128 MANUAL OF HISTOLOGY. 



certain number of the red corpuscles escape being entangled in the 

 coagulum, appearing as a kind of reddish sediment in the lower strata 

 of the serum. 



"When a quantity of blood is beaten and whipped up, the fibrin 

 deposits around the instrument used, and the former remains fluid. In 

 such defibrinated blood the sinking of the red corpuscles mentioned in 78 

 may be best observed. 



80. 



The process of coagulation of blood, moreover, displays much variety. 

 The consideration of each several point connected with it in detail, how- 

 ever, would lead us too far here ; we will only, therefore, touch on some 

 of the most important matters of interest. 



As regards the rate at which the changes take place, we find that they 

 may be hastened or retarded. Retardation is easiest produced, as a rule. 

 The coagulation of blood is accelerated by setting the fluid in rapid motion, 

 as, for instance, by means of whipping or beating. The blood of men is 

 said to coagulate in general, more slowly than that of women. Further, 

 arterial blood solidifies more rapidly than venous, whose greater amount 

 of carbonic acid exercises a retarding influence on the process. 



Again, atmospheric air accelerates the clotting Of blood, which explains 

 the fact that, the finer the stream of blood flowing from the orifice of a 

 vessel, or the flatter the dish in which it is caught, the more rapidly does 

 it become solid. Hewson's experiences, also, are in harmony with this, 

 who found that air injected into the vessels of a living animal frequently 

 furthered coagulation. However, we may prevent the access of air to the 

 blood of a dead animal with all caution, without being able to preserve it 

 in a fluid state. Thus we see that it may coagulate without the influence 

 of the oxygen of the air, as it does also in an atmosphere of carbonic 

 acid, hydrogen, or nitrogen gases. 



As to the influence of temperature, we find that warmth favours the 

 process in general, while cold retards it. Coagulation may take place at 

 any point above freezing, and if we subject fluid blood to the action of 

 great cold it may be frozen before coagulation sets in, subsequently under- 

 going this change on being cautiously thawed. 



How far changes in the composition of the blood may influence the 

 rate of coagulation has not yet been sufficiently accurately ascertained. 

 One important item in the process appears to be the nature of the fibrin 

 itself. Thus the blood of certain animals, as, for instance, of the horse, 

 solidifies slowly, whilst that of the sheep does so more rapidly. The 

 annals of medicine also record extraordinary cases of extremely late coagu- 

 lation, which are probably only to be explained likewise by some modifica- 

 tion in the constitution of the fibrin. 



The character, also, of the crassamentum is liable to vary greatly; some- 

 times it is uncommonly small and hard, sometimes large, soft, and fragile. 

 Poorness in corpuscles may cause the first of these states, an increase in 

 the latter the second, in that a superabundance of cells other things 

 being equal must be looked on as a hindrance to the contraction of the 

 fibrin, whilst in an opposite state but slight resistance to it is offered. 

 A larger proportion than usual of water in the blood gives rise also to 

 a softer coagulum. 



Beside all this, there occur also very incomplete cases of coagulation 

 where the process remains stationary in one of its earlier stages; indeed, 



