THE SACRAL AND COCCYGEAL VERTEBRA 61 



lateral angle; below this angle is a notch, which is converted into a foramen by 

 articulation with the transverse process of the upper piece of the coccyx, and 

 transmits the anterior division of the fifth sacral nerve. This lower, sharp border 

 gives attachment to the greater and lesser sacrosciatic ligaments, and to some 

 fibres of the Gluteus maximus posteriorly, and to the Coccygeus in front. 



Base (basis oss. sacri). The base of the sacrum, which is broad and expanded, 

 is directed upward and forward. In the middle is seen a large oval articular 

 surface, which is connected with the under surface of the body of the last lumbar 

 vertebra by a fibrocartilaginous disk. It is bounded behind by the large, tri- 

 angular orifice of the sacral canal. The orifice is formed behind by the laminae 

 and spinous process of the first sacral vertebra: the superior articular processes 

 project from it on each side; they are oval, concave, directed backward and inward, 

 like the superior articular processes of a lumbar vertebra; and in front of each 

 articular process is an intervertebral notch, which forms the lower part of the 

 foramen between the last lumbar and first sacral vertebra. Lastly, on each side 

 of the large oval articular plate is a broad and flat triangular surface of bone, 

 which extends outward, supports the Psoas magnus muscle and lumbosacral 

 cord, and is continuous on each side with the iliac fossa. This is called the ala of 

 the sacrum (ala sacralis), and gives attachment to a few of the fibres of the Iliacus 

 muscle. The posterior part of the ala represents the transverse process of the 

 first sacral segment. 



Apex (apex oss. sacri). The apex, directed downward and slightly forward, 

 presents a small, oval, concave surface for articulation with the coccyx. 



The Sacral Canal (canalis sacralis) runs throughout the greater part of 

 the bone; it is large and triangular in form above, small and flattened, from 

 before backward, below. In this situation its posterior wall is incomplete, from 

 the non-development of the laminae and spinous processes (hiatus sacralis). 

 It lodges the sacral nerves, and is perforated by the anterior and posterior sacral 

 foramina, through which these pass out. It constitutes the sacral continuation 

 of the vertebral canal (Fig. 24). 



Differences in the Sacrum of the Male and Female. The sacrum in the female 

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

 upper half of the bone being nearly straight, the lower half presenting the greatest amount 

 of curvature. The bone is also directed more obliquely backward, which increases the size 

 of the pelvic cavity; but the sacrovertebral angle projects less. 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. This bone, in some cases, consists of six pieces; occasionally the number is 

 reduced to four. Sometimes the bodies of the first and second segments are not joined or the 

 laminae and spinous processes have not coalesced. Occasionally the upper pair of transverse 

 tubercles are not joined to the rest of the bone on one or both sides; and, lastly, the sacral canal 

 may be open for nearly the lower half of the bone, in consequence of the imperfect development 

 of the laminae and spinous processes. The sacrum, also, varies considerably with respect to 

 its degree of curvature. 



Articulations. With/owr bones: the last lumbar vertebra, coccyx, and the two innominate 

 bones. 



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



Coccyx (os coccygis}. The coccyx (Fig. 26) is usually formed of four small 

 segments of bone, the most rudimentary parts of the vertebral column (vertebrae 

 coccygeae). In each of the first three segments may be traced a rudimentary 

 body, articular and transverse processes; the last piece (sometimes the third) 

 is a mere nodule of bone, without distinct processes. All the segments are desti- 

 tute of pedicles, laminse, and spinous processes, and consequently of interverte- 



