484 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



collects ; the clot then gradually shrinks away from the sides of the vessel 

 and contracts in every direction, and as the coagulum decreases in bulk 

 the fluid serum increases, until, finally, the now opaque, small, firm clot 

 swims in a large amount of clear, yellowish serum. 



When horses' blood is not artificially cooled, and in blood of other 

 mammals, the process of coagulation follows a somewhat different course. 

 Here also it is only the fibrin which coagulates, and not the blood in toto; 

 but the process takes place so rapidly that the corpuscles do not have 

 time to settle, but remain entangled in the fibrin, and the coagulum, 

 instead of being colorless, is of a deep-red color from the contained red 

 blood-corpuscles, the whole forming a red, gelatinous mass, the crassa- 

 mentum, or clot. Here, also, the process of coagulation commences at the 

 free surface of the blood and at the surfaces in contact with the walls of 

 the vessel by the formation of a delicate pellicle, which rapidly thickens 

 until in from seven to fourteen minutes the entire mass of blood is trans- 

 formed into a stiff jelly ; and here, also, after the formation of the clot, 

 there is a gradually progressive contraction, with exudation of serum, 

 until finally there results a small, red, firm coagulum, floating in the 

 yellowish serum. 



Blood of different species of animals varies in the rapidity with which 

 coagulation takes place. In the following series coagulation occurs in 

 gradually increasing rapidity from the first to the last, horse, cat, 

 dog, ox, pig, goat, sheep, the time varying from twenty-five to one 

 and a half minutes. Normal horses' blood always clots so slowly 

 that the corpuscles have time partially to settle to the bottom 

 of the vessel which contains it. Immediately before coagulation, there- 

 fore, the horse's blood forms different-colored layers, the upper and 

 smallest being yellowish, while the lower is red. When coagulation 

 takes place the same arrangement holds, and the clot of horses' blood 

 has, therefore, a yellowish upper surface, the so-called buffy coat, forming 

 what used to be called an inflammatory clot (crusta phlogistic a}. Every- 

 thing, therefore, which accelerates the settling of the red blood-cells, 

 or which delays coagulation, will tend to develop the buffy coat on 

 the clot. 



In addition to cooling, coagulation nuiy be retarded by pumping out 

 the oxygen from the blood, by saturation with carbon dioxide (explaining 

 the tardy coagulation of the blood in animals dying from suffocation, and 

 why coagulation occurs sooner in venous than arterial blood), addition 

 of certain salts, such as sulphate, borate, and carbonate of sodium, 

 chloride of sodium, sulphate of magnesium, nitrate, acetate, and car- 

 bonate of potassium, and chloride of potassium, through the addition 

 of small amounts of caustic potash or ammonia, sugar and gum solutions, 

 or by acidulation with dilute acetic or nitric acids. By exact neutraliza- 



