120 
ANNIVERSARY ADDRESS 
exists also in some of the Mammalia, as for instance in the walrus, 
and is useful in protecting the eye against intense light, while it 
does not prevent vision. In the human eye it is represented in a 
very rudimentary form by the red fold called the “caruncle,” which 
varies in size not only in races, but also in individuals. The 
nictitating membrane is always associated with a special gland for 
lubrication. 
The Eyes oe Reptiles present no very special feature. The 
muscles which serve as adjustments are strong and striped. In the 
tortoise and some of the lizards the external covering of the eye¬ 
ball contains a circle of bony plates such as just described as 
existing in birds, but it is of much less development. Snakes, 
geckos, and amphisbsenas have no eyelids, but their cornea is pro¬ 
tected by a transparent capsule. The chamseleons have a single 
eyelid consisting of a muscular ring with a circular opening, and 
they can move their eyes very freely. Other reptiles have an upper 
and lower eyelid, and the land-lizards can raise their lower eyelid 
like a transparent curtain. The turtles have a nictitating mem¬ 
brane. The processus falciform,is of fishes and the pecten of birds 
are represented in the lizards by certain folds of the choroid. 
The Eyes oe Amphibians are sometimes without eyelids. The 
salamanders have two. The batrachians, or frogs and toads, can 
withdraw the eyeball into a socket. They have an upper eyelid, 
and, with the exception of the Surinam toad, also a nictitating 
membrane. Toads have a rudimentary lower eyelid. The lens of 
amphibians has not such a high refracting power as that of fish. 
The Imperfections of the Human Eye. 
I have thus far briefly sketched some of the modifications of the 
eye in the different groups of the Animal Kingdom, and we find in 
them all a practical adaptation to the wants of the organism. We 
must, however, bear in mind that the faculty of vision is dependent 
not only on the eye but also on the percipient centre, i.e. the brain. 
Thus when we come to consider the imperfections of even the human 
eye as an optical instrument, we shall find that they are all cor¬ 
rected either by the retina itself or by our interpretation of the 
sensations conveyed to us. Brass and glass instruments with 
similar defects require brass and glass adjustments, but the brain 
counteracts the errors of its own instrument. 
I have already referred to the blind spot on the retina of the 
vertebrate eye. It is situated in the human eye about one-tenth 
of an inch nearer the inner (nasal) side than the yellow spot, and is 
about one-sixteenth of an inch in diameter. We can recognize its 
