CHEMISTRY OF DIGESTION AND NUTRITION. 341 



secreted may be increased by the action of diuretics, such as potassium nitrate 

 and caffein. 



Tin 1 inorganic suits of urine consist chiefly of the chlorides, phosphates, 

 and sulphates of the alkalies and the alkaline earths. It may be said in 

 general that they arise partly from the salts ingested with the food, which 

 salts are eliminated from the Mood by the kidney in the water-secretion, and 

 in part they are formed in the destructive metabolism thai takes place in the 

 body, particularly that involving the proteids and related bodies. Sodium 

 chloride occurs in the largest quantities, averaging about 1 5 grams per day, of 

 which the larger part, doubtless, is derived directly from the -alt taken in the 

 food. The phosphates occur in combination with Ca and Mg, but chiefly as 

 the acid phosphates of Xa or K. The acid reaction of the urine is usually 

 attributed to these latter substances. The phosphates result in part from the 

 destruction of phosphorus-containing tissues in the body, but chiefly from 

 the phosphates of the food. The sulphates of urine are found partly in an 

 oxidized form as simple sulphates or conjugated with organic compounds, as 

 described above, and in small part in a neutral or unoxidized form, such as 

 potassium sulphocyanide, or ethyl-sulphide, (C 2 H 5 ) 2 S. The total quantity of 

 sulphuric acid eliminated is estimated to average about 2.5 grams per day. 

 Sulphur constitutes a constant element of the proteid molecule, and the quan- 

 tity of it eliminated in the urine may be used, as in the ease of nitrogen, to 

 determine the total destruction of proteid within a given period. 



Functions of the Skin. — The physiological activities of the skin are 

 varied. It forms, in the first place, a sensory surface covering the body, and 

 interposed, as it were, between the external world and the inner mechanism. 

 Nerve-fibres of pressure, temperature, and pain are distributed over its sur- 

 face, and by means of these fibres reflexes of various kinds are effected which 

 keep the body adapted to changes in its environment. The physiology of the 

 skin from this standpoint is discussed in the section on Cutaneous Sensations. 

 Again, the skin plays a part of immense value to the body in regulating the 

 body-temperature. This regulation, which is effected by variations in the 

 blood-supply or the sweat-secretion, is described at appropriate places in the 

 sections on Animal Heat, Circulation, and Secretion. In the female, during 

 the period of lactation, the mammary glands, which must be reckoned aiming 

 the organs of the skin, form an important secretion, the milk ; the physiology 

 of this gland is described in the sections on Secretion ami Reproduction. In this 

 section we are concerned with the physiology of the skin from a different stand- 

 point — namely, as an excretory organ. The excretions of the -kin are formed 

 in the sweat-glands and the sebaceous glands. The sweat-glands arc distrib- 

 uted more or less thickly over the entire surface of the body, with the excep- 

 tion of the prepuce and glans penis, while the sebaceous glands, usually in ( - 



Election with the hairs, are also found everywhere except upon the palms of 

 the hands and the soles of the feet. 



Sweat. — Sweat, or perspiration, which is the secretion of the sweat-glands, 

 is a colorless liquid with a peculiar odor and a salty taste. Its specific gravity 



