This is a point of much importance, for the evacuation of the lower bowel is intimately connected with feelings of comfort and health, and in illness the insuring of its accomplishment forms an essential part of the physician's duty.
The movements of the intestine cause the various excretions and indigestible parts of the food to pass toward the sigmoid flexure of the colon, where their onward motion is checked for a time by the strong circular muscle of the rectum (called the superior, or tertius sphincter by Hyrtl), which does not carry on the peristaltic wave. The materials here get packed into a more or less solid mass, which is gradually augmented after each meal.
The lower outlet of the alimentary canal is closed by two distinct sphincter muscles. One thin external superficial muscle, made up of striated fibres, belongs to the perineal group, and has little influence on the closure of the anus. The deep or internal sphincter, which is much stronger, surrounds the gut for rather more than an inch (3 centimetres, Henle) in height, and is one-quarter inch thick. It is made of smooth muscle, and therefore capable of prolonged (tonic) contraction. It would appear, however, that- this strong sphincter is merely a supernumerary guard to the anal orifice, but rarely called into action, for during the interval of rest between the acts of defecation, the faeces do not come in contact with the portion of intestine surrounded by this muscle. The rectum for quite one inch above the sphincter is perfectly empty, being kept free from feculent particles partly by a fold of the intestinal wall and partly by the repeated action of the voluntary muscles in the neighborhood, which, by intensifying the angle that exists at this point and flattening this inch of rectum, can squeeze back the approaching matters. Any one familiar with the digital examination of the unevacuated rectum, knows that no faeces are met with for about two inches.
Considerable accumulation may take place in the sigmoid flexure without much discomfort ensuing, but when the rectum is distended, an urgent sensation of wanting to empty it is experienced, and the voluntary movements mentioned above are performed by the levator ani and the neighboring perineal muscles, with the object of preventing any substance reaching the part of the rectum immediately above the sphincter.
If the rectum be distended with fluid, the occasional anal elevation does not suffice to keep it back, and a continuous and combined action of the sphincters and levator ani, etc., is necessary to ward off the expulsion of the contents.
When the lower bowel is habitually emptied at the same hour daily - a habit which should be carefully exercised - the sensations of requirement to go to stool occur with great punctuality, or can be readily induced by the will, so that normal defecation is reputed to be, and practically is, a voluntary act. But not completely so, for, somewhat like swallowing, the later stages of defecation consist essentially of a series of involuntary reflex events which we can initiate by the will, but when it is once started, are powerless to modify until the reflex sequence is completed.
Under ordinary circumstances, the evacuation of the faeces is commenced by the voluntary pressure exercised on the abdominal contents by the respiratory muscles. The diaphragm is depressed, the outlet of the air passages firmly closed, and the expiratory muscles thrown into action, while at the same moment the muscles which close the pelvic outlet relax, and allow the anus to descend, so that the inferior angle of the rectum is straightened, and a voluntary inhibition of the sphincter is brought about. This voluntary expiratory effort seldom requires to be continued for more than three or four seconds before some fecal matter reaches the part of the rectum just above the sphincter. When this has occurred, no further abdominal pressure is necessary (except when the masses of faeces are large and hard), for the local stimulus starts a series of reflex acts which carry on the operation.
Fig. 56. Auerbach's plexus from between the muscle coats of the intestine, with low power.
Fig. 57. A nodal point of Auerbach's plexus under high power, showing the nerve cells.
These consist of an increased peristaltic contraction of the colon and sigmoid flexure, the waves of which pass along the rectum. These waves are accompanied by synchronous rhythmical relaxation of the sphincter, which replaces its normal condition of tonic contraction.
The effect of the voluntary effort, and the amount of the abdominal pressure required, depend upon the consistence of the faeces. When quite fluid, they constantly tend to come in contact with the sensitive point of the rectum, and a voluntary effort is required to prevent the reflex series of events from taking place; a momentary relaxation of the sphincter with voluntary abdominal pressure is sufficient to eject the contents of the bowel. On the other hand, when the faeces are firm, time is required in order that the slowly acting smooth muscle may pass the mass onward. In common constipation, the difficulty is to get the solid mass down to the sensitive exciting point, in which case a few drachms of warm fluid, used as an enema, may awaken the necessary reflex movements.