CLASS AVES. 9 



into two arcades, the internal of which is composed 

 of the palatine bones, and the external of the maxil- 

 laries and jugals, and which are supported on a move- 

 able tympanic bone ; and on the upper part, this 

 same face is articulated, or united to the skull by- 

 elastic laminae : this mode of union leaves them, at all 

 times, some degree of mobility. 



The horn, which invests the two mandibles, serves 

 the place of teeth, and is sometimes prickled, so as 

 to represent them. Its form, as well as that of the 

 mandibles which sustain it, varies infinitely, according 

 to the nature of the food which each species takes. 



The digestion of birds is proportioned to the ac- 

 tivity of their life, and the force of their respiration. 

 The stomach is composed of three parts ; the crop, 

 which is a folding of the oesophagus ; the succentorial 

 ventricle, a membranous stomach, furnished in the 

 thickness of its surface with a multitude of glands, 

 the secretion of which imbibes the food ; and finally, 

 the gizzard, armed with two powerful muscles, which 

 two radiated tendons unite, and lined within with a 

 cartilaginous coating. The food is ground there the 

 more easily, by the bird swallowing little stones to 

 augment the force of the trituration. 



In the majority of species which live only on flesh, 

 or on fish, the muscles and the surface of the gizzard 

 are reduced to an extreme weakness ; and it has the 

 appearance of making only a single bag with the 

 succentorial ventricle. 



The dilatation of the crop is also sometimes alto- 

 gether wanting. 



