THE CAPILLARIES. 



169 



veins are called capillary, on account of their minute size ; 

 and intermediate vessels, on account of their position. 



Fig. 49.* 



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 anas- 

 tomoses of the minute arteries and 

 veins. 



The diameter of the capillary 

 vessels varies somewhat in the 

 different textures of the body, the most common size 

 being about 3 * O th of an inch. Among the smallest may 

 be mentioned those of the brain, and of the follicles 

 of the mucous membrane of the intestines ; among the 

 largest, those of the skin, and especially those of the 

 medulla of bones. 



The form of the capillary network presents considerable 

 variety in the different textures of the body : the varieties 

 consisting principally of modifications of two chief kinds 

 of mesh, the rounded and the elongated. That kind in 

 which the meshes or interspaces have a roundish form is 

 the most common, and prevails in those parts in which the 



* Fig. 49. 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. 



