164 HANDBOOK OF PHYSIOLOGY. 



spread out; a cover glass is then placed upon it, and glacial acetic acid 

 added by means of a capillary pipette. The blood at once turns of a 

 brownish color. The slide is then heated, and the acid mixture evapo- 

 rated to dryuess 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 crys- 

 tals of a rhombic shape, sometimes arranged in bundles, will be seen if 

 the slide be subjected to microscopic examination (fig. 140). 



The formation of these haemin crystals is of great interest and im- 

 portance from a medico-legal point of view, as it constitutes the most 

 certain and delicate test we have for the presence of blood (not of ne- 

 cessity the blood of man) in a stain on clothes, etc. It exceeds in deli- 

 cacy even the spectroscopic test. Compounds similar in composition to 

 haemin, but containing hydrobromic or hydriodic acid, instead of hydro- 

 chloric, may be also readily obtained. 



B. The Carbon Dioxide Gas in the Blood. Of this gas in the 

 blood part exists in a state of simple solution in the plasma, and is given 

 up in vacuo (35.2 per cent), and the rest in a state of weak chemical 

 ^combination. Of the latter, part is in loose combination with the 



haemoglobin and part is more firmly united with the alkalies, possibly 

 with the carbonates in the form of bicarbonate. The amount which can 

 be absorbed depends on the alkalescence of the blood. 



C. The Nitrogen in the Blood. The whole of the small quantity 

 of the nitrogen contained in the blood is simply dissolved in the fluid 

 plasma. 



Chemical Composition of the Blood in Bulk. Analyses of the 

 blood as a whole differ slightly, but the following table may be taken to 

 represent the average composition : 



Water 704 



Solids- 

 Corpuscles 130 



Proteids (of serum) 70 



Fibrin (of clot) 2.2 



Fatty matters (of serum) . . . . 1,4 



Inorganic salts (of serum) . . . . 6 



Gases, kreatin, urea and other extractive ) 



matter, glucose and accidental substances J b<4 ~ 



216 



1000 

 Variations in the Composition of healthy Blood. 



The conditions which appear most to influence the composition of 

 the blood in health are these: Sex, Pregnancy, Age, and Temperament. 

 The composition of the blood is also, of course, much influenced by diet. 



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



