THE BLOOD-PLATELETS 945 



by Deetjen. The drop of blood is received directly from the vessels 

 on to a sheet of solid agar jelly which is made with 0-6 per cent, sodium 

 chloride solution with the addition of sodium metaphosphate and 

 bipotassium phosphate. When examined on this medium large 

 numbers of platelets are seen, each of them provided with numerous 

 processes (Fig. 363). Their central part is more strongly refracting 

 than the periphery and stains with basic dyes, so that it has been 

 regarded as a nucleus. Similar platelets are observed when the blood 

 is received into normal salt solution, and, as the mixture clots, the 

 filaments of fibrin can be seen 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 



FIG. 364. Blood-corpuscles and blood-platelets, within a small vein. 



(SCHAFER after OSLER.) 



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 

 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. 364). 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 Buck- 

 master has shown, a film of blood examined in a platinum loop and 

 kept carefully at the temperature of the body presents no platelets on 



60 



