250 
Histology of the Eye. 
rWoTithly Microscopical 
L Journal, Nov. 1, 1«69. 
In fish, batrachia, and reptiles, the vascular net which pervades 
the capsula hyaloidea represents the retinal vascular system of 
mammals, but in birds this hyaloid net is wanting ; and the great 
development of the pecten was thought by Miiller to be a compen- 
satory provision for both its absence and that of retinal vessels. 
I will now pass on to notice — and I can only do so very 
briefly — the characteristic modifications which the retinal elements 
undergo in the five vertebrate orders. 
In Fish, the retina is distinguished by the occurrence of cones 
of a peculiar kind — double or twin cones, as they are commonly 
called — by the large quantity of connective tissue it contains, and 
by the presence of very large branched connective tissue corpuscles 
in the intergranule layer. 
The twin-cones have distinct outer segments or shafts. Their 
symmetrical appendages are joined together down one side, and at 
their inner end they sometimes appear to be actually continuous. 
Each twin has, I think, its own outer granule, and detaches a 
separate fibre inwards. 
The Batrachian retina is distinguished by the large size of its. 
elementary tissues : the rods are very large. The cones, which are 
smaller, contain a pale yellow or colourless bead. Twin-cones have 
been discovered in it by Schultze. 
Amongst Reptiles, lizards possess cones only ; these contain a 
pale yellow bead (in all I have examined). They are single and 
twin ; but the twin-cones difi'er in many respects from those of fish. 
They are unsymmetrical in form, and one is beaded while the other 
is headless. Their union is much less intimate than that of the 
fish's twin-cones. A little violence frequently disassociates them. 
The chameleon, iguana, gecko, and many other lizards, have a 
fovea centralis, from which the primitive bacillary fibres radiate 
towards the periphery of the retina, and pursue an oblique course 
from the outer towards the inner surface of the retina, crossing the 
vertical radial connective tissue fibres, which enables us easily to 
distinguish the nervous and connective tissue fibres in this region. 
In many lizards, a well-developed, conical, or sword-like pecten 
stands forwards from the optic nerve in the vitreous humour 
towards the lens. In the common alligator, and in the Nile croco- 
dile, there is no projecting pecten, but the optic disc is marked 
with a brown pigment. 
The blind worm's retina closely resembles that of typical lizards, 
especially in the presence of a pale cone-bead. 
A cone-bead is wanting in snakes. In other respects, their 
retina resembles that of lizards. 
The common English snake has no pecten: the viper has a 
rudiment of one ; and the boa's optic nerve has a minute globular 
one. 
