Class 5. Avcs. 



443 



some distance from the tip of the beak, and may even be at its 

 base ; the internal uares open into the mouth, rather far forwards, 

 in a groove, which is partly covered by hiteral longitudinal folds 

 (e/. Reptilia). On the outer wall of the nasal-cavity there is a 

 cartilaginous projection often spirally folded; which corresponds 

 with the t u r b i n a 1 of Reptilia ; there are also two other more or 

 less well-developed folds* The eye and its accessory structures are 

 also very like those of Reptiles. The optic bulb is of very consider- 

 able size. The front wall of the sclerotic, in which there is a circle 

 of bony plates (the sclerotic ring), has the form of a frustrum of a 

 cone ; the absent apical portion of the cone is replaced by the cornea, 

 which is often ^•ery convex, whilst its base is formed by the posterior 

 curved portion of the sclerotic ; if the frustrum is long, and its wall, 

 as sometimes happens [e.g., in the Owls, Fig. 371, B) is somewhat 

 arched inwards, the form of the eyeball differs much from the 



Fig'. 371.- -4 Eye of a Bird, diajrrammatio transverse section, r cornea, ch choroid, 

 H eiliiiry processes, i' iris, I lens, ii optie nerve, o transverse section of bony plates, p peoten, 

 I- retina: s external oonneotive tissue portion, s' internal cartilaginous portion of the 

 solerotio. B Section tlirough the eye of an Owl, in order to show its peculiar shape. — Orig. 



ordinary spherical type, whilst in others it deviates but little from 

 this. A lai'ge membranous process, which is folded and pigmented, 

 the pecteu, arises from the retina at the point of enti-ance of the optic 

 nerve, and projects freely into the vitreous humour. Of the two 

 eyelids, the lower is much larger and more movable than the 

 upper (as in Reptiles) ; there is a well-developed nictitating 

 membrane, which can be flicked across the eye by a special 

 muscle. The Harderian gland opens at the anterior corner of the 



* Into the nasiJ cavity opens the duct of a large nasal gland, which usually 

 lies alx>ve the frontal bone (m Sea-gulls and others, in a longitudinal depression at the 

 edg« of the orbit) ; this gland ocoiu:^ in many BeptUes, but is situated in a different 

 region of the head. 



