CIRCULATION' OF THE BLOOD. 63 



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. 



11. The thin walls of the capillaries allow the liquid part of the 

 blood to soak out and nourish the tissues, and to soak back into the 

 capillaries bearing waste matter. 



12. The veins are thin walled, and collapse when empty, while the 

 arteries are thick walled, and stand open when empty of blood. 



13. Arteries carry blood from the heart, while veins carry it toward 

 the heart. 



14. The veins have valves which allow the blood to pass toward the 

 heart, but not away from it. 



15. Any intermittent pressure on the veins aids the blood flow. 



1 6. The blood flow is most rapid in the arteries, slower in the veins, 

 slowest in the capillaries. 



17. Gravity influences circulation. 



Questions. i . Why do the large arteries lie deep ? 



2. In which direction should the limbs be stroked to promote cir- 

 culation ? 



3. How does slapping the hands around the body warm the 

 fingers? 



4. How can a horse or a cow be comfortable with the head down 

 for a long time ? 



5. Why are the walls of the left ventricle thicker than those of the 

 right ? 



