210 THE BLOOD 



The red and white corpuscles having been excluded as possible 

 sources of the thrombocytes, their origin remains much in. the dark. 

 Wright, however, has suggested that they arise from the cytoplasm 

 of the giant cells, the so-called megakaryocytes, which are found in, 

 the marrow of the bones. It is believed that these cells send out 

 pseudopodia which become detached and are carried away in the blood- 

 stream. The observations of Duke^ and others tend to show that the 

 life of the thrombocytes is very short. 



When the blood is shed, the platelets quickly agglutinate, forming 

 globular or irregular masses. Their formerly pointed processes be- 

 come stubby and break off, while their central portions swell up and 

 rupture. Eventually, therefore, the platelets are reduced to chips 

 of insignificant size, many of which soon disappear altogether by dis- 

 solution, but the regions in which the thrombocytes have undergone 

 this disintegration, soon become the seats of active fibrin-proliferation. 

 In this way definite centers of coagulation are formed, from which the 

 different shreds of fibrin gradually extend through the blood in all 

 directions. Practically all the platelets take part in this process so 

 that they finally become intricate constituents of the network of fibrin. 

 The red and white corpuscles, on the other hand, remain normal, 

 because the shreds of fibrin pass by them without actually imbibing 

 them. It has been proven by Biirker^ that the number of the throm- 

 bocytes is proportional to the mass of the fibrin formed, and that this 

 reaction may be varied by changes in temperature as well as by the 

 addition of chemicals. Thus, any agent tending to cause a destruc- 

 tion of the thrombocytes, also hastens the coagulation of the blood, 

 while any substance possessing preservative qualities, not only retards 

 this process, but actually prevents it. The latter end may be attained 

 very readily by the addition of hirudin,^ because this substance pre- 

 serves the thrombocytes. It must be conceded, therefore, that, quite 

 irrespective of the red and white corpuscles, the disintegration of the 

 platelets gives rise to an agent which plays a most important part in 

 the coagulation of the blood. This activating substance is designated 

 as thrombokinase. 



1 Jour, of the Amer. Med. Assoc, Iv, 1910. 



2 Pfltiger's Archiv, cii, 1904, 36. 



3 A crystallized form of the extract of leeches. The heads of these animals 

 contain the active principle, an albuminous body. 



