CH. XXXVII.] NERVOUS MECHANISM OF LARGE INTESTINE 563 



If the bowels are opened once a day, the interval between a 

 meal and the evacuation of its residue varies between nine and thirty- 

 two hours, the time depending on the hours of meals and that of 

 defaecation. Food taken less than nine hours previously would not 

 have reached far enough. 



If the call to defaecation is resisted, the desire soon passes away, 

 and may not recur until the next regular period arrives for the 

 opening of the bowels, twenty-four hours later. During this time 

 the rectum contains faeces, there being no retro-peristalsis to carry 

 them back into the colon. This is one of the commonest causes of 

 constipation, for the retained faeces continue to lose water, and get 

 harder, and more difficult to expel. 



Nervous Mechanism. The large intestine resembles the rest of 

 the alimentary canal in having a double nerve supply. 



(1) The sympathetic. These fibres leave the cord by the lower 

 lumbar anterior roots; these pass through the lateral chain, and 

 reach their cell-stations in the inferior mesenteric ganglion ; the post- 

 ganglionic fibres arising there, pass by the colonic nerves to the 

 colon, and by the hypogastric nerve to the rectum and internal anal 

 sphincter. 



(2) The nervus erigens. This takes the place of the vagus, 

 which forms the second source of nerve supply to the stomach and 

 small intestine. This nerve is excitatory to both coats of the 

 muscular wall, whereas the sympathetic is inhibitory to the internal 

 sphincter. 



The fibres which pass to the rectum by the pelvic nerves or nervi 

 erigentes arise from the third sacral nerve, and have their cell- 

 stations in the haemorrhoidal nerve plexus, which is the name given 

 to this portion of the plexus of Auerbach. 



The voluntary muscles, namely, the external anal sphincter and 

 the levator ani, are supplied by the fourth sacral nerve, which arises 

 from nerve-cells in the conus terminalis of the spinal cord. 



If Starling's experiment of pinching a spot in the large intestine 

 is performed, much the same result follows as in the small intestine ; 

 the wave of inhibition which travels downwards is well seen, but the 

 upward wave of contraction is not so marked as in the small intestine. 

 Stimulation of the sympathetic (hypogastric) nerve-fibres produces 

 movements of the colon and rectum, and inhibition of the internal 

 sphincter ; that is the main phenomenon in the act of defaecation. If 

 the lower part of the spinal cord is destroyed, defaecation still occurs, 

 but it is an unconscious act, and the reflex is imperfectly executed ; 

 the hypogastric part of the mechanism is intact, and probably the 

 reflex centre concerned is, as in the small intestine, in the peripheral 

 ganglia of Auerbach's plexus ; but the destruction of the conus 

 terminalis prevents the normal reflexes taking place in which the 



