THE SACKAL VERTEBRAE. 



15 



ROMONTORV 



the projection called the inferior lateral angle, below which the breadth of the bone 

 is suddenly contracted so as to form a notch with the adjacent part of the coccyx. 

 The base, or upper surface of the first sacral vertebra, bears considerable 



resemblance to the 

 upper surface of 

 the last lumbar 

 (fig. 23, 9 and 10). 

 In the middle it 

 presents the reni- 

 form surface of 

 the body, behind 

 which is the tri- 

 angular aperture 

 of the sacral canal, 

 bounded by the 

 depressed laminse. 

 On each side of the 

 aperture is an arti- 

 cular process, simi- 

 lar to the superior process of a lumbar vertebra, but 

 of large size, and bearing a well-developed mamillary 

 process. In front of this is a groove which forms with 

 the lower notch of the last lumbar vertebra an inter - 

 vertebral foramen. The external portion of the base 

 presents posteriorly an eminence corresponding to the 

 lumbar transverse process, and in front of that a 

 triangular smooth surface, continuous with the iliac fossa 

 of the hip-bone, and known as the ala of the sacrum. 



The lower end or apex, formed by the small inferior 

 surface of the body of the fifth sacral vertebra, is trans- 

 versely oval, and articulates with the coccyx. 



The sacral canal is curved with the bone, and gra- 

 dually narrows as it descends ; in transverse section 

 ifc is t^ee-sided above, but flattened and rather semi- 

 lunar below. It terminates on the posterior surface of 

 the bone between the sacral cornua, where the lamiiias 

 of the last two sacral vertebras are 

 imperfect. From this canal there 

 pass outwards in the substance of 

 the bone four pairs of interverte- 

 bral foramina, closed externally 

 by the lateral masses, but opening 

 on the surfaces by the anterior 

 and posterior sacral foramina. 



Fig. 14.--SACRUM AND 

 FROM THE RIGHT SIDE. 



by D. Gunn.) f 



CRAL CANAL 



The sacrum of the female is 

 broader in proportion to its length, 

 and usually flatter than that of the 

 male ; but the curvature varies 

 greatly in different skeletons. 



The sacrum of man is charac- 

 terized by its great breadth in comparison with its length. 



LATERAL 



Fig. 15. TRANSVERSE SECTION OF SACRUM, PASSING 



THROUGH THE FIRST PAIR OF FORAMINA. (G. D. T. ) 



This proportion is expressed by 



the sacral index, which is ascertained by the following calculation, 



100 X breadth 



The 



length 

 average sacral index in the male European is 112, in the Negro 106, in the Australian 99, 



