486 TEXT-BOOK OF PHYSIOLOGY. 



another sensation due to the passage of a small quantity of urine into the 

 urethra. In a young child the arrival of the reflex impulses in the spinal 

 cord is immediately followed by an inhibition of the sphincter center and 

 a stimulation of the detrusor center, as a result of which the sphincter muscle 

 relaxes and the detrusor muscle contracts, thus expelling the urine. In 

 the adult if the act of urination is to be permitted volitional impulses 

 descend the cord which cause a contraction of the abdominal muscles which 

 through pressure on the bladder assist in the expulsion of the urine. If the 

 act of urination is to be suppressed volitional impulses descend the cord and 

 cause a contraction of the sphincter urethrae muscle and thus temporarily 

 prevent the discharge of the urine. After urination the entrance of urine 

 from the ureter brings about a reflex contraction of the sphincter muscle 

 by stimulation of the lumbar sphincter center and an inhibition of the 

 detrusor muscle by stimulation of the lumbar inhibitor center in con- 

 sequence of which the urine is received and retained until the pressure of the 

 accumulated urine again causes its expulsion. 



PERSPIRATION; SEBUM. 



The perspiration or sweat, the chief secretion of the skin, is a clear colorless 

 fluid, slightly acid in reaction and saline to the taste. Its specific gravity 

 varies from 1.003 to 1-006. Unless collected from the soles of the feet and 

 the palms of the hand, it is apt to be mixed with epithelial cells and sebum. 

 The total quantity of perspiration secreted daily has been variously estimated 

 at from 700 to 1000 grams; the exact amount, however, is difficult of determi- 

 nation, for the reason that the rate of secretion varies greatly with variations in 

 temperature, food, drink, season of the year, etc. 



Chemic analysis of the sweat shows that it contains but from 0.5 to 2.5 

 per cent, of solid constituents, the variation in the percentage depending on 

 the quantity of water secreted. The solids consist of traces of urea, neutral 

 fats, lactic and sudoric acids in combination with alkaline bases, and inorganic 

 salts (Fovel). Other observers, however, have not been able to detect the 

 presence of either lactic or sudoric acid. Urea is a constant ingredient, 

 though its percentage is extremely small, possibly not more than o.i per 

 cent. The amount, however, may be very much increased in uremic 

 conditions, the result of acute or chronic disease of the kidneys. The inor- 

 ganic constituents consist mainly of sodium chlorid and alkaline and 

 earthy phosphates. Carbonic acid is also present in the free state as well 

 as in combination with alkaline bases. 



The very small quantity of the solid constituents in the sweat, taken in 

 connection with the fact that it is excreted most abundantly when the external 

 temperature is high, indicates that it is not so important as an excrementi- 

 tious fluid as it is as a means for the regulation of the temperature of 

 the body. 



The sweat is a product of the secretory activity of specialized glands, 

 the sweat-glands, embedded in the skin, to the histologic structures of 

 which they bear a special relation. 

 THE SKIN. 



The skin is a complexly organized structure investing the entire external 

 surface of the body. Its total area varies from 1.17 to 1.35 square meters in 



