THE EYE. 445 
surface too convex, and thus alter the focus of the sight. This is the 
“‘buck-eye,” which leads to shying, and is perfectly incurable. The 
membranes are, first, the cornea, a perfectly transparent coat, placed 
in front of the eye, and inserted, ‘like a watch- glass, in the sclerotic coat 
covering the posterior four-fifths of the globe. The latter is a white 
fibrous ‘membrane, strong and inelastic, so as to afford protection to the 
parts within it from external violence. This forms the white of the eye, 
which, however, is only occasionally visible in the horse. Beneath the 
sclerotic is the choroid coat, consisting of a network of blood vessels, 
and lined with a black pigment, which again has on its internal surface, 
at the part opposite the pupil, a greenish. white iridescent lining, called 
tapetum lucidum, or luminous carpet. Lastly, within the whole of this 
surface is spread a beautiful expansion of the optic nerve, called the 
retina, which receives the impressions derived from the rays of light, 
forming a distinct figure upon it exactly similar to the objects which are 
presented to it, except in point of size, and in being inverted. Beyond 
these parts, there is a provision made for moderating the rays of light, 
according to their intensity. This is effected by means of an opaque 
septum, pierced with an oval hole; the former being called the iris, and 
the latter the pupil. The substance of the iris itself is composed of con- 
tractile tissue, which has the power of expanding or contracting the pupil 
in obedience to the impression produced upon the retina; and thus, if the 
eye is examined in a strong light, the pupil will appear large when shaded 
by the hand, but contracts immediately on exposing the eye. The horse’s 
iris is brown, varying somewhat in shade in different individuals, and at 
the upper part of the pupil it presents one or two little floating appen- 
dages, which serve to moderate the sun’s rays. Sometimes the brown 
colour is absent, and the iris is either partially or entirely white, in which 
case it is called a ‘‘ wall eye ;” but though this is considered unsightly, it 
does not interfere with vision. The iris is stretched across the chamber 
of the aqueous humour, and is thus enabled to act freely. There are 
many other delicate structures worthy of being examined, but want of 
space must prevent any further allusion to them. 
THE APPENDAGES of the eye are: Ist. The conjunctiva or here 
protecting the exposed surface of the eye. 2d. The eyelids. 3d. The 
membrana nictitans or haw. 4th. The muscles of the eye. 4th. The 
lachrymal apparatus. The conjunctiva covers the whole front of the eye, 
being thin, and perfectly transparent in a healthy state, but on the occur- 
rence of inflammation speedily becoming red and puffy. Itis reflected from 
this face to the inside of the eyelids, and the whole membrane is extremely 
liable to inflammation from any external irritation. The eyelids have 
nothing very remarkable about them, being merely cartilaginous shutters 
covered with fine skin, and lined with conjunctiva, and raised and 
lowered by muscles peculiar to them. The membrana nictitans or haw is 
a cartilage lying just within the inner corner of the eye, but capable of 
being thrust outwards so as to partially cover it when the muscles retract 
the eye, and for want of space drive it forward. This happens whenever 
the eye is irritated either by an insect or by the dust or hayseeds which 
are so often deposited upon the conjunctiva, and which, causing the eye to 
be drawn back, displace the fat deposited in the back of the orbit, and 
this again pushes forward the haw. For this reason in all irritable states 
of the eye the haw is prominent: but it by no means follows that its 
removal will diminish the irritation ; on the contrary, the usual effect is to 
increase it; and the operation is not only useless, but injurious. The 
