T20 THE CIRCULATION. 



crease in the size of these vessels. A blush is another exam- 

 ple of this, but the excitement comes through the nervous 

 system, and the cause is some transient emotion, either of 

 pleasure or pain. Another example is sometimes seen in 

 purplish faces of men addicted to drinking brandy ; in 

 them the condition is a congestion of the capillary circula- 

 tion, and is permanent, the vessels having lost their power 

 of elastic contraction. 



42. Rapidity of the Circulation. That the blood 

 moves with great rapidity is evident from the almost instant 

 effects of certain poisons, as prussic acid, which act 

 through the blood. Experiments upon the horse, dog, 

 and other inferior animals, have been made to measure 

 its velocity. If a substance, which is capable of a distinct 

 chemical reaction (as potassium ferrocyanide, or barium 

 nitrate), be introduced into a vein of a horse on one side, 

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

 its presence may be detected at the end of twenty or thirty- 

 two seconds. 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 suf- 

 ficiently 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 circumstances, all the blood makes one 

 complete rotation every two minutes; passing successively 

 through the heart, the capillaries of the lungs, the arteries, 

 the capillaries of the extremities, and through the veins. 



42. Show what time is required for a given portion of blood to travel once 

 around the body. 



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



