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9

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ap

The Correlation Between Struc-
ture and Function in the De-

velopment of the Special
Senses of the White Rat.

A DISSERTATION

PRESENTED
TO THE

Facuutty oF Princeton UNIVERSITY
IN CANDIDACY FOR THE DEGREE

oF Doctor or PHILOSOPHY

By
H. H. LANE

NORMAN
UNIVERSITY OF OKLAHOMA
1917

Accepted by the Department of Biology,
June, 1915.

HARLOW
OKLAHOMA

tl i en

CONTENTS

Page
I ree ewe 5
ee InVesLieagions 2.2002 8 6
a MIeLNOGS. LL 9

A brief statement of the chief results obtained in this
i ee en ae ee 13
The nature and function of the neurofibrillae_________ 15
Experimental and structural data—introductory_-_-_-_-_~_ 21
Experimental data on the sense of touch_____________ 22
’ Structural observations on the tactile apparatus______ 28
Summary of results on the sense of touch___________-_ oe
Experimental data on the sense of equilibrium_______~_ 35
Structural observations on the organs of equilibrium__ 38
Summary of results on equilibrium__________________ 42
Experimental data on the sense of smell_____________ 44
Structural observations on the organ of smell_______~_ 48
ey Of results on smel]l_______________________ 51
Experimental data on the sense of taste______________ 53
Structural observations on the organs of taste________ 56
mnery oO. results on taste___.___.._______.______ 58
Experimental data on the sense of hearing___________ 59
Structural observations on the organ of hearing______ 60
mumeary of results on hearing.__________.________- 63
Experimental data on the sense of sight_____________-_ 64
Structural observations on the organ of sight_______~_ 64
env oresuits On sight_..___.___._.._______________ 69

The cause of development and of differentiation in the
SEE 2 70
Secn) connections in the rat._._.._..__._...._...___-_ 75
a 81
el A Se

IN 86

a

Oklahoma University Studies No. 8.

| The Correlation Between Structure and
Function in the Development of the
Special Senses of the White Rat.

H. H. Lane

Two periods in the ontogenetic development of
an organism were recognized more than thirty
years ago by Wilhelm Roux, the first that during
which the organs are formed, and the second that
of the development of function in the organs pre-
viously laid down. Within the organism as a whole
there is, of course, no sharp line of demarcation
between these two periods, for different organs,
or even systems, are formed and become functional
at different stages in ontogeny; or the two periods
may overlap in the development of the same organ,
as Harrison has shown to be the case with muscle
fibers. The order of appearance of the organs in
an embryo is also of significance, since it sometimes
happens that the presence and functional activity
of one is a necessary precursor to the formation of
another organ.

The immediate problem of this investigation
is the determination of the nature and amount of
the correlation between structure and function in
| the development of the special senses in the white
| rat during both prenatal and early postnatal life.
| The work has been done in the Biological Labora-

tory of Princeton University. I wish to extend my
sincerest thanks for the many courtesies and facili-
ties afforded me, as well as for many helpful sug-
| gestions and criticisms, to Professor E. G. Conklin,
| by whom the problem was suggested to me; to

6 H. H. LANE

Doctor Stewart Paton, a pioneer investigator in
neurobiology, whose rich experience and knowledge
of the field has been constantly at my service; and
to Professor C. F. W. McClure, whose deep
acquaintance with the facts of comparative anat-
omy has made it possible for him to offer many
valuable suggestions in the course of the investiga-
tion. Also I acknowledge my great indebtedness
to Professor L. W. Cole, of the University of Colo-
rado, for very material assistance by way of cita-
tions to the literature; and finally I am under many
obligations to the State University of Oklahoma
for a year’s leave of absence to pursue this investi-
gation.

The method of attack adopted may be stated
in general terms as follows:

To determine by physiological experimentation
just when the embryo or young rat first becomes
possessed of the senses of touch, taste, smell, equili-
brium, hearing and sight, and by a histological
examination of the nervous system, both central
and peripheral, and of the sense-organs, to discover
the structural development exhibited by the parts
concerned in each case at the time when the func-
tion is first apparent.

Previous Investigations.

No previous investigation along exactly paral-
lel lines is known to me, and only a very few work-
ers have concerned themselves with allied problems
and methods of attack; and they have for the most
part dealt with the lower vertebrates. An extended
review of their papers can therefore be dispensed
with here.

WINTREBERT (’04, ’05) has published a num-
ber of short papers recording the results of his
experiments and observations upon a few species

CORRELATION OF STRUCTURE AND FUNCTION 7

of batrachians, notably the frog and the axo-
lotl Working on very young embryos of Rana
esculenta, for instance, at the time when the
tail bud had just made its appearance and when
the myotomes of the anterior part only of the
trunk had become contractile, he made a transverse
incision just caudad to the contractile myotomes,
of such a depth as to transect the neural tube, the
notochord, and a considerable portion of the endo-
dermal tissue. Under these conditions he found
that, within a few minutes after the operation, a
simple pricking of the end of the tail with a needle
results in an immediate contraction of the trunk
‘anterior to the incision. The stimulus was trans-
mitted only through the uninjured ectoderm of the
ventral body wall. The power of reacting under
the conditions of the experiment was present for a
period of only four days in the ontogeny; after that
the power was lost. He concludes, therefore, that
there is a period of ‘“‘primitive sensitivity,’”’ charac-
terized physiologically by its independence of mus-
cular differentiation and of nervous connection
between the motor plates and the neural tube.
PATON (’07) undertook to determine the extent
to which the heart beat and “the earliest responses
to external stimulation .. . are
eee event upon the functional gutiviy of a nervous
system.” The forms studied ranged from amphi-
oxus to Lacerta, though Pristiurus and Scyllium
gave the clearest results. He found “that the func-
tional activities of the body represented by the beat
of the heart and the primitive movements of aband
adduction of the body begin at a time when
these phenomena may as yet neither be Naini
as myogenic nor neurogenic in origin”
“That general motility or reactions to wtiedult are
initiated within the different organs, such as the

8 H. H. LANE

myotome or heart, and are at first autochthonous
but later fall under the regulating influence of the
nervous system.” . . . . “The appearance of
neurofibrils may generally be considered to be an
indication that physiological activity has already
actually begun, or will soon begin in the tract in
which they have been differentiated.’’ é
“One of the chief histological characteristics of the
fully differentiated nerve is that it contains neuro-
fibrils, and every bit of evidence so far accumulated
points to the appearance of these structures as
marking the period of greatest physiological activity
in any given nerve.” . . . . “It seems to be
not at all improbable that impulses, centrifugal as
well as centripetal in origin, may play an important
part in the differentiation of the neurofibrils.”’
COGHILL (714) (and more recently HERRICK
and COGHILL (715) ) has made a study of the
reflex mechanism concerned in the production of
the first swimming movements in the larva of
Amblystoma. He finds that in the very young
larva the Rohon-Beard cells are both extro-and
proprio-ceptive elements of a very primitive, but
complex, reflex are through which an extero-cep-
tive stimulus passes cephalad on one side (the right,
for instance) of the cord to commissural neurones
near the posterior end of the medulla, and thence
to the ventral horn cells of the opposite (left) side,
from which in turn the motor impulse travels to
the myotomes. By the contraction of the latter a
proprio-ceptive stimulus is imparted to the Rohon-
Beard cells of the left side of the cord that trans-
mit the impulse by way of secondary neurones to
the same commissure and thence to the ventral
horn cells of the right side. These then produce a
contraction of the myotomes with which they are
connected. In this way, as the result of this unique

CORRELATION OF STRUCTURE AND FUNCTION 9

arrangement, alternate wave-like contractions of
the myotomes on the two sides of the body are
brought about resulting in swimming movements
on the part of the embryo as a whole.

SMALL (799) has studied experimentally the
psychic development of the white rat, during a
period extending from the first to the twenty-eighth
day after birth. Several references to this paper
will be found at various places in this paper.

WATSON (’03) likewise has studied the devel-
opment of the psychic faculties in the rat by means
of a series of “standard problems,” and finds that
psychic maturity is attained by the twenty-fourth
day after birth. Correlated studies were made
upon the establishment of medullation in both the
peripheral and central nervous systems of this ani-
mal, with the result of very conclusively disprov-
ing Flechsig’s hypothesis. A further account of
Watson’s results will be found in the appropriate
sections of this paper.

Some other papers germane to minor questions
raised by my own observations will be considered
in connection with the points to which they have
relation.

Material and Methods.

The material used in this investigation con-
sists of fifteen different stages in the development
of the white rat, ranging from embryos with a crown-
rump measurement of 714 mm. to young sixteen or
seventeen days after birth, at which time all the
special senses have attained functional activity. In
the case of the prenatal stages the mother was first
killed by severing the cervical cord by a quick cut
with a pair of large bone forceps. The use of
anesthetics was avoided for fear of possible delete-
rious effect upon the embryos or fetuses. The

10 H. H. LANE

abdomen of the mother was then immediately
opened by a median incision through the ventral
body-wall, extending from the public symphysis as
far cephalad as necessary. The uterus was removed
and placed at once into a dish containing the nec-
essary amount of a solution made up according to
the following formula:

Caleium. chloride: 2002302227 0.2 gram
Potassium chloride: 2 ohne 0.2 gram
poglum chioride. 2202 eae 9.0 gram
Sodium bicarbonate___...._____ 0.1 gram
Dextrose se ow ee ee 1.0 gram
Distilled’ Waters) 020 ike une 1000.0 cc.

The dish and its contents had previously been
warmed to 38° C., and together with the uterus was
transferred to a warm chamber where the embryos
were removed and subjected to experiment. The
temperature and humidity of the warm chamber
were very nearly constant at all times, an effort
being made to have conditions as favorable as pos-
siblé for the success of the experiment. It was
found that mammalian embryos are very suscept-
ible to the shock of sudden changes in tempera-
ture, frequently only a few moments’ exposure to
room temperature sufficing to kill them.

Careful records were made at the time of the
details of the procedure in each experiment and
of the results. For the most part the embryos were
preserved in a solution of neutral formol made up
as follows: The ordinary 40% formalin was neu-
tralized or made slightly alkaline by an excess of
magnesium carbonate. One part of this stock solu-
tion was added to nine parts of tapwater, making
a 4% solution of neutral formol. The embryos
remained in this solution for not less than four
days—in several cases much longer, even for sev-
eral months. They were then subjected to the fol-

CORRELATION OF STRUCTURE AND FUNCTION 11

lowing procedure which is a slightly modified
form of the Bielschowski-Paton (’07) method:

1. Upon removal from the neutral formol the
specimen is washed over night in running water,
then rinsed three or four times with distilled water
and put into a three-fourths of one per cent
(0.75%) solution of AgNO, in the dark. In this
it is left for a varying number of days, depending
upon the room temperature, until it acquires a light
brown color.

2. The specimen is again rinsed in distilled
water and put for two hours in the dark into a
‘solution made according to the following formula:

DS OES SS ee ee BU), Ce:
es ee 4 drops
Concentrated NH,OH_.~~-...--- 12 drops

The addition of the NaOH to the silver solu-
tion produces a dark brown precipitate which is
dissolved by the NH,OH. In this solution the
specimen becomes mahogany colored and more or
less translucent.

3. The specimen is again rinsed in distilled
water and placed for fifteen minutes in the follow-
ing solution to dissolve any connective tissue that
may be present:

penton |). Water 22 20 ce.
Glacial Acetic Acid__._._._.._._ 10 drops

In this it becomes yellowish brown in color.

4. Again the specimen is rinsed in distilled
water and put for twelve to twenty-four hours in
the dark into a solution composed of:

1% aqueous solution of hydrochinone_. 20 cc.
Beemeurren formol. 0 a 2 ce.

The time in this solution is determined by the
size of the specimen.

12 H. H. LANE

5. Once more the specimen is rinsed in dis-
distilled water, gradually dehydrated in _ alcohol,
cleared in benzol or chloroform (not xylol), im-
bedded in paraffin, and sectioned. Sections five to
seven micra in thickness are best for most purposes.
The sections are mounted on slides by the usual
Mayer’s albumen fixative method and after being
thoroughly dried are painted over with a 0.5%
solution of celloidin, to prevent loss of sections in
later processes.

6. After the removal of the paraffin the sec-
tions are passed down through the alcohols, rinsed
in distilled water, and placed in the dark for two
hours in a 0.1% solution of gold chloride neutral-
ized with lithium carbonate. After the gold has
been reduced and the sections have a dark grayish
blue color, they are quickly rinsed in distilled water
and then put for ten minutes in a 5% solution of
sodium hyposulphite.

7. The sections are now washed for two hours
or longer in running water, passed up through the
alcohols to absolute, where they are counterstained
in a 1% solution of eosin in absolute alcohol, cleared
in xylol, mounted in neutral balsam, and covered
in the usual way.

If the solutions are made up fresh as needed,
all glass-ware kept perfectly clean, and if in every
step of the process, except No. 3, care be taken to
have all the fluids used neutral or slightly alkaline
in reaction, uniformly good results may be expected
by this method.

One very satisfactory modification of this
method consists in the omission of the gold chloride
and subsequent treatment, i. e., steps No. 6 and 7.
The sections when mounted on the slide were al-
lowed to dry, the paraffin removed and the sections
cleared in xylol, and covered with neutral balsam

CORRELATION OF STRUCTURE AND FUNCTION 13

and cover-glass in the usual way. The result is
greater contrast between the nerve-fibers and the
other tissues than that seen after treatment with
gold chloride. In my own preparations, especially
in the case of very long series, it was the practice to
finish the odd-numbered slides by this method and
and the even-numbered slides with the gold chloride.
In addition to being in some respects more satisfac-
tory for study, the slides finished by the shorter
method require a much briefer time and less labor

for their preparation.

As a control method I have used the Ree oe:
Huber (713) pyridine process, with decalcification in
7% nitric acid. The published accounts of this tech-
nique are so recent and so readily accessible that a
description of it is unnecessary here. Its chief ad-
vantages are the beautiful contrast between the
nerve-fibers and the other tissues, and the fact that
decalcification is possible, so that whole heads of
young rats may be studied in serial sections. The
chief disadvantages encountered in its use are, first,
the long time required, and, second, the tendency of
the brain tissues to swell when washing in distilled
water after the pyridine. It has not been possible
so to modify the Bielschowski-Paton method as to
permit of decalcification.

A Brief Statement of the Chief Results Obtained in
This Investigation.

A few points stand out prominently as the
result of the investigation that is described in detail
in the succeeding sections of this paper.

1. The exceedingly early establishment of the
general structural relationships between the nervous
system and the other organs of the body. This
comes at a time when the spatial relations are such

14 : H. H. LANE

as to make easy the proper extension of the sensory
and motor nerves to their respective peripheral
areas of distribution.

2. Both the central and the neruhenn por-
tions of the nervous system are laid down, at least
in ground-plan, for a longer or shorter time before
their functional activity begins.

3. In the establishment of a sensory chain of
neurones from the periphery to the center, the
exteroceptive end-organ is the last link to be com-
pleted.

4. Immediately upon the completion of the
structural development of the peripheral end-organ
the definitive function of that Saad sensory
chain is established.

5. <A functional activity once established may
be further perfected through the gradual addition
of other neurones to those at first constituting the
receptive path and the association paths within the
brain.

6. The gradual perfecting of the co-ordinating
powers of the central nervous system which is a
later development is to be explained in the same
way.

7. The whole nervous mechanism up to the
point, at least, where the definitive functions first
appear, develops not from the effects of extrinsic
stimuli, but along predetermined lines as the result
of inherent forces probably to be thought of as the
product of the hereditary constitution of the fertilized
egg.

It is, of course, altogether probable that the
normal course of events is influenced by many fac-
tors which gradually and in succession enter into
the situation to complicate matters, such as the
establishment of the circulatory and lymphatic sys-
tems, the excretory system, and probably some of

CORRELATION OF STRUCTURE AND FUNCTION 15

the ductless glands. But these are all factors inher-
ent in the primary organization of the individual;
they are therefore intrinsic factors in ontogeny.
Extrinsic factors such as the stimuli of light, sound,
or those of a chemical, electrical, or mechanical
nature, etc., play little or no part in the establish-
ment of the various functional activities of the
nervous system, though they may later have an in-
fluence during the time when co-ordination is being
gradually perfected.

The Nature and Function of the Neurofibrillae.

In the following description of structural con-
ditions there will be noted the assumption that the
presence of neurofibrillae is an indication that the
nerve or tract concerned is capable of functioning.
It becomes necessary therefore at this point to dis-
cuss the grounds for this assumption.

Neurofibrillae may be demonstrated by means
of any one of several technical methods including
those of Apathy, Bethe, Cajal, Bielschowski, Don-
aggio, Paton, Ranson and Huber. They are constant
structures in that by proper means they can always
be found in the neurones of all parts of the adult
nervous system of the vertebrates and in many at
least of the invertebrates. Nevertheless Auerbach
has denied their existence and Pighieri considers
them to be mere artifacts, the inconstant products
of the precipitation of various substances by the
reagents used in fixation. On the other hand, and
with good reason, Apathy, Bethe, Cajal, and others,
hold them to be normal constituents of the neurone.
Hatai asserts that they constitute a reticulum lying
in all parts of the neurone, the cross-meshes not
ordinarily being seen, though he thinks himself able
to demonstrate them by means of a special tech-

16 H. H. LANE

nique. In short it is the old controversy in special
dress as to whether the structures visible in pro-
toplasm that has been subjected to various fixing
reagents are to be regarded as actually present as
such in the living state or are the more or less
altered and distorted products of such structures of
the cell. Were our knowledge of the actual nature
of living protoplasm more profound, possibly this
particular form of the question would be answered.
In the light of present-day results of cytological
research it would appear that some structures seen
under the microscope after the fixation of the tissue
exhibit more of the effects of the fixatives than they
do of the structure of the living protoplasm. It is
highly probable that some of the granules and reti-
cula so frequently seen and described are more or
less the precipitation products of different constitu-
ents of the living protoplasm; on the other hand it
can be demonstrated that mitochondria, spindle-
fibers, and chromosomes, at least, are actually pres-
ent as such in the living state. In some cases
however what appear to be granules in fixed mate-
rial exist in life as isolated portions of the living
colloidal gel having a different degree of viscosity
from the surrounding substance. Thus the neurofi-
brillae are either to be regarded as rows of such
colloidal particles held more or less closely together
in a linear arrangement by means of another con-
stituent of the protoplasm differing from them in its
degree of viscosity, or the fibrillae may consist en-
tirely of such a viscid substance which has the form
of strands differing chemically and physically from
the other elements of the surrounding protoplasm.

If these considerations hold true, then the view
of Koltzoff, upheld for the neurones by Gold-
schmidt, Sztits, and others, namely, that the form
of any cell is determined by the shape of a solid

CORRELATION OF STRUCTURE AND FUNCTION 17

framework within it, must be materially modified.
The so-called ‘“Stiitzgeriist’”’ of the cell is not an
unyielding structure like the bony skeleton of a
mammal or the steel-frame of a skyscraper, but
rather a meshwork of a substance a little more
viscid than some other portions of the protoplasm.
The “skeletal theory” of cell structure proposed by
Koltzoff seems to rely for its proof upon the analogy
of the action of liquid masses when in contact with
solid bodies. Thus Plateau has shown that lquid
masses conform in their shape to that of the solid
body with which they may be in contact.

The idea that in a neurone the neurofibrillae
constitute this “Stiitzgeriist” has been advanced by
Goldschmidt and others, but it has met with some
serious objections. Thus Marinesco (’15) in a recent
paper argues very soundly that (1) Koltzoff and
Goldschmidt have not shown conclusively that the
neuronal cytoplasm is a fluid, which it should be
on the basis of the analogy just mentioned. (2)
Brownian movements are not exhibited by the col-
loidal particles found in the cytoplasm of the neu-
rone until the viscosity of the hyaloplasm has been
reduced, as has been shown experimentally. (8) On
the basis of his own observations, Marinesco main-
tains that the hyaloplasm and the neurofibrils are
both more or less fluid gels which differ only in the
degree of their viscosity, the neurofibrils being the
more stable. (4) The tearing away of a spinal or
cranial nerve usually results in the total destruction
of the neurofibrillar structures; yet, on the other
hand tumefaction followed by an atrophy of the
neurone does not lead to such profound modifica-
tions of the cellular form as the theory of Koltzoff
and Goldschmidt would seem to demand. (5) The
destruction of the neurofibrils that follows shortly
after the death of an animal does not result in the

18 H. H. LANE

collapse of the cell-body of the neurone. (6) The
neurones of animals that have undergone hiberna-
tion or freezing exhibit marked changes in their
neurofibrillae, so that the latter can hardly be con-
sidered to possess such a permanent character as
the theory demands. In fact such observations have
served to establish the fact that the neurofibrils
undergo continual change. (7) After transsection of
nerve trunk, the neurofibrils peripheral to the sec-
tion undergo regressive modifications that end ulti-
mately in their complete destruction, yet the axis
cylinder as a whole does not crumble, collapse, or
otherwise fall into such a dissolution as it should
exhibit if the neurofibrils play only a mechanical
role in the support of the hyaloplasm. (8) In va-
rious pathological states more or less extensive
lesions occur in the neurofibrillar reticulum without
and corresponding modifications of the cellular
form.

It would appear therefore that the neurofibrils
cannot be regarded as the ‘‘Stiitzgeriist’’ of the
neurone. What then is their function?

This is a question much more easily raised than
answered in the present state of our knowledge.
Apathy, Bethe, Paton, and others, hold the view
that they function in the conduction of the nervous
impulse. It is at least questionable whether the
evidence which they advance is conclusive. Indeed,
it seems probable that too much reliance has been
placed upon the analogy so frequently made be-
tween the nervous system and a telegraph system
in which the nerves and the nerve fibers of the
former correspond to the cables of the latter, and
the neurofibrillae to the individual wires. The ex-
periments of Ducceschi and Bethe on the effect of
the compression of nerve-fibers do not appear suf-
ficiently conclusive.

CORRELATION OF STRUCTURE AND FUNCTION 19

A clue to the true conditions may be furnished
by the fact that the relative proportions in the
amount of the fibrillae and of the perifibrillar sub-
stance differ in medullated and in non-medullated
nerve-fibers. It is well established that medullated
fibers are better conductors than the non-medul-
lated. This has generally been ascribed to an in-
sulating power on the part of the myelin sheath
though recent work would appear to render it prob-
able that the latter may serve rather a_ trophic
function. The fact that medullation does not begin
anywhere within the central nervous system of the
white rat until several days after birth leads to the
suspicion that possibly the importance of medulla-
tion from the standpoint of insulation may have
been overestimated in the past. On the other hand,
the better conducting-power of the medullated fibers
may be ascribed to the fact that they are made up
of relatively a larger proportion of neurofibrils than
of perifibrillar substance, the latter in fact being
quite inconspicuous, while in the so-called non-
medullated fibers the reverse is the case. In the
latter type of fibers, the fibrillae form a small and
inconspicuous core with a very thick sheath of peri-
fibrillar substance surrounding them. It is evident
that since the neurofibrillae and the perifibrillar
substance are the only parts of the nerve-fiber that
have a continuous distribution throughout the whole
extent of the fiber, one or the other must be the
conducting element. Where there is a variation in
their relative amounts, therefore, the better power
of conduction will lie with that fiber which has the
greatest amount of the conducting element. Since
then the medullated fiber is the better conductor
and at the same time has a relatively greater quan-
tity of neurofibrillae than of perifibrillar material,

20 H. H. LANE

as compared with the reverse condition in the non-
medullated fiber, the conclusion is inevitable that
the neurofibrillae, and not the perifibrillar sub-
stance, constitute. the conducting element of the
neurone. Since the perifibrillar substance certainly
is just as much a continuous layer in the transverse
direction as in the longitudinal one, and since fur-
thermore the same is not true of the fibrillae, they
being continuous only in the longitudinal direction,
the fact, which has been demonstrated experimen-
tally, namely, that an electric current is not trans-
mitted across a nerve but only in a longitudinal
direction, adds to the probability of the correctness
of our conclusion. Furthermore it may be argued
that in the so-called non-medullated fibers it is the
perifibrillar substance that furnishes the needed in-
sulation for the fibrillae, while in the so-called med-
ullated fibers the lack of perifibrillar substance is
compensated by the addition of the myelin sheath.
To these considerations may be added the further
argument which follows from the results of Paton’s
work on Pristiwrus and others of the lower verte-
brates, namely, that the functional activity of any
part of the nervous system is never fully established
until after the completion of fibrillation.

If therefore an absolutely conclusive answer
cannot now be given to the question of the function
of the neurofibrillae, it is most probable, in the light
of our present knowledge in this field, that the power
of conducting nerve-impulses lies rather in the neu-
rofibrillae than elsewhere. At any rate, the pres-
ence of neurofibrillae may be taken as an indicator,
a criterion of the functional state of the neurone,
and as such it will be used in the description of
structural conditions given below.

CORRELATION OF STRUCTURE AND FUNCTION 21

Experimental and Structural Data.

While the experiments described below show
that there is a certain amount of overlapping of the
periods when the various senses make their appear-
ance, still in a general way it may be said that the
order in which they attain functional activity is as
follows:

Touch.
Equilibrium.
Smell.
Taste.
Hearing.
Sight.

It will be seen that certain minor inconsist-
encies appear in the record of the experiments on
successive days. This is due to the fact that when
hungry or after a period of rest the animals re-
spond to certain stimuli more readily than they do
after a full meal or when fatigued. In general,
however, it is clear that after the first indication of
functional activity on the part of any of the special
senses, succeeding days reveal a gradual perfecting
of the animal’s powers. This is undoubtedly to be
ascribed to a gradual increase in the perfection of
the association centers, as well as to the addition of
an increasing number of exteroceptive end-organs.
For example, the sense of touch is at first mani-
fested most clearly in the snout region at a time
when there are present the anlagen of only a dozen
or so vibrissae on either side. As development pro-
ceeds other vibrissae are added to those first present
and the ordinary body hairs acquire a sensory in-
nervation, as does also the integument generally.

D> OU ye 8 DO

22 H. H. LANE

EXPERIMENTS ON THE SENSE OF TOUCH.

The 714 mm. embryos gave no evidence of hav-
ing a sense of touch, although they were stimulated
with a fine sable brush, and gently pricked with
the point of a fine needle on various parts of the
body, the limb-buds, and the head. Electrical stimu-
lation with an induction-coil produced no apparent
reaction except a variation in the rate of the heart-
beat.

By the time the embryos had reached a length
of 16 mm. (crown-rump measurement) slight but
readily perceptible movements of the body were
noted upon pricking with a fine-pointed needle.
These were most marked when the stimulus was ap-
plied about the flanks and sides of the body and
the snout. Stimulation with a fine sable brush
failed to evoke any response. That the response to
the needle-prick on the snout was due to nerve-
innervation and not to direct stimulation of a motor
mechanism is shown conclusively (1) by the fact
that the movement called forth involved the turning
of the head as a whole, and (2) by the additional
fact that the sections show no sign as yet of the
histogenesis of muscle in the snout region. Further-
more the reaction was too promptly made to permit
of the stimulus being transmitted through the gen-
eral protoplasm.

In embryos of 23 to 28 mm., crown-rump
length, stimulation with the brush, as well as gentle
pricking with the needle-point, about the shoulder,
upper arm, hip, rump, and thigh, resulted in move-
ments of the limbs or body-wall as the case might
be. Stimulation of the vibrissal region of the snout

CORRELATION OF STRUCTURE AND FUNCTION 23

produced movements of the head decidedly more
vigorous than before recorded.

Fetuses 3.5 cm. in length from tip of snout to
base of tail were very active, squirming and kick-
ing about while yet within the uterus. Upon removal
they were found to be very sensitive to stimulation
with the brush, needle-prick, and induction-current,
fully as much so, apparently, as newly-born young.
They responded to stimulation on the flanks, sides
of body, front and hind-limbs, toes, tail, neck and
head, by more or less violent wriggings and twist-
ings of the body, movements of the limbs, spreading
of the toes, etc. One hour after removal from the
uterus, they responded with faint squeaks upon
stimulation with the needle-prick, undoubtedly show-
ing the presence of pain-sensation. Gentle stimula-
tion of the flank with a sable brush caused the
body to be bent laterad into the form of a C, that
is, the head and posterior end of the body were
turned toward the side stimulated. Upon a pro-
longation of the stimulus, the anterior and posterior
extremities of the body were jerked back to or
slightly beyond a straight line corresponding to the
longitudinal axis of the body. When the stimula-
tion was still further prolonged, writhing and jerk-
ing movements were made that persisted for a few
seconds after cessation of the stimulation. Milder
stimulation, as with a single hair, called forth little
or no response except when applied to the snout,
the region of the vibrissae being decidedly more
sensitive than other regions of the head or body.

When young rats (4.3 cm. long) only a few
hours old were gently stimulated by touching the
sides of the body with a sable brush, they responded
by contortions of the body, movements of the limbs,
both fore and hind, and gave vent to audible
squeaks. But here again it was found that the

24 H. H. LANE

region of the vibrissae was especially sensitive.
SMALL (799) notes for this stage (the earliest
heretofore examined) in the rats which he had
under observation that ‘“‘they give little response to
light pressure, as with a hair,—except upon the
nose, which seems to be very sensitive. Mass pres-
sure is not noticed unless comparatively strong.”

Rats thirty to thirty-six hours old when stimu-
lated with a small sable brush were found to be
very sensitive about the vibrissae, flanks, and mid-
dorsal line of the body. When stimulated on the
flanks, some (the larger and more vigorous indi-
viduals) responded quickly with an attempt to push
away the brush with the hind-foot of the same
side, the toes being spread well apart; others
(smaller in size but belonging to the same litter)
made a less vigorous response, more apparent when
the brush was applied to the ventral part of the
flank. Apparently voluntary (?) scratching move-
ments with the hind foot were noted at times in the
larger individuals. There were distinct reactions
to the needle-prick on the foot, shin, thigh, tail,
hand, fore-arm, upper arm, shoulder, sides, flanks,
top and sides of the head, cheeks, and region of the
vibrissae. Electrical stimulation with an induction-
coil applied to the back of the head, along the en-
tire length of the spinal column, of the trunk and
tail, the legs, sides of the body and belly, all re-
sulted in decided reactions, the movements amount-
ing to contortions in many cases. Stimulation of the
feet in the same way produced a spreading of the
toes toa slight but appreciable extent.

In the case of 55-hour-old rats, stimulation
with a sable brush resulted in attempts to remove
the irritating object by kicking and scratching move-
ments of the hind legs and feet. They were most
sensitive on the sides of the body and flanks, though

CORRELATION OF STRUCTURE AND FUNCTION 25

nearly as much so on the limbs. Stimulation of the
top of the head and back produced the same re-
sponses but only after the lapse of several seconds,
that is, after prolonged stimulation. Touching the
region of the vibrissae resulted in twitching move-
ments of the upper lips. Stimulation of the tail
resulted in its being tucked in underneath the body
and between the hind legs. Brushing the median
side of the hind foot resulted usually in violent con-
tortions of the hind quarters together with the
drawing up of the hind feet along the sides of the
body; frequently when the left hind foot was stimu-
lated with the brush, the right one was used to
scratch the right flank, or vice versa; a few seconds
later the stimulated foot was also drawn up in the
same way to scratch the flank on its side. The
median side of the foot seemed to be more sensitive
than the outer side.

Gentle pricking with the needle on the frontal,
occipital and parietal regions of the head resulted
in immediate response; the whole body was vio-
lently contorted and movements of the hind feet as
though to brush off the irritation were noted. Prick-
ing of the lower jaw or snout resulted in violent
attempts to wipe off the irritating object with the
fore paws, usually both paws being used simul-
taneously, and even overlapping each other, the
one on the side stimulated being underneath the
other. These movements were followed after repe-
tition of the stimulus by an opening of the mouth,
extension of the tongue, and movements as of swal-
lowing. When the hind feet were in such a position
that they could not be well used, the front feet
were employed to wipe off the irritation on the
back of the head. The tail, feet, legs, and fore-
quarters were very sensitive to needle-pricks; the

26 H. H. LANE

hind-quarters less so. Reactions to electric stimuli
were violent.

Small’s results on this stage were in part mark-
edly different from those recorded here. He says:
“Irritating fluid (HCl) produced instantaneous re-
sponses from all. In addition to the motor reactions,
there were vocal expressions and a striking acceler-
ation of respiration. Reactions to the other stimuli
were slow, varying from ten to fifty seconds.”

In the case of three-day-old rats the skin
seems not so sensitive as in earlier stages. Upon
stimulation with sable brush on hind-quarters, hind-
legs, flanks, sides of body, and back, no noticeable
response was called forth. Brushing the shoulders
and fore-limbs, sometimes, but not always, occa-
sioned a movement of the hind-limbs as though to
scratch or push off the irritating object. The same
sort of stimulation applied to the top and sides of
the head produced no apparent response. Stimula-
tion, with the brush, of the snout and the region of
the vibrissae, if prolonged, produced squeaks and
movements of the head as though to avoid the irri-
tating object, but no movements of the forelimbs
and paws.

Gentle pricking with a needle of the rump,
thighs, hind legs and tail produced no appreciable
response. Pricking of the hind feet resulted in a
violent attempt to tuck them under the body. Prick-
ing along the vertebral column from the lumbar
region cephalad, over the head, sides of the body
(but not the flanks), fore-limbs and paws resulted
in violent contortions of the whole body, rather
than in any specific response of the parts directly
stimulated. Response to electrical stimulation was
not so violent as in earlier stages.

With five-day-old rats practically identical re-
actions were obtained. Small’s results, again, are

CORRELATION OF STRUCTURE AND FUNCTION 27

not wholly in accord with those recorded here. He
notes for the 5th to 8th days inclusively that the
“Dermal sensitivity becomes more acute, though
susceptibility to pressure is still greater on the nose
than elsewhere on the body. Especially, greater
when tickling is involved. A bristle drawn across
the body elicits scarcely any response; but applied
with the same pressure to the nose, it evokes
squeaking and vigorous head-shaking”’ (on the 7th
day). “When the toes are touched the rats squeak
and jump so as to lift the body nearly off the floor.
One, thus insulted, crawled away two inches.”’

Rats nine days old were very sensitive to
touch all over the body, legs, and head, responding
not only by muscular movements but also by
squeaks.

When gently pinched on the cheeks and sides
of the heads in front of the ears they sought to
push away the offending object with their fore-
paws. When pinched gently on the top or back of
the head the hind foot of the same side was brought
forward to push away the forceps; the same reac-
tion was evoked when the sides and flanks were
gently pinched. Pinching of the toes sometimes
produced an instant response—squeaking and re-
traction of the legs—but sometimes the response
was very slow or even absent. The pinna of the ear
was very sensitive to touch. Pinching the tail, rump,
etc., resulted in squeaking and turning movements
of the whole animal—sometimes it whirled end-for-
end almost instantly.

At an age of twelve days there was not mani-
fested such sensitiveness to light pressure, e. g., of a
brush, as at previous times. A needle prick, except
about the base of the vibrissae, must be accom-
panied by considerable pressure to evoke a marked
response. Pricking about the region of the vibrissae

28 H. H. LANE

resulted in violent responses and vigorous rubbing
of the region on both sides of the head with both
fore-paws. Small notes that the ‘‘Dermal sensitiv-
ity [is] considerably heightened. One jumped vio-
lently when touched with the sharp corner of a
piece of paper. Flanks, sides, back and feet are
equally sensitive.”’

In rats sixteen days old the vibrissae were very
long and in constant use. When pinched about the
face with fine forceps, one grabbed them with its
jaws and bit them forcibly enough to make a dis-
tinctly audible gritting sound. Otherwise this stage
revealed practically the same conditions as are re-
corded for the preceding stage.

Structural Observations on the Tactile Apparatus.

In the 7144 mm. embryos a large number of as-
sociation fibers are already present in the cord, and
brain stem. The anlagen of the vibrissae are not
yet apparent. The innervation in the snout region
comprises two branches of the fifth nerve. Of these
the ramus ophthalmicus profundus trigemini has
the form of a small bundle of fibers, tapering off to
a single fiber at the distal end, deep within the
mesenchyme, connection with the more superficial
tissue not yet having been established. The ramus
maxillaris trigemini extends slightly anterior to the
optic cup, but ends before reaching the surface
ectoderm. Sensory fibers of the spinal nerves do
not extend to the ‘periphery.

In the 16 mm. embryos, about a dozen anlagen
of vibrissae are present on each side of the snout.
The vibrissae themselves do not extend to the sur-
face. The ramus maxillaris trigemini in the form
of a large trunk with many fibers runs to the snout
region where it breaks up into a “brush” by the

CORRELATION OF STRUCTURE AND FUNCTION 29

spreading apart of its branches until finally some of
the fibers end in a “basket” or reticulum in the
follicles around the bases of the vibrissae. The
ramus mandibularis trigemini has a similar distribu-
tion to the vibrissae on the lower jaw. Proximad
the maxillaris enters the Gasserian ganglion through
which none of its fibers can be individually traced,
although it is possible to find many of them in con-
nection with ganglion cells. In other words, the
distal fibers of The trigeminus, so far as can be
determined, are axones of neurones located in the
Gasserian ganglion. The ganglion itself is con-
nected with the anterolateral margin of the myel-
encephalon by a large trunk of fibers which run
dorsad along the anterolateral face of the hind-
brain for a considerable distance, then turn sharply
caudad to enter the medulla in which they join or
constitute a large ventro-lateral tract.

In the 23 mm. embryo the number of anlagen
of vibrissae has increased to more than thirty on each
side of the snout. Those present in the earlier stage
described above are much farther advanced in their
development than those whose appearance is more
recent. In the former the vibrissa itself is distinct
from the follicle and the usual structural character-
istics of both are shown. The relatively simple
“basket” of fibers noted in the follicles in the 16
mm. stage is represented now by a much more com-
plex felted layer of fibers buried between two lay-
ers of the follicular cells and forming a fibrous
lamina about equal in thickness to the follicular
layer between it and the root of the vibrissa.
Whereas in the 16 mm. stage only one or at most
a very few fibers were distributed to each vibrissa,
in the 23 mm. embryo each of the older vibrissae is
innervated by a large number of fibers forming a
well defined branch of the ramus mazxillaris tri-

30 H. H. LANE

gemini. Moreover, the general ectoderm covering
the snout has extending toward it a few branches of
the same nerve. These fibers do not exist in suf-
ficient numbers to constitute branches anywhere
near the size of those innervating the vibrissae, and
they follow in nearly every instance a course par-
allel to a small blood-vessel running toward the
surface between the rows of vibrissae. So far as
could be determined these cutaneous nerve fibers
have not yet come into contact with the ectodermal
layer covering the snout.

The motor branch of the trigeminus can be
distinguished from the sensory branch at this time;
it is completely fibrillated and can be traced proxi-
mad into the medulla in which it runs slightly pos-
teriorly in a ventro-dorsal direction to its nucleus.
The latter is well defined; apparently it has no
correlation tracts connecting it with any other part
of the brain. The proximal connections of the sen-
sory bundles of the trigeminus are the same as
noted in the 16 mm. embryo. They enter the
medulla just posterior to the motor branch and turn
sharply caudad in the ventro-lateral marginal velum
of the hind-brain. No correlation tract between the
trigeminus and the corpora quadrigemina can be
detected. All portions of the trigeminus seem to
be completely fibrillated. Correlated with the in-
creased size of the trigeminal trunks there is a
marked increase in the number of the neurones in
the Gasserian ganglion from which fibrillated proc-
esses can be seen extending either proximad, or
distad, or both. There is also noticeable a tendency
for the fibers which extend through the ganglion
for any considerable distance to be arranged in
definite tracts which lie nearly parallel with each
other and to the long axis of the ganglion itself.

CORRELATION OF STRUCTURE AND FUNCTION 31

The chief advance noted in 26 to 28 mm. em-
bryos in connection with the trigeminal system,
aside from a continuation of the lines of develop-
ment just described for the 23 mm. stage, consists
in the appearance of numerous fibers passing be-
tween the medulla and the corpora quadrigemina;
that is, co-ordination between these two parts of
the brain is certainly possible now, if not earlier.
These correlational fibers are to a certain extent
grouped into a great number of small bundles each
with only ten to twenty, or possibly in some cases
more, fibres. Consequently it is manifestly impos-
sible to identify by name at this stage the tracts
that will be present in this region in the adult.

In the 3.5 cm. fetuses the motor branch of the
trigeminus is much larger than in the preceding
stages, but runs in the same way to its nucleus of
origin in the medulla. This nucleus likewise is
larger in extent than before, though its cells are
not apparently more numerous. They are, however,
more widely separted from one another and in the
spaces between them numerous correlation fibers
from the lower levels of the medulla pass cephalad
to the midbrain, or vice versa. The sensory fibers of
the trigeminus are likewise more numerous than in
the preceding stages. Many of them upon entering
the medulla follow the same course as that already
described for them in younger embryos; others en-
ter more deeply into the substance of the medulla,
some in fact almost reaching the floor of the fourth
ventricle. Anteriorly a well defined portion of the
sensory root fibers of the trigeminus pass in a dorsal
direction through the antero-ventral portion of the
medulla to the posterior corpora quadrigemina, and
there are indications, though rather slight as yet,
of a portion of this tract passing on into the an-
terior corpora quadrigemina.

32 H. H. LANE

The snout region of the newly born rat not
only shows a greatly increased number of the vibris-
sae, but the anlagen of the ordinary body hairs are
almost innumerable and the dermis contains a rich
plexus of nerves which also extends through the
stratum germinativum into the stratum intermedium
of the epidermis. This is, of course, the usual type
of ending for the organs of touch and general sensi-
tivity, and those functions are therefore completely
provided for at birth in the white rat. The fibrillar
basket in the follicles of vibrissae can be seen to
have the form of elongated cylinders at the base of
which a bundle of nerve fibers leave the follicle at
some distance, however, distad to the base of the
latter structure, which extends deeply into the der-
mis. The distal ends of the various trigeminal
branches are composed of a greater number of
fibers than heretofore, and of course the same is
true also of the main trunks of this nerve. Of its
central connections nothing new can be said, and
aside from an increase in the number of fibers in
the tracts leading to the cerebellum and to the
corpora quadrigemina, a condition well marked by
the ninth day after birth, no further appreciable
advances are made in the structure of the tactile
system during the time under consideration in this
paper.

In general, meaning by that to include other
forms as well as the rat, the relations of the sensory
roots of the trigeminus with the cortex are at best
little understood. According to Edinger, “‘The cor-
tical area and the central path of the sensory por-
tion of the nervus trigeminus from the cortex to the
capsula are yet unknown. Following pathological
experiences, its fibers must lie in the posterior third
of the capsule. The cortical tract of the trigeminal
ends, in rabbits at least, in the ventral portion of

CORRELATION OF STRUCTURE AND FUNCTION 83

the thalamus. Leading up to it is a large bundle
from the opposite nucleus of the bulb. And in this
nucleus itself terminate the processes from the cells
of the Gasserian ganglion. The ascending root con-
tains the tactile nerves of the face as is shown by
pathology.”

Summary of Results on the Sense of Touch.

1. 744mm. embryos:

a.

b.

This stage, the youngest examined, gives no evi-
dence of the possession of the sense of touch.

A large number of correlation or coordination
fibers are already present in the cord and brain
stem. Both sensory and motor fibers of the spinal
nerves are present though the former do not reach
the periphery. The snout region is innervated by
two branches of the trigeminus nerve, which,
however, end within the mesenchyme i. e., do not
reach the periphery. No anlagen of vibrissae
are apparent.

2. 16mm. embryos:

a.

The tactile sense is present on the flanks and
snout as evidenced by motor responses to needle
pricks. There is no response to stimulation with
a sable brush.

About a dozen anlagen of vibrissae are found on
each side of the snout; these are innervated by
branches of the maxillary division of the tri-
geminus, which end in a basket-like reticulum in
the vibrissal follicle.

3. 23 to 28mm. embryos:

a.

b.

They respond to stimulation with a fine sable
brush as well as with a needle-prick; the snout
region is most sensitive, though stimulation
about the shoulder, upper arm, hip, rump, and
thigh, also evokes motor responses.

There is a noticeable increase in the number of
the vibrissae as well as greater complexity in the
neurofibral basket in each vibrissal follicle. The

34

H. H. LANE

number of neuro-fibers of the trigeminus inner-
vating the vibrissae is greatly increased. The
general integument of the snout region has not
yet received the terminations of other branches of
the trigeminus, though many such are extending
toward it, for the most part parallelling blood
vessels in their course. In the 23-26 mm. em-
bryos association paths exist between the medulla
and the mid-brain.

4. 3.5 cm. fetus and new-born rats:

a.

The tactile sense is still better developed over
practically the whole of the body, tail and limbs.
The snout is the most sensitive as shown by
response to stimulation with a single hair. Pain
or discomfort is now shown by squeaks.

There is an increased number of vibrissae on the
snout; the anlagen of ordinary body-hairs are very
numerous, and the integument contains a rich
plexus of nerve-fibers extending (in the snout)
through the stratum germinativum into the strat-
um intermedium. There is an increased number
of sensory fibers of the trigeminus ending in the
snout region. The central connections are better
marked and more extensive than in the preceding
stages. The fibrillar baskets in the vibrissal fol-
licles are now elongated, felted cylinders, from
the base of which the neurofibers in a relatively
large bundle emerge some distance distad to the
base of the follicle itself.

5. Older stages:

a.

Throughout the older stages examined there is in
general no particular advance in tactile sensi-
bility over that just described. There is a con-
tinued superiority of the snout region over the rest
of the surface in sensitiveness to tactile stimuli
and the use of the vibrissae as “feelers” is more
and more marked.

The structural advance in the tactile apparatus
during these later stages is confined to an increase
in the perfection of the mechanism already de-
scribed.

CORRELATION OF STRUCTURE AND FUNCTION 35

EXPERIMENTS ON THE SENSE OF
EQUILIBRIUM.

The earliest indications of a sense of equili-
brium were observed in the case of the 3.5 cm.
fetuses. One hour after their removal from the
mothers’ uterus they were able to sit upright on the
belly with the forepaws placed well apart and the
head up. At intervals the head was raised and
moved from side to side, then returned to a resting
position with the ‘‘chin” on the bottom of the dish
or on one fore-leg. They were able to regain this
position after stimulation with a brush applied to
the flank had caused them to bend the body laterad
into the form of a C, and upon prolongation of the
stimulus writhing and jerking movements of a some-
what violent character had followed. Turning them
over on their backs did not result in attempts to
right themselves, except very rarely, and then the
efforts were very feeble.

Young rats nine to ten hours after birth
crawled awkwardly about over one another, and
nosed about in an evident attempt to find the moth-
er’s nipples. Without artificial stimulation they
would roll over onto the belly, sides, or back at
will; turned the head from side to side; kept their
tails tucked beneath the body between the hind
Iegs. When turned over on their backs by the ex-
perimenter they made awkward righting move-
ments, which sometimes succeeded. They had much
better use of their forequarters than of the hind;
they could spread their front legs apart so as to
support the head in an upright position. At eleven
to twelve hours of age they were able to crawl over
the edge of a Petri dish and to wriggle their way

36 H. H. LANE

through an inch of cotton wool. At thirty hours of
age the tendency to lie on the belly rather than on
the side was more strongly marked than hereto-
fore. They were rather restless, crawling about
from place to place. Apparently voluntary scratch-
ing movements with the hind foot were noted. The
tail was held extended posteriorly, that is, was not
tucked forward beneath the body as in the case of
newly born rats. Frequent twitchings were noticed
over various parts of the body, especially on the
shoulders, hips, and flanks. In the case of 55-hour-
old rats it was noted that when at rest they took
various positions, but seemed to prefer to lie on the
belly with the head held either in a straight line
with the body-axis, or turned to one side, or even
with the snout tucked down between the fore-legs.
They were able to roll over voluntarily from one
side to the other. Their movements were but little
better co-drdinated than in the preceding stages.
The 78-hour-old rats, when put into a Petri dish in
the warm chamber lay flat on their bellies with the
head extended in the line of the long axis of the
body; the fore-limbs were spread more or less wide-
ly apart, and usually the paw and the fore-arm to
the elbow rested upon the supporting surface. The
hind-limbs were also spread well apart but not so
widely as the fore-limbs. Occasionally all four (4)
limbs were drawn under the body in such a way as
to hold it slightly elevated above the surface of the
dish. The tail generally extended straight back-
wards. They seemed rather “nervous,” frequently
changing their position, twitching various parts of
the sides and legs, moving the latter forward and
backward, turning the whole body so as to face
now in one direction and now in another, and the
tail was occasionally directed forward so as to lie
a'cngside the body. When rolled over on the back

CORRELATION OF STRUCTURE AND FUNCTION 37

or side, they usually remained comparatively quiet
for a few seconds, and then rolled back and re-
gained the usual position with the belly down.

The young rats on the fifth day after birth had
better co-6rdination of their movements than in pre-
vious stages, though still far from complete. For in-
stance, the left hind foot of one was much inflamed
and swollen and the young rat spent much of his
time at intervals in licking this foot. It was also
observed deliberately washing its face by licking its
perfectly normal front paw and then rubbing the
face with it. These actions were performed not
once but repeatedly.

When placed with the hind quarters hanging
down over the edge of a small box-lid, they made
only feeble and futile efforts to keep from falling
off. They would occasionally raise a hind foot as
though to catch hold of the upper edge of the box-
lid, but finally would usually fall off without any
further attempt, apparently, to save themselves.
Occasional ‘“‘stretching’’ movements were observed
while the hind quarters overhung the edge of the
box; they may have been attempts to regain a more
comfortable position on top of the box-lid. There
were also apparently voluntary attempts at scratch-
ing the flanks with the hind feet.

Nine-day-old rats crawled about with consid-
erable agility; occasionally raised their heads to
sniff. When at rest they lay on the belly with all
four limbs spread well apart. When placed at the
edge of the table top they moved along it with the
feet and vibrissae of one side tracing the edge.
When an attempt was made to push them head first
over the table edge they braced themselves with
their feet and pushed back with all their power.
They righted themselves immediately when placed
on their backs, although the movements were not

38 H. H. LANE

thoroughly co-drdinated, particularly those of the
hind limbs and quarters.

At twelve days of age the rats walked in the
manner of an adult, though the movements were
still lacking somewhat in co-6rdination; the latter
was, however, noticeably better developed than
heretofore. They hung back downwards from the
experimenter’s finger, holding on with fore-paws
and head, or with all four feet and head, for a few
seconds, and then finally managed to pull them-
selves over to the top side of the finger and thence
to the back of the hand. When placed on a slide
box they crawled around feeling the edge with their
vibrissae and the ventral surface of the jaws. Some-
times they would stick their heads far over the
edge of the box and then would turn around and
crawl to near the center, where they would remain
until stimulated to further movement. Small noticed
twelve-day-old rats crawling to the edge of a
table stopping, reaching over as far as possible
without falling, “throwing up the head and sniffing
in the very characteristic way of rats when orienting
themselves,” and then retreating.

At sixteen days of age their movements were
all well co-6rdinated. They crawled readily; their
equilibrium was well established. They moved eas-
ily from end to end along the experimenter’s finger
without showing any sign of falling or of losing
their balance.

Structural Observations on the Organs of
Equilibrium.

In the 7144 mm. embryos there is no trace of
semicircular canals. The auditory vesicle is large
and spherical, its wall epithelial in character. There
is a large endolymphatic duct running dorsad and
ending blindly in the mesenchyme of the dorsal part

=> te '.

CORRELATION OF STRUCTURE AND FUNCTION 39

of the head. The eighth nerve is a short trunk only
that cannot yet be separated into its vestibular and
cochlear portions, and none of its fibers could be
detected reaching the cells composing the auditory
vesicle.

In the 16 mm. embryos the ear has reached a
stage corresponding very nearly to that of the 20
mm. human embryo, as described by Streeter. The
three semicircular canals are well formed, and the
vestibular nerve sends a completely fibrillated
branch to the ampulla of the superior canal as well
as one to the ampulla of the lateral canal, these
two branches arising from a common trunk a short
distance from their terminations. Shortly before the
division just mentioned, the vestibular nerve gives
off the utricular branch. A longer fourth branch
runs to the ampulla of the posterior semicircular
canal. The sacculus is a bud-like projection from
the posterior side of the utriculus and from it the
cochlea arises by a slightly constricted neck, the
ductus reuniens. The cochlea extends in a generally
ventrad direction, making one complete turn at its
distal end. The innervation of the sacculus seems
to be by a branch of the cochlear and not the vesti-
bular nerve as Streeter maintains for Homo; at
least the vestibular branch to the posterior semi-
circular canal is not connected at this stage with
the sacculus. The common trunk of the vestibular
nerve emerges from the otic capsule and after
reaching the brain cavity enters a ganglion (Acces-
sory Ganglion?) from which it emerges before
passing into the myelencephalon, to end in its nu-
cleus close beneath the floor of the fourth ventricle.
Distally the fibers of the vestibular nerve can be
seen penetrating in among the cells that are be-
ginning to elongate to become the sensory cells of
the cristae acusticae.

40 H. H. LANE

In the 23 mm. embryo the cristae acusticae are
prominent ridges about as high as broad at the
base, and with the top of the ridge arched over in
a very regular curve. The differentiation of the
cells composing the epithelium covering this struc-
ture has not gone far enough to enable one to dis-
tinguish the sensory and supporting elements. How-
ever, it is clear that there is an outer layer of cells
rather regularly arranged everlying a basilar layer
in which the cells have no very definite arrange-
ment. A small amount of endolymph is present in
the ampullae. The core of the cristae is made up
of a mass of mesenchymatous cells among which a
few unmedullated fibers of the vestibular nerve
make their way to end among the cells of the epi-
thelial layers. The latter, moreover, do not yet
have the cilia or sensory processes found later. The
vestibular nerve has the same relations with the
medulla as those described for the 16 mm. stage. A
tract from the same general region of the medulla
in which the vestibular nerve ends runs dorsad into
the cerebellum, but any actual relationship between
the two could not be determined in these prepara-
tions.

In the rat at birth and during the first day the
semicircular canals are much larger than in the
preceding stages. The cristae acusticae are not only
larger but their cells are differentiated into a su-
perficial layer composed of stoutly columnar cells;
and a supporting layer of very slender columnar
cells, in many instances much longer than the sen-
sory cells. Each sensory cell is inclosed in a “stock-
ade’”’ of nerve fibers in such a way that a mechanical
pressure exerted at any point must result in the
stimulation of one or more nerve fibers. The pecu-
liar terminal process projecting into the endolymph
undoubtedly serves as a lever that magnifies the

CORRELATION OF STRUCTURE AND FUNCTION 41

sensitivity of the cell to movements of that fluid.
Centrally the root of the vestibular nerve can be
followed through the skull into the accessory nu-
cleus thence on into the medulla, in the manner
already described. Through the medulla its fibers
course dorso-mesad, finally ending in a _ nucleus
through which there also run correlational fibers
caudad in the medulla and cephalad into the cere-
bellum. Medullation has not occurred in any of these
tracts.

No further differentiations of any importance
have been detected in connection with this appara-
tus in later stages.

The functions of the cerebellum, like so many
‘other parts of the brain, are not thoroughly known;
nevertheless it is generally agreed that the cere-
bellum contains the center ‘‘for the maintenance of
the mechani¢al equilibrium of the body” (Sherring-
ton, p. 348). If this be true, it is evident that the
central connections for the main organ of equili-
brium, the system of semicircular canals, are estab-
lished at or shortly before birth. At this time also
the maculae and cristae acusticae have their char-
acteristic structural features developed to a fune-
tional extent. Were this the whole of the mechanism
concerned in maintaining equilibrium it would ap-
pear that the rat at birth could maintain its proper
orientation without difficulty, and this is indeed true
to a large extent. But the fact that the ability to
maintain equilibrium improves during the succeed-
ing two weeks or more of postnatal life indicates
that other factors are involved. One of these is
undoubtedly muscle tonus, which probably comes
as an effect of use. Moreover, it will be recalled
that it was only at the time when the eyes become
functional that the power of equilibration is per-
fected. This accords perfectly with the results of

42 H. H. LANE

investigations elsewhere on this sense. It is a well-
established fact that the sense of sight has a very
important relation to the maintenance of equili-
brium.

The vibrissae are also used as organs of orien-
tation in the rat and constitute another element in
the mechanism of equilibration.

Summary of Results on Equilibrium.

1. 744mm. embryos:

a. There were no experimental results indicating a
sense of equilibrum at this stage in development.

b. There are no traces as yet of semicircular canals.
2. 16mm. embryos:

a. There was no experimental evidence of a sense of
equilibrium in this stage.

b. The semicircular canals are well formed, and the
ampullae are innervated by fibrillated branches of
the vestibular nerve. The region of the cristae
acusticae is indicated merely by an elongation of
the endothelial cells.

3. 23 to 28 mm. embryos.

a. There was again no experimental evidence of a
sense of equilibrium in these stages.

b. The differentiation of the cells of the cristae acus-
ticae is proceeding, but the sensory and support-
ing elements are not yet distinguishable. There
are slight indications of a central connection with
the cerebellum.

4. 3.5 fetus:

a. The earliest observed indication of a sense of
equilibrium occured at this stage. One hour after
removal from the uterus the young were able to
maintain an upright position of head and body,
and to regain this position when disturbed. When

CORRELATION OF STRUCTURE AND FUNCTION 43

turned over on the dorsum infrequent and feeble
efforts were made to right themselves.

The structural features are practically identical
with those of young rats during the first day after
birth.

5. First day after birth:

a.

During the first day after birth the young rats
crawled awkwardly about; turned the head from
side to side; made awkward righting movements
when turned over on the dorsum, which sometimes
succeeded.

The semicircular canals are larger than in the
earlier stages described; the cristae acusticae
have the sensory and supporting cells clearly dif-
ferentiated; the former are inclosed each in a
“stockade” of nerve fibers in such a way as to
transmit easily any stimulus produced by a change
in the position of the animal. The central con-
nections of the vestibular nerve are well defined.

6. Later stages:

la.

Throughout ‘the later stages there was mani-
fested a gradual perfection in the sense of equili-
brium, accompanied by a gradually increasing
power of coordination of movements.

The later stages witness the addition, through
the establishment of various correlational tracts,
of other factors concerned in the perfecting of
the power of equilibration, notably (1) muscle
tonus, (2) the use of the vibrissae, and (3) sight.

44 H. H. LANE

HXPERIMENTAL OBSERVATIONS ON THE
SENSE OF SMELL.

The youngest stage tested for the sense of
smell was that of the 3.5 cm. fetuses. When re-
moved from the uterus and the fetal envelopes they
were placed in a dry dish in the warm chamber,
and respiration was soon set up; the mouth opened
widely and closed, as though yawning, or possibly
gasping for breath—one could hardly decide which,
possibly both. They wriggled and nosed one an-
other about as though in an effort to find the moth-
er’s nipples,— this occurred however only after the
lapse of two hours or more from the time of their
removal from the uterus. No perceptible reaction
to various odors was detected, though when the
brush dipped in an odoriferous substance acciden-
tally touched the snout there was evident discom-
fort manifested—probably a tactile response how-
ever.

Young rats during the first day after birth
seemed to perceive odors as evidenced by turning
the head and movements of the snout as though
snifing. It was rather difficult to be sure of the
results, however, since the responses occurred only
after the lapse of a considerable time—15 to 30
seconds—and may have been ‘“‘spontaneous” move-
ments, i. e., due to other unknown stimuli. Small’s
observations on this point are in the main corrobor-
ative of the results recorded here. He notes:
“Smell. 5 rats. All sensed violet, as indicated by
expressive movements. Reaction, slow—about 15
seconds. One only objected. All showed dislike to
cheese, if movement away could be so interpreted.
Instantaneous convulsive reaction to HCl.”

CORRELATION OF STRUCTURE AND FUNCTION 45

During the second day after birth no percepti-
ble advance in the sense of smell was noted. The
same turning movements of the head and twitching
of the snout were evident, though the reaction time
was still long—15 seconds or more—so that it was
impossible to be sure of the relation of cause to
effect. Small notes that in his rats there “‘seems
to be slight advance in sense of smell, for they
made no objection to the odor of cheese. Other
odors elicited same responses as first day.”’

On the third day after birth the olfactory sense
seemed a little better developed. The reaction time
was shorter, ten seconds or less in some cases. Dis-
tinct sniffing movements of the nostrils followed
the presentation of a piece of cheese. Small re-
cords for his animals at the corresponding stage
that “only one of the five showed aversion to violet,
and two to clove and asafoetida. Spirits of camphor
and pennyroyal brought expressions of disapproval
from all. Irritating fluid (HCl) produced instan-
taneous responses from all. In addition to the motor
reactions, there were vocal expressions and a strik-
ing acceleration of respiration. Reactions to the
other stimuli were slow, varying from ten to fifteen
seconds.”’

On the fourth day after birth the olfactory re-
sponses to various foreign odors (violet-water, cow’s
milk warmed until it steamed, xylol, tobacco smoke)
were more clearly defined than on the preceding
days, except that the reaction time was if anything,
longer, ranging from ten to twenty seconds, or even
more. Small’s record for this stage is a follows:

‘Smell. Reactions to violet, camphor, penny-
royal, and clove, show less aversion; those to asa-
foetida are quicker and show more dislike. In four
cases out of five there seemed to be a pleasurable
response to cheese-odor—in one case accompanied

46 H. H. LANE

by what sounded like a pleasant squeak. The fifth
one paid no attention. In case of camphor and
pennyroyal, it was easy to distinguish between the
act of sensing the odor and the affective response.
They sensed pennyroyal quickly—about 5 secs.—
sniffed with deep respiration—then slowly averted
the head.”’

Rats on the fifth day after birth when first
removed from the nest were disappointing in their
responses to odors. At times they seemed to dis-
criminate between those which might be considered
pleasant and unpleasant, and then again, showed
utter indifference to them. On the whole the
results were so contradictory that it was impossible
to feel sure on this point. The reaction time in all
cases was so long that one could not determine
whether the movements may not have been due to
other stimuli than those of odors. However, after
they had been kept away from the nest several
hours, and the mother had again been handled for
a few minutes, they exhibited a noticeable increase
in the degree of their activity upon being taken up
in the hand for replacement into the nest. They
“nosed”? around and nibbled at various places on
the palm and fingers as though seeking the moth-
er’s nipples. This may have been due to hunger
and a feeling of warmth in the hand. SMALL’S
record for the corresponding period in his observa-
tions is terse—‘“‘nothing new in regard to the spe-
cial senses.”’ However, on the 7th day he notes
that ‘“‘the tests for smell seem to show a growing
indifference to all but the positively painful stimuli
—irritating fluids, e. g., HCl.” For the 8th day
his statements appear rather contradictory; he
says: “Reactions to odors become more individual.
On the whole they tend to become indifferent.

CORRELATION OF STRUCTURE AND FUNCTION 47

Glacial acetic acid and carbolic acid gave negative
reactions.”

On the ninth day after birth the young rats,
would raise their heads to sniff when tobacco smoke
was blown over them. An extra strong puff from a
pipe was followed by reflex movements and a weak
regurgitation. They reached after a brush dipped in
xylol and held one-fourth inch in front of the nose
(they eyes were not yet open), until the head had
been extended nearly half an inch directly for-
ward; then they stopped and paid no further at-
tention to the odor, withdrawing the head to the
resting position. The snout at this time is well
developed, having very much the form of that in
the adult. SMALL records that during the 9th to
llth days, “the special senses show no new fea-
tures.”’

In twelve-day-old rats there was again a very
marked olfactory response, when a brush dipped
in xylol was held a short distance in front of the
nose. The head was raised, turned from side to
side, and the nostrils alternately dilated and con-
tracted as though sniffing the odor. Upon gradu-
ally removing the brush to a distance of six to
eight inches, the rats moved forward a few inches
(two to three) sniffing with the snout elevated as
they moved. A similar response was made to the
odor of alcohol. SMALL noticed twelve-day-old
rats “throwing up the head and sniffing in the very
characteristic way of rats when orienting them-
selves.” On the fourteenth and fifteenth days,
SMALL noted their ability to ‘sense odors at a much
greater distance than previously.”

By the sixteenth day young rats appear to
have the sense of smell as well developed as have
the adults. They move directly toward cheese and
miscellaneous food stuffs put into the cage for the

48 H. H. LANE

mother to eat. SMALL notes that his rats at this.
age “recoiled quickly from camphor. Moved quick- —
ly toward brown-bread, dog-biscuit, and honey held
at a distance of one inch. Appeared not to dislike
iodoform or wintergreen.”

In short, the sense of smell may be present, in
a rudimentary form, at most, at birth or within a
few hours thereafter. It is gradually perfected
during the course of the first two and a half weeks
of postnatal life as the rat’s relations to its environ-
ment become more complex. Probably the earliest
odor sensed under normal conditions is the body
odor of the mother. Since the nest is saturated
with that odor, and other surroundings have it to a
less degree, the very young rats may be more
strongly influenced to remain quietly in the nest
during the mother’s absence than would otherwise
be the case. At any rate such a hypothesis would »
account for the very early appearance of this par-
ticular sense, though of course temperature and
contact sensations probably also enter into the sit-
uation.

Structural Observations on the Organ of Smell.

In the 714mm. embryos the olfactory pits are
well developed and open widely to the exterior.
The future olfactory area is indicated by three
pockets in the dorsal portion of the pit. There is
no rhinencephalon nor an olfactory nerve.

In the 16 mm. embryos the olfactory vesicle is
large and the number of pockets in its dorsal area
has increased to eight. The olfactory epithelium
is much thickened in the olfactory area but other-
wise no indication of the distinctive histogenesis of
the olfactory cells is apparent. The olfactory
nerve is present and has the form of a short brush
of nerve-trunks converging to a small area of union

CORRELATION OF STRUCTURE AND FUNCTION 49

with the olfactory lobe. The latter is a short evag-
ination from the anterior end of the prosencepha-
lon and contains a large ventricular cavity, which
opens widely at its posterior end into the lateral
ventricle of the cerebral hemisphere. In the region
of the olfactory vesicle the olfactory nerve
branches are distributed to various parts of the
olfactory epithelium and to Jacobson’s organ. From
their earliest appearance the olfactory nerves differ
greatly from all other nerves. The fibrillation is
not so distinct and there is a large intermingling of
mesenchymal (?) cells which in later stages con-
stitute the sheath cells of the nerve branches; in
fact, from the 28 mm. stage onward the fibrils are
entirely inclosed by the sheath cells, giving the
olfactory nerve a characteristic appearance by
which it can be distinguished at a glance from all
other nerves in the preparation.

In the 23mm. embryo the fibrillation of the
olfactory nerve is most clearly seen; in the 26 mm.
embryo, the fibers are entirely inclosed by sheath
cells except at the distal end of the most anterior
branch. The sheath cells are elongated parallel
to the long axis of the nerve and have distinctly
elongated nuclei. The olfactory epithelium is still
several cell layers thick but many of the cells
whose distal ends constitute the surface of the
olfactory pockets are becoming distinctly columnar,
some at least extending fully half-way or more
through the entire thickness of the epithelium.
Their nuclei are elongated while those of the
shorter more deeply situated cells are rounded or
oval. In the 23 mm. embryo the cells of the rhinen-
cephalon resemble mesenchymatous tissue, having
large oval nuclei and numerous branching proto-
plasmic processes. The definite formation of
axones can be detected. In the 26mm. embryo, on

50 H. H. LANE

the other hand, the tractus lobi olfactorius is
plainly indicated as a distinct bundle of non-med-
ullated fibers, which runs from the anterior com-
missure, of which it forms a part, on either side
in a_ ventro-latero-anterior direction toward the
olfactory lobe, which, however, it does not reach.
Other olfactory tracts in the brain are not dis-
tinguishable at this time.

In the 3.5 cm. fetus the tractus lobi olfactorii
is a rather large bundle of fibers that begins in
the lobus olfactorius posterior, and runs in a dorso-
posterior direction for some distance and then turns
obliquely mesad, dividing into two smaller bundles,
which reunite after running almost parallel for a
short distance. The reunited bundle runs in a
postero-dorso-mesal direction until it merges with
the anterior commissure, of which it forms the
anterobasal portion. It then passes across to the
opposite side of the brain, where it leaves the com-
missure and passes in an antero-latero-ventral direc-
tion to the dorsal portion of the olfactory lobe of
that side. Its distal end in both cases is enlarged
and spread out into the form of a brush. Other
olfactory tracts are not distinguishable. The olfac-
tory nerve branches are distinctly associated with if
not covered by the sheath ceils already described.

In the rat during the first day after birth the
olfactory epithelium comprises sustentacular cells
many of which appear to be ciliated; a few typical
olfactory cells are shown by the silver method.
They are long and slender with relatively large
nuclei and have a process from the basal end
which enters the adjacent olfactory nerve branch.
The tractus lobi olfactorii is somewhat larger and
perhaps better defined than in earlier stages. It
extends well forward into the olfactory lobe but
not as yet into the bulbus.

CORRELATION OF STRUCTURE AND FUNCTION 51

By the third day after birth it has reached not
only the bulbus but apparently to the region of the
glomeruli. Its fibrils are more distinct, especially
at its distal end than they were previously, and the
whole tract is much larger.

Succeeding days simply bring about the fur-
ther development of these various parts along the
lines already described.

1 ay

a.

2. 3.d.

Summary of Results on Smell.

to 28 mm. embryos.

No practicable means for testing the sense of
smell in these earlier stages was devised.

During these stages the olfactory apparatus is
being gradually laid down, both as regards its
central and its peripheral portions. The histo-
logical differentiation of the olfactory epithelium
has not advanced sufficiently far to enable the
sensory cells proper to be identified.

fetus:

No absolutely certain response to olfactory stim-
uli was obtained in this stage.

Both the central and distal portions of the olfac-
tory apparatus show appreciable development over
the preceding stages, but sensory cells in the olfac-
tory epithelium are not apparently fully differen-
tiated.

3. First day after birth:

a.

Apparent responses to olfactory stimuli were
obtained in rats of this age, though the reaction
time was long.

The olfactory epithelium contains a few cells
which are apparently fully differentiated as sen-
sory cells. The central connections are better
developed than before.

52 H. H. LANE

4. Later stages:

a. There is, on the whole, a gradual perfecting of the
olfactory sense from day to day.

b. Pari passu, there is a gradual perfecting of the
olfactory apparatus.

CORRELATION OF STRUCTURE AND FUNCTION 53

EXPERIMENTAL OBSERVATIONS ON THE
SENSE OF TASTE.

The 3.5cm. fetuses were able to make very
feeble swallowing movements, but otherwise no
results were obtained with liquids placed in their
mouths.

In the case of young rats during the first day
after birth, various experiments were tried to test
their sense of taste. With a saturated solution of
cane sugar in tap water, presented on a camel’s
hair brush, the first response was an attempt to
push the brush away with the forepaws, probably
on account of a tickling sensation when the brush
touched the lips and snout. There was no audible
squeaking noted. After the brush had been inserted
into the mouth, they sucked away at it for several
seconds, and upon the attempt being made to
remove the brush, they held on with the jaws so
firmly that the head and fore-quarters could be
lifted from the dish in which they lay without them
loosening their hold.

With a saturated solution of sodium chloride
in tapwater, there were evident signs of discom-
fort displayed, and distinct attempts were made
with the fore-paws to push the brush away. These
movements were accompanied by quite audible
squeaking. After the brush had been forcibly
inserted into the mouth, sucking and swallowing
movements followed, with no further evidence of
discomfort.

With a solution of 1% acetic acid in tapwater,
the evidence of distaste or at least of annoyance
was even more marked. No sucking movements
could be perceived following the insertion of the

54 H. H. LANE

brush into the mouth; more persistent efforts were
made to keep the mouth closed and thus to keep
out the annoying object. The squeaking was
louder and longer than in the other tests, and the
movements of the fore-paws to push the brush
away from the mouth were made with greater
persistence and force. These experiments were all
tried on several different individuals with like
results in all cases.

SMALL’S observations on rats of this age were
as follows:

“Taste. Tested with sugar-solution, warm
milk, and strong salt solution. These were applied
to the lips with fine brush. In each case, the rats
squeaked and wiped at the offending stuff with fore-
paws. Movements rather incoordinated. The move-
ments are: brushing and pushing away with the
fore-paws; averting the head; movement of the
whole body. In case of the salt solution, the reac-
tions were more vigorous, accompanied by voiding
of urine.

“Clear water called out the same characteris-
tic reactions.

“From this similarity of response, I infer that
there is no differentiation of tastes, as pleasant and
unpleasant. They are all unpleasant.”

Rats of the second day showed no perceptible
advance in taste over the previous day.

On the third day taste seemed a little better
developed; warm milk and sugar solution were
received without protest and swallowing reactions
followed. When the brush was wet in Ringer’s
solution and applied to the lips the front legs were
used in efforts to brush away the irritation, and no
attempt to nurse could be detected. On the fourth
day the gustatory responses could not be more
clearly determined than on the preceding day.

CORRELATION OF STRUCTURE AND FUNCTION 55

On the fifth day, several liquid substances,—milk,
sugar solution, salt solution, dilute vinegar, and
even tap water—when presented to their lips on a
brush caused expressions of discomfort, such as
averting the head, wiping away the brush with the
fore-paws, squeakings, and, if the stimulation was
prolonged, wriggling away on the part of the
whole animal. SMALL notes for the corresponding
period: ‘‘Nothing new in regard to the special
senses.”’

The nine-day old rats displayed nothing new
in regard to the sense of taste; the same was true
at twelve days of age. The seventeen day old
rats pretended at least to eat various kinds of food
that had been placed in the cage for the mother.
Did not exhibit any particular choice as to what
they tried; the whole performance may have been
merely an imitation of their mother’s actions, for
their eyes were now open and functional—a condi-
tion not occurring in the previous stages. SMALL
records for this stage that ‘‘one ate honey when
a drop was put into his mouth. ‘Tried to gnaw
brown-bread when a crumb was put into his mouth.
After that when the brown-bread came _ within
smelling range he would go toward it. Chewed
a tiny piece, holding it in his paws in a well-bred
rat’s way. I gave a little piece to another one.
He took it in both paws and chewed it. The
others scented it and tried to help, but he quickly
drew away with his treasure. There seems to be
immediate association between smell and taste.
Though not conclusive, the evidence points that
way. Another one declined to eat sealing-wax
after smelling it, and spat it out when a piece was
put into his mouth.”

56 H. H. LANE

Observations on the Organs of Taste.

In the 7144 mm. embryos no trace of any part
of the glossopharyngeus nerve could be detected
running to the mandibular arch. The mandibular
branch of the trigeminus is a large bundle of fibres,
which ends as a well-defined brush in the mesen-
chyme of the mandibular arch. A branch of the
facial nerve runs into the base of the hyoid arch,
and is likewise fibrillated.

In the 16mm. embryos the lingual branch of
the glossopharyngeus nerve runs to the posterior
part of the tongue and its fibers are distributed
among the muscles of the superficial layer. The
mandibular branch of the trigeminus innervates, so
far as can be determined, the rest of the tongue.
No indications of taste-buds are present. Over the
surface of the anterior two-thirds of the tongue,
however, there are at least ten longitudinal rows. of
dome-shaped papillae, each consisting of a single lay-
er of cubical cells forming the dome, and in the nearly
spherical central cavity of the papilla there is a
small number of nearly spherical cells. In a few
instances nerve fibers can be detected entering the
open base of the papilla and ending in a glomer-
ulus or plexus around the central cells. A _ single
median circumvallate papilla is fairly well-defined
on the posterior portion of the tongue, and in this
there is a relatively large plexus of nerve-fibers
belonging to the glossopharyngeus. No taste-buds
can be detected in the epithelial covering of this
papilla. |

In the 23mm. embryo conditions are practi-
cally the same as those just described, allowing, of
course, for an increase in size in all the parts men-
tioned. In the circumvallate papilla, some of the
nerve fibers now extend toward, if indeed they do

CORRELATION OF STRUCTURE AND FUNCTION 57

not end among the epithelial cells of its surface.
No taste-buds could be demonstrated. Over the
anterior two-thirds of the upper surface of the
tongue the dome-shaped papillae are present and
show little if any advancement over the preceding
stage described, except an increase in size.

In the one-day-old rat these dome-shaped
papillae are much larger in size than before; their
outermost layer of cells is somewhat flattened and
covered with a thin cuticle; the inner cells are tak-
ing on a form and arrangement suggestive of a
taste-bud, but only one such structure is present in
each papilla, and that is situated in the center of
the distal surface of the papilla. Nerve fibers run
in among the central cells.

In the five-day old rat the dome-shaped papil-
lae are larger and the number of nerve fibers run-
ning to each is much greater than before. Other-
wise they appear very much the same.

In the nine-day old rat, however, quite a
marked advance can be seen. The papillae on the
whole are larger; their surface epithelium is much
thinner and arched into a dome and in its center
has appeared a small orifice surrounded by special
cells. Beneath this outer layer, the cells of the
stratum germinativum are elongated and arranged
in somewhat the same manner as the elements of a
taste-bud. The innervation is by fibrils of the trig-
eminus. At this time also the circumvallate pap-
illa has numerous taste-buds of the usual type
lying within its epithelial layer. Taste-buds
also occur in the walls of the outer margin of the
groove surrounding the circumvallate papilla.

58

H. H. LANE

Summary of Results on Taste.

Before Birth:

NS

The 3.5 cm. fetus were able to swallow, but neither
in them nor in any preceding stages were there
obtained any evidence of a sense of taste.

At no time previous to birth could taste-buds or
other fully differentiated organs of taste be dem-
onstrated.

First day after birth:

a.

The results of tests for a sense of taste at this
stage were very uncertain; apparently anything
applied to the mouth produces a sense of discom-
fort. Sugar-solution, however was received with
much less objection than salt or acid solutions,
and may possibly have been perceived as having
an agreeable taste.

True taste-buds are not demonstrable in the pre-
parations in hand; the dome-shaped papillae
(fungiform) over the anterior part of the tongue
are developing an organ of sense faintly sugges-
tive in its general form and arrangement of a
taste-bud, though decidedly not a typical one.

Older stages:

a.

Though it was exceedingly difficult to distinguish
between annoyance or discomfort and a sense of
taste, it was apparent, especially in the later
stages, that this sense was present and gradually
being perfected.

There is likewise a gradual increase in the histo-
logical differentiation of the organs of taste until
by the ninth day, at least, taste-buds are distinctly
formed on the sides of the circumvallate papilla,
and a decidedly different organ in the dome-shaped
(fungiform) papillae.

CORRELATION OF STRUCTURE AND FUNCTION 59

HXPERIMENTAL DATA ON THE SENSE OF
HEARING.

Absolutely no response to sound was noted
before the twelfth day after birth. At that time
a sharp clapping of the hands occasionally seemed
to produce a response, i. e., the raising of the
pinnae and turning of the head so as to face the
direction of the sound. At other trials there was
no apparent response. The same results were
obtained by the ringing of a small hand bell. The
shrill sound made by drawing in the breath sharply
between the nearly closed lips several times was
followed by a “‘nervous start,’ quite as characteris-
tic but not quite so pronounced as that made by
much older rats.

On the sixteenth day hearing is well estab-
lished. Previous to this time the external auditory
meatus is more or less closed by a cellular plug
which would effectually obstruct the passage of all
sound waves except in the case of very loud or
very shrill noises. Attempts to remove this obstruc-
tion always resulted in so much hemorrhage and
pain, or in so much damage to the ear, that no suc-
cess was attained in attempts to secure an unob-
structed passage for sound waves previous to the
time when the meatus opens by natural means, i. e.,
the degeneration of the cells composing the plug.

SMALL’S observations and those of WATSON also
are in complete agreement with those recorded here.

SMALL’s record is as follows:

“Hearing. The bursting of a bag three feet
away caused them to jump quite out of the nest.
Later, clapping hands sharply at a distance of 10
feet caused the quick recoil peculiar to rats. Did

60 H. H. LANE

not run. A sharp “sh” at 3 ft. brought their heads
up. Word “rats” in a low tone at 1 ft. caused a
slight jump. Rustling of paper produced the same
result. Whistling brought up the head as if listen-
ing. Even at the very dawn of ear-consciousness
there seem to be differences of emotional reaction
to different elements in the ‘big buzzing confusion’
around them. Every concussion elicits a startled
movement; the gentle, prolonged note, e g.,
whistle, on the contrary, produces a reaction indi-
cative of unscared attention.”’

Observations on the Organ of Hearing.

In the 744mm. embryos the auditory vesicle
is large and spherical with an epithelial wall and a
well-defined endolymphatic duct running dorsad
and ending blindly in the mesenchyme of the dor-
sal part of the head. The acustico-facialis gang-
lion is a large and definitely delimited mass of
cells, with numerous fibres connecting it with the
myelencephalon but no fibres have as yet reached the
cells composing the wall of the auditory vesicle.

In the 16 mm. embryo the ear has developed to
the stage very nearly corresponding to Streeter’s
20mm. human embryo. The endolymphatic duct is
long, slender, and ends distally in an enlarged sac-
cus endolymphaticus. The utriculus and sacculus
are distinctly formed, the latter being a bud-like
projection from the posterior side of the former.
From the sacculus, the cochlea arises by a slightly
constricted neck, the ductus reuniens, and extends
generally ventrad, making one complete coil at its
distal end. The spiral ganglion extends along the
median one-half or two-thirds of the cochlea, or at
least it cannot be detected for some considerable dis-
tance from either the proximal or distal ends of the

CORRELATION OF STRUCTURE AND FUNCTION 61

cochlea. It is made up of ovate bipolar cells. Those
in the distal portion of the ganglion have as yet no
fibrillated processes; those more proximal in posi-
tion have fibrillated processes at both poles, but
the distal ends do not enter the epithelium of the
cochlear canal, though in places they come into
contact with the cochlear epithelium. No trace of
an organ of Corti can be seen.

In the 23 mm. embryo, the organ of Corti is
indicated merely by two low ridges in the endo-
thelial lining of the cochlear canal. One of these
is broader than the other and represents the begin-
ing of the membrana tectoria and adjacent parts;
the other more narrow will become the sensory
portion of the organ of Corti. Except that the
cells of this region are columnar and longer than
those elsewhere lining the canal, no histological
differentiation can be perceived. These ridges are
present only in the proximal portion of the coch-
lear coil, not yet having appeared toward the dis-
tal end, except as indicated by a general thickening
of the endothelium on one side of the cochlear
canal.

In the one-day old rat the external auditory
meatus is indicated in the sections as a long flat-
tened and folded tube whose lumen is entirely
obliterated by a plug of cells similar in all respects
to those which compose the raphe palpebrarum
or area of separation between the eyelids before
the latter are open. The organ of Corti shows
an advance in size over the preceding stage
described; the membrana tectoria is rather well
developed. The ridge that will give rise to the
sensory portions of the organ of Corti is well
marked but differentiation into hair-cells and rods
has not yet occurred. The organ is not equally
well developed throughout its whole extent, the

62 H. H. LANE

median portion showing greater differentation than
either end. The limbus spiralis has made its
appearance. The fibers of the cochlear nerve have
not yet established any visible relation with the
sensory portion of the organ of Corti.

In the five-day-old rat the external auditory
meatus is not only larger but its walls are still more
complicated by the development of folds or ridges,
so that a cross-section of it may be \Y-shaped, Y-
shaped, oo-shaped, or i shaped. Along its cen-
tral extent a lumen is beginning to appear through
the disintegration of the cells composing the raphe;
at either end it is still plugged with a solid mass
of ectodermal cells. Internally the scala vestibuli,
scala tympani, and cochlear duct are all present;
the organ of Corti is larger and differentiation of
the hair-cells and rod-cells is beginning though
they are not distinct as yet.

In the nine-day old rat the limbus spiralis is
well defined; the scala vestibuli and scala tympani
are much larger than in the preceding stages.
There is no vas prominens; the ligamentum spirale
is well developed; the lamina spiralis membrana-
cea is complete; the lamina spiralis ossea is not yet
even chondrified. Hensen’s cells are very large,
shortly columnar, and 4 or 5 in number in each
section; a row of cubical cells (the cells of Clau-
dius) are distinguishable from the adjacent endothe-
lium. The sulcus spiralis has not yet formed. The
cells of the organ of Corti proper have not attained
their definitive differentiation; however, neurofi-
brils from the ganglion spirale reach their bases.
The lumen of the external auditory meatus is still
obliterated both proximally and distally by the
plugs of ectodermal cells.

In the thirteen-day old rat the ear shows a
general advance in all its parts. The vas promin-
ens has developed; the lamina spiralis ossea is

CORRELATION OF STRUCTURE AND FUNCTION 63

chondrified. The organ of Corti has differentiated
fully for at least the greater part of its extent.
The tunnel of Corti is large and bounded by the
inner and outer supporting rods. The inner and
outer hair cells can be seen with the fibrils from the
ganglion spirale ending about them. The lumen
of the external auditory meatus is more or less
open throughout its whole length, though the detri-
tus of the old cellular plug still remains. The
latter is however no longer cellular, but rather
seems to have undergone liquefaction, and at no
point apparently completely fills the meatus. The
structural conditions at this stage would indicate
the possibility of the perception of some sounds at
least.

In the sixteen-day old rat, aside from growth
in size, the conditions are _ practically those
described for the thirteen-day old rat. The detri-
tus in the meatus has mostly disappeared. The
organ of Corti has differentiated for most or all
of its extent. So far as the structural conditions
are concerned, the apparatus would appear to be
able to respond to any sound stimulus.

Summary of Results on Hearing.

1. Absolutely no response to sound was noted before
the twelfth day after birth. From that date until
the sixteenth or seventeenth day there is a gradual
increase in the ability to perceive sound.

2. Previous to the twelfth day the portions of the ear
concerned with the perception of sound have been
undergoing a gradual development but had not yet
reached that degree of differentiation of the organ
of Corti necessary for the perception of sound. By
the twelfth or thirteenth day, the organ of Corti
is apparently differentiated for at least part of its
extent, though the lumen of the external auditory
meatus is not fully opened. The next few days
witness the completion of the differentiation of the
apparatus of hearing.

64 H. H. LANE

HXPERIMENTAL DATA ON THE SENSE OF
SIGHT.

Absolutely no response to light was obtained
before the opening of the eyes on the sixteenth or
seventeenth day. At practically all the preceding
stages the eye had been tested with an electric
flash-light without appreciable result.

On the sixteenth day one only out of a litter
of six had its eyes open and functional; it re-
sponded quickly to movements made before its
face and turned its head from side to side to follow
moving objects with its eyes. The others did not
get their eyes open until the seventeenth day, at
which time sight was fully established in them also.

SMALL says of his experiments on this stage:

‘Sight. When brought into a strong light they
did not wink or show uneasiness, though they soon
closed their eyes, and seemed to become drowsy.
A stroke of the hand one inch in front of the
face caused winking and a slight recoil of the
head.”

Observations on the Organ of Sight.

In the 714 mm. embryo the eye is in the
stage of the optic cup; the lens is a hollow vesicle
lying deeply within the mouth of the cup. There
is no apparent differentiation in the retinal layer.
The area between the lens and the ectoderm is filled
with loose, spongy mesenchyme. The choroid fissure
is not closed. No fibrillation can be detected in the
optic stalk, i. e., there is no optic nerve as yet devel-
oped.

In the 16mm. embryo the lens is a solid oval
body with the diameter in the future pupillary

CORRELATION OF STRUCTURE AND FUNCTION 65

axis somewhat the longest. Its anterior surface
consists of a relatively thin layer of columnar cells.
The margin of the optic cup shows a decided but
rather gradual thinning out of the retinal layers,
but no other indication of the ciliary body is appar-
ent. The surface of the retina toward the cavity
of the cup is supplied by a rich plexus of small
blood-vessels and the fibrillar layer of the retina
has begun to differentiate. The remainder of the
retina shows no further differentiation into distinct
layers. Its nuclei are relatively large and oval
in outline; they are apparently more numerous
than in the preceding stage.

The surface of the lens is covered with a
very rich plexus of small blood-vessels. The cham-
ber of the vitreous humor is relatively small and
shallow, and contains a small amount of substance
that in the preparations has the appearance of a
sparse network of fibers, somewhat like a _ very
loose mesenchyme minus the nuclei. There is no
anterior chamber. The area between the lens and
the ectoderm is filled with a rather densely packed
mesenchyme, in which on the side next to the lens
numerous blood-vessels can be seen. The develop-
ment of the eyelids and socket is indicated by the
presence of a groove on the surface of the head
surrounding the optic area and dipping in to a
distance of one-third or more around the optic
cup. The arteria centralis retinae is a relatively
large and distinct vessel forming a central core in
the optic stalk where the latter joins the optic cup.
Less than 180 » outside the cup the artery emerges
from the stalk through the remnant of the choroid
fissure and thence posteriorly the two are inde-
pendent of each other. The cavity of the optic
stalk is still present and while throughout about
one-half of its length its walls are in contact with

66 H. H. LANE

each other, still it can be easily traced into the
ventricle of the brain. The optic nerve is develop-
ing in the form of numerous fibers in the ventral
portion of the optic stalk. Cross sections show it
to be large toward its retinal end, indicating that
processes from the retinal cells are growing toward
the brain.

In the 23 mm. embryo, the lens is more nearly
spherical, if anything the diameter in the pupillary
axis is the shorter. The anterior surface consists
of a layer of columnar cells. The posterior surface of
the lens is covered by a rich plexus of small blood-
vessels. The retinal wall is thicker than in the
preceding stages. The fibrillar layer is better
developed. Many of the cells of the retina proper
are columnar and some extend nearly or entirely
through that structure. The third of the retina
nearest to the vitreous humor is made up of cells
oval in form and not densely packed together; the
other two-thirds is composed of cells more colum-
nar in form and more closely packed together. The
cavity of the vitreous humor is a little larger than
in preceding stages and has, in the preparations,
the same sort of a reticular content through which
passes a cone-shaped plexus of small blood-vessels
from the optic nerve to the lens. The ciliary body
is indicated by a very decided thinning out of the
wall of the optic cup, and consists of columnar
cells arranged in one plane, i. e., none of the folds
so characteristic of the ciliary body have yet begun
to form. The region of the future cornea and
anterior chamber is occupied by a rather densely
packed body of fibrous mesenchyme. Between this
and the eyelids, which now cover the eye com-
pletely, there is a distinct cavity. The eyelids have
fused into a continuous layer but the line of their
future separation is indicated by a raphe bounded

CORRELATION OF STRUCTURE AND FUNCTION 67

on either side by a row of columnar ectodermal
cells, the stratum germinativum of the margins of
the lids. The optic stalk is wholly replaced by
the optic nerve, that is fibers from the retina extend
entirely to the brain. The arteria centralis retinae
is relatively smaller than in preceding stages and
enters the optic nerve almost at the exact point
where the latter enters the eyeball.

In the rat during the first day after birth the
shape of the lens is about the same as in the pre-
ceding stage described. The epithelial layer over
its anterior surface now extends about two-thirds
of the way around it and consists, at least in its
most anterior area, of cells which are cubical or
even slightly flattened instead of columnar. The
surface of the lens is still richly supplied with a
plexus of small blood vessels. The vitreous humor
is greater in amount and in the preparations
appears more granular or homogeneous rather
than reticular, and through it there runs the coni-
cal plexus of bloodvessels from the retina to the
lens, already mentioned. The ciliary body has two
distinct folds with slight traces of others. The
retina exhibits indications of six or seven different
layers though its histological differentiation is oth-
erwise hardly more than begun. There is no ante-
rior chamber; the region of the cornea is thick
and composed of a densely packed fibrous mesen-
chyme. The eyelids are much better developed
than heretofore though still united by a raphe of
ectodermal cells. Anlagen of eyelashes are present
in considerable numbers. Glands are making their
appearance along the inner margins of the lids in
the form of tubular ingrowths of epidermis, while
in the mesenchyme, sphincter and other muscles
are in process of differentiation. Within the brain
the optic tracts are a distinct bundle of fibers that

68 H. H. LANE

pass up through the optic thalami into the anterior
corpora quadrigemina. Many other tracts are pres-
ent also, but in the absence of medullation it is
extremely difficult to trace them with certainty;
however, it seems probable, to say the least, that
by birth or shortly thereafter all the tracts within
the brain connected with the primary optic centers,
i. e., those in the corpus geniculatum laterale, the
superficial portion of the anterior corpora quadri-
gemina, and the pulvinar, are laid down. The
relations of these centers to cortex of the occipital
lobes could not be made out.

In the five-day-old rat the lens has practically
the same structure as at birth. The plexus of
bloodvessels on its posterior surface is still well
marked. The cavity of the vitreous humor is some-
what larger and its contents somewhat greater and
denser than in earlier stages. Differentiation of
the retina is proceeding but rods and cones are not
yet distinguishable. The ciliary body exhibits an
increased number of folds. The anterior chamber
is present and the iris is beginning to be formed.
The development of the lids shows a decided
advance in every respect, but the raphe is still a
thick layer of cells.

In the nine-day old rat the optic nerve and
the lens are practically in their definitive condi-
tion, though the latter still is supplied with its
plexus of blood vessels over its entire posterior sur-
face. The ciliary body has at least seven folds;
the anterior chamber and the iris are better devel-
oped than before. The cavity of the vitreous
humor is much more extensive. All layers of the
retina are distinguishable, the rods and cones ap-
parently being in process of formation.

In the twelve-day-old rat the most noticeable
advances are to be found in the ciliary body and

CORRELATION OF STRUCTURE AND FUNCTION 69

the retina. The former is distinctly marked off
from the latter, in which the rods and cones are
fairly well defined. The lens still possesses its
plexus of blood vessels as heretofore described. The
cornea and the secondary structures in that region
are all in process of advanced development. The
lids are however still closed and no light can reach
the eye.

In the sixteen-day old rat (frequently not until
the seventeenth day), the cells of the raphe palpe-
brarum have degenerated and the lids are separ-
ate. The rods and cones are much better differenti-
ated than heretofore, and are undoubtedly functional ;
ali the other retinal elements are also apparently fully
formed. The plexus of bloodvessels to the lens
is still present, though much reduced.

Summary of Results on Sight.

1. Absolutely no response to light was obtained before
the opening of the eyes on the sixteenth or seven-
teenth day.

2. Before the twelfth day after birth the eye is under-
going the usual course of development. At this time
(twelfth day) the rods and cones are fairly well
defined, but the accessory structures are less fully
developed and the closed lids prevent the entrance of
any but possibly the very brightest light. By the
sixteenth or seventeenth day, the lids open and the
function of sight is fully established.

70 H. H. LANE

THE CAUSES OF DEVELOPMENT AND OF
DIFFERENTIATION IN THE NERVOUS
SYSTEM.

One of the most striking facts in connection
with this investigation is one which must have been
forced upon the attention of all who have studied
the interrelations of organs in the development of
the embryo of any vertebrate species, namely, the
early appearance of the peripheral portion of the
nervous system. At a time when it is inconceiv-
able that distinctly nervous functions can be possi-
ble or at least of any importance to the embryo,
the chief nerve trunks are all laid down, together
with most or all of their important branches. For
example, the vestibular and cochlear nerves are
well developed in the 23mm. rat fetus, not to
speak of still earlier stages, while it is absolutely
impossible that the function of hearing can have
been established. Indeed, if our experiments can
be relied upon, the very first indication of an abil-
ity to detect sound comes not earlier than the
twelfth day after birth. Yet here in the fetus which
has passed through only about two-thirds of its pre-
natal life, the nerve of hearing is apparently fully
formed, at least fibrillation is complete, and this
as we have seen is most certainly to be regarded
as an indication of the establishment of the power
of functional activity.

The anlagen of the vibrissae in the 16mm.
embryo have not more than reached the surface
of the epidermis of the snout and it can hardly be
supposed that the fetus has need of a delicate sense
of touch to maintain itself within the amniotic sac.

CORRELATION OF STRUCTURE AND FUNCTION 71

And yet, the maxillaris and mandibularis branches
of the trigeminus are completely fibrillated and end
in very large and complex basket-like networks
in the follicles of the vibrissae. It is inconceivable
that in these and other cases that might be cited
the nerves and end-organs develop in response to
functional activities or even functional needs on the
part of the fetus at this or any preceding stage in
its existence. The condition as stated exists, how-
ever, and demands an explanation.

The earliest stages in the development of the
peripheral nerves have been studied experimentally
by HARRISON (710) and others, and the results
obtained, especially from the cultivation of tissues
im vitro, shed a flood of light upon the question
raised above. HARRISON finds that all _ tissues
exhibit a specificity in their tendency to undergo
each its own peculiar type of histogenesis, as the
result of which certain cells in vitro become muscle
cells, others epithelial, others connective tissues,
others nervous, etc. This tendency is inherent in
the cells concerned and reveals itself irrespective
of the nature of the external conditions, so long as
the latter are not detrimental to the well-being of
the cells themselves. In short, a neuroblast is
potentially a nerve-cell long before it is definitively
such, owing to an internal organization that has
been handed down to it through all the cell-genera-
tions that have intervened between the neuroblast
stage and the odsperm. Indeed, it is not altogether
a mere inference, as the work of Whitman, Wilson,
Conklin, Lillie, and others, has shown that its pos-
sibilities were predetermined or prelocalized in the
egg at or before fertilization. In short the devel-
opment of the nervous system in general, and the
differentiation of its constituent parts in particular,
as is likewise true of all other organ systems as

72 H. H. LANE

well, are the products of a predetermination in the
odsperm; a process of endogenesis, as Conklin has
termed it, and not of epigenesis.

Furthermore the fate of each neuroblast and
its products is likewise predetermined and there
follows the histogenetic differentiation of the neu-
rones of the central nuclei, motor or sensory as the
case may be, and of the peripheral ganglia, as the
result of an “immanent force” that needs no direct
outside stimulus for its production. Thus HAR-
RISON shows in the case of the formation of the
axone that the outgrowth takes place:

“without the application of any external phy-
sical force and . . . . occurs even when
the normal surroundings are radically modi-
fied. That the original direction taken by the
outgrowing fiber is already determined for
each cell before the outgrowth actually begins,
so that when it does begin it is dependent
upon forces acting from within, follows first
from the fact that the nerve fibers within the
embryo tend to grow out in a given direction
even when quite different surroundings are
substituted for the normal, and secondly, from
the fact that the nerve fibers which grow into
the clotted lymph, are there surrounded on all
sides by an isotropic medium, which cannot
conceivably be held to produce movement in a
definite direction.”’

In other words these structures are repre-
sented by something in the odsperm, whose nature
can only be conjectured, and they appear not as
direct responses to the needs of the embryo, but in
anticipation of those needs, because of the inherited
tendencies and forces immanent in the odsperm and
localized as development proceeds in the parts con-

CORRELATION OF STRUCTURE AND FUNCTION 73

cerned. They are “racial or inherent adaptations
which are not first called forth by the contingent
stimulus to which they are the appropriate and
useful response” (Conklin (’15) ).

This early establishment of peripheral con-
nections on the part of the nervous system receives
its proximate explanation in certain mechanical
conditions that exist at an early stage in embryonic
development but not later. Assuming the truth of
the neurone hypothesis, the question of how any
certain nerve reaches unerringly its proper termina-
tion,—a question that has provoked much discus-
sion,—receives an easy answer.

HARRISON’S experiments show that each neu-
rone sends out its axone in a predetermined manner
and direction; that this axone is in the form of a
protoplasmic process or pseudopodium which ex-
tends outward from the neuroblast toward its peri-
pheral termination; that this process grows from a
terminal bulb—Cajal’s coéne d’accroisement—which
with its changing pseudopods reaches out constant-
ly in various directions, but ultimately extending
through a distance of a millimetre or more until it
reaches the muscle-plate or epithelium with which
it is destined to connect. That this activity must
take place early in embryonic life 1s naturally what
one would expect, since it is only in these early stages
that the neuroblasts of the neural tube lie within the
specified distance—about a millimetre or less—from
the parts they are destined to innervate. On the
basis of adaptation and natural selection it is plain
that only those embryos that thus early establish these
connections can develop properly and so survive.

As HARRISON points out very clearly from his
own results, and as has been shown by the observa-
tions recorded above on the white rat, the neuro-
blasts that thus early come into direct relation with

74 H. H. LANE

their peripheral end-organs are relatively few in
number, but having once made the connection they
elongate whenever and wherever needed as the
growth and shifting of parts goes on so that when
the ultimate relationships have become established
the nerve paths have also been marked out, and
later nerve processes growing out from neighboring
neuroblasts, in relation to the greater’ functional
needs of the embryo or as opportunity is afforded
them, find their course already determined for them
and have no trouble in reaching their own particu-
lar end-organs. This early growth period of the
neuronal processes is clearly a stereotropic response,
as HARRISON’S work shows; the later connections
of the fibers with the individual cells of the end-
organs is probably due, as HARRISON says, to
chemotaxis. It is hardly possible on any other
grounds to explain how it comes about that where
both sensory and motor fibers pass out in the same
nerve trunk the latter turn aside to terminate in
muscle cells, while the former pass on their way to
end in epithelial sense cells.

CORRELATION OF STRUCTURE AND FUNCTION 175

CORTICAL CONNECTIONS IN THE RAT.

The cortical connections with the lower centers
of the brain in mammals are chiefly made through
the corona radiata. In this, fibers from all parts of
the cortex are gathered together and pass caudad
into the thalamus, the cerebellum, the medulla, and
the cord; without doubt other fibers arising in the
lower centers pass cephalad via the same route to
reach the cortex. In the higher mammals there are
other bundles confined to the hemispheres which
connect more or less distant parts of the pallium
with each other. In the rat, however, in common
with many others of the lower mammals, these cor-
tical association tracts are very poorly developed.
The corona radiata on the other hand is a promi-
nent structure even at birth and it may be assumed
from the known relations in higher mammals and
man that its presence may be regarded as conclu-
Sive evidence that connections between the lower
centers and the cortex have already been estab-
lished. While there are probably no medullated fiber
paths present in the brain of the rat at birth, nor
indeed for several days after that event, neverthe-
less the medulla, the cerebellum, the mid-brain, and
the diencephalon contain many non-medullated fib-
ers, and the optic tracts, the olfactory tracts and
lobes, the anterior, posterior, and habenular com-
missures, the corpus callosum, the external capsule
and the deeper layers at least of the cortex exhibit
the same condition. Hence, in view of the sparse-
ness of association tracts in the cortex of the adult
rat, and of the facts just stated, it seems not im-
probable that most of the associations possible later
in life are already established at birth or very soon

76 H. H. LANE

thereafter. Certainly by the tenth day after birth
the cortical connections are pretty definitely estab-
lished, though owing to their primitive and probably
rather diffuse character it has not been possible
with my preparations to map them out. This is
probably due to the lack of medullation of these
fibers, and might be considered as vitiating our con-
tention. However, WATSON (’03) has shown con-
clusively that such an objection has no force in this
connection. Briefly put WATSON found that though

“at birth (and during the first twenty-four
hours after birth) the rat is not only capable
of making many co-6rdinated movements, but
is also capable of receiving sense impressions

. no medullated fibers are present in
either the peripheral or the central nervous
systems.”’

Furthermore, during the first day after birth,
“there must be, too, some pathway between
sensory and motor nerves, because the rat
moves when his tail is pinched, sucks when the
stimulus of the mother’s teats touches his mouth,
scratches his nose with his forepaw when he
smells something unpleasant. . . Granting
now (and the evidence seems conclusive) that
we have motor responses to sensory stimuli at
birth, we must admit a pathway from skin to
muscle. Such a pathway involves peripheral
sensory neurones, central neurones, and finally
motor neurones. During the first few days, at
least, impulses must travel over the unmedul-
lated axis cylinders of all these neurones. Co-
Ordination in the movements mentioned above
grows rapidly better. At eight days
the rats are able to crawl vigorously and,
when crawling to show some selection of path

CORRELATION OF STRUCTURE AND FUNCTION 77

by sniffing and going in different directions.
: Sensitivity for smell, taste, and dermal
stimuli has increased rapidly since the first day.

Whether or not at eight days the
cortex is necessary for the responses which the
rats make to the various stimuli may be a
question. (If the rats were really smelling out
a path, it would of course be necessary.) But,
assuming that the cortex is not involved in
these movements, we still have to account for
the neural pathway in the lower centers over
which these impulses can travel. Granting that
the fibers carrying the impulses from a given
sensory area are all medullated, and granting
that the motor fibers which go to the corre-
sponding muscles in any particular case .are
also medullated, if nevertheless medullation is
lacking in some or all of the pathways within
the central nervous system, then, so far as the
physiological reaction taken as a whole is con- .
cerned, we have function without medulla-
tion.”’

A further quotation perhaps may be permissi-
ble because of its bearing on other aspects of the
problem dealt with in this paper. WATSON finds
that during the period from the tenth to the thir-
teenth day after birth in the rat there is present the
capacity for

“forming and retaining definite associations.
The solving of the problems given to the rats at
the above ages would require the use of the
olfactory tract (probably at thirteen days the
auditory tract was also involved), some sec-
ondary tract to the cortex, the cortex itself, the
pyramidal tract, and of course the peripheral
nerves. If we examine the medullation process

78 H. H. LANE

at this age, we find that the olfactory tract is
entirely unmedullated, that a secondary med-
ullated tract to the cortex does not exist, that
the cortex is entirely unmedullated, and that
the pyramidal tract contains but few medul-
lated fibers.”’

He concludes therefore that ‘“‘medullated fibers in
the cortex of the rat are not a conditio sine qua non
of the rat’s forming and retaining definite associa-
tions.”’

If then, as WATSON has shown so conclusively,
medullated fibers are not necessary for the estab-
lishment of associations in the cortex, and if, as my
preparations show, a multitude of such non-medul-
lated fibers are present at birth and many more
within a few days thereafter in both the brain stem
and the hemispheres, then it must be granted that
the central links in the chain of neurones constitut-
ing the connections between the exteroceptive or-
gans and the motor mechanism are present, and
functional at this time, if the presence of neurofi-
brillae be a safe criterion. Why then is not the rat
at birth able to see or hear, as well as to feel and to
maintain his equilibrium? The optic and auditory
nerves are completely fibrillated long before birth;
the central connections are probably already estab-
lished at birth or within a few days thereafter. The
motor mechanisms that would be involved in the
response to stimuli of light or sound waves are in
good working order even before birth. Even the eye
muscles are differentiating and their innervation es-
tablished as early as the 16 mm. stage in the rat.

In short, the chain of neurones from the ex-
teroceptive organ to the motor mechanism is com-
plete for sight and hearing, possibly at birth, cer-
tainly within a few days after, and several days

CORRELATION OF STRUCTURE AND FUNCTION 79

before the function is established. The block in the
circuit is the extero-receptive sense-organ. The rat
cannot hear before the twelfth or thirteenth day,
nor see before the sixteenth or seventeenth day
after birth because it is not until those dates re-
spectively that the ear and eye have reached a
functional condition. Looking back over the experi-
ments and structural observations recorded above
on the senses of touch, equilibrium, smell and taste,
it will be found that there too, im each case the
function is established when and only so soon as the
proper peripheral sense organ has reached its func-
tional state.

_ The course followed in the development of the
special senses and their correlated mechanisms is
not just what one would expect on a priori grounds.
After the earlier differentiation of the neural tube,
the central connections between sensory and motor
nerves are established in the cord, at least, and
probably in the medulla also before or simultan-
eously with the appearance of such nerves, which
very soon establish their distal connections. This is
followed almost immediately by the completion of
the motor mechanism; then comes the establish-
ment of the central connections with the higher
portions of the brain, and last of all the peripheral
end-organs attain functional capacity. Then, and
not until then, are stimuli from the outside world
able to start a reaction that travels from sense-
organ to central connections and thence out to the
motor mechanism. That the apparatus as first estab-
lished is not perfected has been shown above, but
its later development follows in the paths already
laid down, and consists probably in the successive
addition of neurones to the class of those already
functional.

80 H. H. LANE

This order of development is not what is de-
manded by the Lamarckian hypothesis. If structure
were to follow from the effects of extrinsic stimuli,
the logical order would be: peripheral sense organ,
sensory nerve, central connections, motor nerve, and
finally, motor end-organ. But such is clearly not
the case. It follows therefore that the forces which
bring about the development of the mechanism of
the special senses and their motor connections, are
intrinsic; they are forces brought into the organism
by heredity, that is, they are inherent in the germ-
plasm. The whole process is due to germinal or-
ganization though doubtless with enough plasticity
to allow for a considerable degree of adaptation to
minor environmental changes, and in a secondary
sense controlled in a measure by the correlated de-
velopment of the circulatory, lymphatic, excretory,
and other systems of the body. Such adaptive modi-
fications however do not affect the fundamental
course of development; they concern only its minor
retails.

CORRELATION OF STRUCTURE AND FUNCTION 81

GENERAL SUMMARY.

1. Fifteen stages in the development of the
white rat, ranging from 714 mm. embryos to young
seventeen days after birth, have been examined
from the standpoints both of structure and of func-
tion in an attempt at a correlation between the two
as regards the development of the special senses.

2. The nature of the neurofibrillae is discussed
and the theses are supported: (1) that they are ac-
tual structures of the living neurone; (2) that (a)
either they are composed of rows of colloidal par-
ticles held more or less closely together in a linear
arrangement by means of another constituent of the
protoplasm differing from them in its degree of
viscosity, or (b) they consist entirely of a viscid
substance having the form of strands differing
chemically and physically from the other elements
of the surrounding protoplasm; and (3) that they
are not to be regarded as the so-called “Sttitz-
gerust’”’ of the neurone.

3. The function of the neurofibrillae is dis-
cussed and the conclusion is reached that they
constitute the conducting elements of the neurone.

4. Experimental and structural data are pre-
sented on the sense and apparatus of touch which
are interpreted as showing:

(a) That both the sensory and motor nerves,
as well as the central correlation paths between
them, are laid down very early in embryonic life—
were in fact present in the earliest stage studied,
the 714 mm. embryo.

(b) That the sense of touch is established
somewhat later in embryonic life, at or before the

82 H. H. LANE

16 mm. stage, upon the development of a tactile
end-organ; in the first instance this organ has the
form of a neurofibrillar basket or reticulum in the
vibrissal follicle.

(c) That the sense of touch is increased and
perfected through (1) the addition of new vibrissal
organs, and (2) through the innervation of the in-
tegument itself.

5. Experimental and structural data are pre-
sented on the sense and apparatus of equilibrium,
which show:

(a) That a sense of equilibrium is first appar-
ent upon the completion of the end-organ con-
cerned, viz., the sensory cells of the cristae acusticae
in the ampullae of the semicircular canals.

(b) That the power of equilibration is gradu-
ally perfected through increased powers of co-
ordination with the tactile apparatus (vibrissae),
and the organs of sight (eyes) as well as the estab-
lishment of the general muscle tonus.

6. Experimental and structural data are given
for the sense and apparatus of smell. It is shown:

(a) That no certain response to odors was
made until the olfactory epithelium contains fully
differentiated olfactory sense-cells.

(b) That the olfactory tracts in the brain de-
velop previous to and independent of the peripheral
organ of smell.

7. Experimental and observational data on
the sense and apparatus of taste are set forth,
which show:

(a) That the trigeminal and glossopharyn-
geal nerves are both concerned with taste.

CORRELATION OF STRUCTURE AND FUNCTION 83

(b) That both these nerves and their central
connections are completed long before birth and
long before a sense of taste is present.

(c) That taste comes some time after birth
upon the development (1) of peculiar gustatory
organs on the anterior part of the tongue, and (2)
of taste-buds on and around the circumvallate pa-
pilla.

8. Experimental and observational data on
the sense and apparatus of hearing are presented,
that indicate:

(a) The early establishment of the auditory
(cochlear) nerve and its central connections.

(b) The late development of the organ of
Corti.

(c) The dependence of the sense of hearing
upon the establishment of the definitive structural
conditions in the end-organ of hearing (organ of
Corti).

9. Experimental and observational data on
the sense and apparatus of sight are given which in-
dicate:

(a) The early establishment of the optic nerve
and its central connections.

(b) The very late differentiation of the reti-
nal elements and the accessory structures of the eye
in general.

(c) The fact that sight is not possible until
the whole apparatus is in working order, of which
the last element to be perfected is the sensory end-
organ.

10. The causes of development and of differ-
entation in the nervous system are discussed and
the conclusions reached:

84 H. H. LANE

(a) That the development of the nervous sys-
tem in general, and the differentiation of its con-
stituent parts, are the products of endogenesis, or
predetermination in the odsperm, and not of epi-
genesis.

(b) That these structures appear not as direct
responses to the needs of the embryo, but in an-
ticipation of those needs; not under the influence
of their specific, definitive environmental stimuli,
but because of the inherited organization and forces
in the odsperm.

(c) That the early establishment of the peri-
pheral connections on the part of the nervous
system receives its explanation in the mechanical
conditions existing at an early stage in embryonic
development but not later, viz., that it is only
in these early stages that the distance between the
neural tube and the surface of the embryo is within
the limit of independent growth of the neuronal
processes.

11. The fact is shown that in the case of each
sense the chain of neurones from the exteroceptive
organ to the motor mechanism is completed and the
sensory function established only when the proper
peripheral sense-organ has reached its functional
state.

12. It is pointed out that this unexpected order
of development is contrary to any hypothesis of ex-
trinsic causes, and that the forces concerned in the
development of the mechanism of the special senses
and their motor associations are those inherent in
the organization of the germ-plasm, and may be
only secondarily modified or controlled by other
factors.

CORRELATION OF STRUCTURE AND FUNCTION 85

HPILOGUE.

The author is fully aware that this paper is
only of a preliminary nature, outlining the field and
establishing a few land-marks or base-lines for fur-
ther investigation. As fast as the material can be
obtained the relatively wide gaps in the above-
given account of the rat will be filled in, and in
addition comparative studies on other mammals are
already planned and partially under way. Thanks
are due to the National Academy of Sciences for a
grant of $500.00 with which has already been pur-
chased the equipment necessary to continue this
work. Circumstances render it desirable, however,
and the results so far obtained seem sufficiently .
important to warrant their early publication rather
than to await the more detailed account that can
come only after the expenditure of a much greater
amount of time and labor than has been so far
available for this study.

86

H. H. LANE

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{

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