9 6 



THE FROG 



network into which the blood is passed by the arteries, and 

 from which it is received into the veins. 



FIG. 24. Blood-vessels of the web of a frog's foot seen under a low magnifying power. 



a. small arteries ; v. small veins. The minute tubes joining the arteries to the 



veins are the capillaries. The arrows show the direction of the circulation. 



In the small portion marked off, the pigment cells, which occur throughout 



the web, are also represented. (From Huxley's Physiology.} 



Thus by means of the microscope we are able to take the 

 final step in demonstrating the circulation. The fact that 



