DIGESTION. 203 



the feces are expelled. These complex muscle actions are also aided by the 

 voluntary contractions of the diaphragm and abdominal muscles. 



Nerve Mechanism of Defecation. The act of defecation is primarily 

 reflex though somewhat influenced by voluntary efforts. The reflex charac- 

 ter of the act is especially noticeable in young children in whom by reason of 

 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 nerves 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 an- 

 other their activities. If the act of defecation is to take place there is an 

 inhibition of the nerve-centers maintaining the tonus or contraction of the 

 two sphincter muscles and a stimulation of the nerve-centers exciting or 

 augmenting the contraction of the rectal muscles with the result of a dis- 

 charge of the fecal mass. 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 efferent nerve-fibers for the external sphincter 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 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 nerve 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 



