124 AVES. 



general form vary infinitely. The touch must be feeble in all such 

 parts as are covered with them, and as the beak is almost always 

 corneous, and has but little sensibility, and the toes are invested with 

 scales above, and a callous skin underneath, 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 in an inferior vividness of tints, 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 even that of this organ in the 

 Mammalia. 



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

 cation most commonly furnished with pecuhar muscles, which is 

 called the inferior larynx; this is the spot where the voice of birds is 

 produced; the immense volume of air contained in the air sacs con- 

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

 motions, to its modifications. The superior larynx, which is ex- 

 tremely simple, has but little to do with it. 



The horny substance which invests the two mandibles, performs 

 the office of teeth, and is sometimes so jagged as to resemble them; 

 its form, as well as that of the mandibles which support it, varies 

 extremely, and according to the kind of food used by each species. 



The digestion of Birds is in proportion to the activity of their life, 

 and the force of their respiration. The stomach is composed of 

 three parts: the crop, which is an enlargement of the oesophagus; a 

 membranous stomach, in the thickness of whose parietes are a mul- 

 titude of glands whose juices humect the aliment; and finally, the 

 gizzard, armed with two powerful muscles, united by two radiated 

 tendons, and lined internally with a kind of cartilaginous velvet. 

 The food is the more easily ground there, as Birds constantly swal- 

 low small stones, in order to increase its triturative power. 



In the greater part of the species which feed exclusively on flesh 

 or fish, the muscles and villous coat of the gizzard are greatly atten- 

 uated; and it seems to make but a single sac with the membranous 

 stomach. 



The dilatation of the crop is also sometimes wanting. 



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