THE EYE OF THE OX. 
223 
1 / 
Fig. 12. 
and is formed by an expansion of tlie optic nerve (Plate XI., 
fig. 3, n), commencing at the entrance of the optic nerve 
into the eyeball, and extending as far as l, where it terminates 
by a finely -jagged border. The retina is an exceedingly 
delicate structure, and is admirably supported, as if on a 
water-bed, by a globular mass of almost fluid consistence. 
It is itself semi-transparent, and of a greyish-white colour ; 
and is traversed by many branches of a blood-vessel (whence 
its derivation from rete), which is seen at o, in Plate XI., 
fig. 3, passing through the midst of the optic nerve to supply 
the retina with blood. The retina consists of two layers : 
Jacob’s membrane, and the retina proper; Jacob’s membrane 
(so named, because first described by Dr. Jacob) is the outer 
one of the two, and is seen as a thin film, by a careful dis- 
section made in water : it is formed of a number of rod or 
club-shaped particles, lying side by side, and with the long 
axis vertical, the broad end of the particles 
being in contact with the choroid ; the 
other end, of course, in opposition with 
the retina proper. The appearance of the 
membrane seen from without is represented 
in fig. 12, b ; that of its vertical section in 
fig. 13, a; and that of the particles when 
detached at fig. 12, a. 
The retina proper may be described, in a general way, as 
made up of three parts : a fibrous, a vesicular, and a granular 
layer (fig. 13, b, c, cl). The fibrous layer, cl, 
is derived from the optic nerve ; the fibres 
of which radiate from the end of the nerve, 
where it perforates the sclerotic and choroid, 
and forms a thin sheet of membrane : the 
vesicular layer, c, is composed of a number 
of dehcate bodies called vesicles ( vesicula , a 
little bladder or blister), seen separately in Plate XI., fig. 6, a. 
In the fibrous and vesicular layers the blood-vessels of the 
retina are distributed. The granular layer, b, is that which 
lies immediately beneath Jacob’s membrane, a, and consists 
of particles, such as those in Plate XI., fig. 6, b. 
I have said that the retina is supported and kept extended 
on a semi-fluid material; this is called the vitreous humour 
(■ vitrum , glass), and occupies the whole of the space marked m 
in Plate XL, fig. 3. It is of the consistence of soft jelly, and is 
formed of an exceedingly fine and dehcate web of fibrous 
tissue, which is at the same time transparent, and wliicli con- 
tains within its meshes a watery fluid. It is bounded, at its 
circumference, where it comes in contact with the retina, by a 
thin and transparent membrane (the fine next to the scarlet 
Fig. 13. 
