386 AX AWERKAX TEXT-BOOK OF PHYSIOLOGY. 



start normal peristalsis. Violent movements may be produced also by shut- 

 ting off the blood-supply, and again temporarily when the supply is re-estab- 

 lished. A condition of dyspnoea may also start movements in the intestines 

 or in some eases inhibit movements which are already in progress, the stimu- 

 lus in this ease seeming to act upon the central nervous system and to stimu- 

 late both the motor and the inhibitory fibres. Oxygen gas within the bowels 

 tend- to suspend the movements of the intestine, while C0 2 , CH 4 , and H 2 S 

 act as stimuli, increasing the movements. Organic acids, such as acetic, 

 propionic, formic, and caprylic, which may be formed normally within the 

 intestine as the result of bacterial action, act also as strong stimulants. 1 



Defecation. — The undigested and indigestible parts of the food, together 

 with some of the debris and secretions from the alimentary tract, are carried 

 slowly through the large intestine by its peristaltic movements and eventually 

 reach the sigmoid flexure and rectum. Here the nearly solid material stimu- 

 lates by its pressure the sensory nerves of the rectum and produces a distinct 

 sensation and desire to defecate. The fecal material is retained within the rectum 

 by the action of the two sphincter muscles which close the anal opening. One 

 of these muscles, the internal sphincter, is a strong band of the circular layer 

 of involuntary muscles which forms one of the coats of the rectum. When 

 the rectum contains fecal material this muscle seems to be thrown into a 

 condition of tonic contraction until the act of defecation begins, when it is 

 relaxed. The sphincter is composed of involuntary muscle and is innervated 

 by fibres arising partly from the sympathetic system, and in part through 

 the nervus erigens, from the sacral spinal nerves. The external sphincter ani 

 is composed of striated muscle-tissue and is under the control of the will to 

 a certain extent. When, however, the stimulus from the rectum is sufficiently 

 intense, voluntary control is overcome and this sphincter is also relaxed. 

 The act of defecation is in part voluntary and in part involuntary. The 

 involuntary factor is found in the contractions of the strongly developed mus- 

 culature of the rectum, especially the circular layer, which serves to force the 

 feces onward, and the relaxation of the internal sphincter. It seems that these 

 two acts are mainly caused by reflex stimulation from the lumbar spinal cord, 

 although it is probable that the rectum, like the rest of the alimentary tract, 

 i- capable of automatic contractions. The rectal muscles receive a double 

 nervous supply, containing physiologically both motor and inhibitory fibres. 

 Some of these fibres come from the nervus erigens by way of the hypogastric 

 plexus, and some arise from the lumbar cord and pass through the correspond- 

 in.: sympathetic ganglia, inferior mesenteric ganglion, and hypogastric nerve. 

 It has been asserted that stimulation of the nervus erigens causes contrac- 

 tion of the longitudinal muscles and inhibition of the circular muscles, while 

 stimulation of the hypogastric nerve causes contraction of the circular muscles 

 and inhibition of the longitudinal layer. This division of activity is not 

 confirmed by the recent experiments of Langley and Anderson. 2 



The voluntary factor in defecation consist- in the inhibition of the external 

 1 Buk;ii : Archivfiir exper. PaihoLogie und Pharmakologie, L888, lid. 24, S. 153. 2 Op. eit. 



