So much of the Rectum as lies beneath the cul de sac of the peritoneum, may be consi dered as appertaining to the anal region, and must, therefore, be noticed at present ; below the reflection of that membrane, this intestine descends obliquely forwards between the sacrum and bladder, in the male as far as the prostate gland, and in the female as far as the vagina; it is there on a level with the inferior extremity of the coccyx, and then it bends downwards and backwards, and ends in the anal opening ; the perinzeal portion of the Rectum, therefore, is convex forwards and concave towards the coccyx; hence in introducing into this intestine the bougie, enema pipe, or even the finger, it should be directed at first upwards, and for wards, and then upwards and backwards : in the child, however, this precaution is not ne cessary, as the course of this intestine is not so much curved, the name of Rectum being then more correctly applied than in the adult.
In order to examine the several parts con tained in the anal region, the thighs should be fully separated, flexed, and fixed on the pelvis; the first object which attracts attention is the Anus.
This opening is situated in the median line, at the bottom as it were of a deep excavation, which is bounded on either side by the tube rosity of the ischium, with the superincumbent muscular and adipose substance ; in the erect position it appears at a great depth from the surface, in consequence of the approximation of the nates. In the adult the anus is from one inch to an inch and a half distant from the point of the coccyx, and three inches from the arch of the pubis; it is in some measure, but not perfectly, fixed in its situation, anteriorly by an indirect attachment to the interosseous or triangular ligament of the urethra, and pos teriorly by a dense fibrous tissue, which forms a sort of raphb between it and the coccyx, and to which the muscles and integuments adhere. In the natural and healthy state, the anus pre sents the appearance of a small rounded, or rather elliptical orifice, whose border is thrown into numerous small plaits, or rugm, which during the extended state of the opening are effaced ; these rugw are occasionally so deep as to admit of the escape of a small quantity of fluid. As the skin approaches the margin of this opening it becomes very fine and delicate, is gathered into those several radiated folds or plaits, which sink into it, and in the same man ner as at the other outlets of the body, it be comes continuous with the lining mucous mem brane of the intestine, there being no exact line of demarcation, except that of an increased vascularity, to distinguish the one from the other. This plaited condition of the skin which lines this opening arises from the close contraction of the subjacent muscle, and is doubtless de signed to admit of the more easy dilatability of the anus during defcecation; this opening, however, is never equal to the diameter of the rectum at a little distance above it. In the
child the integument surrounding the anus is smooth and red, in the adult it is of a deep brown colour and studded with several fine hairs, which, however, are usually absent in the female. In this situation also the cutaneous follicles are very distinct and numerous, but not so prominent as in the scrotum; they secrete a mucous or sebaceous matter which gives to the skin a shining or oily appearance, and adapts it to the functions of the part: from the absence or from the vitiated condition of this secretion, painful and troublesome excoriations not unfrequently ensue. In the healthly state the margin of the anus feels firm and resisting, and together with the surrounding muscles forms a floor or support to the inferior part of the pelvis, in the centre of which floor the rectum and its contents are maintained, and on either side a mass of cellular and adipose substance.
Muscles.—The muscular apparatus connected with the lower extremity of the rectum consists of the superficial and the deep sphincters of the anus, also the right and left levatores ani, to which may be added the two transversi perinxi, and the two coccygcei muscles.
The first two, namely, the sphincter muscles, surround the anus, and may be regarded as a modification, or as a particular development of the general circular muscular tunic, which is continued around the whole alimentary tube from the mouth to the anus, and which in dif ferent situations exhibits a considerable increase in colour and consistence, for example, in the lips, around the fauces, the oesophagus, the pylorus, &c. The name of these muscles in dicates their principal function, while the other muscles which have been alluded to proceed from certain fixed points to be inserted into the lower extremity of the rectum, and must, therefore, rather serve to retain the anus in its situation or to restore it to its natural condition, when in the exercise of its functions it has been con siderably dilated, or slightly displaced by the expulsive efforts of the diaphragm and abdo minal muscles. We shall first examine the descriptive anatomy of these individual muscles, and then consider their several powers or pur poses in the economy of the surrounding organs. Although there are two sphincter muscles of the anus, yet this name is generally applied to the more superficial of these ; we shall distinguish these muscles by the names of sphincter ani cutaneus vel ellipticus, and sphincter ani pro ficndus vel orbicularis.