220 



COMPARATIVE PHYSIOLOGY. 



cut anywhere gives rise to a 

 flow of Ulood, owing to sec- 

 tion of some of these tubes, 

 which, it will be remembered, 

 are not visible to the unaided 

 eye. It is estimated that their 

 united area is several hun- 

 dred (500 to 800) times that of 

 the arteries. 



If we suppose the epithe- 

 lial lining pushed out of a 

 small artery we have, so far 

 as structure alone goes, a 

 good idea of a capUlary — i. e., 

 its walls are but one cell 

 thick, and these cells though 

 long are extremely thin, so 

 that it is quite easy to under- 

 stand how it is that the amoe- 

 boid corpuscles can, under 

 certain circumstances, push 

 their way through its proba- 

 bly semi-fluid walls. 



From what has been said, 

 it will be seen that the whole 

 collection of vascular tubes 

 may be compared to two inverted funnels or cones with the 



Fig. 189.— Various layers of the walls of a 

 small artery (Landois). e, endothelium; 

 i. e, internal elastic lamina; 9. m, circu- 

 lar muscular fibers of the middle coat; 

 e. t, connective tissue of the outer coat, 

 or T. adventitia. 



Fig. 190. 



Fig. 191. 



Pig. 190.— Vein with valves lying open (Dalton). 



Fig. 191.— Vein with valves closed, the blood passing on by a lateral branch below 

 (Dalton). 



