384 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



tissues by the veins ; on the contrary, the arterial 

 pipes are continuous through the capillaries with the 

 veins. Yet the capillaries are quite distinct from both 

 in several respects: 1. In the mode of branching. The 

 arteries branch and rebranch tissueward, growing ever 

 smaller and more numerous; the veins run together 

 heartward, growing ever larger and fewer ; the capil- 

 laries branch and run together again, without change of 

 size and without definite direction, forming a fine net- 

 work (Fig. 264). 2. In the course of the current. The 

 course of the current in arteries and in veins are alike 

 fixed. In capillaries it may go in any direction, although 

 it works deviously toward the veins. 3. In structure. 

 The walls of the capillaries are of extreme thinness, and 

 therefore permeable to the blood plasm, and in case of 

 engorgement even to the leucocytes. 4. In function. 

 The function of the arteries is to carry the blood, without 

 loss by leakage, to the tissues; the function of the veins is 

 to bring it back without loss to the heart ; the function 

 of the capillaries is to allow the blood to exude into the 

 tissues. In any good system of irrigation the pipes car- 

 rying the water to the soil and bringing it back from 

 the soil must be impervious, but those running among 

 the soil must be permeable. The blood system is such 

 a system of pipes. So jealously is this function of 

 nutrition restricted to the capillaries that even the 

 arteries and veins themselves are not nourished by the 

 blood that flows through them, but must have their own 

 arterioles, veinlets, and capillaries (vasa vasorum) for 

 that purpose. In the capillaries, then, the nutritive 

 matters of the plasma and the oxygen of the red glob- 

 ules exude into the tissues (exosmose), and the waste 

 matters of the tissues and the C0 2 transude into the 

 blood (endosmose). But the exact nature of this pro- 

 cess is yet obscure. 



