THE PELVIC AND PERINEAL MUSCLES. 



561 



the anterior portion is composed of two morphologically distinct muscles, one of which arises 

 from the pubis and anterior part of the white line and is inserted into the median fibrous raphe, 

 whence it is termed the w. pubo-coccygeus ; while the other, situated beneath, — i.e., superficial to 

 the pubo-coccygeus, — consists of those fibres which arise from the pubis and are inserted into 

 the rectum, and is termed the ni. pubo-rectalis. 



It may be added that in the lower mammals the muscles corresponding to the ilio-coc- 

 cygeus and pubo-coccygeus are inserted into the caudal vertebrae and act as lateral flexors of 

 the tail. 



COCCYGEUS (Figs. 551, 603). 



2. 



Attachments. — The coccygeus, which forms the posterior and lesser portion 

 of the diaphragma pelvis, lies immediately behind the levator ani. It arises from 

 the spine of the ischium and is inserted into the sides of the sacrum and coccyx. 



Nerve-Supply. — From the third and fourth sacral nerves. 



Action. — To assist the levator ani in raising the pelvic floor. It also flexes 

 the coccyx laterally. 



Variations. — Occasionally the insertion of the coccygeus is confined to the sides of the 

 sacrum, in which cases its coccygeal area is occupied by fibres of the levator ani. The muscle 

 is sometimes termed the ischio-coccygeus, and is represented in the lower mammals by a muscle 

 attached to t!ie caudal vertebrae and acting as a lateral fle.xor of the tail. 



The Sacro- Coccygeus Anterior. — Occasionally muscular fibres are to be found arising from 

 the ventral surface of the sacrum and inserting into the coccyx. They form what is termed the 

 sacro-coccygeus anterior or curvator coccygis, and apparently belong to the hyposkeletal group 

 of muscles. 



3. Pyriformis (Figs. 551, 552, 602.) 



Attachments. — The pyriformis (m. piriformis) arises from the ventral surface 

 of the sacrum, between the first, second, third, and fourth sacral foramina. It 

 passes laterally through 



the great sciatic fora- Fig. 



men, receiving a bundle 

 of fibres from the upper 

 margin of the foramen, 

 and is inserted into the 

 summit of the great 

 trochanter, its tendon 

 shortly before its inser- 

 tion becoming closely 

 united with that of the 

 obturator internus. 



Nerve - Supply. 

 — By branches from the 

 sacral plexus from the 

 first and second sacral 

 nerves. 



Action. — To ro- 

 tate the thigh outward 

 and to draw it slightly 

 outward and back- 

 ward. 



Relations. — By 

 its anterior surface, 

 while within the pelvis, 



the pyriformis is in relation to the sacral plexus, the anterior branches of the internal 

 iliac vessels, and the rectum. It lies immediately above the upper border of the 

 coccygeus muscle. Outside the pelvis it is usually separated from the capsule of the 

 hip-joint by the gluteus minimus and is covered by the gluteus medius. Above the 

 upper border of the muscle at its exit from the greater sciatic foramen are the gluteal 

 vessels and the superior gluteal nerve, while below its lower border, between this and 

 the superior gemellus, are the sciatic and internal pudic vessels and the pudic, sciatic, 

 small sciatic, and inferior gluteal nerves. A bursa, the biirsa tn. pyriformis, inter- 

 venes between the tendon of the muscle and the summit of the great trochanter. 



36 



^•' 



I 



Obturator internus / 

 Greater sacro-sciatic 

 ligament 



Greater sacro- 

 sciatic foramen 

 Dorsum of ilium 



Greater sacro- 

 sciatic foramen 



Pyriformis 



Obturator internus 

 Capsule of hip-joint 



Gemein (cut) 

 Obturator externus 



Deep dissection, showing insertion of pyriform, internal and external obturator 



muscles. 



