1210 ON THE STRUCTURE OF THE EYE 
sinus, described by Blainville {Anat. Comp. t. i. p. 425) * 
as occurring in the inner part of this singular fleshy mass, 
is large and distinct in the Swordfish, where it appears like 
the pelvis of a kidney, with numerous openings, probably 
of the ciliary veins, terminating in it. Blainville consi- 
ders the choroid gland of fishes as a vascular ganglion, or 
sinus, for the reception of the ciliary veins. I may observe, 
that, on cutting open this body, in the Swordfish, we find 
its outer part composed of a compact parenchymatous tex- 
ture, and, as we approach its inner sinus or pelvis, it con- 
sists of distinct straight parallel fibres, which may possibly 
be ducts or vessels, leading to the central cavity. After 
remaining long in spirits, the inner concave margin of the 
organ splits spontaneously, and always in the direction of 
these parallel fibres. The vascular membrane of the choroid, 
which lies immediately beneath the bed of fat, is distinctly 
separable, in the Swordfish, into two layers. The outer, 
or venous layer, has a bluish-grey colour, and a strongly 
marked corded or striated appearance, produced by the 
straight veins which compose it. The inner, or arterial 
layer, termed the Tunica Ruyshiana, and composed of the 
ramifications of the ciUary arteries, has a deep brownish- 
purple colour on the surface where the pigment rested. 
At the place of the ciliary ligament, this inner layer of 
the choroid assumes a yellowish-white colour, and conti- 
nues of this light colour over the whole of the uvea to 
its free margin, where it unites at an acute angle with 
the iris. This white part of the choroid, and the white 
surface of the uvea, are covered, in the natural state, like 
every other part of the choroid, with a thick layer of a 
brownish-black pigment. There are no ciliary processes 
projecting in towards the margin of the lens, but in their 
place we find the choroid distended by the white adipose 
band I have already mentioned as surrounding the fore 
