36 THE BLOOD. 



death, blood remains longer fluid in the small veins than in the heart 

 and great vessels ; and even in these the coagulation is usually slow. 

 Agitation of exposed blood accelerates coagulation by increasing its ex- 

 posure to foreign contact. 



4. "Water, in a proportion not exceeding twice the bulk of the blood, 

 hastens coagulation ; a larger quantity retards it. Blood also coagu- 

 lates more speedily when the serum is of low specific gravity, indicative 

 of much water in proportion to the saline ingredients. 



5. Almost every substance that has been tried, except the caustic 

 alkalies, when added to the blood in minute proportion, hastens its- 

 coagulation ; although many of the same substances, when mixed with 

 it in somewhat larger quantity, have an opposite effect. The salts of 

 the alkalies and earths, added in the proportion of two or three per cent, 

 and upwards, retard, and, when above a certain quantity, suspend or 

 prevent coagulation ; but, though the process be thus suspended, it 

 speedily ensues on diluting the mixture with water. Caustic potash and 

 soda permanently destroy the coagulability of the blood. Acids delay 

 or prevent coagulatiou. 



G. Certain states of the system. — Faintness occasioned by loss of 

 blood favours coagulation ; states of excitement are said to have, though 

 not invariably, the opposite effect. Impeded aeration of the blood in 

 disease, or in suffocative 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 in 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. Nasse states that women's blood begins to 

 coagulate 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 disengage- 

 ment 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 -weW 

 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 o^jerate in 

 promoting or opposing coagulation. 



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

 hy John Hunter and some other British physiologists, and which has been advo- 

 cated by Briicke,* the blood has a natural tendency to coagulate ; or, if we may use 

 the language suggested by later researches, tlie para-globulin and fibrinogen 

 naturally tend to combine ; within the body this tendency is held in clieck by some 

 inhibitory or restraining influence exercised by the coats of the vessels and the 

 living tissues in contact with the blood ; but when Ijlood is withdrawn from its 

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

 late being no longer opposed, is allowed to prevail. At the same time it is not 



* British and Foreign Medico-Chirnrgical Review, vol. xix. 1S57. 



