26 THE BLOOD. 



with a little sulphate of soda, which prevented coagulation, and, after the mixture had 

 been allowed to stand for a time, the corpuscles gravitated to the bottom of the vessel. 

 The clear fluid was then decanted and diluted with twice its quantity of water, when 

 fibrin became coagulated. 



The facts thus demonstrated by Hewson were confirmed by Miiller, in 1832. He suc- 

 ceeded in separating the plasma from the corpuscles in the blood of the frog by simple 

 filtration, first diluting it with a saccharine solution. The great size of the corpuscles in 

 this animal prevents their passage through a filter, and the clear fluid which is thus sepa- 

 rated soon forms a colorless coagulum. 



From these observations, it is evident that the coagulation of the blood is due to the 

 formation of fibrin. Coagulation of this substance first causes the whole mass of blood 

 to assume a gelatinous consistence ; and, by virtue of its contractile properties, it soon 

 expresses the serum, while the red corpuscles are retained. One of the causes which 

 operate to retain the corpuscles in the clot is the adhesive matter which covers their 

 surface after they escape from the vessels, which produces the arrangement in rows like 

 piles of coin, which we have already noted under the head of microscopical appearances. 

 This undoubtedly prevents those which are near the surface from escaping from the clot 

 during its contraction. 



Circumstances which modify Coagulation out of the Body. The conditions which 

 modify coagulation of the blood have been closely studied by Hewson, Davy, Thackrah, 

 Kobin and Verdeil, and others. They are, in brief, the following : 



Blood flowing slowly from a small orifice is more rapidly coagulated than when it 

 is discharged in a full stream from a large orifice. If it be received into a shallow 

 vessel, it coagulates much more rapidly than when received into a deep vessel. If the 

 vessel be rough, coagulation is more rapid than if it be smooth and polished. If the 

 blood, as it flows, be received on a cloth or a bundle of twigs, it coagulates almost in- 

 stantaneously. In short, it appears that all circumstances which favor exposure of the 

 blood to the air hasten its coagulation. The blood will coagulate more rapidly in a va- 

 cuum than in the air. 



Coagulation of the blood is prevented by rapid freezing, but it takes place afterward 

 when the fluid is carefully thawed. Between 32 and 140 Fahr., elevation of tempera- 

 ture increases the rapidity of coagulation. According to Eichardson, agitation of the 

 blood in closed vessels retards, and in open vessels hastens coagulation. 



Various chemical substances retard or prevent coagulation. Among them we may 

 mention the following : solutions of potash and of soda ; carbonate of soda ; carbonate 

 of ammonia ; carbonate of potash ; ammonia ; sulphate of soda. In the menstrual flow, 

 the blood is kept fluid by mixture with the abundant secretions of the vaginal mucous 

 membrane. 



Coagulation of the Blood in the Organism. The blood coagulates in the vessels after 

 death, though less rapidly than when removed from the body. As a general proposition, 

 it may be stated that this takes place in from twelve to twenty-four hours after circula- 

 tion has ceased. Under these circumstances, the blood is found chiefly in the venous 

 system, as the arteries are generally emptied by post-mortem contraction of their mus- 

 cular coat ; but, in the veins, coagulation is slow and imperfect. Coagula are found, 

 however, in the left side of the heart and in the aorta, but they are much smaller than 

 those in the right side of the heart and in the large veins. These coagula present the 

 general characters we have already described. They are frequently covered by a soft, 

 whitish film, analogous to the buffy-coat, and are dark in their interior. 



It was supposed by John Hunter that coagulation of the blood did not take place in 

 animals killed by lightning, or by prolonged muscular exertion, as when hunted to death ; 

 but it appears from the observations of others that this view is not correct. J. Davy 



