The External Sphincter Ani.—This is a muscular ring which surrounds the lower extremity of the anus. Of an elliptical shape, its fibres extend from the apex of the coccyx to the posterior part of the rectum which it completely surrounds. In front the fibres cross each other; those on the right pass to the left, those on the left pass to the right. They there inclose the orifice of the vagina and form the Constrictor Vogince.—The fibres of this muscle begin where those of the sphincter ani cross; they are attached to the inferior extremity of the vagina enveloping the bulb of the vagina. When they reach the anterior part of the vagina, the two fasciae surround the orifice of the urethra. They then reach the clitoris, to the roots of which they are attached. and blend with the superior end of the ischio-cavernous muscles. The con strictor vaginw and sphincter ani form a figure of eight.
The Transversue Perinei.—A pair of symmetrical muscles; they are formed by two small fleshy bundles, which spring from the internal sur face of the ischiatic tuberosities below the ischio-cavernous muscles, and between the insertions of the middle and superficial perinea] aponeuroses. They then pass into the space which separates the vulva from the anus.
There they blend with the fibres of the opposite transversus, with the sphincter ani, and with the constrictor vagina: The Ischio-cavernous.—These are a pair of symmetrical muscles situated on the inferior border and internal surface of the ischio-pubic rami. They originate at the ischiatic tuberosities, and the ischio-pubic rami, envelop the root of the corpus cavernosum of the clitoris, and end at the fibrous envelope of the corpus cavernosum, very close to the superior at tachment of the constrictor vagina.' The two fascia of the constrictor vaginx are encircled by the twofold vaginal division of the middle perinea] aponeurosis:—the iscbio-cavernous, by the ischio-pubic division of the superficial perineal aponeurosis.
Finally, the superior part of the glutei maximi muscles cover, with the coccygeal and sacral fibres, the whole of that part of the perinea] strait which lies between the inferior edge of the greater and lesser sciatic ligaments, and two lines which run from the apex of the coccyX to the middle of the ischiatic tuberosities. Some nerves and numerous vessels are distributed to the pelvic floor.
The peritoneum forms the deepest part of the pelvic floor. It covers the pelvic aponeurosis, from which it is separated by a layer of cellular tissue, which communicates freely with the cellular tissue of the broad ligament, and which is most abundant in two points: 1. Between the skin and the superficial perineal aponeurosis, i.e., within the pubic arch.
2. In what is called the ischio-rectal cavity—the ischio-rectal fossa.
Ischio-rectal Foss(e.—These are two cavities symmetrically placed out side of the anus and of the extremity of the rectum. . Each fossa has two walls, two extremities, a base and an apex.
Posterior Wall.—It is formed by the posterior and inferior part of the aponeurosis of the obturator internus. It ends above at a fibrous band
called the pubo-sciatic.
Superior-internal Irall.—It is formed by the levator ani and the sphincter ani.
The anterior extremity is formed by the union of the internal and ex ternal walls with the posterior border of the perineal aponeuroses.
The posterior extremity is formed by the interior border of the gluteus maximus.
The base is turned towards the skin, and extends, on the one hand, from the inferior border of the gluteus maximus to the posterior edge of the perineal aponeuroses; on the other, from the ischia to the anus.
The apex results from the union of the internal and supero-internal walls.
The ischio-rectal fossa is filled with adipose tissue, the quantity of which varies with the individual.
The C'occygeal Gland.
This small gland was discovered by Luschka. It is the size of a pea, and is situated at the point of the coccyx between the tendons of the levator muscles at their cocygeal insertion and at the superior extremity of the sphincter ani. Nerves pass to it.
Its structure and functions are not well known.
Modifications produced in the bony Pelvis by the Presence of the soft Parts.
In the greater pelvis, the presence of the iliac and psoas muscles modifies the depth of the iliac foss', and transforms the superior strait into a cur vilinear triangle. The base of this triangle is iu front, and its truncated apex points backward.
The diameters of the pelvis suffer the following changes: 1st. The antero-posterior diameter loses about .2 of an inch by the presence of the bladder and rectum.
2d. The oblique diameters, on account of the presence of the pyra midalis and obturator muscles, also lose about .2 of an inch.
3d. The transverse diameter loses about .6 of an inch, on account of the psoas-iliacus muscles.
All the diameters are therefore shortened to nearly the same degree.
In the cavity, the presence of the soft tissues diminishes all the diam eters by about .2 of an inch As Pajot says, the floor of the pelvis is extensible, and therefore, during confinement, the pelvis forms a canal, of which the opening is always at the superior strait, but the terminal extremity of which is formed by the distended vulva. The canal is therefore much longer and much more curved than the bony pelvis. The posterior wall is formed of two parts, one bony and about 9.6 inches long; — the other, soft and extensible, which may reach 5.8 inches. The lateral walls are also two, one bony, the other formed by the lateral regions of a greatly extended perina?um. Finally, the anterior wall is formed almost entirely by the pubic sym physis, and the body of the pubes, and accessorily by the urethra and the upper part of the vagina. The central line, which the foetus must tra verse is, then, nearly semi-circular, and the axis about which it must turn in leaving this canal is represented by the symphysis. It is also this line which the hand of the obstetrician must follow in crossing the pelvis from below upward, and from without inward.