Circulation of the Blood. 69 



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 separate streams are flowing through it. 



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

 tion, but prevent its return. 



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

 blood out into the arteries. 



7. The heart works about 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 jerky, in the capillaries 

 slow and steady. 



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 alternating 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 aid circulation ? 



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 ? 



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

 right ? 



6. Trace a drop of blood from the tip of a finger around the circuit 

 to the same point again. 



7. Does the pulse at the wrist occur at exactly the same time as at 

 the temple ? Or at the same time as the heart-beat ? 



