THE SACRUM AND THE COCCYX 29 



between the transverse processes the anterior to the spaces 

 between two ribs. 



The lateral mass is that part external to the foramina, broad 

 above and narrow below. It is made up of broadened trans- 

 verse processes, rudiments of which are seen outside the posterior 

 sacral foramina; the first pair are large, the second are smaller, 

 on each side these two and the upper half of the third, by an 

 uneven auricular lateral surface, enter into the formation of 

 the sacroiliac joint; the fourth and fifth give attachment to 

 portions of the gluteus maximus, and the greater and lesser 

 sacrosciatic ligaments. Anteriorly are four shallow grooves, 

 separated by ridges which give attachment to the pyriformis 

 muscle. Behind the auricular surface it is very rough for 

 the attachment of the posterior sacroiliac ligament. Lower 

 down the bone terminates in the inferior lateral angle, below 

 which is a half-notch, forming a foramen with the coccyx for 

 the fifth sacral nerve. 



The base shows the reniform first sacral body, behind which 

 is the triangular aperture of the sacral canal; on each side of 

 this is a large articular process similar to the superior ones 

 of the lumbar vertebra?, bearing a large mammillary process. 

 In front of this is a vertebral groove which helps form the 

 last lumbar intervertebral foramen. Externally is a modified 

 transverse process, and in front of that a smooth triangular 

 surface continuous with the iliac fossa, the ala of the sacrum. 



The apex is the body of the fifth sacral vertebra, transversely 

 oval; it articulates with the coccyx. The sacral canal curves 

 with the bone, and becomes smaller as it descends. A trans- 

 verse section is triangular above, but flattened below, its 

 posterior wall, however, being still arched. From it there 

 pass out four pairs of intervertebral foramina, opening anteriorly 

 and posteriorly into the anterior and posterior sacral foramina, 

 and closed externally by the lateral masses. 



Differences in the Sacrum of the Male and Female. In 

 the female it is wider, is less curved, the upper half nearly 

 straight, is more obliquely placed, and forms a more marked 

 promontory than in the male. 



The coccyx is pyramidal. Its vertebrae are very rudimentary, 

 four in number (rarely fur or three}. Of the first one, the 

 pedicles and superior articular cornua project upward, and 

 help form the last intervertebral foramen. The short trans- 



