BIRD'S-EYE VIEWS, 



BY DR. ELLIOTT COUES, U. S. A. 

 (Concluded from page 513.) 



In our last "View" we saw all the appendages of a hirers 

 eye ; and now we come to look at the eye itself. "Eye- 

 ball" and "globe of the eye" are very convenient terms, 

 constantly in our mouths ; but they are not strictly accurate 

 ones. Probably there are no perfectly spherical eyes. In 

 our own species, the eye is made up of a segment of a large 

 sphere, representing about five-sixths of its superficies ; the 

 other sixth is a smaller segment of a small sphere, joined in 

 front to the former. Most mammal's eyes are not very dif- 

 ferent in this respect from our own. Bird's eyes are much 

 further removed from perfect sphericit}'. The greater part 

 of the ball is saucer-shaped, — almost discoidal; and there is 

 a very convex prominence, more or less hemispherical in 

 shape, in front. The whole eye may be likened to an acorn 

 of one of those oaks that bear a fruit with a heavy broad 

 shallow cup, and short blunt kernel, or to a thick old-fash- 

 ioned watch with a very- convex crystal. 



This shape is one of the distinguishing characters of a 

 bird's eye : the figure (Fig. 2) will give a better idea of it 

 than any description. It represents a vertical section through 

 the middle of an eye in profile, and shows nearly all the struct- 

 ures and or2:ans that be demonstrated in the ball. Before 

 making use of it, however, the reader must be reminded 

 of the two following points : First, the distinctness of the 

 several memlu-anes forming the ball is greatly exaggerated ; 

 for otherwise the different membranes could not be repre- 

 sented as such. Secondly, the ciliary processes, optic nerve, 

 and marsupium, do not fall wholly within the line of a ver- 

 tical section ; they lie curving obliquely against the inside 

 of the walls of the hollow spheroid. But no idea whatever 

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