PHENOMENA OF THE CAPILLARY CIRCULATION. 85 



esting as was the action of the heart when first seen by Harvey, involving some of the 

 most important phenomena of the circulation. It can be seen how the arterioles regu- 

 late the supply of blood to the tissues ; how the blood distributes itself by the capilla- 

 ries: and, finally, having performed its office, how it is collected and carried off by the 



veins. 



FIG. 29. Web of the frog's hind-foot; magnified. (Wagner.) 

 a, a, veins ; &, 6, &, arteries. 



In studying the circulation under the microscope, the anatomical division of the 

 blood into corpuscles and a clear plasma is observed. This is peculiarly evident in cold- 

 blooded animals, the corpuscles being comparatively large and floating in a plasma 

 which forms a distinct layer next the walls of the vessel. The leucocytes, which are 

 much fewer than the red corpuscles, are generally found in the layer of plasma. * 



FIG. 30. Circulation in the web of the frog's foot. (Wagner.) 



, are pigmentary matter. <z, a venous trunk, composed of three principal 

 branches (&, 6, 6), and covered with a plexus of smaller vessels (c, c). 



1 Various methods of preparing the animal for examination have been employed. The one we have found most 

 convenient, in examining the circulation in the frog, is to break up the medulla with a needle, an operation which 



