THE BLOOD AND CIRCULATION 



265 



their walls become thinner and finally lose their muscular 

 coat altogether as they pass into the minute capillaries. 

 If the web of a frog's foot be examined with a compound 

 microscope one may witness the fascinating spectacle of 

 the veins, arteries and capillaries with their flowing cur- 

 rents of blood, and follow the red and white corpuscles 

 as they pass, single file, in their course from the arteries 

 to the veins. It is through the thin and delicate walls 

 of the capillaries that the principal exchanges of material 



FIG. 196. FIG. 197. 



FIG. 196. Diagram of circulation through the capillaries of a frog's 

 foot showing corpuscles in capillaries, p, pigment cells. 



FIG. 197. Valves in a vein. A, valves as they appear when a vein is 

 cut open; B, section of vein to show closing of valve to prevent backward 

 flow of the blood; C, section showing position of valve when blood is flow- 

 ing normally. 



occur between the blood and the tissues. Food and oxygen 

 diffuse from the blood into the tissues and carbon dioxide 

 and other waste products diffuse from the tissues into 

 the blood. The white corpuscles may pass through these 

 delicate walls also, as may sometimes be seen in the web 

 of the frog's foot. The veins, whose function it is to 

 carry the blood back to the heart, are furnished with cup- 

 shaped valves which allow blood to flow past them toward 

 the heart while they fill and block its passage if it should 

 be forced in the opposite direction. Many veins lie 

 nearer the surface than the arteries and they may be 



