THE GLUTEAL REGION. 429 



chanter, behind, but often partly blended with, the tendon of the Obturator inter- 

 nus and Gemelli muscles. 



Relations. By its (interior surface, within the pelvis, with the Rectum (espe- 

 cially on the left side), the sacral plexus of nerves, and the branches of the internal 

 iliac vessels; external to the pelvis, with the posterior surface of the ischium and 

 capsular ligament of the hip-joint; by its posterior surface, within the pelvis, with 

 the sacrum, and external to it, with the Gluteus maximus ; by its upper border, 

 with the Gluteus medius, from which it is separated by the gluteal vessels and 

 superior gluteal nerve ; by its lower border, with the Gemellus superior and 

 Coccygeus, the sciatic vessels and nerves, the internal pudic vessels and nerve, and 

 muscular branches from the sacral plexus, passing from the pelvis in the interval 

 between the two muscles. 



The Obturator membrane (Fig. 166) is a thin layer of interlacing fibres which 

 closes the obturator foramen. It is attached, externally, to the margin of the 

 foramen ; internally, to the posterior surface of the ischio-pubic ramus, below and 

 internal to the margin of the foramen. It is occasionally incomplete, and presents 

 at its upper and outer part a small canal, which is bounded below by a thickened 

 band of fibres, for the passage of the obturator vessels and nerve. Both obturator 

 muscles are connected with this membrane. 



Dissection. The next muscle, as well as the origin of the Pyriformis, can only be seen 

 when the pelvis is divided and the viscera removed. 



The Obturator internus, like the preceding muscle, is situated partly within the 

 cavity of the pelvis, and partly at the back of the hip-joint. It arises from the 

 inner surface of the anterior and external wall of the pelvis, where it surrounds 

 the greater part of the obturator foramen, being attached to the descending ramus 

 of the os pubis and the ramus of the ischium, and at the side to the inner surface 

 of the innominate bone below and behind the pelvic brim, reaching from the upper 

 part of the great sacro-sciatic foramen above and behind to the thyroid foramen 

 below and in front. It also arises from the inner surface of the obturator mem- 

 brane except at its posterior part, from the tendinous arch which completes the 

 canal for the passage of the obturator vessels and nerve and to a slight extent from 

 the obturator layer of the pelvic fascia, which covers it. The fibres converge 

 rapidly, and are directed backward and downward, and terminate in four or five 

 tendinous bands, which are found on its deep surface ; these bands are reflected at 

 a right angle over the inner surface of the tuberosity of the ischium, which is 

 grooved for their reception ; the groove is covered with cartilage, and lined by a 

 synovial bursa. The muscle leaves the pelvis by the lesser sacro-sciatic foramen ; 

 and the tendinous bands unite into a single flattened tendon, which passes horizon- 

 tally outward, and, after receiving the attachment of the Gemelli, is inserted into 

 the fore part of the inner surface of the great trochanter in front of the Obturator 

 externus. A synovial bursa, narrow and elongated in form, is usually found 

 between the tendon of this muscle and the capsular ligament of the hip: it 

 occasionally communicates with the bursa between the tendon and the tuberosity 

 of the ischium, the two forming a single sac. 



In order to display the peculiar appearances presented by the tendon of this muscle, it 

 must be divided near its insertion and reflected inward. 



Eelations. Within the pelvis this muscle is in relation, by its anterior surface, 

 with the obturator membrane and inner surface of the anterior wall of the pelvis: 

 by its posterior surface, with the pelvic and obturator fasciae, which separate it 

 from the Levator ani ; and it is crossed by the internal pudic vessels and nerve. 

 This surface forms the outer boundary of the ischio-rectal fossa. ^External to the 

 pelvis it is covered by the Gluteus maximus, crossed by the great sciatic nerve, 

 and rests on the back part of the hip-joint. As the "tendon of the Obturator 

 internus emerges from the lesser sacro-sciatic foramen it is overlapped by the two 

 Gemelli, while nearer its insertion the Gemelli pass in front of it and form a 

 groove in which the tendon lies. 



