ORGANS OF PERCEPTION. 18] 
ing what is called the White of the eye. In some animals, 
however, (as the Myxine glutinosa of Linn.) this membrane 
is so thick and opake, as it passes over the eye, scarcely 
differing from the skin on the rest of the body, as to render 
the eye beneath useless as an organ of sight. 
As connected with the eye, we may here take notice of 
the eye-brows or supercilia. These form a screen to pro- 
tect the eye from too much light,—intercept the sweat flow- 
ing down the forehead,—and greatly contribute to the ex- 
pression of the countenance. 
The observations which we have hitherto made, illustra- 
tive of the structure of the organs of vision, relate to the 
more perfect animals. Among the Gasteropodous Mollusca, 
the eye is too minute to admit of accurate dissection. It 
appears as a black spot, convex, hewever, on the surface, 
and furnished with a nerve from the cerebral portion of the 
brain. Among the Annulose animals, black spots are observ- 
ed in many species, but to which no nerves have been traced ; 
while in others the nervous filaments have been detected. 
Even in the apparent absence of the nerves, these spots 
have retained the name of Eyes, and the analogy on which 
their claim to be so denominated rests, is far from remote. 
Such black spots in the annulose animals are termed simple, 
in opposition to other organs which are termed compound 
eyes. The surface of the compound eyes is convex, and, 
when viewed through a microscope, appears to consist of a 
number of hexagonal facets, slightly convex, forming a hard 
elastic membrane. Each facet is concave internally, but 
always appearing thicker in the middle than at the edges. 
Behind this external compound plate, which may be re- 
garded as a cornea, or aggregation of lenses, there is a 
close covering of an opake substance. On the posterior 
part of this last substance, there is a fine delicate membrane 
of a black colour, having behind it an expansion of the 
