STRUCTURE OF THE VERTEBRATES 243 



suprarenals and the phrenic veins. The vena cava then passes 

 through the liver and empties into the auricle. 



The hepatic portal system is composed of the following major 

 branches: (1) the superior mesenteric; (2) splenic; (3) inferior 

 mesenteric; (4) the gastric; and (5) the pyloric vein. The com- 

 pleted hepatic portal vessel is short, and as it reaches the liver 

 it divides into right and left branches. The right is the larger 

 vessel. In the liver the vessels break into capillaries and sinuses. 

 The blood from the liver is collected into several small veins, 

 and the two large hepatic veins which enter the vena cava before 

 it passes through the diaphragm. 



The heart consists of the four chambers typical of the mam- 

 mals, the auricles being anterior and the ventricles posterior. It 

 is roughly triangular in shape, the apex pointing downward, and 

 located usually in the fifth intercostal space; that is, between 

 the fifth and sixth rib cartilages. About one-third of the heart 

 lies to the right of the median line, the axis of the organ passing 

 from right to left. The aortic arch usually lies behind the 

 sternum. There is considerable range in the size and shape of 

 the heart, both being correlated with the general shape of the 

 body, the tall thin individual tending to have a slender heart, 

 while the broad type tends to have a shorter rounder one. 



D. Lymphatic System 



Although the veins and arteries of the vertebrate form a 

 closed system, a certain amount of the blood fluid passes through 

 the capillary walls, or is forced through their inter-cellular 

 spaces. This fluid is the lymph and lacks most of the corpuscles, 

 or cells, of the blood, and some of the coagulating elements. The 

 lymph bathes each cell of the body and carries food m.aterials 

 and free oxygen. This fluid is collected from the tissues by a 

 definite system of vessels, called the lymphatics, and is returned 

 by them to the veins. Failure of the lymphatic system to func- 

 tion properly causes an accumulation of fluid in the tissues or 

 body cavities, and the affected region becomes swollen or edema- 

 tous. There are many other causes of edema (infections, heart 

 trouble, and others) but during embryonic life lymphatic edema 

 is perhaps most frequent. 



