KIDNEYS, SKIN, AND GENERAL EXCRETION. 393 



That the phenomena of secretion are controlled by nerves 

 is evident for several reasons. States of the emotions, ex- 

 cessive pain, may affect the amount of such secretion 

 through this, innervation. It has, however, not been possi- 

 ble so far to trace these nerves to their final terminations. 

 There is no objection, though, against believing that these 

 nerves end in or near the epithelium cells of the tubules and 

 capsule, and that the increased or decreased secretion is not 

 wholly due to vascular changes. A number of drugs known 

 as diuretics affect the kidneys, causing an increase in its se- 

 cretion, but whether due to vascular changes or to a direct 

 stimulus of the secreting cells is not clear. 



THE KIDNEY SECRETION. 



The amount of urine eliminated in one day is about 

 1,600 grams. Its specific gravity is 1,020, that is, slightly 

 heavier than water. Its composition consists of 96 per cent. 

 water and 4 per cent solids. These 4 per cent, solids are: 



(1) Urea and its derivatives; uric acid, kreatin and xan- 

 thin. 



(2) Aromatic bodies such as hippuric acid, kresol, indol, 



(3) Oxalic acid in the form of calcium oxalate. 



(4) Pigments. 



(5) Mineral salts; sodium chloride, phosphates and 

 sulphates of potassium, calcium, magnesium, ammonia in 

 the form of urate. In addition to these solids slight quan- 

 tities of CO 2 are dissolved in the liquid. 



The most important of these solid bodies is the urea. Its 

 chemical composition is given in the formula CO (NH 2 ) 2 - 

 It is the final product of nearly all the albumens and albu- 

 minoids which have been used up in the tissues. It is, 

 therefore, the substance in which the nitrogen of the foods 

 is eliminated. Reference to the chemical formula will show 

 that it contains a relatively large amount of nitrogen. 

 This substance is interesting as having *been the first or- 

 ganic substance which was produced artificially in the la- 



