The action of the levator ani muscles is two-fold, and not confined to the mere ele vation of the anus, as its name would im ply. First, they act as a moveable floor to the abdomen and pelvis, which can antago nize the diaphragm ; these two fleshy planes being opposed to each other, can, by a slight action of one or both, materially alter the perpendicular axis of the abdomen, which extends between them. This axis is at its great est length during the state of expiration, and is most diminished when both these muscles are forcibly contracting. The levatores ani, how ever, have less influence in effecting this change than the diaphragm ; they serve chiefly to support the lower region of the pelvis and the several viscera this cavity contains against the combined protruding forces of the diaphragm and abdominal muscles in violent exertions of the body, or in forcible efforts of respiration, or in the evacuation of the contents of the rec tum and bladder ; and, secondly, they not only raise, but dilate the anus, by drawing out its circumference so as to overcome the sphinc ters ; at the same time they compress and assist in emptying the rectum, particularly the dilated pouch, which is a little above the anus; they also resist the prolapsus of the mucous coat of the intestine, and raise it after it has been to a certain extent protruded by the action of the abdominal muscles. They raise and draw forward the coccyx after it has been forced back by abdominal pressure in parturition, or in the ordinary evacuation of the bowels, and further, by raising and compressing the trigone of the bladder, they assist in expelling its contents, and for the same reason they can also empty the vesiculw seminales of their fluid. The anterior portions of these muscles are intimately con nected to the membranous part of the urethra, and are variously modified in different indi viduals and in different animals ; we consider those muscular fasciculi which have been de scribed differently by anatomical writers under different names, compressores urethrae, &c., as parts of or appendages to these muscles : these urethral portions of the levatores ani can cer tainly compress the membranous part of the urethra and empty its canal ; they can even interrupt or suddenly stop the stream of urine, and thus they may occasionally aid the neck of the bladder in retaining the contents of that organ.
The Ischio-coccypri muscles are situated at the posterior inferior part of the pelvis ; they are thin, flat, and triangular, composed of a mix ture of fleshy and tendinous fibres. The apex or origin of each is attached to the spine of the Ischiurn, and its base is inserted into all the side of the coccyx, and a small portion of the sacrum ; they are partly covered by the great sciatic ligaments. The superior and posterior border is connected to the lesser sciatic liga ment, and the anterior border is in part con tinuous with the levator ani muscle ; the an terior or pelvic surface is connected to the rectum and the surrounding adipose substance. This pair of muscles appear as a prolongation. of the levatores ani, and are of use in com pleting the inferior boundary of the pelvis; they thus support the rectum and the pelvic viscera, and they also serve to retain the coccyx and restore it to its situation when protruded by the diaphragm and abdominal muscles in the pro cess of parturition, and in the act of defcation,.
or when drawn too much forward by the levatores ani muscles. If the several muscles in this region be now partially removed on one side, the lower extremity of the rectum will become more distinct, and will be found surrounded by a quantity of loose, fatty, cellular tissue, sepa rating it from the surrounding muscles and bones ; this contains many nervous filaments and numerous bloodvessels, particularly veins. (See INTESTINAL CANAL.) Anteriorly in the male subject a small triangular space, the bulbo-rectal hollow, will now become distinct; this is situated between the anus and mem branous portion of the urethra ; the base of it is at the skin of the perinmum ; the apex at the prostate : to the last the rectum will be seen rather intimately connected. The bulb and the membranous portion of the urethra bound this space in front, and the rectum behind.
(See PERINEUM and URETHRA.) Rectum.— In addition to the several muscles which have now been severally noticed, and which thus serve not only to retain and support the rectum and anus, but which even enter into the structure of the former, we have further to consider the parts more immediately composing the parietes of the lower extremity of the in testine ; these are the longitudinal muscular fibres,—the mucous membrane, and the sub mucous cellular tissue. The longitudinal fibres of the alimentary tube exist through its whole extent, but like the circular are differently mo dified in different situations; thus along the ceso phagus they are very fully developed, also along the arches of the stomach ; in both these situa tions the fibres are strong and somewhat red ; whereas on the parietes of the small intestines they are very indistinct and pale ; on the ccecum and colon they are still pale, but very distinct, being collected into three flat fasciculi or bands. On the as on the oesophagus, they are again fully developed as to thickness and num ber ; their colour is still rather pale. In the two superior thirds of this intestine, or as low down as the prostate gland, they predominate over the circular fibres, which are internal, whereas in the lower third the latter prevail ; the former terminate, some by becoming con tinuous or intermingled with the fibres of the levatores ani, others with the cutaneous sphinc ter so low as the border of the anus, and some are inserted into the submucous tissue of the intestine ; these fibres are continuous superiorly with those of the colon ; they serve to continue that successive series of contractions or shorten ings of the intestine, which essentially assist in the process of defwcation. As the longitudinal fibres of the rectum resemble those of the ceso phagus, so the inferior circular fasciculi or the sphincters are like the muscles of the pharynx, not merely in their increased strength and colour, but also in their vital power. Over the lon gitudinal fibres the will has no control, whereas the inferior circular are to a certain extent under its influence. Here, then, as in the organs of deglutition, we perceive the animal and organic powers still distinct as to their elementary na ture, but becoming intimately, nay inseparably associated for wise and obvious purposes.