URINE AND ITS CONSTITUENTS. 703 



of fat is increased by adding cream, or by removing part of the lower layer 

 from milk which has separated into two layers (top milk). This mixture is 

 diluted with water to lower the percentage of protein ; milk-sugar and lime- 

 water are then added in different proportions, according to the quantity desired. 

 Although the difference in the composition of human and cows' milk is con- 

 siderable, the fuel value of both is nearly the same, about 315 calories to the 

 pound of milk. 



58. URINE AND ITS CONSTITUENTS. 



Excretion 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, 

 while passing through the system, deposits proteins and other sub- 

 stances, receiving in exchange the products formed by the oxidation 

 of the various tissues. These products are either gases (chiefly car- 

 bon dioxide), liquids (chiefly water), or solids held in solution by the 

 water. These waste materials must necessarily be eliminated from 

 the system, and this result is accomplished principally by the kidneys. 



The urine is the most important animal excretion ; in it are elimi- 

 nated the nitrogenous waste materials as well as most of the water 

 and soluble mineral substances. A study of the composition of the 

 urine will give important information regarding metabolism, the 

 nature of the chemical processes taking place within the body, as 

 also of the condition of the urinary organs. 



General properties. Normal human urine, when in a fresh state, 

 is a clear, transparent aqueous liquid, of a lighter or deeper amber 

 color, having a peculiar, faintly aromatic odor, a bitter, saline taste, 

 a distinct acid reaction on blue litmus-paper, and a specific gravity 

 heavier than water (averaging about 1.020). 



In urine, shortly after cooling, especially if it be concentrated, a 

 light, cloudy film of mucus is formed, which slowly sinks to the 

 bottom ; the acid reaction gradually increases, small yellowish-red 



QUESTIONS. Mention the five principal constituents of milk. To what 

 group of compounds does casein belong, how is it obtained, and what are its 

 reactions? Give tests for milk-sugar, and state how it may be distinguished 

 from grape-sugar. What physical and chemical changes does milk suffer on 

 standing? Describe the processes used for preserving milk; what are their 

 advantages and disadvantages? Give approximately the quantities of the 

 chief components of cream, skimmed milk, butter, buttermilk, curd, whey, and 

 cheese ; also state how the materials are obtained from milk. Describe the 

 advantages of the combined use of the lactometer and creamometer in testing 

 milk. What are the differences between human and cows' milk ? What is 

 paracasein? Give a process for the complete quantitative analysis of milk. 



