946 PHYSIOLOGY 



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 appear- 

 ance. 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 20 C. All the 

 various fixing fluids which have been recommended for the display 

 of blood- platelets owe their virtues, not to the fact that they preserve, 

 but to the fact that they produce platelets. These may therefore 

 be regarded as precipitates produced in the plasma directly it 

 undergoes alterations, their appearance being the first sign of 

 changes in this fluid. They consist of some substance probably 

 belonging to the class of nucleo-proteins, and like these yield a precipi- 

 tate on gastric digestion, and have a specific affinity for basic dyes. 

 Even after their first appearance they are unstable, tending to undergo 

 further alteration and to give off substances to the surrounding plasma 

 which play an important part in the formation of fibrin-ferment and 

 in the coagulation of the blood. 



