6 CLASS AVES. 



The tarsus and metatarsus are represented by a single 

 bone, terminated at the bottom by three pulleys. 



There are generally three toes in front, and a thumb 

 behind ; the latter is sometimes wanting ; and in the 

 martins is directed forward. In the climbers, on 

 the contrary, the external toe and the thumb are 

 directed backward. The number of articulations 

 increases in each toe, commencing with the thumb, 

 which has two, and finishing with the external toe, 

 which has five. 



Birds are in general covered with feathers, a sort 

 of tegument the best adapted to protect them from 

 the effects of the rapid variations of temperature to 

 which their movements expose them. The air cavities 

 which occupy the interior of their body, and which 

 even occupy the place of marrow in the bones, aug- 

 ment their specific lightness. The sternal portion of 

 the ribs, like the vertebral, is ossified, to give more 

 force to the dilatation of the chest. 



The eyes of birds are so disposed, as to enable them 

 to distinguish objects both far and near equally well ; 

 and a vascular and folding membrane placed at the 

 bottom of the globe, at the edge of the cristalline, 

 assists probably in displacing that lens. The ante- 

 rior surface of the globe is moreover strengthened by 

 a circle of bony pieces ; and besides the two ordi- 

 nary eyelids, there is always a third placed at the 

 internal angle, and which, by means of a remarkable 

 muscular apparatus, is able to cover the front of the 

 eye like a curtain. The cornea is very convex, but 

 the cristalline is flat, and the vitreous humour small. 



