RAT, A REPRESENTATIVE MAMMAL 129 



esophagus, stomach, duodenum, liver, pancreas and spleen; superior 

 mesenteric artery to the pancreas, small intestine, cecum, and colon; 

 infer'ior mesenteric artery to the colon and rectum; lumbar arteries, 

 paired to the back; renal arteries, paired to the kidneys; and finally 

 at the posterior, the division into the two common iliac arteries in the 

 pelvic region. These each branch into external and internal iliac ar- 

 teries and supply the pelvis and hind limbs. The aorta proceeds pos- 

 teriorly as the caudal (coccygeal) artery to supply the tail. 



In the tissues of the organs over the body the arteries continue to 

 branch and rebranch until their diameter becomes small enough to 

 allow them to pass between individual cells. These minute vessels 

 are called capillaries and while in them the blood exchanges oxygen 

 and food material for carbon dioxide and excretory wastes from the 

 cells of the tissues. After passing among the cells of an organ the 

 capillaries converge to form small veins (venules), which join each 

 other in forming the larger ones. 



The large veins joining the heart have already been mentioned. 

 They are the postcava from the posterior and precavae from the an- 

 terior. The single postcava is formed by the union of the caudal and 

 the paired common iliacs in the pelvis. This vessel lies in the mid- 

 dorsal line, receiving paired renal veins from the kidneys, genital 

 veins from the gonads, and the suprarenal veins from suprarenal 

 glands. At the level of the liver, hepatic veins join the postcava. 

 Anterior to this it enters the right atrium. The blood in the stomach, 

 intestines, pancreas, spleen is collected by the branches of the hepatic 

 portal vein, which delivers the blood to the liver where it distributes 

 through the special capillaries (sinusoids) and is collected by the 

 hepatic veins. The precaval veins are formed one on each side by 

 the subclavian veins from the forelimbs, the internal and external 

 jugular veins from the head and neck, and the vertebral from the 

 brain. The precavae, like the postcava, enter the right atrium of the 

 heart bringing in the venous blood. 



Blood. — Generally speaking, thirty rats are approximately equal 

 to one human metabolically. If this holds true in regard to the 

 quantity of blood in the body of a rat, it has approximately 170 c.c. 

 of blood since it is held that the human body carries between 5 and 

 6 liters. Blood is a scarlet substance composed of a fluid in which 

 cells float. Its specific gravity is about 1.055. The liquid basis is 

 called plasma, and the cells are known as corpuscles. Plasma con- 



