64 THE VITAL PROCESSES 



right hand and arm, clenching and unclenching the fist and flexing the 

 arm at the elbow. Place the hands again side by side and, after wait- 

 ing a minute, observe the increase in the size of the veins in the 

 hand exercised. How is this accounted for ? 



To show the Effect of Gravity on the Circulation. Hold one hand 

 high above the head, at the same time letting the other hand hang 

 loosely by the side. Observe the difference in the color of the hands 

 and the degree to which the large veins are filled. Repeat the experi- 

 ment, reversing the position of the hands. What results are observed? 

 In what parts of the body does gravity aid in the return of the blood to 

 the heart? In what parts does it hinder? Where fainting is caused by 

 lack of blood in the brain (the usual cause), is it better to let the patient 

 lie down flat or to force him into a sitting posture? 



To study the Circulation in a Frog's Foot (Optional). A com- 

 pound microscope is needed in this study and for extended examination 

 it is best to destroy the frog's brain. This is done by inserting some 

 blunt-pointed instrument into the skull cavity from the neck and moving 

 it about. A small frog, on account of the thinness of its webs, gives the 

 best results. It should be attached to a thin board which has an open- 

 ing in one end over which the web of the foot may be stretched. 

 Threads should extend from two of the toes to pins driven into the 

 board to secure the necessary tension of the web, and the foot and 

 lower leg should be kept moist. Using a two-thirds-inch objective, 

 observe the branching of the small arteries into the capillaries and the' 

 union of the capillaries to form the small veins. The appearance is 

 truly wonderful, but allowance must be made for the fact that the 

 motion of the blood is magnified, as well as the different structures, and 

 that it appears to move much faster than it really does. With a still 

 higher power, the movements of the corpuscles through the capillaries 

 may be studied. 



NOTE. To perform this experiment without destroying the brain, 

 the frog is first carefully wrapped with strips of wet cloth and securely 

 tied to the board. The wrapping, while preventing movements of the 

 frog, must not interfere with the circulation. 



