Chapter VII: The Structure of the Brain.28

FIG. 28. FIG. 29. FIG. 30.

(All after Huguenin.)]

=Embryological Sketch.=--The brain is a sort of _pons asinorum_ in anatomy until one gets a certain general conception of it as a clue. Then it becomes a comparatively simple affair. The clue is given by comparative anatomy and especially by embryology. At a certain moment in the development of all the higher vertebrates the cerebro-spinal axis is formed by a hollow tube containing fluid and terminated in front by an enlargement separated by transverse constrictions into three 'cerebral vesicles,' so called (see Fig. 28). The walls of these vesicles thicken in most places, change in others into a thin vascular tissue, and in others again send out processes which produce an appearance of farther subdivision. The middle vesicle or mid-brain (_Mb_ in the figures) is the least affected by change. Its upper walls thicken into the optic lobes, or _corpora quadrigemina_ as they are named in man; its lower walls become the so-called peduncles or _crura_ of the brain; and its cavity dwindles into the aqueduct of Silvius. A section through the adult human mid-brain is shown in Fig. 31.

FIG. 31.--The 'nates' are the anterior corpora quadrigemina, the
spot above _aq_ is a section of the sylvian aqueduct, and the
tegmentum and two 'feet' together make the Crura. These are marked
_C.C._, and a cross (+) marks the aqueduct, in Fig. 32.
]

The anterior and posterior vesicles undergo much more considerable change. The walls of the posterior vesicle thicken enormously in their foremost portion and form the _cerebellum_ on top (_Cb_ in all the figures) and the _pons Varolii_ below (_P.V._ in Fig. 33). In its hindmost portions the posterior vesicle thickens below into the medulla oblongata (_Mo_ in all the figures), whilst on top its walls thin out and melt, so that one can pass a probe into the cavity without breaking through any truly nervous tissue. The cavity which one thus enters from without is named the fourth ventricle (4 in Figs. 32 and 33). One can run the probe forward through it, passing first under the cerebellum and then under a thin sheet of nervous tissue (the _valve of Vieussens_) just anterior thereto, as far as the _aqueduct of Silvius_. Passing through this, the probe emerges forward into what was once the cavity of the anterior vesicle. But the covering has melted away at this place, and the cavity now forms a deep compressed pit or groove between the two walls of the vesicle, and is called the _third ventricle_ (3 in Figs. 32 and 33). The 'aqueduct of Sylvius' is in consequence of this connection often called the _iter a tertio ad quartum ventriculum_. The walls of the vesicle form the _optic thalami_ (_Th_ in all the figures).

From the anterior vesicle just in front of the thalami there buds out on either side an enlargement, into which the cavity of the vesicle continues, and which becomes the _hemisphere_ of that side. In man its walls thicken enormously and form folds, the so-called _convolutions_, on their surface. At the same time they grow backwards rather than forwards of their starting-point just in front of the thalamus, arching over the latter; and growing fastest along their top circumference, they end by bending downwards and forwards again when they have passed the rear end of the thalamus. When fully developed in man, they overlay and cover in all the other parts of the brain. Their cavities form the _lateral ventricles_, easier to understand by a dissection than by a description. A probe can be passed into either of them from the third ventricle at its anterior end; and like the third ventricle, their wall is melted down along a certain line, forming a long cleft through which they can be entered without rupturing the nervous tissue. This cleft, on account of the growth of the hemisphere outwards, backwards, and then downwards from its starting point, has got rolled in and tucked away beneath the apparent surface.[29]

At first the two hemispheres are connected only with their respective thalami. But during the fourth and fifth months of embryonic life they become connected with each other above the thalami through the growth between them of a massive system of transverse fibres which crosses the median line like a great bridge and is called the _corpus callosum_. These fibres radiate in the walls of both hemispheres and form a direct connection between the convolutions of the right and of the left side. Beneath the corpus callosum another system of fibres called the _fornix_ is formed, between which and the corpus callosum there is a peculiar connection. Just in front of the thalami, where the hemispheres begin their growth, a ganglionic mass called the _corpus striatum_ (_C.S._, Figs. 32 and 33) is formed in their wall. It is complex in structure, consisting of two main parts, called _nucleus lenticularis_ and _nucleus candatus_ respectively. The figures, with their respective explanations, will give a better idea of the farther details of structure than any verbal description; so, after some practical directions for dissecting the organ, I will pass to a brief account of the physiological relations of its different parts to each other.

=Dissection of Sheep's Brain.=--The way really to understand the
brain is to dissect it. The brains of mammals differ only in their
proportions, and from the sheep's one can learn all that is
essential in man's. The student is therefore strongly urged to
dissect a sheep's brain. Full directions of the order of procedure
are given in the human dissecting books, e.g. Holden's Practical
Anatomy (Churchill), Morrell's Student's Manual of Comparative
Anatomy and Guide to Dissection (Longmans), and Foster and
Langley's Practical Physiology (Macmillan). For the use of classes
who cannot procure these books I subjoin a few practical notes. The
instruments needed are a small saw, a chisel with a shoulder, and a
hammer with a hook on its handle, all three of which form part of
the regular medical autopsy-kit and can be had of
surgical-instrument-makers. In addition a scalpel, a pair of
scissors, a pair of dissecting-forceps, and a silver probe are
required. The solitary student can find home-made substitutes for
all these things but the forceps, which he ought to buy.

The first thing is to get off the skull-cap. Make two saw-cuts,
through the prominent portion of each condyle (or articular surface
bounding the hole at the back of the skull, where the spinal cord
enters) and passing forwards to the temples of the animal. Then
make two cuts, one on each side, which cross these and meet in an
angle on the frontal bone. By actual trial, one will find the best
direction for the saw-cuts. It is hard to saw entirely through the
skull-bone without in some places also sawing into the brain. Here
is where the chisel comes in--one can break by a smart blow on it
with the hammer any parts of the skull not quite sawn through. When
the skull-cap is ready to come off one will feel it 'wobble.'
Insert then the hook under its forward end and pull firmly. The
bony skull-cap alone will come away, leaving the periosteum of the
inner surface adhering to that of the base of the skull, enveloping
the brain, and forming the so-called _dura mater_ or outer one of
its 'meninges.' This dura mater should be slit open round the
margins, when the brain will be exposed wrapped in its nearest
membrane, the _pia mater_, full of blood-vessels whose branches
penetrate the tissues.

The brain in its pia mater should now be carefully 'shelled out.'
Usually it is best to begin at the forward end, turning it up there
and gradually working backwards. The _olfactory lobes_ are liable
to be torn; they must be carefully scooped from the pits in the
base of the skull to which they adhere by the branches which they
send through the bone into the nose-cavity. It is well to have a
little blunt curved instrument expressly for this purpose. Next the
_optic nerves_ tie the brain down, and must be cut through--close
to the chiasma is easiest. After that comes the _pituitary body_,
which has to be left behind. It is attached by a neck, the
so-called _infundibulum_, into the upper part of which the cavity
of the third ventricle is prolonged downwards for a short distance.
It has no known function and is probably a 'rudimentary organ.'
Other nerves, into the detail of which I shall not go, must be cut
successively. Their places in the human brain are shown in Fig. 34.
When they are divided, and the portion of dura mater (tentorium)
which projects between the hemispheres and the cerebellum is cut
through at its edges, the brain comes readily out.

FIG. 31.--The human brain from below, with its nerves numbered,
after Henle I, olfactory; II, optic; III, oculo-motorius; IV,
trochlearis; V, trifacial; VI, abducens oculi; VII, facial; VIII,
auditory; IX, glosso-pharyngeal; X, pneumogastric; XI, spinal
accessory; XII, hypoglossal; _nc_I, first cervical, etc.
]

It is best examined fresh. If numbers of brains have to be prepared
and kept, I have found it a good plan to put them first in a
solution of chloride of zinc, just dense enough at first to float
them, and to leave them for a fortnight or less. This softens the
pia mater, which can then be removed in large shreds, after which
it is enough to place them in quite weak alcohol to preserve them
indefinitely, tough, elastic, and in their natural shape, though
bleached to a uniform white color. Before immersion in the chloride
all the more superficial adhesions of the parts must be broken
through, to bring the fluid into contact with a maximum of
surface. If the brain is used fresh, the pia mater had better be
removed carefully in most places with the forceps, scalpel, and
scissors. Over the grooves between the cerebellum and hemispheres,
and between the cerebellum and medulla oblongata, thin cobwebby
moist transparent vestiges of the _arachnoid_ membrane will be
found.

The subdivisions may now be examined in due order. For the
convolutions, blood-vessels, and nerves the more special books must
be consulted.

First, looked at from above, with the deep _longitudinal fissure_
between them, the hemispheres are seen partly overlapping the
intricately wrinkled _cerebellum_, which juts out behind, and
covers in turn almost all the medulla oblongata. Drawing the
hemispheres apart, the brilliant white _corpus callosum_ is
revealed, some half an inch below their surface. There is no median
partition in the cerebellum, but a median elevation instead.

Looking at the brain from below, one still sees the longitudinal
fissure in the median line in front, and on either side of it the
_olfactory lobes_, much larger than in man; the _optic tracts_ and
_commissure_ or _'chiasma'_; the _infundibulum_ cut through just
behind them; and behind that the single _corpus albicans_ or
_mamillare_, whose function is unknown and which is double in man.
Next the _crura_ appear, converging upon the pons as if carrying
fibres back from either side. The _pons_ itself succeeds, much less
prominent than in man; and finally behind it comes the medulla
oblongata, broad and flat and relatively large. The pons looks like
a sort of collar uniting the two halves of the cerebellum, and
surrounding the medulla, whose fibres by the time they have emerged
anteriorly from beneath the collar have divided into the two crura.
The inner relations are, however, somewhat less simple than what
this description may suggest.

Now turn forward the cerebellum; pull out the vascular _choroid
plexuses_ of the pia, which fill the fourth ventricle; and bring
the upper surface of the _medulla oblongata_ into view. The _fourth
ventricle_ is a triangular depression terminating in a posterior
point called the _calamus scriptorius_. (Here a very fine probe may
pass into the central canal of the spinal cord.) The lateral
boundary of the ventricle on either side is formed by the
_restiform body_ or _column_, which runs into the cerebellum,
forming its _inferior_ or _posterior peduncle_ on that side.
Including the calamus scriptorius by their divergence, the
posterior columns of the spinal cord continue into the medulla as
the _fasciculi graciles_. These are at first separated from the
broad restiform bodies by a slight groove. But this disappears
anteriorly, and the 'slender' and 'ropelike' strands soon become
outwardly indistinguishable.

Turn next to the ventral surface of the medulla, and note the
_anterior pyramids_, two roundish cords, one on either side of the
slight _median groove_. The pyramids are crossed and closed over
anteriorly by the _pons Varolii_, a broad transverse band which
surrounds them like a collar, and runs up into the cerebellum on
either side, forming its _middle peduncles_. The pons has a slight
median depression and its posterior edge is formed by the
_trapezium_ on either side. The trapezium consists of fibres which,
instead of surrounding the pyramid, seem to start from alongside of
it. It is not visible in man. The _olivary bodies_ are small
eminences on the medulla lying just laterally of the pyramids and
below the trapezium.

FIG. 35.--Fourth ventricle, etc. (Henle). _III_, third ventricle;
_IV_, fourth ventricle; _P_, anterior, middle, and posterior
peduncles of cerebellum cut through; _Cr_, restiform body; _Fg_,
funiculus gracilis; _Cq_, corpora quadrigemina.
]

Now cut through the peduncles of the cerebellum, close to their
entrance into that organ. They give one surface of section on each
side, though they receive contributions from three directions. The
posterior and middle portions we have seen: the _anterior
peduncles_ pass forward to the _corpora quadrigemina_. The thin
white layer of nerve-tissue between them and continuous with them
is called the _valve of Vieussens_. It covers part of the canal
from the fourth ventricle to the third. The cerebellum being
removed, examine it, and cut sections to show the peculiar
distribution of white and gray matter, forming an appearance called
the _arbor vitæ_ in the books.

Now bend up the posterior edge of the hemispheres, exposing the
corpora quadrigemina (of which the anterior pair are dubbed the
_nates_ and the posterior the _testes_), and noticing the _pineal
gland_, a small median organ situated just in front of them and
probably, like the pituitary body, a vestige of something useful in
premammalian times. The rounded posterior edge of the corpus
callosum is visible now passing from one hemisphere to the other.
Turn it still farther up, letting the medulla, etc., hang down as
much as possible and trace the under surface from this edge
forward. It is broad behind but narrows forward, becoming
continuous with the _fornix_. The anterior stem, so to speak, of
this organ plunges down just in front of the _optic thalami_, which
now appear with the fornix arching over them, and the median _third
ventricle_ between them. The margins of the fornix, as they pass
backwards, diverge laterally farther than the margins of the corpus
callosum, and under the name of _corpora fimbriata_ are carried
into the lateral ventricles, as will be seen again.

It takes a good topographical mind to understand these ventricles
clearly, even when they are followed with eye and hand. A verbal
description is absolutely useless. The essential thing to remember
is that they are offshoots from the original cavity (now the third
ventricle) of the anterior vesicle, and that a great split has
occurred in the walls of the hemispheres so that they (the lateral
ventricles) now communicate with the exterior along a cleft which
appears sickle shaped, as it were, and folded in.

The student will probably examine the relations of the parts in
various ways. But he will do well to begin in any case by cutting
horizontal slices off the hemispheres almost down to the level of
the corpus callosum, and examining the distribution of gray and
white matter on the surfaces of section, any one of which is the
so-called _centrum ovale_. Then let him cut down in a fore-and-aft
direction along the edge of the corpus callosum, till he comes
'through' and draw the hemispherical margin of the cut outwards--he
will see a space which is the ventricle, and which farther cutting
along the side and removing of its hemisphere-roof will lay more
bare. The most conspicuous object on its floor is the _nucleus
caudatus_ of the _corpus striatum_.

FIG. 36.--Horizontal section of human brain just above the
thalami.--_Ccl_, corpus callosum in section; _Cs_, corpus striatum;
_Sl_, septum lucidum; _Cf_, columns of the fornix; _Tho_, optic
thalami; _Cn_, pineal gland. (After Henle.)
]

Cut the corpus callosum transversely through near its posterior
edge and bend the anterior portion of it forwards and sideways. The
rear edge (_splenium_) left _in situ_ bends round and downwards and
becomes continuous with the _fornix_. The anterior part is also
continuous with the fornix, but more along the median line, where a
thinnish membrane, the _septum lucidum_, triangular in shape,
reaching from the one body to the other, practically forms a sort
of partition between the contiguous portion of the lateral
ventricles on the two sides. Break through the _septum_ if need be
and expose the upper surface of the fornix, broad behind and narrow
in front where its _anterior pillars_ plunge down in front of the
third ventricle (from a thickening in whose anterior walls they
were originally formed), and finally penetrate the corpus albicans.
Cut these pillars through and fold them back, exposing the thalamic
portion of the brain, and noting the under surface of the fornix.
Its diverging _posterior pillars_ run backwards, downwards, and
then forwards again, forming with their sharp edges the _corpora
fimbriata_, which bound the cleft by which the ventricle lies open.
The semi-cylindrical welts behind the _corpora fimbriata_ and
parallel thereto in the wall of the ventricle are the _hippocampi_.
Imagine the fornix and corpus callosum shortened in the
fore-and-aft direction to a transverse cord; imagine the
hemispheres not having grown backwards and downwards round the
thalamus; and the corpus fimbriatum on either side would then be
the upper or anterior margin of a split in the wall of the
hemispheric ventricle of which the lower and posterior margin would
be the posterior border of the corpus striatum where it grows out
of the thalamus.

The little notches just behind the anterior pillar of the fornix
and between them and the thalami are the so-called _foramina of
Monro_ through which the plexus of vessels, etc., passes from the
median to the lateral ventricles.

See the thick _middle commissure_ joining the two thalami, just as
the corpus callosum and fornix join the hemispheres. These are all
embryological aftergrowths. Seek also the _anterior commissure_
crossing just in front of the anterior pillars of the fornix, as
well as the _posterior commissure_ with its lateral prolongations
along the thalami, just below the pineal gland.

On a median section, note the thinnish _anterior wall_ of the third
ventricle and its prolongation downwards into the _infundibulum_.

Turn up or cut off the rear end of one hemisphere so as to see
clearly the optic tracts turning upwards towards the rear corner of
the thalamus. The _corpora geniculata_ to which they also go,
distinct in man, are less so in the sheep. The lower ones are
visible between the optic-tract band and the 'testes,' however.

* * * * *

The brain's principal parts are thus passed in review. A
longitudinal section of the whole organ through the median line
will be found most instructive (Fig. 37). The student should also
(on a _fresh_ brain, or one hardened in bichromate of potash or
ammonia to save the contrast of color between white and gray
matter) make transverse sections through the _nates_ and _crura_,
and through the

FIG. 37.--Median section of human brain below the hemispheres.
_Th_, thalamus; _Cg_, corpora quadrigemina; _V^{III}_, third
ventricle; _Com_, middle commissure; _F_, columns of fornix; _Inf_,
infundibulum; _Op.n_, optic nerve; _Pit_, pituitary body; _Av_,
arbor vitæ. (After Obersteiner).
]

hemispheres just in front of the corpus albicans. The latter
section shows on each side the _nucleus lenticularis_ of the corpus
striatum, and also the _inner capsule_ (see Fig. 38, _Nl_, and
_Ic_).

FIG. 38.--Transverse section through right hemisphere (after
Gegenbaur). _Cc_, corpus callosum; _Pf_, pillars of fornix; _Ic_,
internal capsule; _V_, third ventricle; _Nl_, nucleus lenticularis.
]

When all is said and done, the fact remains that, for the beginner, the understanding of the brain's structure is not an easy thing. It must be gone over and forgotten and learned again many times before it is definitively assimilated by the mind. But patience and repetition, here as elsewhere, will bear their perfect fruit.