In the act of defcation the offices of the se veral muscles connected with the anus may be summed up as follows :—When the contents of the rectum, particularly if of a solid con sistence, are being expelled, the whole rectum descends, and the perinTum becomes promi nent in consequence of the viscera being forced against it by the contraction of the diaphragm and abdominal muscles. The presence of the faeces irritates the muscular fibres of the rectum; the longitudinal fibres shorten the intestine, while the successive actions of the circular urge down the faecal mass ; these two orders of muscular fibres are the true antagonists to the sphincters. During this stage, however, the sphincters are relaxed, and the anus be comes dilated, partly by the contents of the rec tum distending it, and partly by the levatores ani muscles, which are nevertheless in a suffi ciently relaxed condition to allow the protru sion of the rectum and anus, while they still support the latter to a certain extent, and thus exert a sort of check against its forcible de scent ; they also tend to open the orifice of the anus. During this forcible expulsion, a small portion of the mucous lining is frequently protruded. The expulsion of the last portion of fwculent matter is then effected by the sub sequent strong and gradual contraction of the levatores ani compressing the rectal pouch, and raising the rectum and anus to their former position, and lastly, the sudden action of the sphincters clears and closes the orifice.
The mucous membrane lining the rectum is in every respect highly organized, it is thrown into several folds, and is larger and looser than the other coats, hence portions can be easily removed by operation, and are not unfrequently detached by gangrene. As it approaches the anus, it is very red, soft, and fungous, being highly vascular, presenting the orifices of several glands, follicles, or lacunae. It is here very loosely connected to the muscu lar fibres, and is frequently found thrown into irregular folds ; these are protruded somewhat during defbecation, and when morbidly enlarged or thickened, are not unfrequently the source of considerable pain and inconvenience. As the mucous membrane is not. contractile, these folds are necessarily increased when the longitudinal fibres of the rectum contract and shorten the intestine ; they are then protruded together with the faecal matter. Immediately above the plaited margin of the anus the skin and mu cous membrane become continuous ; the ter.; mination of the cuticle appears rather abrupt, just within the internal sphincter. Some de scribe it as continued higher up, and gradually lost on the surface. I have not been able to • exhibit it satisfactorily higher than the point indicated, nor does it appear to me that it extends through this orifice by any means to the same extent as through the other outlets of the body, the mouth, nose, urethra, or va gina. In the latter passage in particular it is very distinct, even in health ; and in disease, as in cases of prolapsus uteri, its develop ment becomes considerable; whereas in pro lapsus ani, that is, a protrusion of the mucous lining of the rectum, at a little distance above . the anus, I have not found the protruded mass to become covered with cuticle : I have seen cases of long standing in which the surface presented the same soft, vascular tissue as it does at first. It does not controvert this state ment to find tumours about the margin of the anus (hemorrhoids or polypi) covered with a thickened or developed cuticle ; for in such cases the cutaneous covering is derived from the elongation of the surrounding skin, which has increased in density from exposure to the air, and from continued irritation. The same will apply to cases of artificial anus, no matter in what situation : in all these the villous surface, which protrudes during the peristaltic action, retains its mucous charac ters, and does not become covered with cu ticle. From these facts it may be inferred that cuticle is never developed in any situation in which it did not originally exist, but that circumstances favour tlfe increase or more full development of it in those situations where it naturally occurs, even though its normal con dition be extremely delicate and fine.
Nerves and vessels.—The submucous tissue in the vicinity of the anus is very loose, and the seat of nervous and vascular plexuses ; in the latter the venous system predominates.
A consideration of the functions of the rec tum and of its surrounding muscles, its re markable irritability and sensibility in health, as well as its sympathies in disease, would lead us to infer what dissection proves to exist, namely, that this organ is largely supplied with nerves ; numerous branches are furnished to it from the sacral plexus, which is formed by the union of the inferior spinal nerves, also from the hypogastric plexus, which is chiefly com posed of filaments of the sympathetic. The sacral plexus of spinal nerves furnishes, • in addition to many others, the hemorrhoidal, vesical, and pudic branches ; the hemorrhoidal nerves are directed principally towards the in ferior part of the rectum, some ascend to the colon, others descend even to the sphincter ani : they divide into numerous filaments, which are chiefly distributed to the muscular fibres of the rectum and the adjacent muscles; the vesical nerves in their course to the bladder give some filaments to the rectum, and the in ferior or perinceal division of the pudic nerves also send several branches to the levator and sphincter ani muscles. The hypogastric plexus of nerves is composed of filaments from the sacral plexus, which interlace with some from the inferior mesenteric plexus, and with nume rous branches from the sacral ganglions of the sympathetic nerves. This plexus supplies the rectum, as also the other pelvic viscera; the branches accompany the bloodvessels, and are distributed principally to the mucous and sub mucous tissue. The cellular tissue, also, about the coccyx, and the adjacent muscular fibres receive some filaments from the coccygeal plexus of the sympathetic. This supply of nerves from these two very different sources, the one presiding over voluntary, the other over in voluntary motion, corresponds with the well known functions of this organ, and causes its muscles to be classed by the physiologist under the head of mixed muscles, that is, partaking of the common characters of the animal or voluntary, and the organic or involuntary sys tems. Its supply of spinal nerves serves to explain not only the influence which the will can exert over its functions, but also the impaired or altered state of its powers in case of disease or injury of the brain or spinal cord ; thus irrita tion of the latter may cause morbid irritability and contraction of the rectum, and, necessarily, constipation of the bowels ; or, again, paralysis of the spinal cord from injury-or compression may lead to perfect atony of the sphincters, and to the involuntary discharge of the contents of the rectum. The general distribution of the branches of the sacral and hypogastric plexuses to the several pelvic viscera, and to the muscles, &c. in the perinTum, associates these different organs with each other, which is so necessary to their functions, and with the urinary and genera tive organs, connecting more particularly the muscles of the anus with the muscular coat of the urinary bladder and with the parts about its cervix. This interlacement and subsequent general distribution of these nerves serve also to establish those several sympathies which are found to exist in acute and chronic diseases of the rectum and anus, between this intestine and the other pelvic viscera. In some the uterus and vagina partake of the irritation, in others the urinary bladder is almost incessantly irri tated to expel its contents; or, on the other hand, when the sympathetic irritation engages its cervix and the parts in its vicinity, the most painful retention of urine is endured. Chronic disease of this intestine is also very generally attended with occasional attacks of pain and irritation in different portions of the alimentary canal, as also with pain in the sacrum and loins, and in various other direc tions, which may in most cases be explained by referring them to nervous irritation extending in the course of some of the nervous communica tions which are found to exist in such num bers in the pelvis.