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HANDBOOK OF PHYSIOLOGY. 



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

 the 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 size than those formed by the anastomoses of the minute ar- 

 teries 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- 



FIG. 104. 



FIG. 105. 



FIG. 104. Blood-vessels of an intestinal villus, representing the arrangement of capillaries be- 

 tween the ultimate venous and arterial branches; a, a, the arteries; b, the vein. 



FIG. 105. Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothe- 

 lial membrane of which they are composed. (Klein and Noble Smith.) 



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

 form a continuous transparent membrane (Fig. 105). 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 endo- 

 thelial cells which compose the capillary wall, pseudo-stomata may be 

 seen (p. 22). The endothelial cells are often continuous at various 

 points with processes of adjacent connective-tissue corpuscles. 



Capillaries are surrounded by a delicate nerve-plexus resembling, in 

 miniature, that of the larger blood-vessels. 



The diameter of the capillary vessels varies somewhat in the differ- 

 ent textures of the body, the most common size being about -g-oV^th of 



