( IliCTLATION OF THE BLOOD. 



133 



other, and give off the capillaries which, by their anastomoses, com- 

 pose a continuous and uniform network, from which the venous radicles 

 take their rise (Fig. 114). The point at which the 

 arteries terminate and the minute veins commence, 

 cannot be exactly defined, for the transition is 

 gradual; but the capillary 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 net- 

 work that they compose are more uniform in shape 

 and size 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 com- 

 posed of a single layer of elongated or radiate, flat- 

 tened and nucleated cells, so joined and dovetailed 

 together as to form a continuous transparent mem- 

 brane (Fig. 115). 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 nu- 

 cleated, 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-stomafa may be seen 



FIG. 114. Blood-vessels of 

 an intestinal villus, repre- 

 senting the arrangement of 

 capillaries between the ulti- 

 mate venous and arterial 

 branches ; a, a, the arteries ; 

 6, the vein. 



FIG. 115. Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothe- 

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



resembling those in serous membranes (p. 296). The endothelial cells are 

 often continuous at various points with processes of adjacent connective- 

 tissue corpuscles. 



