THE BLOOD. 477 



appearances have sometimes been wrongly described as due 

 to disease. 



The liquid in which the blood-particles are seen to flow in 

 the living body is not easily obtained for microscopic examina- 

 tion at rest, from its tendency to clot. In the horse it may 

 rather readily be procured, from the blood-discs gravitating 

 to the bottom of the jar in which blood is contained, so as to 

 leave a tolerably free layer of pure liquor sanguinis. It is of 

 a yellowish colour, mucilaginous consistence, and, on clotting, 

 separates into fibrin and serum. The constitution of these 

 elements of blood we shall refer to, after having alluded to the 

 process of coagulation. 



COAGULATION OF BLOOD. Moving within the living body, 

 blood has no tendency to clot, but when at rest in a jar, it is 

 found first to give off a vapour with a characteristic odour. 

 In two or three minutes, and much sooner in the dog, a 

 pellicle forms on the surface, extending from any solid object, 

 such as the sides of the vessel or a rod placed in blood. The 

 pellicle descends on the surface of the mass of blood where 

 it is in contact with the jar, and the process of consolida- 

 tion then advances towards the centre. When the whole 

 blood is in a trembling mass, it is said to be gelatinized, 

 and this occurs in from one to three minutes in the dog, 

 from a half to one and a half minutes in the sheep, from five 

 to ten minutes in the horse, and from two to ten minutes in 

 the ox. When gelatinization is complete the clot begins to 

 shrink, and serum exudes all over its surface, so that in three 

 or four hours a contracted red coagulum is found floating in 

 an abundant yellow fluid. This separation of serum goes on 

 to a certain extent for a day or two, but it is most complete 

 when the coagulation is slow, and vice versa. Thus, blood 

 coagulates more rapidly in an open basin than in a vessel 

 with a narrow neck, and the serum is most abundant, hence 



