AVES. 



115 



portion of the ribs is ossified, in order to give more power to the dilatation 

 of the chest. To each rib is annexed a small bone, which soon becomes 

 soldered to it, and is directed obliquely towards the next one, thereby 

 giving additional solidity to the thorax. 



The eye is so constructed, in birds, as to distinguish, with equal 

 facility, objects at a distance, or in its immediate vicinity ; a vascular 

 and plaited membrane, which stretches from the bottom of the globe to 

 the edge of the crystalline, probably assists in effecting this, by displacing 

 that lens. The anterior surface of the ball is also strengthened by a 

 circle of bony pieces ; and besides the two ordinary eye-lids, there is 

 always a third one placed at the internal angle, which, by a remarkable 

 muscular apparatus, can be drawn over the eye like a curtain. 



The breadth of the bony openings of the nostrils determines the 

 strength of the beak; and the cartilages, membranes, and other tegu- 

 ments which narrow down those apertures, influence the power of 

 smell, and the nature of the food. 



There is but little muscular substance in the tongue, which is sup- 

 ported by a bone articulated with the hyoid ; in most birds this organ is 

 not very delicate. 



The feathers, as well as the quills, which only differ in size, are com- 

 posed of a stem, hollow at base, and of laminae, which are themselves 

 furnished with smaller ones ; their tissue, lustre, strength, and general 

 form vary infinitely. The touch must be feeble in all such parts as are 

 covered with them ; as the beak is almost always horny, and has but 

 little sensibility, and the toes are invested with scales above, and a 

 callous skin underneath, so that sense can have but little activity in this 

 class of animals. 



Birds moult twice a year. In certain species, the winter plumage 

 differs in its colours from that of summer ; in the greater number, the 

 female differs from the male having less vividness of tint ; and when 

 this is the case, the young of both sexes resemble the former. When 

 the adult male and female are of the same colour, the young ones have 

 a livery peculiar to them. 



The brain of birds has the same general characters as that of other 

 Oviparous Vertebrata, but is distinguished by its very great propor- 

 tionate size, which often surpasses the proportion of this organ in the 

 Mammalia. 



The rings of the trachea are entire ; there is a glottis at its bifurca- 

 tion most commonly furnished with peculiar muscles, which is called 

 the inferior larynx ; this is the spot where the voice of birds is pro- 

 duced ; the immense volume of air contained in the air sacs contributes 

 to its strength, and the trachea, by its various forms and motions, to its 



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