Part 29

3. =Lymphatics of the Thoracic Limbs.=--The lymphatics of the forelimbs begin as a number of vessels on the dorsum and palm of the hand. These all pass toward the radial side, and at about the base of the first metacarpal the vessels from both sides of the hand unite to form two or three main trunks. These pass towards the elbow, accompanying the vena cephalica (Fig. 130). There are usually two of these main trunks, one lying on each side of the vena cephalica. They accompany this vein over the shoulder, following that portion which finally joins the external jugular. In the hollow of the shoulder they enter the =cervical lymph-glands=, one or two lymph-glands imbedded in the fat lying beneath the clavotrapezius and levator scapulæ ventralis in this region. From the cervical lymph-glands a single trunk passes mediocaudad, unites with the lymphatics from the head, and usually (on the left side) with the cranial end of the thoracic duct, and enters the external jugular vein at its junction with the subclavian. The common trunk from the head and arm may enter the jugular separately, but close to the termination of the thoracic duct. On the right side the common trunk is joined by one or more small vessels coming from the lymphatic glands of the thorax and forming the right lymphatic duct.

There is said to be also a deep system of lymphatics in the arm, in addition to the superficial system above described; this is said to accompany the branches of the brachial vein. If this system is present in the cat, it is much less easily demonstrated than the superficial system.

4. =Lymphatics of the Thorax and Abdomen.=--In the thorax are a number of lymphatic glands which are apparently not constant in number and position. One is commonly found dorsad of the bifurcation of the superior vena cava, one ventrad of the mammary vein, several small ones about the bifurcation of the trachea, one or more in the region of the aortic arch, and a number of small ones scattered in the mediastinum. The vessels from these glands finally join either the right lymphatic duct or the thoracic duct; the details have not been worked out and are probably variable.

In the abdominal cavity a number of =mesenteric= glands, of considerable size, are found in the mesentery and in the mesocolon. In the mesocolon these are usually separate, forming a chain of glands following the colon about one and a half centimeters from it. In the mesentery the glands are mostly united into a very large one, formerly known as the =pancreas aselli=; this is the largest lymph-gland in the body. It is a curved structure, four or five centimeters in length, lying in the central region of the mesentery.

Into the mesenteric glands pass lymphatic vessels from the viscera of the abdomen. From the mesenteric glands one or two large lymphatic vessels pass craniodorsad, at first with the portal vein, then dorsad of it. This vessel reaches the dorsal side of the aorta near the cranial end of the kidney, where it enters a large fusiform vessel, the =receptaculum chyli=, which stretches from the cranial end of the kidney craniad between the crura of the diaphragm into the thorax. The receptaculum receives other lymphatics from the various organs of the abdominal cavity as well as vessels coming from the pelvic region, and from the =iliac= glands. These lie beside the iliac artery, receive lymphatics from the hind limbs, and send lymphatics craniad to the receptaculum chyli. The latter forms the beginning of the =thoracic duct=.

The thoracic duct (Fig. 118, 5, page 282) enters the thoracic cavity between the crura of the diaphragm as a continuation of the receptaculum chyli. It lies on the dorsal side of the aorta (_a_), somewhat to the left. It passes along the aorta, gradually coming to lie more on its left side, and finally leaves that vessel near the aortic arch. The duct continues thence straight craniad along the left side of the œsophagus till it reaches the external jugular vein at its junction with the subclavian. In this region the thoracic duct usually unites with the lymphatics coming from the head and arm, then enters the external jugular vein, or the innominate at the point of union of external jugular and subclavian.

In its passage through the thorax the duct frequently divides into two or three divisions which pass along parallel to each other for a distance, then reunite. Such division into two or three parts is especially frequent just before the union with the jugular vein. The numerous valves in the thoracic duct give it a beaded appearance.

5. =Lymphatics of the Pelvic Limbs.=--A number of lymphatic vessels are formed on the dorsum and the sole of the foot. These pass to the lateral side, forming a network on the lateral and dorsal surface of the ankle-joint, and finally unite into two main trunks which accompany the vena saphena parva. In the popliteal space these enter the =popliteal gland= which lies imbedded in the fat of that region. From the popliteal gland one or two trunks accompany the superficial division of the vena saphena parva across the lateral surface of the biceps muscle, and finally thus reach the pelvic cavity. Here they join the lymphatics of the pelvic region, which all pour their lymph at last into the thoracic duct. Inguinal glands are either very small or not present in the cat.

The pelvic limbs have perhaps a deep system of lymphatics, accompanying the deep veins; if so, they are not easily demonstrable.

[Illustration: FIG. 133.--CRANIAL PORTION OF THE SPINAL CORD, WITH THE CERVICAL ENLARGEMENT. DORSAL VIEW.

1-8, the eight cervical nerves. _I_-_II_, first two thoracic nerves. _a_, dorsal roots (ventral roots not shown); _b_, spinal ganglia; _c_, dorsal rami; _d_, ventral rami; _e_, posterior median sulcus; _f_, posterior lateral sulcus.]

NERVOUS SYSTEM.

I. THE CENTRAL NERVOUS SYSTEM.

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.

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.

The central canal of that part of the medulla which is overhung by the cerebellum is greatly widened (Fig. 141, _h_). The cavity thus formed is flattened and triangular, with the apex of the triangle caudad, and is the caudal part of the =fourth ventricle= (Fig. 141, _h_). The roof is very thin (Fig. 143, _n_) and is intimately connected with the pia mater, so that in removing the latter the thin roof of the fourth ventricle is removed with it and the cavity of the fourth ventricle is left exposed. The thin roof of the fourth ventricle is known as the =velum medullare posterius= (Fig. 143, _n_).

The ventral (anterior) fissure (Fig. 138, _p_) of the cord passes onto the medulla oblongata and ends at the pons (_i_), its end being slightly deeper and forming what is known as the =foramen cæcum= (_q_). The dorsal (posterior) sulcus (Fig. 141, _a_) is well marked; it ends at the caudal boundary of the fourth ventricle (_h_).