THE VERTEBRAL COLUMN. 



to side. It is traversed horizontally by four ridges, which indicate the places of 

 union of the bodies of the five sacral vertebras, and at the extremities of which are 

 situated on each side four foramina called anterior sacral. These foramina lead 

 externally into grooves, and dimmish in size from above downwards. 



The dorsal surface is convex, very uneven, and somewhat narrower than the 

 ventral. It presents along the median line three or four small eminences, the 

 spinous processes, usually more or less connected, so as to form a ridge. Below the 

 spinous process is a triangular opening, the termination of the spinal canal, the 

 lateral margins of which are formed by the imperfect laminae of the fourth and fifth 

 sacral vertebrae, and are produced downwards into a pair of tubercles, the sacral 



UPPER APERTURE OF SACRAL- CANAL 

 SUP. ARTIC. PROC. 



AURICULAR 

 SURFACE 



DEPRESSIONS 

 FOR POSTERIOR 

 SACRO ILIAC 

 LIGAMENTS 



POST. SAC. FORANI. 

 NF. LAT. ANGLE 



SACRAL CORNU 



J-OWER APERTURE OF SACRAL. CANAL. 



Fig. 13. THE SACRUM, FROM BEHIND. (Drawn by D. Gunn.) $ 



cornua, which represent the inferior articular processes of the last sacral vertebra, 

 and are connected to the cornua of the coccyx. On each side of the ridge of spines 

 the surface formed by the united laminae is slightly hollowed, thus giving rise to 

 the sacral groove, which prolongs the vertebral groove of the moveable part of the 

 column ; and beyond this are the four posterior sacral foramina, opposite to, but 

 smaller than the anterior. Immediately internal to each foramen is a slight 

 eminence, which represents the articular and mamillary processes of the vertebras 

 above, while external to the foramen a more strongly marked elevation corresponds 

 to the transverse process. 



The part of the sacrum external to the foramina constitutes the lateral mass, and 

 is broad and thick above, but narrowed below. The outer aspect of the upper part 

 presents in front a large uneven surface, covered in the recent state with cartilage, 

 which articulates with the ilium, and is called from its shape the auricular surface : 

 behind this the bone is rough and marked with strong depressions for the attachment 

 of ligaments. Lower down, the margin becomes narrowed and sinuous, terminating in 



