•>0 



THE SKELETOX 



one on each side for the hmiina, inferior articular process, and the lateral half of 

 the spinous process (fig. 28). The pedicles may fail to join the laminae; more 

 rarely the laniin;e fail to fuse (fig. 16). 



The sacral vertebrae. — In addition to the three primary vertediral centres, 

 the three upper sacral vertebrae have each an extra pair corresponding to the 

 costal processes of the seventh cervical vertebra; they appear at the seventh month. 

 These processes are very large in the first sacral, smaller in the second, and very 

 small in the third. Although the various primary centres of the sacral vertebra? 

 appear much later than in other regions of the column, yet they are all visiljle at 

 Itirth. The centrum of each sacral vertelira develops a superior and an inferior 



Fig. 27. — Ax Immature Cervical Vertebra. 



NEURO-CENTRAL SUTURE 



epiphysial meniscus, and eventually the five vertebrae fuse to form a single bone, 

 the sacrum. Even in advanced life the intervertebral discs between the sacral 

 vertebrae persist in the centre of the bone. The ear-shaped lateral articular facet 

 on the side of the sacrum arises from two additional centres on each side, about the 

 eighteenth year. The total number of ossific centres for the sacrum is thirty-five. 



The coccygeal vertebrae. — These are cartilaginous at birth. A few months 

 later the first segment ossifies. The remaining three ossify from above downwards 

 before the twentieth year. By the twentieth year the first three have usually 

 coalesced. The fourth fuses with them later, and the coccyx ankyloses with the 

 sacrum, as a rule, late in life. 



Fig. 28.— Ossification of the Fifth Lumbar Vertebra. 



NEURO-CENTRAL SUTURE 

 CENTRUM 



The Serial Morphology of the Vertebrce 



Although at first sight many of the vertebrae exhibit peculiarities, nevertheless a study of 

 tlie mode by which they develop, and their variations, indicates the serial homology of the con- 

 stituent parts of the vertebrae in each region of the column. 



The centrum, or body of the vertebra, is that part which immediately surround.s tlie noto- 

 chord. This part is present in all the vertebras of man, but the centrum of the atlas is dissociated 

 from its neural arch, and ankylu.sed to the body of tlu; axis. The reasons for regarding the odon- 

 toid process as the body of the atlas are these : In the embryo the notochord passes through it 

 on its way to the base of the cranium. Between the odontoid process and the body ol' the axis, 

 there is a swelling of the notochord in the early embryo as in other interveitebral i-egions. This 

 swelling is later indicated by a small intervertebral disc hidden in the bone, Imt i)ersistent even in 

 old age. The odontoid process arises from primary centres, and in chelonians it remains as a 

 separate ossicle throughout life ; in Ornithorhynchus it remains distinct for a long time, and it has 

 been found separate even in an adult man. Lastly, in man and many mammals, an epiphysial 

 I)late develops between it and the true body of the axis. 



