AVES BIRDS. 611 



the cornea ; and this latter plan is again adopted in Birds, to main- 

 tain their eyes in a shape precisely the converse of the former. In 

 the Owls these ossicles are F*x 278. 



most largely developed; in 

 such birds they form a broad 

 zone (Jig. 278), extending 

 from the margin of the cor- 

 nea, embracing the anterior 

 conical portion of the eye, and 

 imbedded between two fi- 

 brous layers of the sclerotic. 

 The figure which is thus 

 given to the eye, from the in- 

 creased space obtained, is evi- 

 dently 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 cor- 

 respondence 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 eye-ball ; 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 recedes, 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. 



But the most beautiful piece of mechanism, if we may be par- 

 doned the expression, met with in the eye of a bird, is destined to 

 regulate the focal distance between the crystalline lens and the 

 sentient surface of the retina, in order to insure the clearest pos- 

 sible delineation either of near or distant objects. The provision 

 for this purpose is peculiar to the class under our notice ; and con- 

 sists of a vascular organ, called the marsupium, or pecten, which is 



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



2 R 2 



