226 



COMPARATIVE PHYSIOLOGY. 



Fig. 196.— Circnlatioa in the wob of a frog's foot (Wagner). V, venons trunk com- 

 posed of the three principal branches (?j, v, v") covered with a plexus of smaller ves- 

 sels. The whole is dotted over with pigment masses. 



THE CHARACTERS OF THE BLOOD-FLOW. 



If an artery be opened, tlie blood is seen to flow from it in 

 a constant stream, with periodic exaggerations, which, it is 

 found, answer to the heart-beats ; in the case of veins and 

 xjapillaries the flow is also constant, but shows none of the 

 spurting of the arterial^ stream, nor has the cardiac beat appar- 

 ently an equal modifying effect upon it. 



We have already explained why the flow should be constant, 

 though it would be well to be clearer as to the peripheral re- 

 sistance. The amount of friction from linings so smooth as 

 those of the blood-vessels can not be considerable. Whence, 

 then, arises that friction which, keeps the arterial vessels always 

 distended by its backward influence ? The microscopic study 

 of the circulation helps to answer this question. The plas- 

 ticity of the corpuscles and of the vessel walls themselves must 

 be taken into account, in consequence of which a dragging 

 influence is exerted whenever the corpuscles touch the wall, 

 which must constantly happen with, vast numbers of them in 

 the smallest vessels and especially in the capillaries. The 

 arrangement of capillaries into a mesh-work, must also, in 



