AVES. 263 



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). Eye- 

 lashes are almost universally absent ; but, in addition to the 

 ordinary upper and lower eyelids, Birds possess a third mem- 

 branous eyelid the membrana nictitans which is placed on 

 the inner side of the eye. This nictitating membrane is some- 

 times transparent, sometimes pearly white, and it can be drawn 

 over the front of the eye like a curtain, moderating the too 

 great intensity of the light. As regards the organ of hearing, 

 the chief point to remark is that Birds have mostly no external 

 ear, by means of which the undulations of sound can be col- 

 lected and transmitted to the internal ear. In some birds, 

 however, as the Ostrich, the external opening af 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 apparently 

 seldom very acute 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 communi- 

 cate from side to side. In the curious Apteryx of New Zea- 

 land, the nostrils are placed at the extreme end of the elon- 

 gated beak. Sometimes the nostrils are defended by bristles, 

 and sometimes by a cartilaginous scale. 



Before passing on to a consideration of the divisions of 

 Birds, a few words may be said on the migrations of Birds. 

 In temperate and cold climates, few birds remain constantly in 

 the same region in which they were originally hatched. Those 

 which do so are called " permanent birds." Other birds, such 

 as the Woodpeckers, migrate from place to place without fol- 

 lowing any very definite course. These are called " wander- 

 ing birds," and their movements are chiefly conditioned by the 

 scarcity or abundance of food in any particular locality. Other 

 birds, however, at certain seasons of the year, undertake long 

 journeys, usually uniting for this purpose into larger or smaller 

 flocks. Such birds of which the Swallows are a familiar in- 

 stance are properly called " migratory birds," and their move- 

 ments are conditioned by the necessity of having a certain 

 average temperature, without which they cannot live. Thus 



