LIGAMENTS OF PELVIC BONES. 543 



iliac vessels. The opposite surface rests on the sacrum, and is covered 

 by the great gluteal muscle outside the pelvis. The upper border is near 

 the hip-bone, the gluteal vessels and the superior gluteal nerve being be- 

 tween : and the lower border is contiguous to the coccygeus muscle, the 

 sacral plexus, and the sciatic and pudic vessels intervening. 



Action. The pyriformis belongs to the group of external rotators of the 

 hip-joint; and its use will be given with the description of the rest of the 

 muscle in the dissection of the Buttock. 



The OBTURATOR INTERNUS MUSCLE has its origin in the pelvis, and 

 insertion at the great trochanter of the femur, like the preceding ; but the 

 part outside is almost parallel in direction with that inside the pelvis. 



The muscle arises by a broad fleshy attachment from the obturator mem- 

 brane, except a small part behind ; from the pelvic fascia covering its sur- 

 face ; slightly from the bone anterior to the thyroid hole, but from all the 

 smooth inclined surface of the pelvis behind and above that aperture, 

 though opposite the small sacro-sciatic foramen a thin layer of fat separates 

 the fleshy fibres from the bone. The fibres are directed backwards and 

 somewhat downwards, arid end in three or four tendinous pieces, which 

 turn over the sharp edge of the hip-bone corresponding with the small 

 sacro-sciatic notch. Outside the pelvis the tendons blend into one, which 

 is inserted into the great trochanter. 



The muscle is in contact by one surface with the wall of the pelvis and 

 the obturator membrane; by the other surface with the pelvic fascia, and 

 towards its lower border with the pudic vessels and nerve. 



Action. The muscle draws the trochanter towards the back of the hip- 

 bone over which it bends, and rotates out the hip joint. For further no- 

 tice of its use, see the dissection of the Buttock. 



COCCYGEDS MUSCLE. The position and the connections of this muscle 

 may be studied with advantage in the interior of the pelvis. The muscle 

 is described at p. 501. 



LIGAMENTS OF THE PELVIS. 



The following are the articulations between the bones of the pelvis : 

 The several pieces of the sacrum and coccyx are united with one another. 

 The sacrum is joined at its base to the last lumbar vertebra, at its apex to 

 the coccyx, and laterally to the two innominate bones. And the innomi- 

 nate bones are connected together in front, as well as to the sacrum and 

 the spinal column posteriorly. 



UNION OF PIECES OF SACRUM AND COCCYX. Whilst the pieces of 

 the sacrum and coccyx remain separate they are articulated as in the 

 other vertebrae by an anterior and posterior common ligament, witli a disk 

 of intervertebral substance for the bodies; and by other ligaments for the 

 neural arch and processes (p. 346). 



After the sacral vertebrae, have coalesced, only rudiments of the liga- 

 ments of the bodies are to be recognized ; but when the pieces of the coccyx 

 unite by bone, those ligaments disappear in the adult male. 



SACRO-VERTEBRAL ARTICULATION. The base of the sacrum is articu- 

 lated with the last lumbar vertebra by ligaments similar to those uniting 

 one vertebra to another (p. 346) ; and by one special ligament the sacro- 

 vertebral. 



Dissection. For the best manner of bringing these different ligaments 



