164 COMPARATIVE PHYSIOLOOr. 



fewness of red corpuscles. This is the buffy-coat, or, as it oc- 

 curs in inflammatory conditions of the blood, was termed by 

 older writers, the crusta phlogistica. It is to be distinguished 

 from the lighter red of certain parts of a clot, often the result of 

 greater exposure to the air and more complete oxidation in con- 

 sequence. The white blood-cells, being lighter than the red, are 

 also more abundant in the upper part of the clot (buffy-coat). 

 If the coagulation of a drop of blood withdrawn from one's 

 own finger be watched under the microscope, the red corpuscles 

 may be seen to run into heaps, like rows of coins lying against 

 each other (rouleaux, Fig. 141), and threads of the greatest 

 fineness are observed to radiate throughout the mass, gradually 

 increasing in number, and, at last, including the whole in a 

 meshwork which slowly contracts. It is the formation of this 

 fibrin which is the essential factor in clotting; the inclusion of 

 the blood-cells and the extrusion of the serum naturally result- 

 ing from its formation and contraction. 



The great mass of every clot consists, however, of corpuscles ; 

 the quantity of fibrin, though variable, not amounting to more 

 usually than about "2 per cent in mammals. The formation of 

 the clot does not occupy more than a few minutes (two to seven) 

 in most mammals, including man, but its contraction lasts a 

 very considerable time, so that serum may continue to exude 

 from the clot for hours. It is thus seen that, instead of the 

 pla,sma and corpuscles of the blood as it exists within the living 

 body, coagulation has resulted in the formation of two new 

 products— serum and fibrin — difi'ering both physically and 

 chemically. These facts may be put in tabular form thus : 



Blood as it flows ( Liquor sanguinis (plasma). 

 in the vessels. ( Corpuscles. 



Blood after co- S Coagulum | '^(}^^^^^^^ 

 agulation. ') Serum. 



As fibrin may be seen to arise in the form of threads, under 

 the microscope, in coagulating blood, and since no trace of it in 

 any form has been detected in the plasma, and the process can 

 be accounted for Otherwise, it seems unjustifiable to assume that 

 fibrin exists preformed in the blood, or arises in any way prior 

 to actual coagulation. 



Fibrin belongs to the class of bodies known as proteids, and 

 can be distinguished from the other subdivisions of this group 

 of substances by certain chemical as well as physical character- 



