THE CIRCULATION" OF THE BLOOD. 183 



The Capillaries. 



Distribution. In all vascular textures except some parts of the cor- 

 pora cavernosa of the penis, and of the uterine placenta, and of the 

 spleen, the transmission of the blood from the minute branches of the 

 arteries to the minute veins is effected through a network of capillaries. 

 They may be seen in all minutely injected preparations. 



The point at which the arteries terminate and the minute veins com- 

 mence, cannot be exactly denned, for the transition is gradual; but the 



Fig. 156. Ramification of nerves and termination in the muscular coat of a small artery of the 



frog. (Arnold.) 



capillary 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 ize than those formed by the anastomoses of the minute 

 arteries and veins. 



Structure. This is much more simple than that of the arteries or 

 veins. Their walls are composed of a single layer of elongated or radi- 

 ate, flattened and nucleated cells, so joined and dovetailed together as 

 to form a continuous transparent membrane (fig. 157). 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. 

 In some cases this external membrane is nucleated, and may then be 

 regarded as a miniature representative of the tunica adventitia of arteries. 

 Here and there at the junction of two or more of the delicate endothe- 

 lial cells which compose the capillary wall, pseudo-stomata may be seen. 



