VERTEBRATA. 



25 



save that they are all early and firmly anchylosed, with the excep- 

 tion of the tympanic bone. 



The bones of the face correspond in number and position to those 

 of the mammalia, especially the order rodentia. They are movably 

 connected with the bones of the cranium, and remain separate to a 

 much later period. The upper mandible is chiefly characterised by 

 the presence of the intermaxillary bone. This consists of three 

 processes, a central, and two lateral ; the former passes up between 

 the superior maxillary bones, and becomes joined to the nasal and 

 ethmoid ; this union is ligamentous in the parrot, and those birds 

 that apply the upper mandible to the purposes of climbing. The 

 two lateral processes of the inter-maxillary bone, extend upwards 

 and backwards external to the superior maxillary bones, to which 

 they are firmly united. 



There are few more ingenious or beautiful pieces of mechanism 

 than that by which the mouth of a bird is opened. The tympanic 

 bone as before observed, never anchyloses with the other elements 

 of the temporal; on the contrary, it articulates with the zygomatic 

 portion of the latter bone, by two transverse condyles above ; below 

 it articulates by a broad surface with the upper and back part of the 

 lower jaw, while in front and near its lower extremity, it has 

 applied to it the long, slender malar bone which reaches to the 

 superior maxilla. When the tympanic bone is drawn forwards, 

 either by the action of the pterygoid muscles attached to it, or by 

 the depression of the lower jaw, the malar bone is advanced against 

 the superior mandible, which is elevated at the same moment that 

 the lower one is depressed. The moment the pressure ceases to 

 exist below, the elasticity of the union of the intermaxillary bone 

 with the cranium restores the upper mandible to its situation. 



The inferior maxilla originally consists of six pieces on each 

 side, named anterior dental, two condyloid, angular, supra-angular, 

 and opercular. As this bone, together with the upper jaw, forms 

 the chief organ of prehension in birds, it is but natural to expect 

 that it shall present numerous modifications, indicative of the food 

 and habits of each species. 



There is more uniformity observed in the skulls of birds than in 

 any other class of the vertebras department. It generally presents 

 the form of a five sided pyramid, the base represented by the occiput, 

 and the apex by the bill. In the Raptores it is short, broad, and 

 deep ; nearly spherical in the Warblers ; flattened, and of great 

 breadth, in the Scansores ; narrow, and slightly raised in theRaso- 

 rial birds ; and remarkable for its great length in the Waders. 



Thorax. — The extent and energy of the respiratory function of 

 birds, are clearly indicated by the peculiar and perfect condition of 

 this part of their osseous system. The ribs, as in the mammalia, 

 are arranged into true and false, with this difference, that the false 

 ribs are placed both above and below the true ones. There is 

 some, though not very great variety in the number of ribs presented 

 by this class : for instance, in the Insessores we meet with seven or 



