THE BLOOD 113 



erties displayed by the blood rather than because they can 

 be chemically identified. Illustrations of such are afforded 

 by the internal secretions, enzymes, and the immune bodies. 

 Individual peculiarities of metabolism, susceptibility, and 

 resistance must depend on substances of this obscure 

 class. 



Coagulation. Blood when shed shows the familiar 

 property of clotting. This is a most valuable quality, 

 since it provides for the automatic checking of ordinary 

 bleeding and also forms a protective shield, the scab, be- 

 neath which the healing process may go on. The im- 

 portance of coagulability is emphasized by the rather fre- 

 quent observation of cases in which it is lacking, and in 

 which serious or even fatal hemorrhages follow trifling 

 injuries. The essence of the process is a chemical change 

 affecting one of the minor proteins of the plasma in such a 

 way that it passes into a solid form and cements together 

 the red corpuscles. The original protein is the one already 

 named as fibrinogen. The modified form after coagulation 

 is called fibrin. The actual amount of fibrin is extremely 

 small (perhaps 2 to 4 parts in 1000 of blood), but it is not 

 difficult, when we consider its gummy character, to under- 

 stand how it can convert a liquid medium into a stiff jelly 

 by knitting together the suspended corpuscles. 



The entirely natural impression that coagulation is the 

 result of exposure to the air is erroneous. The matter has 

 been the subject of the most painstaking studies, of which 

 we can give only the briefest summary. The formation of 

 fibrin is an instance of enzyme action, and forcibly recalls 

 the curdling of milk in the stomach under the influence of 

 rennin. The resemblance is not complete in every re- 

 spect, but is still suggestive. If we are to assume that an 

 enzyme exists in the blood at the time of clotting and not 

 before, we must establish its origin. Reducing the facts 

 to the barest essentials, we may make the following state- 

 ment: normal blood contains in great numbers minute and 

 extremely perishable bodies known as the blood-plates. 

 / These are much smaller than the red corpuscles. When 



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