Part 74

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.

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.

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.]

a, dorsal root; b, spinal ganglion; c, dorsal ramus; d, ventral ramus; e, ventral root.]