RESPIRATORY SYSTEM. 715 
the hind-legs, and give off in the abdomen several branches to the 
abdominal walls, the pelvic cavity, the bladder, and the uterus, 
The two superior venz cave bring blood’ from the head, neck, 
thorax, and fore-limbs. Each is formed from the union of— 
a subclavian from the shoulder and fore-limb, 
an external jugular from the face and ear, 
an internal jugular from the brain, 
an anterior intercostal from the spaces between the anterior ribs, 
an internal mammary from the ventral wall of the thorax ; 
and the right superior vena cava also receives an azygos cardinal vein, 
which runs along the mid-dorsal line and collects blood from the 
posterior intercostal spaces. 
The inferior vena cava is a large median vein lying beside the aorta 
beneath the backbone. Anteriorly it is embedded in the liver, and 
receives the hepatic veins. Thence it passes through the diaphragm 
into the right auricle. Posteriorly the inferior vena cava has the 
following components :— : 
Internal iliacs from the back of the thighs, forming by their union 
the beginning of the inferior vena cava ; 
femoral veins from the inner borders of the thighs, continued into 
external iliacs which open into the inferior vena cava ; 
paired ilio-lumbars from the posterior abdominal walls ; 
spermatic or ovarian veins from the reproductive organs ; 
renal veins from the kidneys. 
There is no renal-portal system. 
The food which has been digested—rendered soluble and diffusible— 
passes from the food canal into the vascular system by two paths— 
(a) All except the fatty material is absorbed by veins from the 
stomach and intestine. These unite in a main trunk, the 
portal vein. The components of the portal vein are—the lieno- 
gastric from the stomach (and also from the spleen), the 
duodenal from the duodenum (and also from the pancreas), 
the anterior mesenteric from the intestine, the posterior 
mesenteric from .the rectum. The portal vein breaks up 
into branches in the liver, whence the modified blood passes 
by hepatic veins into the inferior vena cava. 
(6) The fat passes through the intestinal villi into the lymphatic 
vessels, which combine to form a thoracic duct, which runs 
forward and opens into the left subclavian vein at its junction 
with the left external jugular. Here and there lie lymphatic 
glands. 
Respiratory system.—The lungs are pink, spongy bodies, 
lying in the thorax, connected with the exterior by the 
bronchial tubes and the trachea, and with the heart by 
blood vessels. The pleural membrane which invests the 
surface of the lungs is reflected from the sides of the 
thoracic cavity. When the lungs expand, the pleural 
cavity—between the two folds of pleural membrane—is 
