46 HUMAN ANATOMY. 



The apex is small and articulates with the coccyx. 



The anterior concave surface presents four ridges, indicating 

 the original separations, which terminate externally in eight 

 anterior sacral foramina, with wide, shallow grooves, for the 

 exit of the anterior sacral nerves. ' To the outer side of these 

 foramina is the lateral mass for origin of pyriformis muscle. 



The posterior surface presents rudimentary spinous articu- 

 lar and transverse processes, the laminae of the last two segments 

 being deficient.* On the other side of the spinous process is a 

 broad, shallow concavity, the sacral groove, for the origin of the 

 erector spinaa muscle, and external to the articular processes are 

 four posterior sacral foramina for the exit of posterior sacral 

 nerves. 



The lateral surfaces have each a rough articular surface for 

 articulation, with ossa innominata, and below presents a deep 

 notch on each side of the apex, which is converted by articulation 

 with the transverse process of the coccyx into the fifth sacral 

 foramen, for exit of anterior division of the fifth sacral nerve. 



The sacral canal, continuous above with the spinal foramen 

 of the vertebrae, runs through the centre. Large and triangular 

 above, small and flattened below, its lower posterior wall is de- 

 ficient. It lodges the sacral nerves, and out of it pass the an- 

 terior and posterior sacral foramina. 



It articulates with four bones, two ossa innominata, last 

 lumbar vertebra, and coccyx. 



Muscular attachments are seven iliacus, pyriformis, coccyg- 

 eus, glutens maximus, latissimus dorsi, multifidus spinae, and 

 erector spiiiae. 



Its ossific centres are thirty-five, as follows : bodies, includ- 

 ing interarticular plates, three each (15) ; arches, two each (10) ; 

 lateral masses, six (6) ; epiphyseal plates of lateral surfaces, 

 four (4). Total, 35. 



THE COCCYX is a small triangular bone, resembling a 

 cuckoo's beak, and composed of four rudimentary vertebrae more 

 or less co-ossified. 



The base articulates with the sacral apex, being prolonged 

 upward into two coriiua to complete the fifth posterior sacral 

 foramen for exit of posterior branch of fifth sacral nerve. 



Laterally the base is prolonged into rudimentary transverse 

 processes, completing with the notch of the sacrum the fifth 

 anterior sacral foramina for exit of the anterior branch of fifth 

 sacral nerve. 



* The articular processes of the fourth and fifth unite to form the 

 sacral cornua for articulation with the cornua of the coccyx. 



