OSSEOUS SYSTEM OF AVES. 



4S 



centrums, from three points,* and in the Bird, as in other Ovipara, 

 becoming confluent with that, 9, which stands in the same relation 

 to the third cranial segment ; the pit for the pituitary body marks 

 the boundary ; but the essential distinction of these centrums is 

 given by the neural and hfemal arches. The neural arch of the 

 parietal vertebra retains the same characters which it first mani- 

 fested in Fishes. Besides the neurapophyses, 6 (alisphenoids), 

 impressed by the mesencephalic ganglia and transmitting the chief 

 part of the trigeminal nerves, besides the vastly expanded and 

 again, as in Fishes, divided neural spine, 7 (parietal bones), the 

 parapophysis, 8 (mastoid) is originally distinct. It has a similar 

 proportional size to that in the Crocodile (vol. i. figs. 93, 95, g) ; 

 but OAving to the raised dome of the neural arch, is relatively 

 lower in position ; it extends apophysially downward and outAvarcl, 

 is ossified with the petrosal, and forms a large proportion of the 

 outer wall of the otocrane. Owing to the breadth and shortness 

 of the bird's brain, and the displacement of the optic lobes, the 

 neurapophyses of the mesencephalon, 6, converge toward each 

 other anteriorly, and support part of the neural spine of the 

 prosencephalon, ii, as well as their own, 7. On comparing a side 



Skull of young Ostrich. 



view of the cranium of the Bird, fig. 28, with that of the Tortoise 

 (vol. i. fig. 91), and Crocodile (fig. 95), the greater developement 

 of the epencephalon brings its neural arch, 2, 3, into view, which is 

 obscured by the growth of the apophysial part of 8, in the cold- 

 blooded Ovipara : but the connections of 8 with 2 behind, with 7 

 above, and with the ear-capsule within, are the same ; the latter, 

 however, being ossified in the Bird, but retaining its gristly state 

 in the Tortoise. 



' cccxxx. and xxiii. 



