Natural bowel movement is preceded by sensations which clearly indicate the necessity for evacuation of the bowels. The mechanism of this instinctive notification of the necessity for giving attention to the needs of the body is very interesting. We have already learned that the colon is subdivided into four separate compartments and that the feces are dealt with by each of these in succession. In the cecum the consistency of the feces is increased to such a degree that the mass can be handled by the muscular wall of the bowel. The ascending colon pushes the feces through the hepatic flexure into the transverse colon. In this horizontal portion of the canal the feces rest for a time for further extraction of water. From the transverse colon the feces are pushed up the incline to the splenic flexure, and through this narrow gateway into the descending colon, along which, in the course of an hour or two, it finds its way to the capacious loop of the pelvic colon, through which it passes quickly to the lower end. Here its further progress is arrested by the tightly folded canal, just as a current of water through a rubber tube may be controlled by a sharp bend in the tube.
The pelvic loop gradually fills, and in filling is raised until the bowel is unfolded and thus opened. Now, unless the feces have been so long retained that they have become hard and dry, the bowel contents are pushed on into the rectum.
Up to this point the progress of the food material after leaving the mouth, during its passage through the long food canal, has been unattended by any sensation whatever. The process has been wholly automatic, and, though controlled in a way showing marvelous intelligence, wholly independent of the consciousness. But now there is felt an unpleasant sense of weight in the region of the rectum This sensation increases as the rectum becomes fuller, and there is a more or less urgent desire to evacuate the bowels.
This is the "call" of Nature for bowel movement, evoked by the contact of the feces with the nerves of the rectum and distention of its walls.
The fuller the rectum becomes, the more pressing is the desire for evacuation. The "call" appears only when the feces have reached the rectum.
It will now be easily seen how the "call" and the bowel movement may be directly influenced by numerous factors. Let us briefly notice some of the most important of these, which will be discussed more fully in a later chapter.
If the food taken is insufficient in bulk, the pelvic loop will be only partly filled, and hence will not rise high enough to permit the feces to pass into the rectum, and hence there will be no "call" and no movement. It is evident, also, that if the amount of food taken is small, the pelvic loop may be so long a time in filling that the feces which first entered, will become so dry and compact that they may form a mechanical obstruction, and thus the onward movement necessary to reach the rectum will be prevented, even though the bowel may rise, and the gate which guards the entrance to the rectum may be open. In starvation, no "call" appears because there is nothing with which to fill the loop and open the rectal gate.
Bodily movement has a certain amount of influence upon the position of the loop and the entrance of feces into the rectum, especially deep breathing exercises, and bodily exercises which produce deep breathing. In deep breathing, the diaphragm is pushed down upon the abdominal viscera, compressing the colon as well as other parts against the abdominal wall. By this means the feces in the loop may be pushed through the fold into the rectum, thus evoking a "call."
The increased depth of breathing and the compression of the abdomen resulting from movement when one first awakens in the morning are no doubt the reasons why many persons experience a "call" almost immediately upon awakening after a full night's rest. During sleep the pelvic loop has been quietly filling and rising, but the pressure has not been quite sufficient to cause the feces to pass into the rectum. A push from the diaphragm and the abdominal muscles gives the little extra help needed and the "call" comes.
By straining movements, such as accompany bowel evacuation, sufficient fecal matter may be pushed over into the rectum to create an effective "call," when not previously felt. Hence the importance of going regularly to stool even though no "call" is experienced.
A cold morning bath helps in the same direction, both by causing deep respiratory movements, which increase the intra-abdominal pressure, and by causing a reflex contraction of the colon.
These facts are mentioned here in order to bring the explanation of the "call" within the range of common every-day experience, and to show its very important bearing upon the practical management of cases of constipation.
The act of swallowing a glass of water, especially the drinking of cold water, and above all other things the taking of food, by setting up peristaltic movements may produce a "call," provided there is at the time a quantity of feces in the pelvic loop. If the loop is empty, food taking or anything else which sets up intestinal peristalsis will serve to help the feces along toward the pelvic colon, thus leading to a "call" a little later. The immediate effect of any such stimuli will of course depend upon the position of the fecal mass in the colon. If, for example, there is slight delay at the hepatic flexure, perhaps as the result of neglect of usual exercise or spending a day in bed, the use of measures to promote intestinal action may seem to produce no effect, whereas a bowel movement the next morning may be the result of the impulse given to the fecal mass by means of which the stagnation in the ascending colon was overcome.