24 THE BLOOD. 



then begins, and little drops of clear serum make their appearance on the 

 surface of the clot. This fluid increases in quantity, and in ten or twelve 

 hours separation is complete (Nasse). The clot, which is heavier, sinks to 

 the bottom of the vessel, unless it contain bubbles of gas or the surface be 

 very concave. In most of the warm-blooded animals, the blood coagulates 

 more rapidly than in man. Coagulation is particularly rapid in blood taken 

 from birds, and sometimes it takes place almost instantaneously. Coagula- 

 tion is more rapid in arterial than in venous blood. In the former, the pro- 

 portion of fibrin formed is notably greater and the characters of the fibrin 

 are somewhat different. A solution of sodium chloride dissolves the fibrin 

 of venous blood, but does not dissolve the fibrin of an arterial clot. 



The relative proportions of the serum and clot are very variable, unless 

 that portion of the serum which is retained between the meshes of the coag- 

 ulated mass be included in the estimate. As the clot is composed of corpus- 

 cles and fibrin, and as these in their moist state represent, in general terms, 

 about one-half of the blood, it may be stated that after coagulation, the 

 actual proportions of the clot and serum are about equal. Simply taking the 

 serum which separates spontaneously, there is a large quantity when the clot 

 is densely contracted, and a very small quantity, when it is loose and soft. 

 Usually the clot retains about one-fifth of the serum. 



On removing the clot, after the separation of the serum is complete, it pre- 

 sents a gelatinous consistence, and is more or less firm according to the degree 

 of contraction which has taken place. As a general rule, when coagulation has 

 been rapid, the clot is soft and but slightly contracted. When, on the other 

 hand, coagulation has been slow, the clot contracts for a long time and is much 

 denser. When coagulation is slow, the clot frequently presents what is known 

 as the cupped appearance, having a concave surface, a phenomenon which de- 

 pends merely on the degree of its contraction. It also presents a marked dif- 

 ference in color at its upper portion. The blood having remained fluid for 

 some time, the red corpuscles settle, by reason of their greater weight, leaving a 

 colorless layer on the top. This is the buffy-coat spoken of by some authors. 

 Examined microscopically, the bufiy-coat presents fibrils of coagulated fibrin 

 with some of the white corpuscles of the blood. On removing a clot of ve- 

 nous blood from the serum, the upper surface is florid from contact with the 

 air, while the rest of it is dark ; and on making a section, if coagulation have 

 not been too rapid, the gravitation of the red corpuscles is apparent. If 

 the clot be cut into small pieces, it will undergo farther contraction and ex- 

 press a part of the contained serum. If the clot be washed under a stream of 

 water, at the same time kneading it with the fingers, nearly all the red cor- 

 puscles may be removed, leaving the meshes of fibrin. 



After coagulation, if the serum be carefully removed, it is found to be a fluid 

 of a color varying between a light amber and a clear red. This color de- 

 pends upon a peculiar coloring matter which has never been isolated. The 

 specific gravity of the serum is about 1028, somewhat less than that of 

 the entire mass of blood. It presents all the constituents of the plasma, or 

 liquor sanguinis, with the exception of the fibrin-factors. It can hardly 



