EXCRETION 475 



there is a return to the former condition, namely, a contraction of the sphincter 

 muscles and a gradually inhibition of the detrusor muscle. If the act of 

 urination is to be suppressed volitional impulses descend the cord and cause 

 an increased contraction of the sphincter urethrae muscle, whereby the action 

 of the reflex mechanism is for a while opposed. 



The nerve mechanism therefore involves both efferent and afferent nerves 

 as well as nerve-centers in the lumbo-sacral region of the spinal cord. 



Efferent Nerves. The efferent nerve-fibers for the sphincter urethrae 

 muscle have their origin in the spinal cord from which they pass by way of 

 the third and fourth sacral nerves, the pelvic nerve and the inferior hemor- 

 rhoidal nerve directly to the muscle. 



The efferent nerve-fibers, for the detrusor muscle, including the specialized 

 portion, the internal sphincter, have their origin in nerve-cells in the lumbo- 

 sacral region of the spinal cord and pass to their destination by two paths. 

 The fibers in the first path leave the spinal cord by way of the second to the 

 fifth lumbar nerves, then pass into and through the sympathetic chain, 

 through the inferior splanchnics to the inferior mesenteric ganglion around 

 the cells of which their terminal branches arborize; from the cells of this 

 ganglion new fibers emerge which pass through the hypo gastric nerves to the 

 muscles. The fibers of the second path leave the spinal cord by way of the 

 second to the fourth sacral nerves, then pass into the pelvic or erigens nerve 

 to small ganglia (pelvic ganglia) along the sides of the bladder around the 

 cells of which their terminal branches arborize; from the cells of these ganglia 

 new nerve-fibers emerge which pass directly to the muscles. In both paths 

 the nerves coming from the cord are pre-ganglionic, those coming from the 

 ganglia, post-ganglionic. 



The central mechanism that excites and coordinates the activities of the 

 vesical muscles is situated in the lumbo-sacral region of the spinal cord and 

 is designated the vesical or urination center. 



Afferent Nerves. The afferent nerve-fibers that excite the central mech- 

 anism to activity, are contained in both the nerve-paths described in fore- 

 going paragraphs and enter the spinal cord in the dorsal roots of the lumbar 

 and spinal nerves. 



Though the origin, course and distribution of the nerves composing this 

 mechanism are fairly well known, their mode of action is somewhat obscure 

 and the results of experimentation not always in accord. According to v. 

 Zeissl stimulation of the peripheral ends of the divided hypogastric nerves 

 causes mainly a contraction of the sphincter muscles and a relaxation of the 

 detrusor muscle, while a stimulation of the peripheral ends of the divided 

 pelvic nerves causes a vigorous contraction of the detrusor muscle and a re- 

 laxation of the sphincter muscles. The lumbar centers would therefore 

 cause a reception and a retention of the urine, and the sacral centers would 

 cause 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 



