The Sphincter ani internus vel orbicularis ( Sphincter intestinale, Winsl.) is of much less extent than the former, and is situated more deeply ; it is closely connected to the mucous membrane, or the fine lining integument, and appears a particular development of the circular fibres of the intestine, like those which surround the pyloric extremity of the stomach. This cir cular muscular ring consists of several fine and pale fasciculi of fibres, which are closely con nected together, and when contracted form a thick ring around the intestine immediately with in the anus; this muscle may be exposed either by detaching the lining membrane which is but loosely attached to it, or removing the rectum from the subject, everting and distending it. The mucous membrane being then detached, the muscle will be distinct ; its upper border is continuous with the circular fibres of the rectum, and a distinct cellular line separates it from the cutaneous sphincter ; anteriorly it is connected with the levatores ani muscles.
The action of this muscle must be to assist the former sphincter in closing the lower ex tremity of the rectum and supporting its con tents ; in the process of defmcation it assists in the expulsion of the residual portions of the faecal matter, by the sudden or almost spas modic action which succeeds its relaxation ; moreover, it strongly opposes the entrance of any foreign body by the anus ; so that from its power of resisting the ingress or egress of any substance, it may be considered as constituting a perfect pylorus.
The subcutaneous adipose tissue in perineo is very abundant in some situations; close to the anus, or between the sphincter and the skin, there is but very little ; hence abscesses but seldom form there, except of very limited size, such as small furunculi, or as the result of circumscribed inflammation in some of the fol licles around the opening; whereas at either side of the anus and rectum there always exists a considerable quantity of cellular and adipose matter, the former remarkable for the large size of its cells, which are intersected by irre gular bands or fibres from the perinmal fascia, and which give the whole some degree of elasticity ; the adipose substance is abundant, very soft, loose, sometimes reddish, and fills those large spaces which exist on either side of the rectum. In - no part of the body do ab scesses so frequently form as in these ischio rectal spaces ; and as such abscesses are very generally attended with consequences tedious, troublesome, and dangerous, it may be right to make a few remarks on the anatomy of these regions. • Each Ischio-rectal space is a deep triangular hollow, the base being situated towards the integuments, the apex towards the cavity of the pelvis ; the outer side is formed by the ischium, and the inner by the rectum with its muscles; this intestine, together with the attachments of the levatores ani behind and before, separates the two spaces from each other, but the cellular membrane of one side communicates with that of the opposite, and hence in cases of diffused or extensive suppu the fluid is occasionally observed to pass from one side to the other ; anteriorly the transversus perinmi, and posteriorly the coccygeus muscles bound this hollow. Each
of these triangular recesses is lined on all sides,_ except towards the skin, by fasciae, a view of which may be obtained by dissecting out of either all the contained adeps. There may then be observed near the apex, or the deepest part of the recess, a strong and tense aponeu rotic line, which is the inferior folded surface of the pelvic fascia, which in this situation sends off its inferior or descending layer ; this latter immediately divides into two laminw, an internal and an external ; the latter is called the obturator, the former the ischio-rectal fascia; the former is very strong and distinct, the latter very thin and cellular.
The obturator fascia descends a little ob liquely outwards and is inserted into the falci form process of the great sacro-sciatic liga ment, and into the tuberosity and ramus of the ischium. It is very dense, being composed of strong aponeurotic fibres, and it conceals and separates from the perineum the obturator in ternus muscle, and the internal pudic nerves and vessels, the perinea,' and hemorrhoidal branches of which pierce it as they proceed to their destination. The internal layer, or the Ishio-rectal fascia, is much weaker and more cellular than the last ; from the tioned aponeurotic line it descends obliquely inwards along the lower and outer surface of the levator ani as far as the sphincter, when it becomes thin and cellular, and is lost in the surrounding adipose tissue. Thus, by the unfolding or division of the inferior layer of the pelvic fascia into these two laminae, the obturator and ischio-rectal fasciae, these re cesses are completely lined, and by the degeneration of the last named aponeurosis into cellular and fibrous bands, which inter lace in every direction, the large mass of adi pose substance is enclosed and supported, whilst a general firmness and elasticity is imparted to the whole region. Towards the posterior part of each of these regions a cul de sac is enclosed between these fasciae and overlapped by the glutams maxims, on the surface of which the fasciae become extended, and ulti mately lost. A somewhat similar but smaller cul de sac exists anteriorly behind each trans versus perini muscle. An inspection of the Ischio-rectal spaces will serve to explain not only the great size to which abscesses here attain, but also the difficulty in effecting a cure when they have been of long standing and of considerable magnitude ; the constantly-vary ing form of the rectum on one side, the im moveable surface of the pelvis on the opposite, a muscle above, and the integuments below, all tend to prevent the possibility of effecting any permanent apposition between the sides of the cavity, while very generally the state of the constitution is equally unfavourable to any healthy action in the part. These several facts have impressed surgeons with the propriety of opening all such abscesses in a very early stage, otherwise a large cavity will be formed, the rectum denuded, and very frequently opened by ulceration.