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sclerotic. The ophthalmic artery breaks up in the choroid 
gland. The other artery appears to pass forwards with 
the optic nerve to the retina, but we are uncertain as to its 
precise distribution. 
The sclerotic consists of two layers, an external layer 
of tough fibrous tissue into which the eye muscles are 
inserted, and an internal cartilaginous layer of some thick- 
ness. The cartilaginous sclerotic is perforated for the 
entrance of the optic nerve and the blood vessels, the 
fibrous layer becoming continuous with the connective 
tissue surrounding the latter structures. The cartilaginous 
sclerotic ceases at some distance from the pupil, and the 
fibrous layer with another layer which seems to be a con- 
tinuation forwards of part of the choroid fuse with the 
skin of the head to form the cornea. In suitably prepared 
sections all these layers can be distinguished in the cornea, 
and the structure of the skin in that region does not differ 
materially from that in other parts of the head. 
The Argentea.—Internal to the sclerotic and in close 
contact with its internal surface is the peculiar layer so 
named. It covers the whole internal surface of the 
sclerotic as far forwards as the iris. It has a white silvery 
appearance by reflected light, but is opaque to trans- 
mitted light. No structure beyond wavy bundles of very 
fine fibres can be made out in it. The silvery appearance 
is said to be due to minute crystals imbedded in a cellular 
tissue. 
The Choroid.—This is the usual vascular and pig- 
mented layer. It lies between the argentea and the 
retina, is closely adherent to the latter, and comes away 
with it when removed. Anteriorly it passes into the iris. 
The Choroid Gland.—This structure is situated in the 
posterior wall of the bulb between the argentea and the 
choroid. It lies to the nasal side of the entrance of the 
