138 



ELEMENTARY PHYSIOLOGY 



produce it, hence the reason why the walls of the right ventricle 

 are so much thinner than those of the left. 



The shortest course which the blood can possibly take is, 

 however, not through the lungs, but through the coronary system, or 

 system of the heart. In this instance it enters the coronary arteries 

 just outside the semilunars of the aorta, passes through a capillary 

 network, and is then collected up by the branches of the coronary 

 veins, which pour it direct into the right auricle. Thus the blood 

 completes a circulation without ever leaving the heart. 



The course of the blood in the body was first demonstrated by 

 Harvey in the seventeenth century, and the following were among 

 the proofs he gave as the results of his experiments : 



1. The heart propels in half an hour more blood than would 

 make up the weight of the whole body. 



2. The spurting of blood from the arteries corresponds with 

 the beats of the heart. 



3. Tie the great veins and the heart becomes pale and bloodless. 



4. Tie the great arteries, and the heart is unduly distended. 



5. All the valves are so arranged as to allow of a passage of 

 blood in one direction only. 



6. Poisons injected into a single blood-vessel soon produce 

 an effect on the whole body. 



To these may also be added 



7. When an artery is cut, the bleeding may be stopped by press- 

 ing on the part above the wound, i.e. on the side nearer the heart. 



8. When a vein is cut, we stop the bleeding by pressing 

 on the portion below the wound. 



9. The blood may actually be seen in motion, by the aid of a 

 microscope, in the capillaries of the web of a frog's foot. 



BLOOD- 

 VESSELS 



Arteries . . 



Capillaries . - 



Veins . 



Coats . 



SUMMARY. 



Take blood from the heart. 

 (I. Outer areolar. 

 2. Middle muscular and elastic 



thick. 



1 3. Inner epithelial. 

 Assist in propelling blood by their elastic 



recoil. 



Connect small arteries with small veins. 

 Arranged like networks. 

 Very thin walls. 

 ( Diameter about ^^ inch. 

 Convey blood to the heart. 

 Coats same as in arteries, but less muscu- 

 lar and elastic tissue. 



Generally provided with valves like the 

 semilunars. 



