224 FUNCTIONS OF NUTRITION. 



The earthy phosphates of the plasma, which by themselves are insol- 

 uble in alkaline or neutral fluids, are held in solution in the blood by 

 union with its albuminous ingredients. 



Coagulation of the Blood. 



Within a few moments after blood has been withdrawn from the 

 vessels, it presents the remarkable phenomenon of coagulation or clot- 

 ting. This process commences at nearly the same time throughout the 

 whole mass, which becomes first somewhat diminished in fluidity, so 

 that its surface may be gently depressed with the end of the finger 

 or a glass rod. It then becomes rapidly thicker, and at last solidifies 

 into a uniformly red, opaque, gelatinous mass, which takes the form 

 of the vessel in which the blood was contained. Coagulation usually 

 commences, in man, in from ten to twelve minutes after the blood 

 has been drawn, and is completed in about twenty minutes. In most 

 animals, it is more rapid than this, taking place in the dog, ox, and 

 sheep often within five minutes. In the horse, on the other hand, it is 

 exceptionally slow, requiring a longer time than in man. 



The coagulation of the blood is dependent on the presence of its 

 fibrine-producing ingredient. This may be demonstrated in various 

 ways. First, if freshly drawn frog's blood be mixed with a solution 

 of sugar of one-half per cent., and placed on a filter, the blood-glob- 

 ules will be retained ; and the transparent colorless filtered fluid after 

 a time coagulates like fresh blood. Secondly, if horse's blood, which 

 coagulates slowly, be drawn from the veins into a cylindrical vessel 

 and allowed to remain at rest, by the time coagulation takes place the 

 blood-globules will have partially subsided, leaving at the surface a 

 layer which is colorless and semi-transparent, but as firmly coagulated 

 as the rest. Thirdly*, if horse's blood, freshly drawn into such a vessel, 

 be surrounded by a freezing mixture, and kept at the temperature of 

 C., coagulation is suspended, and the globules sink towards the bot- 

 tom, leaving a colorless fluid above. If this be removed by decantation, 

 and allowed to rise in temperature a few degrees, it coagulates like fresh 

 blood. 



These facts show that the blood-globules take no direct part in 

 coagulation ; and that, when present, they are simply entangled in 

 the solidifying clot. 



Finally, if the freshly drawn blood of man, or of any warm-blooded 

 animal, be stirred with a bundle of twigs or glass rods, the fibrine 

 coagulates in comparatively small mass on the surface of the foreign 

 bodies; and the globules entangled in it may be washed out with- 

 out changing its essential character. 



It is the fibrinogen, therefore, which, by its coagulation, induces the 

 solidification of the entire blood. As it is uniformly distributed through- 

 out, when coagulation takes place its filaments entangle in their meshes 

 the globules and albuminous fluids of the plasma. A very small quan- 

 tity of fibrine is sufficient to include in its solidification all the fluid and 



