534 THE MECHANICAL PROCESSES OF DIGESTION [CH. XXXV. 



as the glottis is closed, the whole of their pressure is exercised on the 

 abdominal contents. The sphincter of the rectum being relaxed, the 

 evacuation of its contents takes place accordingly, the effect being 

 increased by the peristaltic action of the intestine. 



Nervous Mechanism. The anal sphincter muscle is normally in a 

 state of tonic contraction. The nervous centre which governs this 

 contraction is situated in the lumbar region of the spinal cord, inas- 

 much as in cases of division of the cord above this region the sphinc- 

 ter regains, after a time, to some extent the tonicity which is lost 

 immediately after the operation. By an effort of the will, acting on 

 the centre, the contraction may be relaxed or increased. Such volun- 

 tary control over the act is obviously impossible when the cord is 

 divided. In ordinary cases the apparatus is set in action by the 

 gradual accumulation of faeces in the sigmoid flexure and rectum, 

 pressing by the peristaltic action of these parts of the large intestine 

 against the sphincter, and causing by reflex action its relaxation ; 

 this sensory impulse acts upon the brain and reflexly through the 

 spinal centre. At the same time that the sphincter is inhibited or 

 relaxed, impulses pass to the muscles of the lower intestine increas- 

 ing their peristalsis, and to the abdominal muscles as well. 



Both inhibitory and motor fibres for the lower part of the intes- 

 tine leave the cord by anterior roots lower than those which contain 

 the fibres for the small intestine. The cell-stations are situated in 

 the inferior mesenteric ganglia, or along the course of the colonic or 

 hypogastric nerves. The lower portion of the large intestine resembles 

 the oasophagus in being more under external nervous control than 

 the small intestine. 



