118 
ON THE STRUCTURE OF THE EYE 
the sclerotic, and are firmly inserted into the osseous plate. 
On the upper half of the plate, about half an inch from its 
ragged posterior margin, there is a perforation about two 
lines in diameter, for the passage of the arteries and veins 
of the eyebalL On the inside of this perforation there is a 
bony ridge, about a line in thickness, for the protection of 
the vessels passing through it. This hard inner layer of 
the sclerotic is continued in a membranous form, and in 
firm union with the outer layer, from the ragged posterior 
margin, to the optic nerve. The broad anterior inflected 
margin of the plate affords attachment to the iris and the 
cornea, and gives lodgment to a white adipose thick band 
surrounding the eye, and placed anteriorly to the ciliary 
ligament. Immediately within the osseous plate we ob- 
serve a very dehcate black coloured membrane, adhering 
loosely to it as far as the ciliary ligament, where it becomes 
inseparably united with the vascular layer of the choroid. 
This thin membrane lies immediately over and in contact 
with the fatty substance enveloping the choroid gland, and 
occupies the situation of the white silvery pigment which 
forms the outer layer of the choroid coat in many osseous 
fishes. Near the ciliary ligament this black membrane ac- 
quires a light grey colour and metaOic lustre ; but at the cili- 
ary ligament, where it unites inseparably with the vascular 
layer of the choroid for a line and a half, it again becomes 
dull, and has a deep purple colour. On the fore part of 
the ciliary ligament it resumes its metallic lustre, becomes 
detached from the choroid, and can be traced with ease 
dipping into an angular recess between the uvea and the 
iris, and again mounting to descend over the fore part of 
the iris. In this animal there is a wide angular space, a 
line in breadth, between the uvea and iris, wliich contains 
a white cellular substance. The iris is thick and firm, like 
cartilage, and^ on removing its metallic pigment from the 
