174 PHILOSOPHY OF ZOOLOGY. 
ed together by acompact cellular substance. It is thickest 
in the middle, becoming thinner towards the edges, wher 
it unites with the sclerotic coat. The line of junction ex- 
hibits considerable differences. In man, the edges are be- 
velled, and the cornea slides under the sclerotic coat. In 
the hare, the sclerotic coat divides at the edges, and embra- 
ces, like a forceps, the margin of the cornea. In the 
whale, the fibres of the sclerotic coat pass into the sub- 
stance of the cornea, in the form of very delicate white 
lines. 
The cornea is transparent. It possesses few, if any, 
bloodvessels or nerves. It is, however, porous, and readily 
admits the escape of the aqueous humour, as appears by 
the eye speedily becoming flat after death. 
In insects, this membrane is hard and scaly, and supplies 
the place of the crystalline humour ; while in the cuttle- 
fish it is regarded as absent. 
These two coats may be considered as forming the case 
of the eye. There are other membranes on their central as- 
pect, which are usually classed along with them, which we 
shall now consider. 
Immediately within the sclerotica, is the Choroides, a thin 
delicate membrane of a vascular structure. It lines the 
concave surface of the sclerotic coat, with which it is con- 
nected by cellular tissue, and by the nerves and blood- 
vessels which terminate in its substance. On its concave 
surface, its substance is compact and. uniform in its texture, 
and has been regarded as a particular membrane, and 
termed Membrana Ruyschiana. 
Towards the anterior part of the sclerotic coat, near its 
termination, the choroides is more intimately united with 
it, by means of a circular band of dense cellular substance, 
moistened by a whitish mucus, termed the Ciliary Liga- 
ment. From this band, numerous processes arise, which 
