288 MANUAL OF PHYSIOLOGY. 



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

 of blood being required in the parts where energetic duties are 

 performed. 



FIG. 127. 



Capillary network of Fat Tissue. (Klein.) 



The venous radicles arise from the capillary network, com- 

 mencing as tributaries which unite in much the same way as the 

 arterioles divide, but they form wider and more numerous chan- 

 nels. They rapidly congregate into comparatively large vessels, 

 which frequently intercommunicate so as to form coarse and ir- 

 regular 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 

 differing but little from those of the arteries. The middle coat, 

 however, in the large veins is distinguished from that of the large 

 arteries by being much thinner, owing to the paucity of yellow 

 elastic tissue. It is also characterized by its relative richness in 



