434 TEXT-BOOK OF PHYSIOLOGY. 



which collectively encircle the entire organ. As these fibers by their 

 contraction expel the urine from the bladder, they are known col- 

 lectively as the detrusor urina muscle. At the exit of the bladder 

 the circular fibers are somewhat increased in number, giving rise to 

 the appearance of a distinct muscle which has been termed the 

 sphincter vesica muscle. The urethra just beyond the bladder is 

 provided with a distinct circular muscle composed of striated fibers, 

 the sphincter urethra muscle. When the urine passes into the blad- 

 der it is retained there and prevented from escaping by the contrac- 

 tion of this latter muscle. Under normal conditions the urine ac- 

 cumulates before its presence gives rise to a characteristic sensation 

 and the desire for urination. 



Nerve Mechanism of Urination. The muscle mechanisms 

 which retain as well as expel the urine are under the control of the 

 nerve system. The sphincter urethrae muscle, by which the orifice 

 of the bladder is closed, is kept in a state of tonic contraction by nerve 

 impulses coming from the spinal cord through the anterior roots of 

 the third and fourth sacral nerves. The detrusor urinae muscle is 

 excited to contraction by impulses coming likewise through the sacral 

 nerves from the cord. TTipjQpntpr_f>f origin f nr t^ p sp ^""^ sets_ n f 

 motpr nerves are located in the cord in the neighborhood of the fifth 

 lumbar vertebra. The expulsion of the urine is largely a reflex act, 

 though under the control of the will. When the desire to urinate is 

 experienced, nerve impulses are coming through sensory nerves from 

 the mucous membrane of the bladder which are reflected to the centers 

 governing the sphincter urethrae and detrusor urinae muscles and to 

 the brain. The effect of the reflected impulses is to inhibit the 

 sphincter center and to stimulate the detrusor center. If the act of 

 urination is to be permitted, volitional impulses descend through 

 the^spinal cord which have "the effect of still further inhibiting the 

 sphincter center and stimulating the detrusor center, the result being 

 a relaxation of the sphincter muscle and a contraction of the detrusor 

 muscle and the expulsion of the urine. If the act of urination is to be 

 suppressed, volitional impulses inhibit the detrusor center and stimu- 

 late the sphincter. 



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 determination, for the reason 



