59O MANUAL OF ZOOLOGY. 



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 

 internal 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 

 of simple perforations, which sometimes communicate from 

 side to side by the deficiency of the septum narium. In the 

 singular Apteryx of New Zealand, the nostrils are placed at the 

 extreme end or tip of the elongated upper mandible. Some- 

 times the nostrils are defended by bristles, and sometimes by 

 a scale (Rasores). Taste must be absent, or almost absent, in 

 the great majority of birds, the tongue being nothing more than 

 a horny sheath surrounding a process of the hyoid bone, and 

 serving for deglutition or to seize the prey. In the Parrots, 

 however, the tongue is thick and fleshy, and some perception 

 of taste may be present. Touch or tactile sensibility, too, as 

 already remarked, is very poorly developed in Birds. The 

 body is entirely, or almost entirely, covered with feathers ; the 

 anterior limbs are converted into wings, and rendered thereby 



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

 inner side of the eye. It occurs in some Fishes (e. g. t some 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." 



