50 



THE SKELETON. 



Cornua. 



of the laminae with the bodies in front and with each other behind varies in different 

 segments. The junction between the laminae and the bodies takes place first in 

 the lower vertebrae as early as the second year, but is not effected in the upper- 

 most until the fifth or sixth year. About the sixteenth year the epiphyses for 

 the upper and under surfaces of the bodies are formed, and between the eighteenth 

 and twentieth years those for each lateral surface of the sacrum make their 

 appearance. The bodies of the sacral vertebras are, during early life, separated 

 from each other by intervertebral disks. But about the eighteenth year the two 

 lowest segments become joined together by ossification extending through the 

 disk. This process gradually extends upward until all the segments become 

 united, and the bone is completely formed from the twenty-fifth to the thirtieth 

 year of life. 



Articulations. With four bones: the last lumbar vertebra, coccyx, and the 

 two innominate bones. 



Attachment of Muscles. To eight pairs : in front, the Pyriformis and Coccyg- 

 eus, and a portion of the Iliacus to the base of the bone ; behind, the Gluteus 

 maximus, Latissimus dorsi, Multifidus spinae, and Erector spinae, and sometimes 

 the Extensor coccygis. 



The Coccyx. 



The Coccyx (xoxxvz, cuckoo), so called from having been compared to a cuc- 

 koo's beak (Fig. 21), is usually formed of four small segments of bone, the most 



rudimentary parts of the vertebral column. In each 

 of the first three segments may be traced a rudi- 

 mentary body, articular and transverse processes ; the 

 last piece (sometimes the third) is a mere nodule of 

 bone, without distinct processes. All the segments are 

 destitute of pedicles, laminae, and spinous processes, 

 and, consequently, of intervertebral foramina and spinal 

 canal. The first segment is the largest ; it resembles 

 the lowermost sacral vertebra, and often exists as a 

 separate piece ; the last three, diminishing in size from 

 above downward, are usually blended together so as to 

 form a single bone. The gradual diminution in the 

 size of the pieces gives this bone a triangular form, the 

 base of the triangle joining the end of the sacrum. It 

 presents for examination an anterior and posterior sur- 

 face, two borders, a base, and an apex. The anterior 

 surface is slightly concave, and marked with three 

 transverse grooves, indicating the points of junction of 

 the different pieces. It has attached to it the anterior 

 sacro-coccygeal ligament and Levator ani muscle, and 

 supports the lower end of the rectum. The posterior 

 surface is convex, marked by transverse grooves similar 

 to those on the anterior surface ; and presents on each 

 side a lineal row of tubercles, the rudimentary articular 

 processes of the coccygeal vertebrae. Of these, the supe- 

 rior pair are large, and are called the cornua of the 

 coccyx; they project upward, and articulate with the 

 cornua of the sacrum, the junction between these two 

 bones completing the fifth posterior sacral foramen for the transmission of the pos- 

 terior division of the fifth sacral nerve. The lateral borders are thin, and present a 

 series of small eminences, which represent the transverse processes of the 

 coccygeal vertebrae. Of these, the first on each side is the largest, flattened 

 from before backward, and often ascends to join the lower part of the thin 

 lateral edge of the sacrum, thus completing the fifth anterior sacral foramen 

 for the transmission of the anterior division of the fifth sacral nerve; the others 

 diminish in size from above downward, and are often wanting. The borders 



Anterior surface. 



Posterior surface. 

 FIG. 21. Coccyx. 



