266 PHYSIOLOGY. 



heart.] The reason why the blood moves slower in the arte- 

 ries as it recedes from the heart, is owing, probably, to 

 friction, and the increased capacity of the vessels, for the 

 arterial system is compared to a cone, whose apex is at the 

 heart. The course of blood in the arteries is intermittent, 

 or by jets ; not that it does not flow all the time, but its flow 

 is more or less rapid, according as the ventricles are con- 

 tracting or dilating. 



24^The force with which the blood is thrown from the 

 heart is variously estimated. Hales computed that the left 

 ventricle of a horse exerted a force equal to 113 pounds, and 

 that of a man at 51 pounds. Some think that the contrac- 

 tion must overcome the whole pressure of the air upon the 

 body, which is equal to forty thousand pounds. As we can- 

 not, however, correctly estimate the influence of breathing 

 and other causes, we cannot, with any degree of certainty, 

 tell what degree of power is exerted by the heart. Dr. 

 Arnott thinks, however, that the heart acts with a force of 

 about six pounds on every square inch, and as the left cham- 

 ber of the heart has about ten square inches, the whole force 

 exerted is sixty pounds. ) 



25. The Blood. The blood is not necessarily red, it may 

 be white, as in the fish ; transparent, as in the insect; yel- 

 lowish, as in the reptile ; and indeed there is no animal in 

 which the blood is red in all the parts of its body. 



26. In a short time after blood is taken from the body it 

 separates into two portions, by a process called coagulation, 

 vi2^1. a watery portion called serum, and a solid portion 

 called coagulum or clot ; the white substance which forms 

 the upper part of the clot is called fibrin, and the red mass 

 under it, the red particles. The fibrin is the material from 

 which all the solids of the body are formed^ (See Fig. 6.) 



27. The red particles owe their colour, it is supposed, to 

 the presence of iron; though some say it depends on an ani- 

 mal substance of a gelatinous nature. These are usually 

 described as being minute globules, but the latest microsco- 



