208 TEXT-BOOK OF PHYSIOLOGY 



the imperfect development of the brain there is a lack of volitional control. 

 During the intervals of defecation the anal orifice is tightly closed by the 

 tonic contraction of the internal non-striated sphincter and the external 

 striated sphincter muscles, thus preventing the escape of gases or semi-liquid 

 material. The tonic contraction of both muscles is maintained by the 

 activity of nerve-centers located in the lumbar region of the spinal cord. 

 The circular and longitudinal fibers of the rectum proper are at the same 

 time in a relaxed or inhibited condition, the result of an inhibition, or a want 

 of stimulation, of their governing nerve-center or centers in the lumbar region 

 of the spinal cord. When the desire to evacuate the bowel is experienced, 

 impressions are being made by the feces on the afferent nerve endings in the 

 mucous membrane of the sigmoid flexure and of the rectum. The nerve 

 impulses thus developed are transmitted to the defecation or rectal nerve- 

 centers in the spinal cord and to the cerebrum and influence in one direction 

 or another their activities. 



In the young child the arrival of the transmitted impulses in the spinal 

 cord is immediately followed by an inhibition of the sphincter centers and a 

 stimulation of the rectal muscle centers, as a consequence of which, the 

 sphincter muscles relax and the expulsive muscles contract thus discharging 

 the feces. 



In the adult if the act of defecation is to be permitted the same mechan- 

 ism is brought into action. In their expulsive efforts, these latter muscles 

 are assisted by the contraction of the diaphragm, abdominal, and other 

 muscles in response to volitional efforts. After the expulsion of the feces, 

 there is a return to the former condition, namely, a relaxation or inhibition 

 of the rectal muscles and a contraction of the sphincter muscles. If the act of 

 defecation is to be suppressed, the controlling influence of the nerve-center 

 on the contraction of the external sphincter may by an act of volition be 

 strengthened and the action of the reflex mechanism for a while antagonized. 

 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 external sphincter mus- 

 cle 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 hemorrhoidal 

 nerve directly to the muscle. 



The efferent nerve-fibers, for the longitudinally and circularly arranged 

 muscle-fibers of the rectum, 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 hypogastric 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 along the sides of the rectum 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 



