THE SPINE. 55 



sacrum. It presents for examination an anterior and posterior surface, 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,, the 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 linear row of tubercles, the rudimentary articular processes of the 

 coccygeal vertebra. Of these, the superior pair are very large ; and are called 

 the cornua of the coccyx; they project upwards, and articulate with the cornua of 

 the sacrum, the junction between these two bones completing the fifth sacral 

 foramen for the transmission of the posterior branch of the fifth sacral nerve. 

 The lateral borders are thin, and present a series of small eminences, which re- 

 present the transverse processes of the coccygeal vertebra. Of these, the first on 

 each side is of large size, flattened from before backwards ; and often ascends to 

 join the lower part of the thin lateral edge of the sacrum, thus completing the 

 fifth sacral foramen : the others diminish in size from above downwards, and are 

 often wanting. The borders of the coccyx are narrow, and give attachment on 

 each side to the sacro-sciatic ligaments and Coccygeus muscle. The base presents 

 an oval surface for articulation with the sacrum. The apex is rounded, and has 

 attached to it the tendon of the external Sphincter muscle. It is occasionally 

 bifid, and sometimes deflected to one or other side. 



Development. The coccyx is developed by four centres, one for each piece. 

 Occasionally, one of the first three pieces of this bone is developed by two centres, 

 placed side by side. The ossific nuclei make their appearance in the following 

 order: in the first segment, at birth; in the second piece, at from five to ten years; 

 in the third, from ten to fifteen years ; in the fourth, from fifteen to twenty years. 

 As age advances, these various segments become united in the following order : 

 the first two pieces join; then the third and fourth; and, lastly, the bone is com- 

 pleted by the union of the second and third. At a late period of life, especially 

 in females, the coccyx becomes joined to the end of the sacrum. 



Articulation. With the sacrum. 



Attachment of Muscles. On either side, the Coccygeus; behind, the Gluteus 

 maxim us ; at its apex, the Sphincter ani ; and in front, the Levator ani. 



OF THE SPINE IN GENERAL. 



The spinal column, formed by the junction of the vertebras, is situated in the 

 median line, at the posterior part of the trunk : its average length is about two 

 feet two or three inches; the lumbar region contributing seven parts of that 

 length, the dorsal eleven, and the cervical five. 



Viewed in front, it presents two pyramids joined together at their bases, the 

 upper one being formed by all the vertebra from the second cervical to the last 

 lumbar ; the lower one by the sacrum, and coccyx. Viewed somewhat more closely, 

 the uppermost pyramid is seen to be formed of three smaller pyramids. Of these, 

 the most superior one consists of the six lower cervical vertebra; its apex being 

 formed by the axis or second cervical ; its base, by the first dorsal. The second 

 pyramid, which is inverted, is formed by the four upper dorsal vertebra, the base 

 being at the first dorsal, the smaller end at the fourth. The third pyramid com- 

 mences at the fourth dorsal, and gradually increases in size to the fifth lumbar. 



Viewed laterally (fig. 21), the spinal column presents several curves, which 

 correspond to the different regions of the column, and are called cervical, dorsal, 

 lumbar, and pelvic. The cervical curve commences at the apex of the odontoid 

 process, and terminates at the middle of the second dorsal vertebra ; it is convex 

 in front, but the least marked of all the curves. The dorsal curvature, which is 

 concave forwards, commences at the middle of the second, and terminates at the 

 middle of the twelfth dorsal. Its most prominent point behind corresponds to 

 the body of the seventh or eighth vertebra. The lumbar curve commences at the 

 middle of the last dorsal, and terminates at the sacro->vertebral angle. It is convex 



