CHAP. XXVII.] COAGULATION OF THE BLOOD. 293 



and to beware of subjecting patients to those excessive losses of 

 blood, which, experience teaches us, too often inflict upon the 

 general nutrition of the body a shock so severe, that it is more or 

 less seriously affected by it ever after. 



II. The Phenomenon of Coagulation. We have already described 

 the separation of the blood into serum and crassamentum. In this 

 consists the phenomenon of coagulation. In a few minutes after 

 blood has been allowed to rest in a vessel, its surface assumes the 

 appearance of a jelly, on which, after a little more time, drops of 

 serum appear to ooze out here and there : these drops multiply 

 and coalesce, so as to cover the jelly-like surface with a layer of 

 serum, which increases so much in quantity, as coagulation ad- 

 vances, that the clot is at last found covered, and more or less 

 completely surrounded by serum. 



The crassamentum, or clot, is a solid mass, .varying much in con- 

 sistence : sometimes soft and tremulous, like jelly ; at other times, 

 firm and tough almost as leather. If a section of it be made, it 

 will be found in most instances coloured throughout, but always 

 most deeply so at its lower half or third, the heavy red particles 

 gravitating to the lowest part; that portion which is exposed to the 

 air having always a scarlet tint. The surface of the clot is always 

 slightly concave ; sometimes it is remarkably so, and exhibits the 

 appearance of a hollow cup; and on these occasions, the upper 

 layers of the clot generally consist of fibrine only, which is of a 

 whitish yellow or buff colour, with an intermixture of colourless 

 corpuscles entangled in its meshes. Hence, blood which presents 

 these appearances, is said to be " cupped and buffed." The phe- 

 nomenon is due to the more complete subsidence of the blood-par- 

 ticles as the clot is being formed ; so that its upper layers are left 

 quite free from colouring-matter. The clot, when this state is 

 present, is generally small, tough, and well contracted, and it floats 

 in a large quantity of serum. 



The period required for the completion of coagulation, varies 

 very much : it commences in about two minutes after the blood 

 has been collected in a vessel, and is rarely completed for half an 

 hour afterwards, but more frequently the clot is not perfectly 

 formed in less than one or two hours. After it has been formed, 

 it will continue to contract for many hours, and to press out the 

 serum, which will thus increase in quantity while the bulk of the 

 clot undergoes diminution. 



Coagulation appears to take place more rapidly under the influ- 

 ence of a high temperature; 1 14 to 120, according to Hewson : it 



