THE CIRCULATION OF THE BLOOD. 927 
allowing the corpuscles to be squeezed through; the changes in 
the velocity of the current, especially in the capillaries; its pos- 
sible ‘arrest or retrocession; the velocity in one so much greater 
than in its neighbor, without very obvious cause—all this and 
much more forms, as we have said, a remarkable lesson for the 
thinking student. This, like all microscopic views, especially 
if motion is represented, has its fallacies. It is to be remem- 
Fig. 206.—Circulation in the web of the frog's foot (Wagner). V, venous trunk composed of 
the three principal branches (v, v, v), covered with a plexus of smaller vessels. The whole 
is dotted over with pigment masses, 
bered that the movements are all magnified, or else one is apt 
to suppose the capillary circulation extremely rapid, whereas 
it is like that of the most sluggish part of a stream, and very 
irregular. 
THE CHARACTERS OF THE BLOOD-FLOW. 
If an artery be opened, the 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 
capillaries 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 
