I THE SACRAL AND COCCYGEAL VERTEBRA 111 



Structure. The sacrum consists of cancellous tissue enveloped by a thin layer of compact bone. 



Articulations. The sacrum articulates with four bones; the last lumbar vertebra above, the 

 coccyx below, and the hip bone on either side. 



Differences in the Sacrum of the Male and Female. In the female the sacrum is shorter and 

 wider than in the male; the lower half forms a greater angle with the upper; the upper half is 

 nearly straight, the lower half presenting the greatest amount of curvature. The bone is also 

 directed more obliquely backward; this increases the size of the pelvic cavity and renders the 

 sacrovertebral angle more prominent. In the male the curvature is more evenly distributed 

 over the whole length of the bone, and is altogether greater than in the female. 



Variations. The sacrum, in some cases, consists of six pieces; occasionally the number is 

 reduced to four. The bodies of the first and second vertebra? may fail to unite. Sometimes 

 the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, 

 or the sacral canal may be open throughout a considerable part of its length, in consequence of 

 the imperfect development of the lamina? and spinous processes. The sacrum, also, varies con- 

 siderably with respect to its degree of curvature. 



The Coccyx (os coccygis). The coccyx (Fig. 100) is usually formed of four 

 rudimentary vertebrae; the number may however be increased to five or diminished 

 to three. In each of the first three segments may be traced a rudimentary body 

 and articular and transverse processes; the last piece (sometimes the third) is a 

 mere nodule of bone. All the segments are destitute of pedicles, laminae, and 

 spinous processes. The first is the largest; it resembles the lowest sacral vertebra, 

 and often exists as a separate piece; the last three diminish in size from above 

 downward, and are usually fused with one another. 



Surfaces. The anterior surface is slightly concave, and marked with three trans- 

 verse grooves which indicate the junctions of the different segments. It gives 

 attachment to the anterior sacrococcygeal ligament and the Levatores ani, and 

 supports part of the rectum. The posterior surface is convex, marked by transverse 

 grooves similar to those on the anterior surface, and presents on either side a linear 

 row of tubercles, the rudimentary articular processes of the coccygeal vertebrae. 

 Of these, the superior pair are large, and are called the coccygeal cornua; they 

 project upward, and articulate with the cornua of the sacrum, and on either side 

 complete the foramen for the transmission of the posterior division of the fifth 

 sacral nerve. 



Borders. The lateral borders are thin, and exhibit a series of small eminences, 

 which represent the transverse processes of the coccygeal vertebrae. Of these, 

 the first is the largest; it is flattened from before backward, and often ascends 

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

 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 of the coccyx are narrow, and give attachment on either side to the sacro- 

 tuberous and sacrospinous ligaments, to the Coccygeus in front of the ligaments, 

 and to the Glutseus maximus behind them. 



Base. The base presents an oval surface for articulation with the sacrum. 



Apex. The apex is rounded, and has attached to it the tendon of the Sphincter 

 ani externus. It may be bifid, and is sometimes deflected to one or other side. 





Ossification of the Vertebral Column. Each cartilaginous vertebra isossified from three primary 

 centers (Fig. 101), two for the vertebral arch and one for the body. 1 Ossification of the vertebral 

 arches begins in the upper cervical vertebrae about the seventh or eighth week of fetal life, and grad- 

 ually extends down the column. The ossific granules first appear in the situations where the trans- 

 verse processes afterward project, and spread backward to the spinous process forward into the 

 pedicles, and lateralward into the transverse and articular processes. Ossificationof thebodies begins 

 about the eighth week in the lower thoracic region, and subsequently extends upward and down- 

 ward along the column. The center for the body does not give rise to the whole of the body of 

 the adult vertebra, the postero-lateral portions of which are ossified by extensions from the verte- 

 bral arch centers. The body of the vertebra during the first few years of life shows, therefore, 



1 A vertebra is occasionally found in which the body consists of two lateral portions a condition which proves that 

 ne body is sometimes ossified from two primary centers, one on either side of the middle line. 



