238 THE ARTICULATIONS 



The intervertebral fibro -cartilage is a small oval disc, three-quarters of an 

 inch (about 2 cm.) wide, and a little less from before backward, closely con- 

 nected with the surrounding ligaments. It resembles the other discs in struc- 

 ture, but is sqfter and more jelly-like, though the laminse of the fibrous portion 

 are well marked. 



The anterior sacro-coccygeal ligament is a prolongation of the glistening 

 fibrous structure on the front of the sacrrnn. It is really the lower extremity of 

 the anterior longitudinal ligament, which is thicker over this joint than over 

 the central part of either of the bones. 



The posterior sacro-coccygeal ligament (fig. 274) is divided into two layers 

 of which one (the deep) is a direct continuation of the posterior longitudinal 

 ligament of the column, consisting of a narrow band of closely packed fibres, 

 which become blended at the lower border of the first segment of the coccyx 

 with the filum terminale and deep posterior ligament. 



The superficial layer of the posterior sacro-coccygeal ligament (or supra- 

 cornual ligament), (fig. 274) is the prolongation of the supraspinous which be- 

 comes inseparably blended with the aponeurosis of the sacro-spinalis (erector 

 spince) opposite the laminse of the third sacral vertebra, and is thus prolonged 

 downward upon the back of the coccyx, passing over and roofing in the lower 

 end of the spinal canal where the laminse are deficient. 



The median fibres (the supraspinous ligament) extend over the back of the coccyx to its 

 tip, blending with the deep fibres of the posterior sacro-coccygeal ligament and filum terminale; 

 the deeper fibres run across from the stunted laminae on one side to the next below on the oppo- 

 site side, and from the sacral cornua on one side to the coccygeal on the opposite, some passing 

 between the two cornua of the same side, and bridging the aperture through which the fifth 

 sacral nerve passes. Its posterior surface gives origin to the gluteus maximus muscle. 



The lateral sacro-coccygeal or intertransverse ligament (fig. 274) is merely a quantity of 

 fibrous tissue which passes from the transverse process of the coccyx to the lateral edge of the 

 sacrum below its angle. It is connected with the sacrosciatic ligaments at their attachments, 

 and the fifth sacral nerve escapes behind it. It is perforated by twigs from the lateral sacral 

 artery and the coccygeal nerve. 



The arterial supply of the sacro-coccygeal joint is from th<e lateral sacral and middle sacral 

 arteries. 



The nerves come from the fourth and fifth sacral and coccygeal nerves. 



The movements permitted at this joint are of a simple forward and backward, or hinge- 

 like character. In the act of defecation, the bone is pushed back by the faecal mass, and, in 

 parturition, by the fcetus; but this backward movement is controlled by the upward and forward 

 pull of the levator ani and ooccygeus. The external sphincter also tends to pull the coccyx 

 forward. 



I 



(c) Intercoccygeal Joints 



The several segments of the coccyx are held together by the anterior and 

 posterior longitudinal ligaments, which completely cover the bony nodules on 

 their anterior and posterior aspects. Laterally, the sacro-sciatic ligaments, 

 being attached to nearly the whole length of the coccyx, serve to connect them. 

 Between the first and second pieces of the coccyx there is a very perfect amphiar- 

 throdial joint, with a well-marked intervertebral substance. 



Movements. — But little movement occurs as a rule at the sacro-coccygeal and inter- 

 coccygeal joints, but when the head of the child is passing through the pelvic outlet at birth, 

 the tip of the coccyx is displaced backward, it may be to the extent of one inch. 



(d) The Symphysis Pubis 



Class. — False Synchondrosis. 



The bones entering into this joint are the pubic portions of the hip-bones. 

 This joint is shorter and ])r<):ider in the female than in the male. The ligaments, 

 which completely surround the articulation, are: — 



ior. Anterior. 



Arcuate. Posterior. 



Interpubic cartilage. 



The superior ligament f figs. 275 and 270) is a well-marked stratum of yellowish 

 fibres vvlii{;h ext ('iids iatciraiward along the crest of the pubis on each side, blending 

 in the middle line with the interosseous cartilage. 



