288 VERTEBEATE ANIMALS. 



are liberated from the egg in a, perfectly helpless and naked con- 

 dition, and require to be fed by their parents for a longer or shorter 

 time, before they are able to take care of themselves. Most of 

 these Birds {Aves altricex), such as our common Song-birds, reside in 

 trees, and build more or less elaborate nests. 



As regards their nvrixms system, the brain of Birds is relatively 

 larger than the brain of Eeptiles, but it is destitute of those folds or 

 convolutions which form so marked a feature in the brain of most 

 Mammals. The organs of sense, with the exception of touch and 

 taste, are well developed in Birds, vision especially being generally 

 extremely acute. The eyes are always well developed, and in no 

 Bird are they ever wanting or rudimentary. The chief peculiarity 

 of the eye of Birds is, that its anterior jwirtion {cornea) forms the 

 segment of a much smaller circle than does the eyeball proper ; so 

 that the whole eye assumes a conical shape. Another peculiarity is, 

 that the form of the eye is maintained by means of a circle of from 

 thirteen to twenty bony plates, which are placed in the front portion 

 of the fibrous coat of the eye {sclerotic). Eyelashes are almost uni- 

 versally absent ; but in addition to the ordinary upper and lower 

 eyelids. Birds possess a third membranous eyelid — the memhrana 

 nictitans — which is placed on the inner side of the eye. This nictitat- 

 ing membrane is sometimes transparent, sometimes pearly white, 

 and it can be drawn over the front of the eye like a curtain, mod- 

 erating the too great intensity of the light. As regards the organ 

 of hearing, the chief point to remark is, that Birds possess no external 

 ear, by means of which the undulations of sound can be collected 

 and transmitted to the internal ear. In some Birds, however, as the 

 Ostrich, the external opening of the organ of hearing is provided 

 with a circle of feathers, which can be raised and depressed at will. 

 In the nocturnal Birds, also (such as Owls), the external opening of 

 the ear is protected by a musculo-membranous valve, foreshadowing 

 the gristly external ear of Mammals. The sense of smell is appa- 

 rently seldom very acvite in Birds ; and even the Birds of Prey appear 

 to seek their food mainly by the sight. The external nostrils are 

 usually placed on the sides of the upper mandible, near its base, and 

 form simple perforations which sometimes communicate from side 

 to side. In the curious Apten/x of New Zealand, the nostrils are 

 placed at the extreme end of the elongated beak. Sometimes the 

 nostrils are defended by bristles, and sometimes by a cartilaginous 

 scale. 



Before passing on to a (/i )nsideration of the divisions of Birds, a few 

 words may be said on the migrations of Birds. In temperate and 

 cold climates, onlj' certain birds remain constantly in the same region 

 in which they were originally hatched. Those which do so are called 



