880 PHYSIOLOGY 



often to radiate from a disintegrated blood platelet as from a centre. That 

 the blood platelets are concerned in the production of clots is shown by 

 the fact that in the living vessels blood platelets aggregate round any 

 injured spot in the vessel wall, and later fuse together so as to form an 

 adherent thrombus or clot which covers the seat of injury and helps to 

 repair the damage and to prevent the escape of the contents of the blood 

 vessel. Blood platelets have been- found only in mammalian blood and 

 are certainly absent from frogs' blood as well as from the blood of fishes 

 and birds, nor can they be demonstrated in any of the serous fluids even in 



FIG. 379. Blood corpuscles and 'blood platelets, within a small vein. 

 (ScHAFER after OSLER. ) 



mammals. Certain nucleated spindle-shaped cells have been described 

 in frogs' blood as blood platelets, but these are probably immature 

 red blood corpuscles and not homologous with the blood platelets of 

 mammals. (Blood platelets themselves were regarded by Hay em as stages 

 in the formation of red blood corpuscles.) If the blood of an animal be 

 defibrinated by repeated bleeding, whipping and returning to the 

 veins of the animal, it will be found for the next few days to be quite 

 free from blood platelets. There is no doubt therefore that it is possible 

 by the most varied means to demonstrate the existence of blood platelets 

 in shed blood. According to the method adopted, so do the number and 

 form of these platelets vary. Moreover they can be seen to be deposited 

 from the circulating blood in the living animal on any injured portion of 

 the vessel wall or on any foreign body introduced into the blood current 

 (Fig. 379). On the other hand, it is possible to obtain blood in an un- 

 coagulated state from the vessels in which no trace of platelets is to be 

 observed. As Buckmaster has shown, a film of blood examined in a 

 platinum loop and kept carefully at the temperature of the body presents 

 no platelets on microscopic examination ; and the same absence of platelets 

 is to be noted when blood is received into sterile blood serum of the same 

 species of animal and kept at the body temperature. On allowing these 

 specimens of blood to cool, blood platelets make their appearance. Many 

 specimens of non-coagulable plasma, such as peptone plasma or oxalate 

 plasma, can be separated by means of the centrifuge from all formed ele- 

 ments. If the plasma be cooled to C. for twenty-four hours, a precipitate 

 indistinguishable from blood platelets is found to have been produced 

 under the action of cold. We are therefore probably justified in con- 

 cluding that the blood platelets do not form a constituent of normal living 

 blood, but are produced in the plasma either on contact with foreign bodies 

 or lowering of its temperature from 37 C. to' 18 or 20 C. All the various 



