468 ABDOMEN 



of areolar tissue which forms a loose connection between the 

 mucous and muscular coats, except in the region of the 

 trigone, where the connection is much closer. The blood- 

 vessels and nerves ramify in the submucous layer before they 

 enter the mucous coat. 



The Mucous Coat. -The mucous coat has already been 

 examined (p. 425). When the bladder is distended it is 

 smooth in all areas, but when the bladder is empty or 

 partially empty it is smooth in the area of the trigone only. 

 In all other parts it is thrown into a series of irregular folds 

 (Fig. 200). 



Dissection. Remove the levator ani, leaving small portions 

 attached to its bony origins, viz., the body of the pubis and the 

 spine of the ischium. Take away all the remains of the parietal 

 pelvic fascia from the side wall of the pelvis, and the obturator 

 internus muscle will be exposed. 



M. Obturator Internus. The internal obturator muscle 

 clothes the side wall of the pelvis on its inner aspect. It is fan- 

 shaped and takes an extensive origin, viz. (i) from the cir- 

 cumference of the obturator foramen, except above, where the 

 obturator vessels and nerves quit the pelvis; (2) from the 

 pelvic surface of the obturator membrane; (3) from the 

 surface of bone behind the obturator foramen, as far back as 

 the greater sciatic notch. A few fibres are derived also from 

 the parietal pelvic fascia which covers it. From these origins 

 the fibres converge towards the lesser sciatic notch, and end 

 in a tendon which issues from the pelvis through the lesser 

 sciatic foramen. In the gluteal region the tendon is inserted, 

 together with the two gemelli, into the medial margin of the 

 upper border of the greater trochanter of the femur. The 

 margin of the lesser sciatic notch over which the tendon glides 

 is coated with smooth cartilage, which is raised into three or 

 four parallel ridges ; the ridges fit into fissures in the deep 

 surface of the tendon. A mucous bursa intervenes between 

 the tendon and the cartilage. 



The obturator internus is supplied by a special branch 

 from the front of the upper part of the sacral plexus (p. 463). 

 In the erect posture it is a lateral rotator of the femur, but 

 when the hip joint is flexed it is an abductor of the femur. 



M. Piriformis. The piriformis lies on the anterior 

 aspect of the posterior pelvic wall. It arises by three pro- 

 cesses from the anterior surface of the sacrum, in the region 



