BLOOD CORPUSCLES. 51 



but comes back only part way. It stops where it reaches the 

 valves, filling the vein full to this point, but leaving it col- 

 lapsed beyond, as shown by the groove. Remove the second 

 finger, and the vein immediately fills from the side nearer the 

 tip of the fingers. 



These experiments show that the blood in the veins moves 

 freely toward the body, but cannot flow outward to the 

 extremities. 



With the finger-tips press on the back of the hand. Note 

 the temporary white spot caused by the pressure. 



Observe the color of the finger-nails in the closed (but not 

 clinched) hand. Now straighten the fingers as far back as 

 possible, and again note the color of the nails. Press the 

 nail on its surface, ends, sides, and note the changes in color. 



A Drop of Frog's Blood. Kill a frog as directed on 

 page 13. If blood enough is not obtained from the wound 

 already made, open the body cavity and cut across the heart. 



Place a small drop of blood on each of several slides and 

 quickly cover with coverslips. 



First examine with a low power, one inch, or half-inch 

 objective. Then with a one-fifth inch objective. There wilJ 

 be seen : 



1. The Colored Corpuscles. They are yellowish pink in 

 color, elliptical in shape. Some of them, seen edgewise, will 

 be seen to be flattened, with a bulging center. This bulge is 

 caused by the Nucleus. 



2. The Colorless Corpuscles, few in number compared with 

 the colored corpuscles, much smaller, ordinarily circular in 

 outline, because they are usually spherical. They are almost 

 always dotted in appearance. If watched closely for some 

 time they may be seen to change their shape, like the Ameba. 

 Watch these changes. Make drawings of the outline at in- 

 tervals of ten seconds. 



