132 



THE CIRCULATION. 



FIG. 48. 



spleen, the transmission of the blood from the minute branches 

 of the arteries to the minute veins is effected through a net- 

 work of microscopic vessels, in the meshes of which the proper 

 substance of the tissue lies (Fig. 48). This may be seen in all 

 minutely injected preparations ; and during life, by the aid of 

 the microscope, in any transparent vascular parts, such as the 

 web of the frog's foot, the tail or external branchiae of the tad- 

 pole, or the wing of the bat. 



The ramifications of the minute arteries form repeated an- 

 astomoses with each other and give off the capillaries which, 

 by their anastomoses, compose a continuous and uniform net- 

 work, from which the venous radicles, on the other hand, take 

 their rise. The reticulated vessels connecting the arteries and 

 veins are called capillary, on account 

 of their minute size ; and intermedi- 

 ate vessels, on account of their po- 

 sition. The point at which the ar- 

 teries terminate and the minute veins 

 commence, cannot be exactly defined, 

 for the transition is gradual ; but the 

 intermediate network has, neverthe- 

 less, 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 ghape 

 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 elon- 

 gated or radiate, flattened and nu- 

 cleated cells, so joined and dovetailed 

 together as to form a continuous 

 transparent membrane (Fig. 49). 

 Outside these cells, in the larger cap- 

 illaries, 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 --th of an inch. Among the smallest may be men- 



Bloodvessels of an intestinal 

 villas, representing the ar- 

 rangement of capillaries be- 

 tween the ultimate venous and 

 arterial branches ; a, a, the ar- 

 teries; 6, the vein. 



