372 PHYSIOLOGICAL CHEMISTRY. 



514. What physical and what chemical changes does milk suffer on 

 standing? 



515. What acid is formed in milk on standing, and how does this 

 acid act on the casein? 



516. Describe the processes used for preventing the decomposition of 

 milk. What are their advantages and their disadvantages? 



517. Give the quantities of the chief components of cream, skimmed 

 milk, butter, buttermilk, curd, whey, and cheese, and state how these 

 substances are obtained. 



518. Why does the specific gravity of milk not indicate its purity and 

 richness ? 



519. Describe the advantages of the combined use of the lactometer 

 and creamometer in testing milk. 



520. Give a process for the complete quantitative analysis of milk. 



53. URINE AND ITS NORMAL CONSTITUENTS. 



Secretion of urine. It has been explained in a former chapter 

 how blood absorbs the digested food as chyle, how this is acted 

 upon by the atmospheric oxygen in the lungs, and how this 

 arterial blood, whilst passing through the system, deposits 

 albuminous and other substances, receiving in exchange the 

 products formed by the oxidation of the various tissues. These 

 products are either gases (chiefly carbon dioxide), liquids (chiefly 

 water), and solids held in solution by the water. These waste 

 solids must necessarily be eliminated from the system, and the 

 organs which accomplish this result are the kidneys. 



The process of separating the waste materials from the blood 

 is chiefly of a physical nature, partly a transudation or filtration, 

 and parti}' a diffusion or osmosis. The conditions essential for 

 such an exchange are given in the kidneys. Blood is separated 

 by delicate membranes from a thin, aqueous, saline solution; 

 the interchange taking place is chiefly a passage of the waste 

 crystalline products of the blood into the aqueous solution, 

 which is thereby gradually converted into urine, that liquid, 

 which is finally discharged, carrying off nearly the total quantity 

 of all the nitrogen taken into the system in the form of nitroge- 

 nous food. 



General properties. Normal human urine, when in a fresh 

 state, is a clear and transparent aqueous liquid, of a lighter or 



