168 THE CIRCULATION 



is capable of a distinct chemical reaction (as potassium ferro- 

 cyanide, or barium nitrate) be introduced into a vein on one 

 side of a horse, and at the end of twenty or thirty-two seconds 

 blood be taken from a distant vein on the other side, its 

 presence may be detected. In man, the blood moves with 

 greater speed, and the circuit is completed in twenty-four 

 seconds. 



43. What length of time is required for all the blood of the 

 body to make a complete round of the circulation? This 

 question cannot be answered with absolute accuracy, since the 

 amount of the blood is subject to continual variations. But, 

 if we assume this to be one-eighth of the weight of the body 

 about eighteen pounds it will be sufficiently correct for our 

 purpose. Now to complete the circuit, this blood must pass 

 -once through the left ventricle, the capacity of which is two 

 ounces. Accordingly, we find that, under ordinary circum- 

 stances, all the blood makes one complete rotation every two 

 minutes passing successively through the heart, the capil- 

 laries of the lungs, the arteries, the capillaries of the extremi- 

 ties, and through the veins. 



44. Assimilation. The crowning act of the circulation 

 the furnishing of supplies to the different parts of the body 

 is effected by means of the capillaries. The organs have been 

 wasted by use ; the blood has been enriched by the products 

 of digestion. Here, within the meshes of the capillary net- 

 work, the needy tissues and the needed nutriment are brought 

 together. By some mysterious chemistry, each tissue selects 

 and withdraws from the blood the materials it requires, and 

 converts them into a substance like itself. This conversion 

 of lifeless food into Jiving tissue is called assimilation. The 

 process probably takes place at all times, but the period espe- 

 cially favorable for it is during sleej). Then the circulation is 

 slower, and more regular, and most of the functions are at 

 rest. The body is then like some trusty ship, which, after a 

 long voyage, is "hauled up for repairs." (Read Note 15.) 



48. Time required for all the blood to circulate completely around ? 



44. What is meant by assimilation ? What can you say of its use, etc. ? Time ? 



