734 AYES. 



the other. The obvious consequence of this figure of the globe of the 

 eye is to allow room for a greater proportion of aqueous fluid, and for the 

 removal of the crystalline lens from the seat of sensation, and thus pro- 

 duce a greater convergence of the rays of light, by which the animal is 

 enabled to discern the objects placed near it, and to see with a weaker 

 light ; and hence Owls, which require this sort of vision so much, possess 

 the structure fitted to effect it in so remarkable a degree. 



(2080.) But it is evident that, in order to retain this conical shape 

 of the eyeball, some further mechanical arrangements are necessary, 

 which in the spherical form of the human eye are not requisite. In 

 Fishes, where the eyeball is constructed upon entirely opposite prin- 

 ciples, being compressed anteriorly, cartilaginous supports are found im- 

 bedded in the sclerotic tunic, which in some cases is absolutely ossified 

 into a bony cup. In many Reptiles the same end is obtained by placing 

 a circle of bony plates around the cornea ; and this latter plan is again 

 adopted in Birds, to maintain their eyes in a shape precisely the con- 

 verse of the former. In the Owls these ossicles are most largely deve- 

 loped ; in such birds they form a broad zone (fig. 364), extending from 

 the margin of the cornea, embracing the anterior conical portion of the 

 eye, and imbedded between two fibrous layers of the sclerotic. The 

 figure which is thus given to the eye, from the increased space obtained, 

 is evidently calculated to allow the humours, forming the refracting 

 media whereby the rays of light are brought to a focus upon the retina, 

 to become materially changed in shape ; and both the convexity of the 

 cornea and the position of the lens may thus be altered so as to adjust 

 them in correspondence with the distance at which an object is viewed. 

 The cornea is rendered more convex, and the shape of the aqueous 

 humour consequently adapted to examine objects close at hand, by the 

 simple action of the muscles that move the eyeball : for these, seeing 

 that the edges of the pieces composing the bony circle overlap each 

 other so as to be slightly moveable, as they compress the globe of the 

 eye, cause the protrusion of the aqueous humour, and the cornea becomes 

 prominent; or if the bird surveys 

 things that are remote, the cornea re- 

 cedes and becomes flattened, an effect 

 caused by the recession of the aqueous 

 humour, and, as some authors assert*, 

 by muscular fibres disposed around 

 the circumference of the cornea, and 

 attached to its inner layer, which draw 

 back the cornea in a manner analogous 

 to the action of the muscles of the 

 diaphragm upon its tendinous centre. 



(2081.) But the most beautiful Section of the eye of an Owl. 



* Vide Cyclop, of Anat. and Phys. p. 304. 



