734 PHYSIOLOGY OF DIGESTION AND SECRETION. 



than that of the external. Like the rest of the rectum, it receives 

 motor fibers from the hypogastric plexus by way of the nervus 

 erigens, and inhibitory fibers from the same plexus by way of the 

 hypogastric nerve. It has been possible to show experimentally 

 that each of these sets of fibers may be acted upon reflexly, for 

 example, by stimulation of the sensory nerves in the sciatic. 

 The reflex takes place in this case through the lower portion of 

 the cord. Both the hypogastric nerve and the N. erigens con- 

 tain also afferent fibers. Stimulation of the central end of the 

 severed N. erigens gives a reflex inhibition through the hypo- 

 gastric nerve, and stimulation of the central stump of the cut 

 hypogastric causes a reflex contraction through the N. erigens. 

 The act of defecation as it occurs normally is partly a voluntary 

 and partly an involuntary act. The involuntary act consists in 

 peristaltic contractions of the rectum or, indeed, of the whole 

 colon, together with an inhibition of the sphincters. Whether 

 the inhibition of the sphincters is normally entirely an involuntary 

 reflex cannot be stated definitely. No doubt the sensory stimuli 

 arising from the accumulation of fecal material would eventually 

 cause in this way a relaxation of the sphincters, but the act of 

 defecation usually takes place before such a strong necessity arises. 

 It is initiated usually by a voluntary act, and it is possible that in 

 such cases the relaxation of both sphincters may be effected by 

 voluntary inhibition acting upon the spinal centers. 



The voluntary factor in defecation consists mainly in the 

 contraction of the abdominal muscles. When these latter 

 muscles are contracted and at the same time the diaphragm is 

 prevented from moving upward by the closure of the glottis, 

 the increased abdominal pressure is brought to bear upon the 

 abdominal and pelvic viscera, and aids strongly in pressing the 

 contents of the descending colon and sigmoid flexure into the 

 rectum. The pressure in the abdominal cavity is still further 

 increased if a deep inspiration is first made and then maintained 

 during the contraction of the abdominal muscles. Hertz, on 

 the basis of his skiagraphic observations, insists that simul- 

 taneously with the contraction of the abdominal muscles and 

 the closure of the glottis the diaphragm is also contracted and 

 thus aids in bringing pressure to bear upon the pelvic organs. 

 Although the act of defecation is normally initiated by voluntary 

 effort, it may also be carried out as a purely involuntary reflex 

 when the sensory stimulus is sufficiently strong. Goltz* has 

 shown that in dogs in which the spinal cord had been severed 

 in the lower thoracic region defecation was performed normally. 

 In later experiments, in which the entire spinal cord was removed 



* "Archiv f. die gesammte Physiologie," 8, 160, 1874; 63, 362, 1896. 



