THE SACRAL VERTEBRAE. 



13 



there may be twelve dorsal and six lumbar vertebras, or thirteen dorsal and five lumbar. 

 Sometimes an intermediate or transitional form is met with, as in the so-called hnnbo-sacral 

 vertebra, in which one side is united to the sacrum, while the other has a free transverse 

 process (fig. 23, 11) ; such a vertebra may be the 24th or 2oth. The study of the development 

 of the vertebral column throws light on the origin of these varieties. It appears from the 

 researches of Rosenberg that in the foetus the 26th vertebra is originally the first sacral, and 

 that in the course of growth the hip-bones move headwards so as to become attached 

 also to the 25th, which consequently becomes incorporated in the sacrum. This shifting 

 may proceed farther, so that the 24th vertebra is included ; or it may be unsymmetrical, 

 giving rise to a lumbo-sacral vertebra. Similarly, a thirteenth rib arises from the 

 persistence and growth of a cartilaginous rudiment which is regularly present in the 

 embryo, but usually becomes incorporated in the transverse process of the first lumbar 

 vertebra. (J. Struthers, " Variations of Ribs and Vertebrae," Journ. Anat., ix ; P. Topinard, 

 li Anomalies de nombre de la colonne vertebrale," Rev. d'Anthropol., 1877 ; E. Rosenberg, 

 " Entwickelung der Wirbelsaule," Morph. Jahrb., i.) 



3UP. ARTIC. PROC. 



TRAIMSV. PROC. OF 



FIRST VERT. 



Fig. 12. THE SACRUM, FROM BEFORE. (Drawn by D. Gunn.) | 



Sacral vertebrae. These by their union in the adult form the os sacrum, but 

 in youth they present the elements of five distinct vertebras. The sacrum is placed 

 below the last lumbar vertebra, and articulates laterally with the two hip-bones, 

 thus completing, together with the coccyx, the wall of the pelvis above and behind. 

 The uppermost vertebra is the largest, those which follow become rapidly smaller, and 

 the fifth is rudimentary. Hence the sacrum has the form of a triangle with itg 

 base directed upwards. It is concave and smooth in front, convex and uneven 

 behind. The direction of its surfaces is very oblique, its ventral aspect looking 

 considerably downwards, and forming above, at the place where it joins the last 

 lumbar vertebra, the projection termed the promontory. The dorsal or posterior 

 surface looks upwards as well as backwards. 



The ventral surface is concave from above downwards, and slightly so from side 



