Movements of the reetum.—The contraction of the muscular coat of the rectum, like that of the other segments of the digestive tube, has for its object the onward propulsion of its contents. But since the rectum ends the intes tinal canal, it forms the portal by which these contents are altogether dismissed from the body. Health and comfort alike require that this act of dismissal should be both inter mittent and infrequent. While the mere con sistence of the fmces is generally such as to demand the application of a comparatively powerful force in order to effect their rapid removal. Hence the entire mechanical action of the rectum is naturally divisible into two stages :—one which propels, and one which expels, the various substances occupying its interior. The latter of these two processes is called defreation ; and, as we shall see, in volves the aid of various agents which are strictly extraneous to the bowel itself.
Those movements of the rectum which are seen in living or newly-killed animals, can only be regarded as confirming the conclusions that might fairly be inferred from the much greater thickness of the muscular coat in this particu lar segment of the large intestine.
When empty, the bowel remains in a state of rest, from which it can scarcely be aroused to peristalsis by the application of any local irri tation. But when occupied by a moderate quan tity of fkces, it will often respond to a direct mechanical stimulus ; and still more energetic ally to the electrical irritation of its nerves. On applying the rapid and powerful shocks of the electro-magnetic machine to those large branches of the sympathetic which are distri buted in the muscular coat of the rectum, a violent contraction of this tube frequently occurs. In this intense but irregular move ment, we may generally observe a shortening and a constnction :—acts which no doubt re present the specific contractions that form the functions of the longitudinal and transverse fibres respectively. Thus the bowel suddenly becomes straighter and shorter; and hence appears as though it were jerked downwards towards its most fixed point at the lower open ing ot the pelvis. This shortening is generally accompanied by a less violent and more uniform movement :—in short, by a peristalsis; which offers the ordinary progressive constriction, and is evidently the principal agent in the pro pulsion of the fmcal pellets. But both of these movenients are very irregular. The former commonly alternates with_a relaxation, which appears to pull the rectum upwards, and exat,s gerate its curves. While the latter occupies various parts of the bowel with very unequal intensity and duration; and, occasionally, even seems to take a retrograde course. In most of these details, we may observe the same close analogy between the rectum and the (-esopha gus, which has already been rernarked in their median 8.nd terminal situation, and (to a lesser degree) in the nature and. thickness of
their muscular coat.
The normal movement of the bowel differs from the above in the fact of its being a more exact and co-ordinate action ; and, therefore, a much more effective one. But while obser vation and experiment both agree in repre. senting it as a peristalsis, which is quite capable of slowly expelling fwees of moderate consist ence, nothing is more certain than that it is rarely called upon to exert such an inde pendent and unaided force.
Under all ordinary circumstances, its influ ence is assisted by the action of various vo luntary muscles. These may be divided into two classes,which differ in their situation and action.
The first are those large planes of muscle which form the anterior, lateral, and superior, walls of the belly; and thus, by their contrac tion, exert a forcible pressure on the intestines contained in this cavity.
The second are the similar structures that close the outlet of the pelvis. These have for their office to support the end of the canal. And their muscular nature enables them to effect this support by a texture, the passive or active contraction of which can alway s increase the reaction or resistance to the abdominal pressure in exact correspondence with the varying demands made upon it. They thus fix the end of the intestinal canal, while it is being emptied of its contents by the pressure of the muscles of the belly.
The mechanism of the abdominal pressure having already been described in treating of the act of vomiting*, but little need here be added with reference to the special conditions under which it is called upon to aid that of defaecation. Of course the chief of these c.m ditions consists in the application of a stimulus to the large intestine itself. And though any irritation of the mucous membrane of the rectum seems capable of producing that vio lent straining which marks the exertion of the abdominal pressure, still its strictly co ordinate character is well illustrated by the preference apparently shown to a stimulus which acts directly on the muscular fibres of the bowel themselves. Thus a mechanical distention of the rectum appears to be a more efficient stimulus to the total expulsive act than the application of' any ordinary irritant. While, vice versa', there are good reasons for conjecturing, that mere distention of the belly is capable of arousing a sluggish large intestine to expel its contents.t And a still more frequent association of movement is probably exemplified in the tenesmus pro duced by dysentery and other disorders, which involve great irritation of the recturn. For the sensations of the patient, and the uncon trollable impulse which follows them, seem to indicate that the irritation of the mucous membrane. is often accompanied by violent contraction of the muscular coat of the in testine, both of which unite to excite the subsequent abdominal pressure.