THE PELVIC AND PERINEAL MUSCLES. 563 



scrotum (or fascia of the labia majora) and on either side of this with the corre- 

 sponding layer of the abdomen. Behind it unites with the posterior border of the 

 trigonum urogenitale on a line extending between the two ischial tuberosities, and 

 thence is continued backward, forming a single sheet with the superficial layer, to 

 unite with the superficial fascia of the gluteal region. This posterior portion of the 

 superficial perineal fascia may conveniently be termed the circumanal fascia. 



By the union of the deep layer of the superficial fascia with the triangular liga- 

 ment behind, an almost completely enclosed space is formed between the two struc- 

 tures ; it is open only anteriorly where it communicates with the areolar spaces 

 between the superficial and deep layers of the abdominal fasciae. This space is the 

 superficial perineal interspace, and contains the bulb and spongy portion of the 

 urethra, the corpora cavernosa, and certain of the perineal muscles. 



The Trigonum Urogenitale. — The trigonum urogenitale, more usually called 

 the triangular ligament of the perineum, is formed by the deep fascia of the peri- 

 neum, and, like the superficial fascia, is composed of two layers, the superior and 

 inferior (fasciae trigoni urogenitalis superior et inferior). At the sides both layers are 

 attached to the rami of the pubes and ischia, in front to either edge of the lower 

 border of the pubis, and behind they unite with each other and with the deep layer 

 of the superficial fascia along a line extending transversely across the perineum 

 between the tuberosities of the ischia. Between the two layers there is a completely 

 closed space, the deep perineal interspace., in which are to be found the membranous 

 portion of the urethra, the bulbo-urethral glands, the pudic vessels and nerves, and, 

 in front, the subpubic or arcuate ligament of the pubis. 



At their lateral insertions the layers of the trigone are continuous with the 

 obturator fascia, and the superior layer is fused above with the portion of the fascia 

 endopelvina which invests the lower surface of the prostate gland (or the base of the 

 bladder). The trigone is perforated by the urethra and, in the female, by the vagina, 

 and anteriorly the dorsal vein of the penis passes through it immediately behind the 

 subpubic ligament of the pubis, the fibres of the trigone immediately behind the 

 opening for the vein being thickened to form a transverse band known as the trans- 

 verse ligament of the pelvis. 



I. Sphincter Ani Externus (Fig. 554). 



Attachments. — The external sphincter of the anus consists of a group of 

 fibres which surround the terminal portion of the rectum, the superficial fibres 

 standing in close relationship with the integument. Its fibres arise posteriorly 

 from the coccyx and from the raphe extending from that bone to the anus, and, 

 passing forward around the anus, are inserted into the superficial fascia and the 

 central tendon of the perineum, and may in some cases be continued forward to 

 join with the fibres of the superficial transverse perineal and bulbo-cavernosus 

 muscles. 



The central tendon of the perineum is situated in the median line about 2.5 cm. 

 in front of the anus, and is the point of union of five muscles, — namely, the external 

 sphincter ani, the two superficial transversi perinei, and the bulbo-cavernosi. 



Nerve-Supply. — From the fourth sacral nerve and the inferior hemorrhoidal 

 branches of the pudic. 



Action. — To close the anal aperture. It also serves to fix the central tendon 

 of the perineum during the contraction of the bulbo-cavernosi. 



Variations.— The common embryological origin of the external sphincter ani and the perineal 

 muscles is indicated by the extension forward of the fibres of the former to join the bulbo-caver- 

 nosus, and occasionally a fasciculus of it extends as far forward as the base of the scrotum, 

 forming what has been termed the retractor scroti. 



The longitudinal muscle-fibres of the lower portion of the rectum pass below into a sheet 

 of connective tissue, which divides into three more or less distinct layers extending to the 

 integument. The outer two of these layers traverse the substance of the external sphincter 

 ani, a portion of the outermost one being continued backward to the region of the coccyx on 

 each side of the median line as a moderately strong band known as the ano-coccygeal ligament. 

 By these layers of fibrous tissue the external sphincter is divided, sometimes quite distinctly, 

 into three portions which have been regarded as distinct muscles. One of these lies imme- 

 diately beneath the skin surrounding the anus, and has consequently been termed the sphincter 



