COAGULATION OF THE BLOOD. BUFFY COAT. 533 



take place in shreds or insulated portions ; but that rest is not the cause of its 

 coagulation (as some have supposed), is proved by the fact that, if a portion 

 of blood be included between two ligatures in a living vessel, it will remain 

 fluid for a long time. Again, the coagulation is accelerated by moderate heat, 

 and retarded by cold; but it is not prevented by even extreme cold; for, if 

 blood be frozen immediately that it is drawn, it will coagulate on being thawed. 

 Moreover it is accelerated by exposure to air, but it is not prevented by com- 

 plete exclusion from it, as is proved by its taking place in a vacuum, or in a 

 shut sac wilbin the dead body: complete exclusion from the air, however, 

 retards the change ; as has been shown by causing Blood to flow into a ves- 

 sel containing oil, which will form an impervious coating on its surface, and 

 will occasion the coagulation to take place so slowly, that the Red particles 

 have time to subside, and the upper stratum of the clot is colourless.* A re- 

 markable case has been put on record by Dr. Polli, in which complete coagu- 

 lation of the blood did not take place until fifteen days after it had been with- 

 drawn from the body ; and fifteen days more elapsed before putrefaction 

 commenced. The upper four-fifths of the clot were colourless; the red cor- 

 puscles occupying only the lowest fifth. It is additionally remarkable, that 

 the patient (who was suffering under acute pneumonia) being bled very fre- 

 quently during the succeeding week, the blood gradually lost its indisposition 

 to coagulate.t An extrication of Carbonic acid usually takes place to a slight 

 degree during coagulation ; but this is not a constant occurrence ; and the 

 process is not prevented, even by agitating Carbonic acid with the Blood. 



703. The proportions of Serum and Clot which present themselves after 

 coagulation, are liable to great variation, independently of the amount of the 

 several ingredients characteristic of each ; for the Coagulum may include not 

 only the Fibrine and Red particles, but also a large proportion of the Serum, 

 entangled as it were in its substance. This is particularly the case when the 

 coagulation is rapid ; and the clot then expels little or none of it by subse- 

 quent contraction. On the other hand, if the coagulation be slow, the parti- 

 cles of Fibrine seem to become more completely aggregated, the coagulum is 

 denser at first, and its density is greatly increased by subsequent contraction. 

 When a firm fresh clot is removed from the fluid in which it is immersed, its 

 concretion is found to continue for 24 or even 48 hours, serum being squeezed 

 out in drops upon its surface; and in order, therefore, to form a proper esti- 

 mate of the relative proportions of Crassamentum and Serum, the former 

 should be cut into slices, and laid upon bibulous paper, that the latter may be 

 pressed from it as completely as possible. According to the experiments of 

 Mr. Thrackrah, Coagulation takes place sooner in metallic vessels than in 

 those of glass or earthenware, and the quantity of Serum separated is much 

 less; in one instance, the proportion of Serum to Clot was as 10 to 24?, 

 when the blood coagulated in a glass vessel ; whilst a portion of the same 

 Blood, coagulating in a pewter vessel, gave only 10 of Serum to 175 of Clot. 

 The Specific Gravity of Blood is no measure of its coagulating power; for a 

 high specific gravity may be due to an excess in the amount of globules, 

 which form the heaviest part of the blood ; and may be accompanied by a 

 diminution in the quantity of fibrine, which is the coagulating element. 



704. The Crassamentum not unfrequently exhibits, in certain disordered 

 conditions of the Blood, a layer of Fibrine nearly free from colour; and this is 

 known as the Buffy Coat. The presence of this has been frequently re- 

 garded as a sign of the existence of Inflammation, occasioning an undue pre- 

 dominance of Fibrine; but this idea is far from being correct, since, as will pre- 



' Babington in Medico-Chinirgical Transactions, vol. xvi. 

 f Mr. Paget's Report, in Brit, and For. Med. Rev., xix. p. 252. 



45* 



