ORGAN OF SIGHT IN BIRDS. 141 



The humours of the eye no less correspond to the peculiar 

 vision of the Bird, and the rare medium through which it is 

 destined to move, than the shape of the globe and the texture of 

 its coats. 



The aqueous humour is extremely abundant, owing to the 

 extent of the anterior chamber gained by the convexity of the 

 cornea, and its refractive power must be considerable in the 

 higher regions of the atmosphere. The membrane inclosing it 

 can be more readily demonstrated in Birds than in most Mammals, 

 especially where it adheres to the free edge of the iris. The large 

 size of the ciliary processes may have the same relation to the 

 reproduction of the aqueous, as the marsupium is supposed to have 

 mth reference to the vitreous, humour. 



The crystalline lens is remarkable for its flattened form, espe- 

 cially in the high-soaring Birds of Prey; it is also of a soft 

 texture, and is without the hard nucleus found in Fishes and 

 Reptiles. In the Cormorant and other birds which seek their 

 food in water, the crystalline is of a rounder figure, and this is 

 peculiarly the case in the nearsighted Apteryx and Owls which 

 hunt for prey in obscure light. It is inclosed in a distinct 

 capsule, which adheres very firmly to the depression in the an- 

 terior part of the vitreous humour ; the capsule is itself lodged 

 between two layers of the membrana hyaloidea, which, as they 

 recede from each other to pass — the one in front and the other 

 behind the lens — leave round its circumference the sacculated 

 canal of Petit. 



The vessels of the lens are derived from those of the marsu- 

 pium, which, as before observed, are ramifications of the homo- 

 logue of the arteria centralis retinae : this is not continued as a 

 simple branch from its origin to the marsupium ; but, imme- 

 diately before penetrating the coats of the eye, it breaks into 

 numerous subdivisions, the aggregate of which is greater than the 

 trunk whence they proceed, and these again unite, forming a 

 plexus, s, fig. 59, close to the external side of the optic nerve. 

 The artery of the marsupium proceeds from this plexus, and runs 

 along the base of the folds, giving off at right angles a branch to 

 each fold, which in like manner sends off smaller ramuli, fig. 58. 

 The plexus at the origin of the marsupial artery serves as a 

 reservoir for supplying the blood required for the occasional full 

 injection of the marsupium ; and a snnilar but larger plexus, fig. 

 59, 4, is formed at the origins of the ciliary arteries which supi)Iy 

 the erectile tissue of the ciliary processes and iris. 



The vitreous humour presents few peculiarities worthy of note; 



