419 



As the contents pass along the large intestine, they become sepa- 

 rated into semi-solid globular masses. In the transverse and descending 

 colon, antiperistalsis is very slight, and the predominating movement 

 is a slow true peristalsis. 



The pelvic colon down to the acute flexure (P.R.F., Fig. 216) just 

 above the rectum is the storehouse of faeces. Occasional long- 

 continued waves of contraction force the contents well down into the 

 pelvic colon, and eventually, by rendering the angle of flexure less 

 acute, force some of the contents into the rectum. This leads to a 

 desire to defsecate. These long-drawn movements are subject to the 

 control of a centre in the lumbar spinal cord. They are probably 

 evoked by distension of the gut stimulating the afferent nerve- 

 endings in the pelvic nerve. The times taken for the passage of food 

 through the large intestine in man are marked in hours in Fig. 215. 



FIG. 216. DIAUK.V.M OF RECTUM (Hurst), SHOWING PEL VI-RECTAL FLEXURE (P.R.F.); 

 FAECES IK COLON (F.); HOUSTON'S VALVE (V.H.) ; RECTAL AMPULLA (R.A.); 

 LEVATOR ANI (L.A.); INTERNAL SPHINCTER (I.S.A.); EXTERNAL SPHINCTER 

 (E.8.A.). 



Defaecation. To stimulate the desire to defaecate, the distension 

 of the empty rectum by a small amount of faeces is sufficient (Fig. 216). 

 Such an amount is normally passed into the rectum from the pelvic 

 colon as the result of peristaltic action reflexly induced by the 

 taking of food on an empty stomach. Hence the desire to defaecate 

 after breakfast. The result may also be brought about by physical 

 exercises and a cold bath, or even by the muscular exerfcise involved 

 in dressing. 



That it is the distension of the rectum, and not the actual contact 

 of the faeces with the rectal mucous membrane, which leads to the 

 desire to defaecate has been shown by inflating the rectum with a 

 balloon. If the desire be not obeyed, the wall of the rectum relaxes, 

 the intrarectal pressure falls, and the desire passes away, only to 

 recur when the intrarectal pressure is again raised by the advent of 

 faeces. Defaecation may sometimes be started by voluntary effort. 



