82 PHYSIOLOGY AND TEMPERANCE. 



wound. It sometimes happens that little or no fibrin forms, 

 and the slightest wound bleeds freely. In such a case it 

 is dangerous to have even a tooth extracted. 



4. The Organs of Circulation. The circulation of the 

 blood is carried on by the heart, arteries, capillaries and 

 veins. The blood constitutes the food of the tissues, and in 

 supplying these finds its way to the most remote parts of 

 the body. The circulation of the blood never ceases while 

 life continues. It flows from the heart, bearing the elements 

 of growth and sustenance. It returns to the heart, carrying 

 with it the waste products. The arteries are the channels 

 through which the rich, bright-red blood feeds the tissues. 

 The veins are the tube-like canals through which the dark, 

 impure blood returns. Between the final branches of the 

 small arteries and the small veins lies a great network of 

 capillaries. In these capillaries the elements of the food, 

 digested and carried into the circulation, are incorporated 

 in the living tissue. 



5. The System a Closed Sac. The heart and the 



three varieties of blood-vessels form a cavity in which the 

 blood is confined. The arteries and veins, wifch the capil- 

 laries, are a continuation of the heart. They form a complete 

 circuit, so that the space within is continuous. The blood 

 proper cannot pass through the coats of these vessels at 

 any point. It parts with its nutrient material while it is 

 flowing through the capillaries, but the blood itself cannot 

 pass through the walls, unless they are injured or cut. The 

 prick of a pin, if deep enough, will draw blood, because it 

 pierces one or more of these small vessels. At the same time 

 that it imparts its nourishment to the tissues, it receives from 

 them their worn-out products. The vitiated blood, returning 

 to the heart through the veins, is then sent by a separate 

 system of vessels to the lungs, where it is brought in close 

 relation with the air we breathe. Here the vital change of 



