xlvi THE BLOOD. 



invariably, the opposite effect. Impeded aeration of the blood in disease, 

 or in suflbcative modes of death, makes it slow to coagulate; probably from 

 retention of carbonic acid. In cold-blooded animals, with slow circulation 

 and low respiration, the blood coagulates less rapidly than iu the warm- 

 blooded ; and, among the latter, the tendency of the blood to coagulate is 

 strongest in birds, which have the greatest amount of respiration, and 

 highest temperature. 



7. Coagulation commences earlier, and is sooner completed, in arterial 

 than in venous blood. Dr. Nasse finds that women's blood begins to 

 coagulate nearly two minutes sooner than that of the male sex. 



In general, when blood coagulates quickly, the clot is more bulky and 

 less firm, and the serum is less effectually expressed from it ; so that causes 

 which affect the rapidity of coagulation will also occasion differences in the 

 proportion of the moist clot to the exuded serum. 



There is no sufficient evidence of evolution of heat or of disengagement of 

 carbonic acid from blood during its coagulation, which some have supposed 

 to occur. 



Theory of Coagulation. Although it is certain that the coagulation of the blood 

 consists in solidification of fibrin, and although it seems tolerably well established 

 that this is the result of the combination of two primarily separate animal principles, 

 it is by no means clearly understood how such combination and solidification do not 

 naturally take place within the living body, and how the several conditions already 

 mentioned as influencing the process operate in promoting or opposing coagulation. 



According to the explanation proposed a few years ago by Dr. B. W. Richardson,* 

 the blood is kept liquid within the vessels by ammonia, which maintains the fibrin in 

 solution (or, as it may be now expressed, hinders the union of fibrinogen and 

 globulin), and drawn blood coagulates in consequence of the loss of ammonia which 

 escapes from it on exposure. Drawn blood placed in an atmosphere of ammonia 

 remains fluid; and, according to Dr. Richardson, the natural presence of ammonia in 

 blood may be demonstrated, as well as its extrication from blood on exposure to air 

 or in a vacuum. The more extensive and thorough the exposure, the more speedy is 

 the escape of ammonia, and the quicker is the coagulation. Heat hastens coagulation 

 by hastening the extrication of ammonia, whilst, by retaining it, cold delays or 

 suspends the change. Moreover Dr. R. found that air which had become charged 

 with the volatile element by passing through one portion of blood delayed the coagu- 

 lation of another portion through which it was sent. 



Nothing could at first sight appear more natural and consistent than this explana- 

 tion ; but of the facts on which it mainly rests, some have since been disputed, and 

 others differently interpreted. Thus after a most elaborate research, a late inquirer 

 (Thiry) has been unable to find evidence of free ammonia in fresh blood. Again, 

 Lister finds that blood may be poured from one piece of a vein into another, in a small 

 stream through the air several times, so as to give ample opportunity for the escape of 

 ammonia, and yet remain fluid for hours after. Lastly, the different effects of heat 

 and cold may be ascribed to their influence in promoting or hindering the mutual 

 reaction of the two constituents of the fibrin. 



According to another view, which is fundamentally the same as that entertained 

 by John Hunter and some other British physiologists, and which has recently been 

 advocated by Professor Briicke of Vienna, f the blood has a natural tendency to 

 coagulate ; or, if we may use the language suggested by later researches, the globulin 

 and fibrinogen naturally tend to combine ; within the body this tendency is held in 

 check by some inhibitory or restraining influence exercised by the coats of the 

 vessels and the living tissues in contact with the blood ; but when blood is withdrawn 

 from its natural receptacles, or if these lose their vitality, its intrinsic disposition to 



* The Cause of the Coagulation of the Blood. 1857. 



f British and Foreign Medico-Chirurgical Review, vol. xix. 1857. 



