ENDOSKELETON. ANIMALS. 38 



A. 136. One half of the frontal bone of a Human Foetus, finely 

 injected, and preserved in alcohol, showing its vascularity. 

 0. C. 162 B. Mm. Sir Aatley Cooper, Bart. 



A. 137. The vertebral column of a Human Foetus of the sixth 

 month, injected, showing the commencement of ossification 

 in the bodies and arches of the vertebrae. 0. C. 132 A. 



The parts of the arches which are ossified are the two vertebral 

 lamina? or " neurapophyses." Ossification usually commences in 

 the neurapophyses before it does in the centrum, viz. about the 

 sixth or eighth week, by a single centre in each. The neura- 

 pophyses unite together first at the base of the spinous process, 

 completing the arch in the first year after birth : in the course of 

 the third year, the bases of the ueurapophyses, which have ex- 

 tended into the centrum so as to form its neural angles, unite with 

 the independently ossified part of the centrum. Prior to puberty, 

 the following epiphyses exist, one flat plate forming each articular 

 end of the centrum a terminal epiphysis for the spine, another for 

 the diapophysis, and a third for the metapophysis, where that 

 process exists. 



With regard to the atlas, the neurapophyses begin to be ossified 

 as early as in the other vertebrae, but the so-called body does not 

 begin to ossify until after birth. The neurapophyses unite 

 together between the second and third years, and with the hypa- 

 pophysis between the fifth and sixth years ; the true body of the 

 atlas, called " odontoid process," begins to be ossified about the 

 same time as the body of the dentata, viz. about the sixth month 

 of foetal life, but by two centres placed side by side, which unite 

 together before birth. The proper centrum of the axis coalesces 

 with that of the atlas about the fourth year. The pleurapophysis 

 of the seventh cervical is formed from a separate osseous nucleus, 

 at about the sixth month, which usually unites at one end with 

 the centrum, at the other end with the diapophysis, about the 

 fifth or sixth year. 



The transverse processes of the first lumbar vertebra are some- 

 times developed from a separate centre, when they are " pleura- 

 pophyses " as long as they remain distinct : those of the other 

 vertebra? are exogenously developed, and are " diapophyses." As 

 the pleurapophysial element homologically enters into the formation 

 of the hole in the transverse process of the cervical vertebra?, but 

 is autogenously developed only in the seventh, the pleurapophysis 



D 



