ORGANS OF PERCEPTION. 173 
relative importance of the same organs, as they appear to 
be modified in the inferior tribes. 
(1.) Coats of the Eye.—There are two coats which may 
be considered as constituting the integuments of the Eye, 
and within which, the other essential parts are contained. 
‘These are, in the form of two cups, applied to each other 
at the margins, and hence the eye-ball approaches a globu- 
lar form. The cup, which is situated with its convexity in- 
wards, is termed the Sclerotic coat. Its texture is cellular, 
and is composed of filaments, which are interwoven in every 
direction. It possesses considerable opacity, tenacity, and 
flexibility. It determmes the shape of the eye. Hence, 
in those animals whose eye-balls are more or less depres- 
sed, the sclerotic coat acquires a proportional degree of 
strength, to enable it to retain its form. In birds, for ex- 
ample, its anterior part is divided into two plates, between 
which, a circle of thin hard bones is mterposed. 
The sclerotic coat is perforated posteriorly, for the pas- 
sage of the optic nerve. By some, it is considered as a 
continuation of the dura mater, forming a sheath for the 
protection of the nerve, and afterwards. expanding to sup- 
port its terminal enlargement, denominated the Retina. On 
its internal surface, a thin blackish membrane adheres, which 
has been considered as a prolongation of the pia mater. 
The sclerotic coat not only preserves the form of the 
eye, but furnishes a firm support for the insertion of the 
muscles destined to regulate its motions. It is supplied 
with few bloodvessels, and possesses little sensibility. 
The sclerotic coat protects the posterior part of the eye, 
but it is wanting on the anterior surface. Its placc, how- 
ever, is supplied by another membrane, termed the Cornea, 
which may be compared to the glass of a watch, the case 
resembling the sclerotic coat. 
The Cornea is composed of thin concentric plates, unit- 
