AXIAL SKELETON OP THE OSTEICH. 409 



great lateral projection of the parapophysis, on to which here, as in the more preaxial 

 vertebrae, the preaxial articular surface of the centrum extends itself. 



Superiorly the canal for the artery is bounded by a process of bone, which extends 

 outwards from beside and beneath the prezygapophysis (beiag in fact a dicqwjjlii/sis, 

 or tubercular process)', on the outer side of which is to be seen a prominence still 

 representing the metapojihi/sis (figs. 40 & 41, m). 



These parts exist in the same way in preceding vertebrae ; but they are most con- 

 veniently noticed here on account of the large size they attain on the dorsal and ventral 

 sides respectively of the great canal for the vertebral artery. 



The articular surfaces of the prezygapophyses look more inwardly, and those of the 

 postzygapophyses more outwardly, than do the corresponding surfaces of the seventeenth 

 vertebra; and the postzygapophyses also extend postaxially beyond the centrum to a 

 greater degree (fig. 40, j}^). 



The neural spine may be longer antero-posteriorly, and is wider transversely as well 

 as more extended dorsad than is the neural spine of the preceding vertebra. 



Its anterior excavation is larger and has a nearly vertical postaxial wall, with two 

 prominent ridges, which bound it laterally and proceed preaxially and nearly parallel 

 to near the preaxial margin of the neural arch. 



The posterior excavation of the neural spine is at about its maximum of development 

 in this vertebra, and is overshadowed and protected by the postaxial extent of its dorsal 

 lateral margins, which diverge from the neural spine to the postzygapophyses. 



The pre- and postaxial margins of the neural arch are strongly concave, though the 

 former is not so much so as is the corresponding part in the seventeenth vertebra. 



Sometimes two conspicuous foramina lead into the substance of the bone on either 

 side. One of these is placed a little on the ventral side of the interzygapophysial 

 ridge, rather nearer to the postaxial margin of the pedicle of the neural arch than to 

 its preaxial border ; the other may be sometimes found at the side of the middle of the 

 centrum, directly ventrad to the first foramen. 



The Nineteenth Vertebra. 



The nineteenth vertebra is very like the eighteenth in size and general shape, 

 although it has never any pleurapophysial osseous band, but always an articulated rib 

 (the second rib) instead. 



This absence of course produces a striking difference in the general appearance of the 

 vertebra when it is viewed preaxially. 



The centrum presents a jjreaxial surface which ends in an obtusely pointed process 

 on each side ; and both the dorsal and ventral margins of its articular surface are less 

 concave than are the con-esponding margins of the eighteenth vertebra (fig. 43, ac). 



' The diapophysis is that part to which the tubercle of the rib is attached, while its head joins the 

 parapojyhijsis. 



VOL. viii. — PART vii. March, 1874. 3 m 



