1020 



DIGESTION 



fecal material very shortly after arising, because the food which has 

 found lodgment in the transverse colon during the preceding night, 

 is then aided by gravity and a renewed irritability of the receptors 

 in exciting those long peristaltic waves which finally move it into the 

 vicinity of the anal orifice. The gradually increasing mass of rectal 

 contents finally stimulates the mucosa in a mechanical manner and 

 evokes those muscular responses which are required for its expulsion. 

 If the feeling of fulness experienced at this time, is neglected, the walls 

 of the rectum relax more fully, so that a much greater excitation will 

 be required to make them contract again. In man, Hertz 1 has shown 



Trunk 



2. Lumbar 



I. Lumtor gonylitn 



ILL 



JI.L.ganyl. 



m.L. 



FIG. 518. SCHEMA TO SHOW THE INNERVATION OF THE RECTUM AND INTERNAL SPHINC- 

 TER OF THE Aires, AND THE FORMATION OF THE HvpOGASTRic PLEXUS. (After Frankl- 

 Hochwart and Frohlich.) 



that the intrarectal pressure may rise to 30 and 40 mm. Hg. before the 

 act of defecation is actually initiated. 



While defecation is a reflex phenomenon, it also embraces a definite 

 voluntary factor. The former consists in peristaltic contractions of 

 the rectum and the inhibition of the internal sphincter, whereas the 

 latter comprises the relaxation of the external sphincter and the activa- 

 tion of the abdominal press. Under normal conditions, these reflexes 

 may be counteracted if necessary, by volition, but only until the sen- 

 sory stimuli become so powerful that they are able to overcome the 

 volitional efforts. The reflex center for defecation is situated in 

 the lumbar segment of the spinal cord, whence efferent and afferent 



1 Guy's Hosp. Rep., 1907. 



