Compartment V, the last of the three great abdominal compartments of the alimentary tube, is almost entirely expulsive. No digestive processes are carried on in any part of this compartment; the chemical reactions taking place in it are chiefly of a bacterial nature. There is prob- ably some absorption of water from the pelvic colon, but no absorption of foodstuffs.
Muscular activity is directed to driving the contents of the tube distalwards to the rectum where they can be expelled through Compartment VI to the exterior. The most important movement is a so-called mass movement, by which the contents of the transverse colon, left colic flexure, and descending colon are swept towards and into the pelvic colon.
The mass movement occurs at irregular, infrequent inter- vals and is extremely rapid. It usually begins in the trans- verse colon, but may start from the right colic flexure, or even, rarely, in the ascending colon. During the movement the longitudinal taeniae of the affected portion of the bowel are relaxed so that the sacculi disappear and the tube is left widely open, clear of the obstructing infolded shelves. The mass of semi-solid material is propelled very rapidly round the left colic flexure, down the descending colon, and into the pelvic colon, where its progress is halted by the pelvi- rectal sphincter. Not all the movements are so complete as this—some of them carry the intestinal contents for only part of the way. During its stay in the pelvic colon some water is absorbed from the mass, which thus acquires the consistence of normal faeces.
The pelvirectal sphincter is controlled by the autonomic nervous system. It therefore works in association with the sphincters of the pyloric and ileocolic regions, and a special gastrocolic reflex is described that determines a relaxation of the pelvirectal sphincter when food enters the stomach. There is, however, an important point to be noted in connexion with this innervation. The pyloric and ileocaecal valves receive their innervation from cerebral centres (cerebral para- sympathetic outflow) while the innervation of the pelvi- rectal and recto-anal sphincters is derived from centres in the spinal cord (sacral parasympathetic outflow). The rela- tionship between the pyloric and pelvirectal sphincters is thus not so intimate as that between the pyloric and ileocaecal sphincters. Usually it is only when the first meal of the day enters the stomach that the gastrocolic reflex is particularly effective.
The pelvirectal sphincter relaxes at irregular times during the day to allow the contents of the pelvic colon to enter the rectum; some of these relaxations are related to mass movements in the colon. A sudden distension of the rectum, or a slower distension up to a certain point, may set up a call for defaecation, but the mechanism of defaecation is partly under voluntary control and the call may be voluntarily neglected.
In the uppermost and middle dilatations of the rectum a considerable amount of faeces supported by the rectal valves may accumulate without initiating any marked urge to defaeca- tion. Under normal conditions the lowermost dilatation (ampulla) of the rectum is only partly filled and often is completely empty.
In the act of defecation, which is a voluntary act, the recto- anal sphincter is relaxed, as are also, at first, the levatores ani muscles that normally compress the sides of the rectum. The voluntary muscles of the abdominal wall and the diaphragm are called in to aid the contractions of the walls of the colon and rectum. By these combined compressions faeces are forced through the recto-anal valve into the anal canal.
In the later stages the fibres of the levatores ani inserted into the rectum come into action and pull the rectal wall upwards over the faecal mass.
Normally, a complete evacuation clears the contents of the descending colon, pelvic colon, and rectum.
Compartment VI is the anal canal, and its wall is composed very largely of thick circular involuntary muscle. This muscle, although functioning as an internal sphincter, becomes, during evacuation, a detrusor muscle that aids in the expulsion of the faeces. Part of the wall of the anal canal consists of voluntary muscle-fibres derived from the pelvic diaphragm, and these voluntary fibres when in action pull the canal upwards over the faeces and retract the canal when the faeces have been expelled.
COMPOSITION OF THE FAECES
Analysis of the faeces shows that with a normal mixed diet only a small proportion of the mass is made up of undigested or partially digested food. The main bulk of the faeces consists of disintegrated cells from the wall of the intestinal tube and a mass of (chiefly dead) bacteria. With meals, however, containing a large amount of indigestible cellulose (e.g., green vegetables and coarse cereals), the bulk of the faeces is greatly increased.