740 



PHYSIOLOGY OF DIGESTION AND SECRETION. 



exhibits a long latent period and a slow contraction and relaxa- 

 tion. Both the internal and the external sphincter are normally 

 in tonus and unite in protecting the anal opening. The force 

 of the tonic contraction of the internal is somewhat less (30 to 

 60 per cent.) than that of the external sphincter. The internal 

 sphincter, like the rest of the musculature of the rectum, receives a 

 double nerve supply, one set of fibers coming to it through the 

 pelvic nerve (sacral autonomies) and one set through the hypo- 

 gastric nerve by way of the inferior mesenteric ganglion (thoracic 

 autonomies). The action of the efferent fibers in these nerves 

 has been a matter of dispute, and possibly the results may differ 

 in different animals. According to one account the sphincter 

 responds in the same way as the rest of the circular muscle of the 

 rectum, that is to say, it is thrown into contraction by stimulation 

 of the pelvic nerve, and is inhibited by stimulation of the hypo- 

 gastric nerve. According to others the effects are just the reverse.* 

 Following the accounts given by the first group of observers it is 

 stated 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 



* Consult Frankl-Hochwart and Frohlich, "Archiv f. de ges Physiologie," 

 81, 420, 1900, and GaskeU, loc. cit. 



