AVES. 
160 
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 with 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 i 
{os carrd), that represents the drum of the ear : above, this same face is articulated or 
united to the skull by elastic laminse ; a mode of union which always leaves some i 
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 i 
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 craw, which is ; 
an expansion of the gullet ; the proventriculus, a membranous stomach, furnished in i 
the thickness of its coats with a multitude of glands [variously disposed and shaped in i 
different groups] , the secretion of which humects the aliment ; and lastly, the ' i 
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.] | i 
The dilatation of the craw is also sometimes [even | ^ 
generally] wanting. [This is is commonly situate i i i 
above the furcula, but in the genus Palamedea ^ 
beyond it: in the Grebes, there is a contraction and| i 
intervening space between the proventriculus and j)' 
gizzardf, which in the very peculiar genus Opistho- | i 
comus is developed into a considerable cavity (this bird j 
subsisting mainly on green foliage) : the Totipalmati “i 
have generally an accessory pouch to the stomach, f 
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 ; 
is furnished with numerous glands, which become 
developed in both sexes during the period that they alternately perform the daty(S 
• See note to p. 36 . — Ed. I rented from entering the gizzard till they have been sufficiently *:|| 
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- S 
gansers, and other piscivorous Birds with strong and muscular triculus, to pass its aperture. Q 
gizzards : hence the fishes that they swallow are mechanically pre- ' J 
