88 SKELETON. 



ments arise. After these the posterior surface of the bone slants 

 very considerably to its lateral margin. 



The base of the sacrum presents in its middle an oval surface 

 for articulating with the last lumbar vertebra. Between this 

 surface and the oblique process, may be remarked the groove 

 for the fifth lumbar nerve. The base of the sacrum continually 

 thickens, from the side of the oval surface to the place of junc- 

 tion with the ilium. The anterior margin of this expansion is 

 continuous with the linea ilio-pectinea; the posterior margin is 

 elevated at its extremity, is a substitute for a transverse process, 

 and is placed immediately below the transverse of the last lum- 

 bar vertebra. The point of the sacrum is truncated where it 

 articulates with the os coccygis. The lateral face of the sacrum 

 is thicker above than below; its upper two-thirds present an ir- 

 regular, and somewhat triangular face for joining the ilium; the 

 lower third is very thin, and contributes to form the sacro-scia- 

 tic notch of the pelvis. 



The spinal canal of the sacrum is triangular, and diminishes 

 continually to its lower extremity, where it terminates by a 

 small orifice, notched behind, as mentioned, and exposing the 

 last piece of the bone. The foramina on the anterior and pos- 

 terior surface of the sacrum, communicating with this canal, cor- 

 respond strictly in their uses and positions with the inter-verte- 

 bral foramina of other parts of the spine. 



The sacrum is extremely light for its size, and its texture is 

 in a high degree spongy; but its processes and articular faces 

 are quite as compact as they are in other parts of the spine. 



Of the Coccyx or Caudal Vertebra. 



The os coccygis (coccyx) resembles the sacrum in shape and 

 texture, and is so placed as to continue forwards the line of the 

 curvature of the sacrum. It consists in four pieces, sometimes 

 only three, united to one another by fibro-cartilaginous matter, 

 and it corresponds with the tails of animals. These pieces in 

 the progress of life, are not only anchylosed together, but also 

 with the sacrum; so that all the false vertebrae, from the base of 

 the sacrum to the point of the coccyx, are joined into a single 

 bone. 



