CHARACTERS OF AVES. 439 



from the egg. When development has reached a stage at which 

 external life is possible, it is of course necessary for the chick 

 to be liberated from the egg, the shell of which is often ex- 

 tremely hard and resistant. To this end, in very many in- 

 stances, the young bird is provided with a little calcareous 

 knob on the point of the upper mandible, and by means of this 

 it chips out an aperture through the shell. Having effected its 

 purpose, this temporary appendage then disappears, without 

 leaving a trace behind. 



The state of the young upon exclusion from the egg is very 

 different in different cases, and in accordance with this, Birds 

 have been divided into the two sections of the Autophagi or 

 Aves prczcoces, and the Heterophagi or Aves altrices. In the 

 Autophagi the young bird is able to run about and help itself 

 from the moment of liberation from the egg. In the Hetero- 

 phagi the young are born in a blind and naked state, unable to 

 feed themselves, or even to maintain unassisted the necessary 

 vital heat. In these birds, therefore, the young require to be 

 brooded over and fed by the parents for a longer or shorter 

 period after exclusion from the egg. 



As regards their nervous system, the brain of birds is rela- 

 tively larger, especially as regards the size of the cerebrum 

 proper, than the brain of Reptiles. The cerebellum, though 

 always present, consists simply of the central lobe (the " vermi- 

 form process "), and is not provided with the lateral lobes which 

 occur in the Mammals, or they are only present in a rudimen- 

 tary form. The corpus callosum is absent, and the surface of 

 the cerebral hemispheres is devoid of convolutions. 



As regards the organs of the senses^ the eyes are always well 

 developed, and in no bird are they ever rudimentary or absent. 

 The chief peculiarity of the eye is that the cornea forms a 

 segment of a much smaller sphere than does the eyeball pro- 

 per, so that the anterior part of the eye is obtusely conical, 

 whilst the posterior portion is spheroidal. Another peculiarity 

 is that the form of the eye is maintained by a ring of from 

 thirteen to twenty bony plates, which are placed in the anterior 

 portion of the sclerotic coat. Eyelashes are almost universally 

 absent ; but in addition to the ordinary upper and lower eye- 

 lids, Birds possess a third membranous eyelid the " membrana 

 nictitans " which is sometimes pearly-white, sometimes more or 

 less transparent. This third eyelid is placed on the inner side 

 of the eye, and possesses a special muscular apparatus, by 

 which it can be drawn over the anterior surface of the eye like 

 a curtain, moderating the intensity of the light. As to the 

 organ of hearing, Birds possess no external ear or concha, by 



