THE CIRCULATION 



173 



44. The circulation of the blood, as seen under the micro- 

 scope, in the transparent web of a frog's foot, is a beautiful 

 sight, possessing more than ordinary interest, from the fact 

 that something much grander is taking place in our own bodies. 

 It is like opening a secret page in the history of our own 

 frames. We there see distinctly the three classes of vessels 

 with their moving contents ; first, the artery, with its torrent of 

 blood rushing down from^the heart; secondly, the vein, with 

 its slow, steady stream flowing in the opposite direction ; and 



FIG. 45. WEB OF A FBOQ'S FOOT, 

 slightly magnified 



Fw. 46. MARGIN or FBOG'S WIB, 

 magnified thirty diameters 



between them lies the network of capillaries, so fine that the 

 corpuscles can pass through only " in single file." The current 

 of the capillaries has here an uncertain or swaying motion, 

 hurrying first in one direction, then hesitating, and turning 

 back in the opposite direction, and sometimes the capillaries 

 contract so as to be entirely empty. Certain of the tissues are 

 destitute of capillaries; such are cartilage, hair, and a few 

 others on the exterior of the body. In all other structures, 

 networks of these vessels are spread out in countless numbers ; 

 so abundant is the supply, that it is almost impossible to 

 puncture any part with the point of a needle without lacerat- 



44. The circulation of the blood in the web of a frog's foot ? Describe it. How general 

 is the existence of the tissues ? 



