62 PHYSIOLOGY. 



fainted should be laid flat on his back, that the heart may 

 more easily drive blood to the brain. Many persons go to 

 sleep more readily while sitting than while lying down. 

 A sore hand feels less pain if held up, as in a sling, than 

 when hanging by the side, and a sprained ankle does 

 better rested on a chair, as less blood flows to it. Nearly 

 every one has noted the pain following the pressure of 

 blood when a sore hand, or foot, is suddenly lowered. 



Experiments illustrating the Effect of Gravity on Circulation. 

 Let all the pupils in the class stand. Let one arm hang freely by the 

 side. Hold the other arm straight up as far as the clothing will readily 

 permit. Observe : 



1. The difference in the color of the two hands. 



2. The difference in fullness, both in the feeling of fullness and in 

 the prominence of the veins. 



3. The difference in temperature ; place the backs of the hands 

 against the cheeks. 



The position largely determines the amount of blood in the hand, 

 and the amount of blood determines the temperature, the size, and the 

 color. 



Summary. i. The heart beats about seventy-two times a minute. 



2. The pulse is a wave running along an artery. 



3. The pulse varies with age, health, food, etc. 



4. The heart has two main cavities, one in each half of the heart, 

 and two independent streams are flowing through it. 



5. Valves allow the blood to flow through the heart in one direc- 

 tion, but prevent a reversal of the current. 



6. The heart is a hollow muscle, and. by contraction forces the 

 blood out into the arteries. 



7. The heart works rather less than half the time. 



8. The large arteries, by elastic reaction, push the blood on while 

 the heart is resting. 



9. Circular muscle fibers in the walls of the medium-sized arteries 

 regulate the blood supply to the organs. 



10. In the arteries the blood flow is rapid and intermittent, in the 

 capillaries slow and constant. 



