152 OS COCCYGIS. 



Fig. 37.* resembles the bodies of vertebrae, is 



the continuation of the vertebral 

 cavity which contains the spinal 

 marrow. From the cauda equina, 

 contained in this cavity, the great 

 nerves of the lower extremities pass 

 off, through the large holes on the 

 anterior surface, and some small 

 nerves through the posterior holes. 



In some bones the spinous pro- 

 cesses are entirely deficient, and 

 the cavity above mentioned is completely open behind ; but the 

 contained parts are defended by strong membranes. 



The anterior part of each lateral surface is covered by a 

 plate of cartilage, and articulated to the os ilium. The poste- 

 rior part is rough, and perforated by the fibres of the strong 

 ligaments, which are inserted into it. 



On the posterior surface of the sacrum, the sides of the open 

 part of the vertebral canal terminate, so as to form a notch 

 through which passes the twenty-ninth pair of nerves. 



The OS sacrum is very spongy, and is lighter In proportion to 

 its bulk than any bone in the body : it is defended by the 

 muscles that cover it, and the ligaments which adhere to it. It 

 is articulated, above, to the last lumbar vertebrae ; below, to the 

 OS coccygis by its apex and two cornua ; and on the sides, to 

 the ossa ilia. 



That triangular chain of bones depending from the os 

 sacrum, in which each bone becomes smaller as it descends, 

 till the last ends in a small tubercle, is called os coccygis. It 

 is convex behind, and concave before ; from which crooked 



* The sacrum seen upon its anterior surface. 1, 1. The transv^erse lines 

 marking the original constitution of the bone of four pieces. 2, 2. The anterior 

 sacral foramina. 3. The promontory of the sacrum. 4. The ear-sliaped surface 

 which articulates with the ilium. 5. The sharp edge to which the sacro-ischiatic 

 ligaments are atta'ched. 6. The vertebral articular surface. 7. The broad tri- 

 angular surface which supports the psoas muscle and lumbosacral nerve. 8. 

 The articular process of the right side. 9. The inferior extremity, or apex of 

 the sacrum. 10. One of the sacral cornua. 11. The notch which is converted 

 into a foramen by the coccyx. 



