GENERAL PHYSICAL AND CHEMICAL PROPERTIES 33 



v.seJ for the identification of human blood. It is probable that an- 

 aphylaxis plays an important role in certain pathological reactions. It 

 is well known, for example, that some persons are so susceptible to 

 particular foods that the slightest indulgence in them brings on an 

 attack of urticaria or nettlcrash. It has been suggested that these 

 persons have become sensitized to certain foreign proteins such as 

 those existing in eggs, veal, pork, strawberries, shellfish, or whatever 

 the peccant article of diet may be possibly by absorption at some 

 previous time, owing to gastro-intestinal disturbance, of small quan- 

 tities of the proteins which have escaped complete digestion. 



It is only when proteins are introduced parenterally (i.e., by some 

 other route than the alimentary canal, such as the subcutaneous tissue, 

 the blood, or the serous cavities) that the immunity reactions already 

 described and the phenomenon of anaphylaxis can be experimentally 

 produced. For in digestion the protein molecule is decomposed, and 

 although, as will be seen later on, the decomposition products are not 

 the same for each kind of protein, the factor on which the specificity 

 of the molecule depends does not survive the hvdrolysis. 



COAGULATION OF THE BLOOD. 



Since changes begin in the blood as soon as it is shed, having 

 for their outcome clotting or coagulation, we have to gather from 

 the composition of the stable factors of clotted blood, or of blood 

 which has been artificially prevented from clotting, some notion of 

 the composition of the unaltered fluid as it circulates within the 

 vessels. The first step, therefore, in the study of the chemistry 

 of blood is the study of coagulation. 



When blood is shed, its viscidity soon begins to increase, and 

 after an interval, varying with the kind of blood, the temperature 

 of the air, and other conditions, but in man seldom exceeding ten, 

 or falling below three, minutes, it sets into a firm jelly. This jelly 

 gradually shrinks and squeezes out a straw-coloured liquid, the 

 serum. Under the microscope the serum is seen to contain few or 

 no red corpuscles; these are nearly all in the clot, entangled in the 

 meshes of a kind of network of fine fibrils composed of fibrin. In 

 uncoagulated blood no such fibrils are present ; they have accordingly 

 been formed by a change in some constituent or constituents of 

 the normal blood. Now, it has been shown that there exists in the 

 plasma *he liquid portion of unclotted blood a substance from 

 which fibrin can be derived, while no such substance is present in 

 the corpuscles. In various ways coagulation can be prevented or 

 delayed, and the plasma separated from the corpuscles. For 

 example, the blood of the horse clots very slowly, and a low tem- 

 perature lessens the rapidity of coagulation of every kind of blood. 

 If horse's blood is run into a vessel surrounded by ice and allowed 

 to stand, the corpuscles, being of greater specific gravity than the 

 plasma, gradually sink to the bottom, and the clear straw-yellow 

 plasma can be pipetted off. Or the addition of neutral salts to 



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