OSSEOUS SYSTEM OF AVES. 4t 



the strong quadrate figure which characterizes the same process 

 in the dorsal vertebrae. The neural canal, ib. n, varies in form 

 and diameter in the same vertebras. If, e.g., the sixth cervical 

 of a Stork be sawed lengthwise vertically, the diameter is great- 

 est in the middle, least at the ends ; but if it be sawed length- 

 wise horizontally, the transverse diameter is the reverse, being 

 narroAvest at the centre and widest at the ends. In the Ostrich, 

 the Swan, and many other birds, the canal widens in every direc- 

 tion at its extremities ; and on the dorsal or posterior aspect of 

 the spine, the canal remains open for some extent in the intervals 

 of the vertebrae, the myelon being there protected only by mem- 

 brane and the elastic ligaments which connect the neural spines 

 together. This modification subserves the prevention of compres- 

 sion of the myelon during the frequent, varied, and extensive 

 inflections of the neck in birds. 



The atlas and axis speedily effect a partial coalescence; the 

 body of the first, e.g., as an ^odontoid process' to that of the 

 second, and usually presenting a pair of small facets to articulate 

 with its own neurapophyses, which are mainly supported by the 

 ' hypapophysis ' simulating the entire centrum of the atlas. The 

 back part of the hypapophysis offers a flat surface to the centrum 

 of the axis, beneath which it is slightly produced, being here 

 wedged into a notch between the true bodies of the atlas and 

 axis. The fore part of the hypapophysis combines ^viih the neura- 

 pophyses to form the major part of the cup for the condyle of the 

 occiput, which is completed by the * odontoid.' The atlantal 

 neurapophyses usually diverge as they rise, and are joined toge- 

 ther above by a broad plate slightly arching across from one to 

 the other ; in some {Ajjtenodytes, Dhwrnis) they do not meet : 

 rarely is a neural spine developed. The centrum of the axis is 

 sometimes carinate below mth a slight posterior production (Alca 

 impennis), sometimes produced into a hypapophysis, as long as 

 the neural spine above {Aptenodi/tes, most Baptores.) Postzyga- 

 pophyses of the atlas articulate with the prezygapophyses of the 

 axis. In a Hornbill (Buceros) I have seen complete coalescence 

 of the atlas and axis. 



§ 128. The Skull. — The neural and haemal plates of the embryo- 

 nic trace become modified in the head of the chick by the early ex- 

 pansion of the cerebral part of the neural axis, and by the almost 

 contemporary appearance of the capsules of the organ of hearing, 

 which are speedily followed by the rudiments of the eyeballs. 

 The neural plates are dilated by the primitive vesicles of the 

 ep- mes- and pros-encephalon, the latter speedily showing its 



