Monthly Microscopical "I 
Journal, Nov. 1, 18G9. J 
Histology of the Eye. 
251 
The Chelonian retina agrees very closely with that of birds. 
Both are distinguished by bright cone-beads, and by twin-cones, the 
structure of which, particularly in chelonia, resembles that of lizards, 
and differs in the same way that this does from that of the fish's 
twin-cone. Each twin has certainly its own outer granule, and its 
separate primitive cone-fibre, which, as in lizards, takes an obhquely 
radial direction from the posterior pole of the globe. The cone- 
beads are of three colours — ruby, which are the largest ; and 
orange, passing through pale yellow into pale green : the orange 
and green beads are the most numerous. The inter granule layer, 
contains large branched connective tissue corpuscles, resembling, 
those occurring in the same layer in the fish's retina. 
The Bird's retina, as I have just said, agrees in several par- 
ticulars with that of the chelonia. It has cones with beads of three 
colours, except in the case of nocturnal birds, e. g. owls, in which, 
as Schultze first showed, all the beads are pale, almost colourless, 
a light yellow. It has also twin-cones, like those of reptiles. In 
many birds there exists a very distinct fovea, and in some H. Miiller 
discovered two, one at the posterior pole and the other near the ora 
retinae, the former being affected by the incident pencils in mono- 
cular vision, the latter coming into use in vision with both eyes. 
The primitive bacillary fibres radiate obliquely from the fovea, as 
in man and reptiles. 
The Mammalian retina is marked by the absence of twin-cones 
and of cone-beads. Its elements are smaller than those of the 
lower vertebrates. That of man has a macula lutea, in \Yhich is a 
distinct fovea centralis. The macula lutea occurs also la certain 
apes. The bat, mouse, hedgehog, and certain other animals, 
chiefly of nocturnal habits, have rods only ; in most others cones 
and rods are both present, as in man. The retina is vascular ; the 
distribution of the vessels, however, varies in different families. 
There are two situations where the structure of the retina in 
man and some other vertebrates which I have particularized is 
peculiar; these are the macula lutea and the ova retinae. The 
macula lutea is an oval spot, at the posterior pole, of a yellow 
colour : the coloration is not produced by granular pigment, as in 
that of the choroid, but it is a diffuse stain of the elementary tissues. 
In the centre of the macula is the minute pit — not a perforation — 
the fovea centralis. This pit is produced by the radial divergence of 
the primitive cone-fibres from a central point, and by the thinning 
and outward curving of all the retinal layers (except the bacillary) 
as they approach this point. In the fovea and the macula, except 
at its periphery, cones only occur, and they are more slender and 
longer than in other parts of the retina. The greater length is 
chiefly due to the elongation of the cone appendage. The slender- 
ness of the cones does not allow their appendages to include the 
