BIRD’S-EYE VIEWS. 
BY DR. ELLIOTT COUES, U. 8. A. 

(Concluded from page 513.) 
In our last “View” we saw all the appendages of a bird’s 
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 sphericity. 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 thé 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 organs that can 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 membranes 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 p vor- 
tical section ; they lie curving obliquely against the inside 
of the walls of the hollow spheroid. But no idea ara | 

