the per-
manent teeth and the necessary increase in the size of the face.
It is therefore greater in the adult than in the newly born.

The Para-occipital Process is sometimes present in man, and
projects downwards from the jugular process of the occipital bone.
The rectus capitis lateralis is inserted to it. The process repre-
sents the para-occipital process, which is so highly developed in
four-footed mammals. The para-mastoicl process projects from
the temporal at its outer side (Parsons).

Upgrowth of the Temporal and Occipital Ridges or Curved
Lines. — In lower animals, such as the ape or dog, a great increase
in the development of the temporal and nuchal muscles takes
place with the eruption of the permanent teeth, the area of their
origin from the skull being necessarily enlarged. The ridges of
bone which mark the limit of attachment of these muscles, the
temporal and occipital ridges, ascend on the skull as waves of

174

HUMAN EMBRYOLOGY AND MORPHOLOGY.

bone before the growing muscles. The ridges may meet, as in
apes, along the sagittal and lambdoidal sutures and form crests,
like that on a fireman’s helmet. In man the temporal ridges
and superior curved lines of the occipital bone also ascend with
the eruption of the permanent teeth, but only to a slight extent.
Man retains in the adult the condition seen in young apes.

CHAPTER XIV.

DEVELOPMENT OF THE STRUCTURES CONCERNED
IN THE SENSE OF SIGHT.

The structures concerned in the sense of sight are :

(1) The Eyeball and the Optic Nerve.

(2) The Eyelids and Lachrymal Apparatus.

(3) The Orbit, the Muscles, Nerves, and Vessels contained
in it.

(4) The Nerve Centres and Tracts.

The Eyeball. — The condition of the eye in the third week of
foetal life is shown diagrammatically in Fig. 141. The three

Fig. 141.— Diagram of the Elements -which form the Eyeball.

elements which unite to form the eyeball are as yet separate.
They are :

(1) Epiblast, which forms (a) the epithelium of the cornea, (b)
the lens, and probably (c) the rods and cones of the retina.

176

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(2) Neuroblast, which forms (a) the optic nerves, (b) sensitive
retina, (c) pars ciliaris retinae, ( d ) uvea ( e ) pigmentary layer of
retina.

(3) Mesoblast, which forms (a) outer tunic (sclerotic and
fibrous cornea) ; (b) middle tunic (choroid, ciliary-choroid and
iris) ; (c) the vitreous humour and its capsule — the hyaloid mem-
brane ; (d) the capsule of the lens.

1. Structures derived from the Epiblast. — (a) The lens. —
The lens is developed by a saccular invagination of the epiblast
situated over the optic vesicle (Fig. 142). It becomes a closed

Fig. 142.— Invagination of the Epiblast to form the Lens Vesicle.

Fig. 143.— The manner in which the Lens Vesicle is severed from the Epiblast.

sac by the severance of its connection with the epiblast, its wall
being formed by a single layer of epithelial cells. The cavity of

Fir 144 -The Formation of the Lens Fibres from the Epithelium on the posterior

Wall of the Vesicle.

the lenticular vesicle is gradually obliterated by the cells of the
posterior wall becoming elongated (Fig. 144) until they reach

SENSE OF SIGHT.

177

the anterior wall. Each elongated cell is transformed into a
lens fibre.

The cells of the anterior wall retain their primitive form
(Fi". 144). New lens fibres are added by the cells at the margin
(equator) becoming multiplied and elongated. The lens reaches
its full size in the 1st year of life and then no more fibres are
formed. It will thus be seen that the lens is an area of modified
epidermis. Like the epidermis, it shows a tendency in the aged
to be transformed into keratin. The oldest cells (the central or
nuclear fibres) alter first ; hence the central position of the
cataract which occurs so frequently in old people.

(b) The cornea. — The epithelial covering of the cornea is
continuous with the epidermis and in some animals (snakes, etc.)
it is shed with that structure, rendering the animal blind for the
time being. It becomes transparent. The mesoblast which
grows in between the lens vesicle and epiblast forms the con-
nective-tissue basis of the cornea and also the capsule of the lens
(Fig. 148).

(c) It is probable, although not yet verified, that certain cells
from the epiblast grow into the optic vesicle and afterwards form
the sensory epithelium of the retina — the rods and cones (Gaskell).
Before the incursion of the mesoblast separates them, the optic
vesicle and epiblast are in contact. If this is so, then the rods
and cones — the sensory cells of the retina, the olfactory cells, the
taste cells, the acoustic cells, are all of similar origin — epiblastic.

2. Structures formed from the Optic Vesicles (neuroblastic
element). — (a) The optic nerve is formed out of the stalk of the
optic vesicle. The vesicle is well developed at the commence-
ment of the third week (see Fig. 141) ; even before the medullary
plates have quite met to enclose the cavity of the fore-brain the
optic vesicles have commenced as evaginations of those plates.
They form a great lateral diverticulum on each side of the
fore-brain — a cavity which becomes the third ventricle in the adult.
The condition of the optic nerves at the commencement of the
second month is shown diagrammatically in Fig. 145. The stalk
or neck remains constricted while the vesicle enlarges.

Invagination of the optic vesicle. — Almost as soon as it begins
to grow out the optic vesicle becomes invaginated, one half

M

178

HUMAN EMBRYOLOGY AND MORPHOLOGY.

being pushed within the other. It is invaginated by the lens-bud
in the same manner as a schoolboy’s fist indents a punctured
india-rubber ball. The invaginated vesicle is known as the
optic cup. I he invagination of' the vesicle, which takes place in

Fig. 145.— Diagram showing the condition of the Optic Stalk and Vesicle at the
commencement of the 2nd month. (After His.)

an oblique manner — the pressure being applied from below and
behind, leads to the closure not only of the cavity of the vesicle,
but also to that of the distal half of the stalk (optic nerve). The
mesoblast, surrounding the lens, grows into the invagination and
afterwards forms the vitreous humour. The artery, which is
folded in with the mesoblast, becomes afterwards the central
artery of the retina. Hence the point at which the central artery
enters the optic nerve marks the upper limit of the invagination
of the optic stalk. By the fourth week the optic vesicle no longer
communicates with the cavity of the fore-brain but the recessus
opticus, in the floor of the third ventricle, above the chiasma,
marks the point at which it entered (Fig. 145). The optic
fibres, developed as processes of the neuroblasts of the invaginated
layer, grow into the brain from the retina along the optic stalk.
They thus form the greater number of the fibres in the optic
nerve. The optic fibres also form the chiasma in the floor of
the third ventricle and the optic tracts on the wall of the fore-

cerebr. uesicle / ^ for. Monro

/ / ^ ^

aright op. ues.
| turned down

SENSE OF SIGHT.

179

brain (Fig. 153). It will thus be seen that the optic nerves and
vesicles are of the same origin as the cerebral vesicle — both repre-
senting parts of the wall of the fore-hrain.

( b ) The pigmentary layer of the retina is formed from the
ensheathing or outer layer of the optic cup (Fig. 146). At
first the wall of the optic vesicle is composed of a single layer of
epithelium ; the outer or pigmentary layer of the retina retains
this embryonic form.

uvea

pars ciliar. ret

cauity of
outer or pit

optic, stalk -

or inuag. layer

Fig. 146. — Diagrammatic Section of the Optic Cup and Lens.

(c) The uvea is the layer of pigmented epithelium which covers
the posterior surface of the iris. It is formed out of both outer
and inner layers of the optic cup, and represents the rim of the
cup (Fig. 146).

(cl) The Pars ciliaris retinae is formed out of that part of the
inner or invaginated layer of the optic cup which lies in the
shadow of the iris, and is therefore inaccessible to light rays.
It also retains the primitive columnar form of the epithelium.
The ora serrata marks the junction of the pars ciliaris retinae
and sensitive retina.

Ciliary Processes. — At the commencement of the third month,
the pars ciliaris retinae becomes plicated or puckered into 60 or
70 small folds; mesoblast of the middle tunic (choroid) grows
into the puckers and forms the ciliary processes. It should be
observed that the lens lies within the optic cup and the ciliary
processes are formed round the equator or circumference of the
lens.

(e) The Sensitive Eetina is formed out of the inner or in-
vaginated layer of the optic cup (Fig. 146). At first the inner

180

HUMAN EMBRYOLOGY AND MORPHOLOGY.

wall is composed of a single layer of epithelium. The pars
ciliaris retinae retains this form. The cells of the sensitive
retina elongate, but the process of formation of rods and cones
and other structures in the retina has not been fully followed.
If Gaskell is right then the matter is simple. He believes that
the epithelial cells of the inner layer of the optic cup become
transformed into the fibres of Muller ; the rods, cones, and gang-
lionic cells being derived from the cells which grow into the optic
vesicle from the epiblast. The ganglionic cells, however, are
more probably derivatives of the neuroblasts of the optic vesicle.
At any rate an inner layer of ganglionic cells is formed
which give off the optic fibres as processes. These fibres
converge at the stalk of the vesicle, thus forming the optic
disc ; they grow inwards on the stalk, which becomes the optic
nerve ; some at least, perhaps all, cross in the floor of the 3rd
ventricle forming the chiasma, and pass round, as the optic tracts,
to ganglia situated on the mid-brain. There are also efferent
fibres in the optic nerve, which end round the ganglion cells of
the retina. The inner ganglionic cells of the retina probably
correspond to the cells of a posterior root ganglion. According
to Gaskell the optic vesicles arose in the ancestry of the verte-
brates as diverticula of their alimentary canal; when the alimentary
function of the canal was lost and it became neural, these diverti-
cula became the optic vesicles.

The choroidal Fissure. — Occasionally congenital fissures are seen

cup

lens

'n of pupil

choroidal fissure

Fio. 147.— The Optic Stalk and Cun, viewed on the lower and lateral aspect, showing the

closure of the Choroidal Fissure.

ill the lower and outer segment of the iris (coloboma iridis). A
white line, due to absence of pigment, may be seen in the corre-

SENSE OF SIGHT.

181

sponding segment of the retina when the interior of the eye is
examined. These are due to imperfect closure of the choroidal
fissure. The choroidal fissure is the result of the peculiar mode
in which the optic vesicle is cupped or invaginated. The lens
grows into it from the malar or lower lateral aspect. The lens is
lodged in the anterior part of the depression ; the posterior part
becomes the choroidal fissure (Fig. 147). The margins of the
fissure unite, all traces of it normally disappearing. The margin
of the cup left after the union of the lips of the choroidal fissure
becomes the boundary of the pupil (Fig. 147).

Binocular Vision. — At first the optic vesicles are directed
laterally in the human embryo, and in mammals generally the
eyes are so directed, each eye having its own field of vision.
In the Primates the eyes swing forwards during the second month;
binocular vision is thus made possible. With binocular vision
and the combination of images appear in the highest primates : —

(1) A fovea centralis and macula lutea (L. Johnston) ;

(2) A partial crossing of the optic fibres at the chiasma ;

(3) Certain alterations in the attachments of the oblique
muscles of the eyeball.

The cavity of the Optic Vesicle (Fig. 146) is of some clinical
importance. It is obliterated by the invagination of the vesicle;
the rods and cones formed in the inner or invaginated layer grow
out across the cavity into the outer or ensheathing pigmented
layer of the retina. From accident or disease the retina may be
detached ; the separation takes place between the pigmented
epithelium, which remains in situ, and the rods and cones, which
fall inwards with the nerve layer. Fluid then collects in the site
of the primitive cavity of the optic vesicle.

3. Parts of the Eyeball formed from the Mesoblast. —

As the lens invaginates the optic vesicle and forms the optic
cup, it carries in with it the surrounding mesoblast. Tims
the lens is surrounded and the cup filled by mesoblast (Fig.
148).

The structures formed from the mesoblast are :

(1) The Capsule of the lens. — It is developed out of the meso-
blast which surrounds the lens. At first the capsule is continuous

human embryology and morphology.

182

iii front with the basis ot the cornea ; behind, it is continuous
with the mesoblast of the vitreous humour (Fin-. 148).

outer or \
pigment layer]

inner layer ,
of retina

Vitreous

hyaloid art.-f

>M

optic nerue

conjunctiua

-epiblast

mesoblast

epithet, of cornea
id-j. basis of cornea and

j [capsule of lens,
lens.

lentic. cauity

uuea

pigment layer
of retina

optic fibres / mner layer of retina
( basis of middle and outer
1 tunics of eyeball

Fio. 148.— Diagrammatic Section of the Eye showing the Parts formed from the
Mesoblast. (After His’ Model of the eye of a 3rd month human embryo.)

(2) The vitreous humour. — This is formed out of the mesoblast
which fills the optic cup behind the lens. The closure of the
choroidal fissure cuts the vitreous humour off from the mesoblast
which covers the outer layer of the optic cup and becomes trans-
formed into the tunics of the eyeball. The vitreous humour —
like Wharton’s jelly of the umbilical cord — represents an early
form of embryonic tissue. It consists of cells imbedded in a
jelly-like matrix. All the connective tissues of the body are
originally of this type, and remain as such until the fifth month
(Ferry Hart).

(3) The hyaloid artery. — This is the vessel which supplies the
mesoblast of the optic cup ; it terminates in the capsule of the lens
(Fig. 148). In the 7th month foetus a trace of the artery can
still be seen passing through the vitreous humour from the optic

SENSE OF SIGHT.

183

disc to the lens. With the gradual obliteration of the artery, the
capsule of the lens becomes thin and clears up. A foetus born in
the seventh month is blind, because of the vascular and opaque
capsule of the lens. The anterior part of the capsule — filling the
pupil — is the membrana pupillaris. The part of the hyaloid
artery within the optic nerve persists as the central artery of the
retina. The canal of the artery within the vitreous humour, from
the optic disc to the lens, remains as the hyaloid canal — a lymph
path. The hyaloid artery may persist and cause partial or com-
plete blindness. It disappears some days after birth in cats and
rabbits.

(4) The Aqueous chamber, a space formed in the mesoblast which
lies between the epiblast of the cornea and the lens (Fig. 148). Part
of this mesoblast becomes the anterior capsule of the lens ; part
becomes the connective-tissue basis of the cornea. The aqueous
chamber is simply an enlarged lymph space formed between these
two parts. Up to the time of birth the lens lies almost in contact
with the cornea (Fig. 149).

(5) The choroid, ciliary processes, and iris. — These form the
middle or vascular tunic of the eye, and are developed out of the
mesoblast which covers the optic cup. They form a vascular and
pigmented covering through which the optic cup is nourished.
The ciliary muscle is formed in this tunic.

184

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(6) The sclerotic. — This is the outer covering or tunic of meso-
blast. It is continuous in front with the cornea ; behind with the
optic nerve sheath. In some vertebrates, but not in mammals,
plates of bone are developed in the anterior half of the sclerotic.

The Tapetum lucidum is absent in the human and primate eye.
It gives the metallic lustre seen on the retinal surface of the eye
of the ox, and is formed by a layer of fine fibres which are
developed on the retinal surface of the choroid.

(7) The Capsule of Tenon, the bursa or connective-tissue socket
of the eyeball, and is developed in the mesoblast surrounding the
eyeball. A lymph space separates it from the sclerotic, which
is but slightly marked until after birth. The choanoid muscle
(retractor bulbi or Muller’s muscle) which surrounds the posterior
part of the eyeball as a muscular hood in mammals and verte-
brates generally, has disappeared in man and the higher primates.
Probably its fibrous remnant helps to form the capsule of Tenon.

Formation of the Orbit (Fig. 150). — The orbit is formed
(1) above by the capsule of the fore-brain in which the frontal

position of nasal duct

Fig. 150.— The Origin of the Bones entering into Formation of the Orbit.

bone is developed; (2) externally and below by the maxillary
process (Fig. 1). In the maxillary process the malar bone and
superior maxilla (except the ascending nasal process) are

SENSE OE SIGHT.

185

developed (Fig. 150). (3) The inner wall is formed by the

lateral nasal process, in which the nasals, lachrymals lateral
mass of the ethmoid, are formed. The optic nerve enters the
orbit between the orbito- and pre-sphenoids — derivatives of the
trabeculae cranii, both of which help to form the orbit. The
great wings of the sphenoid are also derived from the trabeculae
(p. 168). The orbital plate of the malar cuts the orbit off from
the temporal fossa ; it is developed in higher primates only.
The nasal duct is formed between the maxillary and nasal pro-
cesses (Figs. 1 and 150). In lower primates and mammals
generally the hamular process of the lachrymal appears on
the margin of the orbit ; the pars facialis lachrymalis is some-
times seen in the human skull (Fig 20, p. 26).

The Eyelids are formed by folds or ridges of epiblast which
rise above and below the cornea. Mesoblast grows into the
folds and forms the tarsal plates. The upper eyelid is formed
from the capsule of the fore-brain, the lower from the maxillary
process. The epiblast on the deep surface of the folds forms the
palpebral conjunctiva. It is continuous with the epiblast of
the cornea. The lid-folds meet in front of the cornea during the
third month and adhere by their edges. The edges become again
separated about the 7th month. From the epiblast between their
adherent edges, buds grow into the lids and form the cilia, Meibo-
mian and other glands in the same manner as hairs and sweat
glands are developed.

The plica semilunaris (Fig. 151), a fold of conjunctiva in
the inner canthus of the eye, is a vestige of the third eyelid
(membrana nictitans) which is so fully developed in birds and
reptiles. It is well seen in the cat, partially crossing the cornea
as the lids are shut. The lachrymal papillae rub in the grooves
at the outer and inner margins of the fold.

The Lachrymal Gland arises as a number of epiblastic buds
which spring from the fornix of the conjunctiva beneath the
upper lid and grow into the tissue of the outer and upper
segment of the orbit. The outer buds form the orbital part
of the gland ; the more internal buds form the palpebral part.
Smaller lachrymal glands may occasionally be found at the
outer angle of the eye. This is the position occupied by the
lachrymal glands of birds and reptiles (Wiedershiem). The

186

HUM AM EMBRYOLOGY AND MORPHOLOGY.

lachrymal canaliculi and sac and nasal duct are formed out of
solid epithelial cords enclosed between the maxillary and lateral
nasal processes (Fig. 151).

inf. canal

carunc.

plica semilunaris
canaliculus

lach. sac
nas. duct

value

Fig. 151.— Diagram of the Plica Semilunaris and Lachrymal Canaliculi.

The Orbital Muscles. — We have already seen that the head
is composed of nine segments, at least four of these being
occipital ; also, that each segment gives rise to a muscle plate.
The muscle plate of the first head segment forms the muscles

to sup. rectus, etc.

-for. Monro
3rd vent,
corp. quad,
motor nucleus
1st segment
— of 2nd segment

4th ventricle
-of 3rd segment

Fig. 152. — Diagram of the Motor Nerves of the Muscles of the Eye derived from the
1st, 2nd, and 3rd Cephalic Segments.

supplied by the third cranial nerve — the motor nerve of the first
cephalic segment. The mesencephalon (crura cerebri) contains
the corresponding segment of the neural canal. The ciliary
muscle and sphincter of the iris also belong to this segment and

SENSE OF SIGHT.

187

are supplied by the 3rd nerve (Fig. 152). The muscle plate of
the 2nd head segment produces the superior oblique. In the
course of evolution the superior oblique of the right side has
shifted to the left and the left to the right (Gaskell), hence the
decussation of the 4th nerves (the motor nerves of this segment)
on the anterior part of the roof of the hind-brain — the valve ol
Vieussens. The muscle plate of the third cephalic segment gives
rise to the external rectus ; the 6th nerve is the motor nerve of
the segment.

The sensory nerves of these three segments are fused together
to form the ophthalmic division of the 5th nerve. The ciliary
ganglion is the splanchnic (sympathetic) ganglion of the first
segment (see Fig. 180, p. 220). The nerves for the choanoid
(Muller’s) muscle, the non-striated muscle of the upper eyelid,
and the dilator fibres of the iris, issue from the upper three dorsal
segments of the spinal cord, and reach the eye by the cervical
sympathetic chain and cavernous plexus. The nerve fibres for
the orbicularis palpebrarum pass out with the facial, but they
arise from cells in the first segment of the neural canal (oculo-
motor nucleus). The ophthalmic division of the fifth nerve
represents the sensory part of the 1st segment; hence the reflec-
tion of pain along this nerve (frontal headache) in disorders of
accommodation, the muscle of accommodation being the ciliary,
and its nerve, the oculo-motor, both also derivatives of the first
segment.

Development of the Nerve Centres concerned with Sight.

— Five parts of the brain are concerned with vision. They
are :

(1) The optic tracts.

(2) The basal centres surrounding the termination of the aque-
duct of Sylvius in the 3rd ventricle.

(3) The optic radiations.

(4) The occipital lobes — in part at least.

(5) The angular gyrus.

(1) The optic tracts are made up of fibres developed from
the ganglionic cells of the retina and also in part of efferent
fibres developed from cells of the basal ganglia in which the

188

HUMAN EMBRYOLOGY AND MORPHOLOGY.

optic tiacts are seen to terminate. The fibres grow in by the
optic stalk, decussate in the floor of the third ventricle between
the origins of the optic vesicles, and thus form the chiasma.
The optic fibres grow backwards on the surface of thala-
mencephalon (see Tig. 153) and on the optic thalamus to reach
the nerve centres which afterwards form the pulvinar, geniculate

bodies and the superior corpora quadrigemina. In these centres
the optic fibres end. From some of the cells of these ganglia the
efferent fibres of the optic tracts are developed.

'i-(2) The basal ganglia. — The corpora quadrigemina. — Almost
in every structure the human embryonic condition resembles
the adult condition of lower vertebrates. A good example
is seen in the corpora quadrigemina. The human foetus
at the commencement of the third month (Fig. 153) shows
the corpora quadrigemina represented by a prominent thicken-
ing in the roof of the cavity of the mid-brain, which forms
subsequently the aqueduct of Sylvius. The thickening is
divided into lateral halves by a median sulcus, each half
being nearly as large as the cerebral vesicle of that period. In
Fig. 154 is shown the condition in an adult lizard; there is

SENSE OF SIGHT.

189

one body on each side — the optic lobes or corpora bigemina. As
the human foetus grows older, each lateral lobe becomes divided
into an upper and lower part by the formation of a transverse
groove, the upper and lower pairs of the corpora quadrigemina being
thus formed. The upper pair are connected with sight. In the
mole they are vestigial, but in compensation the inferior corpora
are well developed as they are connected with the sense of hear-
ing, which is very acute in that animal.

vent.

cerebrum

corp. bigem ( optic lobe)
cerebellum

lamina termin.

optic nerve

Fig. 154. — Mesial Section of the brain of a Lizard showing the resemblance to the
human foetal brain (Fig. 153) especially in the development of the Corpora
Bigemina.

The internal geniculate body also belongs to the mid-brain
(mesencephalon) ; the pulvinar and external geniculate body, in
which the upper division of the optic tract ends, are developed on
the wall of the 3rd ventricle (thalamencephalon).

(3) The optic radiations connect the basal optic centres just
named with the mesial surface of the occipital lobes. The fibres
join the posterior part of the internal capsule and pass under and
round the posterior horn of the lateral ventricle to end in the
cortex of the calcarine fissure and neighbourhood.

(4) The occipital lobe and calcarine fissure. — A mesial view
of the 5th month foetal brain is shown in Fig. 155. The
occipital lobe is already well formed ; its inner aspect shows
the calcarine and parieto-occipital fissures. A section across the
occipital lobe is shown in Fig. 156 ; the posterior horn is large;
the calcarine fissure indents its inner wall, giving rise to the
calcar avis or hippocampus minor.

The calcarine is one of the first fissures to appear on the brain ;

190

HUMAN EMBRYOLOGY AND MORPHOLOGY.

it appears early in the filth month. It is present in all primates
except the very lowest. The optic radiations end in the cortex of

line of section
/v j-x par-occip. fis.

calcar, fis,

calcar auis

calcarine fis.

uncus

fascia dentata

post horn

Fia. 155.

Fio. 150.

Fig. 155. — View of the Mesial Surface of the Brain in the 5th month.

Fig. 156. — Section of the Occipital Lobe at the position marked in Fig. 155.

the fissure. In Fig. 157 the condition of the occipital lobe in
the 4th week is shown. The cerebral vesicle has arisen as a
hollow protrusion from the anterior superior end of the fore-brain
(3rd ventricle). The lateral ventricle is as yet undifferentiated
into horns and only the rudiment of the occipital lobe is present.
The occipital lobe is produced by a backward growth of the
cerebral vesicle, the posterior horn being produced as a diverti-

rudiment of occip. lobe

Fig. 157.— Mesial Section of the Brain at the 4th week showing the Rudiment of

the Occipital Lobe. (After His.)

culum of the cavity of the vesicle. By the 5th month the
occipital lobe has reached far enough back to overlap the
cerebellum.

SENSE OF SIGHT.

191

(5) The Angular Gyrus is connected with the calcarine region by
association fibres. In it is seated the word-seeing and word-
understanding centre. It is developed round the posterior end of
the 1st temporal or parallel fissure (Fig. 176, p. 214). It is part
of the wall of the cerebral vesicle. The first temporal or parallel
fissure appears during the sixth month and is one of the primary
fissures. It is found in the brains of all primates except the
lowest.

Summary. — It will thus be seen that three parts of the neural
tube are specialized in connection with sight.

(1) The optic vesicle, an outgrowth from the fore-brain
(thalamencephalon).

(2) The occipital end of the cerebral vesicle, which may also
be regarded as an outgrowth from the thalamencephalon.

(3) The walls of the 3rd ventricle (thalamencephalon) and
mid-brain (mesencephalon), in which the basal optic ganglia are
developed.

CHAPTER XV.

THE BRAIN AXD SPINAL CORD.

Formation of the Central Canal. — The medullary plates of
epiblast, which form the spinal cord and brain, rise up, meet, and
enclose a canal — the central canal of the spinal cord and brain
(Figs. 158 and 69, p. 90). The lips of the medullary plates meet

Fio. 158. — Medullary Folds uniting to form the Neural Tube in a Human Embryo
of about 14 days. (After Graf Spec.)

and fuse together in the cervical region first (Fig. 158), the process
of union spreading forwards and backwards, the last parts to be
enclosed being the cephalic and caudal extremities. The optic
vesicles start to grow out from the medullary plates before they
have united to enclose the cavity of the fore-brain. The canal
is completely closed by the middle of the 3rd week.

\

chorion

THE BRAIN AND SPINAL CORD.

193

Divisions of the Neural Canal (Fig. 159). — At the end

of the 3rd week the neural tube is divided into four parts. They
are :

(1) An anterior dilatation, the fore-brain, which forms the
3rd and lateral ventricles and their walls.

(2) The mid-brain, which becomes transformed into the aque-
duct of Sylvius and crura cerebri.

(3) The hind-brain, the basis of the 4th ventricle, pons, cere-
bellum and medulla.

(4) The central canal and spinal cord.

Fig. 159.— Diagram"of_the Four Primary Divisions of the Neural Tube.

Cerebro-spinal fluid fills the canal. We do not know how it
is secreted or absorbed, but under certain abnormal conditions it
may collect and give rise to a cystic condition of the neural tube.
The expansion may involve the whole tube or only a part of it.

N

194

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Chick embryos, hatched under abnormal conditions as regards
temperature, frequently show a cystic condition of the neural
tube, which is also accompanied by a dropsical state of the
mesoblastic tissues. If the cystic condition occurs at an early
stage it may dilate the fore-brain, or even the whole cephalic part
of the tube until it bursts. This is probably the pathology of
the condition seen in anencephalic foetuses, children born with
all the parts of the body developed except the brain and roof

of the skull, which are represented merely by a broken mass

of tissue. A cystic condition of the lateral ventricles, which
are formed as diverticula of the fore-brain, occasionally occurs
towards the end of foetal life, and gives rise to the condition
known as hydrocephalus.

Encephalocele and Spina-bifida. — Localised dilatations of the
neural tube may occur. The most common, spina bifida, occurs
in the lumbar region, where the medullary plates are last to
close. The dilatation may affect the arachnoid and dura mater
only ; or the neural tube may be also distended. Another site

is at the anterior or cephalic end of the tube, where the

medullary plates are also late in closing. The encephalocele,
or it may be only a meningocele, formed in this site, projects at
the root of the nose or within the nasal cavity. A meningocele
may also occur at the roof of the 4th ventricle ; it projects at the
occipital fontanelle (Fig. 132, p. 166).

The Spinal Cord. — The Spinal Cord at first extends the whole
length of the spinal column. After the 4th month the spinal
column and canal grow more rapidly than the cord, and at birth
its lower end has become withdrawn to the level of the 3rd
lumbar vertebra. By the third year it only reaches the disc
between the 1st and 2nd lumbar vertebrae. The results of this
inequality of growth are —

(1) The roots of the lumbar and sacral nerves become enor-
mously elongated, forming the cauda equina ; all the nerves are
more or less drawn up, except the 1st and 2nd cervical; the
origins of the lower cervical nerves are drawn up 2 vertebrae (as
indicated by the position of their spines) ; the upper dorsal, 3 ; the
lower dorsal, 4 ; the lower lumbar, 5 ; the coccygeal, 10. These
statistics represent a broad expression of the observations made
by Professor B. W. Beid.

THE BRAIN AND SPINAL CORD.

195

(2) As the caudal termination of the neural canal is never
separated from the epiblast over the coccyx, the posterior end of
the cord with its pial covering (mesoblast) is pulled out into a
line string — the filum terminale. The neural canal extends for
some wav into the filum terminale, and in the foetus shows there
a dilatation or ventricle. The arachnoid and dura mater, meso-
blastic sheaths of the cord, do not suffer in the retraction of the
cord ; they remain widely open to the level of the 2nd sacral
vertebra.

Neurenteric Canal. — The caudal ends of the medullary plates
fold over and include within the neural canal the blastopore
(Fig. 158). The blastopore occurs at the anterior end of the
primitive streak and marks the point at which the cavity of
the hypoblast (that part which becomes the hind-gut) opens
on the epiblast. There is thus set up a communication between
the neural canal and the gut to which the name of neurenteric
canal is given (see p. 121). The neurenteric canal also opens
into the posterior end of the canal of the notochord.

Differentiation of the Spinal Part of the Neural Tube. —
When the medullary plates close in about the 14th day to form

the neural tube, they are composed of a layer of elongated epithelial
(epiblastic) cells. By the 6th week these cells have undergone
the following changes (Fig. 160):

19G

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(1) Some become neuroglia, elongated and branched supporting
cells ; in the outer zone of the tube wall they form a mesh-
work ; in the inner zone, round the central canal, they form the
ependyma ; in the middle zone their interstices are filled with
neuroblasts.

(2) Some of the cells become differentiated into neuroblasts.
These are produced from germinal cells lying in or near the inner
zone (Fig. 160). Two points should be noted (Fig. 160) —

(a) That the cells in the roof and floor scarcely undergo any

proliferation — they retain for a considerable time the
simple embryonic form.

(b) That there are two lateral centres of proliferation — a dorsal

centre or lamina, the cells of which are connected after-
wards with the ingrowing fibres of the dorsal root, and
a ventral centre or lamina, at which the cells of the
anterior horn are produced and from which the fibres
of the anterior root arise.

ant.pyram (from motor cortex )

Fig 161. Diagrammatic Section of the Spinal Cord to show the Parts foimed in the

three Zones of the Embryonic Spinal Cord.

The changes which occur in the Spinal Cord after the 6th week
(see Figs. 160 and 161) are:

A. In' the Outer Neuroglial Zone :

(1) Postero-mesial and postero-lateral columns are produced

post mes. (from post, roots)

/ post /at. (from post, roots)

outer zone

middle zone-

inner zone

THE BRAIN AND SPINAL CORD.

197

by the fibres of the posterior roots. They become myelinated
about the 6th month.

(2) To the inner side and to the outside of the anterior horn
fibres are produced from cells in the anterior horn, which con-
nect together neighbouring spinal segments (association fibres).

(3) Two tracts of fibres grow out from cells in the grey matter
of the cord and go to the cerebellum. They are the direct cere-
bellar tract (Flechsig’s) from the cells of Clarke’s column and
Gower’s tract from cells at the base of the anterior horn.

(4) Fibres produced by the cells of the cerebellar cortex grow
downwards outside the anterior horn (intermediate tract). All
these tracts are medullated before birth.

(5) The pyramidal tracts (crossed and direct) grow down from
the cells of the motor cortex. They are not medullated until
five months after birth.

The white matter of the cord is thus entirely produced by the
ingrowth of nerve fibres within the neuroglial network of the
outer zone. The cerebral motor cortex, through the pyramidal
tracts, comes to dominate the cord ; sensory and cerebellar paths
are formed, and intercommunications set up between the spinal
segments.

The Anterior and Posterior Median Fissures are produced by the
growth of the white matter. It is possible that the posterior
median fissure is formed in part by the inclusion of the dorsal
segment of the central canal of the neural tube.

B. The Middle Zone — filled with neuro-blasts, forms the horns
and grey matter of the cord.

C. The Inner Zone becomes the ependyma which lines the
central canal and the gelatinous tissue which surrounds it.
The columnar cells which line the central canal are ciliated. The
central canal retains the embryonic calibre while the wall
increases enormously in thickness.

THE HIND BRAIN.

That part of the neural tube which forms the hind brain
(Fig. 159) becomes transformed into:

(1) The Medulla Oblongata.

198

human embryology and morphology.

(2) The Pons Varolii.

(3) The Cerebellum.

The Fourth Ventricle. — The cavity or neural canal of the hind
brain becomes the fourth ventricle. In its floor are developed,

Fig. 102. — Section across the Hind Brain of a Human Embryo in the 5th week.

out of the ventral and dorsal parts or laminae (Fig. 162) of the
medullary plates, the pons and medulla. In its roof are developed
the cerebellum, superior and inferior medullary vela.

Inferior Medullary Velum. — When a section is made across the
posterior half of the hind brain of a fifth week human embryo
(Fig. 162), the same parts are seen as in a section of the cord.
The only difference is that the roof plate, which in the cord is
narrow, is here very wide and thin. It will also be noticed that
each medullary plate, shows, as in the spinal cord, a dorsal or
alar lamina and a ventral or basal one. The two laminae of each
side meet at an angle. In the roof plate over the anterior half
of the 4th ventricle is developed the cerebellum and superior
medullary velum ; over the posterior half, the roof plate forms the
inferior medullary velum.

As shown in Fig. 163, the velum is continuous with the
cerebellum above and the roof of the central canal of the cord
below. In the posterior margin of the cerebellar rudiment are
developed: (1) the nodule, (2) the flocculus, (3) the peduncle of
the flocculus between 1 and 2 (Fig. 164). Hence the inferior
medullary velum ends above in these structures.

The obex and ligula, thickenings found on the margins of the
lower angle of the 4th ventricle, mark the attachment of the roof

THE BRAIN AND SPINAL CORD.

19!)

plate or velum to the alar laminae of the medullary plates.
They represent the attached margin of the velum. The velum is
also attached to the restiform body which is developed in the
upper margin of the alar lamina.

rudiment of cerebellum

Fig. 103. — Lateral view of the Cephalic Part of the Neural Tube in a 5th week human

embryo. (After His.)

The velum is to be regarded as a specialized part of the neural
tube, which has been arrested at an early embryonic stage and
specialized for the secretion (and absorption ?) of cerebro-spinal
fluid. The foramen of Majendie and openings at the lateral
recesses of the 4th ventricle, where the velum is produced into
choroid villi or cornucopia, may be caused by the absorption of a
part of the velum or in some cases they may be produced in the
removal of the brain from the skull. In the early embryonic
condition, at least, the ventricle is quite closed. As will be
seen from Fig. 163, the neural tube is bent with its convexity
forwards at the pons. His has suggested that this bend may
have something to do with the production of the wide roof plate
of the 4th ventricle, for if a piece of tubing be bent so, the part
in the concavity of the bend becomes widened out.

Cerebellum. — The condition of the cerebellum in a 4th week
human foetus resembles that of the Frog (Figs. 165 and 16 6).
It is then merely a thickened transverse band in the anterior
part of the roof plate of the 4th ventricle. The vermis or median
lobe is the first part to be developed, the lateral lobes form in the

200

HUMAN EMBRYOLOGY AND MORPHOLOGY.

4th month (Fig. 164) and by the 6th month are larger than the
median lobe.

fat. lobe
cerebellum

uermis

nodule

recess
med. uelum

obex.

cuneatus
gracilis

Fig. 164. — Diagram of the Attachments of the Inferior Medullary Velum in a 5th
month foetus. (After Kollmann.)

Only in the higher primates are the lateral lobes well
developed. In the 2nd and 3rd months the simple transverse
roof plate is transformed by the outgrowth from its surface of
five transverse ridges. As these ridges or foliae grow out (Fig.
166) the anterior two of them come to occupy the upper surface,
while the three posterior ones are thrust to the lower surface.
The folding of the simple cerebellar plate, so that it comes
to present an upper and lower surface, is caused by its growth.
The secondary and tertiary foliae are produced in the last three
months of foetal life.

Fig. 165.— Median Section of the Cerebellum and 4th Ventricle of a Frog.

The three Peduncles of the cerebellum are produced thus —
probably during the latter half of intra-uterine life :

(1) The Superior. — Fibres grow from the cells of the dentate

THE BRAIN AND SPINAL CORD.

201

nucleus of the cerebellum — probably also from the cortex — to the
red nucleus and optic thalamus on the opposite side of the brain.
Some of the ascending antero-lateral cerebellar fibres from the
cord probably enter the cerebellum by the superior peduncles.

The Superior Medullary Velum is part of the root plate of the
4th ventricle which remains between the superior peduncles.
The vestigial laminae which cover it form the lingula (Tig. 166).

corp. quad. horiz. fis.

cerebellum
yramid

uvula

nodule

inf. med. velum

centra! canal

pons

Fig. 166. — Diagrammatic Section of the Cerebellum of a 3rd month Human Foetus
showing the folding of the Cerebellar Plate.

(2) The Middle Peduncles are formed by processes which grow
from the cortical cells of the cerebellum to the nuclei of the Pons
and also by processes from the cells of the Pontine nuclei to the
cerebellum. They are probably connected indirectly with the
frontal lobes through the fronto-cerebellar fibres which lie in
the inner third of the crusta.

(3) The Inferior Peduncles are formed by :

(a) Processes from the cerebellar cortex to the cord (descend-

ing cerebellar tract) ;

( b ) Processes which end in the opposite olive ;

(c) Processes which grow in from Clarke’s column (direct

cerebellar) ;

(d) Processes from the cells of the sensory nuclei of the

postero-mesial and postero-lateral tracts (nucleus gracilis

et cuneatus).

202

HUMAN EMBRYOLOGY AND MORPHOLOGY.

THE MID-BRAIN.

The central canal of the mid-brain forms the aqueduct of
Sylvius (Fig. 167). In its roof are developed the corpora quadri-
gemina. The dorsal and ventral laminae of its medullary plates
form the tegmentum and crusta of the crura cerebri.

The Three Neural Flexures (see Fig. 163). — The pontine
flexure, a convexity forwards of the pons, has already been men-
tioned ; the nuchal flexure is concave forwards and occurs between
the medulla and cord. Both of these are of small import, but
the anterior flexure, whereby, in the third week of foetal life, the
fore-brain is bent downwards and forwards until it comes to lie
on the ventral aspect of the cephalic end of the notochord, leads
to a great alteration in the form and relationships of the fore and
mid brains and is of great importance. The mid-brain, by this
flexure, is brought to be, for a short time, the most anterior part
of the neural canal ; the fore-brain is doubled back under the
notochord. Bound the projecting end of the notochord — project-
ing between the mid and fore brains — is developed the posterior
clinoid processes and dorsum sellae (Fig. 163). The tentorium
cerebelli is developed between the mid-brain and fore-brain, and
lies at first at right angles to the axis of the mid-brain, but the
great subsequent growth of the cerebrum forces it backwards and
downwards until it becomes a horizontal partition between the
cerebral and cerebellar chambers of the skull.

THE FORE-BRAIN.

The Third Ventricle and Structures derived from its
Walls. — The Third Ventricle (see Fig. 1 6 7) is the cavity of the fore-
brain and represents the anterior dilated end ol the neural canal.
From its walls many structures are derived. We have already seen
that the optic vesicles are produced from its ventro-lateral walls ;
from its floor is produced the hypopophysis cerebri — the ancient
mouth of the alimentary canal if Gaskell’s views are right.
From the posterior part of its roof plate is produced the pineal
body — an ancient median eye; from its antero-superior part is
produced a bifid hollow outgrowth — the cerebral vesicles— which
come in time to dominate the whole nervous system.

THE BRAIN AND SPINAL CORD.

203

The Pituitary Body is formed from two elements (Fig. 167):
(1) An epiblastic hollow bud from the stomodaeum (Fig. 22,

p. 30).

roof plate
optic, thclam.

caudate nucleus
cereb. uesicle

for. Monro-

mid brain
corp. quad.

lam. terminalis\
olfact. lobe'

sulcus of Monro
corp. mam.
hypothalam. part 3rd uent.
tuber ciner.

nose-

-notoch.
neural part of pituitary
buccal pituitary
'phar.

Fio. 167. — A schematic figure to show the parts derived from the walls of the

fore-brain. (After His.)

(2) A neuroblastic bud from the floor plate of the 3rd ven-
tricle (fore-brain).

The union of the two processes takes place at the anterior
extremity of the notochord. The epiblast of the stomodaeum
and the floor of the neural tube are in contact from the very
beginning ; subsequently the mesoblast grows in between the fore-
brain and the epiblast of the stomodaeum, but the parts which
form the pituitary adhere. The posterior or neuroblastic bud
becomes solid ; its structure is that of neuroglia into which many
vessels have grown carrying mesoblastic tissue with them.

The anterior or stomodaeal bud embraces the posterior. While
its posterior wall remains quiescent, its anterior throws out solid
processes between which a network of vessels lies. This forms
the glandular part of the pituitary — reticulated rows of cells
surrounding blood channels — similar in structure to the medullarv
part of the supra-renals, carotid bodies and parathyroids. The
trabeculae cranii form round the pituitary. When the basi- and

204

HUMAN EMBRYOLOGY AND MORPHOLOGY.

pre-sphenoids are developed in the trabeculae, the position of the
stalk of the stomodaeal process is seen in the later months of
foetal life between these two bones and forms the canalis cranio-
pharyngeus (Fig. 3).

Pineal Body (see Fig. 167) grows as a hollow bud from the
dorsal plate of the hinder part of the fore-brain during
the 6th week. In fossil reptiles and in some forms still
living it forms a median eye which perforates and appears on
the dorsum of the head between the parietal bones. It differs
from the lateral eyes which grow from the third ventricle as the
optic vesicles in this, that it produces the lens as well as the
retina and optic stalk. The retina is inverted — i.e. the apices of
the rods and cones point towards the vitreous chamber. The
ganglion of the habenula, situated on the dorsal and inner aspect
of the optic thalamus, represents its terminal ganglion. In man
and mammals its development is arrested at an early stage. It
produces a number of diverticula which are filled up by a pro-
liferation of the cells which form the walls of the diverticula. In

pia mater

cereb. uesicle
posit of corp. call,
caudate nuc.

tat. vent,
lentic. nuc.
to for. Monro

cereb. uesicle
arcuate fis.

choroid plex.
caud. nuc.
vel. interpos'd,
lentic. nuleus

desc. horn.

optic thalamus [alar lamina)
3rd uentricle

Fig 168.— Transverse Section of the brain of a Human Foetus at the commencement
of the 3rd month to show the Cerebral Vesicles overlapping the Thalamence-
phalon (schematic).

man it appears to be merely vestigial. It lies in the velum
interpositum, which is forced down on it by the growth back-
wards of the cerebral hemispheres.

THE BRAIN AND SPINAL CORD.

205

The Lamina Cinerea or lamina terminalis (Fig. 167) represents
the anterior end of the neural tube. In the adult it stretches
between the optic chiasma, which is developed on the floor of the
3rd ventricle and the rostrum of the corpus callosum. Its
development will be described later, but it retains with little
alteration its early simple structure. The inter-peduncular space,
which forms the floor of the 3rd ventricle, also retains in the
adult to a considerable extent the simple embryonic form. In it
are developed the corpora albicantia and posterior perforated
space.

The Optic Thalami are formed in the lateral walls of the fore-
brain (thalamencephalon) and in the adult occupy the whole
extent of this wall. At first they are completely exposed on
the outer aspect of the brain (see Fig. 154), but after the cerebral
vesicles grow out from the antero-lateral angles of the fore-brain
(3rd ventricle) they are prolonged backwards and downwards over
the optic thalami and thus bury them (Figs. 173, 174, and 175).
As may be seen from Fig. 169, the optic thalami, with the internal

Fig. 169.— Diagrammatic Section across the 3rd Ventricle of the Adult to show the

Structures formed in its Walls.

capsule, are simply the enlarged upward continuations of the
tegmentum and crusta of the crura cerebri or mid-brain. From
Fig. 167, it will be seen that the optic thalamus is an upward

206

HUMAN EMBRYOLOGY AND MORPHOLOGY.

continuation of the alar lamina of the mid-brain, and the same
may also be said of the caudate nucleus. The sulcus of Monro
(Fig. 167), which runs from the opening of the aqueduct of
Sylvius to the foramen of Munro, on the lateral wall of the 3rd
ventricle, marks off the alar from the basal lamina of the thala-
mencephalon. A section across the 3rd ventricle and optic
thalami, at the end of the second month is shown diagramma-
tically in Fig. 168. The internal capsule has not yet appeared.
The cerebral vesicles already overlap and bury the optic thalami.

The Cerebral Hemispheres. — In the 3rd week very soon
after the medullary plates have closed, a hollow bud, which almost
immediately divides into a right and left half, grows out from the
anterior superior extremity of the fore-brain. These two processes
form the cerebral hemispheres. It will be seen that the cerebrum
represents a super-addition to the neural-tube system. The
lateral ventricles with their horns represent the cavity of the
cerebral vesicles ; the foramina of Monro, which have a common
entrance to the third ventricle, represent the position at which
the primitive cerebral outgrowth took place. The walls of the
vesicles thicken and form the mantle of the brain. The anterior
horn represents the anterior extremity of the vesicle — the point
at which the olfactory lobe is produced ; the descending horn
represents the real posterior extremity of the vesicle ; the posterior
horn, although found in all mammalian brains, is a later diverti-
culum formed in connection with the growth of the occipital lobe.

The primitive simple relationship of the cerebral vesicle which
holds for low vertebrates (see Figs. 154 and 155) and for the
first two months of foetal life, becomes obscured in the third by
the vesicles growing over the optic thalami and burying them.
The wall of the vesicle, which comes in contact with the optic
thalamus, adheres to the outer surface of that body (Figs. 168
and 169). Hence the optic thalamus comes to form part of the
floor of the body of the lateral ventricle and enters into the roof
and inner wall of the descending horn.

Divisions of each Cerebral Vesicle.— A prolonged study of
the adult vertebrate brain has led Elliot Smith to divide the wall
of each cerebral vesicle into three primary parts :

(1) Rhinencephalon (defined on p. 22 ; see kigs. 18 and 1/2).
It represents the oldest part of the cerebral vesicle and composes

THE BRAIN AND SPINAL CORD.

207

nearly the whole of the cerebrum of fishes, amphibians, reptiles,
and birds.

(2) The Corpus Striatum.

(3) The Neo-pallium. This comprises the remainder of the
cerebral vesicle. With the evolution of the higher mammals the
neo-pallium became bigger and bigger until in man it constitutes
by far the greater part of the cerebrum. With its development
the corpus striatum also became increased, while the rhinen-
cephalon became more and more reduced.

The Velum Interpositum. — We have seen that the roof plate

Fio. 170.— A dorsal view of the Fore and Mid-brain at the 5tli week of development
to show the formation of the Velum Interpositum. The Cerebral Vesicles are
laid open and the inflection of their mesial walls shown on the ingrowing- Velum
The Roof Plate of the 3rd Ventricle is also exposed. (Modified from His5

of the 4th ventricle (hind-brain) forms the cerebellum in front,
while its posterior half becomes the inferior medullary velum a

ant horn

inflect, of)
mes. wall,

\J

208

HUMAN EMBRYOLOGY AND MORPHOLOGY.

secretory membrane. The root plate of the third ventricle, from
the foramina of Monro backwards, becomes modified in a similar
manner. It merely forms the ependymal covering of the lower
surface ol the velum interpositum, also a secretory membrane
(Figs. 168 and 169). The anterior part of the roof plate is
produced into the cerebral vesicles over the foramina of Monro,
and covers the apex of the velum interpositum (Fig. 170). The
mesial wall of each cerebral vesicle from the foramen of Monro
back to the posterior extremity of the vesicle (Fig. 170), which
becomes the tip of the descending horn, is also inflected and
becomes a secretory ependyma, covering the velum interpositum
and choroid plexus within the lateral ventricles. Into this
inflection of the embryonic neural wall spreads the mesoblast,
carrying vessels with it. The velum interpositum is thus com-
posed of a basis of mesoblast and its intraventricular parts have
a covering of the ependyma of the neural wall.

The ependymal covering of the entire velum is derived from :

(1) The roof plate of the 3rd ventricle (lower surface) ;

(2) The roof plate of the foramen of Monro ;

(3) An inflection of the mesial wall of the cerebral vesicle.

The choroid plexus, which fringes the velum in the adult,

completely fills the cavities of the lateral ventricles, which for the
first five months are relatively very large and the containing walls
thin. The velum and choroid plexus must play an important part
in the development of the cerebral vesicle in the early period of
growth. The spread of the vesicles backwards and downwards
over the optic thalami (Fig. 173) obscures the original simple
relationship of the velum to the brain ; but, when withdrawn
from the transverse fissure, the velum is seen to rest on the optic
thalami and project within the ventricle from the foramen of
Monro to the tip of the descending horn, and that stretch marks
the line at which the choroidal inflection took place.

The fibrous substance of the velum interpositum is con-
tinuous with the pial covering of the brain, and also with the
edge of the tentorium cerebelli. The veins of Galen are developed
in the velum and join the straight sinus in the tentorium.
Pressure applied to the veins causes dropsy of the lateral
ventricles.

Development of Commissures. — (1) The Anterior Commissure

TI1E BRAIN AND SPINAL CORD.

209

(Fig. 171) is developed in the lamina terminalis — the primitive
anterior wall of the fore-brain. The commissure passes between
the temporo-sphenoidal lobes. These lobes represent the posterior
ends of the cerebral vesicles. At first they are mere dilatations
behind the foramen of Monro. The commissure crosses in the
lamina terminalis below and rather anterior to the foramen of
Monro. This is the earliest and most primitive of the cerebral
(pallial) commissures (Elliot Smith).

corp. callos,

lam. term,
ant. comm
lam . term!

sulcus arcuatus
prim, callosal gyrus,
ransuerse or choroid, fis.
roof plate of 3rd uentricle

pineal

aqueduct Sy/uius
cerebellum
4th cent.

3rd cent.

Fig. 171. — Mesial Aspect of the human Foetal Brain during the 4th month. (After

Minot.)

(2) The Corpus Callosum, the great commissure between the
cerebral hemispheres, forms late ; except in the higher mammals
it is smaller in size than the anterior commissure. It reaches its
fullest development in man, and in him it commences to form at
the end of the third month.

To understand its development the student must be familiar
with the mesial aspect of the brain during the third and fourth
months. On this aspect he should note —

(1) The sulcus arcuatus (Fig. 171), .which indents the mesial
wall of the fore-brain. This fissure in the adult brain becomes
(a) the callosal sulcus between the corpus callosum and callosal
gyrus (gyrus fornicatus) (Fig. 172); (b) the hippocampal fissure,
which indents the posterior extremity (descending horn) of the
cerebral vesicle and causes the hippocampus major (Fig. 172).
The calcarine fissure may also be a derivative of it (Elliot Smith).

o

210

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(2) ihe Choroid or Transverse Fissure

the inflection of the mesial wall of the
interpositum and choroid plexuses.

(Fig. 171), caused by
vesicle on the velum

sept, lucid.

ant. comm.

lamina cinerea
gyrus subcallos.

optic.

transu. or choroid, fis.

uncus* fas. dentata

hippocamp. fis.

ca/los.
trans. fis.

callosal sulcus

supra-ca/los. gyrus

attach, of crura cerebri

Fig. 172. Diagram to show the structures formed in the Lamina Termiualis and
Primitive Callosal Gyrus. (After Elliot Smith.)

(3) The primitive callosal gyrus1 (see Figs. 171 and 172) is that
part of the mesial wall of the cerebral vesicle which lies between
the arcuate and transverse (choroid) fissures. In the lower edge
of this marginal gyrus, the edge which bounds the transverse
fissure, and therefore overlies the velum interpositum, is developed
the Fornix with the fimbria, its posterior continuation, a longi-
tudinal commissure which connects the optic thalamus with the
hippocampal (uncinate) gyrus. The corpus callosum is developed
in the lamina terminalis above the foramen of Monro about the
end of the 3rd month. It afterwards extends backwards, en-
croaching on and displacing the primitive callosal gyrus.

The grey matter of the primitive callosal gyrus becomes
reduced to — (1) The vestigial supra-callosal gyrus, lying on the
upper surface of the corpus callosum (Fig. 172); (2) the gyrus
dentatus ; (3) the gyrus, which unites 1 and 2 round the
splenium of the corpus callosum ; (4) the cortex of the hippo-

1 This gyrus forms part of the Rhinencephalon, and the name is proposed
merely as a provisional one, until comparative anatomists agree as to its proper
designation. At present, Elliot Smith proposes the term “hippocampal forma-
tions ” for the parts of the brain derived from it in the adult.

THE BRAIN AND SPINAL CORD.

211

campus buried in the hippocampal fissure. The longitudinal
striae, the thin layer of grey matter on the upper surface ol the
corpus callosum, and the grey matter on the fornix, are also
derived from the primitive callosal gyrus.

(4) The lamina terminalis is seen in section. It is the
terminal anterior wall of the fore-brain (Fig. 171). The lower
part becomes the lamina cinerea ; in the upper part, the anterior
commissure is developed, and the anterior pillars of the fornix
(Fig. 172). Its dorsal extremity bounds anteriorly the foramina
of Monro. It connects the mesial walls of the cerebral vesicles
(Fig. 1 7 1), and becomes thickened and enlarged. In its dorsal
part, where it is continuous with the primitive callosal gyrus,
the callosal commissure (corpus callosum) commences. The
part of the lamina terminalis which lies between the fornix
and corpus callosum (Fig. 172) forms the septum lucidum. In
this septum a cavity appears — the 5th ventricle. Cases are
known of people with normally functional brains in which the
corpus callosum was found afterwards to be absent.

The corpus callosum connects the cortex of one hemisphere
with the basal ganglia and cortex of the other. It is the
commissure of the neo-pallium. Its fibres are probably collaterals
derived from the pyramidal and other fibres of the cortical
cells. Each fibre grows out and crosses the great median
fissure of the brain in the lamina terminalis.

The Fornix is developed in the inner margin of the primitive
callosal gyrus, which bounds the transverse or choroid fissure
(Fig. 171).

The Corpus Striatum. — As soon as the cerebral vesicle grows
out, the corpus striatum appears as a thickening in its wall at
the outer side of the foramen of Monro (Fig. 173). With the
posterior development of the vesicle it comes to lie in the ven-
tricular floor, from the foramen of Monro to its posterior
extremity.

The anterior extremity of the corpus striatum is continuous
with the olfactory lobe (Fig. 173). Hence in the adult brain
the anterior end of the corpus striatum appears at the base of the
brain in the anterior perforated space, a lamina of grey matter
derived from the olfactory lobe (Fig. 18, p. 22). As the cerebral
vesicle grows over the optic thalamus (Fig. 173) the corpus

HUMAN EMBRYOLOGY AND MORPHOLOGY.

striatum comes to be applied to that body. The posterior
extremity of the cerebral vesicle, in which the tail of the caudate
nucleus is situated (Fig. 173), develops downwards, behind and
below the optic thalamus, forming the descending horn of the

Fig. 173. — Showing the Development of the Corpus Striatum in the floor and outer

wall of the Cerebral Vesicle.

lateral ventricle. Hence the occurrence of the tail of the caudate
nucleus and the amygdaloid nucleus on the roof of the descending
horn, both being intrinsic parts of the corpus striatum. The
corpus striatum is imperfectly separated, during the 4tli and
5 th months of foetal life, into two parts, the caudate and
lenticular nuclei, by the downgrowth, from the cerebral cortex
of the fibres which form the internal capsule, crusta, and pyra-
midal tracts.

Formation of the Island of Reil and Fissure of Sylvius. —

The formation of the corpus striatum, the Island of Beil, and
Fissure of Sylvius are part and parcel of the same process.

When the lateral wall of the cerebral vesicle is examined at
the 5th month (Fig. 175) an area of cortex is seen to be rapidly
becoming submerged by the overgrowth of the surrounding
cortex. The submerged area is the Island of Beil ; it covers
that part of the wall of the cerebral vesicle which is thickened
by the corpus striatum (Fig. 168). The submerged area becomes
triangular in shape, the apex being directed backwards ; it is

THE BRAIN AND SPINAL CORD.

213

bounded by three limiting sulci — an anterior, superior, and in-
ferior. The rising lips of cortex, which bound the limiting sulci,
form the temporal, fronto-parietal, and orbital opercula, and

Pig. 174. — Lateral Aspect of the Cerebral Hemisphere during the 2nd month.

ultimately meet over the submerged area (Fig. 176). The
fissure of Sylvius separates the opercula. It will be readily
grasped that the development of the corpus striatum prevents
the expansion of the insular part of the vesicle, whereas the
thin-walled mantle, out of which the other lobes of the brain
are developed, expands readily and overwhelms the thickened

fronto-par.op.

Fig. 175. — The same Aspect during the 5th month.

area. The corpus striatum begins to form during the 2nd month,
hence as early as that date the insular depression is visible on
the lateral wall of the hemisphere (Fig. 174).

The lower end of the stem of the Sylvian fissure also indents

214

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the Ehinencephalon, separating the uncinate gyrus from the parts
derived from the olfactory lobe.

The student is already familiar with the fact that the Island
of Eeil forms a cortical cap to the corpus striatum. The struc-
tures between the islandic cortex and the foramen of Monro
represent a section of the thickened wall of the cerebral vesicle
(Fig. 169). Convolutions appear on it at the 7th month, when
the rest of the cortex also becomes convoluted. Further, the
larger the area of cerebral cortex in any primate, the larger is
the Island of Eeil ; the more convoluted the cortex, the more
convoluted the Island. Flechsig has shown that the cortex of
the Island is joined to all the cortical areas of the mantle by
bands of association fibres. Hence the Island must be regarded
as playing a highly important part in co-ordinating the functions
of the brain.

The Opercula. — Three opercula grow up and cover the Island
of Eeil (see Figs. 175 and 176): (1) the temporal, (2) the
fronto-parietal, (3) the orbital. Cunningham, whose researches
into this region of the brain have become classical, found that
during the later months (7-9) of foetal life, the orbital operculum
in quite 50% of brains shows a subdivision into two, an upper,
the pars triangularis, and a lower, the pars orbitalis (Figs. 1/6
and 177). The subdivision occurs more frequently on the left

sup. precentral upper Ro/andic

\ post limb. fis. of. Sylu.

% / temp. op.

orbit/op. islandic area

Fig. 176.— The same Aspect during the 7th month.

THE BRAIN AND SPINAL CORD.

215

side than on the right, probably owing to the centres for speech
being situated on the left side. The temporal operculum rises first
(5th month) the others a month later. The opercula which bound
the posterior horizontal limb of the fissure of Sylvius are the first

Fig. 177. — Diagram of the Opercula and Fissure of Sylvius.

In A the orbital operculum is undivided ; in A it is subdivided. (After Cunningham.)

to come in contact. By the end of the first year after birth all
three opercula meet over the Island and completely hide it. At
birth there is still a part of the Island exposed behind the
orbital operculum and in lower human races this is frequently
the condition throughout life. The orbital opercula (pars
triangularis and pars orbitalis) become part of the centre of
speech and represent the latest additions to the human brain.
If the orbital operculum is not subdivided, which is commonly
the condition on the right hemisphere, then the anterior limb of
the fissure of Sylvius is not subdivided into anterior horizontal
and ascending parts (Fig. 177 A and B).

Comparative Anatomy of the Opercula and Island. — The

216

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Island oi Ileil and its opercula are only well developed in the
higher primates. Figs. 178, 179 and 180 represent the stages
in its evolution. In Fig. 178 the condition in dog-like apes is

Fig. 17S.— The Island of Reil and Fissures on the lateral Aspect of the Brain of a

dog-like Ape.

represented. Only the upper and lower limiting sulci of the
Island are present. The Island, which is small, is continuous
anteriorly with the frontal lobe. In anthropoids (gorilla, etc.)
the Island is larger ; the upper and lower limiting sulci are
present ; an imperfect anterior limiting sulcus (fronto-orbital
fissure) is present and partially separates the Island from
the orbital surface of the frontal lobe. In man all three
limiting sulci are present and completely isolate the Island,
and occasionally this is the condition (Fig. 179 B) in the

higher anthropoids, but it is in man only that the orbital

operculum grows up and meets with the other opercula.

This can be the more easily understood when it is remembered

that the orbital part of the 3rd frontal convolution is connected
with speech.

Temporary Fissures. — In the 3rd month of foetal life the
cerebral vesicles have thin walls, and when extracted from the
head show several fissures. These fissures, unlike those which
appear at the sixth and seventh months on the cortex of the
brain, are really inflections of the whole thickness of the cerebral
wall and are variable in number and position. It is possible

THE BRAIN AND SPINAL CORD.

217

that some of them are post-mortem products, for Hochstetter
found that when the brain of a 3rd month foetus, which had
been just removed from the mother, was hardened in situ, these

Fio. 179 A. — The more common Condition of the Island of Reil in Anthropoids.

Fig. 179 B. — The complete isolation of the Island of Reil, the condition seen con-
stantly in the Human Brain and occasionally in the Anthropoid.

temporary fissures were absent. But to this there are at least
two exceptions.

(1) The choroid fissure caused by the ingrowth of the velum
interpositum and choroid plexus (Fig. 170).

(2) The arcuate fissure on the mesial aspect, which has been
already described (Fig. 1 7 1 ). The hippocampal fissure, the
callosal, and probably the calcarine too, are remnants of this
fissure.

The Sylvian depression is also visible from the second month
onwards, but it is in no sense a fissure comparable either to the
temporary or even to the permanent.

The Fissures of the Brain. — The Principal Fissures of the
Brain include: (1) The Sylvian, (2) the Calcarine, (3) the

Hippocampal, (4) the 1st Temporal or Parallel, (5) the Parieto-
occipital, (6) Eolandic, (7) the Intra-parietal, (8) the Inferior
Pre-central, (9) the Collateral, (10) the Calloso-marginal. These,
with the exception of the first three (which have been already
described), appear at the commencement of the 6 th month. At

218

HUMAN EMBRYOLOGY AND MORPHOLOGY.

this date the human brain presents a marked resemblance in
the arrangement of its fissures to that of the dog-like ape
(Figs. 178 and 176).

The fissures and sulci are caused by a rapid increase in number
and size of the cortical cells ; the increase of the area of the
cortex leads to a crumpling up of its surface. The increased rate
of growth appears to affect certain definite areas, hence the fairly
constant forms into which the surface of the brain is thrown.

Affenspalte or Simian Fissure (Fig. 178). — In all ape brains
the anterior margin of the occipital lobe grows upwards and
forwards as an operculum, which covers the posterior margin
of the parietal lobe. The sulcus between the occipital operculum
and parietal lobe is the simian fissure or affenspalte. In the
human brain it is never developed owing to the great growth of
the posterior area of the parietal lobe ; the area which forms the
floor of the sulcus in apes is spread out on the surface of the
human brain. The ramus occipitalis of the intra-parietal fissure
lies in the floor of the simian fissure ; in the human brain the
ramus occipitalis is raised to the surface of the brain.

Sensori-motor Areas of the Brain. — The fissure of Rolando,
which begins by two depressions — an upper and a lower— appears
in the fifth month ; it divides the sensori-motor areas into
anterior and posterior parts. Sherrington and Grunbaum found,
however, in their experiments on the brains of anthropoids, that
the posterior area gave no reaction when excited artificially, and
that the fissure of Rolando formed the posterior boundary
of the motor cortex. The anterior area is further sub-

divided by two fissures (Fig. 176), the inferior pre-central, an
L-shaped fissure which commences as soon or even before the
fissure of Rolando, and the superior pre-central, quite a late
development, and evidently an isolated part of the superior
frontal sulcus. The post-Rolandic area of the sensori-motor

cortex is limited behind by the inferior and superior parts
of the post-central fissure. The superior part is of late origin.
The inferior limb of the post-central fissure is an intrinsic part
of the intra-parietal fissure (Fig. 178). From the pyramidal
cells of the sensori-motor cortex, processes grow out and form the
middle part of the internal capsule. They reach the spinal cord
during the 4th and 5th months of foetal life and become

THE BRAIN AND SPINAL CORD.

219

myelinated about the 5th and 6th months after birth. As
the processes grow downwards in the lateral wall of the cerebral
vesicle they pierce the corpus striatum, dividing it into the
caudate and lenticular nuclei.

The Secondary Sulci, which divide the superior and middle
frontal convolutions, the calloso-marginal, the parietal and occi-
pital gyri, appear in the 9th month. They are for the greater
part peculiar to the human brain.

THE CRANIAL NERVES.

The differentiation of the simple neural tube of the embryo
has thus far been followed into the complicated central nervous
system of the adult. It is now necessary to make a short survey
of the arrangement of the cranial nerves and see what evidence
they afford of a segmental arrangement of the cephalic part of
the neural tube.

The Cranial Nerves. — The segmental arrangement of the
nerves of the body has been already discussed (page 158). Even
although the human trunk is highly specialized the 33 or more
segments of which it is made up can still be recognised from
the arrangement of the spinal nerves ; each segment is constituted
on a similar principle, and it becomes increasingly difficult to
deny that man and the whole kingdom of vertebrates are derived
from a form in which all the segments of the body were
identical.

The head has become even more highly specialized than the
trunk, and in it evidence of segmentation is accordingly more
difficult to detect. Most of the evidence at present at our
disposal indicates the presence of nine segments in the head.
That is to say that the mammalian head is the derivative
of a structure which was made up of nine segments, every
one of which was originally constituted very much alike.
Each had a similar arrangement of nerves and muscles, a similar
arrangement of vessels, and provided with a similar pair of
appendages.

In Fig. 180 is diagrammatised the relationship of the cranial
nerves to the nine segments of the head. The olfactory and

j-jU human embryology and morphology.

optic nerves arise as processes of the neural tube, and are not
comparable to the remaining cranial nerves. Primarily each
segmental cranial nerve appears to have contained sensory and
motor fibres. The sensory fibres, like those of the spinal nerves,

Fig. ISO.— A Diagram to show the Relationship of the Cranial Nerves to the Primitive

Segments of the Head.

are developed from ganglionic cells derived from the neural crest,
and are of two kinds, somatic and visceral. The motor fibres
are developed from cells in the neural tube and are also
of two kinds, somatic and visceral. In the evolution of
the vertebrates there has been much reconstruction in the
arrangement of the segmental fibres, the sensory fibres of several
segments having become grouped together in the 5th nerve,
and the motor fibres of others in such nerves as the 10th
and 12th.

The Segments to which the Cranial Nerves belong. 1st
Cranial Segment. — The motor nerve is the 3rd or oculomiotor.

THE BRAIN AND SPINAL CORD.

091

The ciliary ganglion, a derivative of the Gasserian, represents the
sympathetic ganglion. Ganglion cells representing a vestigial
posterior root may be found on the trunk of the nerve. The
ophthalmic division of the 5th appears to represent its posterior
or sensory root.

2nd Cranial Segment. — The motor nerve is the 4th. The

sensory is represented by the superior maxillary division of the
fifth. Meckel’s ganglion represents the sympathetic ganglion.
It is known to be derived from the same group of nerve cells as
the Gasserian ganglion.

3rd Cranial Segment. — The motor nerve is the 6th and motor
fibres of the fifth. The sensory root is represented by the
inferior maxillary division of the 5th. The otic and sub-

maxillary represent its sympathetic ganglia.

4th Cranial Segment. — The motor nerve is the 7th. The

sensory root is represented by the chorda tympani and great
superficial petrosal, which are developed from the geniculate

ganglion (Dixon). The eighth nerve and its ganglia also belong
to the sensory system of this segment. The great superficial

petrosal represents a splanchnic nerve, the chorda tympani the
nerve on the anterior margin of the 1st visceral cleft (see

p. 34).

5th Cranial Segment. — The motor fibres of this segment have
probably been scattered. Some may still remain in the 9th
cranial nerve (glosso-pharyngeal) which is the chief nerve of the
segment. The ganglia on the trunk of the glosso-pharyngeal
represent the posterior root ganglion. The tympanic branch
and small superficial petrosal represent an afferent (sensory)
splanchnic branch.

6th, 7th, 8th and 9th Cranial Segments.— It has been already
mentioned (pages 152 and 161) that the four posterior cranial seg-
ments are probably trunk segments which have become modified
and added to the head. The anterior or motor nerve roots of these
four segments are combined in the 12th nerve. Motor visceral
fibres, which issue by the anterior roots of spinal nerves, here
issue by the vagus and bulbar part of the spinal accessory (all
of which are properly designated vagal fibres — Sherrington) and
represent the visceral motor fibres of the four posterior cranial
segments. The ganglia on the root and trunk of the vagus

222

HUMAN EMBRYOLOGY AND MORPHOLOGY.

represent part of a posterior root ganglion. From these ganglia
are developed the sensory visceral fibres connected with the fore
gut and all the structures derived from the fore gut or splanchno-
pleure of the fore gut. A vestigial posterior root ganglion may
occur on the 12th nerve.

The circuitous course of the spinal accessory is probably due to
the migration of the trapezius caudalwards from a cephalic to its
present position.

CHAPTER XYI.

DEVELOPMENT OE THE CIRCULATORY SYSTEM.

Veins. — (1) The Superior Vena Cava arises from the following
foetal vessels (Figs. 181 and 182):

(a) The part above the entrance of the vena azygos is the
terminal part of the right primitive jugular or anterior cardinal
vein ;

(b) The part below the entrance of the vena azygos major
arises from the right duct of Cuvier. The condition of these
venous trunks, the anterior and posterior cardinal veins and Ducts
of Cuvier, in a human embryo of the 3rd week is shown in Eig.
182. The condition shown is retained permanently in lower
vertebrates (Fishes, etc.).

[■ Vint. jug.

%

rt. subclau.

prim. jug.

uena

4— from prim. jug.

^ — pericardium
- from rt. d. of Cuuier

azyg. maj, pericardium cardin. ueinj.

from card. -4^ j

pericar.

\ ( aur.

sin. uen.)

nr^nrt

left duct
pleura of Cuuier

/( rt. duct
\of Cuuier

-rt. aur.

Pig- 181. Fig. 1S2.

Fig. 181.-— The Superior Vena Cava of the Adult.

Fig. 182. — The Embryonic Venous Trunks out of which the Superior Vena Cava
is formed.

fhe anterior Cardinal or primitive Jugular Vein, which drains
the anterior half of the body on each side with the posterior

224

HUMAN EMBRYOLOGY AND MORPHOLOGY.

cardinal vein, which drains the posterior half of the body, receive
a tiibutary (segmental vein) from each body segment. The
caidinal veins lie in the mesoblast on the dorsal side of the
coelom at the junction ot the splanchnopleure and somatopleure
(Fig. 183). By their union they form on each side the
Duct of Cuvier which conveys the blood to the sinus venosus
a contractile chamber opening into the primitive auricle. The
sinus venosus remains as a separate chamber of the heart in lower
"vertebrates, but in the course of mammalian development it
becomes partly merged in the right auricle of the heart.

jug. uem

card. vein,
somatopleure

stomodaeum

aorta

prim, auricle
sinus uenosus
pericardium
'uct of Cuuier
pleura.

peritoneum

splanchnopleure /iuer bud

Fig. 183. — Diagram to show the manner in which the Ducts of Cuvier encircle the
Coelom at the junction of the Pericardial and Pleural Parts (Iter venosum).
(After His.)

It is important to notice how each duct of Cuvier reaches the
sinus venosus (see Fig. 183). They pass from the dorsal to
the ventral surface of the body in the somatopleure and thus
encircle the coelom. The right and left ducts of Cuvier lead to
a constriction of the coelom, the fold in which each descends
being known as the lateral or venous meso-cardiuvi (Fig. 205,
p. 251). Ultimately, by the end of the 4th week, the part of
the coelom lying in front of the ducts of Cuvier is cut off from

DEVELOPMENT OF TIIE CIRCULATORY SYSTEM.

the rest; the part so cut off forms the pericardium (Fig. 201).
Thus the ducts of Cuvier are instrumental in separating the
pericardial from the pleural cavity. If the primitive pleuro-
pericardial communication (iter venosum of Lockwood) persists
between them, it occurs as a foramen in the pericardium behind
the part of the superior vena cava derived from the duct of Cuvier.

2. The Vestigial Fold and Oblique Vein of Marshall. — In the
human embryo during the 3rd week and for some weeks after-
wards there is a right and left duct of Cuvier and corresponding
cardinal veins (Fig. 185). A left superior vena cava is present
and may persist. The vestigial fold and oblique vein of Marshall
(Fig. 184) are all that usually remain of the left superior vena

Fig. 185.— Diagram of the Sinus Venosus and Ducts of Cuvier of the human embryo
about the 3rd week.

cava. The right superior vena cava within the pericardium
passes in front of the right pulmonary vessels, and is bound to
them by a mesentery or fold of serous pericardium ; the left has
a similar relationship (Fig. 184); when it disappears this fold
remains in front of the left pulmonary vessels as the vestigial
fold. The intra-pericardial part of the left vena cava or duct of
Cuvier becomes the oblique vein (Fig. 184): it turns round the

p

226

HUMAN EMBRYOLOGY AND MORPHOLOGY.

left auricle to terminate in the left horn of the sinus venosus
(coronary sinus). The extra-pericardial part of the left duct of
Cuvier joins the superior intercostal vein (Fig. 184). Both right
and left superior venae cavae persist in some lower mammals.

The left superior intercostal vein represents the following
embryonic vessels (see Fig. 184):

(a) Anterior part of the left posterior cardinal vein ;

(b) The extra-pericardial part of the left duct of Cuvier ;

(c) The terminal part of the left primitive jugular vein.

3. The Left Innominate Vein opens up as a channel of com-
munication between the two primitive jugular veins, the left
superior vena cava undergoing a simultaneous process of atrophy
(Fig. 184).

4. The Subclavian Veins are developed in the 4th week with
the outgrowth of the fore-limb buds ; they open into the primitive
jugulars (Fig. 184).

5. The Primitive Jugular Veins escape from the cranial cavity
in front of the ear. A trace of the opening may occasionally be
detected at the root of the zygoma behind the post-glenoid spine
(A. Cheatle). The petro-squamous sinus represents the intra-
cranial part of this vein (p. 56). As the internal jugular vein
opens up, the primitive jugular vein within the skull becomes
atrophied. The temporo-maxillary vein and the external jugular
vein probably represent the extra-cranial part of the primitive
jugular vein. It must be remembered that the caudal ward
migration of the heart affects the primitive relationship of the
veins in the neck. They are drawn backwards with it.

Veins formed from the Posterior Cardinals of the Embryo.
— 1. From the branches of the Cardinal Veins. Each posterior
cardinal vein receives on its own side — (a) A branch from each
body segment from the lower cervical to the last caudal. These
become the intercostal, lumbar and sacral veins. (b) The seg-
mental veins of the intermediate cell mass (Fig. 85), which
become the suprarenal, renal, spermatic, ovarian, uterine and
vesical veins. (c) When the hind-limb buds grow out their
veins join that part of the cardinal veins which become the
common iliac. The lateral sacral, ilio-lumbar and ascending
lumbar veins are anastomotic channels formed between the
segmental veins.

DEVELOPMENT OF TIIE CIRCULATORY SYSTEM.

227

From the right cardinal vein are formed (1) the vena azygos
major; (2) the post-renal part of the inferior vena cava (Fig. 186).

The part above the entrance of the right renal becomes the
vena azygos major ; the part below, the inferior vena cava.
Hence it is that the origin of the vena azygos can commonly be
traced to the renal vein. The ascending lumbar vein, which also
ends in the vena azygos major, is, as already mentioned, a new
anastomotic channel.

From the left cardinal arise (Fig. 186) — (1) Part of the left
superior intercostal vein ; (2) Left superior azygos vein ; (3) Left

right duct of Cuuier

Fig. 186. — The Remnants of the Posterior Cardinal Veins in the Adult. The new
channels are shaded. (After Hoehstetter.)

inferior azygos, which commences in the left renal vein, and also
receives the left ascending lumbar. The post-renal part of the
left cardinal disappears in higher mammals; occasionally it persists
in man and very frequently in the rabbit.

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The greater part of the left common iliac vein arises, like the
left innominate, as a communicating channel between the posterior
cardinals. It is formed as the post-renal part of the left cardinal
becomes obliterated (Fig. 186).

The Inferior Vena Cava. — The post-renal part is formed from
the right cardinal vein (Fig. 186). The pre-renal part, with the
mesial part of the left renal vein, is quite a new formation which
opens up a short circuit to the heart for the blood of the lower
half of the body. The formation of this new channel leads to the
retrogression of the thoracic stages of the cardinal veins and their
formation into the azygos veins. The exact date of the origin of
the inferior vena cava in the human foetus is not known ; the new
channel is said to grow out from the ductus venosus of the liver,
and growing downwards opens up a connection with the right
and left cardinal veins at the entrance of the renal vessels
(Fig. 188). Thus in the formation of the inferior vena cava are
included three elements — (1) the terminal or upper part of the
ductus venosus ; (2) the new channel ; (o) the inferior or post-
renal part of the right cardinal.

Occasionally it happens that pressure on the hepatic part of
the vena cava, from tumours, cirrhosis of the liver, etc., leads to
the azygos veins, the early foetal channels, being again opened up.

The Portal Vein.— The Portal Vein is formed out of the two
vitelline veins — the first of all the veins to be developed. They
end in the posterior chamber of the tubular heart of the embryo—
the sinus venosus. The vitelline veins, right and left, arise from
ramifications on the yolk sac and pass in the splanchnopleure to
the sinus venosus (Fig. 187). The nutriment within the yolk sac
is thus carried to the heart and distributed by the heart to the
tissues of the embryo and yolk sac. The commencement of the
left vitelline or omphalo-meseraic vein disappears. The right
(Fig. 187) forms the superior mesenteric vein. It commences
on the yolk sac, of which a remnant (the neck) may remain as
Meckel’s diverticulum.

The terminal parts of the two vitelline veins are joined by
three transverse communications, the upper two of which are in-
cluded in the portal vein. The uppermost of the three afterwards
lies in the transverse fissure of the liver (Fig. 188). lhe middle
communication lies on the dorsal aspect of the duodenum

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

229

(Fig. 188). The splenic vein and inferior mesenteric pass in
the dorsal mesentery (Fig. 187) and join this transverse communi-
cation which may be named the supra-duodenal junction. The

right uit. uein

hepatic bud.
uent. mesentery
left, uitelline uein J;

'yolk sac.

sinus uenosus

- left duct of Cuuier

left umb. uein
stomach
spleen
dorsal mesentery
splenic uein
inf. mes. uein

hind gut

sup. mes. uein

Fig. 187. — The Left Vitelline Vein of an Embryo of the 4th week.

third or lowest of the junctional trunks is situated on the ventral
aspect of the duodenum (Fig. 188). Two parts may be recognised
in the portal vein of the adult: (1) the part which lies behind
the pancreas and duodenum ; (2) the part in the gastro-hepatic
omentum and transverse fissure of the liver.

The retro-duodenal part is formed out of the left vitelline
vein and the supra-duodenal junction (Fig. 188); the omental
stage is formed out of the right vein and the uppermost of the
junctional trunks (Fig. 188). The part of the portal vein within
the transverse fissure of the liver represents the third or upper-
most junction between the left and right vitelline veins.

The Hepatic Veins are formed out of the terminal parts of the
vitelline and umbilical veins. These veins end at first in the

230

HUMAN EMBRYOLOGY AND MORPHOLOGY.

sinus venosus (Fig. 187). The liver is developed between and
aiound the vitelline and umbilical veins, near their termination
in the sinus venosus. The veins are broken up and a fine
intra-hepatic venous network takes their place. Thus it comes
about that the vitelline veins are transformed into the veins of
the portal and hepatic circulation. All the foetal and umbilical
blood is at first poured through the liver.

sup. uen. cava. //'

c:zy. vein

rt. umb.
rt. vit,
inf. ven. cav.
ductus ven. —

in transv. fisX
of liver J

portal vein

.{.— vein of Marshall
stomach

remnants of left umb.

rt vit. vein

left vit. vein
part disappears
left umb. vein

\ rt. umb. vein ( disappears )
duodeno-jej. p/eure

left vit. vein.

left vit. vein

sup. mesent. vein

Fig. 1S8. — Diagram showing the Formation of the Ductus Venosus, and the fate of
the Umbilical and Vitelline veins. The arrows show the parts of the Vitelline
Veins which become the Portal Vein.

The Ductus Venosus is a new channel formed between the
uppermost of the anastomoses between the right and left vitelline
veins and the sinus venosus whereby the greater part of the
umbilical blood is short-circuited to the heart without passing
through the liver. It appears after the liver bud has broken up
the vitelline venous trunks (Fig. 188). After birth, when a
short circuit is no longer required between the foetal circulation
and heart, it becomes reduced to a fibrous cord. It occupies the
posterior part of the longitudinal fissure of the liver and lies
within the hepatic attachment of the gastro-hepatic omentum.

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

231

The pre-renal part of the inferior vena cava is developed as an
outgrowing channel from the ductus venosus (Fig. 189).

The Umbilical Veins. — The umbilical vein at birth consists
of two parts: (1) A part within the umbilical cord; (2) another
within the body, enclosed in the falciform ligament and anterior
half of the longitudinal fissure of the liver. It joins there the
ductus venosus and portal vein (Fig. 189). The condition of the
umbilical veins in a human embryo of three weeks is shown in
Fig. 190. They are formed after the vitelline veins but before
the ducts of Cuvier, which afterwards terminate in the sinus with
the umbilical veins. The veins lie in the somatopleure and
drain the blood of the chorion, a derivative of the somatopleure.
It passes from the chorion to the body wall in the umbilical
cord, which is also formed from the somatopleure as well as
allantois. In front the vein of each side joins the sinus venosus
with the duct of Cuvier. There is a right and left vein, but in

Fig. 189. — Diagram of the Remnants of the Umbilical Vein in the Adult— viewed

from behind.

the cord they have fused into one. Within the body the right
umbilical vein completely disappears during early embryonic life.

The outgrowth of the liver-bud breaks up not only the
vitelline veins but also the umbilical at their junction with the
sinus venosus (Figs. 185 and 188). Thus the umbilical blood as

232

HUMAN EMBRYOLOGY AND MORPHOLOGY.

well as the vitelline comes to be poured into the liver. The left
umbilical vein within the body remains permanently ; when its
terminal part is broken up by the outgrowth of the liver it
becomes united with the uppermost of the transverse communica-

tions between the right and left vitelline veins, which, as already
mentioned, form the part of the portal vein within the transverse
fissure of the liver. The left umbilical vein thus comes into com-
munication with the ductus venosus (see Figs. 188 and 189).

DEVELOPMENT OF THE 'HEART.

L

The Heart. — In the 2nd week, while the embryo is still in
the blastoderm stage, a contractile tube appears in the splanchno-
pleure on each side (Fig. 191). Each tube receives blood from

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

233

the yolk sac by a vitelline vein ; each pumps its blood into an
aortic stem which terminates as the artery of the yolk sac.

pericardiac part of coelom

Fir;. 191. — Transverse section of the Blastoderm showing' the Right and Left Cardiac
Tubes situated in the Splanchnopieure.

These two tubes become fused together to form a simple tubular
heart by the 3rd week. The cardiac tubes unite as the splanchno-
pleures come together (see Figs. 191 and 192). The lining
membrane of the cardiac tubes is probably furnished by an inva-
gination of the hypoblast (Fig. 191). The heart is at first

neural canal

Fir;. 192. — Transverse section at a more advanced stage showing the union of the
Splanchnopleures to form the Mesocardia and the fusion of the Right and Left
Cardiac Tubes.

suspended by a mesentery, which stretches from the fore-gut
(Fig. 192) to the ventral median line of the body wall and
separates the anterior end of the coelom into right and left
halves. The mesentery is formed out of the fused splanchno-

234

HUMAN EMBRYOLOGY AND MORPHOLOGY.

pleures (lig. 192). The part of the cardiac mesentery between
the heart and the lore-gut forms the dorsal mesocardium, the part
between the heart and the ventral wall, the ventral mesocardium.
Ihe anterior end ot the coelom which is occupied by the heart
becomes the pericardium. The pericardium is situate beneath
the primitive pharynx.

Demarcation into Chambers. — In the third week the heart
undergoes three important changes :

(1) Ihe dorsal and ventral mesocardia disappear and the
heart, all but the anterior and posterior extremities, is left free
in the anterior end ol the coelom. This can best be under-
stood by a reference to Fig. 193. The heart is there seen

Fig. 1U3. — Lateral view of the Heart and Pericardium to show the Attachments of
the Dorsal and Ventral Mesocardia (schematic).

suspended between the fore-gut and ventral wall. The tubular
heart is formed behind by the union of the right and left
vitelline veins from the yolk sac and ends in front as the first
or mandibular aortic arches. The posterior end of the meso-
cardium, both the ventral and dorsal parts, persists — the part
which encloses the sinus venosus and this forms the primitive
basis of the diaphragm or septum transversum (Fig. 202, p. 244).

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

235

(2) The tubular heart shows demarcations into four parts
(Fig. 194): (a) Sinus venosus ; (b) Primitive auricle; (c) Primi-
tive ventricle ; (cl) Conus or bulbus arteriosus. In this condition
(third week) the human heart is exactly like that of a fish, viz., a
tubular four-chambered heart which pumps blood into the
branchial or aortic arches.

(3) The disappearance of the mesocardium allows the heart
to become twisted and bent. Two chief bends are formed which
materially help to give the heart its adult shape (Fig. 195):

Fig. 194. — The Primitive Divisions of the Heai't.

(a) The Ventricular Bend. — The ventricular part of the tube is
bent into a Ar-shaped piece, the apex of the V-shaped loop being
turned towards the right.

(b) The Auriculo-ventricular Bend. — The ventricular part is bent
in front of the auricular so that the auricle becomes dorsal to the
ventricle (Fig. 195).

The Sinus Venosus. — The sinus venosus, the first chamber
of the foetal heart, is formed by the union of the vitelline
veins ; the umbilical veins and ducts of Cuvier come subse-
quently to open in it (Fig. 185). The sinus is imbedded in
the persistent posterior part of the mesocardium (Fig. 202).

HUMAN EMBRYOLOGY AND MORPHOLOGY.

rhrough the ventral part of the mesocardium the ducts of
Cuvier reach the sinus from the somatopleure (Fig. 183). In
tishes and in the human embryo the sinus pumps the blood
into the primitive auricle, and its orifice into the auricle is
protected by two valves, right and left (Fig. 197).

— stomodaeum

vent. bendA';

conus arteriosus

aur. uent bend

pericardium
prim. aur.
sinus uenosus
sept, trans
duct of Cuuier

liver bud

pleura,

gut.
uit. vein

umb. vein

Fig. ] 05. — Showing the two chief JBends which occur in the Heart during the 3rd week.

Fate of the Sinus Venosus (Fig. 196). — The sinus venosus
shifts towards the right side of the primitive auricle and ulti-
mately forms part of the right auricle and the coronary sinus.
The part which it forms of the right auricle is indicated by the
entrance of the following vessels which primarily terminate in
the sinus :

(1) The superior vena cava (the right duct of Cuvier);

(2) The inferior vena cava, which also opens into the sinus;

(3) The oblique vein of Marshall (left duct of Cuvier), which
opens into the left horn of the sinus venosus. The left horn of
the sinus becomes the coronary sinus. A groove, the sulcus
terminalis, which is marked on the interior of the right auricle by
a crest, runs down on the anterior wall of the right auricle from

DEVELOPMENT OF THE CIRCULATORY SYSTEM. 237

the superior to the inferior vena cava, and indicates the junction
of the primitive auricle with the sinus venosus.

sup. uen. cau. n

*1 If

p' left sup. mtercost

uein of Marshall

uena. azyg.

sup. uen. cau.-

part of rt. aur.
left uenous ualue

Eustach. ualue

p rignt uenous

ualue) / / | cor' sinus

open, of cor. sinus
Thebesian ualue
inf. uen. caua

Fig. 196.— Showing the Structures formed from the Sinus Venosus.

right uen. ualue
right aur.

inf. uen. cau.

uen. cau.

sept, secundum

primum

left uenous ualue
left aur.

open, sinus, uenosus.
r post

\ endocard . cushion
aur. uent. canal

right uent.

inter-uent. sept.

Fig. 197.— Section of the Heart of a 5th week human foetus showing the Right and
Left Venous Valves which guard the entrance of the Sinus Venosus into the
Primitive Auricle. (After His.)

The Valves of the Sinus Venosus. — Eight and left lateral valves,
(venous valves) guard the entrance of the sinus to the primitive

238

HUMAN EMBRYOLOGY AND MORPHOLOGY.

auricle and prevent the regurgitation of blood when the auricle
contracts (Fig. 197). The right valve becomes reduced, and
ultimately forms the Eustachian valve, and the Thebesian which
is always connected with the Eustachian (Fig. 196). The part of
the Eustachian valve prolonged to the annulus ovalis is a new
formation. What becomes of the left valve is not certain ; it
may disappear completely, but more probably it amalgamates
with, and forms part of the septum primum (see Figs. 196,
197, and 198). The Eustachian and Thebesian valves also help
to indicate the part of the right auricle, which is formed
from the sinus venosus, for they bound the right side of the
entrance of the sinus to the auricle.

The Cardiac Septa. — Every modification of the heart, from its
simple tubular form in fishes to its complete division into separate
pulmonary and systemic pumps, can be seen in the vertebrate
series. In amphibians the primitive auricle becomes divided
into right and left chambers, but the ventricle is undivided. In
crocodilia an incomplete ventricular septum, with a complete
auricular, may be seen. In birds and mammals the auricular
and ventricular septa are complete, and the sinus venosus becomes
part of the right auricle. Occasionally the septa are incompletely
formed in man, conditions found in the lower vertebrates being
thus produced.

The division of the simple tubular heart into right and left
halves is rendered possible by four changes which take place in
its shape :

(1) The ventricle, instead of being a bent tube, becomes
dilated and bag-like.

(2) The opening of the sinus venosus in the primitive auricle
migrates towards its right side, and thus opens in that part of the
primitive auricle which becomes the right (Fig. 197).

(3) The auricle sends out an appendix on each side; these
grow forwards and nearly surround the bulbus arteriosus.

(4) The communication between the common auricle and
ventricle is drawn out into a tube — the auriculo-ventricular canal
<Fig. 198).

The Division of the Heart is effected by five Elements or Septa

(Fig. 198). These are :

1. The endo-cardial cushions.

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

239

2. The Inter- ventricular septum.

3. The Aortic septa.

4. The septum primum of the auricle.

5. The septum secundum of the auricle.

(1) The auricular or auriculo-ventricular canal is divided by
anterior and posterior endocardial cushions, which grow out and
meet, dividing the auricular canal into the right and left auriculo-
ventricular orifices.

(2) The inter- ventricular septum rises from the floor of the
common ventricle and grows upwards towards the aortic orifice
and endocardial cushions (Figs. 197 and 198).

(3) In the aortic bulb (Fig. 198) appear anterior and posterior
aortic ridges, arranged spirally. They fuse together ; their lower
extremities unite with the inter-ventricular septum. The conus

aorta

Fig. 198.— Diagram of the Septa of the Heart viewed on the right side.

or bulbus arteriosus is thus divided into an anterior or pulmonary
part connected with the right ventricle, and a posterior or aortic
part connected with the left ventricle.

Pars Membranacea Septi. — The three septa just described meet

240

HUMAN EMBRYOLOGY AND MORPHOLOGY.

about the 6th week, and are fused together by a thin mem-
brane— the pars membranacea septi (Fig. 198). It is seen
under cover of the septal segment of the tricuspid valve and
is the last part of the septum to form ; occasionally it fails
to form, and a communication exists between the ventricles.
This condition is commonly associated with a partial failure in
the formation of the pulmonary artery. Such a condition may
give rise to cyanosis ; the subjects of it commonly die before 15 of
pulmonary tuberculosis. The walls of the right and left ventricle
in such cases are of equal thickness, for both act as common
systemic and pulmonary pumps:

(4) The septum primum of the auricle springs from the
posterior wall of the primitive auricle, and falls like the blade
of a guillotine on the endocardial cushions with which it fuses
(Figs. 197 and 198). Its upper border separates from the
auricular wall. It forms the septum ovale. As it descends
it involves and fuses with the left venous valve.

(5) The septum secundum grows down from the roof and
posterior wall of the primitive auricle to the right of the
septum primum. It forms the annulus ovalis and the auricular
septum above it (Fig. 198).

The foramen ovale (Fig. 198) is an interval left between
the two auricular septa ; the Eustachian valve becomes connected
with the annulus (septum secundum). In 75 per cent, of
people, following Fawcett’s statistics, the foramen becomes closed
in the first year after birth. In 25 per cent, it remains partially
open, but even when an opening remains the blood could pass
from the right to the left auricle only when the pressure within
the right auricle is greater than within the left. The valvular
action of the septum ovale prevents the passage of blood from
the left to the right auricle. The foramen ovale is an adapta-
tion to the foetal form of circulation.

Thus the primitive auricle is divided into right and left
chambers by two septa — septum primum and secundum ; the
bulbus arteriosus by the anterior and posterior aortic septa ;
the auriculo-ventricular canal by the anterior and posterior endo-
cardial cushions ; and, lastly, the primitive ventricle into right
and left by the inter-ventricular septum.

Tricuspid and Mitral Valves. — It is to be remembered

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

241

that the interior of the ventricular cavities during the second
month is filled with a muscular sponge work, like the ventricle
of the frog’s heart (Fig. 199). Out of this muscular network

are formed :

(1) Chordae tendineae ;

(2) Musculi papillares ;

(3) The moderator band ;

(4) Trabeculae and columnae carneae ;

(5) Musculi pectinati (in the auricular appendages).

Fig. 199. — Section of the Ventricles of the Foetal Heart, showing the Muscular
Sponge Work within their Cavities. (After His.)

The endocardium lining the auricular canal (see Figs. 197
and 199) becomes folded or protruded within the ventricles by
a shortening of the auricular canal. The funnel-shaped fold or in-
flection of endocardium is the basis of the auriculo-ventricular valves.
They are formed at the same time as the septa of the heart.
Originally funnel-shaped, the left or mitral comes to show two
cusps — septal and left, the right or tricuspid three segments —
septal, anterior and posterior. The funnel shape is frequently
partially retained. The cardiac tissue on the outer aspect of
the valves is opened out into a sponge-work continuous with that
of the ventricle. Hence the attachment of the chordae tendinae
on the outer surfaces and ed"es of the valves.

O

The Semilunar Valves. — The bulbus arteriosus is divided by
the aortic septa into an anterior and right part — the pulmonary
aorta, and a posterior and left — the ascending aorta. Just

right aur-uent.
tricuspid

intervent sept.

Q

242

HUMAN EMBRYOLOGY AND MORPHOLOGY.

before the division takes place four cushions of the endocardium
arise at the junction of venticle and aortic bulb (Fig. 200 A). The
four cushions anterior, posterior and two lateral- — become cupped

Fig 200. — The origin of the semilunar valves.

A. The four Endocardial Cushions in the Aortic Bulb.

B. The division of the Lateral Cushions to form two Aortic and two Pulmonary Semi-

lunar Valves.

above and valvular. When the aortic septa are formed, each
lateral cushion is divided in two, thus forming the two postero-
lateral semilunar valves of the pulmonary aorta and the two
antero-lateral of the aorta. The anterior and posterior cushions
remain as the anterior pulmonary semilunar valve and the
posterior aortic valve (Fig. 200 B).

Remnants of the Foetal Circulation in the Adult. — The
nature of these remnants has been already described ; they need
be only enumerated here. They are :

1. The Obliterated Hypogastric Arteries ;

2. The Umbilicus;

3. The Round Ligament of the Liver ;

4. The Fibrous Remnant of the Ductus Venosus ;

5. The Eustachian Valve ;

6. The Foramen Ovale ;

7. The Fibrous Remnant of the Ductus Arteriosus.

The Coelom, Splanchnocele or Cavity of the Body Wall.—

We have already seen that in the primitive blastoderm (Fig. 191)
a cleft is formed between the somatopleure and splanchno-
pleure. It is highly probable that the mesothelium which

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

243

lines the coelom is a derivative of the hypoblast, and that
the coelom was originally a series of segmental diverticula
derived from inflections of the hypoblast ; yet, from a clinical
point of view, it is better to regard it as part of the lymphatic
system, and really a very extensive lymph space. It allows
the heart, lungs and abdominal viscera to undergo movements
with a minimum of friction.

As may be seen from Fig. 72, page 93, the coelom at first
•extends beyond the embryo, between the layers of the somato-
pleure, which go to form the foetal membranes. With the union
■of the right and left layers of the somatopleure at the umbilicus,
part of the blastodermic coelom is shut within the body of
the embryo — the intra-embryonic part of the coelom. It is
out of this part that the pericardium, pleurae, peritoneum and
•tunicae vaginales are formed. The coelom commences in front

244

HUMAN EMBRYOLOGY AND MORPHOLOGY.

below the pharynx and ends behind on the hind gut (Fig. 201).
This cavity is originally divided into a right and left half by
the dorsal and ventral mesentery, formed by the united
splanchnopleures (Fig. 192). The ventral mesentery disappears
in the anterior and posterior parts, so that the right and left
halves of the coelom communicate ; an intermediate part persists
and helps to form the diaphragm (Fig. 202).

The Divisions of the Coelom. — Out of the anterior part of
the coelom (Fig. 201) is formed the pericardium. It lies beneath
and behind the primitive pharynx (Fig. 202). The pericardial
part of the coelom by the third week (Fig. 201) is expanded and
thrust downwards and backwards, and communicates by a narrow
neck or isthmus ( iter venosum) with the pleuro-peritoneal part on
each side of the mesentery. The duct of Cuvier encircles the neck
or isthmus on each side, and causes the constriction (Fig. 183).

dorsal aorta

transuerse sinus
post, meso-

1st aortic arch

left duct of Cuvier
lung bud in mesentery
dorsal mesogast.
tom.

art meso-card.

con. arter.y

pen card. ^ sinus uenosus /
amnion -

septum transv.

yolk sac.

ventral mesentery

Fig. 202.— Diagram to show the manner in which the Heart is fixed within the
Pericardium by the Arterial and Venous Mesocardia in a human embryo of 3
weeks.

This condition is permanent in many fishes. With the origin of
the lungs the part of the coelom which forms the isthmus undergoes
a great expansion by the ingrowth of the lung buds (Fig. 201).
Thus out of the isthmus of the right and left sides are gradually

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

245

formed the pleural cavities. The pleurae expand until, instead
of lying as minute passages above and behind the pericardium,
they come to completely cover it. The abdominal part of the
coelom forms the peritoneal cavity ; a small part on each side
is shut off in the scrotum, the tunica vaginalis.

The Pericardium. — In Fig. 202 the relationship of the heart
to the pericardium is shown during the 3rd week of foetal life.
The heart is tubular ; the dorsal and ventral mesocardia (Fig.
193) have already disappeared except at two points, where the
heart still remains attached. These two points are (Fig. 202):

(1) In front where the bulbus arteriosus passes out under the
pharynx to divide into right and left ventral aortae from which
the right and left visceral aortic arches arise. The mesocardium
which binds it here may be named the arterial mesocardium.

(2) Behind the sinus venosus is imbedded in the mesentery
or posterior mesocardium, through which the great veins reach it.
The mesocardium which binds it behind may be named the
venous mesocardium.

sup. u. cau.

uenous meso-card.-jf
right pul. veins

uenous meso-card.

aorta

arterial meso-cardium
pulm. art.

transverse sinus
left pul. veins

diaph.

diaph.

oblique sinus

Fig. 203.— View of the Interior of the Pericardium showing the attachments of the
heart to its dorsal aspect by the Arterial or Venous Mesocardia.

In Fig. 203 is shown the fixation of the heart within the peri-
cardium of the adult. The arterial and venous mesocardia can
be recognised, only somewhat altered in position and form.

(1) The bulbus arteriosus becomes the ascending aorta and

24G

HUMAN EMBRYOLOGY AND MORPHOLOGY.

pulmonary aorta, and these are attached together to the fibrous
pericardium by the arterial mesocardium (Fig. 203).

(2) The sinus venosus, which becomes part of the right auricle,
and its vessels are still attached by the venous mesocardium.
The heart has become so doubled on itself that the venous meso-
cardium comes almost in contact with the arterial mesocardium,
the narrow space which separates them being the transverse sinus
(Fig. 203). By comparing Figs. 1 93, 202 and 203 it will be
seen that the transverse sinus is formed by the breaking down of
the dorsal mesocardium of the heart.

The venous mesocardium becomes much more extensive by the
ingrowth of the pulmonary veins. These grow in from the lungs,
and pierce the pericardium to reach the left auricle. They reach
the auricle through the mesentery or venous mesocardium of the
sinus venosus (Fig. 202). The ingrowth of the left pulmonary
veins causes a prolongation of the venous mesocardium to the
left side ; when the heart is removed the venous mesocardium is
seen to be F-shaped in section. The oblique sinus lies in the
concavity of the venous mesocardium (Fig. 203).

Ectopia Cordis. — Occasionally children are born with their
hearts exposed on the surface of the chest. In extreme cases,
only the dorsal wall of the pericardium is present, and it is flush
and continuous with the skin of the chest. In such cases there
has been either a failure on the part of the right and left somato-
pleures to unite in the ventral line, or, more probably, they did
unite, but an early cystic condition led to a rupture of the peri-
cardium and overlying layer of the somatopleure. In such cases
the sternum is partially absent, or if present it is cleft, the right
and left halves being widely parted. The pathology of the con-
dition is probably similar to that of- ectopia vesicae.

The Dorsal Aortae. — In the 3rd week there are still two
dorsal aortae (Fig. 205). The blood passes from the two ventral
aortic stems through the aortic visceral arches to end in a dorsal
aorta on each side (Fig. 202). These run backwards side by side,
and at first end in the yolk sac, but with the formation of the
allantois they pass out on the body stalk to the membranes and
placenta. The umbilical arteries are thus the direct continuations
of the dorsal aortae (Young and Robinson) ; the middle sacral artery
is an inter-segmental vessel of later formation (Fig. 221, p. 273).

DEVELOPMENT OF THE CIRCULATORY SYSTEM.

247

About the end of the 3rd week the dorsal aortae fuse together
from about the region of the 4th dorsal segment to the 4th
lumbar. In front of and behind these points they remain
separate.

The fate of the two dorsal aortae in front of the 4th dorsal
vertebra has been already dealt with (p. 35 and Figs. 28 and
29). It remains to consider what becomes of them in the pos-
terior extremity of the body. Following the researches of Young
and Eobinson, the dorsal aortae beyond the 4th lumbar segment,
where they remain apart, form :

(1) The common iliac arteries;

(2) The internal iliac — in part at least;

(3) The hypogastric arteries — intra-abdominal parts of um-
bilical ;

(4) The umbilical arteries — in the umbilical cord.

The authors just mentioned have described in the embryos
of several mammals a system of arterial arches on each side
of the hind gut resembling the aortic arches on each side of
the fore gut (see Fig. 221, p. 273).

Red Corpuscles of the Blood. — The first red corpuscles are
nucleated, capable of division, and are formed in mesoblastic cells
of the splanchnopleure of the yolk sac and in the somatopleure
of the amnion and chorion. The nucleated red corpuscles
gradually become the biconcave, non-nucleated disc, but even
at birth nucleated red corpuscles are still present. In the early
half of foetal life the liver plays a leading part in their forma-
tion ; in the latter half the spleen, red marrow and connective
tissues become the chief sites of their formation. After birth
they are formed almost entirely in the red marrow.

White blood corpuscles are -produced at a later date than the
red blood corpuscles. They also are produced by cells of the
mesoblast.

Lymphatic Vessels. — Lymphatics are developed in the chick by
the end of the 3rd day. It is some time before the lymphatic
system opens into the blood system by the formation of a thoracic
duct.

Certain important changes take place in the formation of the
human lymphatic system about the 5th month. Lymphatic
glands then begin to form in the connective tissue throughout the

248

HUMAN EMBRYOLOGY AND MORPHOLOGY.

body. Peyer’s patches begin to appear in the intestine. Until
then the connective tissue of the body contains a jelly-like ground
substance corresponding to Wharton’s jelly in the umbilical cord.
At the 5th month the mucoid substance in the connective tissue
begins to be absorbed (Berry Hart).

Until the fifth month the marrow of bones is composed of
embryonic mesoblastic tissue (primary marrow). After the fifth
month this becomes invaded by lymphoid tissue (white marrow)
which in certain bones (vertebrae, etc.) becomes transformed
further into red marrow. (Hamar.)

CHAPTER XVII.

THE RESPIRATORY SYSTEM.

Introductory. — In the 3rd week the arrangement of the
heart and branchial arches in the human embryo is that of a
water -breathing animal. In water- breathing animals the
tubular heart expends its force in pumping the blood through
the visceral arches — necessarily weakening the circulation in
the rest of the body beyond them (Figs. 21 A and B). The
blood that returns to the heart and from which the heart
is nourished is purely venous. In amphibians an air-
breathing apparatus appears— hence the division in them of
the auricular chamber, one chamber receiving systemic, the
other pulmonary blood. In amphibians the air is pumped into
the lung by the mouth and pharynx. In birds and mammals
the division of the heart into pulmonary and systematic pumps
becomes complete ; the chest wall is developed as a respiratory
apparatus. The pharynx may be regarded as the respiratory
organ of water breathers, and it is from the floor of this chamber
that the pulmonary or air-breathing structures are developed.
The arrangement of the heart and branchial arches in the human
embryo can be explained only on the supposition that the air-
breathing animals are the descendants of a water-breathing
stock. The addition of the pulmonary system commences in
the human embryo at the 3rd week.

Development of the Pulmonary System. — In the 3rd week,
towards the end of it, a deep groove appears in the floor of
the primitive pharynx and oesophagus. The groove or trough-
like depression of the fore-gut commences between the ventricle
ends of the 4th arch and stretches almost to the stomach
(Fig. 204). The furcula, formed from the ventral parts of the

250

HUMAN EMBRYOLOGY AND MORPHOLOGY.

4th arch, bounds the pulmonary groove ; in its anterior part,
which is the most prominent, is developed the epiglottis; its
lateral margins, which bound the pulmonary groove, form the
aiyteno-epiglottic folds. The margin of the groove Thus be-

tuberculum impar.

1st recess ( sa/iuary glands )
2nd recess (tonsil)
median thyroid bud.
furcula (epiglottis)

3rd recess (thymus)

4th recess (tat. thyroid bud)
— aryteno. epi. fold
coelom (pericardium)
pulmonary groove

stomach

Fig. 204. — Floor of the Pharynx and Oesophagus of a human embryo of 3 weeks
showing the Furcula, Pulmonary Groove, and Diverticulum. (After His.)

comes the upper aperture of the larynx. Two points should
be noted in connection with the relationships of the oesophagus
at the 3rd week: (1) like that of a fish, it is extremely short;
(2) it lies between the right and left cavities of the coelom
(Figs. 204 and 205) in the dorsal attachment of mesocardium
of the sinus venosus (Fig. 202). (3) The part of the coelom

which lies at each side of the oesophagus, is the narrow isthmus
connecting the pericardial and peritoneal cavities which after-
wards become the pleurae (Figs. 201 and 205).

When the pulmonary bud or groove is viewed from the side,
its posterior extremity is seen to end in a deep pocket, the
pulmonary pocket or diverticulum (Fig. 22, p. 30). The wall
of the pocket is lined by a mass of hypoblast, which ulti-
mately forms the epithelial lining of the whole respiratory
tract, from the ciliated epithelium of the trachea to the pavement
epithelium lining the alveoli of the lungs. Found the pulmonary
bud is grouped a mass of mesoblastic tissue out of which the con-
nective-tissue system of the trachea, bronchi and lungs is developed.

TIIE RESPIRATORY SYSTEM.

251

Iii the 4th week the pulmonary pocket produces a larger
right and a smaller left process, the right and left lung buds.
The median part, developed from the hinder end of the groove,
grows out from the pharyngeal door and forms the trachea.
The groove in front forms the larynx. The right bud forms
the right lung and bronchus ; the left, the left lung and bronchus.
The hypoblast becomes the epithelial lining of the respiratory
tract ; the surrounding mesoblast forms the vessels, connective-
tissue covering and coats of the respiratory tubes and lungs.
As the lung buds develop the stomach is forced backwards ;
the oesophagus becomes elongated. The tracheal part of the
bud becomes separated from the oesophagus, but both retain the
same nerve supply — the recurrent branch of the vagus — which
is the nerve of the 5 th arch.

right dorsal aorta

fore-gut
right lung bud

dorsal mesocard.

passage between
pericard. and pleura

left dorsal aorta
dorsal mesent.

card, uein of left side

pleuro-perit. commun
pleura

uen. mesocard.
left d. of Cuuier
pericard.
heart

pericardium

Fig. 205. A section of a human embryo to show the Relationships of the Pulmonary
Buds at the 4tli week, looking backwards. (After Kollmann.)

In Fig. 205 the relationship of the lung buds is shown to
surrounding structures during the 4th week. The following
points should be noted :

(1) As the lung bud grows out it pushes its way into the
isthmus of the coelom — the narrow neck of communication
between the pericardium and peritoneum (Fig. 201). This part

V V

HUMAN EMBRYOLOGY AND MORPHOLOGY.

of the coelom on each side forms the pleura. The part of the
coelomic membrane which is invaginated on the lung bud
becomes the visceral pleura. The invaginating or ensheathing
lining of the isthmus becomes the parietal layer. As the lung
buds grow, they distend the originally small pleural parts of the
coelom until at the time of birth the right and left pleurae almost
meet in front of the heart. They meet after birth under the
sternum, enclosing between them the anterior mediastinum.

(2) As will be seen from Fig. 201, the lung bud sprouts out
from the dorsal mesentery just behind the duct of Cuvier. This
relationship is retained in the adult, the vena azygos and

superior vena cava lying above and in front of the root of
the right lung. If the left duct of Cuvier persisted it would

lie above and in front of the root of the left lung. The

lung bud springs from the pulmonary diverticulum just be-
hind the 5 th visceral arch of the pharynx. This arch is
involved in the formation of the pulmonary groove and pul-
monary diverticulum. It is from this arch (5th aortic arch) that
vessels arise, perforate the lung, and become the pulmonary
arteries. The fifth arch — part of the branchial pharynx — takes
a chief part in the formation of the pulmonary apparatus. The
ductus arteriosus — part of the 5 th arch — lies over the root

of the left lung. At this stage (4th week) the pleural cavity is
still in communication with the peritoneal above the septum
transversum (Fig. 205).

Formation of the Bronchi and Lungs. — The bronchi are
the stalks of the right and left lung buds. The right bud is
the bigger ; the left is probably repressed by the heart turning
to the left side. The right shows three secondary buds — the
forerunners of the upper, middle and lower lobes of the lung;
the left, two, which form the upper and lower lobes.

The condition of the lung buds at the end of the 5th week is
shown in Fig. 206. The right and left bronchi are formed, so
are the chief bronchial ramifications. Each ramification ends in
a bud, which divides again and again and keeps on dividing
until the fourth month. The terminal buds form the infundibula.
Each bud is solid, and carries its sheath of mesoblast. At the
sixth month sacular evaginations occur from the infundibula;
they form the air cells, or alveoli.

THE RESPIRATORY SYSTEM.

253

There are certain peculiarities in the lungs of animals which
are adapted to an upright posture (Man and Anthropoids) : —

Fig. 200. — The condition of the Right and Left Pulmonary Buds in a 5th week

embryo. (After His).

(1) Ramification of the Bronchi.— In quadrupedal mammals
the bronchus does not divide by a process of equal dichotomy, as
in man, but passes backwards in the lung as a main stem, which

Fig. 207.— Scheme of the Bronchial Ramifications in Quadrupedal Mammals. D, the
Dorsal Ramifications ; V the Ventral Ramifications.

grows gradually smaller by giving off four dorsal and four ventral
bronchial branches (Fig. 207). In man this arrangement can
scarcely be recognised. The ventral branches in him have become
larger than the main stems.

254

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(2) The Lobes of the Lungs. — In the embryonic con-
dition (Fig. 206) it is seen that the right and left lung
buds are nearly symmetrical. Aeby supposed the upper lobe
of the right lung to be absent in the left, but that is not
so. Each bronchus gives off three primary buds. All three
remain separate on the right side ; on the left the upper and
middle primary buds arise together (Fig. 206). Hence the upper
lobe of the left lung represents the upper and middle lobes of the
right. In the sheep the upper right lobe springs from the trachea.
The bronchus of the upper right lobe (the reason for it is not clear)
lies above its artery — that is to say, it is eparterial. The other
bronchi are hyparterial.

(3) The Diameters of the Thorax. — The peculiar branching
of the bronchi in man and upright primates is due to the
shape of their lungs, which in turn is due to the shape of
the thorax. In quadrupedal animals such as the horse or dog, in
which the chest rests and is supported between the fore limbs, the
thorax has its greatest diameter in the dorso-ventral direction
(Fig. 208). In upright animals (man, anthropoids, and also in

vertebra

Fig. 20S.— Diagrammatic Section of tlio Thorax of a Quadrupedal Mammal (A), con-
trasted with a corresponding section in Man ( B ).

some water-living mammals, such as seals, etc.) the transverse
-diameter becomes the greater. At birth the diameters of the
•child’s thorax are nearly equal. The thorax is flattened by the
spine becoming invaginated within it ; the thorax thus comes to
lie within the axis of gravity of the upright body.

TIIE RESPIRATORY SYSTEM.

255

(4) The Azygos Lobe. — On the inner side of the right lung
of man the azygos lobe is frequently present, sometimes as a mere
pulmonary projection or trace, sometimes as a lobule. It projects
into and fills a slight recess between the pericardium and
diaphragm, behind the intra-thoracic part of the inferior vena
cava. The lobe is always well developed in quadrupedal
mammals. In them the pericardium is separated from the
diaphragm by a diverticulum of the right pleura — the sinus sub-
pericardiacus (Fig. 209). With the assumption of the upright
posture (in man and anthropoids) the heart sinks until its rests
on the diaphragm, the sub-pericardiac sinus and azygos lobe
being thus obliterated. The reappearance of the azygos lobe in
man is an atavism — that is to say, a recurrence of an ancestral
feature.

Fig. 209.— The Relationship of the Heart to the Diaphragm in Quadrupedal Mammals.

Blood Supply of the Lung. — The pulmonary aorta is formed,
with the ascending part of the aortic arch, out of the bulbus or
conus arteriosus (see page 230). The right and left pulmonary
arteries spring as branches from the right and left 5th aortic
arches (Fig. 29, p. 37). They enter the lung buds, and are
carried backwards with them. The pulmonary veins grow out
from the pulmonary buds and enter the left auricle through the
venous mesocardium about the 3rd month (see page 24G).

Changes at Birth. — When the child begins to breathe at birth
the expansion of the lungs opens up the pulmonary circulation ;

256

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the foramen ovale starts to close and the ductus arteriosus
begins then to contract, and within the 1st month becomes
reduced to a fibrous cord. The ductus arteriosus represents the
dorsal segment of the 5th left arch ; the corresponding part of
the right 5th arch disappears soon after it is formed.

The Larynx. — The larynx is developed in the floor of the
pharynx out of the basal part of the pulmonary diverticulum.
The origin of the upper aperture of the larynx has been already
described (page 250). It is probable that the thyroid cartilage
represents the skeletal parts of the 4th and 5th visceral arches,
but this cannot be regarded as settled. There is no interval to
be recognised between the hyoid bone and thyroid cartilage in
the earlier stages of development. By some, the epiglottis is
believed to be a derivative of the fifth arch.

Only in man and the higher anthropoids are the true vocal
cords covered by stratified epithelium ; but all the muscles of the
human larynx are represented in the larynx of the ape, but in a
less specialized condition.

Fio. 210.— Diagram of the Diaphragm to show the Parts formed by each of the five

Elements.

Occasionally the saccule of the larynx, a development from the
apex of the ventricle, may protrude through the thyro-hyoid
membrane, thus giving rise to an air cyst in the neck. Such

TIIE RESPIRATORY SYSTEM.

257

laryngeal sacs are normally developed in anthropoids after birth,
and attain in them great dimensions.

The Diaphragm. — The diaphragm is developed from five
elements — one mesial from the primitive mesentery, two ventro-
lateral parts formed by adhesion, and two dorso-lateral formed
much later (Fig. 210). In fishes, only the mesial or mesenteric
element is developed ; in reptiles and birds the ventro-lateral
elements are added, these three parts forming the septum
transversum. In mammals the dorso-lateral elements are de-
veloped, and thus the pleural cavities become separated com-
pletely from the peritoneal. Occasionally in man the dorso-
lateral elements may fail ; a communication remains between the
pleura and peritoneum, through which the abdominal contents
may become herniated into the pleural cavity. This occurs eight
times more frecpiently on the left than on the right side. The
explanation will be found in the manner in which the liver is
developed.

Elements entering into the Formation of the Diaphragm.
1. The Mesial or Mesenteric Element (Fig. 210). — When the
splanchnopleures meet below so as to enclose the fore-gut, they

uitelline

uem

dorsal mesentery
coelom
stomach
uitelline uein
somatopleare
liuer bud

central mesent.

( splanchnopl. )

Fig. 211.— Diagrammatic section behind the Embryonic Heart to show the Part of the
primitive Mesentery which forms the mesial Element of the Diaphragm.

form a mesial septum between the right and left halves of the
coelom, with the gut suspended in it (Figs. 210 and 211). The
part between the gut and the spine is the dorsal mesentery ; the
part of the septum between the gut and the ventral wall is the

R

258

HUMAN EMBRYOLOGY AND MORPHOLOGY.

ventral mesentery. In the ventral part of the mesentery below
the fore-gut is swung the heart ; the mesocardia disappear ; but
that part of the mesentery behind the heart, in which the sinus
venosus rests and in which the stomach is developed, persists
(Fig. 201.) The ventral and dorsal parts of the mesentery of
this region form the first or mesial element of the diaphragm. In
this part of the mesentery lie (Figs. 211 and 212) (1) the
stomach, (2) the posterior end of the sinus venosus ; (.3) the
vitelline and umbilical veins which pass in it to the sinus ;
(4) the ducts of Cuvier which enter it to reach the sinus ; (5) the
liver bud which is developed into it; (6) the ductus venosus
and pre-renal part of the inferior vena cava which are de-
veloped in it. Further, on each side of this part of the mesentery
lies the isthmus of the coelom into which the lung buds are
developed and out of which the pleural cavities are formed
(Fig. 205). The mesial element is situated, at first, under the
middle cervical segments of the trunk.

In the course of development the stomach gradually separates
itself from this part of the diaphragm ; the liver bud also frees
itself ; the ducts of Cuvier, with the sinus venosus, also become
separated from it.

2. The formation of the ventro-lateral parts of the diaphragm
is obscure. They appear very early (3rd week), while yet the
vitelline veins terminate in the sinus venosus, and before the liver
bud has grown out (Fig. 187, p. 229). It will be seen from
Figs. 187 and 212 that the vitelline veins pass obliquely in the
ventral mesentery from the yolk sac to the sinus venosus. They
are said to be the active agents in producing the ventro-lateral
parts of the diaphragm. They become so dilated (see Fig. 211)
as to produce their covering of splanchnopleure outwards until
lateral folds are formed which come in contact with, and adhere
to, the somatopleure. In this way the mesentery (mesial part
of the diaphragm) becomes adherent to the lateral somatic walls.
These three parts, mesial and ventro-lateral, form the septum
transversum.

3. The Dorso-lateral Parts. — These parts are formed in the
3rd month, and arise as cresentic folds from the dorsum of the
isthmus. They are situated at first under the cervical region
and their free cresentic edges are directed backwards over the

THE RESPIRATORY SYSTEM.

259

lung buds. They are developed in mammals only, and are
evidently comparatively late structural additions. In the for-
mation of these parts of the diaphragm, which close up the
pleuro-peritoneal communications, probably several other factors
take part. Of these may be mentioned :

(1) The Wolffian ridges, which extend forwards as far as the
cervical region, and project from the roof of the coelom at the
passage between the peritoneum and pleura.

(2) The formation of the supra-renal body (Minot). It is
developed in the mesoblast round this opening — partly in the
septum, partly in the Wolffian ridge.

(3) The growth of the liver within the septum transversum
probably plays an important part. Hence, probably, the greater
frequency of diaphragmatic hernia on the left side, where the
liver growth is less. By these various factors the openings are
closed and the diaphragm completed by the formation of the
region of the arcuate ligaments.

The diaphragm begins to develop in the neck under the 4th
and 5 th cervical segments. Processes from the muscle plates of
these two segments enter the basis of the diaphragm and form
part at least of its muscular substance. The muscle buds carry
their nerve — the phrenic — with them. The descent of the
heart, the retrogression of the stomach, and the development of
the lungs lead to the diaphragm being pushed backwards until
it assumes the adult position.

Development of the Supra-renal Bodies. — In spite of much
research there is still doubt as to the origin and nature of these
bodies. The cortex and medulla are certainly of different origin.
In Elasmobranchs (sharks, etc.) these two elements are separate,
the cortical part forming an inter-renal, unpaired body ; the
medullary part a body situated above the Wolffian kidney (supra-
renal) on each side, and closely connected with a ganglion of the
sympathetic system, in connection with which they are developed.
In vertebrates above fishes the cortical and medullary parts are
combined in one body. The medulla arises from the groups of
cells which form the sympathetic ganglia ; probably from the
primitive cell basis of the semilunar ganglion, which is developed
in the septum transversum, close to the pleuro-peritoneal canal.
Hence the great plexus of nerves which pass from the solar

260

HUMAN EMBRYOLOGY AND MORPHOLOGY.

plexus to the medulla of the supra-renals. By the 3rd month
in the human foetus these cells have lost all evidence of their
origin from nerve cells, and have taken on a chromogenic
function. Probably the intercarotid and coccygeal bodies are
similar to the medulla in origin and nature (Swale Vincent).

The cortical part is of quite different origin. It is developed
within the Wolffian ridge and causes or assists to cause, the
closure of the pleuro-peritoneal canal over which it is formed.
It arises from the endothelium covering the Wolffian ridge — that
part of the ridge which lies above the diaphragm in the pleuro-
peritoneal canal. The endothelium proliferates within the
Wolffian ridge and conies in contact with the medullary element
derived from the sympathetic. The medullary element is at first
the larger and surrounds the cortical ; but soon the increased
growth of the cortex leads to its surrounding and containing the
medulla. The cortical cells range themselves in rows between
blood sinuses. Probably cortical cells invade and replace the
medullary part (Minot). As the kidneys grow forwards they
come in contact with the supra-renal bodies which at first lie on
their ventral surface. The supra-renal is at first larger than the
kidney ; even at birth they are nearly equal in size. The nerves
and arteries enter the bodies on their renal surface ; the veins
emerge on their anterior surface.

Isolated parts of the supra-renal body (accessory supra-renals)
occasionally occur in the broad ligament or in the spermatic cord
above the testicle. Such accessory bodies are probably derived
from the cortical element which is developed within the Wolffian
ridge and body. With the descent of the ovary and testicle,
which bring with them the Wolffian body, adjacent accessory
supra-renals, if such be present, are also brought down, and may
occasionally give rise to peculiar tumours.

CHAPTER XVIII.

THE ORGANS OF DIGESTION.

Development of the Liver. — The human liver, in its develop-
ment, repeats the forms met with in ascending the animal scale.
In Amphioxus the liver is merely a caecal diverticulum of the
digestive canal ; in amphibians the liver is a compound tubular
gland — the hepatic cells being arranged in cylinders round the
bile ducts. In birds and mammals the tubular arrangement is
lost and the lobular form substituted.

transverse sinus

Fio. 212. — The Mesentery of the Fore-gut and its Contents, viewed from the left

side (schematic).

To understand the development of the liver, the condition of
parts at the commencement of the third week must be studied.

262

HUMAN EMBRYOLOGY AND MORPHOLOGY.

At that time, the anterior wall of the yolk-sac and that part of
the lore-gut which becomes the stomach, lie in the septum
transversum (Figs. 211, 212). When the liver bud grows out,
it springs from the junction of the fore-gut and yolk-sac
(Fig. 221); and spreads into the ventral mesentery and septum
transversum. The part of the gut from which it springs after-
wards becomes the second stage of the duodenum It is at first
a hollow diverticulum of the gut hypoblast (Fig. 218). The
diverticulum is surrounded in the mesogastrium by a mass of
mesoblastic cells which form the vessels, capsule and connective
tissue of the liver. It divides almost at once into right and
left solid processes of hypoblastic cells, to form the right and
left lobes of the liver (Fig. 218.)

The hepatic buds are developed just behind the sinus venosus and
between the vitelline veins which are also situated in the ventral
mesentery (Figs. 202 and 212). The veins are broken up by
the ingrowth; from them starts an invasion of venous capillaries,
which, with the mesoblast of the septum transversum, penetrates
the liver buds and breaks the solid processes of hypoblast into
reticulating cylinders. Secondary processes start from the primary
hepatic reticulating cylinders and form smaller and smaller meshes
of hepatic cells. By the third month the hepatic lobules, with
the intra- and sub-lobular arrangement of portal and hepatic
veins, have appeared. The bile ducts probably represent the
lumina of the original tubular hepatic buds. The umbilical veins
also are cut by the hepatic invasion (Figs. 188 and 190).

The liver develops rapidly; in the 2nd and 3rd months it
occupies more than half the abdominal space. At the com-
mencement of the 2nd month, the yolk-sac becomes gradually
smaller and, owing to the development of the duodenum and
jejunum, it gradually retreats from the abdomen and region
of the diaphragm and liver. The second and third stages of
the duodenum and the jejunum are developed behind the ventral
mesentery. As the liver grows it gradually frees itself from the
diaphragmatic part of the ventral mesentery. The right lobe
develops more rapidly than the left and comes in contact with the
inferior vena cava. The stomach also frees itself from the basis
of the diaphragm and comes to lie with the liver in the mesentery.
The common bile duct is developed out of the hollow stalk of the

THE ORGANS OF DIGESTION,

263

hepatic bud. So is the common hepatic duct, while the right
and left hepatic ducts are formed from the stalks of the processes
which form the right and left lobes of the liver. The gall

B.

Fig. 213. — The origin of the Peritoneal Ligaments connected with the Liver.

A. Diagram of the foetal relationship of the ventral mesentery to veins and the

stomach, the liver being removed.

B. The liver viewed from behind to show its relationship to the gastro-hepatic

omentum, part of the ventral mesentery.

bladder arises as a diverticulum of the common bile duct.
(Fig. 218.)

264

HUMAN EMBRYOLOGY AND MORPHOLOGY.

The Ligaments of the Liver. — When the liver grows out
from the septum transversum it withdraws within the ventral
mesentery. Out of the ventral mesentery are formed all the
ligaments of the liver (Fig. 213 A). These are the following:

1. The G-astro-hepatic Omentum is that part of the ventral
mesentery which passes from (1) the oesophagus, (2) lesser cur-
vature or ventral border of stomach, and (3) first stage of
duodenum to (1) the diaphragm, (2) the posterior part of the
longitudinal fissure of the liver, the ductus venosus lying
within its hepatic attachment, and (3) the transverse fissure of
the liver (Fig. 213 B). The portal and umbilical veins lie in the
ventral mesentery (Fig. 213.4); the hepatic artery passes by it
to the liver. The right or free border of the gastro-hepatic
omentum, with the falciform ligament containing the remnant
of the umbilical vein, represents the posterior border of the
primitive ventral mesentery (Fig. 213 A).

2. The Falciform Ligament, containing the umbilical vein, also
represents part of the ventral mesentery (Fig. 213 4). At an
early stage the umbilical veins reached the sinus venosus by
passing through the septum transversum. The development of
the liver led to its being thrust out within the falciform ligament.

3. The coronary, the right and left lateral ligaments, and the
attachments to the vena cava and diaphragm (Fig. 213 5) are
developed as the liver emerges from the septum transversum,
and separates itself from the diaphragm.

Fio. 214.— Diagram of a mammalian Liver viewed from behind and below.

Morphology of the Liver.— The liver of orthograde animals

THE ORGANS OF DIGESTION.

265

(man, anthropoids) differs widely in form and lobulation from
that of mammals generally, but Professor Arthur Thomson has
shown that traces of the fissures and lobes of the typical mam-
malian liver can be seen in the human organ. The liver of a
dog or dog-like ape consists of three main lobes — right, middle
and left, and two accessory lobes — the caudate and Spigelian
(Fig. 214). In man the right and middle lobes have fused, but
traces of the fissure which separates them (the right lateral
fissure) are frequently to be seen in the liver of the newly-born
child (Fig. 215). The caudate lobe has been reduced in man to
a vestige, but in the third month foetus it is of considerable size
(Fig. 215). It projects from the liver at the upper boundary
of the foramen of Winslow; in many animals it rivals the right
lobe in size. The caudate fissure separates the caudate from the
right lobe, and a trace of this fissure is very frequently to be seen
in the human liver (Fig. 215).

Fig. 215. — The lower surface of the Liver of a human foetus during the 3rd month
showing Vestiges of Fissures and Lobes of the typical mammalian Liver.

After the second month the growth of the right lobe is more
rapid than that of the left. At birth the left lobe still touches
the spleen, a relationship more frequently retained in women
than in men. Riedel’s lobe is a linguiform prolongation of
the right lobe below the 10th right costal cartilage caused by
compression.

Gall Bladder. — It is developed from the common stalk
(common bile duct) as a diverticulum in the second month (Fig.

266

HUMAN EMBRYOLOGY AND MORPHOLOGY.

218). It may represent a modified lobule of the liver. In some
animals it is absent. The cystic duct represents the stalk of the
bud.

The Oesophagus and Stomach. — The Oesophagus in a third

week’s foetus is the junctional piece of fore-gut that unites the
pharynx to the scarcely differentiated stomach (Fig. 202, p. 244).
It is extremely short, but with the development of the lungs and
recession of the diaphragm it becomes elongated. In the foetus
the mucous membrane is thrown into four longitudinal folds
which end abruptly at the orifice of the stomach in a circular
epithelial ridge which sometimes persists and acts as a valve
at the cardiac orifice of the stomach. These four ridges can
be seen in adult apes. At one stage of development the cervical
part of the oesophagus is filled with epithelium and thus closed.

The Stomach. — The stomach is developed out of that part of
the fore-gut which lies between the oesophagus and pharynx
in front and the yolk sac, duodenum and liver bud behind
(Fig. 187, p. 229). At first it is contained in that part of the
primitive mesentery to which the name of septum transversum
has been given, and is situated below the lower cervical and
upper dorsal region of the spine (Fig. 204). In the 3rd week
the gastric part of the fore-gut shows a dorsal bulging — the
greater curvature. As the liver and gut are developed,
the stomach separates itself from the septum transversum
and comes to be suspended from the dorsal wall of the
coelom by the dorsal mesogastrium (Fig. 213 A). The gastro-
hepatic omentum is part of the ventral mesogastrium. The
oesophageal end of the stomach is fixed in the septum trans-
versum (diaphragm) ; the outgrowth of the liver bud fixes its
pyloric end in the ventral mesogastrium. Three changes
quickly ensue, the one being partly dependent on the other :

(1) Its dorsal margin grows rapidly so as to produce the
greater curvature; this growth particularly affects the left side
of the cardiac end of the stomach and thrusts the attachment of
the dorsal mesogastrium towards the right side at the cardiac end
of the stomach. The greater part of the fundus or cardiac sac of
the stomach is produced from the left side.

(2) The dorsal mesogastrium being too limited in extent to
allow for the growth of the greater curvature, the stomach

THE ORGANS OF DIGESTION.

267

rotates so that its left surface, with the left vagus nerve, comes
to be anterior ; the other posterior.

(3) To allow this rotation to take place and probably for some
other unknown reason, part of the dorsal mesogastrium, which is
attached to the great curvature or dorsal border of the stomach,
undergoes a rapid growth and forms the great omentum (Fig. 214).

The stomach loses its original vertical position only to a slight
degree ; even in the adult the pyloric orifice lies not far from
the middle line ; the oesophageal slightly to the left of it. The
glands of the stomach begin to form during the third month.
They appear as tubular invagination of the gastric hypoblast.
The peptic cells are formed by the 7th month.

The Spleen. — The Spleen appears in the dorsal mesogastrium
above the cardiac end of the stomach (Fig. 216) and grows
out on the left surface of the mesogastrium (Fig. 217). It

ventral mesent-
(falc. tig.)

umb. vein

uit. duct

g astro- hep. oment.
gastro-spl. oment.
spleen

dorsal mesogastrium
stomach
great oment.

■pancreas

mesentery

rectum

Fio. 216. — The Relationship of the Spleen, Pancreas, and Liver to the Mesogastrium

in the Embryo.

appears at the end of the second month by a collection of
lymphoid follicles in the mesoblast of the mesogastrium. The
tail of the pancreas (Fig. 216) reaches its point of origin. The

human embryology and morphology.

splenic artery is one of the vessels of the mesogastrium (Fig. 219),
its branches end in the developing tissues of the spleen. The
splenic blood spaces are probably formed from the veins which in
the developing spleen are lined by a layer of columnar cells.
The trabecular and muscular tissue and the capsule are derived
from the mesoblast of the dorsal mesogastrium.

o

The gastro-splenic omentum is that part of the dorsal meso-
gastrium which unites the spleen to the stomach (Figs. 216 and
217). It becomes elongated and stretched as the stomach rotates.

left kidney .
spleen

gastro-spl.

oment.

stom.

yastro-hep.

oment.

jlSt%
flMHi

right kidney

.peritoneal canity
■aorta

-lieno-renal lig.

■ splenic artery
-liuer

\V\ \\ \ \ \ \ \ \\\\\\\ \\ \ \\\Y\\\N \\\\\\y

\\ wwwwwwww'W^v \\\v

'-falciform tig. {vent, mesent .)

Fig. 217. — A diagrammatic transverse Section of the Mesogastrium viewed from behind.

The spleen comes to lie against the posterior (right) surface of the
cardiac end of the stomach. The dorsal part of the mesogastrium
between the roof of the coelom and the spleen becomes the lieno-
renal ligament. The rotation of the stomach also leads to the
spleen being thrust towards the left side ; the upper or renal
surface of the spleen becomes applied to the peritoneum covering
the anterior surface of the left kidney (Fig. 217). The part of
the mesogastrium between the spleen and oesophagus adheres to
the diaphragm and forms the suspensory ligament of the spleen.

The Pancreas. — The Pancreas appears during the 4th week as
two, or perhaps three, processes of hypoblast from that part of
the gut which afterwards becomes the second stage of the

THE ORGANS OF DIGESTION.

269

duodenum (Fig. 218). Of the two buds, one is a minor process
which springs from the ventral aspect ot the duodenum in
common with the hepatic diverticulum. This ventral bud only

liuer

right hep. duct

gl. bl.

common bile duct

stomach

iorsal pancr. bud

central pancr. bud
duodenum

Fig. 21S. — The Pancreatic and Hepatic Processes of a 4th week human embryo.

(After Kollmann.)

forms the lower part of the head of the pancreas. The greater
part is formed from a process which springs from the dorsal border
of the duodenum, nearer the stomach than the ventral process,
and grows into the dorsal mesogastrium above the stomach until
it reaches the spleen (Fig. 219). It unites in the mesogastrium
with the ventral bud. The ducts of both processes may persist,
the duct of the dorsal bud (duct of Santorini) opening half an
inch above the opening of the bile duct ; the duct of the ventral
bud (Wirsung’s) opens with the common bile duct. The terminal
part of the duct of Santorini commonly becomes obliterated, and
even if it persists the secretion from the dorsal bud reaches the
duodenum mostly through the duct of the ventral bud — the duct
of Wirsung (Fig. 218). A third pancreatic bud has been observed
in the human embryo. It arises from the ventral aspect of the
gut, and corresponds to a third bud observed in the development
of the pancreas in lower vertebrates.

The developing pancreatic processes are at first hollow, like the
primary liver process, but the secondary processes are solid and
cylindrical. They divide and re-divide, acquire lumina, and form
an acino-tubular gland. The capsular and connective tissue of
the pancreas are derived from the mesoblast of the dorsal
mesentery.

*270

HITMAN EMBRYOLOGY AND MORPHOLOGY.

Relationship of the Pancreas to the Peritoneum and Vessels.
1. In the Embryo. The pancreas develops between the layers
of the dorsal mesogastrium (Fig. 216); it is completely sur-

rounded by peritoneum, and it lies with its tail directed forwards
against the spleen and its head on the dorsal bend of the
duodenum. It is parallel to the great curvature (dorsal border)
of the stomach. This is the relationship during the 5th and 6th
weeks. The dorsal mesogastrium is then attached in the middle
line, in front of the aorta. The coeliac axis (Fig. 219) is the
artery of the mesogastrium and of the structures which it con-
tains. It supplies the fore-gut and its derivatives, between the
septum transversum in front and yolk sac behind. The coronary
artery passes direct to the cardiac end of the stomach ; the
splenic is a short vessel ending on the cardiac dilatation of the
stomach and supplying the spleen ; the hepatic passes on the
right side of the pancreas to the duodenum and pyloric end of
the stomach, and ends in the liver by passing through the ventral
mesentery.

2. In the Adult. — The development of the great omentum

TIIE ORGANS OF DIGESTION.

271

and the rotation of the stomach to the left lead to the pancreas
being pressed against the posterior wall of the abdomen on the
left side. That part of the dorsal mesogastrium which lies
between the stomach and pancreas becomes elongated enormously
to form the great omentum, and hence the two anterior layers of
the great omentum are attached to the great curvature of the
stomach and to the gastro-splenic omentum (Fig. 219). The
two posterior layers of the omentum end on the lower (formerly
ventral) border of the pancreas. The duodenal loop, with the
head of the pancreas in its concavity, is also pressed against
the posterior abdominal wall. During all the changes which
take place in the position of the pancreas and spleen, owing
to the rotation of the stomach and intestine, one structure
remains firm, and that is the coeliac axis. The part of the
mesogastrium in which the spleen and tail of the pancreas are
situated becomes greatly drawn out. Both structures, instead of
being situated near the middle line dorsal to the stomach, come to
occupy a situation in front of the left kidney, the pancreas thus

mesogastrium becomes

Fig. 220.— Diagram to show the Formation of the Lesser Sac of the Peritoneum from

the Dorsal Mesogastrium.

coming to lie across, instead of along, the abdominal cavity. The
mesogastrium is ballooned out towards the left side to form the
lesser sac of the peritoneum, and as the splenic artery lies in the
mesogastrium it also is drawn towards the left, circumventing
the lesser sac of the peritoneum (Fig. 220).

272

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Iii the 5th week the pancreas lies between the layers of the
dorsal mesogastrium ; thus right and left surfaces are covered by
peritoneum. The left surface, which becomes anterior, retains
its covering, but the right becomes applied to the posterior
abdominal wall in front of the aorta, crura of the diaphragm
and left kidney (Fig. 220). Its peritoneal covering gradually
disappears, and thus in the adult the pancreas conies to appear as
if it lay behind and outside the cavity of the peritoneum. The
complete application and fixation of the pancreas to the posterior
abdominal wall only occurs in animals adapted to the upright
posture.

The part of the dorsal mesogastrium between the pancreas and
aorta (Fig. 219) is also applied to the posterior abdominal wall,
and forms the posterior wall of the lesser sac.

The Lesser Sac is really the bursa of the stomach. In its
anterior wall are situated (Fig 220): (1) The gastro-hepatic
omentum or ventral mesentery, which is at first vertical and
median ; (2) The stomach ; (3) The gastro-splenic omentum, a
part of the dorsal mesentery ; (4) The two anterior layers of the
great omentum, also parts of the dorsal mesentery.

In its posterior wall are situated: (1) The lieno-renal liga-
ment (dorsal mesentery); (2) The dorsal mesentery of pancreas.

THE PRIMARY DIVISIONS OF THE ALIMENTARY

CANAL.

At the commencement of the 3rd week the embryo appears to
be situated as a cap on the yolk sac (Fig. 75, p. 96). As the
chorion and amnion represent the extra-embryonic part of the
somatopleure, so the yolk sac represents the extra-embryonic part
of the splanchnopleure (Fig. 72, p. 93). At the end of the 3rd
week the alimentary tract shows three divisions (Fig. 221):

1. A short anterior diverticulum — the fore-gut; it becomes the
pharynx, oesophagus, stomach, and that part of the duodenum
situated in front of the opening of the common bile duct;

2. The hind-gut — a short posterior diverticulum ; it forms the
rectum, sigmoid and descending colon; its terminal pait forms
the cloaca from which the allantois grows out ;

THE ORGANS OF DIGESTION.

273

3. The mid-gut or roof of the yolk sac, from which is formed
that part of the gut between the opening of the common bile duct
in front to the splenic flexure of the colon behind. Only the
fore-gut has a ventral mesentery ; the mid-gut and hind-gut
(excepting the cloaca) are suspended by a dorsal mesentery only.

Fig. 221. — The Form of the Alimentary Canal in a human embryo of the 3rd week.

The Yolk Sac and Meckel’s Diverticulum. — The fore and hind
gut may be regarded as diverticula of the yolk sac. The yolk
sac reaches its maximum size in the 4th week. In the 3rd
week the umbilicus extends along the whole length of the
abdomen, from the septum transversum to the allantois. The
neck of the yolk sac completely fills it. The vitelline arteries,
which afterwards form the superior mesenteric, end on its walls,
the vitelline veins commence on them (Fig. 221).

In the fifth week the form of the alimentary tract is that
shown in Fig. 222. The condition then differs from that shown
in the 3rd week in the following points :

1. The production of the mid-gut as a U-shaped loop from the
roof of the yolk sac ;

2. The formation of a long neck to the yolk sac — the vitello-
intestinal canal ; if it persists it forms a Meckel’s diverticulum ;

3. The yolk sac, by the constriction of the umbilical orifice

s

274

HUMAN EMBRYOLOGY AND MORPHOLOGY.

and formation of the cord, comes to lie on the placenta (Fig. 222).
lhe neck of the yolk sac, the vitello-intestinal duct, lies within
the umbilical cord. By the sixth week the sac is in a state of

retrogression. The U-shaped intestinal loop, which is formed

from the mid-gut, is at first really extra-abdominal, being situated
within a funnel-shaped cavity formed by the somatopleure at the
umbilicus. The vitelline artery, afterwards the superior mesenteric,
is the artery of the U-shaped loop ; it terminates at the vitello-
intestinal canal — the elongated neck of the yolk sac (Fig. 222).

Persistence of Certain Embryonic Structures. — Many of
the features seen in the human embryo at the stage of
development reached during the fifth or sixth weeks may persist.

1. The most common structure to remain is the intestinal
end of the neck of the yolk-sac — Meckel’s diverticulum. Tt
occurs in 2 per cent, of subjects, and commonly forms a finger-
glove-like sac on the free border of the ileum about four feet
above the ileo-caecal valve. Hence we know that this part of
the ileum forms the apex of the U-shaped loop of intestine.

2. The neck of the yolk sac, instead of closing, may remain
open at the umbilicus and form a faecal fistula at birth.

THE ORGANS OF DIGESTION.

275

3. The artery of the yolk-sac, the terminal part of the

superior mesenteric, may persist as a fibrous band which

stretches from the mesentery at the situation of Meckel’s
diverticulum to the umbilicus. Over it, the gut may become
strangulated.

4. The U-shaped loop, instead of retreating within the

abdomen at the end of the second month, may remain within

the umbilical funnel of somatopleure. This gives rise to a
congenital umbilical hernia. Such hernias occur in all degrees ;
they may contain a piece of intestine, or almost the whole of
the abdominal contents. In such cases the somatopleure, or
belly wall, which forms the covering of the hernia, is commonly
thin and transparent.

DERIVATES OF THE HIND-GUT.

The Rectum and cloaca are formed out of the posterior end
of the hind-gut. The manner in which they become sepa-
rated from the allantois and open in the proctodaeum at the
anal depression has been described (page 118).

The Descending Colon and Sigmoid Flexure are also formed out
of the hind-gut. The artery of the hind-gut is the inferior
mesenteric (Fig. 222). Hence it supplies the rectum, sigmoid, and
descending colon. In the fifth week the hind-gut is suspended
from the front of the aorta and spine by the dorsal mesentery of
the hind-gut. This becomes transformed into the meso-rectum,
meso-sigmoid and descending meso-colon. The angle between the
hind-gut and U-shaped loop becomes the splenic flexure (Fig.
222). At the commencement of the third month the U-shaped
loop has become twisted round on the axis of the superior
mesenteric artery (Fig. 223 A), so that the part of the hind-gut
which forms the splenic flexure is turned forwards and to the
left until it touches the spleen (Fig. 226). It carries its
artery, the left colic, with it. At this time the anterior limb
of the U-loop grows rapidly, and is produced into coils of small
intestine — the jejunum and ileum — which press the descending
meso-colon against the kidney and the parietal peritoneum cover-
ing the left kidney (Fig. 223 B). The left surface and layer
of the meso-colon adheres to the pre-renal layer of the peri-

276

HUMAN EMBRYOLOGY AND MORPHOLOGY.

toneum, both layers subsequently being absorbed. Thus the
descending meso-colon, originally situated in the middle line,
comes to be attached in the left lumbar region.

spl. p/ex. colon

B.

Fig. 223 A. — The mesentery of the hind-gut. The Position assumed by the Colon
after the rotation of the Gut has taken place.

Fig. 223 B. — Diagram to show how the descending Meso-colon becomes applied to the
parietal Peritoneum of the left Lumbar Region.

The Intersigmoid Fossa. — The sigmoid flexure, after the
rotation of the gut, forms a loop, with its convexity directed
towards the liver. The meso-sigmoid is originally attached in
the middle line, but the pressure of the developing loop of small
bowel pushes the meso-sigmoid against the posterior abdominal
wall and left iliac fossa. The extent to which it adheres varies ; it
may become completely adherent like the descending meso-colon,

THE ORGANS OF DIGESTION.

277

or only partially. When the sigmoid is lilted up a recess or
fossa may be apparent beneath the meso-sigmoid, to the outer
side of the left common iliac artery, which is due to a failure ol
adhesion between the meso-sigmoid and parietal peritoneum. It
occurs opposite the convexity of the sigmoid loop.

Development of the Colon and Caecum. — In the sixth week
a process or diverticulum is seen to grow out from the free bolder
of the posterior limb of the U-shaped loop (big. 225 A). Ihe

Fig. 224. — Diagram of the Apex of the Caecum at the time of birth and the Diver-
ticula which may be produced in the Fundus of the Caecum afterwards.

diverticulum forms the caecum and appendix. It continues to
grow outwards and downwards. The part of the posterior loop
above the caecal diverticulum becomes increased in diameter and
forms the ascending and transverse parts of the colon. As the
superior mesenteric (vitelline) artery descends in the loop, it
gives off three branches to the posterior limb — the middle colic,
right colic and ileo-colic arteries (Figs. 222 and 223 A). The
mesentery of the U-shaped loop may be divided into two parts,
the fate of the two parts being different :

1. The mesentery of the anterior limb in front of the superior
mesenteric artery — the pre-arterial part. This forms the greater
part of the mesentery of small bowel.

2. The mesentery of the posterior limb, behind the artery — the

278

HUMAN EMBRYOLOGY AND MORPHOLOGY.

post-arterial part. It forms the mesentery of the ascending and
transverse colon and also the lower part of the mesentery of the
small bowel.

jejunum-

colon

ileo-col. art.
■mesentery
artery of appendix
caecum

appendix
ffpost. aspect.)

ileum

uit. intest, canal

A.

appendix

B.

Pig. 225 A. — The Appendix and Peritoneal Folds at the end of the 2nd month of
foetal life. The Intestinal Loop is viewed from the left side.

Fig. 225 B. — Peritoneal Fossae in the lleo-caecal Region.

At the seventh week the great growth of the anterior limb, to
form the coils of the jejunum and ileum, causes the U-shaped
loop to rotate so that the splenic flexure of the colon comes
against the spleen. This brings the transverse meso-colon, con-
taining the middle colic artery, against the part of the meso-
gastrium which forms the great omentum (Fig. 226). These two
layers adhere. The rotation places that part of the loop mesentery,
which forms the ascending meso-colon, against the duodenum,
and at the same time the duodenal loop is pressed into its

THE ORGANS OF DIGESTION.

279

permanent position in front of the right kidney and inferior
vena cava. The caecum thus comes to be situated in front of
the right kidney, near the gall-bladder, and there it remains
until about the time of birth, when both the caecum and
ascending colon undergo a gradual migration towards the
right iliac fossa. The cause of this migration is not known,
but it occurs only in animals adapted to the upright posture.
Tims the attachment of the ascending meso-colon is formed by a
secondary adhesion to the parietal peritoneum during the migra-
tion of the colon and caecum. The appendix, during the
migration, may be caught behind the colon ; it is then lodged and
fixed in the ascending meso-colon.

O

stom.

ciuoden.
caec.

post-art. pt. mesent.
pre-art. part mes.

great oment.
flex.

-jej. fossa
sup. mes. art.
colic artery

inf. mesent.
mter-sig. fossa

Fig. 226.— To show the Rotation of the Intestinal Loop and Formation of the

Duodeno-jejunal Fossa.

The Appendix. — At first and until the third month, the caecal
diverticulum is of the same calibre throughout, but from the
third month onwards, the appendix remains small while the
caecum grows, keeping pace in diameter with the colon. At
birth the appendix is still the tapered apex of the caecal diverti-
culum (Fig. 224), but during childhood, an outer, or an inner,
sacculation, or both together, arise in the fundus of the caecum
and thrust the appendix backwards and to the left into an
asymmetrical position.

Although a distinctly marked appendix is only seen in man,
the anthropoids, lemur, and opossum, still a corresponding

280

HUMAN EMBRYOLOGY AND MORPHOLOGY.

lymphoid structure is present generally in mammals. The ap-
pendix is a lymphoid diverticulum of the caecal apex (B. J.
Berry). It must be regarded as a lymphoid structure, and
although it can be dispensed with, is not therefore to be regarded
as vestigial in nature any more than is the tonsil. The
caecum is largest, as is also the bowel, in herbivorous animals.

Ileo-caecal Fossae. — When the caecal diverticulum grows out
from the hinder limb of the U-shaped loop it carries with it
three folds (see Fig. 224) :

1. The ileo-colic fold, a process from the right side of the
mesentery containing the anterior caecal artery ;

2. The bloodless fold, a process from the coat of the ileum ;

3. The mesentery of the appendix, a process from the left side
of the mesentery, containing the artery to the appendix (Fig.
225 A).

These three folds give rise to three fossae (Fig. 225 B) :

1. The ileo-colic, between the termination of ileum and ileo-
colic fold ;

2. The ileo-caecal, between the bloodless fold and mesentery of
the appendix ;

3. The retro-caecal, between the mesentery of the appendix and
commencement of the ascending meso-colon.

The Duodenum. — The part of the duodenum above the
entrance of the common bile-duct is formed from fore-gut, the
part behind from the mid-gut. The liver and pancreatic buds
arise from its ventral border at the junction of these two parts.
At first it is entirely covered by peritoneum and suspended in the
mesentery, in which it forms a minor loop (Figs. 219, 222). In
its concavity rests the head of the pancreas.

In the 7th week the U-shaped loop of intestine rotates so as to
bring the ascending colon, with its mesentery, against the duodenal
loop, which, with the head of the pancreas, is thus pressed
against the right kidney and inferior vena cava (Fig. 226). The
peritoneum covering the right aspect of the head of the pancreas
and duodenal loop adheres to the parietal peritoneum covering
the kidney, and disappears. The transverse meso-colon gains an
attachment to the front of the duodenum.

The Duodeno-jejunal Fossa is formed as the U-shaped loop of
bowel rotates, dragging the transverse meso-colon after it (Fig.

THE ORGANS OF DIGESTION.

281

226). The inferior mesenteric vein passes forwards along the
dorsal mesentery to the splenic vein, which is also then in the dorsal
mesentery (Fig. 187, p. 229). The mesentery of the hind-gut is
stretched as the ascending colon migrates to the right ; the
inferior mesenteric vein checks it and gives rise to a fold, which
bounds the fossa (Fig. 228).

The Mesentery of the small gut is formed out of the primitive
mesentery of the U-shaped intestinal loop, chiefly from that
part of it (the pre-arterial) which lies between the superior
mesenteric artery and the anterior limb of the loop (Fig. 223 A).
After the rotation, the aspect of the mesentery, which was directed
towards the right, becomes left and anterior. During the rotation
of the gut the superior mesenteric artery comes to lie in front of
the third stage of the duodenum. At first the mesentery is
attached in front of the spine only at the origin of the superior
mesenteric artery. Its oblique attachment to the posterior
abdominal wall, from the duodenum to the right iliac fossa, is
a secondary adhesion, formed after the rotation of the gut, and
this extensive attachment is found only in animals adapted to
the upright posture.

CHAPTER XIX.

THE BODY WALL, RIBS AND STERNUM.

Bilateral Symmetry of the Body. — From a developmental
point of view the body is made up of two symmetrical halves ;
each half of the blastoderm, taking the medullary groove as the
line of division, contributes equally to the formation of the body.
Each produces a half of the nervous system, each a half of the
vascular, muscular and alimentary systems, so that each individual
is in reality made up of two identical halves, right and left.

The Ventral Line of the Body. — The two halves are united
along the ventral line from the mouth to the anus (see Fig. 227).
In this line are developed the symphysis of the lower jaw, the
body of the hyoid bone (copula), the white line of the neck and
angle of the thyroid cartilage, the sternum, the supra-umbilical
part of the linea alba, umbilicus, infra-umbilical part of the linea
alba, symphysis pubis, the septum of the penis, and of the scrotum
and perineal raphe. The ventral line is continued forwards on
the face between the parts derived from the mesial nasal processes
(Chap. I.).

The idea was at one time prevalent that the whole of this
line was formed by the fusion of one somatopleure with the
other ; the median ventral line was the suture formed by the
union. Such is not the case. The blastoderm, which lies at first
like a cap on the yolk-sac (Fig. 75, p. 96), is produced or folded
anteriorly to form the fore-gut and the part of the body above the
umbilicus ; it is produced posteriorly to form the hind-gut and
the part of the body below the umbilicus (Fig. 75). The
blastoderm grows out from the umbilicus to form the embryo in
much the same way as a soap bubble is blown from the bowl ol

THE BODY WALL, BIBS AND STERNUM.

283

a pipe. In an embryo, at the commencement of the 3rd week,
the greater part of the ventral line is occupied by the umbilicus
(Fig. 228). At that time the umbilicus is 3 mm. long, the

Symphysis menti-
uent. line of neck-

uent. line of thorax—

supra-umb. tinea alba-

umbilicus

infra-umb. linea alba

symph. pubis-

uulu. deft
perin. raphe
anus

C

coccyx

Fig. 227.

stomodaeum

/ cent, line of )
\pharyn. regionj

Fig. 22S.

Fig. 227.— Diagram of the Structures formed in the Median Ventral Line of the Body.
Fig. 228.— The Median Ventral Line in an embryo of three weeks, to contrast with
the Con-esponding Line in the Adult.

entire ventral line being about 4 mm. At the 6th week the
ventral line measures 15 mm., the umbilicus retains its former
size, about 3 mm.

At first the somatopleure shows no trace of segmentation.
The paraxial masses of mesoblast become segmented early and
form the muscle plates (Fig. 126, p. 156). From each muscle
plate of the primitive segments a process grows down into
the somatopleure. The somatopleure thus becomes segmented
secondarily, the process of segmentation spreading from the
dorsal to the ventral side of the plate, but along the median
ventral line of the body wall a band of the primitive mesoblastic

284

HUMAN EMBRYOLOGY AND MORPHOLOGY.

tissue remains unchanged and undifferentiated. In the ventral
band between the left somatopleure and the right are formed
the sternum and the linea alba (Fig. 227). In lower vertebrates,
in fishes, and to a less marked extent in amphibians and reptiles,
the somatopleure becomes segmented from end to end of the trunk,
each muscle segment being distinctly recognizable in the adult.

axial

somatopleure

uertebra-

ion

am wrnmm n

ir

. • V .* i • , \ • 1 1 7. * ’ \ ’ 1 •

inn

1 1 .IIS

im

. *

I [J

il MI

ISIS ..

o

uentral median tine

/

myotome

Jnter-segment

(inter-cost, muse )

axial myotome soJato.pl. myotome ‘Tenn™?™ ^ °f

Fig. 229. — Scheme of the Manner in which the Somatopleure is segmented.

Formation of Ribs. — Ribs, like all true skeletal bones, pass
through three stages : 1. They are represented by a membranous
basis in the fibrous tissue (septa) between the muscular seg-
ments of the somatopleure (Fig. 229). 2. The fibrous basis

of the rib becomes cartilaginous. 3. Ossification of the cartilage
begins in the 7th week, but the process of ossification leaves the
ventral parts of the costal segments untouched ; they form the
costal cartilages ; in lower forms they may become ossified and
form sternal ribs. In lower vertebrates, such as reptiles, each rib
articulates with the neural arch of a vertebra by two heads,
dorsal and ventral (Fig. 123, p. 153). In man and mammals
only the first and the lowest two ribs retain the vertebral
attachment. The heads of the upper ribs (2nd to 10th)
migrate forwards and gain attachments to the disc and vertebra
in front of their own segments. The dorsal head of the rib in
man is represented by the tuberosity (see also p. 152).

The Sternum. — In man and anthropoids the sternum has
become flat and highly modified with the alterations in the shape
of the thorax (Fig. 208, p. 254). With the adaptation to the upright
posture the thorax becomes flattened from back to front ; its
transverse diameter is as great, or greater, than the antero-

THE BODY WALL, RIBS AND STERNUM.

285

posterior. The sternum also becomes wider and shorter. To
understand its true nature it is necessary to note the characters
of the sternum of a pronograde mammal, such as the dog or ape
(Fig. 230). In such the sternum is typically made up of seven
segments :

1. A modified anterior segment, the pre-sternum.

2. Five narrow, cylindrical segments or sternebrae, forming the
body of the sternum.

3. The ensiform process, a hind segment, complex in nature
and ending in the middle ventral line. The ensiform process
usually bifurcates and probably belongs to several segments.

Fig. 230. — The Form of Sternum in a Pronograde (quadrupedal) Mammal.

Fig. 231. — The Form of Sternum in a Mammal adapted to the orthograde (upright)
Posture. The Points of Ossification are also shown.

The chief changes in the human sternum are :

1. Each segment has become flat and wide;

2. The segments of the body fuse together during the years of
adolescence, the fusion beginning behind and passing forwards ;

3. The 4th sternebra of the body is usually vestigial; is probably
made up of two or more fused segments.

In low primates 8 or 9 pairs of ribs may reach the sternum,

suprasternal

Fig. 230.

Fig. 231.

286

HUMAN EMBRYOLOGY AND MORPHOLOGY.

six or more sternebrae being then present. In man the number
has been reduced to seven pairs, the sternal ends of the seventh
pair lying in front of the base of the ensiform process. It is not
uncommon to find the 8th rib reaching the sternum, especially on
the right side; it is rare to find the 7th pair fail to reach the
sternum. The more frequent presence of an 8th sternal rib on
the right side is due to right-handedness (Cunningham) or the
pressure of the underlying liver requiring support (Tredgold).

Development of the Sternum. — The sternum is developed in
the mesoblast of the ventral median line between the first 8 or 9
dorsal segments (Paterson). It is developed in two parts — a
right from the right somatopleure, a left from the left somato-
pleure. These two halves, the right and left fibrous sternal bars,
fuse gradually in the middle line, the process of fusion commencing
at the presternum and spreading back (Fig. 232). The bifurcated
end of the ensiform process represents the posterior extremities
of the sternal bars.

epi coracoid element

Fig. 232.— The Sternal Bars in a human embryo of six weeks (after Paterson).

The sternum is described here as a structure arising in-
dependently in the median ventral line. This, however, is not
the commonly accepted view. Kuge’s researches led him to the
conclusion that the segments of the sternal bars were produced as

TIIE BODY WALL, IilBS AND STERNUM.

287

buds from the ventral ends of the ribs, to which they correspond,
and his conclusions are supported by the evidence of comparative
anatomy.

In its development the sternum passes through three stages —
fibrous, cartilaginous and bony.

1. Fibrous Stage. — At the 6th week (Fig. 232) the costal
cartilages are already chondrified. The mesoblast on each side of
the median line, in which they end, has become condensed, and
forms the membranous basis of the two sternal bars (Paterson).
The bars begin to fuse together in front.

2. Cartilaginous Stage. — The mesoblast of each sternal bar
begins to chondrify in the intervals between the ends of the
costal cartilages. The process of chondrification and fusion proceed
apace, and by the commencement of the third month the segments
of each side have united to form the cartilaginous sternal bars
(Paterson). At the same time, the two bars gradually unite, the
cellular tissue between them becoming chondrified. Thus a single
cartilaginous plate formed out of the two cartilaginous bars is
formed. Fibrous joints are subsequently formed between the
presternum and mesosternum and between the mesosternum and
ensiform process. A fibrous and synovial joint is also developed
at the union of the costal cartilages with the sternum, except in
the case of the first pair.

3. Ossification. — A centre appears for each sternebra ; those for
the third and fourth of the mesosternum are usually double, one
being placed on each side. The centres for the 4th segment are
more frequently absent than present (Paterson). The centre for
the presternum (there may be two or even more) appears about
the 4th month ; the centres behind appear in order ; that for the
4th sternebra of the mesosternum appearing about the time of
birth, that for the ensiform after birth. The process of fusion of
segments begins behind about puberty ; the segments of the
mesosternum are united together by the 30 th year. Occasionally
a median foramen may be seen in the sternum ; it is due to im-
perfect union of the sternal bars.

The Presternum is complex in nature. It probably represents
more than the simple sternebra between the first and second pairs
of ribs. Part of it may belong to the segment in front, the last
cervical. When the last (7th) cervical rib is fully developed it

288

HUMAN EMBRYOLOGY AND MORPHOLOGY.

reaches the presternum. Ic may also contain another element. In
more primitive types of vertebrates, in fact in all below the higher
mammals, a part of the coracoid element of the shoulder girdle,
the epi-coracoid (pre-coracoid), is situated at the cephalic end of
the sternum (Fig. 244). The supra-sternal bones, not infrequently
seen on the upper border of the human manubrium sterni (Fig.
231), appear to represent the epi-coracoids. These bones are
probably always present, but their presence is difficult to detect
because they are commonly more or less completely fused with
the presternum (Paterson). Occasionally the first segment of the
meso-sternum joins the manubrium instead of the body of the
sternum- — a union frequently seen in some anthropoids.

The presternum is the first part of the sternum found in
ascending the scale of vertebrate animals, and is developed as a
median ventral support for the shoulder girdle.

CHAPTER XX.

THE LIMBS.

The Limbs. — The limbs begin to appear at the end of the 3rd
week. A slight elevation or ridge is then seen to run along the
dorsal border of the somatopleure, at some distance from the row

4th vise, arch

V**

rad. border-
heart

somatopleun

th C. segment

arm bud
1st D.

u. Ihar border
ridge

tibial border
leg bud

fib. border

5th L.

Figs. 233. — Lateral view of a human embryo at the 28th day, showing the
Limb Buds, Lateral Ridges, and Primitive Segments.

of primitive segments formed in the paraxial mesoblast (Fig. 233).
The limb buds spring from this ridge as flat processes with an
upper, dorsal, or extensor surface, and a lower, ventral or flexor
surface. The two borders are anterior or cephalic and posterior
or caudal (Fig. 233). It is generally held that the lateral ridge,
of which the limb buds are specialized parts, represents a

T

HUMAN EMBRYOLOGY AND MORPHOLOGY.

290

continuous row of lateral fins. If this view is right, then the
fore and hind limbs represent highly specialized fin-rays.

A section shows each bud to be composed of undifferentiated
mesoblast, with a covering of epiblast (Figs. Ill A p. 139, and
237). It represents in structure a process of the undifferen-
tiated somatopleure or body- wall. Processes grow into the limb
bud from the muscle plate of each segment which is situated
opposite the origin of the bud. Each corresponding segment of
the spinal cord also sends to the limb bud a nerve process. At
least seven body segments contribute to the formation of each
limb.

Changes in External Conformation— In the 4th week (Fig.
234) the limb buds are unsegmented; in the 5th a constriction

marks the hand off ; the position of the elbow being indicated in
the same week. In the 7th week the fingers appear as thicken-
ings in the wcbbecl hand, the middle digit being indicated first.
They become free at the end of the second month ; occasionally

thigh

they remain attached, the child being born with its fingeis in
a syndactylous condition. The shoulder remains buried in the
body-wall ; the skeletal structures of the shoulder are the first
to appear. The corresponding parts of the lower limb appear
in the same order as in the arm.

THE LIMBS.

291

The Internal Differentiation of Tissues begins at the basal part of
the limb and spreads towards the fingers, the terminal phalanges
being the last of the skeletal parts to become differentiated. The
mesoblast becomes condensed in the axis of the bud and forms the
fibrous basis of the limb bones. The skeletal basis of mesoblast
is continuous, but where joints are to be formed there occur
opener formations in the arrangement of the cells. In the 5th
week the fibrous basis of the bones begins to chondrify, and this
is soon followed by the appearance of centres of ossification in the
shafts of the long bones (7th week). The mesoblast between the
chondrified bases of the bones opens out into a cavity and forms
the synovial membranes of the joints. The joints are all formed
before the 3rd month is well begun. During the 6th to the
8th week the mesoblast differentiates into muscles, bones, vessels
and the sheaths of nerves ; the tissue left over, not included in
these structures, forms their sheaths, and the fasciae and con-
nective tissue of the limb. The processes of the nerve cells to
form the nerves, and of the muscle plates to form the muscles,
grow in very early (see Fig. 237).

Fig. 236. — The Corresponding Points (A, B, C, and D) in the Ilium and Scapula.

Torsion and Rotation of the Limbs. — As the limbs are
developed, the extensor surfaces of the knee and elbow are
directed upwards. If the body of an adult were placed in the prone
position, it would be necessary, in order to restore the limbs to
their embryonic position, (1) to draw them out at right angles to

292

HUMAN EMBRYOLOGY AND MORPHOLOGY.

the axis of the body, (2) to rotate the leg outwards so that the
extensor surface of the knee is directed upwards, with the great
toe in front and the little toe behind. (3) The arm, on the
other hand, would require to be rotated inwards to bring the
elbow (extensor surface) into the dorsal position. The rotation
which brings the embryonic limbs into the adult position occurs
at the junction of the limb girdle with the trunk.

Rotation at the Limb Girdle. — To understand the extent of
this rotation it is best to compare the scapula and ilium and pick
out their corresponding points. The extensors of the thigh and
arm may be taken as guides. The long head of the triceps and
rectus femoris of the quadriceps manifestly correspond ; their
points of origin — the anterior border of the ilium and axillary

border of the scapula— may be regarded as homologous points.
The other corresponding points are shown in fig. 236. The
sacral articular surface of the ilium corresponds to part of the
supra-spinous fossa. To restore the limb girdles to their primitive
and corresponding positions, the scapula has to be rotated so that

THE LIMBS.

293

its axillary or posterior border comes to occupy the position of its
spine, while the ilium has to be placed at right angles to the spine
and its anterior border rotated outwards until it occupies a posi-
tion corresponding to the axillary border of the scapula.

There is a manifestly spiral twist in the humerus, but it is
doubtful if this be in any way due to the torsion which the limb
undergoes.

Nerve Supply of the Limbs. — The Arm. — It is important to
note that the limb buds arise from the ventro-lateral aspect of the
trunk (Fig. 237) at the junction of the somatopleure with the
paraxial mesoblast. Therefore the nerves of the limbs are
the nerves of the ventro-lateral zone — the lateral cutaneous

dorsal spinal muse, l

(epi-axial)

ventral spinal muse .7

kidney

uentro-lateral muscles ( in
somatopleure )

becomes trapezius, etc.
post. prim. diu.

dorsal limb muse.

dorsal ramus

axial structures
of limb

uent. ramus'
uentral limb muse.

Fig. 238.— Schematic Section showing the Origin and Arrangement of the Muscles
and Nerves of the Limbs. (After Kollmann.)

branches of the segmental nerves (Fig. 238). The muscles are
derived from the ventro-lateral sheet, which gives rise to all
the muscles of the body- wall (Fig. 238). As soon as the limb
buds appear, bundles of fibres from the anterior and posterior
nerve roots of the corresponding body segments enter them and
keep time with their growth. The limb nerves are at first so
large in comparison with the size of the limb bud that they are
crushed together and form a plexus. As they enter the bud, the

294

HUMAN EMBRYOLOGY AND MORPHOLOGY.

nerves encounter the condensed skeletal inesoblast at its base and
divide into a dorsal or extensor set and a ventral or flexor set
(Figs. 237 and 238).

I he nerve supply assists to indicate the body segments from
which the arm is developed (Fig. 239). The 4th cervical is the

Fig. 23P. — The Distribution of the Posterior Roots of the Spinal Nerves on the Flexor

Aspect of the Ann.

most anterior, the 3rd dorsal, sometimes it is the 2nd, is the most
posterior segment. Hence the arm is produced from seven, or
more commonly eight segments in all. Each segment contributes
from its nerve, its muscle plate and probably also its artery.
The typical distribution of a segmental nerve to the limb bud is
shown diagrammatically in Fig. 238. Each segmental nerve, as is

THE LIMBS.

295

the case with the typical lateral cutaneous nerves, divides into
a dorsal division for the extensor muscles (musculo-spiral, posterior
circumflex, supra-scapular, etc.) and ventral for the flexor muscles
(median, ulnar, musculo-cutaneous) (Fig. 288).

Clinical and experimental research has shown that each of the
seven or eight segments contributes to the cutaneous supply of
the limb. The classical investigations of Sherrington in the
segmental distribution of the sensory nerves in the limbs of apes,
showed that they are arranged in a definite and orderly manner
(Fig. 240). The sensory distribution of the spinal nerves in the

Fig. 240. — Diagram to show the typical Manner in which the Posterior Nerve Roots
are distributed in the Lower Limb (based on Sherrington’s researches into the
sensory distribution of the limb nerves of apes).

human arm is shown diagrammatically in Fig. 239. The distribu-
tion of the motor nerves of each segment is the following :

5th Cervical. — To extensor muscles which raise the shoulder,
extend the arm, fore-arm and proximal phalanges.

6th Cervical.— To adductors and extensors of the arm, flexors
of the thumb.

7th Cervical. — Rotators outwards of the shoulder and flexors of
the middle phalanges.

8th Cervical. — Flexors of the fingers and extensors of the
carpus.

1st Dorsal. — Flexors of fingers, the interossei, adductors of
thumb.

It will be thus seen that in the motor segments, not muscles
but actions are represented ; hence the formation of the plexuses

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HUMAN EMBRYOLOGY AND MORPHOLOGY.

that each nerve may contain functional elements derived from
several segments. Each muscle, it must be remembered, is
supplied from two, three or more segments of the cord.

Only three anomalous points in the arrangement of nerves in
the upper limb require attention : (1) The segments which

supply nerves for the arm are nearly constant. The extent to

Fig 241.— Flexor Aspect of the Lower Limb showing the Sensory Distribution of the

Segmental or Spinal nerves.

which the 4th cervical and 3rd dorsal contribute varies ; the
degree of variation is markedly less than in the lower limb.
(2) A part of the musculo-cutaneous nerve frequently joins the

T1IE LIMBS.

297

median below the insertion of the coraco-brachialis ; this
communication is frequently seen in lower primates ; its meaning
is not known. (3) A communication between median and ulnar
in the forearm is also common and is seen constantly in some
primates. The communicating branch passes with the deep
branch of the ulnar nerve to the palm.

Nerve Supply of the Lower Limb. — Usually ten segments
contribute to the nerve supply of the lower limb — the 12th
dorsal to the 4th sacral (Fig. 241). The sensory nerves are
derived from these segments; the motor nerves begin at the 1st
lumbar segment and end at the 3rd sacral. There is a consider-

o

able variation in the number of body segments or vertebrae to
which the lower limb is attached; usually it is the 25th vertebra
which becomes the 1st sacral, but it may be the 26th or 24th
(page 144). Of these three forms the first is the normal type
(25th); the second the post-fixed type (26th), the third the
pre-fixed type (24th). There is even a greater variation in the
segments which contribute nerves to the limb ; the normal motor
segments are the 1st lumbar to the 3rd sacral ; in the post-fixed
type (more common than the next) the motor segments commence
at the 2nd lumbar and cease at the 4th sacral ; in the pre-fixed
type the motor segments commence at the 12th dorsal and end
at the 2nd sacral. The spinal nerve which bifurcates and joins
both lumbar and sacral plexuses is known as the nervus furcalis.
In the normal type it is the 4th lumbar; in the pre-fixed type
it is the 3rd lumbar ; in the post-fixed type the 5th lumbar.

The nervus bigeminus, normally the 4th sacral, may also vary
in a corresponding manner.

The nerves to the extensor surface of the lower limb, the
anterior crural, external popliteal, etc., represent the dorsal
divisions of lateral cutaneous nerves (Fig. 238). The nerves to
the adductor and flexor aspects, the obturator and internal
popliteal, represent the ventral divisions. In a considerable
number of individuals, the dorsal division (external popliteal)
and ventral (internal popliteal) of the great sciatic separate in
the pelvis, the external popliteal perforating the pyriforinis.

The muscles of the lower extremity are supplied in the normal
type from the following segments :

Motor Segments. — 1st Lumbar. Psoas and Uiacus.

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HUMAN EMBRYOLOGY AND MORPHOLOGY.

2nd Lumbar. Psoas and iliacus, sartorius, pectineus, ad-

ductor brevis.

3rd Lumbar. Adductors, quadriceps, obturator externus.

4th Lumbar. Adductors, quadriceps, obturator externus,

also external rotators of hip, gluteus maximus, hamstrings.

5th Lumbar. External rotators of hip, gluteus maximus,
gluteus medius and minimus. Muscles of leg and foot.

1st Sacral. External rotators of hip, gluteus maximus,
gluteus medius and minimus. Muscles of leg and foot.

2nd Sacral. External rotators of hip, gluteus medius and
minimus, flexor hallucis, extensors of foot and toes.

3rd Sacral. Pyriformis, muscles of calf and sole.

4th Sacral. Muscles of perineum.

It will be remembered that the perineal region is developed
behind the limb buds of the lower extremities ; hence its nerve
supply from the most posterior nerve segments (3rd and 4th
sacral).

Sherrington found that the posterior roots of the limb nerves
were distributed a regular and simple manner in apes. His
results are applied to the lower limb of a human foetus in
Fig. 240. The actual distribution in man, which has been
partially worked out by clinicians, varies considerably from what
might be expected from Sherrington’s results (compare Figs.
240 and 241).

In the human leg and foot there is a tendency for the nerve
fibres destined for the outer digits to proceed in the external
saphenous nerve instead of by the musculo-cutaneous. The
external saphenous nerve may supply the 4th and 5th digits, in
a maimer similar to the ulnar nerve in the hand ; more fre-
quently it is confined to the outer side of the 5th digit.

Pelvic and Shoulder Girdles.— In the basal part of each limb
bud a cartilaginous arch is developed. It consists of a dorsal
and ventral part, the joint cavity for the articulation of the limb
being situated at the junction of the two parts. Fishes retain
this simple primitive form of girdle.

The Pelvic Girdle has undergone less modification from the
primitive type (Fig. 242) than the shoulder girdle. The primi-

THE LIMBS.

299

tive type of pelvic girdle, such as is seen in the crocodile or
lizard, and of which the mammalian type is a derivative, is
shown d iagramm atically in Fig. 242. For comparison the human
girdle in a 5th week foetus is shown in Fig. 243.

The dorsal element consists of the ilium ; it is attached by
ligaments to the costal process of one or more sacral vertebrae.
In the ventral portion of the cartilaginous arch are developed two
elements, the pubes and ischium, both of which take part in the
formation of the acetabulum (Fig. 242). Both reach the ventral
median line in which cartilage is developed.

Fig. 243. — The Pelvic Girdle of a Human Foetus at the 5tli week. (After Kollmann )

In man the following changes may be noted: (1) The costal
processes ot the sacral vertebrae (2 ^ usually) have fused together
to form the lateral sacral mass, with these the ilium articulates ;

300

HUMAN EMBRYOLOGY AND MORPHOLOGY.

(2) the vertebral border (crest) has become enormously elongated
and gives attachment to abdominal muscles, cutting off the fibres
of insertion of the external oblique which form the chief part of
Poupart’s ligament ; (3) the ischium does not reach the ventral line.
In most birds, neither ischium nor pubes reach the ventral line.
The pubes fail to meet in cases of ectopia vesicae. The symphysis
pubis is formed in the ventral line. The cotyloid bone — os
acetabuli — is formed in the Y-shaped cartilage between the three
elements. It ossifies in the 13th year. Professor Howes has
pointed out that it is this ossification which forms the pubic
part of the acetabulum, and that it is really part of the pubes.

Shoulder Girdle. — The duckbill (ornithorynchus) shows the
most generalized type of mammalian shoulder girdle ; it resembles
closely the primitive reptilian type ; from such a form the various
types of mammalian shoulder girdle were probably evolved.

The dorsal part of the arch consists of (1) scapula, (2) supra-
scapula (Fig. 244). The supra-scapula is represented in man by
the cartilage along the vertebral border ; it ossifies in the early

clauicle ( derma!)

Fio. 1244. — The Shoulder Girdle of Ornithorynchus.

years of manhood. The supra-spinous part of the scapula appears
first in higher mammals ; it is produced late in the development
of the scapula (in the 3rd month of foetal life) by the upgrowth
of the supra-spinous blade of the scapula ; it is not repiesented
in the pelvic girdle. The spine of the scapula is represented in
the pelvic girdle by the ilio-pectineal line of the ilium ; but there
is no pelvic representative of the acromion process. The dorsal

THE LIMBS.

301

segment of the pelvic girdle becomes fixed to the costal processes ;
the corresponding part of the scapula remains free.

In the typical reptilian shoulder girdle, as in the pelvic
(Fig. 242), two elements are formed in the ventral part of
the arch — a posterior part — the coracoid, corresponding to the
ischium, and an anterior — the pre-coracoid, corresponding to
the pubes. Both elements reach the ventral median line
in which the presternmn is developed. In Ornithorynchus
the coracoid element is represented by two bones — the cora-
coid and epi-coracoid. The coracoid helps to form the glenoid
cavity at its dorsal extremity ; its ventral articulates with
the presternum. In man and all higher mammals, in which
mobility of the fore limb is of advantage for speed and free move-
ment, the coracoid element is much reduced. It forms merely a
process on the scapula, which it joins in man about the 15th year.
It still enters into the formation of the glenoid cavity, the articular
part (supra-glenoid) having a separate centre of ossification which
appears in the 12 th year. It is possible that the costo-coracoid
ligament may be derived from the ventral part of the coracoid

inter-clau. lig. ( inter-clavicle )
represent pre-or epi-coracoid

memb. clavicle (primitive clau.)
cartilage

acrom.

coraco-clav. ligs.

costo-cor. lig.

post asp. manub !

cartil. c/av.
meniscus
'supra-stern.

coracoid

glenoid

right scap. fr. behind

Fig. 245. — The Parts in the Shoulder Girdle of a human foetus which correspond

with those of Ornithoi-ynchus.

element — the part which articulates with the sternum in the
duckbill. The pre-coracoid in the shoulder girdle of the lizard
corresponds to the pubic element in the pelvis. The pre-coracoid,
which, like all the primitive elements of the pelvic and shoulder

302

HUMAN EMBRYOLOGY AND MOIiPIJOLOGY.

girdles is formed in cartilage, has been partly or entirely replaced
in all mammals by the development over it of the clavicle, a
dermal or membrane-formed bone, the first of all the bones
to ossify. There is thus no true representative of the clavicle
in the pelvis. The inter-clavicle so strongly developed in
the ornithorynchus and in the “ merry-thought ” of the fowl
is also a dermal bone. It is represented in man by the inter-
clavicular ligament. It is highly probable that the clavicle does
not completely replace the pre-coracoid. The epi-coracoid of
ornithorynchus probably represents part of this element (Fig.
244). It is also represented in the shoulder girdle of man by
three structures (see Fig. 245). (1) By the cartilaginous inner

end of the clavicle ; (2) by the inter-articular cartilage ; (3) by
the supra-sternal bones, all of which appear in cartilage.

In order to give greater mobility and speed to some four-footed
mammals, the clavicle lias been reduced to a ligamentous band,
except at its extremities (rabbit, dog, etc.). In climbing animals,
and those in which the power of grasping or embracing is highly
developed, the clavicles are fully developed.

The acromion process is ossified from several centres which
appear in the years of adolescence ; the epiphysis so formed may
be united to the spine by fibrous tissue only. This occurs in
over 8% of subjects (Symington), and may be mistaken for a
fracture of the process. The coraco-clavicular ligaments may be
derived from the pre-coracoid element.

The Hand and Foot. — The hand and foot of man, as is the
case in all primates, retain the primitive arrangement of elements
much more closely than do most other mammalian orders. The
primitive type of hand or foot, out of which the various forms
found in mammals have been modified, are seen in such reptiles
as the lizard or tortoise (Fig. 246). In the hand of man the
same bones are to be seen as in the tortoise (Fig. 246), and the
same arrangement with some exceptions. The elements in the foot
of a typical lizard resemble closely the arrangement seen in its
hand ; the same elements are present even in the highly modified
human foot. The hand and foot bones have undergone great
specialization in most mammals. In the evolution of the horse,
for instance, one lateral digit after another has become vestigial,
leaving the central digit enormously enlarged and specialized to

THE LIMBS.

303

form the lower part of the extremities. In ruminants the 3rd
and 4th digits have become specialized ; the rest of the digits
have become reduced until only traces of them are left ; in rodents

radius or tibia.

radiate or tibiate.
carpal V.
meta-carp

V. tv.

ulna or fibula,
intermedium
ulnare or fibulare
carpal I.
cent rale
carpal III.

Fig. 246. — The Carpal Bones of a Tortoise.

the hallux is vestigial. The hallux and pollux are the mammalian
digits most liable to undergo retrogression. In man, on the other
hand, the hallux and pollux find their greatest development.

Comparison of the Tarsus and Carpus. — Both are the de-
rivatives of such a typical form as is shown in Fig. 246. In the
typical tarsus or carpus there occur the following bones :

1. Radiale or Tibiale forms the scaphoid in the hand and
astragalus in the foot.

cuboid (4. 5 tarsale

^ os cct/cis ( fibulare )
os trigonum {intermedium)
if astrag. ( tibiale )

■A— scaphoid (centrale)

Fig. 247.— The Os Trigonum and Bones of the Tarsus.

2. Intermedium forms the semilunar in the hand ; in the foot
it is much reduced and usually unites with the astragalus to form

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HUMAN EMBRYOLOGY AND MORPHOLOGY.

the external tubercle of that bone. It may remain separate and
form the os trigonum (Fig. 247).

3. Ulnare becomes the cuneiform in the hand ; the os calcis in
the foot. During the fibrous and early cartilaginous stages in the
development of the human tarsus, the os calcis is in contact with
the fibula. In the hand the ulnare and intermedium are bound
by fibrous bands to the ulna (Fig. 246); these bands assist to
form the triangular fibro-cartilage.

4. Carpale or Tarsale I. becomes the trapezium in the hand,
the internal cuneiform in the foot. In the prehensile foot of apes,
the hallucial articular surface is directed inwards for the move-
able big toe. This is also the case during the foetal development
of the human foot (Leboucq). At no period of development is
the hallux of man directed inwards and separated from the other
toes. In man the great toe resumes a primitive position and
its metatarsal lies parallel with those of the metatarsal series.

5. Carpale or Tarsale II. forms the trapezoid in the hand, the
middle cuneiform in the foot.

6. Carpale or Tarsale III. forms the os magnum in the hand,
the external cuneiform in the foot.

7. Carpale or Tarsale IV. and V. have united in both hand and
foot to form the unciform and cuboid. This union is seen in
mammals generally. In the cat and carnivores the scaphoid and
semilunar unite together.

The Os Centrale is situated between the first and second rows
of the carpal or tarsal bones (Fig. 246). In the foot it forms the
scaphoid — a bone which plays an important part in the formation
of the plantar arch — but is yet remarkably late in beginning to
ossify, viz., about the 3rd year. It appears in the membranous
stage of the human carpus, but at the end of the 2nd month it
has joined the dorsal and distal aspect of the scaphoid of the hand.
It may be occasionally detected as a tubercle on the dorsal aspect
of the scaphoid or even as a separate bone. It is a separate bone
in the carpus of all primates except the gorilla, chimpanzee, and
man. There are two centralia in lower vertebrate forms.

The Pisiform (ulnare laterale of Forsyth Major) is of doubtful
nature. It is possible that in a very early stage of the evolution
of mammals there were more than five digits — one behind the
little finger — post minimi digiti; and another on the radial side

THE LIMBS.

305

of the hand — a pre-hallux. Supernumerary digits, when they
appear, are commonly situated on the radial side of the thumb or
ulnar side of the little finger, but they may represent merely a
fission of the normal pollex or little finger. The pisiform has been
regarded as the vestige of a post-minimal digit ; the sesamoid on
the trapezium, in which a slip of the extensor ossis metacarpi
pollicis ends, as a remnant of a pre-hallux. It is possible also to
regard the pisiform as a sesamoid developed in the tendon of the
flexor carpi ulnaris — for that muscle is originally a flexor of the
metacarpus and ends on the 5 th metacarpal — the pisi-metacarpal
ligament representing the terminal part of the tendon. The pisi-
form, however, is developed with the rest of the carpal bones and
before the tendon of the flexor carpi ulnaris. In mammals generally,
but not in man, the pisiform articulates with the ulna as well as the
cuneiform, and its synovial facet opens into the wrist joint. It
may be represented in the foot by the heel epiphysis of the os
calcis. The gastrocnemius, which represents the flexor carpi
ulnaris in the leg, is also primitively a flexor of the metatarsus ;
the long plantar ligament, from which it is separated by the
growth of the heel, represents the continuation of its tendon.

The Eversion of the Foot and Development of the Arch. —
The human foot has been highly modified for upright progression.
The chief modifications are :

(1) Gradual eversion of the foot, so that the sole can be
applied to the ground. Even at birth — and for some time after
— and always up to and before the 7th month of foetal life, the
soles of the feet are inverted, so that when the foetal limbs are in
their natural position they are directed towards the belly of the
child. In club foot the natural process of eversion does not take
place. The ape’s foot is kept normally in the inverted position,
an adaptation for prehension. The following factors assist in
producing eversion :

(a) The neck of the astragalus (Fig. 248), which in the foetal
foot is long and directed downwards and inwards at
an angle to the axis of its body, becomes relatively
shorter and directed more in line with the axis of the
articular surface of its body (Fig. 248). Further, the
outer border of the tibial articular surface of the astra-
galus is prominent in the foetus ; the inner border is

u

306

HUMAN EMBRYOLOGY AND MORPHOLOGY.

much the lower ; a growth upwards of the inner border
causes the astragalus and foot to rotate outwards (Lazarus).

Fig. 248. — The Foetal and Adult (in dotted outline) Forms of the Astralagus

contrasted.

(b) The bones on the inner side of the foot, particularly the

scaphoid and internal cuneiform, grow more rapidly than
those on the outer side of the foot — especially after
birth. This tends to evert the foot and also to produce
the longitudinal arch.

(c) A special evertor of the foot is produced — the Peroneus

tertius — a muscle peculiar to man. It is developed from
the outer and lower fibular fibres of the extensor longus
digitorum and represents part of the tendon of that
muscle to the 5 th toe. The peroneus brevis and longus
may also assist, especially the latter, which in apes is
a grasping muscle, acting as a flexor of the metatarsal
bone of the hallux.

(2) The tarsal bones of the human foot — especially the astra-
galus and os calcis — are of great size when compared with the
tarsal bones of other primates; while the digital or phalangeal
elements, except in the case of the great toe, which is relatively of
great size, have undergone retrogression. This is especially the
case in the human little toe; some of its muscles are not in-
frequently fibrous, and the terminal phalanx may not be separated
from the middle phalanx. The terminal phalanx is the last to be
differentiated in development of the fingers and toes.

(3) The plantar arches, both longitudinal and transverse, are
produced. The arch of the foot is a human character. At birth
the child is flat-footed; the head of the astragalus touches the

THE LIMBS.

307

ground. The arch is produced as the child learns to walk. The
chief factor in its production is the growth of the tarsal bones—
especially of the scaphoid and internal cuneiform — and 1st meta-
tarsal. Hence in rickets, where the normal tarsal growth is
disturbed, the occurrence of flat foot. Amongst the structures
which help to maintain the arch are :

(a) The growth of the os calcis into the heel separates the

tendon of the plantaris from its prolongation in the sole
— the middle part of the plantar fascia, which assists
in maintaining the arch. In lower primates the two parts
are continuous, the tendon of the plantaris plying across
the os calcis in a cartilage-lined groove.

(b) The internal lateral ligament of the ankle (anterior part)

and the inferior calcaneo-scaphoid ligaments undergo
great development in man.

(c) The flexor brevis digitorum which in lower primates arises

principally from the long flexor tendons in the sole, has
its origin completely transferred to the os calcis in man.
It can thus act more powerfully in maintaining the
arch. The flexor accessorius, a detached part of the
flexor longus hallucis, is specially well developed and
helps to maintain the arch of the foot.

( d ) The tibialis posticus, originally a flexor of the metatarsus,

corresponding to the flexor carpi radialis in the hand,
obtains a secondary attachment to the scaphoid. The
tibialis anticus, which answers to the extensor ossis
metacarpi pollicis, becomes permanently inserted to the
internal cuneiform and metatarsal. Both of these
muscles, thus modified, help to maintain the arch of
the foot. So does the tarsal part of the tendon of the
tibialis posticus.

(e) The long plantar ligament, originally a part of the tendon

of insertion of the gastrocnemius — also assists to main-
tain the arch.

(4) The development of the great toe and the peculiar
arrangement of its muscles must also be regarded as adaptations
in the foot to upright posture and progression.

Comparison of the Muscles of the Upper and Lower
Extremities. — As a mental exercise it may be worth the

308

HUMAN EMBRYOLOGY AND MORPHOLOGY.

student’s while to briefly review the corresponding muscles in
the two limbs. He has already seen that the arm and leg buds
are similar in origin and made up of similar elements. Each is
subsequently modified for its special function. The key to the
homology of the muscles lies in their relationship to the digits,
and their position on the limb.

Pollex and Hallux. — The extensor ossis metacarpi pollicis
corresponds to the tibialis anticus. The thumb muscle has
commonly a carpal insertion as well as metacarpal. The extensor
brevis or primi internodii pollicis is constant in man only; it is a
segment of the extensor ossis metacarpi pollicis.

The extensor brevis hallucis is not represented in the thumb.

Second Digit. — In the lower primates each finger has two
extensors — a deep and superficial. The deep in the second
digit becomes the extensor indicis ; in the little finger it forms
the extensor minimi digiti. The deep extensor muscles have
disappeared in man from the 3rd and 4th digits, but occasion-
ally reappear. In the leg the deep extensors have migrated to the
foot, and form the extensor brevis digitorum. That for the little
toe, however, has not descended ; it is always vestigial, if present,
and is commonly absent. It runs beneath or with the peroneus
brevis, and is known as the peroneus quartus or peroneus quinti
digiti.

Flexors and Extensors of the Metacarpus. — These have
retained their primitive insertions in the hand; their modifications
in the foot have been already mentioned. Both at the knee and
elbow joint the origins of these muscles have undergone much
shifting and migration.

The comparison already made between the scapula and ilium
(p. 291), will help the student to understand the correspondence
between the muscles of the thigh and arm.

Vestigial and Abnormal Muscles in the Limbs and
Trunk. — (1) The muscles of the human ear and scalp may be
described as vestigial when compared to the development in
other mammals. Although their action on the ear and scalp is
feeble, yet they serve as most important bases into which certain
psychological states are reflected.

THE LIMBS.

309

(2) The levator claviculae (omo-trachelian) is a muscle which
passes from the upper transverse cervical processes to the outer
end of the clavicle or acromion process. It is well developed in
climbing primates. It is not a common muscle in man. It can
be recognised during life in the posterior triangle of the neck.

scapula

long head triceps
teres major
dorsi

uestigial /at condyl. of man
•condyl. of primates
head triceps

internal intermusc. sept,
condyle.

olecranon

Fig. 249. — Latissimo-conclyloideus Muscle.

(3) The latissimo-condyloideus (dorso-epitrochlearis), a climbing
muscle, is always represented in man, commonly by a fibrous
bundle between the tendon of the latissimus dorsi and the long
head of the triceps (Fig. 249). It may be occasionally muscular.
In apes it passes from the latissimus dorsi at the axilla to the
inner aspect of the elbow and arm, which it retracts in climbing.
It belongs to the same sheet as the coraco-brachialis. The
ligament of Struthers — a strip of fibrous tissue over the internal
intermuscular septum, above the internal condyle — represents
part of the tendon of this muscle. The muscular slips occa-
sionally found crossing the brachial or axillary artery from the
latissimus dorsi to the coraco-brachialis or biceps are derivates
of this muscle. Other slips found crossing the floor of the axilla,
between the adjacent borders of the pectoralis major and
latissimus dorsi, are parts of the muscular sheet out of which
these two muscles are developed.

(4) The pectoralis externus arises from the 4-5-6 ribs and

310

HUMAN EMBRYOLOGY AND MORPHOLOGY.

costal cartilages, beneath the axillary border of the pectoralis
major. This is its normal condition in most mammals, but in
man it is commonly fused with, and forms part of, the pectoralis
major.

(5) The sternalis is a new muscle. The pectoralis major is
formed from the same ventral longitudinal sheet as the rectus
abdominis and sterno-mastoid. The fibres of the sternalis, which
lie along the sides of the sternum, superficial to the origin of the
pectoralis major, represent a persistent part of the primitive
longitudinal sheet. The sternalis is a derivative of the sphincter
colli, part platysma-sheet (Parsons).

(6) In the sterno-mastoid, four elements are recognised : sterno-
mastoid, sterno-occipital, cleido-mastoid, cleido-occipital. The
cleido-occipital fibres, which form part of the same sheet as the
trapezius, are often absent. On the other hand the cleido-
occipital fibres may be continuous with the trapezius.

(7) The pectoralis minor is sometimes inserted to the capsule
of the shoulder and great tuberosity of the humerus. This is
the primitive insertion of the muscle. The coracoid insertion is
a secondary attachment seen only in man and some of the higher
primates. When the pectoralis minor is inserted to the
coracoid, the former fibres of insertion become fused with, and
forms part of, the coraco- humeral ligament. The coraco-humeral
ligament represents the ischio-capsular of the hip joint.

(8) In some apes (such as the Gibbons) the biceps has four
heads — the two usual, the long and short, and two others, one
from the inner border of the humerus and one from the bicipi-
tal groove. These two extra heads appear frequently in man.

(9) The epitroclileo-anconeus is frequently present. It crosses
the ulnar nerve from the internal condyle to the olecranon.

(10) The palmaris longus and its homologue in the leg, the
plantaris, are vestigial, aberrant in form, and often absent. The
plantar and palmar fasciae represent their divorced tendons.
The plantaris and palmaris undergo retrograde changes in the
Primates with the transformation of claws to nails.

(11) Each digit (fingers and toes) in lower Primates, such as
monkeys, is provided with three short muscles which arose from
the carpus or tarsus. The three muscles are (Fig. 250): (1) a
short flexor on the radial side of the digit ; (2) a short flexor

THE LIMBS.

311

on the ulnar side ; (3) a contrahens or adductor muscle (always
absent in the middle digit). The ten short flexor muscles
form a deeper sheet than the four contrahentes. Of this form

i

pi. inter. 3

I

opponens and [ fior. inter 4 d0r/ 3

flex. breu. min.
digiti

1st meta-tarsal

tib. head flex,
breu. hat.

cp cp cp p?

contrah I/, contrah IV. contrah II. , contrah I

abd. min. dig.

deep plant, nerue

\ adduct hal.

add. halluc

Fig. 250. — The Morphology of the Short Muscles of the Digits. The Muscles shaded
are those of the ape’s hand or foot ; the positions of the corresponding muscles in
the human hand or foot are indicated by dotted outlines.

the arrangement of the short muscles in the human hand is
a derivative. The remnants in the human hand and foot of
the contrahentes are: (1) The adductors of the 1st digit (pollex
or hallux) ; (2) fibrous remnants of the others occur over the
deep plantar or carpal arch (Fig. 2 50). The short flexors in
man have become (1) the seven interossei ; (2) the flexores
breves (ulnar and radial) and opponens of the first digit ; (3)
the flexor brevis and opponens of the fifth digit (see Fig. 250).
The ulnar flexors of the thumb and great toe are absent or
fibrous.

(12) The Pyramidalis is often absent in man or vestigial. It
is the tensor of the linea alba.

(13) Eemnants of the extensors and flexors of the tail may

occur between the sacrum and coccyx (page 136).

(14) The Coccygeus is vestigial; its superficial part forms the
small sacro-sciatic ligament.

(15) Fibres of the biceps of the thigh may be followed into the
great sacro-sciatic ligament. This ligament, which is almost peculiar
to man — in other primates it is quite thin and slender — may con-
tain fibres derived from the caudo-femoral group of muscles, such
as the tenuissimus, a long strap-like muscle which passes from the
coccyx to the femur and leg in lower mammals. The sacro-

312

HUMAN EMBRYOLOGY AND MORPHOLOGY.

sciatic ligament is mainly derived from the great median sheet,
out of which the middle layer of the lumbar fascia is also formed.
Parsons regards the short head of the biceps as a derivative of
the tenuissimus, while others regard it as part of the muscular
sheet which forms the peroneal muscles. Amongst Primates, it
is found only in man, the anthropoids, and some South American
apes. The short head of the biceps corresponds to the brachialis
anticus in the arm, and is supplied by the external popliteal
nerve.

(16) The psoas parvus is also vestigial. It acts primarily as
a flexor of the pelvis on the spine. It begins to disappear with
the assumption of the erect posture.

(17) The scansorius is a separated segment of the gluteus
medius and minimus. It rises from the anterior border of the
ilium and passes to the great trochanter. It corresponds to the
teres minor. It is not constant in any animal.

(18) The flexor brevis digitorum to the little toe and the
adductor transversus of the great toe are often fibrous.

Vessels of the Limbs. — The Vas Aberrans. — This vessel gives
rise to a number of anomalous arrangements of the brachial
artery. If a newly-born child be well injected, a number of
branches derived from the axillary, brachial and main arteries of
the fore-arm will be found to form an anastomatic chain along the
superficial aspect of the median nerve. The upper end of the
chain is formed by a branch from the axillary ; the lower end is
completed by a branch of the ulnar or radial at the elbow.
In South American apes this anastomosis always opens up to
form a supplemental brachial artery — the vas aberrans — ending
usually as the radial. From the manner of its origin and dis-
position in front of the median nerve, it will be seen that the
vas may arise from the axillary or upper part of the brachial,
and terminate in the radial, ulnar or brachial, according to the
branches which participate. It may be so large as to simulate
a large branch of the axillary or a division of the brachial. Such
a condition is spoken of as high division of the brachial.
The vas may supplant the brachial artery altogether, which
is then represented by a trunk which ends high up in the
arm by giving off the superior and inferior profunda arteries.
The brachial vessel formed from the vas is recognised by its

THE LIMBS.

313

superficial position to the median nerve. In lower apes, the
brachial artery commonly divides a little above the elbow. This
position of division is rare in man. In cases of high division the
radial, or even the ulnar, may be superficially placed in the fore-
arm.

Internal Saphenous Artery. — In most mammals — in all
primates except man — the anastomotica magna of the femoral
artery is as large as the popliteal and passes over the inner side of
the tibia with the internal saphenous vein to reach the dorsum of
the foot, where it forms the dorsalis pedis artery. This vessel is
known in them as the internal saphenous artery, and corresponds
to the radial artery in the forearm and wrist. The superficial
branch of the anastomotica magna rarely assumes such a develop-
ment in man ; but the course taken by the internal saphenous
artery explains the position of its accompanying vein, the internal
saphenous, in front of the internal malleolus. In man only is the
internal saphenous vein continued up the thigh to a saphenous
opening in the groin. Its primitive termination is in the femoral
vein at the lower end of Hunter’s canal.

The Superficial Plantar Arch, formed from the internal plantar
artery, is seldom complete in man. The pressure to which it
is subjected in the standing posture has led to its partial
obliteration. It corresponds to the superficial palmar arch.

The Supra-condylar Process is well developed in lemurs, the
lowest primates, and in mammals of many orders. Its function is
unknown. It occasionally appears in man. It is developed from
the humerus about two inches above the internal condyle as
a hook-like process of bone. It lies in front of the internal
inter-muscular septum, and when well developed the brachial
artery and median nerve may pass beneath it, as they do in such
animals as the squirrel and cat.

Third Trochanter. — A tubercle may appear in the gluteal ridge
which receives this name. It is cartilaginous until the 20th
year, when a separate centre of ossification appears in it (Dixon).
It is well developed in the horse.

Ligaments and Joints of the Limbs. — The Inter-articular
Cartilages are of doubtful origin. In their first appearance, by
the condensation of the mesoblast in the axis of the limb bud, the
bones are continuous. The joints are formed between the bones,

314

HUMAN EMBRYOLOGY AND MORPHOLOGY.

by the mesoblastic tissue remaining fibrous, while that which
forms the bases of the bones undergoes chondrification. The
mesoblastic tissue thus left between the cartilages opens out and
forms a synovial cavity and a fibrous capsule. Hence the peri-
osteum of the 'bones is continuous with and derived from the same
primitive layer as the joint capsules. The inter-articular
cartilages of the knee, for instance, may be simply masses of
mesoblast left between the bones, originally part of the condensed
mesoblastic bar out of which the femur, tibia and fibula were
developed. The inter-articular cartilage of the temporo-maxillary
joint may be a derivative of Meckel’s cartilage ; that of the
sterno-clavicular of the pre-coracoid. The triangular fibro-
cartilage of the wrist joint is derived from the ulno-carpal
ligaments, and interosseous membrane (Parsons). The cartilage
may also contain a post-minimal element (Corner). The cartilages
of the knee are of obscure derivation ; they may be parts of
muscles formerly inserted on the upper end of the tibia (Sutton).

Parsons has shown that in all primates, except man, the
external semilunar cartilage is circular in form and is attached
to the posterior crucial ligament.

Pm. 251. Showing the Origin of the Ligamentum Teres and Reflected Bundle of

the Capsular Ligament.

The Ligamentum Teres is to be regarded as a part of the capsule
of the hip joint which has been cut off by the outgrowth of the
articular surface of the head of the femur (Fig. 251). The
crucial ligaments are semi-isolated fasiculi from the capsule of
the knee joint, separated in a similar manner to the ligamentum
teres by the outgrowth of the condyles of the femur (Fig 252).
At first the ligamentum mucosum, crucial ligaments and posterior

THE LIMBS.

315

part of the capsule are continuous and form parts of the capsule
of the joint ; they come to occupy a position within the joint as
the femoral condyles grow backwards.

Fig. 252. — Showing the Origin of the Crucial Ligaments of the Knee.

In Eeptiles and in lower mammals the fibula enters into the
formation of the knee joint as the ulna does in that of the elbow.
This is at no time the condition in the human foetus (Grunbaum).
The external malleolus in primates is generally shorter than the
internal ; this is the case in man until the 3rd month of foetal
life ; after the 3rd month the external is the longer (Wilgress).

INDEX.

[The numbers refer to the pages.]

Abnormalities of muscles, 308.
Accessory processes of vertebrae, 153.
Accessory thyroid bodies, 47.

Acoustic ganglia, 60.

Acromion process, 302.

Affenspalte, 218.

Agger nasi, 26.

Alar cartilages, 4.

Alar cartilages of nose, 7.

Allantois, 97, 124.

Ameloblasts, 64.

Amnion, 92.

Anal fascia, 140.

Angular gyrus, 191.

Anterior commissure, 209.

Antrum of Highmore, 12.

Antrum of mastoid, 55.

Aortic arches, 35.

Aortic septa, 239.

Appendix, 279.

Aqueductus cochlea, 59.

Aqueous chamber, 183.

Arch of aorta on right side, 37.

Arch of foot, Development of, 305.
Arm, Nerve supply of, 293.

Arteries of body segment, 157.
Asterion, 164.

Astragalus, 305.

Atlas, 151.

Atresia vaginae, 117.

Attic of tympanum, 55.

Axis, 151.

Azygos lobe of lung, 255.

Bartholin, Glands of, 127.
Basal ganglia, 189.

Basi-hyal, 33.

Bilaminar blastoderm, 88.
Binocular vision, 181.

Bladder, 124.

Blastodermic vesicle, 87.
Blood, 247.

Body-stalk, 96.

Body, Symmetry of, 282.
Body, Ventral line of, 282.
Brachycephalic skull, 172.
Brain capsule, 172.

Brain, Fissures of, 217.
Branchial arches, 28.

Bregma, 165.

Broca, Area of, 22.

Bronchi, Formation of, 252.
Bronchi, Ramification of, 253.
Bulla ethmoidalis, 25.

Caecum, Development of, 277.
Calcarine fissure, 189.

Canalis cranio-pharyngeus, 19.
Cardiac septa, 238.

Cardiac tube, Flexures of, 235.

INDEX.

317

Cardinal veins, 22G.

Carotid bodies, 48.

Carpale, I., II., III., IV., V., 304.
Cartilages of Jacobson, 4.

Cartilaginous part of skull, Develop-
ment of, 165.

Caudate lobe of liver, 265.

Central canal, Formation of, 192.
Cephalic index, 171.

Cerato-hyal, 33.

Cerebellum, 199.

Cerebellum, Peduncles of, 200.
Cerebral hemispheres, 206.

Cerebral vesicle, 206.

Cervical fascia, 141.

Cervical sinus, Formation of, 32.
Choroid, 183.

Choroid fissure, 210.

Choroidal fissure, 180.

Chorion, 92, 97.

Chorionic vesicle, 97.

Ciliary muscle, 183.

Ciliary processes, 179, 183.

Circulatory system, Development of,
223.

Cleft membrane, 30.

Cleft palate, 1.

Cloaca, 103, 275.

Cloaca, Parts formed from, 124.
Coccygeus, 311.

Coccyx, 135.

Cochlear ganglion, 60.

Coelom, 91, 242.

Coelom, Divisions of, 244.

Coloboma iridis, 180.

Colon, Development of, 277.
Commissures, Development of, 208.
Congenital umbilical hernia, 101, 275.
Contrahentes, 311.

Coracoid, 301.

Cornea, 177.

Corpora quadrigemina, 189.

Corpus callosum, 209.

Corpus callosum, Peduncles of, 22.
Corpus luteum, 85.

Corpus striatum, 207, 211.

Cowper, Glands of, 126.

Cranial nerves, 219.

Cranial nerves, Segmental arrange-
ment of, 220.

Cranium, 161.

Crusta petrosa, 66.

Cuticular membrane, 64.

Darwin’s tubercle, 53.

Decidua, 91.

Decidua reflexa, 92.

Decidua serotina, 92, 96.

Decidua vera, 92.

Dental papilla, 64.

Dental sac, 66.

Dental shelf, 64.

Dentigerous and other cysts of the
jaw, 67.

Dentine, Origin of, 65.

Dentitions, Number of, 67.

Dermal bones, Development of, 162.
Dermal papillae, Formation of, 71.
Descending colon, 275.

Descending thoracic aorta, 36.
Diaphragm, Development of, 257.
Dolichocephalic skull, 172.

Dorsal aortae, 36, 38, 246.

Dorsal mesogastrium, 266.
Dorso-cervical region, Variations of,
145.

Dorso-epitrochlearis, 309.
Dorso-lumbar region, Variations of,
145.

Double ureter, 110.

Ducts of Cuvier, 224.

Duct of Gartner, 106.

Ductus arteriosus, 37.

Ductus endolymphaticus, 57.

Ductus venosus, 230.

Duodeno-jejunal fossa, 280.
Duodenum, 280.

Dura mater, 164.

Ear, muscles of, 308.

Ectoderm, see Epiblast.

Ectopia cordis, 246.

Ectopia vesicae, 125.

Embryo and membranes, 91.

Enamel, Origin of, 64.

Encephalocele, 194.

318

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Endo-cardial cushions, 238.
Endoderm, see Hypoblast.

Ensiform process, 285.

Epiblast, 89.

Epiglottis, 42.

Epi-hyal, 33.

Epiotic centre, 59.

Epipteric bone, 171.
Epitrochleo-anconeus, 310.
Epoophoron, 106.

Ethmoidal sinuses, 25.

Ethmoid, Lateral mass of, 7.
Ethmoid, Vertical plate of, 4.
Eustachian tube, 54.

External auditory meatus, 50.
External cleft depression, 31.
External ear, 51.

External ear, Muscles of, 53.
External genitals of female, 122.
External genitals of male, 122.
Eyeball, 175.

Eyelids, 185.

Face, Development of, 1.

Face, Malformations of, 1.

Face and scalp, Muscles of, 38.

Facial angle, 172.

Falciform ligament, 264.

Fallopian tube, 85.

Fasciae, 138.

Female pronucleus, 86.

Femoral hernia, 133.

Filum terminale, 195.

Fimbria, 210.

Fissures of brain, 217.

Fissure of Sylvius, 170, 212.

Flexor brevis digitorum, 307.

Flexor brevis of little toe, 312.
Floccular fossa, 60.

Foetal circulation, Remnants of, 242.
Fontanelles, 164.

Foot, 303.

Foot, Eversion of, 305.

Foramen caecum, 46.

Foramen ovale, 240.

Foramina in bone, Formation of, 170.
Foramina and canals, Formation of in
bone, 11.

Fore-brain, 202.

Fore gut, 272.

Fornix, 210, 211.

Fossa of Rosenm filler, 33.

Frontal bone, 162.

Frontal sinus, 24.

Frontal sinus, Infundibulum of, 24.
Funicular process, 131.

Gall bladder, 265.

Gastro-hepatic omentum, 264.
Gastro-splenic omentum, 268.
Geniculate ganglion, 60.

Genital cord, 112.

Genital folds, 122.

Genital ridge, 93, 103.

Genital tubercle, 122.

Genital Wolffian tubules, 107.
Germinal epithelium, 84, 94.
Germinal spot, 86.

Germinal vesicle, 86.

Great omentum, 267.

Great sacro-sciatic ligament, 311.
Gubernaculum testis, 128.
Gubernaculum testis, Formation of,
129.

Gyrus subeallosus, 22.

Hairs, 71.

Hallux, 308.

Hamulus of lachrymal, 26.

Hand, 303.

Hare lip, 1.

Hassall, Corpuscles of, 46.

Head folds of amnion, 95.

Heart, Demarcation of into chambers,
234.

Heart, Development of, 232.

Heart, Valves of, 240.

Hepatic veins, 229.

Hernia, 133.

Hiatus semilunaris, 23.

Hind brain, 197.

Hind gut, 272.

Hippocampal gyrus, 23.

Horse-shoe kidney, 110.

Human teeth, Morphology of, 67.
Hyaloid artery, 182.

INDEX.

319

Hyaloid canal, 183.

Hydrocephalus, 165.

Hymen, 117.

Hyoid, 32.

Hyoid arch, 33.

Hypoblast, 30, 89.

Hypospadias, 123.

Ileo-caecal fossa, 280.

Ileo-colic bloodless fold, 280.

Ileo-colic fold, 280.

Ileo-colic fossa, 280.

Ilium, 299.

Imperforate anus, 120.

Incus, 10, 15.

Inferior medullary velum, 198.
Inferior turbinate bone, 7.

Inferior vena cava, 228.

Inguinal hernia, 133.

Inter-articular cartilages, 313.
Intermedium, 303.

Internal auditory meatus, 62.

Internal cleft recess, 31.

Internal cuneiform, 306.

Internal geniculate body, 189.

Internal lateral ligament of ankle,
307.

Internal pterygoid plate, 9.

Internal saphenous artery, 313.
Inter-parietal bone, 171.

Inter-sigmoid fossa, 276.
Inter-ventricular septum, 239.

Iris, 183.

Ischium, 299.

Island of Reil, Formation of, 212.
Isthmus of thyroid, 46.

Jacobson’s organ, 21.

Jugular vein, 223.

Kidney, Origin of, 108.

Lachrymal bone, 27.

Lachrymal gland, 185.

Lambda, 164.

Lamina einerea, 205.

Lamina terminalis, 205, 211.

Larynx, 256.

Lateral nasal process, 6.

Lateral nasal process, Cartilage of, 6.
Lateral recess of pharynx, 33.
Latissimo-condyloideus, 309.

Left innominate vein, 226.

Leg, Nerve supply of, 297.

Lens, 176.

Lens, Capsule of, 181.

Lesser sac, 272.

Levator claviculae, 309.

Levator glandulae thyroideae, 47.
Lieno-renal .ligament, 268.

Ligament of Struthers, 309.
Ligamentum teres, 314.

Limbs, 289.

Limbs, Arteries of, 312.

Limbs, Joints of, 313.

Limbs, Ligaments of, 313.

Limbs, Nerve supply of, 293.

Limbs, Torsion and rotation of, 291.
Limbs, Veins of, 312.

Limbs, Vessels of, 312.

Limiting sulci of Island of Reil, 213.
Lingual papillae, 41.

Lingual tonsil, 45.

Lingula, 170.

Littre, Glands of, 127.

Liver, Development of, 261.

Liver, Ligaments of, 264.

Liver, Morphology of, 264.

Lobes of lungs, 254.

Long plantar ligament, 307.

Lower jaw, Development and ossifi-
cation of, 14.

Lumbar plexus, 297.

Lung, Blood supply of, 255.

Lung buds, 250.

Lungs, Development of, 249.
Lymphatic vessels, 247.

Macrostoma, 2.

Male pronucleus, 87.

Malleus, 15.

Mammae, 74.

Mamma, Lymphatics of, 77.

Mamma, Origin of glandular tissue,
75.

Mamma, Peripheral remnants of, 78.

320

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Mammalian cranium, Evolution of,
161.

Mammary gland, Position of, 78.
Mammary line, 75.

Mammillary processes of vertebrae,
154.

Mandibular processes and arch, 13.
Marginal gyrus, '210.

Mastoid, 59.

Maxillary processes, 8.

Maxillary process, Nerves and
Arteries of, 11.

Maxillo-turbinal, 7.

Meckel’s cartilage, 13, 14, 33.

Meckel's diverticulum, 273, 274.
Medulla oblongata, 197.

Medullary plate, 90.

Membrana pupillaris, 183.

Membrana tympani, 17, 33, 56.
Membranous labyrinth, Origin of, 57.
Meningocele, 166, 194.

Mesentery of appendix, 280.
Mesentery of small gut, 281.

Mesial nasal processes, 3.

Mesoblast, 89.

Meso-cephalic skull, 172.

Mesoderm, see Mesoblast.
Mesonephros, 102, 105.

Mesorchium, 132.

Meso-salphinx, 105.

Metacarpus, Flexors and extensors of,
308.

Metopic suture, 163.

Microcephaly, 165.

Mid-brain, 202.

Mid-gut, 273.

Morula, 87.

Mullerian ducts, 104, 110, 114.
Muller’s muscle, 184.

Muscles, Abnormalities of, 308.
Muscles of body segment, 157.

Muscles of extremities, 307.

Nails, 73.

Nasal air sinuses, Development of,
23.

Nasal cavities, 20, 23.

Nasal duct, 6, 26.

Nasal processes, 3.

Nasal processes, Arteries and nerves
of, 7.

Naso-palatine foramen, 5.

Neck, Development of, 28.
Neo-pallium, 207.

Nerves of body segment, 158.

Neural canal, 89.

Neural canal, Divisions of, 193.

Neural crest, 159.

Neural flexures, 202,

Neurenteric canal, 121, 195.
Notochord, 90, 145.

Oblique vein of Marshall, 225.
Obturator fascia, 140.

Occipital bone, 165.

Occipital lobe, 189.

Occipito-atlanto - axial articulations,
151.

Odonto-blasts, 65.

Oesophagus, 266.

Olfactory lobe, 21.

Olfactory nerves, 21.

Olfactory peduncle, 22.

Olfactory pits, 21.

Olfactory plates, 20.

Olfactory sense epithelium, Origin
of, 20.

Olfactory structures, 20.

Olfactory tract, Termination of, 22.
Omo-trachelian, 309.

Opercula, 214.

Opisthotic centre, 59.

Optic lobes, 189.

Optic nerve, 177.

Optic radiations, 189.

Optic thalami, 205.

Optic tracts, 187.

Optic vesicle, 177.

Oral plate, 18.

Ora serrata, 179.

Orbital muscles, 186.

Orbital plate, 185.

Orbit, Formation of, 184.

Organ of Corti, 58.

Organ of Geraldds, 107.

Organ of hearing, Development of, 49

INDEX.

321

Organ of Rosenmuller, 106.

Os anti epilepticum, 171.

Os articulare, 15.

Os calcis, 304, 306, 307.

Os centrale, 304.

Os Japonicum, 12.

Ostium abdominale, 110.

Ova, Discharge of, 84.

Ova, Origin of, 93.

Ovario-pelvic ligament, 128.

Ovary, Descent of, 80.

Ovary, Position of, S2.

Oviducts, 110.

Ovum, 83.

Ovum, Course of, in the Fallopian
tube, 85.

Ovum, Development of, 80.

Ovum, History of, within the Fallo-
pian tube, 86.

Ovum, Implantation of, 94.

Palatal rugae, 12.

Palate bone, 9.

Palato-glossus, 33.

Palato-quadrate bar, 10.

Palmaris longus, 310.

Pancreas, 268.

Pancreas, Relationship to peritoneum,
270.

Papillae foliatae, 42.

Parachordal cartilages, 166.
Paradidymis, 107.

Para-mastoid process, 173.
Para-thyroids, 47.

Paraxial mesoblast, 90.

Parietal bone, 163.

Paroophoron, 106.

Parotid gland, 42.

Parovarium, 106.

Pars ciliaris retinae, 179.

Pars facialis lachrymalis, 27, 185.

Pars membranacea septi, 239.
Pectoralis externus, 309.

Pectoralis minor, 310.

Pelvic fascia, 138, 140.

Pelvic floor, 136.

Pelvic floor, Development of, 136.
Pelvic girdle, 298.

Pericardium, 244.

Pericranium, 164.

Peridental membrane, 66.

Perineal body, 118.

Perineum, 118.

Permanent teeth, Origin of, 66.
Peroneus brevis, 306.

Peroneus longus, 306.

Peroneus tertius, 306.

Petro-mastoid 58, 167.

Petro-mastoid, Origin of, 58.
Petro-mastoid, Ossification of, 59.
Pharyngeal recess, 44.

Pharyngeal tonsil, 44.

Pharynx, Development of, 28.
Pharynx, Structures developed from,
39.

Pineal body, 204.

Pisiform, 304.

Pituitary body, 170, 203.

Pituitary body, Origin of, 19.
Placenta, Formation of, 97.

Plantar arches, 306.

Plantaris, 310.

Platysma myoides, 38.

Platysma sheet, 39.

Pleura, 251.

Plicae fimbriatae, 42.

Plica gubernatrix, 128.

Plica semilunaris, 185.

Plica triangularis, 44.

Plica vascularis, 128.

Polar body, 86.

Pollex, 308.

Pons varolii, 198.

Portal vein, 228.

Post anal gut, 120.

Posterior cardinal vein, 223.

Posterior root ganglion, 159.
Post-frontal, 171.

Pre-chorion, 92.

Pre-coracoid, 301.

Premaxillary bones, 4.

Pre-sternum, 285, 287.

Primitive callosal gyrus, 210.
Primitive groove, 89.

Primitive jugular vein, 56, 226.
Primitive perineal depression, 119.

X

322

HUMAN EMBRYOLOGY

AND MORPHOLOGY.

Primitive segments, 147.

Primitive streak, 89.

Primitive utricle, 57.

Primordial ova, 84, 94.

Processus vaginalis, 131.

Proctodaeum, 119.

Pronephros, 105.

Pro-otic centre, 59.

Prostate, 126.

Proto-vertebrae, 147.

Psoas parvus, 312.

Pterion, 164.

Pterotic, 59.

Pterygo-palatine bar, 9.

Pubes, 299.

Pulmonary artery, 37.

Pulp of teeth, 66.

Pyramidalis, 311.

Pyramid of thyroid, 46.

Pyriform fossa, 33.

Radiale, 303.

Rathke’s pocket, 19.

Rauber’s layer, 88.

Recto-vesical fascia, 140.

Rectum, 275.

Renal Wolffian tubules, 106, 107.
Retina, 179.

Retro-caecal fossa, 280.
Rhinencephalon, 22, 206.

Ribs, 152.

Ribs, Formation of, 284.

Ribs, vestigial, 152.

Riedel’s lobe, 265.

Roof of skull, Development of, 162.

Sacral plexus, 297.

Sacro-coccygeal region, Variations of,
145.

Sacro-lumbar regions, Variations of,
144.

Salivary glands, Origin of, 42.

Scalp, Muscles of, 308.

Scansorius, 312.

Scaphoid, 306.

Scapula, 300.

Sclerotic, 184.

Sclerotome, 146.

Scrotum, 124.

Sebaceous glands, 74.

Secondary sulci, 219.

Second digit, 308.

Seessel’s pocket, 43.

Segmentation of body, 155.

Segment, Constitution of, ,155.
Sensori-motor areas of brain, 218.
Septal cartilage, 4.

Septum primum of auricle, 240.
Septum secundum of auricle, 240.
Septum transversum, 257, 258.
Shoulder girdle, 298.

Sigmoid flexure, 275.

Simian fissure, 218.

Sinus subpericardiacus, 255.

Sinus venosus, 235.

Sinus venosus, Valves of, 237.
Skeleton of body segment, 156.

Skene’s tubules, 126.

Skin, 70.

Skull, Development of, 161.

Skull, Primitive membranous, 161.
Somatopleure, 90, 224.

Spermatozoa, 93.

Sphenoidal sinus, 25.

Sphenoidal turbinate bone, 25.
Sphenoid, Development of, 169.
Spigelian lobe of liver, 265.
Spina-bifida, 194.

Spinal column, 142.

Spinal column, Curves of, 143.

Spinal column, Evolution and develop-
ment of, 145.

Spinal cord, 194.

Spinal cord, Development of, 195.
Spine, Pyramids of, 142.
Splanchnocele, 242.

Splanchnopleure, 90.

Spleen, 267.

Squamosal, 163.

Stapes, 59.

Sternalis, 310.

Sternebrae, 285.

Sterno-mastoid, 310.

Sternum, 284.

Sternum, Development of, 286.
Stomach, 266.

INDEX.

323

Stomodaeum, 17.

Stylo-hyal, 33.

Styloid process, 33.

Subclavian arteries, 37.

Subclavian veins, 226.

Sulcus arcuatus, 209.

Sulcus terminalis, 40.

Superficial plantar arch, 313.

Superior medullary velum, 201.
Superior vena cava, 223.
Supra-condylar process, 313.
Supra-occipital, 163.

Supra-renal bodies, Development of,
259.

Supra-scapula, 300.

Supra-sternal bones, 288.
Supra-tonsillar recess, 44.

Sweat glands, 75.

Sylvian fissure, 170.

Sympathetic nerves of body segment,
160.

Tail fold of amnion, 95.

Tail, Muscles of, 311.

Tapetum lucidum, 184.

Tarsale, I., II., III., IV., V., 304.
Tarsus, 303.

Teeth, Eruption of, 69.

Teeth, Development and morphology
of, 63.

Teeth, Nature of, 67.

Teeth, Boots of, 68.

Tegmen tympani, 54.

Temporal ridges, 173.

Temporary fissures of brain, 216.
Temporo-maxillary articulation, 15.
Tenon, Capsule of, 184.

Testes, 127.

Testicle, Descent and development of,
127, 131.

Third trochanter, 313.

Thorax, Diameters of, 254.

Thymus, 45.

Thyreo-glossal duct, 46.

Thyroid, 40, 46.

Thyroid cartilage, 34.

Tibialis posticus, 307.

Tongue, 39.

Tongue, Lymphatics of, 42.

Tongue, Muscles of, 41.

Tonsil, 33, 44.

Tooth, Structure of, 63.

Trabeculae cranii, 168.

Transverse fissure, 210.

Transverse and spinous processes of
vertebrae, 154.

Triangular ligament, 140.

Trigonum olfactorium, 22.

Tuberculum impar, 40.

Tympanic bulla, 55.

Tympanic plate and articular emin-
ence, Development of, 17.
Tympanic ring, 17.

Tympano-hyal, 33.

Tympanum, 54.

Ulnare, 304.

Umbilical cord, Formation of, 99.
Umbilical faecal fistula, 101.

Umbilical urinary fistula, 101.
Umbilical veins, 231.

Umbilicus, Formation of, 101.
Unstable regions of spine, 144.
Urachus, 124.

Urethra, Female, 124.

Uro-genital cleft, 121.

Uro-genital sinus, 115, 125.

Uro-genital system, 102.

Uterus, Formation of, 113.

Uterus, Bound ligament of, 112.

Uvea, 179.

Vagina, Formation of, 113.

Vasa aberrantia, 107.

Vas aberrans, 312.

Veins of trunk, Development of, 223.
Velum interpositum, 207.

Ventral aortic stems, 36.

Ventral mesogastrium, 266.

Vermian fossa, 60.

Vertebra, Development of, 148.
Vertebrae of mammalia, 150.
Vesico-vaginal septum, 116.

Vestibular ganglion, 61.

Vestigial fold, 225.

Vestigial turbinates, 25.

324

HUMAN EMBRYOLOGY AND MORPHOLOGY.

Visceral arches, 28.

Visceral arches, Arteries of, 35.
Visceral arches, Cartilages of, 33.
Visceral arches, Muscles of, 38.
Visceral arches, Nerves of, 34.
Visceral clefts, 30.

Vitelline duct, 100.
Vitello-intestinal canal, 273.
Vitreous humour, 182.

Vomer, 4.

Wharton’s jelly, 101.
Wolffian body, 102.
Wolffian ducts, 103.
Wolffian tubules, 103.
Wormian bones, 171.

Yolk sac, 273.

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