282 MANUAL OF PHYSIOLOGY. 



nitely with the capillary mesh works of the neighboring arterioles, 

 so that any given capillary area appears to be one continuous 

 network of tubules, connected here and there with distinct ar- 

 terioles, and thus is fed with blood from several different sources. 

 The walls of the capillaries are soft and elastic, and permeable 

 not only to the fluid portion of the blood, but also, under certain 

 circumstances, to the solids. 



It is, in fact, in this part of the circulation that its essential 

 function is carried on, viz., the establishment of a free interchange 

 between the tissues and the blood. 



The characters of the capillary network vary in different tissues 

 and different organs: the closeness and wideness of the meshes 

 may be said to be in proportion to the functional activity or in- 



FIG. 127. 



Capillary network of Fat Tissue. (Klein.) 



activity of the organ or tissue in question, a greater amount of 

 blood being required in the parts where energetic duties are per- 

 formed. 



The venous radicles arise from the capillary network, commenc- 

 ing as tributaries which unite in much the same way as the ar- 

 terioles divide, but they form wider and more numerous channels. 

 They rapidly congregate into comparatively large vessels, which 

 frequently intercommunicate so as to form coarse and irregular 

 plexuses. The general arrangement of the structures in the walls 

 of the veins is like that of the arteries; they also have three 

 coats, the external, middle, and internal ; the tissues of each dif- 



