COAGULATING PRINCIPLE OF THE BLOOD. 25 



Characters of the Clot. On removing the clot, after the separation of the serum is 

 complete, it presents 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, coagu- 

 lation has been slow, it 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 depends merely on the extent of its contraction. 

 It also presents a marked difference in color at its upper portion. The blood having 

 remained fluid for some time, the red corpuscles settle, by virtue of their greater weight, 

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

 Although this frequently presents itself in the blood drawn in inflammations, it is by no 

 means pathognomonic of this condition, and is liable to occur whenever coagulation is 

 slow or has been retarded by artificial means. It is always present in the blood of the 

 horse. Examined microscopically, the buffy-coat presents fibrils of coagulated fibrin 

 with some of the white corpuscles of the blood. On removing a clot of venous 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 the coagulation have not been too rapid, the gravi- 

 tation of the red corpuscles is apparent. The section, which is at first almost black, soon 

 becomes red from contact with the atmosphere. If the clot be cut into small pieces, it 

 will undergo farther contraction, and express 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, we 

 may remove almost all the red corpuscles, leaving the meshes of fibrin, which, on micro- 

 scopical examination, presents the fibrillated appearance to which we have already referred. 



Characters of the Serum. After coagulation, if the serum be carefully removed, it 

 is found to be a fluid of a color varying from a light amber to quite a deep, but clear red. 

 This depends 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 contains all the principles found in the plasma, or liquor sanguinis, with the exception 

 of the elements of fibrin. It can hardly be called a physiological fluid, as it is formed 

 only after coagulation of the blood and never exists isolated in the body. The effusions 

 which are commonly called serum, although they resemble this fluid in some particulars, 

 are not identical with it, being formed by a process of transudation rather than separa- 

 tion from the blood, as in coagulation. The serum must not be confounded with the 

 plasma or liquor sanguinis, which is the natural clear portion of the blood. 



Coagulating Principle of the Blood. Acquainted, as we are, with the properties of 

 fibrin, it is evident that this substance is the agent which produces coagulation of the 

 blood. In fact, whatever coagulates spontaneously is called fibrin, and whatever requires 

 some agent to produce this change of consistence is called by another name. But, before 

 the properties of fibrin were fully understood, the question of the coagulating principle 

 was a matter of much discussion. Malpighi was probably the first to isolate fibrin, which 

 he did by washing the clot in a stream of water, which removed the corpuscles and left a 

 whitish, fibrous net-work. His experiments are set forth in an article in which he at- 

 tempted to show that the so-called polypi of the heart were formed of fibrin, although it 

 was not then called by that name. These observations were soon confirmed by others ; 

 and it then became a question whether this substance existed as a fluid in the liquor san- 

 guinis, or was furnished by the corpuscles after the removal of blood from the vessels. 

 This was decided by Hewson, whose simple and conclusive experiments leave no doubt 

 that coagulation of the blood is due to fibrin, and that this substance is entirely distinct 

 from, and independent of the corpuscles. This observer, taking advantage of the prop- 

 erty possessed by certain saline substances of preventing the coagulation of the blood, 

 was the first to separate the liquor sanguinis from the corpuscles. He mixed fresh blood 



