472 HANDBOOK OF PHYSIOLOGY. 



secreted ; by the length of time which has elapsed since the last meal ; 

 and by several other accidental circumstances. The existence of these 

 causes of difference in the composition of the urine has led to the secre- 

 tion being described under the three heads of Urina sanguinis, Urina 

 potus, and Urina cibi. The first of these names signifies the urine, or 

 that part of it which is secreted from the blood at times in which 

 neither food nor drink has been recently taken, and is applied especially 

 to the urine which is evacuated in the morning before breakfast. The 

 term urina potus indicates the urine secreted shortly after the intro- 

 duction of any considerable quantity of fluid into the body: and the 

 urina cibi, the portions secreted during the period immediately succeed- 

 ing a meal of solid food. The last kind contains a larger quantity of 

 solid matter than either of the others ; the first or second, being largely 

 diluted with water, possesses a comparatively low specific gravity. Of 

 these three kinds, the morning urine is the best calculated for analysis 

 in health, since it represents the simple secretion unmixed with the 

 elements of food or drink; if it be not used, the whole of the urine 

 passed during a period of twenty-four hours should be taken. The 

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

 widely on both sides of the usual average. It may vary 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; sometimes descending, in albu- 

 minuria, 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 experiments, 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 fluid ounces (1^ to 2 litres). 



Abnormal Constituents. In disease, or after the ingestion of special 

 foods, various abnormal substances occur in urine, of which the follow- 

 ing may be mentioned Serum- albumin, Globulin, Ferments (appar- 

 ently present in health also), Proteases, Mood, Sugar, Bile acids and 

 pigments, Casts, Fats, various Salts taken as a medicine, Micro-organ- 

 isms of various kinds, and other matters. 



The Solids of the Urine. 



Urea (CH^NgO). Urea is the principal solid constituent of the 

 urine, forming nearly one-half of the total quantity. It is also the 

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

 nitrogen which is derived from the metabolic changes in the tissues as 

 well as that which is derived from any superfluous food is excreted 



