128 



THE SKELETON". 



the sacral canal is exposed. External to the laminae are a linear series of in- 

 distinct tubercles representing the articular processes; the upper pair are large, 

 well developed, and correspond in shape and direction to the superior articu- 

 lating processes of a lumbar vertebra; the second and third are small; the 



Fig. 83. Sacrum, Posterior Surface. 



Jiott.sa cra.1 fir . 



fourth and fifth (usually blended together) are situated on each side of the 

 sacral canal. They are called the sacral cornua, and articulate with the cornua 

 of the coccyx. External to the articular processes are the four 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 are a series of tubercles, the rudimentary posterior 

 transverse processes of the sacral vertebrae. 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 be- 

 tween 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 Spinae. . 



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 iliurn. This is called the auricular 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 liga- 

 ments, 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 



