AVES BIRDS. 597 



are examined under very favourable circumstances, as for example in 

 the skull of a young Ostrich, it is by no means difficult to distin- 

 guish them, and by comparing them with those of other Vertebrata, 

 to observe the modifications they have undergone both in form and 

 position. In the annexed figure the principal pieces, both of the 

 cranium and face, have been indicated by the same figures as were 

 used to point out the correspondent bones in the skulls of the Cro- 

 codile (Jig. 246) and the Serpent (Jig. 249), so that it would be 

 needless again to enumerate them in this place. 



(663.) The muscular system of the feathered tribes, as far as acti- 

 vity and energy of motion is concerned, contrasts strikingly with that 

 of the Vertebrata we have as yet considered ; for, with the exception 

 of Insects, no animals in creation are comparable to Birds, either in 

 the vigour or velocity of their movements. 



This perfection of muscular power, which is obviously essential to 

 enable the bird to sustain itself in the air, and there perform the 

 varied evolutions connected with flight, is no doubt mainly con- 

 nected with the highly arterialized condition of the blood, and the 

 completeness of the respiratory apparatus. Neither is it uninterest- 

 ing to observe, that while in the Insect respiration was effected by 

 the admission of air to every part of the system by means of tra- 

 cheal tubes, in Birds likewise the air freely penetrates to the interior 

 of the body, and, as we shall afterwards find, is there most exten- 

 sively diffused. 



(664.) In the construction of the alimentary system there are 

 many interesting peculiarities to invite our notice. Their mouth 

 constitutes the apparatus whereby the prehension of food is ac- 

 complished ; it is in no instance provided with teeth, or adapted to 

 masticate food, but forms a beak encased in a dense, horny sheath, 

 which, from the varieties of form that it assumes in different genera, 

 becomes adapted to very various purposes. 



In the Rapacious tribes, for instance, the bill is a strong and for- 

 midable hook, calculated to tear in pieces the animals devoured. In 

 Granivorous Birds it is a simple forceps for picking up the seeds of 

 vegetables. In the Snipe and the Curlew it forms a probe, whereby 

 insects are extracted from the soft and marshy ground. In the 

 Parrot it is partially an assistant in climbing, as well as an organ 

 for seizing food ; and, not to mention innumerable other modifica- 

 tions, in the Flamingo and Duck tribes it constitutes a shovel, by 

 the aid of which alimentary matters are obtained. 



(665.) The sense of taste, even in these highly gifted animals, is 



