THE COCCYX. 



99 



thin and tapering below, where they furnish attachments for the powerful sacro- 

 tuberous ligaments (O.T. . great sacro-sciatic). The iliac articular surfaces are 

 described as auricular in shape (facies auricularis), and overlie the lateral parts 

 formed by the first three sacral vertebrae, though this arrangement is liable to con- 

 siderable variation. Posterior to the auricular surface the bone is rough and pitted 

 by three distinct depressions for the attachment of the strong sacro-iliac ligaments. 

 Inferiorly, the edge formed by the lateral parts of the fourth and fifth sacral 

 vertebrae becomes gradually thinner, and at the inferior lateral angle changes its 

 direction and sweeps medially towards the body of the fifth sacral segment. 



The apex, or lower end of the sacrum, is formed by the small oval body of the 

 fifth sacral vertebra, which articulates with the coccyx. 



The sacral canal follows the curve of the bone ; more or less triangular in shape 

 above, it becomes compressed and flattened dorso-ventrally below. Inferiorly, its 

 posterior wall is deficient owing to the imperfect ossification of the laminae of the 

 fifth, and, it may be, of the fourth sacral segments. Passing obliquely downwards 

 and laterally from this canal into the lateral parts on either side are the four pairs 

 of intervertebral foramina, each of which is connected laterally with a V-shaped 

 canal which terminates in front and behind in the anterior and posterior sacral 

 foramina. The posterior limb of the V is shorter and narrower than the anterior. 



The female sacrum is proportionately broader than the male, its curves are 

 liable to great individual variation ; usually it is flattened above, and somewhat 

 abruptly curved below, as contrasted with the male sacrum, in which the curve is 

 more uniformly distributed throughout the bone. In the female the absolute 

 depth of the curve is less than in the male. The iliac articular surface of the 

 female sacrum is smaller than, and of a different shape from, that of the male ; in 

 the majority of cases it only extends over two sacral segments, whereas in the male 

 it invariably includes a part, and at times the whole of the third segment (Derry). 



The variation in the proportions of the breadth to the length of the sacrum is 



expressed by the formula 



breadth x 100 



= Sacral Index. Sacra with an index above 



length 



100 are platyhieric and are generally characteristic of the higher races, those with 

 an index below 100 are dolichohieric and are more commonly met with in the lower 

 races of men. The average European index is 112'4 for males and 116'8 for females. 



Os Coccygls. 



The coccyx consists of four sometimes five, less frequently three rudimentary 

 vertebrae, which tend to become fused. The first piece is larger than the others ; 

 it has an oval hollow 

 facet on its superior sur- 

 face, which articulates 

 with the body of the last 

 sacral segment. Pos- 

 teriorly, two processes, 

 cornua coccygea, which 

 lie in series with the 

 articular processes of the 

 sacrum, extend upwards 

 and unite with the sacral 

 cornua, thus bridging 

 over the notch for the 



exit of the fifth sacral 



FIG. 114. THE COCCYX. 

 nerve, and converting 



it into a foramen, the A " Posterior Surface ' R Anterior Surface ' 



last Of the intervertebral L T ' lsverse P rocess - 2 ' Transverse process. 3. For Sacrum. 4. Cornu. 



series. From the sides of the body project rudimentary transverse processes, which 

 may, or may not, unite with the sacrum close to the lower lateral angles ; in the 

 latter case the fifth anterior sacral foramina are enclosed. Inferiorly, the body of the 

 bone articulates with the succeeding vertebra. The second coccygeal vertebra displays 



