IGO AVES. 



of Birds is formed ; the enormous volume of air contained in the air-cavities contri- 

 butes to the strength of this voice, and the trachea, by its various forms and move- 

 ments, to its intonations. The upper larynx, which is extremely simple, has little to 

 do vv^ith it. 



The face, or upper mandible of Birds, formed principally by the intermaxillaries, is 

 prolonged backwards into two arcades, the internal of which is composed by the pala- 

 tine and pterygoid bones, the external by the maxillaries and jugals, and which are 

 both supported on a moveable tympanic bone, commonly termed the square bone 

 {os carre), that represents the drum of the ear: above, this same face is articulated or 

 united to the skull by elastic laminae ; a mode of union which always leaves some 

 mobility. 



The horny substance which invests the two mandibles supplies the place of teeth, 

 and is occasionally serrated, so as to represent them.* Its form, as also that of the 

 mandibles which support it, varies excessively, according to the sort of food 

 resorted to. 



The digestion of Birds is in proportion to the energy of their vitality, and the 

 amount of respiration. The stomach is composed of three parts : the craio, which is 

 an expansion of the gullet ; the proventriculus, a membranous stomach, furnished in 

 the thickness of its coats with a multitude of glands [variously disposed and shaped in 

 different groups], the secretion of which humects the aliment; and lastly, the 

 gizzard, armed with two powerful muscles united by two radiating tendons, and inter- 

 nally lined by a coating of cartilage. The food is more readily ground there, as Birds 

 are in the habit of swallowing small stones to augment its triturating power. 



In the greater number of species which subsist only on flesh or fish, the muscles 

 and the internal lining of the gizzard are reduced to extreme tenuity, so that it appears 

 to make but one sac with the proventriculus. [The same is noticeable in the Bustards, 



which subsist mainly upon herbage : a series of inter- 

 mediate gradations, however, occurring from these to 

 the most powerfully muscular gizzards.] 



The dilatation of the craw is also sometimes [even 

 generally] wanting. [This is is commonly situate 

 above the furcula, but in the genus Palumedea 

 beyond it : in the Grebes, there is a contraction and 

 intervening space between the proventriculus and 

 gizzardf, which in the very peculiar genus Opistho- 

 comus is developed into a considerable cavity (this bird 

 subsisting mainly on green foliage) : the Totipalmati 

 have generally an accessory pouch to the stomach, 

 analogous to that of the Loricated Reptiles. It may 

 also be mentioned here, that in the Parrots and 

 Pigeons, both exclusively vegetable feeders, the craw 

 F.g. 7o.-Pigeon-s Craw. jg fumishcd with numcrous glands, which become 



developed in both sexes during the period that they alternately perform the duty 



* See note to p. 36. — Ed. I vented from entering the gizzaril till they have been sufiiciently 



t The same contraction is noticeable, to a less extent, in the Mer- reduced, by the action of the gastric juice elaborated in the proven- 



gansers, and other piscivorous Birds with strong and muscular triculus, to pass its aperture. 



gizzards; hence the fishes that they swallow are mechanically pre- ' 



