450 LABORATORY MANUAL OF HUMAN ANATOMY 



anteriorly into the central tendinous point of the diaphragm, be- 

 coming continuous with some of the fibres of the M. bulbocaverno- 

 sus. 



(c) Internal circular layer, made up of annular fibres surrounding the 

 terminal part of the rectum. 



Note that the external sphincter is made up of two symmetrical 

 halves. 



Clean out the fat of the ischiorectal fossa, carefully pre- 

 serving the structures found therein. Proceed to the study of 

 it, noting exactly the position of its boundaries. 



Ischiorectal Fossa (Fossa ischiorectalis). (Figs. 224, 232, 274, 278.) 



(a) Levator ani muscle (M. levator ani}. 



(b) Obturator internus muscle (M. obturator internus}. 



(c) Inferior fascia of pelvic diaphragm (fascia diaphragmatis pelvis in- 



ferior) (0. T. anal fascia). 



(d) Obturator fascia (fascia obturatoria) . 



At the junction of (c) and (d) as seen from below, in the angle 

 between them is seen the tendinous arch of the levator ani muscle 

 (arcus tendineus M. levatoris ani) (0. T. white line of pelvic 

 fascia). 



(e) Posterior border of urogenital diaphragm (diaphragma urogenitale) . 



(f) Gluteus maximus muscle (M. glutaeus maximus). 



(g) Sacrotuberous ligament (ligamentum sacrotuberosum) (0. T. great 



sacrosciatic ligament). 



After having located accurately the boundaries of this fossa, 

 proceed to the study of its contents : 



(a) Inferior hemorrhoidal artery (A. haemorrhoidalis inferior). (Vide 



Spalteholz, Fig. 473.) 



Determine the course of the inferior hemorrhoidal veins (Vv. 

 haemorrhoidales inferiores). What is the plexus haemorrhoidalis? 

 With what vessels do the above anastomose? 



(b) Inferior hemorrhoidal nerve (N. haemorrhoidalis inferior). 



(c) Anococcygeal nerves (Nn. anococcygei) (0. T. perineal branches of 



fourth sacral nerve). 



Trace the arteria haemorrhoidalis inferior back to the inter- 

 nal pudendal artery (A. pudenda internet), its parent stem. 

 Note that it is accompanied by the V. pudenda interna and the 

 N. pudendus. These three structures run in the " canal of Al- 

 cock," which is formed by the fusion of the fascia obturatoria 

 with the falciform process of the ligamentum sacrotuberosum. 



