THE BLOOD-PLATELETS 837 



often to radiate from a disintegrated blood-plate 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 only been found 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. 368. Blood- corpuscles and blood-platelets, within a small vein. 

 (SCRAPER 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 Hayem as stages 

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

 defibrinated by bleeding, continually whipping the blood and returning it 

 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. 368). On the other hand, it is possible to 

 obtain blood in an uncoagulated 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 elements. 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 concluding 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 



