OSTEOLOGY. 



posterior sacral foramina ; they are smaller in size, and less regular in form than 

 the anterior, and transmit the posterior branches of the sacral nerves. On the 

 outer side of the posterior sacral foramina is a series of tubercles, the rudimentary 



Fig. 16. Sacrum: Posterior Surface. 



posterior transverse processes of the sacral vertebra?. The first pair of transverse 

 tubercles are of large size, very distinct, and correspond with each superior angle 

 of the bone ; the second, small in size, enter into the formation of the sacro-iliac 

 articulation ; the third give attachment to the oblique sacro-iliac ligaments ; and 

 the fourth and fifth to the great sacro-ischiatic ligaments. The interspace between 

 the spinous and transverse processes on the back of the sacrum, presents a wide 

 shallow concavity, called the sacral groove; it is continuous above with the 

 vertebral groove, and lodges the origin of the Erector spinse. 



The Lateral surface, broad above, becomes narrowed into a thin edge below. 

 Its upper half presents in front a broad ear-shaped surface for articulation with 

 the ilium. This is called the auricular or ear-shaped surface, and in the fresh 

 state is coated with cartilage. It is bounded posteriorly by deep and uneven 

 impressions, for the attachment of the posterior sacro-iliac ligaments. The lower 

 half is thin and sharp, and gives attachment to the greater and lesser sacro-ischiatic 

 ligaments, and to some fibres of the Gluteus maximus ; below, it presents a deep 

 notch, which is converted into a foramen by articulation with the transverse 

 process of the upper piece of the coccyx, and transmits the anterior branch of the 

 fifth sacral nerve. 



The Base of the sacrum, which is broad and expanded, is directed upwards and 

 forwards. In the middle is seen an oval articular surface, which corresponds with 

 the under surface of the body of the last lumbar vertebra, bounded behind by 

 the large triangular orifice of the sacral canal. This orifice is formed behind by 

 the spinous process and laminae of the first sacral vertebra, whilst projecting from 



