COAGULATION OF THE BLOOD. 25 



be called a physiological fluid, as it is formed only after coagulation of the 

 blood. 



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 reason of its contractile properties, it soon expresses the 

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

 ate to retain the corpuscles in the clot is the adhesive matter which covers 

 their surface after they escape from the vessels. 



Conditions which modify Coagulation. 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 instantaneously. In short, it appears that all conditions which favor 

 evaporation from the blood hasten its coagulation. The blood will coagu- 

 late more rapidly in a vacuum 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. 

 (zero and 60 C.), elevation of temperature increases the rapidity of coagula- 

 tion. Agitation of the blood in closed vessels retards, and in open vessels, 

 hastens coagulation. 



Various chemical substances retard or prevent coagulation. Among them 

 may be mentioned the following: solutions of potassium or of sodium 

 hydrate ; sodium carbonate ; ammonium carbonate ; potassium carbonate ; 

 ammonia; sodium sulphate. In the menstrual flow, the blood is kept 

 fluid by mixture with the abundant secretions of the vaginal mucous mem- 

 brane. 



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 between 

 twelve and twenty-four hours after circulation has ceased. Under these 

 conditions, the blood is found chiefly in the venous system, as the arte- 

 ries are usually emptied by post-mortem contraction of their muscular 

 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 already described. 

 They are frequently covered by a soft, whitish film 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 reported a case of death by lightning, in 

 which a loose coagulum was found in the heart twenty-four hours after. In 

 this case decomposition was very far advanced, and it is probable that the 



