Sphincter ani cutaneus (o-cp constringo,) coccygeo-anal, sphincter externus, constrictor ani) is the first muscle which meets the eye of the anatomist in the dissection of this region. It may be exposed by dividing the integuments from the coccyx to near the back part of the anus, and thence extending an incision on each side, and about half an inch distant from the edge of this opening to its forepart, whence it should be continued indefinitely along the me dian line of the perinwum ; the integument should then be carefully dissected off from either side of this elliptical incision.
The muscle thus exposed is thin and flat, of an elongated and elliptical form, and cleft in the centre to embrace the opening of the anus ; it arises posteriorly fleshy and cellular from the point of the coccyx, and from a tense fibrous or cellular tissue, called the recto-coccygaal liga ment, which extends from the coccyx to the back part of the anus, where it divides and is lost in the cellular tissue on either side. From this origin the fibres of the sphincter collect into a rounded fasciculus, which proceeds for wards and downwards, increasing in size, and at the back of the anus divides into two bands which pass one on either side of this opening, each spreading out till it is an inch or even more in breadth ; again converging in front of the anus, these bands unite into a fasciculus, which in the male is very long and passes for wards and upwards between the skin and the acceletatores urinw muscles, to be partly in serted into the median line of the superficial fascia of the perinwum, and partly confounded and interlaced with the transversi perinwi, and with the muscles which cover the bulb of the urethra; through the medium of these last it is even attached to the common cellulo tendinous central point of the perinwum, be tween the rectum and the bulb, whereby it is enabled to act on this part of the urinary canal. This anterior insertion is very variable in different persons ; in some it stops abruptly at _the bulb, while in others it continues to run forwards between the skin and the acceleratores as far as the dartos, in which it terminates. In the female this anterior fasciculus is much shorter, and ends in the sphincter or con strictor vaginae ; in the male its attachment to the muscles of the bulb is often deficient, so that in the course of the dissection, when the superficial fascia has been removed, this ex tremity of the muscle will be found detached and its insertion isolated. The entire of the inferior surface of this muscle is in contact with the integuments, its superior surface is related to the levatores ani, acceleratores urines, and transversi perini muscles ; in front of the anus it is confounded with the two latter, and immediately behind it with the formerly named muscles; its external border is of uncertain ex tent, and is imbedded in adeps, while its inter nal edge is in close relation with the delicate inflected anal skin, being separated only by a fine cellular tissue. This muscle is composed entirely of fleshy fibres, occasionally intersected by cellular and imperfect tendinous bands; these fibres are placed in concentric arches, those of opposite sides unite at acute angles, and sometimes interlace before and behind the anus ; the fasciculi are frequently separated by considerable intervals, so that they appear like different muscles ; some of the internal fibres assume a circular arrangement ; in the female this muscle is shorter, broader, and more rounded, particularly in front. In structure
and appearance this muscle presents great diversity ; in some it is red, strong, and large, in others, so pale and weak as to be difficult of perfect demonstration ; it is also probable that during life great differences exist as to its power of contraction.
The use of this muscle is obviously to close the anus, the skin of which it throws into small rug ; hence when the sphincter is paralysed, there is incontinence of the contents of the rec tum ; the most internal fibres will tend to close the opening more perfectly than the external or elliptical, which will reduce it rather to a cleft or fissure; this muscle can also raise the anus somewhat, and at the same time draw back and compress the bulb of the urethra; it will also express the secretion from the anal glands and follicles. The sphincter ani may be properly said to belong to the class of mixed muscles, both as relates to its structure and function ; as to the former, its paleness, scattered fibres, connection with the commencement of the mu cous surface, and absence of true tendon ally it to the muscular system of organic life; while on the other hand the parallel direction of its fas ciculi, and the arrangement of many of the latter in the surrounding adeps, assimilate it to the muscles of voluntary motion. In its functions also it appears to border on the province of each division of the muscular system; thus without the efforts of the will, or even without any in ternal cognizance, it continues in a state of al most permanent contraction, and as uncon sciously relaxes when the functions of the part impress upon its sensibility the necessity of so doing ; while on the other hand the will can exert a considerable control over its powers, and can cause it to contract with considerable and continued energy, as well as throw it into a state of atony and relaxation. Although this muscle belongs to the same class with the other sphincters, the orbiculares oris and palpebra rum, yet it manifests a considerable difference in its vitality. The natural and, therefore, the usual condition of these other sphincters is relaxation ; hence the mouth continues open, and partly from the same cause too, the eyelids are apart ; whereas the natural condition of the sphincter ani when at rest is contraction, and hence the anal opening is always closed, although the muscle is still capable of contracting with con siderably more energy when any of the contents of the rectum suddenly approach the orifice, or when any irritation exists in its vicinity.