7iG SURGICAL ANATOMY OF THE PERINEUM. 



two or three cutaneous branches of the small sciatic nerve ; these turn round the 

 inferior border of the Gluteus maximus, and are distributed to the integument 

 in this region. 



Fig. 389. Dissection of Perineum and Ischio-rectal Region. 



The External sphincter is a thin flat plane of muscular fibres, elliptical in 

 shape, and intimately adherent to the integument surrounding the margin of the 

 anus. It measures about three or four inches in length, from its anterior to its 

 posterior extremity, being about an inch in breadth, opposite the anus. It arises 

 from the tip of the coccyx, by a narrow tendinous band, and from the superficial 

 fascia in front of that bone; and is inserted into the tendinous centre of the 

 perineum, joining with the Transversus perinei, and the other muscles inserted 

 into this part. Like other sphincter muscles, it consists of two planes of muscular 

 fibre, which surround the margin of the anus, and join at the commissure before 

 and behind. 



Relations. By its superficial surface, with the integument ; by its deep surface it 

 is in contact with the Internal sphincter ; and is separated from the Levator ani 

 by loose areolar tissue. 



The Sphincter ani is a voluntary muscle, supplied by the hemorrhoidal branch 

 of the fourth sacral nerve. This muscle is divided in the operation for fistula in 

 ano ; and also in some cases of fissure of the rectum, especially if attended with 

 much pain or spasm. The object of its division is to keep the parts at rest and 

 in contact during the healing process. 



The Internal sphincter is a muscular ring, about half an inch in breadth, which 

 surrounds the lower extremity of the rectum, about an inch from the margin of 

 the anus. This muscle is about two lines in thickness, and is formed by an 

 aggregation of the involuntary circular fibres of the intestine. It is paler in 

 color, and less coarse in texture, than the External sphincter. 



The Ischio-rectal Fossa is situated -between the end of the rectum and the 

 tuberosity of the ischium on each side. It is triangular in shape, its base 

 directed to the surface is formed by the integument of the ischio-rectal region ; its 

 apex, directed upwards, corresponds to the point of division of the obturator 

 fascia, and the thin membrane given off from it, which covers the outer surface 

 of the Levator ani (ischio-rectal fascia). Its dimensions are about an inch in 

 breadth, at the base, and about two inches in depth, being deeper behind than in 

 front. It is bounded, internally, by the Sphincter ani, Levator ani, and Coccygeus 

 muscles ; externally, by the tuberosity of the ischium, and the obturator fascia, 

 which covers the inner surface of the Obturator intern us muscle ; in front, it is 

 limited by the line of junction of the superficial and deep perineal fascia ; and 

 behind, by the margin of the Gluteus maximus, and the great sacro-sciatio 



