STRUCTURE AND FUNCTION OF THE KIDNEYS. 363 



enced by the condition of the nervous system, being sometimes greatly 

 increased, e. g., in hysteria, and in some other nervous affections; and 

 at other times diminished. In some diseases it is enormously increased; 

 and its increase may be either attended with an augmented quantity of 

 solid matter, as in ordinary diabetes, or may be nearly the sole change, 

 as in the affection termed diabetes insipidus. In other diseases, e. g. , 

 the various forms of albuminuria, the quantity may be considerably di- 

 minished. A febrile condition almost always diminishes the quantity of 

 water ; and a like diminution is caused by any affection which draws off 

 a large quantity of fluid from the body through any other channel than 

 that of the kidneys, e. g., the bowels or the skin. 



Method of estimating the Solids. A useful rule for approximately 

 estimating the total solids in any given specimen of healthy urine is to 

 multiply the last two figures representing the specific gravity by 2.33. 

 Thus in urine of sp. gr. 1025, 2.33 x 25 = 58.25 grains of solids, are con- 

 tained in 1000 grains of the urine. In using this method it must be 

 remembered that the limits of error are much wider in diseased than in 

 healthy urine. 



Variations in the Specific Gravity. The average specific gravity 

 of the human urine is about 1020. The relative quantity of water and 

 of solid constituents of which it is composed is materially influenced by 

 the condition and occupation of the body during the time at which it is 

 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 terms 

 urina potus indicates the urine secreted shortly after the introduction of 

 any considerable quantity of fluid into the body; and the urina cibi, the 

 portions secreted during the period immediately succeeding 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 rep- 

 resents 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. Tne 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 circum- 

 stances, may make even greater differences than these. In disease, the 



