162 THE BLOOD 



Even in thin layers blood is not transparent, because much of the 

 light entering it is reflected from the surfaces of the formed elements. 

 Its opacity, therefore, is caused chiefly by the red corpuscles. 



Odor and Taste. — Blood possesses a salty taste and a faint odor. 

 The latter is caused by volatile fatty acids held in solution, and be- 

 comes more distinct when concentrated sulphuric acid is added to the 

 blood. While both factors vary somewhat even in animals of the 

 same species, the odor of blood is usually sufficiently strong so that 

 an animal, when wounded, may easily be followed by another pos- 

 sessing a keen sense of smell. 



The Temperature. — The factor responsible for the temperature of 

 the blood is the metabolism of the tissues. The heat given off by the 

 cells is retained in full measure by the blood as long 

 as it traverses well-protected channels, but is dissipated 

 by it as soOn as it reaches the more exposed parts of 

 the body. Thus, it is found that the highest tempera- 

 ture prevails in the intrahepatic veins and the lowest 

 in the blood-vessels of the fingers, nose and ears. 

 Differences between 36° C. (97.7° F.) and 39.7° C. 

 (103° F.) have been recorded; a fair average value for 

 the blood in central channels is 38° C. (100° F.). 



_, j^ Specific Gravity. — This factor may be determined 



Pycnometer. by means of a pycnometer. A small flask of glass, 

 large enough to contain from 3 to 5 c.c. of blood, is 

 weighed when empty and when filled with distilled water. The 

 value so obtained is compared later on with the weight of this flask 

 when flUed with blood. It need scarcely be mentioned that the 

 temperature must be the same for the two weighings. If the 

 amount of blood is very small, short capillary tubes of glass may 

 be employed, and, if large, flasks of a greater capacity than 5 c.c. 

 The larger pycnometers are equipped with a thermometer as well as 

 with an extra bulbular enlargement for the reception of that portion 

 of the blood which is forced out when the flask is filled. A second 

 method consists in permitting a drop of blood to fall into a fluid 

 of known speciflc gravity. Various mixtures have been advocated for 

 this purpose, for example, glycerin and water, or benzol and chloro- 

 form.^ To begin with, the specific gravity -should be adjusted at about 

 1.050. The procedure consists in quickly increasing or decreasing the 

 specific gravity of this medium by the addition of a certain quantity 

 of one or the other of these liquids, until the individual droplets of 

 blood are held in the central mass of the mixture. At this moment 

 their densities may be said to be practically the same. It is then only 

 necessary to determine the specific gravity of the mixture by means of 

 an ordinary hydrometer corrected to the mixture used. If a compara- 

 tive study is made with different samples of blood, care must be taken 

 to make the collections always from the same blood-vessel and under 



» Hammershlag, Zeitschr. fiir klin. Med., xx, 1892, 444. 



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