THE BLOOD-VASCULAR SYSTEM 



111 



into the sinusoids, which in turn unite in the tributaries of the 

 hepatic veins. In lower vertebrates and in the embryos of mammals 

 where the kidney is a mesonephros (p. 126), a second system of 

 venous capillaries occurs in that organ and is known as the renal 

 portal system. 



* es_ cd 



>-vpcl 



Fig. 62. Dorsal aspect of heart of rabbit: ad. as, right and left atria; 

 apd, aps, right and left pulmonary arteries; cd, cs, right and left common 

 carotid arteries; in, innominate artery; la, arterial ligament (cut); sd, ss, 

 right and left subclavian arteries; vcd, vcs, right and left superior caval 

 veins; vci, inferior caval vein; vpd, vps, right and left pulmonary veins. 



The ultimate function of the vascular system is connected with 

 interchange of materials between the tissues and the liquid flowing 

 in the vessels. This is brought about through the medium of 

 microscopic capillaries, the delicate walls of which act as semiperme- 

 able membranes permitting diffusion through them. The gross parts 

 of the system are concerned with transportation of dissolved 

 materials from one part of the body to another. The propulsive 



