232 TEXT-BOOK OF PHYSIOLOGY 



variations in physiologic processes. The specific gravity is diminished by 

 the ingestion of liquids and abstinence from solid food. It is increased by 

 abstinence from liquids, by the ingestion of dry food, and by the elimination 

 of large quantities of water by the lungs, skin, and kidneys. 



Inasmuch as the specific gravity of the blood varies from the normal in 

 one direction or the other in certain pathologic states, it is deemed desirable 

 for clinical purposes to determine the extent of this variation. Among the 

 methods suggested for this purpose that of Hammerschlag is the one most 

 generally resorted to. It is based on the principle, that a fluid in which a 

 drop of blood neither rises nor falls must have the same specific gravity as 

 the blood itself. As the specific gravity of the blood varies in different 

 pathologic states it is essential that the fluid employed is of such a character 

 that its specific gravity can be quickly varied in one direction or the other. 

 To meet this indication a fluid, a mixture of chloroform (specific gravity 

 1.526) and benzol (specific gravity 0.889) is first prepared in such propor- 

 tions that the mixture has a specific gravity of about 1.040. With a pipette 

 a drop of blood is then placed in the mixture. If the drop rises the specific 

 gravity of the mixture is greater than that of the blood. Benzol is then 

 gradually added until the drop remains stationary. At this moment the 

 specific gravity of the mixture is the same as that of the blood. If the 

 drop falls the specific gravity of the mixture is less than that of the blood. 

 Chloroform is then gradually added until the drop remains stationary. 

 At this moment the specific gravity of the mixture is the same as that of 

 the blood. In either case the specific gravity of the mixture is determined 

 with a suitable hydrometer and the figure observed attributed to the blood. 



The Reaction. In accordance with the ideas of physical chemistry the 

 reaction^ of any fluid is dependent on the relative proportions of H + ions 

 and OH ions present. If the H + ions are in excess the fluid is acid in 

 reaction; if the OH ions are in excess the fluid is alkaline in reaction; 

 if both are present in equal proportions the fluid is neutral in reaction. 

 The degree of the reaction of any fluid containing a number of chemic 

 compounds in solution and undergoing ionization will therefore be de- 

 pendent on the relative proportion of the H + ions and the OH ions that 

 make their appearance. 



Tested by the methods of physical chemistry the plasma of the blood 

 and the lymph as well is neutral in reaction, a condition essential to the 

 maintenance of tissue vitality. A neutral reaction, much less an acid 

 reaction would on the contrary be destructive to tissue vitality. The 

 plasma and the lymph contain as normal constituents, though doubtless 

 in a state of more or less complete ionization, a number of inorganic 

 salts derived from the food such as sodium, potassium, calcium, and 

 magnesium salts. The alkaline bases of the salts absorb or combine 

 with the acids, e.g., sulphuric and phosphoric acid, which arise from the 

 metabolism of the proteins and thus promote their elimination from the 

 body. In so doing they prevent the accumulation of the^H + ions and thus 

 tend to maintain the relative preponderance of the OH ions and hence 

 the alkalinity of the blood. 



As the alkaline bases under normal conditions are in excess of the acids, 

 they are in general termed the " alkaline reserve." The quantity of the 



