THE URIXE. 325 



the most important of these substances are those which contain nitrogen. 

 This element indicates their derivation from the albumenoid ingredients 

 of the body, and they present in other respects a mutual analogy in 

 chemical properties and composition. They accordingly form a group 

 of organic substances, resembling each other in origin, constitution, 

 and physiological destination. They are eliminated from the body by 

 the urine, of which they form the characteristic ingredients. 



The urine is therefore solely an excretion. It is a solution of the 

 nitrogenous excrementitious matters of the body ; and by its abundance 

 and composition it indicates the activity of metamorphosis in the nitro- 

 genous ingredients of the tissues and fluids. It also contains most of 

 the mineral salts discharged from the body ; and by the water which 

 holds these matters in solution it represents a large proportion of the 

 fluids passing through the system. Furthermore, accidental or abnor- 

 mal ingredients, introduced into the blood, are usually eliminated by 

 this channel, and appear as temporary ingredients of the urine. The 

 constitution arid variations of the urine during health, and its altera- 

 tion in disease, are regulated by the corresponding changes of nutrition 

 in the body at large. It is therefore one of the most essential products 

 of the animal system, and its formation is second in importance only to 

 the function of respiration. 



Physical Properties of the Urine. 



The urine is a clear, amber-colored fluid, of a watery consistency 

 and distinctly acid reaction. It is usually so nearly transparent, that 

 no turbidity is perceptible by ordinary diffused light. It contains, 

 however, a small quantity of mucus from the urinary bladder, which 

 becomes visible as a faint opalescence when a sunbeam is made to pass 

 through it in a lateral direction. After remaining for some hours at 

 rest in a cylindrical vessel, the mucus subsides, forming a light cloud 

 at the bottom and leaving the supernatant fluid clear. The average 

 specific gravity of healthy urine, in the adult, is from 1020 to 1025 ; 

 and its daily quantity about 1200 cubic centimetres. 



Variations in Quantity, Acidity, and Specific Gravity. The urine 

 is habitually discharged from the bladder five or six times in the twenty- 

 four hours, each specimen showing more or less variation in its physical 

 properties. This depends on the changing conditions of the body, as 

 to rest, exercise, food, drink, sleep, and wakefulness. In the same 

 person, leading a uniform mode of life, the diurnal variations of the 

 urine follow each other with considerable regularity ; though they may 

 not be altogether the same in different individuals. As a rule, the 

 urine which collects during the night and is first discharged in the 

 morning is strongly colored, of high specific gravity, with a very dis- 

 tinct acid reaction. During the forenoon it is pale and of diminished 

 density ; .its specific gravity often falling so low as 1018 or 1015. At 

 the same time, its acidity diminishes or disappears ; so that it may be 

 either faintly acid, neutral, or slightly alkaline. Its density and depth 



