TRANSACTIONS OF THE SECTIONS. _ 141 
swan, goose, and duck, the entrance of the nerve is very nearly in the centre of the 
ball. So in reptiles :—in the turtle, who only requires to see immediately before 
and under him, the outer and upper portions of the retina are very much the larger. 
In the more active alligator, frog, toad, and chameleon, while the upper portion 
maintains its size, the outer and inner parts are more nearly equal. In those crea- 
tures whose habitation is for the most part underground, as the shrew and the 
mole, the eyes are so small as to have led Majendie to assert that the mole is without 
the organ altogether, which is not the fact; for I have found all the essentials of an 
eye, even true retinal elements, optic nerve, and a well-developed choroid. Yet the 
organ is so minute and concealed by the skin and hair, as probably only enables the 
creature to discern the light, which is all that it requires; for, living underground, 
where it seeks its prey, it obviously must depend upon the acuteness of other senses 
than of sight for its living. ‘Though in the individual there is usually some propor- 
tion between the size of the eye and the body, taking different classes and genera, 
the size of the animal is a very little guide to that of the eye, the proportions between 
the two being determined by other considerations than that of the bulk alone of the 
creature ; for though, as a whole, the eye in fish bears a larger proportion to the 
whole body than it does in other divisions of the animal kingdom, and the eyes of 
birds are, as a class, much larger than those of mammalia or reptiles ; yet amongst 
the different genera of all these classes there are very great differences, determined, 
apparently, by the following considerations, amongst others not so obvious. When 
the creature lives in feeble light, yet moves actively about, and is guided in its loco- 
motion by the sense of sight, as in nocturnal birds and animals and fish, the eye is 
very large, apparently to take in a large quantity of the feeble light; on the con- 
_ trary, where the creature is guided in its movements by other senses, then the eye 
is very small, as in the bat, the mole, the shrew, and the eel. Where vision pene- 
trates to a long distance, and where the eye enjoys great power of overcoming the 
aberration of parallax, the eye is large, as in rapacious birds. When the brain and 
intellect are more developed, the size of the eye diminishes, and the two eyes become 
more parallel, as in man and the higher mammalia. Where animals are feeble, 
timid, have but little defensive power, and are preyed upon, the eye is usually very 
large,—as in the hare, the conies, the whole deer tribe, and many of the other rumi- 
nants. Where the animal is not predaceous, and its size and strength are such as 
to protect it from being preyed upon, the eyes are commonly small,—as in the whale 
and the elephant: in the latter the eye is even smaller than it is in the horse, and 
searcely larger than in the eagle. 
On the Structure of the Retina at the Punctum Centrale, or Foramen of 
Semmering. By T. NuNNELEy. 
The author defined this punctum centrale retina as a small dark circular spot, ,};th 
of an inch large, situated exactly in the axis of the eye, and ;',th of an inch external 
to the entrance of the optic nerve in the living human eye; after which he proceeded 
in great detail to give what he considered to be the correct anatomy of this part of 
the retina. 
On the Structure of the Choroid Coat of the Eye, and more particularly on 
the Character and Arrangement of the Pigmentary Matter. By T. 
NuNNELEY. 
The choroid coat is the dark tissue interposed between the delicate sentient retina 
and the hard, dense sclerotic, anil co-extensive with the latter. It begins at the 
entrance of the optic nerve by a round aperture, with a distinct edge, in close appo- 
sition with the nerve, but not organically connected with it, and passing forward as 
far as the junction of the sclerotic and cornea, where, as choroid proper, it termi- 
nates. It there comes in connexion with the ciliary circle or muscle, the ciliary body 
and the iris. The choroid is essentially a vascular membrane, being made up of 
blood-vessels, colouring matter, and a modified white fibrous tissue. The choroid 
universally pervades the pigmentary nigrum, and is of a deep bronze colour, ap- 
proaching to black, The pigment was described as consisting of two distinct forms 
