212 TEXT-BOOK OF PHYSIOLOGY. 



passage of the feces into the rectum there is a relaxation of the sphinc- 

 ter muscles and a contraction of the longitudinal and circular mus- 

 cular fibers, in consequence of which the feces are expelled. These 

 complex muscular actions are also aided by the voluntary contrac- 

 tions of the diaphragm and abdominal muscles. 



Nerve Mechanism of Defecation. The act of defecation is 

 primarily reflex, though somewhat influenced by voluntary efforts. 

 UncTer normal conditions the sphincter muscles governing the anal 

 orifice are firmly contracted, thus preventing the escape of gases or 

 semi-solid matter. This tonic contraction is maintained by a nerve- 

 center located in the lumbar region of the spinal cord. This center 

 can be inhibited and the sphincter muscle relaxed, either reflexly, by 

 impressions made on the mucous membrane of the rectum, or directly, 

 by volitional impulses from the brain. When the desire to evacuate 

 the bowels is experienced, the impressions made by the feces on the 

 afferent nerves of the rectal mucous membrane develop nerve im- 

 pulses, which are transmitted to the rectal center and to the brain. 

 If the act of defecation is to take place, the inhibitory effect of the 

 rectal impulses is strengthened by volitional impulses descending 

 the cord, followed by a relaxation of the sphincter. At the same time 

 motor impulses are transmitted to the rectal and abdominal muscles, 

 which, cooperating, expel the rectal contents. If the act is to be 

 suppressed, the controlling influence of the rectal or sphincter center 

 is strengthened and the reflex mechanism for a while held in abey- 

 ance. The exact course of the afferent and efferent nerves concerned 

 in this reflex is yet a subject of investigation. 



