24: SCHULTZE, ON THE RETINA. 
The structure of the retina in mammals and other verte- 
brates is then compared with that of the human eye regarded 
as a typical form. 
Apes, as is well known, possess a macula lutea, and in 
other respects their retina seems to agree very closely with 
that of man, even in the comparatively great thickness of the 
*€ cone-fibres.”” 
Among the other mammalia a very remarkable and, as it 
would seem, hitherto unnoticed diversity, with respect to the 
distribution of “ rods” and “ cones,” exists. Whilst most of 
our larger domestic animals, especially the sheep, ox, pig, 
horse, and dog, present an arrangement of those elements re- 
sembling that which is observed in the human subject and im 
apes, except, of course, in the absence of the macula lutea, the 
cones, according to the author’s observations, are entirely 
wanting in bats, the hedgehog, mole, mouse, and guinea-pig. 
A sort of intermediate condition is met with in the cat, rabbit, 
and rat, in which animals are found either very slender true 
“cones,” as in the cat, or merely indications of them, as in 
- the rabbit. But in any case the “rods” preponderate so 
much that the “cones” among them may readily be over- 
looked. According to Ritter, the “cones ” are also wanting 
in Balena mysticetus. 
In the rat the “ rods ” are the longest and slenderest yet 
met with by the author. In the other vertebrate classes the 
proportion of “rods ”’ and “ cones” to each other approaches 
nearest to that observed in the mammalian retina in the 
osseous fishes. In the ray and shark “rods” only exist. In 
Petromyzon elements of one kind only occur in the bacillary 
layer ; but whether these be “ cones” or “ rods” is undeter- 
mined, nor is it determined whether, as supposed by some, 
both elements may not really be present. The osseous fishes 
afford excellent materials for the study of the ‘‘ cone ”’-fibres ; 
which at one time M. Schultze regarded as belonging to the 
connective-tissue framework of the retina, and to represent in 
the outer granule-layer the “ radial fibres of Miller ” in the 
other layers of the retina; but of their nervous nature, as of 
the corresponding fibres in the human retina, he is now 
thoroughly convinced. 
The structure of the retina in birds, reptiles, and amphibia, 
differs in a very peculiar manner from that of mammals and 
fish. In the bird’s retina the proportion of “cones” to 
“rods” is in the reverse proportion to that in the mammalia. 
In other words, the retina of the bird, as regards the distribution 
of “rods” and “ cones,’’ approaches that which is observed 
in the human macula lutea, masmuch as the “cones” pre- 
