616 AVES [CH. 



plated by scales which are raised horny plates of skin, similar to the 

 scales of Reptiles. The fifth toe corresponding to the little toe of 

 the human foot is always absent. 



The most characteristic features about a bird, next to the limbs 



and feathers, are certainly the head and neck. The 

 Neck ad and skull * s high an( ^ arched behind in order to make 



room for the comparatively large brain ; in front it 

 slopes gradually downwards to the pointed beak, which is encased 

 in a hard horny sheath. The bones which underlie this beak are 

 (above) the premaxilla and (below) the dentary bone of the lower 

 jaw. No modern bird possesses teeth, and the maxilla, which 

 usually carries most of the teeth in animals which have them, is 

 very small and confined to the cheek behind the gape, whereas the 

 premaxilla is very large. Behind the maxilla two other slender 

 bones, the j ugal and quadratojugal, complete the lower temporal 

 arcade as in Chelonia and Crocodilia, but the jugal never sends up 

 a process behind the orbit and the postorbital is a mere process of 

 the frontal bone, so that the orbit and the temporal fossa open into 

 one another. The eyes are of great size : a bird has little or no sense 

 of gmell, and governs its life mainly by the sense of sight : in corre- 

 spondence with this the orbits or eye-sockets are so enlarged that the 

 skull between them is reduced to a thin vertical plate, the inter - 

 orbital septum, in which there is no brain cavity. This great 

 development of the eye-sockets and the obliteration of the brain 

 cavity between them is not, however, confined to Birds : it is found 

 as already mentioned in many Reptiles also, and is indeed one 

 of the several points in which a bird's skull may be said to be 

 Reptilian. It is however characteristic of Birds, as opposed to 

 Reptiles, that this interorbital septum is largely converted into 

 bone. In its hinder and upper portions it is composed of orbito- 

 sphenoid bones, like those found in Teleostean fishes, which support 

 the exit of the optic nerve, but in its lower part it is composed of a 

 vertically compressed presphenoid bone corresponding to that 

 which ossifies the front part of the floor of the cranium in 

 Mammalia. In front the interorbital septum is continuous with the 

 internasal or ethmoid septum : this latter is ossified by a meseth- 

 moid bone, which unites, but not quite immovably, with the 

 presphenoid. The hinder part of the floor of the cranium is ossified 

 by the basioccipital and basisphenoid bones, and the front of the 

 latter is drawn out into a long spur called the basisphenoidal rostrum. 

 Underlying the basisphenoid there is a membrane bone called the 



