OS COCCYGIS. 153 



pyramidal figure, which was thought to resemble a cuckoo's 

 beak, the name is derived. 



There arc four pieces in people of middle age. In children, 

 they are almost wholly cartilaginous. In old subjects, all the 

 bones are united, and become frequently one continued bone 

 with the OS sacrum. 



The highest of the four bones is the largest, with shoulders 

 extended farther to each side than the end of the os sacrum ; 

 which enlargement may serve as a distinguishing mark to fix 

 the limits of either bone. The upper surface of this bone is a 

 little hollow. From the back of that bulbous part called its 

 shoulders, a process often rises up on each side, to join with 

 the OS sacrum. Sometimes these shoulders are joined to the 

 sides of the open end of the vertebral canal, to form the hole 

 in each side common to these two bones, for the passage of the 

 twenty-ninth pair of spinal nerves. Immediately below the 

 shoulders of the os coccygis, a notch may be remarked on each 

 side, where the thirtieth pair of the spinal nerves passes. The 

 lower end of this bone is formed into a small head, which very 

 often is hollow in the middle. 



The three lower bones gradually become smaller, and are 

 spongy, but are strengthened by a strong ligament, which 

 covers and connects them. Their ends, by which they are 

 articulated, are formed in the same manner as those of the first 

 bone. 



Between each of these four bones of young subjects a carti- 

 lage is interposed ; therefore their articulation is analogous to 

 that of the bodies of the vertebrae of the neck ; for the lower 

 end of the os sacrum, and of each of the three superior bones 

 of the OS coccygis, has a small depression in the middle ; and 

 the upper part of all the bones of the os coccygis, is a little 

 concave, and, consequently, the interposed cartilages are thick- 

 est in the middle, to fill up both cavities ; by which they 

 connect the bones more firmly. When the cartilages ossify, 

 the upper end of each bone is formed into a cavity, exactly 

 adapted to the protuberant lower end of the bone immediately 

 above. From this sort of articulation, it is evident that, unless 



