358 HAND-BOOK OF PHYSIOLOGY. 



In accordance with the various circumstances above-mentioned, the 

 specific gravity of the urine may, consistently with health, range widely 

 on both sides of the usual average. The average healthy range may be 

 stated at from 1015 in the winter to 1025 in the summer; but variations 

 of diet and exercise, and many other circumstances, may make even greater 

 differences than these. In disease, the variation may be greater; some- 

 times descending, in albuminuria, to 1004, and frequently ascending in 

 diabetes, when the urine is loaded with sugar, to 1050, or even to 1060. 



Quantity. The total quantity of urine passed in twenty-four hours 

 is affected by numerous circumstances. On taking the mean of many 

 observations by several experimenters, the average quantity voided in 

 twenty-four hours by healthy male adults from 20 to 40 years of age has 

 been found to amount to about 52 -J- fluid ounces (1-J- to 2 litres). 



Abnormal Constituents. In disease, or after the ingestion of 

 special foods, various abnormal substances occur in urine, of which the 

 following may be mentioned serum-albumin, globulin, ferments (ap- 

 parently present in health also), blood, sugar, bile acids, and pigments, 

 fats, oxalates, various salts taken as medicine, and other matters, as bac- 

 teria and renal casts. 



THE SOLIDS OF THE UKINE. 



Urea (CH 4 N 2 0). Urea is the principal solid constituent of the 

 urine, forming nearly one-half of the whole quantity of solid matter. It 

 is also the most important ingredient, since it is the chief substance by 

 which the nitrogen of decomposed tissue and superfluous food is excreted 



from the body. For its removal, the secre- 

 tion of urine seems especially provided; and 

 by its retention in the blood the most per- 

 nicious effects are produced. 



Properties. Urea, like the other solid 

 constituents of the urine, exists in a state 

 of solution. But it may be procured in the 

 solid state, and then appears in the form of 

 delicate silvery acicular crystals, which, 

 under the microscope, appear as four-sided 

 prisms (Fig. 247). It is obtained in this 

 FIG. 247.-Crystaisof urea. state b J evaporating urine carefully to the 



consistence of honey, acting on the inspis- 

 sated mass with four parts of alcohol, then evaporating the alcoholic 

 solution, and purifying the residue by repeated solution in water or alco- 

 hol, and finally allowing it to crystallize. It readily combines with 

 some acids, like a weak base; and may thus be conveniently procured 

 in the form of crystals of nitrate or oxalate of urea. 



