128 ADVANCED PHYSIOLOGY 



they have entirely disappeared. The only way to see them 

 at all is to draw the blood into some preserving fluid 

 which will prevent them from breaking to pieces. The 

 function of the blood platelets is not definitely known, 

 although they probably have something to do with blood 

 clotting. 



BLOOD CLOTTING 



Everyone is familiar with substances which may exist 

 either as liquids or solids. The most familiar example is 

 water, which becomes solid if the temperature falls below the 

 freezing point. The solidifying is dependent on temperature, 

 and the material can be changed from solid to liquid by 

 heating, or from liquid to solid by cooling. 



So, too, blood has the property of existing in a liquid and ir 

 a solid form. When it becomes solid, we speak of it as 

 clotted. This clotting, however, differs decidedly from the 

 freezing of water, since it is not due to cooling, for blood will 

 clot even if kept warm. Indeed, the comparison of clotting 

 with the freezing of water is not a good one, for the blood 

 greatly changes its nature when clotting and can never be 

 brought back again into the condition of liquid blood. If 

 blood is drawn directly from the blood vessels into a small 

 dish, it will be found at first to be fluid, like water; if allowed 

 to stand a few minutes it becomes jelly-like. Presently it- 

 forms such a firm jelly that the dish can be turned upsidf 

 down without displacing any of the blood. If it stands for 

 some time longer the jelly mass will shrink and a yellowish 

 liquid ooze out of it. This liquid is known as serum, and thr 

 contracted jelly, which holds most of the red corpuscles, is tht 

 clot. If now the clot is taken out and thoroughly washed in 

 water, all the corpuscles can be separated from it, and the 

 material left will be a tangled mass of white, elastic threads 

 This substance is called fibrin. It must be understood thai 

 the fibrin does not exist while the blood is in the vessels, bu1 



