XVI INTRODUCTION. 



birds, are thus cnaLlcd to see equally well in the Jiffcrenfc 

 media of air and water. 



The raarsupium, a plaited and vascular membrane, 

 extending from the retina to the edge of the lens, is sup- 

 posed, by some, to assist in this focal adjustment; by others 

 it is considered to be simply a nervous prolongation intend- 

 ed to increase the extent of the visual surface. The pupil is 

 always round. The iris is usually narroAv, and often highly 

 coloured. Certain colours prevail in certain tribes or fami- 

 lies; such as dark-brown in the Falcons, yellow in the 

 Hawks; but colour is not constant even in diiferent species 

 of the same genus. There are two horizontal eyelids, 

 the lower of which is the larojest and most moveable, 

 only a few birds being able to depress the upper eyelid 

 to any extent, viz. Owls and Capriniulgidge. There is a 

 third, or vertical, eyelid called the nyctitating membrane. 

 This is fixed in the inner canthus of the eye, and is semi- 

 transparent, being a fold of the Tunica conjunctiva, and 

 can be drawn like a curtain over tlie front of the eye. 

 Very few birds possess eyelashes, only Hornbills, some 

 Cuckoos, a few Parrots, Ostriches &c. 



Hearing is sufficiently acute in all birds, and is highly 

 developed in Owls, in which the auditory aperture is often 

 immense, and, in some species, is also furnished with aa 

 external conch. There is only one auditory ossicle, which 

 connects the drum, or membranum tynipani, with the 

 fenestra ovalis ; and the cavities communicate with each 

 other throuo;h the air-cells of the skull. The external 

 aperture is usually covered by comparatively loosely -barbed 

 feathers, as has been already alluded to. The Eustachian 

 tubes terminate in a common opening on the palate. 



The sense of smell is also highly developed. The 

 posterior nares unite to form a single cleft in the palatal 



