376 SENSES OP SMELL AND SIGHT. 



supported by the sides of the cavity. The posterior nares unite 

 towards the centre of the palatal arch, so as to form in it a single 

 longitudinal cleft. Birds of prey, especially those that live on 

 carrion, have the olfactory organ more developed than granivor- 

 ous (grain-eating) or insectivorous Birds ; most authors assure us 

 that among the former, the delicacy of this sense is such, that 

 it enables them to discover their prey, even when at a con- 

 siderable distance ; but the experiments of some naturalists seem 

 to prove, that in these animals smell scarcely exists, and that 

 they are almost entirely guided by sight. The organs of hearing 

 are less complicated than in the Mammalia ; the external ear is 

 generally wanting in Birds ; and the auditory canal consists of 

 little more than a membranous tube placed between the os 

 quadratum and a projecting part of the occipital. 



340. The organs of sight, on the contrary, appear more 

 perfect than in the preceding class ; the eyes of Birds are larger 

 in proportion to the size of the head, and some new parts are 

 developed. The retina is very thick, and there proceeds from 

 it a black membrane, folded like a fan, which advances towards 

 the crystalline lens. Physiologists are not agreed respecting the 

 nature of this appendage, which is termed the marsupium. It is 

 regarded by some as a part of the choroid coat; and by others as 

 a nervous prolongation, intended to increase the extent of the 

 visual surface. The pupil is always round ; the iris very con- 

 tracted ; the cornea transparent, large and convex ; and the 

 sclerotic strengthened in front by a circle of bony plates lodged 

 in its thickness. There are two horizontal eyelids, the lower 

 of which is the largest and most moveable ; and there is also 

 a third vertical, semi-transparent eyelid, which occupies the 

 internal angle of the eye, and may be drawn across so as to cover 

 its whole surface. There are always lachrymal glands. In many 

 Birds the range of sight is extremely long ; there are some which, 

 when raised in the air to heights at which, notwithstanding 

 their size, we can scarcely distinguish them, clearly perceive 

 small animals on which they feed, and pounce upon them with 

 an unerring aim. In these the crystalline lens is less convex 

 ana less dense than in the Birds which rise but little from the 



