156 



THE CIRCULATION. 



account of their minute size ; and intermediate vessels, on 

 account of their position. The point at which the arteries 



terminate and the minute veins 

 commence, cannot be exactly de- 

 nned, for the transition is gradual ; 

 but the intermediate network has, 

 nevertheless, this peculiarity, that 

 the small vessels which compose 

 it maintain the same diameter 

 throughout; they do not diminish 

 in diameter in one direction, like 

 arteries and veins ; and the 

 meshes of the network that they 

 compose are more uniform in 

 shape and size than those formed 

 by the anastomoses of the minute 

 arteries and veins. 



The structure of the capillaries 

 is much more simple than that of 

 the arteries or veins. Their walls 

 are composed of a single layer of elongated or radiate, 

 flattened and nucleated cells, so joined and dovetailed 

 together as to form a continuous transparent membrane 

 (fig. 49). Outside these cells, in the larger capillaries, 

 there is a structureless, or very finely fibrillated membrane, 

 on the inner surface of which they are laid down. 



The diameter of the capillary vessels varies somewhat in 

 the different textures of the body, the most common size 

 being about T oVo tn f an ^ ncn - Among the smallest may 

 be mentioned those of the brain, and of the follicles 

 of the mucous membrane of the intestines ; among the 



* Fig. 48. Blood-vessels of an intestinal villus, representing the 

 arrangement of capillaries between the ultimate venous and arterial 

 branches ; a, a, the arteries ; b, the vein. 



