THE BONES OF THE TRUNK 



35 



^ Appeal in the yd month 

 ' of intra-uterine life 



^i Appeal about the 

 6th Month 



^ Appear after the 

 5th Month 



Netual Arch 



coccygeal vertebra, \Wth the intervention of an intervertebral 

 disc until advanced life, when ankylosis takes place. 



The sacral canal is situated behind the bodies of the first four 

 sacral vertebrse, as a rule. It is triangular in the upper part, but 

 somewhat crescentic below^. It is closed in front by the ankylosed 

 bodies, and behind by the ankylosed laminae. Along each side 

 it presents four intervertebral foramina for the passage of nerves. 

 These are bounded externally by the lateral mass, but each opens 

 on the ventral and dorsal siurfaces by the anterior and posterior 

 sacral foramina, which represent the limbs of a capital V, the apex 

 of which corresponds \\ath an 



intervertebral foramen. The '"PPjSL'Kt'^e^l^ yS "^ 

 superior aperture or inlet is i 



large, triangular, and wide 

 transversely. It represents 

 the spinal or neural foramen 

 of the first sacral vertebra. 

 The inferior aperture or out- 

 let is compara- ( 



tively small and i^J^i,^^]J 

 somewhat trian- ^d join about] 



, , . the 25th Yeai. I 



gular, and is usu- V 



ally situated on the back of 

 the body of the fifth sacral 

 vertebra. It is bounded 

 above by the tubercle which 

 represents the spinous pro- 

 cess of the fourth sacral 

 vertebra, and on either side 

 by (i) the imperfectly de- 

 veloped lamina of the fifth 

 sacral vertebra, and (2) the 

 sacral comu. The outlet 

 transmits the fifth pair of 

 sacral nerves and the two 

 coccygeal nerves. The con- 

 tents of the canal are the sacral and coccygeal nerves, and the 

 filum terminale of the spinal cord. 



The sacrum derives its blood-supply from the lateral sacral and 

 middle sacral arteries. 



Articulations.— SM/>mor/y with the fifth lumbar vertebra, in- 

 ferior ly with the cocc5rx, and at either side with the os innominatum. 



Varieties. — (i) The number of sacral segments may be six, or more rarely 

 four. Increase in the number is usually due to the incorporation of the first 

 coccygeal vertebra, or sometimes the fifth lumbar. The decrease may be due 

 to the fifth sacral vertebra forming a part of the coccyx, or to the first sacral 

 forming a sixth lumbar. (2) The bodies of the first and second sacral vertebrae 

 may remain permanently separate, though ankylosis has taken place in all 

 their other parts. (3) The first sacral vertebra may be normal on one side. 



Transverse Process 



Costal Process 



Body 



Fig. 26. — Ossification of the Sacrum. 



A, Anterior View; B, First Sacfal Vertebra 



in early life (Superior View). 



