528 THE MECHANICAL PROCESSES OF DIGESTION. [CH. xxxv. 



vagus. The inferior mesenteric nerves are inhibitory in function, 

 and the pelvic nerves take the place of the vagal fibres as 

 excitatory : this refers to both coats of the muscular wall. If 

 one pinches any particular spot, the upward wave of contraction 

 is not so marked as in the small intestine, but the downward 

 travelling wave of inhibition is well seen. 



Duration of Intestinal Digestion. The time occupied by the 

 journey of a given portion of food from the stomach to the anus, 

 varies considerably even in health, and on this account probably 

 it is that such different opinions have been expressed in regard 

 to the subject. About twelve hours are occupied by the journey 

 of an ordinary meal through the small intestine, and twenty -four 

 to thirty-six hours by the passage through the lanje bowel. 



Drugs given for relief of diarrhoea or constipation act in various 

 ways ; some influence the amount of secretion and thus increase or 

 diminish the fluidity of the intestinal contents ; others acting on 

 the muscular tissue or its nerves increase or diminish peristalsis. 



Defsecation. The act of the expulsion of fa3ces is in part due 

 to an increased reflex peristaltic action of the lower part of the 

 large intestine, namely of the sigmoid flexure and rectum, and in 

 part to the action of the abdominal muscles. In the case of active 

 voluntary efforts, there is usually, first an inspiration, as in the case 

 of coughing, sneezing, and vomiting ; the glottis is then closed, and 

 the diaphragm fixed. The abdominal muscles are contracted as 

 in expiration ; bat 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, inasmuch as in cases of division of the cord above 

 this region the sphincter 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 voluntary control over the act is 

 obviously impossible when the cord is divided. In ordinary 

 cases the apparatus is set in action by the gradual accumula- 

 tion 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 



