THE COCCYX. 



127 



slightly raised edges and a rough, irregular surface. The auricular surface is formed 

 chiefly by the lateral mass of the first sacral {vertebra fulcralis, as having the most 

 to do in supporting the pelvis), to a less extent by that of the second, and very little 

 by that of the third. Behind this articular portion lies the rough ligamentous sur- 

 face, which slants backward and inward, and affords origin for the posterior sacro- 

 iliac ligaments. * 



Differences depending upon Sex. — The female sacrum is relatively broader 

 than the male. The sacral index ^ or the ratio of the breadth to the length (^^^^^^^)> 

 is 112 for the white male and 116 for the female. Such a rule is, however, not abso- 

 lute, there being many doubtful cases, but a narrow sacrum is almost invariably 

 male. Another, and very reliable, guide, especially in conjunction with the first, is 

 the curve. There are contradictory statements among authors, but the truth is, as 

 originally shown by Ward, that the male sacrum is the more regularly curved, while 

 the anterior surface of the female bone runs in nearly a straight line from the prom- 

 ontory to the middle of the third piece and then suddenly changes its direction. 



Variations. — The sacrum often consists of six vertebrae. Such a one may 

 be recognized even when the lower part is wanting, so that the vertebrae cannot be 

 counted. If a line across the front, connecting the lowest points of the auricular 

 surfaces, passes below the middle of the thi"d sacral, the sacrum is of six pieces ; if 

 above, of five.^ Sacra consisting of only four vertebrae are rare. 



THE COCCYX. 



This bone is composed of four or five^ flattened plates representing vertebral 

 bodies. It is an elongated triangle with the apex below. The base, joined by fibro- 

 cartilage to the apex of the sacrum, is oblique, the posterior border being higher 

 than the front, so that the coccyx slants forward from the sacrum. The anterior 

 surface of the coccyx is, moreover, very slightly concave. T\\q first vertebra consists 

 of a thin body, about twice as broad as long, from the back of which on each side the 

 rudiment of an arch extends up-yard as a straight process, the coccygeal cornu, which 



Surface for sacru 

 Transverse process' 



Coccygeus 



Levator an 



Anterior 



Fig. 155. 



Posterior 



Cornu 



Transverse process 



Gluteus maximus 



•Sphincter ani 



The COCCYX. 



overlaps the back of the body of the last sacral vertebra and joins the sacral cornu. 

 A short lateral projection from the side of the body represents the transverse process; 

 perhaps the costal element also. On the upper border of this process, at its origin, is 

 a 7iotch, which usually forms a foramen with the sacrum for the anterior division oi 

 the fifth sacral nerve. Very faint rudiments of these two pairs of processes are 

 sometimes to be made out on the second vertebra, which is much smaller than the 

 first, but also broad and flat. The succeeding ones are much smaller and ill-defined. 

 Constrictions on the surfaces and notches on the edges mark the outlines of the 



' Bacarisse : Le sacrum suivant le sexe et suivant las races. These, Paris, 1873. 

 * According to Steinbach, there are five in man and four or five in woman. Die Zahl der 

 Caudalwirbel beim Menschen. Inaugural Dissertation, Berlin, 1899. 



