1. =The Spinal Cord. Medulla spinalis= (Figs. 133-136).--The spinal cord
is that portion of the nervous system which occupies the vertebral
canal; it is continuous craniad with the brain. It has the form of a
somewhat flattened cylinder and extends from the foramen magnum into the
caudal region. It diminishes in diameter after entering the sacral
region.
The cord has a cervical and a lumbar enlargement, the former (Fig. 133)
marking the origin of the nerves which pass to the fore limb, and the
latter (Fig. 136) the origin of those which pass to the hind limb.
The =cervical= enlargement (Fig. 133) lies in that part of the vertebral
canal bounded by the fourth to seventh cervical and first thoracic
vertebræ. Caudad of the first thoracic vertebra the cord continues of
nearly uniform diameter to the lumbar enlargement (Fig. 136) which
stretches from the third to the seventh lumbar vertebræ (inclusive).
Caudad of the seventh lumbar vertebra it diminishes uniformly in
diameter and ends in a slender cord, the =filum terminale= (Fig. 136,
_C_), which may be traced into the caudal region.
The surface of the cord is marked by a number of longitudinal grooves or
=sulci=, and =fissures=. The most prominent of these is the =anterior
median fissure= (Fig. 134, _c_), along the ventral median line; into
this the pia mater dips. The =posterior median sulcus= (Fig. 133, _e_;
Fig. 134, _a_) is a shallow furrow along the dorsal median line. The
anterior fissure and the posterior sulcus thus divide the cord into
lateral halves. Each half is subdivided by the =anterior= and =posterior
lateral sulci=.
The =posterior= lateral sulcus (Fig. 133, _f_; Fig. 134, _b_) lies at
the side of the posterior median sulcus. It is broad and shallow and
has the posterior roots of the spinal nerves emerging from its bottom.
The =anterior= lateral sulcus exists only after the forcible pulling out
of the anterior (ventral) roots of the spinal nerves. It then marks the
line along which they originate from the cord.
In cross-section (Fig. 134) the substance of the cord appears to the
naked eye as composed of a darker central “=gray matter=” (_g_) and of
an outer “=white matter=” (_f_). The gray matter has in section the form
of the letter H. The cord is really tubular, having a =central canal=
(_h_) which appears in section in the middle of the cross-bar of the H,
while the two ends of each vertical bar extend toward the anterior and
posterior lateral fissures.
The white matter is divided into funiculi (or columns) by the fissures
and sulci, so that there is in each half of the cord an =anterior=, a
=posterior=, and a =lateral= white funiculus.
In the cervical region (Fig. 134) there appears between the posterior
lateral and posterior median sulci an =intermediate= sulcus which
divides the posterior funiculus in this region into two. The median
slenderer of these is the =fasciculus gracilis= (_d_) or column of Goll.
The lateral and thicker is the =fasciculus cuneatus= (_e_) or column of
Burdach.
_The membranes of the spinal cord_:
The =dura mater= is a tough fibrous membrane directly continuous with
the dura mater of the brain. It differs from the dura mater of the brain
in two particulars:
1. At the foramen magnum it splits into two layers, one of which lines
the bony vertebral canal and forms its periosteum, while the other
covers the cord.
2. It is separated from the cord by a considerable space. The dura mater
is continuous with the fibrous sheaths of the spinal nerves at their
points of exit. Along the sides of the cord it is connected to the pia
mater by a delicate strand of connective tissue probably equivalent to
the “ligamentum denticulatum” of man.
The =arachnoid= is a delicate cellular membrane lying beneath the dura
mater, between it and the pia mater. It forms a continuous investment
for the cord, is not vascular, and is said not to dip into the fissures
of the cord.
[Illustration: FIG. 134.--CROSS-SECTION OF THE SPINAL CORD IN THE
CERVICAL REGION.
_a_, posterior median sulcus; _b_, posterior lateral sulcus; _c_,
anterior median fissure; _d_, fasciculus gracilis; _e_, fasciculus
cuneatus; _f_, white matter; _g_, gray matter; _h_, central canal.]
[Illustration: FIG. 135.--SECTION OF SPINAL CORD, SHOWING THE ORIGIN
OF A PAIR OF SPINAL NERVES.
_a_, dorsal root; _b_, spinal ganglion; _c_, dorsal ramus; _d_,
ventral ramus; _e_, ventral root.]
[Illustration: FIG. 136.--CAUDAL PORTION OF THE SPINAL CORD.
_A_, lumbar enlargement; _B_, cauda equina; _C_, filum terminale; 5-7,
fifth to seventh lumbar nerves. _I_-_III_, the three sacral nerves.
_a_, dorsal roots; _b_, spinal ganglia; _c_, dorsal rami; _d_, ventral
rami.]
The =pia mater= invests the cord closely and contains some
blood-vessels. It is a delicate membrane which dips into the fissures
and sulci of the cord and is connected to it by numerous strands of
connective tissue that pass from it into the substance of the cord. The
nerves pierce it.
=Spinal Nerves.=--From the spinal cord arise the spinal nerves. Of these
there are about thirty-eight pairs in the cat. Eight are =cervical=,
thirteen =thoracic=, seven =lumbar=, three =sacral=, and seven or eight
=caudal=. Those leaving the cervical (Fig. 133, 5-8 and _I_) and lumbar
(Fig. 136) enlargements are larger than the others. The =first= cervical
nerve leaves the vertebral canal through the atlantal foramen, the
=second= leaves between the arches of the atlas and axis, while all
the others leave the vertebral canal by way of the intervertebral
foramina. Each nerve arises from the cord by a =dorsal= and a =ventral=
root. The dorsal root is chiefly sensory, the ventral motor in
character. The =dorsal= root (radix posterior) (Figs. 133, 135, and 136,
_a_) begins as a number (twelve or more) of separate nerve-bundles which
emerge from the posterior lateral groove. These roots lie nearly in a
single plane and pass laterad, converging to penetrate a ganglion, the
=spinal ganglion= (_b_) (or ganglion of the posterior root). All the
spinal ganglia except the first and second are situated in the
intervertebral foramina or within the vertebral canal. The first and
second are situated among the muscles surrounding the place of exit of
the nerves.
The =ventral root= (radix anterior) (Fig. 135, _e_) arises as a larger
number of small fibre-bundles which do not lie in a single plane, so
that in a transverse section several rootlets may appear in a single
section. The rootlets converge to form a single mass which joins the
dorsal root just as it emerges from the spinal ganglion. The nerve
formed by the junction of the ventral and dorsal roots is one of the
=spinal nerves=.
The direction in which the nerves leave the cord varies. In the cervical
region (Fig. 133) and cranial part of the lumbar region it is nearly
laterad; at the cervical and lumbar enlargements it is laterocaudad. The
nerves in the sacral and caudal regions pass almost directly caudad to
reach the intervertebral foramina and form thus a brush which surrounds
the filum terminale (Fig. 136, _C_) and is called the =cauda equina=
(Fig. 136, _B_).
Each spinal nerve immediately after leaving the intervertebral foramen
divides into two branches, a =dorsal= or =posterior= branch or =ramus=
(_c_), and a =ventral= or =anterior ramus= (_d_). The dorsal ramus is in
each case small (except in the first and second cervical nerves), and is
distributed to the longitudinal muscles and integument of the back.
The ventral rami are larger and each is connected a short distance
beyond its origin with the sympathetic system by a short =ramus
communicans= or communicating branch. Each is then distributed to the
integument and muscles of the ventral part of the body, including the
limbs.
The ventral rami which pass to the limbs are much larger than the
others. The ventral rami are further distinguished from the dorsal by
the fact that they frequently unite with one another to form plexuses.
The peripheral distribution of the spinal nerves is described later.
2. =The Brain. Encephalon.=--The brain is that portion of the central
nervous system that is included within the cranial cavity. It is a
direct continuation of the spinal cord, and presents many of the same
essential characters as the latter, with great modifications in details.
The structure of the brain can best be understood if it be considered as
a modified continuation of the spinal cord, and the relation of the
parts to the essential parts of the cord noted. The spinal cord is a
nearly straight tube, with a central cavity and thick walls. The brain
is likewise tubular, with the cavities enlarged or subdivided in places,
with the walls greatly thickened, and with a number of bends and
constrictions in the tube. The relation of the structure of the brain to
that of the spinal cord is most easily perceived by an examination of
the brain of some lower vertebrate, as the frog or shark, and such an
examination should be made before proceeding to the study of the more
complicated brain of the cat.
In the following account of the brain of the cat all parts will be
described as far as possible in relation with the tubular structure of
the brain. The brain will be considered as a hollow structure, having
central cavities, and the solid portions will be brought into relation
as parts of the roof, sides, or floor of the cavities. The cavities of
the brain, forming a direct continuation of the central canal of the
spinal cord, are known as =ventricles= (=ventriculi=).
[Illustration: FIG. 137.--DORSAL SURFACE OF THE BRAIN.
_I_, olfactory bulbs; _II_, cerebral hemispheres; _III_, cerebellum;
_IV_, medulla. _a_, præsylvian sulcus; _b_, cruciate sulcus; _c_,
sulcus ansatus; _d_, coronal sulcus; _e_, anterior sulcus; _f_,
posterior sulcus; _g_, suprasylvian sulcus; _h_, separate parts of the
lateral sulcus; _i_, end of splenial sulcus; _j_, vermis of
cerebellum; _k_, hemispheres of cerebellum; _l_, roof of fourth
ventricle; _m_, first cervical nerve. 1, orbital lobe (or gyrus); 2,
sigmoid gyrus; 3, anterior sylvian gyrus; 4, ectosylvian gyrus; 5,
suprasylvian gyrus; 6, posterior sylvian gyrus; 7, parts of marginal
gyrus; 8, gyrus compositus posterior.]
In a general view of the cat’s brain from the dorsal side (Fig. 137)
four subdivisions are discernible. At the caudal end is a small
stalk-like portion (_IV_) which is clearly a slightly modified
continuation of the spinal cord; this is the =myelencephalon= or
=medulla oblongata=. Just craniad of this, rising high above it, is an
irregularly lobulated rounded portion, the =cerebellum= (_III_). This is
produced as a thickening of the wall of the original tube; it forms the
dorsal part of the =metencephalon=. These two portions of the brain have
a common cavity, lying within the medulla oblongata and ventrad of the
cerebellum, and formed by a widening of the cavity of the spinal cord;
it is known as the =fourth= ventricle. Craniad of the cerebellum,
separated from it by a deep transverse fissure, are two very large
portions, the =hemispheres= of the =cerebrum= (_II_), separated from
each other by a deep longitudinal fissure; they constitute together the
=telencephalon=. The hemispheres are produced by a sort of forking of
the original central tube;--by outgrowths on the sides of the tube, the
central cavity extending into the outgrowths. The two outgrowths extend
dorsad, as well as craniad and caudad from the original place of origin,
forming the larger part of the brain; the extensions of the central
cavity which they contain are known as the =lateral ventricles=.
Finally, in front of the two hemispheres are seen the two small
=olfactory bulbs= (_I_) which are mere extensions of the two hemispheres
and contains cavities which are extensions of the lateral ventricles.
In a ventral view of the entire brain (Fig. 138) certain parts are
visible which in the dorsal view are hidden by the large cerebral
hemispheres. At the caudal end is seen, as before, the medulla
oblongata, or myelencephalon, and craniad of this, on the ventral side
of the cerebellum, a broad transverse tract, the =pons= (_i_); this,
like the cerebellum, forms part of the metencephalon. All this part of
the brain formed by the myelencephalon and metencephalon lies caudad of
(behind) the rest of the brain and is marked off from it by a great
fissure; it therefore receives as a whole the name hindbrain or
=rhombencephalon=.
Just craniad of the pons are seen two short diverging arm-like bands of
fibres (_g_), enclosing a small triangular space between them. These two
arms with the space between them are all that is visible of a division
of the brain which in dorsal view is completely covered by the backward
projection of the cerebral hemispheres. This is the midbrain or
=mesencephalon=. In this portion of the brain the continuation of the
central cavity is a narrow canal which receives the name =cerebral
aqueduct= (=aqueductus cerebri=) or aqueduct of Sylvius.
[Illustration: FIG. 138.--VENTRAL SURFACE OF THE BRAIN.
_I_-_XII_, the twelve cranial nerves in order; _I_, olfactory bulb;
_II_, N. opticus; _III_, N. oculomotorius; _IV_, N. trochlearis; _V_,
N. trigeminus (1, mandibular nerve; 2, maxillary nerve; 3, ophthalmic
nerve; 4, sensory root); _VI_, N. abducens; _VII_, N. facialis;
_VIII_, N. acusticus; _IX_, N. glossopharyngeus; _X_, N. vagus; _XI_,
N. accessorius; _XII_, N. hypoglossus. _a_, tractus olfactorius; _b_,
substantia perforata anterior; _c_, optic chiasma; _d_, infundibulum;
_e_, hypophysis; _f_, lobus pyriformis, or tractus postrhinalis; _g_,
pedunculi cerebri; _h_, substantia perforata posterior; _i_, pons;
_j_, sulcus basilaris of pons; _k_, semilunar (or Gasserian) ganglion;
_l_, trapezium; _m_, area ovalis; _n_, area elliptica; _o_, pyramidal
tracts; _p_, anterior or ventral fissure; _q_, foramen cæcum; _r_,
anterior lateral sulcus; _s_, first cervical nerve.]
Just craniad of the midbrain is a small rather irregular area, bounded
craniad by the band-like =optic chiasma= (_c_), from which arise the
optic nerves (_II_), and showing caudad of this band a rounded irregular
projection (_e_). This is a fourth division of the brain which is
likewise covered dorsad by the hemispheres; it is the ’tween-brain or
=diencephalon=. Its cavity is known as the third ventricle. Just craniad
of the ’tween-brain, in the median line, is a deep fissure, showing that
here in reality is the termination of the original tube, the further
extension craniad being due to the pushing forward of the lateral
outgrowths, or cerebral hemispheres, which extend in ventral view a
considerable distance further craniad. The hemispheres lie also caudad,
laterad, and dorsad of the midbrain and ’tween-brain, so that these
parts are almost enveloped by the hemispheres. The hemispheres and
’tween-brain are sometimes taken together as forming a single portion,
the forebrain or =prosencephalon=.
[Illustration: FIG. 139.--DIAGRAM OF BRAIN FROM ABOVE.