32 PHYSIOLOGICAL CHEMISTRY. 



matters are usually separated by alcohol, and oils may be dissolved out 

 by ether. When a chemical decomposition is unavoidable, it must be 

 kept in sight and afterward corrected. Thus, the sodium glycocholate 

 of the bile is separated from certain other ingredients by precipitating 

 it with plumbic acetate, forming lead glycocholate ; but this is afterward 

 decomposed in turn by sodium carbonate, reproducing the original 

 sodium glycocholate. Certain organic materials of peculiar physiological 

 activity are extracted by means of glycerine, which preserves them in- 

 definitely in an unaltered condition ; and as a general rule the improve- 

 ments in this branch of investigation consist in exact regulation of the 

 temperature, the avoidance of strong acid and alkaline reagents, the 

 employment of mild solvents and precipitating solutions, and in especial 

 care that the substance to be examined is obtained in a fresh condition, 

 unchanged by cadaveric alterations. By this means we may form a 

 tolerably correct estimate of the nature, quantity, and properties of the 

 constituent materials of the living organism. 



The manner in which these ingredients are associated together is also 

 deserving of notice. In every animal solid and fluid, there is a number 

 of different substances present in certain proportions, so united with 

 each other that the mixture presents a homogeneous appearance. But 

 this union is of a complicated character ; and the presence of each in- 

 gredient depends, to a certain extent, upon that of the others. Some 

 of them, such as the alkaline carbonates and phosphates, are in direct 

 solution in tfc$ water. Some, which are insoluble in water, are retained 

 in solution by the presence of other soluble substances. Thus, the 

 insoluble lime phosphate of the urine is held in solution by the acid 

 sodium biphosphate, also present as an ingredient. In the alkaline 

 blood-plasma, on the other hand, the lime phosphate is liquefied by 

 union with the albumen, which is itself soluble in the water of the 

 plasma. The same substance may be fluid in one part of the body, 

 and solid in another part. Thus, in the blood and secretions the 

 water is fluid, and holds other substances in solution ; while in the 

 bones and cartilages it is solid, by its union with the animal and 

 saline ingredients, abundantly present in the solid form. In the blood, 

 the lime phosphate is fluid by solution in the albumen ; but in the 

 bones it forms a solid substance with the animal matter of the 

 osseous tissue ; and the union of the two is as intimate and homo- 

 geneous in the bones as in the blood. An animal ingredient, there- 

 fore, never exists alone in any part of the body, but is always 

 associated with a number of others, by homogeneous mixture or 

 mutual solution. 



The proportion in which each ingredient is present, in any animal 

 solid or fluid, is, as a rule, characteristic of that tissue or secretion, 

 and contributes largely to its physiological characters. Thus, water 

 is present in large quantity in the perspiration and the saliva, but in 

 small quantity in the bones and teeth. Sodium chloride is compara- 

 tively abundant in the blood and deficient in the muscles, On the 



