10(30 



URINARY AND REPRODUCTIVE ORGANS 



rectal mucous membrane. It is supplied by the inferior ha?morrhoidal branches 

 of the internal juidic artery; its veins, of large size and very lial)le to varicose 

 dilatation, terminate in the inferior hsemorrlioidal and pudic veins; its lymphatics 

 open into the inguinal glands; and its nerves are derived from the perinseal, the 

 pudic, and the fourth sacral. 



Action. — The external sphincter, by virtue of its elasticity, gives some passive 

 aid to the internal sphincter in maintaining the closure of the anus and the 

 retention of the contents of the rectal ad-de-sac under ordinary conditions; but 

 its active and more important function is to close the anal aperture firmly during 

 the contraction of the levatores ani and other constrictor muscles of the al)domino- , 

 pelvic cavity in powerful muscular efforts not connected -with defecation (as in 

 vomiting, urinaticm, parturition, expiration, etc.). This it effects partly through 

 the approximation of the opposite sides of the anus by its longitudinal fibres, and 

 partly by the ring-like contraction of its circular fibres. It also aids in flexing the 

 coccyx, and in fixing the tendinous centre of the perina?um during the contraction 

 of the bulbo-cavernosi. 



THE PELVIC FASCI.E AXD MUSCLES 



In order to understand the constitution of the ischio-rectal fossae and their 

 relation to adjacent parts, it is necessary to review the muscular elements of the 

 pelvic Avail, and the arrangement of the pelvic fasciae. 



Fig. 648. — Diagram of the Pelvic Fascia. 



Transversalis fascia 



Obturator fascia 



' White line' 



Recto-vesical fascia 



Obturator fascia 

 Ischio-rectal fascia 



Alcock's canal with 

 pudic vessels 



Abdominal muscles 



BORDER OF ACETABULUM 



Apex of ischio-rectal 

 fossa 



Levator ani 



ISCHIAL TUBEROSITY 



Sphincter ani 



The osseo-ligamentous framework of the walls of the true pelvis is constituted 

 by the two ossa innominata below the ilio-pubic line and posterior border of the 

 pubic crest, by the sacrum and coccyx, and by the sacro-sciatic, sacro-iliac, and 

 interpubic ligaments, and the obturator membrane. Supplementing these struct- 

 ures on each side of the median line, are four muscles: the obturator internus, the 

 pyriformis, the coccygeus, and the levator ani, together with their fascial invest- 

 ments. 



The obturator internus and pyriformis have already been described (pages 355- 

 358). The fascia covering the inner aspect of the former muscle bears a very 

 important relation to the intrapelvic structures. 



