Part 75

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=).

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.

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=.

FIG. 140.--DIAGRAM OF BRAIN FROM THE SIDE.

1, myelencephalon; 2, metencephalon; 3, mesencephalon; 4, diencephalon; 5, telencephalon. a, canal of the spinal cord; b, fourth ventricle; c, cerebral aqueduct (aqueduct of Sylvius); d, third ventricle; e, lateral ventricle; f, interventricular foramen (or foramen of Monroe); g, lamina terminalis; h, olfactory bulb.]

There are thus altogether five principal subdivisions of the brain, each enclosing a portion of the central cavity. These are the =myelencephalon=, the =metencephalon= (these two enclosing the =fourth= ventricle), the =mesencephalon= (enclosing the =cerebral aqueduct=), the =diencephalon= (enclosing the =third= ventricle), and the two cerebral hemispheres, constituting together the =telencephalon=, and enclosing the two =lateral ventricles=. A plan of the brain, considered as a tube enclosing cavities, is given in Figs. 139 and 140. Fig. 139 shows the plan considered as viewed from the dorsal side, Fig. 140 in a lateral view. These figures are of course pure diagrams, showing only what may be called the plan of structure, and omitting all details. In the actual conditions many important modifications even of the main features of the plan are met with.

In the following description the parts of the brain will be taken up in this order:

A. RHOMBENCEPHALON (primitive hindbrain). 1. Myelencephalon. 2. Metencephalon.

B. MESENCEPHALON (primitive midbrain). 3. Mesencephalon.

C. PROSENCEPHALON (primitive forebrain). 4. Diencephalon. 5. Telencephalon.

A. RHOMBENCEPHALON.

1. Myelencephalon.--The myelencephalon or medulla oblongata is transitional between the spinal cord and the brain. It has in general the structural peculiarities of the cord, but these undergo in it a gradual transformation into the arrangements characteristic of the brain.

It has the form of a flattened and truncated cone, which widens craniad and is limited at the cranial end by the pons (Fig. 138, i) ventrally and laterally; by the cerebellum (Fig. 137, III) dorsally. The cranial portion of its dorsal surface is overhung by the cerebellum. The points of origin of the roots of the first pair of cervical nerves (Fig. 138, s) may be taken as indicating the boundary between the spinal cord and the medulla oblongata; there is no other external marking to show the limits of the two.