126 THE BLOOD 



at a high temperature. The excess of salt is washed away with water from 

 the dried residue, and the specimen may then be dried and mounted. A 

 large number of small, dark, reddish black crystals of a rhombic shape, 

 sometimes arranged in bundles, will be seen if the slide be subjected to micro- 

 scopic examination, figure 1233,. 



The formation of these hemin crystals is of great interest and importance 

 from a medico-legal point cf view, as it constitutes the most certain and 

 delicate test we have for the presence of blood (not of necessity the blood 

 of man) in a stain on clothes, etc. It exceeds in delicacy even the spectro- 

 scopic test. Compounds similar in composition to hemin, but containing 

 hydrobromic or hydriodic acid, instead of hydrochloric, may be also readily 

 obtained. 



Variations in the Composition of Healthy Blood. The conditions 

 which appear most to influence the composition of the blood in health are 

 these : Diet, Exercise, Sex, Pregnancy, and Age. 



Sex. The blood of men differs from that cf women, chiefly in being of 

 somewhat higher specific gravity, from its containing a relatively larger 

 quantity of red corpuscles. 



Pregnancy. The blood cf pregnant women has rather lower than the 

 average specific gravity. The quantity of the colorless corpuscles is increased 

 in the later months, especially in primiparae; it is also claimed that the 

 fibrin is increased in amount. 



Age. The blood of the fetus is very rich in solid matter, and especially 

 in colored corpuscles; and this condition, gradually diminishing, continues 

 for some weeks after birth. The quantity of solid matter then falls during 

 childhood below the average, rises during adult life, and in old age falls again. 



Diet. Such differences in the composition of the blood as are due to the 

 temporary presence of various matters absorbed with the food and drink, 

 as well as the more lasting changes which must result from generous or poor 

 diet respectively, need be here only referred to. 



Effects oj Bleeding. The result of bleeding is to diminish the specific 

 gravity of the blood, and so quickly that in a single venesection the portion 

 of blood last drawn has often a less specific gravity than that of the blood that 

 flowed first. This is, of course, due to absorption of fluid from the tissues 

 of the body. The physiological import of this fact, namely, the instant 

 absorption of liquid from the tissues, is the same as that of the intense thirst 

 which is so common after either loss of blood, or the abstraction from it of 

 watery fluid, as in cholera, diabetes, and the like. 



For some little time after bleeding, the want of colored corpuscles is well 

 marked, but with this exception: no considerable alteration seems to be 

 produced in the composition of the blood for more than a very short time; 

 the loss of the other constituents, including the colorless corpuscles, being 

 very quickly repaired. 



