THE SACRAL AND COCCYGEAL VERTEBRA 



59 



middle are seen four transverse ridges (lineae transversae), indicating the original 

 division of the bone into five separate pieces. The portions of bone intervening 

 between the ridges correspond to the bodies of the vertebrae. The body of the first 

 segment is of large size, and in form resembles that of a lumbar vertebra; the 

 succeeding ones diminish in size from above downward, are flattened from before 

 backward, and curved so as to accommodate themselves to the form of the sacrum, 

 being concave in front, convex behind. At each end of the ridges above men- 

 tioned are seen the anterior sacral foramina (foramina sacralia anteriora), analogous 

 to the intervertebral foramina, four in number on each side, somewhat circular 

 in form, diminishing in size from above downward, and directed outward and 

 forward; they transmit the anterior branches of the sacral nerves and the lateral 

 sacral arteries. External to these fora- 

 mina is the lateral mass (pars lateralis), 

 consisting at an early period of life of 

 separate segments; these become blended, 

 in the adult, with the bodies, with each 

 other, and with the posterior transverse 

 processes. Each lateral mass is traversed 

 by four broad, shallow grooves, which 

 lodge the anterior divisions of the sacral 

 nerves as they pass outward, the grooves 

 being separated by prominent ridges of 

 bone, which give attachment to the slips 

 of the Pyriformis muscle. 



If a vertical section is made through 

 the centre of the sacrum (Fig. 24), the 

 bodies are seen to be united at their cir- 

 cumference by bone, a wide interval being 

 left centrally, which, in the recent state, 

 is filled by intervertebral substance. 

 In some bones this union is more com- 

 plete between the lower segments than 

 between the upper ones. 



Posterior or Dorsal Surface (fades dor- 

 salis). The posterior surface (Fig. 25) 

 is convex and much narrower than the 

 anterior. In the middle line are three 

 or four tubercles, which rep resent the rudi- 

 mentary spinous processes of the sacral 

 vertebrae. Of these tubercles, the first 

 is usually prominent, and perfectly dis- 

 tinct from the rest; the second and 

 third are either separate or united into a 

 tubercular ridge (crista sacralis media), which diminishes in size from above 

 downward; the fourth usually, and the fifth always, remaining undeveloped. 

 The gap which results from failure of the laminae to meet in the mid-line is called 

 the hiatus sacralis. External to the spinous processes on each side are the lamime, 

 broad and well marked in the first three pieces; sometimes the fourth, and gener- 

 ally the fifth, are only partially developed and fail to meet in the median line. 

 These partially developed laminae are prolonged downward as rounded processes, 

 the sacral cornua (cornua sacralia), and are connected to the cornua of the coccyx. 

 Between them the bony wall of the lower end of the sacral canal is imperfect. 

 External to the laminae is a linear series of indistinct tubercles representing the 

 articular processes (cristae sacrales articulares); the upper pair are large, well 



FIG. 24. Vertical section of the sacrum. 



