CHARACTERS OF AVES. 497 



- 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 nervotis system, the brain of Birds is rela- 

 tively larger, especially as regards the size of the cerebrum 

 proper, than the brain of Reptiles, but its chief mass con- 

 sists of the corpora striata. The cerebellum, though always 

 present, consists simply of the central lobe (the "vermiform 

 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, most birds possess no external ear or concha, 

 by which sounds can be collected and transmitted to the in- 

 ternal ear. In some birds, however, as in the Ostrich and 

 Bustard, the external meatus auditorius is surrounded by a 

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

 The Nocturnal Birds, also, especially Owls, have the external 

 meatus auditorius protected by a musculo-membranous valve, 

 which foreshadows the cartilaginous concha of the majority of 

 Mammals. The external nostrils in Birds are usually placed 

 on the sides of the upper mandible, near its base, in the form 



* The membrana nictitans is simply a fold of the conjunctiva on the inner 

 side of the eye. It occurs in some Fishes (e.g., Sharks), in some Reptiles 

 and Amphibians, in Birds, in Monotremes and Marsupials, and in some of 

 the higher Mammals. In Man, however, in Monkeys, and in most of the 

 higher Mammals, it is rudimentary, and constitutes the so-called " plica 

 semilunaris." 



2 I 



