56 SACRUM. 



The one or two centres of the anterior arch appear during the first year, 

 and become consolidated with the lateral pieces during the fifth or sixth 

 year. The axis develops its lateral pieces at the same time with the rest 

 of the vertebrae ; they join posteriorly soon after birth, and with the body 

 during the fourth or fifth year. The centres for the body and odontoid 

 process appear during the sixth month, and are consolidated during the 

 third year. The body of the axis is more largely developed at birth than 

 that of the other vertebrae. The costal segments of the vertebra prominens 

 appear during the second month, and become united to the body at the 

 fifth or sixth year. These processes sometimes remain permanently sepa- 

 rate, and constitute a cervical rib. The transverse process of the Jirst 

 lumbar vertebra has sometimes a distinct centre, which may remain per- 

 manently separate, in that case forming a lumbar rib. 



The ossification of the arches of the vertebrae commences from above, 

 and proceeds gradually downwards ; hence arrest of development give? 

 rise to spina bifida, generally in the loins. Ossification of the bodies, on 

 the contrary, commences from the centre, and proceeds from that point 

 towards the extremities of the column; hence imperfection of the bodies 

 occurs either in the upper or lower vertebras. 



Attachment of Muscles. To the atlas are attached ten pairs of muscles; 

 the longus colli, rectus anticus minor, rectus lateralis, rectus posticus 

 minor, obliquus superior and inferior, splenius colli, levator anguli scapulae, 

 first interspinales, and first intertransversales. 



To the axis are attached twelve pairs, viz. : the longus colli, intertrans- 

 versales, obliquus inferior, rectus posticus major, supraspinalis, interspi- 

 nales, semi-spinalis colli, multifidus spinse, levator anguli scapula?, splenius 

 colli, transversalis colli, and scalenus posticus. 



To the remaining vertebra collectively, thirty-three pairs ; viz. poste- 

 riorly, the trapezius, latissimus dorsi, levator anguli scapulae, rhomboideus 

 minor and major, serratus posticus superior and inferior, splenius, sacro- 

 lumbalis, longissimus dorsi, spinalis dorsi, cervicalis ascendens, trans- 

 versalis colli, trachelo-mastoideus, complexus, semi-spinalis dorsi and 

 colli, multifidus spinae, supraspinalis, interspinales, intertransversales, 

 levatores costarum : anteriorly, the rectus anticus major, longus colli, 

 scalenus anticus and posticus, psoas magnus, psoas parvus, quadratus 

 lumborum, diaphragm, obliquus internus and transversalis. 



THE SACRUM is a triangular bone, situated at the lower extremity of the 

 vertebral column, and formed by the consolidation of five false vertebrae. 

 It is divisible into an anterior and posterior surface, two lateral and a 

 superior border, and an inferior extremity. 



The anterior surface is concave, and marked by four transverse lines, 

 which indicate its original constitution of five separate pieces. At the 

 extremities of these lines, on each side, are the four anterior sacral fora- 

 mina, which diminish in size from above downwards, and transmit the 

 anterior sacral nerves. The projection of the superior piece is the sacro- 

 vertebral angle or promontory. 



The posterior surface is narrower than the anterior and convex. Upon 

 the middle line is a rough crest formed by the rudiments of four spinous 

 processes, the fifth remaining undeveloped and exposing the lower termi- 

 nation of the sacral canal. Immediately external to and parallel with the 

 median crest, is a range of five small tubercles which represent the poste- 



