SACRO-COCCYGEAL. 205 



It is in relation, anteriorly, with the Coccygeus muscle ; posteriorly, it is covered 

 by the posterior ligament, and crossed by the pudic vessels and nerve. Its supe- 

 rior border forms the lower boundary of the great sacro-sciatic foramen ; its infe- 

 rior border, part of the lesser sacro-sciatic foramen. 



These two ligaments convert the sacro-sciatic notches into foramina. The 

 superior or great sacro-sciatic foramen is bounded, in front and above, by the pos- 

 terior border of the os innominatum ; behind, by the great sacro-sciatic ligament ; 

 and below, by the lesser ligament. It is partially filled up, in the recent state, 

 by the Pyriformis muscle. Above this muscle, the gluteal vessels and nerve 

 emerge from the pelvis ; and below it, the ischiatic vessels and nerves, the internal 

 pudic vessels and nerve, and the nerve to the Obturator internus. The inferior 

 or lesser sacro-sciatic foramen is bounded, in front, by the tuber ischii ; above, by 

 the spine and lesser ligament ; behind, by the greater ligament. It transmits 

 the tendon of the Obturator internus muscle, its nerve, and the pudic vessels and 

 nerve. 



3. ARTICULATION' OF THE SACRUM AND COCCYX. 



This articulation is an amphiarthrodial joint, formed between the oval surface 

 on the summit of the sacrum, and the base of the coccyx. It is analogous to the 

 joints between the bodies of the vertebrae, and is connected by similar ligaments. 

 They are the 



Anterior Sacro-coccygeal. 



Posterior Sacro-coccygeal. 



Interarticular Fibro-cartilage. 



The Anterior Sacro-coccygeal Ligament consists of a few irregular fibres, which 

 descend from the anterior surface of the sacrum to the front of the coccyx, becom- 

 ing blended with' the periosteum. 



The Posterior Sacro-coccygeal Ligament is a flat band of ligamentous fibres, of a 

 pearly tint, which arises from the margin of the lower orifice of the sacral canal, 

 and descends to be inserted into the posterior surface of the coccyx. This liga- 

 ment completes the lower and back part of the sacral canal. Its superficial fibres 

 are much longer than the deep-seated ; the latter extend from the apex of the 

 sacrum to the upper cornua of the coccyx. This ligament is in relation in front 

 with the arachnoid membrane of the sacral canal, a portion of the sacrum, and 

 almost the whole of the posterior surface of the coccyx ; behind with the Gluteus 

 maxim us. 



An Interarticular Fibro-cartilage is interposed between the contiguous surfaces 

 of the sacrum and coccyx ; it differs from that interposed between the bodies of 

 the vertebrae, in being thinner, and its central part more firm in texture. It is 

 somewhat thicker in front and behind, than at the sides. Occasionally a synovial 

 membrane is found where the coccyx is freely movable, which is more especially 

 the case during pregnancy. 



The different segments of the coccyx are connected together by an extension 

 downwards of the anterior and posterior sacro-coccygeal ligaments, a thin annular 

 disk of nbro-cartilage being interposed between each of the bones. In the adult 

 male all the pieces become ossified ; but in the female, this does not commonly 

 occur until a later period of life. The separate segments of the coccyx are first 

 united, and at a more advanced age the joint between the sacrum and the 

 coccyx. 



Actions. The movements which take place between the sacrum and coccyx, 

 and between the different pieces of the latter bone, are slightly forwards and back, 

 wards ; they are very limited. Their mobility increases during pregnancy. 



4. ARTICULATION OF THE PUBES. 

 The articulation between the pubic bones is an amphiarthrodial joint, formed 



