STRUCTURE. 33 



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 backwards. The number of articulations 

 increases in each toe, commencing with the thumb, which has 

 two, and finishing with the extreme toe, which has five. 



Walking and Running Feet. 



Ring-Dove. 



Little Bustard. 



Birds are in general covered with feathers, a sort of tegument 

 best adapted to protect them from the effects of the rapid varia- 

 tions 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, augment 

 their specific lightness. The sternal portion of the ribs, like the 

 vertebral, is ossified, to give more force to the dilation of the chest. 



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

 guish objects both far and near equally well ; and a vascular 

 and folding membrane placed at the bottom of the globes, at 

 the edge of the crystalline, assists probably in displacing that 

 lens. The anterior surface of that globe is moreover strengthened 

 by a circle of bony pieces ; and besides the two ordinary eye- 

 lids, there is always a third placed at the internal angles, 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- 

 curved, but the crystalline is flat, and the vitreous humour small. 



The ear of birds has but one little bone between the tympanum 

 and the oval aperture. Their cochlea is a cone scarcely bent ; 

 but their semi-circular canals are large, and lodged in a part of 

 the skull, where they are surrounded on all sides with air cavi- 

 ties which communicate with the area. Night-birds alone have 

 a large external ear, which, nevertheless, is not so prominent as 



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