1384 



CLINICAL AND TOPOGRAPHICAL ANATOMY 



The uro-genital diaphragm (or trigone) (fig. 400), the lower diaphragm of the 

 pelvic floor, is both morphologically and functionally different from the upper. 

 The uro-genital diaphragm is a sphincter muscular layer, derived (with the sphincter 

 ani externus) from the primitive sphincter cloacoe. The uro-genital diaphragm is 

 composed of superior and inferior fascial layers, enclosing the membranous urethra, 

 the sphincter urethrse membranacese and the transversus perinei profundus. 

 Superficial to the uro-genital diaphragm is the superficial perineal interspace 

 (fig. 400). This is covered by the superficial perineal (Colles') fascia, and in- 

 cludes the crura and bulb of the corpora cavernosa, with associated muscles, 

 vessels and nerves. 



The space in the pelvic floor on each side below the pelvic diaphragm is the 

 ischio-rectal fossa (figs. 399, 400, 1114). In the posterior or rectal triangle, where 

 the urogenital diaphragm is absent, the ischio-rectal fossae form large wedge-shaped 

 spaces. The lower wall or base is formed chiefly by the corresponding skin and 

 superficial fascia, and partly by the external sphincter ani; the medial wall by the 



Fig. 1114. — Coronal Section of the Ischio-rectal Fossa. (G. Elliot Smith.) 



Obturator 

 internus 



Roof of ischio-rectal fossa 

 (lamina terminalis) 



Levator ani 



Internal pudic 

 vessels and nerve 



Falciform process 



External sphincter ani 



Gluteus maximus 



muscles (levator ani and coccygeus) and inferior fascia of the pelvic diaphragm; 

 the lateral wall by the obturator internus muscle, with the corresponding obturator 

 fascia (with Alcock's canal, including the pudic vessels and nerves). The apex of 

 the fossa is above, where medial and lateral walls meet. The narrow fibrous roof 

 strip joining the medial and lateral walls just above the level of the internal 

 pudic vessels and nerves has been called the lamina termi7ialis (Elliot Smith, 

 fig. 1114). Posteriorly the fossa is bounded by the gluteus maximus and lig. 

 sacro-tuberosum. Anteriorly on each side the ischio-rectal fossae extend as narrow 

 spaces between the pelvic diaphragm above, the uro-genital diaphragm below, and 

 the pelvic wall laterally (figs. 400, 401, 402). 



Contents. — The ischiorectal fossa is filled with loose adipose tissue continuous with the 

 subcutaiioouH fat of the buttock. It is traversed by the inferior hasmorrhoidal branches of 

 the internal pudic artery, with tlie associated veins and nerves, passing to the external anal 

 Bphincter, the skin and the adja(;ent mucosa. The superficial vessels and nerves, as they 

 run forward to pierce the superficial perineal fascia, lie in this space, as well as the inferior 

 clunial (perforating cutaneous) branches and branches of the fourth sacral nerve. The inferior 



