OF THE SWORDFISH. 
119 
fore part, it presents a black surface. The white adipose 
band surrounding the eye before the ciliary Hgament, rests 
on a peculiar thin membrane, which separates it from the 
outer dark layer of the choroid, and extends forward as far 
as the cornea. The dark coloured external layer of the cho- 
roid, in the Xiphias, somewhat resembles the thin black 
lining of the sclerotic of quadrupeds, which was thought 
to be a continuation of the pia mater. There is no silvery 
pigment in this animal covering the choroid gland, which 
we find so abundant in many osseous fishes, as the cod and 
haddock. 
Between the thin dark outer layer of the choroid and its 
thick vascular layer, there is an immense deposit, of a white 
semifluid fatty substance, half an inch in thickness, on the 
back part of the eye, and extending as far forward as the 
ciliary ligament. The fatty deposit is thickest on the back 
part, near the optic nerve, and becomes gradually thinner 
towards its anterior termination ; it is traversed in every part 
by numerous large branches from the ocular artery, and by 
the ciliary veins returning to the choroid gland. These 
vessels are filled with red blood, and are so numerous as to 
give a red tinge to the oily substance through which they 
are ramified. The great bed of fat completely envelopes 
that singular organ the choroid gland; it enables the great 
trunks of the ciliary arteries to subdivide, before penetrat- 
ing the choroid, to form its innermost layer ; and it enables 
the internal parts of the eye to assume a sphericity which 
the osseous plate of the sclerotic is far from possessing. 
The choroid gland has the same deep blood-red colour, 
firm consistence, and vascular texture, as in other fishes ; 
it is composed of two parts surrounding the optic nerve, 
the one shaped like a horse-shoe, and the other nearly 
straight. These two pieces are flat, about half an inch 
broad, and nearly a quarter of an inch thick. The venous 
