THE FRONTIERS OF MEDICINE 



investigated more thoroughly, and their functional in- 

 gredients determined. Already one can take a concentrated 

 powder instead of eating large quantities of raw liver; 

 and at least one liver extract is being prepared as a pure 

 chemical. 



The knowledge of the platelets in the blood is even more 

 recent than that of the red cells. These platelets are seen 

 only with great difficulty, and can be counted only by 

 special technics which have been developed, none of which 

 is of any great accuracy. The platelets are concerned with 

 the ability of the blood to clot promptly when it escapes 

 from a blood vessel. It is difficult to conceive of anything 

 more important to a person's life than the ability to stop 

 bleeding. Because of the importance of this reaction, 

 nature has apparently developed a number of effective 

 mechanisms. The blood contains a substance called fibrin, 

 which is concerned in the formation of the clot. Unques- 

 tionably in association with the fluid constituents of the 

 blood other materials may be involved. Most physiologists 

 believe that the platelets have a prominent activity in this 

 direction. 



If the number of platelets is greatly reduced, the person 

 bleeds easily under the skin. In some infectious diseases 

 the number of platelets is promptly reduced, and one of 

 the most significant manifestations of these diseases 

 is the collection of large black and blue spots over the 

 body. 



The medical profession knows a good deal about the 

 blood platelets, but there is far more unknown than known. 

 It knows a great deal about the clotting of blood, but it 

 needs to know much more. Tests have been devised which 

 indicate that the blood clots normally in from one to three 

 minutes after bleeding commences. The clotting can be 

 hastened by squeezing the bleeding spot, by pressing on the 

 blood vessel from which the blood comes, by applying 



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