Part 73

3. =Vena cava inferior= (Fig. 129, c; Fig. 126, a).

The inferior vena cava (Fig. 126, a) is formed at about the level of the last lumbar vertebra by the union of the two =common iliac= veins (Fig. 126, w). It passes craniad near the dorsal median line, lying at first dorsad of the aorta (b), then to the right, then ventrad. It enters the substance of the liver in the dorsal part of the caudate lobe of the latter, passes through the liver, and then through the diaphragm near the ventrolateral edge of the central tendon. It passes then craniad in the thoracic cavity (Fig. 129, c) ventrad of the caudal lobe of the right lung, and enters the right auricle.

It receives the following branches (Fig. 126): =lumbar=, =iliolumbalis= (v), =spermatica interna= (u), =renal= (t), =adrenolumbalis= (s), =phrenic=, and =hepatic= veins. All but the last two named accompany the arteries of the same name.

=V. phrenica.=--The phrenic veins gather the blood from the diaphragm and empty into the vena cava as it penetrates the diaphragm.

=V. hepatica.=--The hepatic veins vary in number. They gather the blood from the liver (sent in by the portal vein and hepatic arteries) and enter the vena cava just caudad of the diaphragm.

VENA PORTÆ. THE PORTAL VEIN (Fig. 131).--The =portal= vein is the large vein carrying the blood from the abdominal viscera to the liver. Within the liver the portal vein breaks up into capillaries; these collect to form the =hepatic= veins, which enter the vena cava inferior. The portal vein (a) is formed near the pyloric end of the stomach (1) by the union of the =superior mesenteric= (b) and =gastrosplenic= (c) veins and passes thence along the ventral border of the foramen epiploicum (foramen of Winslow) to the liver (2), where it divides, going to the lobes of the liver. On its way to the liver it may receive the =pancreaticoduodenalis= (d), =gastroepiploica= (e), and =coronaria ventriculi= (f). These may unite with the portal separately, or any two or all three may unite to form a single trunk before entering the portal vein.

1. =V. coronaria ventriculi= (f) gathers the blood from the lesser curvature of the stomach and anastomoses with the gastrosplenic veins. It usually empties into the portal vein near the pylorus.

2. =V. pancreaticoduodenalis= (d) receives the blood from the pancreas and first part of the duodenum, and empties into the vena portæ near to or with the preceding.

a, portal vein; b, superior mesenteric; c, gastrosplenic; d, pancreaticoduodenalis; e, gastroepiploica; f, coronaria ventriculi; g, h, i, branches of gastrosplenic; j, inferior mesenteric. 1, stomach; 2, liver; 3, duodenum; 4, spleen.]

3. =V. gastroepiploica= (e) comes from the greater curvature of the stomach and ascending limb of the great omentum. It empties into the vena portæ ventrodextrad of the opening of the coronaria ventriculi, or sometimes in common with it.

4. =V. gastrolienalis= (c).--The gastrosplenic is one of the veins which unite to make up the portal vein. It is formed by three branches: one (g) from the horizontal or gastric portion of the pancreas, one (h) from the stomach and gastric end of the spleen, the third and largest (i) from the caudal end of the spleen and descending limb of the great omentum. The three branches unite and the common trunk joins the superior mesenteric vein (b) to form the portal vein as above described.

5. =V. mesenterica superior= (b).--The superior mesenteric vein receives the blood from the small and large intestines. It is formed by numerous branches which follow and have the same name as the corresponding arteries. It receives also the =inferior mesenteric= (j) from the large intestine. The numerous branches unite, and the main trunk thus formed joins the gastrosplenic (c), and the two united become the portal vein (a).

VENA ILIACA COMMUNIS. COMMON ILIAC VEIN (Fig. 126, w; Fig. 127, page 310).

The common iliac veins (Fig. 126, w) are the two large vessels which unite in the sacral region to form the inferior vena cava (a). Each is a large vessel, usually four or five centimeters in length, which is formed by the junction of the large =external iliac= vein with the smaller =hypogastric= vein (z). The left common iliac usually receives the =V. sacralis media= (o); other lateral branches are as a rule not received by the common iliac veins.

There is much variation as to the formation of the common iliac veins and their union to form the vena cava. On this subject the paper by McClure, in the American Naturalist, vol. XXXIV. pp. 135-198 (March, 1900), may be consulted with profit. The more typical variations may be classified as follows: (1) The two common iliac veins may be longer than usual, so that the vena cava is formed farther craniad than usual. (2) There may be in the abdominal region two separate veins (=venæ cardinales=) representing the inferior vena cava, these uniting in the region of the kidneys. (3) The vena sacralis media may enter into the left common iliac (the usual condition); or the right common iliac (as in Fig. 126); or it may fork, one branch passing to the left, the other to the right common iliac.

Branches of the common iliac veins:

1. =V. sacralis media= (Fig. 126, o).--This follows the course of the corresponding artery and usually enters the left common iliac.

2. =V. hypogastrica= (or =iliaca interna=) (z).--The hypogastric or internal iliac vein joins the medial side of the external iliac to form the common iliac (w). Its branches follow the branches of the corresponding artery except that the vein from the bladder (umbilical vein) joins the =V. hemorrhoidalis media=. Also the =vena glutea inferior= receives a large superficial communicating branch (Fig. 163, k′) from the =vena saphena parva=. This branch leaves the saphena parva (Fig. 163, k) at the popliteal space, and passes over the lateral surface of the biceps muscle to its proximal end. Here it passes between the biceps and caudofemoralis to join the vena glutea inferior.

3. =V. iliaca externa= (Fig. 127, page 310).--This vein follows the corresponding artery, collecting the blood from the posterior extremity. Its branches are the same and have in general the same distribution as those of the artery. The vein has, however, certain branches in addition to those of the artery, resulting in a somewhat different general arrangement of vessels; it will be necessary therefore to give an account of the system of veins from the foot and lower leg.

The blood is collected from the foot into two sets of veins which may be distinguished as the deep and the superficial sets. The veins of the deep set are those which accompany the branches of the popliteal and anterior tibial arteries. Those of the superficial set are larger and carry most of the blood. On the dorsum of the foot blood is collected from the third, fourth, and fifth digits by two or three superficial metatarsal veins, which unite opposite the proximal end of the metatarsus into a single vein. This receives a similar branch coming from the dorsum of the second digit, and has numerous branches in the ankle region which form here a network, communicating with the vena saphena parva (Fig. 163, k), and with branches from the plantar side of the foot. From this network two or three main veins pass proximad on the medial side of the leg, along with the branches of the arteria saphena, and unite with a similar vein from the plantar side of the foot to form the vena =saphena magna= (Fig. 127, k, page 310), which accompanies the arteria saphena to join the femoral vein (a).

On the plantar side of the foot small veins pass from the digits to a superficial =venous plantar arch=, which lies just proximad of the fibrous pad on the sole of the foot. At its medial side this arch extends proximad and dorsad to join that branch of the vena saphena magna which comes from the dorsum of the second digit. On the lateral side it extends proximad along the lateral margin of the foot, sends a communicating branch dorsad to the branches of the V. saphena magna, and continues along the lateral border of the foot to the ankle. It forms the beginning of the =V. saphena parva= (Fig. 163, k, page 401). This receives branches from the lateral surface of the ankle, which anastomose with those of the saphena magna. The saphena parva then passes proximad along the lateral surface of the leg, close to its ventral border, to the popliteal space. At the distal end of the popliteal space it divides into two branches. One passes inward through the fat of the popliteal space, receiving branches from the lymphatic gland situated here, and joins the popliteal vein. The other (k′) remains superficial and continues proximad over the lateral surface of the biceps to the proximal end of that muscle. Here it passes between the biceps and caudofemoralis to join the inferior gluteal vein.

IV. THE LYMPHATIC SYSTEM. SYSTEMA LYMPHATICUM.

The lymphatic system of the cat has not been worked out in detail, so that only the main features of the system are given in the following account.

The lymphatic system consists of a number of vessels, the =lymphatics=, containing a colorless fluid called =lymph=, and of =lymphatic glands=, connected with the lymphatic vessels. The lymphatic vessels are found throughout the body as slender tubes, frequently united into networks, and containing many valves. They take origin from the spaces in the connective tissue, so that they are at first without definite walls. The fluid in the connective-tissue spaces gradually flows together into tubes with definite walls, and these tubes, the lymphatic vessels, finally join the venous system. In their passage through the lymphatic glands the vessels break up into fine channels which again unite on leaving the gland. In the gland lymph-cells are added to the fluid, and probably other changes are produced.

Most or all of the lymphatic vessels of the body finally unite into two trunks before entering the veins. The largest trunk is the =thoracic duct= (Fig. 118, 5, page 282), which collects the lymph from the entire caudal half of the body, and from the left forelimb and the left side of the thorax, head, and neck. It passes along the dorsosinistral side of the thoracic aorta, lying against its surface, and enters the left external jugular vein at its junction with the subclavian. The second main trunk is the =right lymphatic duct=; this collects the lymph from the right side of the thorax, the right forelimb, and the right side of the neck and head; it enters the right external jugular.

1. =Lymphatics of the Head.=--A number of lymphatic vessels arise on the sides of the face, especially in the upper and lower lips. They form a superficial network of vessels, lying over the ventral half of the masseter muscle and an area ventrad of it. These lymphatic vessels all enter two large lymphatic glands (Fig. 131, 12, page 322) lying at the ventrocaudal angle of the masseter muscle, covering the union of the anterior and posterior facial veins. Into these glands pass also a number of lymph vessels from the back of the head. From these two lymphatic glands two or three small lymphatic vessels pass caudad, lying on the surface of the external jugular vein. Near the point of the shoulder, some distance craniad of the junction of the external and internal jugular veins, these vessels enter another small lymphatic gland lying on the dorsal surface of the external jugular vein. Thence one or two vessels continue caudad, still on the surface of the external jugular, and finally unite with the deep lymphatics of the head and neck and those of the arm to enter the caudal end of the external jugular vein, usually (on the left side) after junction with the =thoracic duct=, from the thorax.

The deep lymphatics of the head come from the internal parts of the head,--tongue, pharynx, etc., and enter a large lymphatic gland situated close to the caudoventral surface of the tympanic bulla, at the side of the pharynx. Thence a large lymphatic trunk passes caudad at the side of the trachea, along with the common carotid artery and internal jugular vein, unites with the superficial vessels from the head, above described, joins (on the left side) the thoracic duct, and enters the external jugular vein.

2. =Lymphatics of the Neck.=--Two or three lymph-glands are found in the fatty mass in the neck beneath the clavotrapezius and levator scapulæ ventralis muscles. These receive lymphatics from the neck and side of the thorax. The vessels which arise from them join those from the head and arm to enter the external jugular vein.