OBSERVATIONS ON SOUNDNESS. 
261 
folds, formed by the plaiting of the middle and internal layer 
of the choroid; they are covered with a thick layer of pig- 
mentum nigrum; when this is washed off, the processes are 
rather white. 
The next tunic is the retina. It is composed of three 
layers—external, or Jacob’s membrane; middle, or nervous 
membrane; and the internal, or the vascular membrane. 
Jacob’s membrane, when examined by the microscope, is 
found to be composed of cells having a tessellated arrange¬ 
ment ; it is said to be a serous membrane. The nervous mem¬ 
brane is the expansion of the optic nerve ; it envelopes the 
vitreous humour. The vascular membrane consists of the 
ramifications of the arteria centralis retinae and its vein ; its 
branches are continued anteriorly into the zonula ciliaris; it 
is a branch of the ophthalmic artery. 
The zonula ciliaris is a thin, vascular layer, which connects 
the anterior margin of the retina with the anterior surface of 
the lens near to its circumference; the under surface of the 
zonula is in contact with the hyaloid membrane^ and around 
the lens forms the anterior, fluted wall of the canal of 
Petit. 
The humours. —The aqueous humour is situated in the 
anterior and posterior chambers ; the anterior chamber is the 
space intervening between the iris and pupil behind. The 
chambers are lined by a thin layer, which is the secreting 
membrane of the aqueous humour; it is a weak albuminous 
fluid, with an alkaline reaction ; its sp. gr. is little more than 
water. The posterior chamber is the space bounded by the 
posterior surface of the iris and pupil in front, and by the 
ciliary processes, zonula ciliaris, and lens behind. 
The vitreous humour forms the greater bulk of the globe of 
the eye. It is an albuminous and very transparent fluid, 
enclosed in a membrane of extreme delicacy, the hyaloid. 
From the inner surface of this membrane many thin lamellae 
are pointed inwards, and form compartments or cells in 
which the fluid is contained. A tubular canal has been found 
in the centre of the vitreous humour, through which a minute 
artery is conducted from the arteria centralis retinae to the 
capsule of the lens. 
The crystaUiue humour or lens is situated immediately 
behind the pupillary opening, and is surrounded by the ciliary 
processes. It is more convex on its posterior surface than on 
the anterior; it is imbedded in the anterior part of the 
vitreous humour, separated from it by the hyaloid membrane. 
It is invested by a transparent, elastic membrane, the capsule 
of the lens, which contains a small quantity of fluid, called 
