PHYSIOLOGY OF THE EYE OF THE HORSE. 515 
while the tapetum lucidum, by means of its bright hue, reflects 
the rays, and brings them more forcibly on the retina. Man 
has no such construction in the choroid coat. The retina is the 
focal point of that membrane on which the picture is painted ; 
and the sensation of its presence is conveyed to the sensorium 
by nervous energy. The cornea is composed of horny plates, 
with fluid interposing, which in the living state is transparent; 
but on death taking place. Sir Charles Bell says evaporation 
occurs; the layers of horn become compressed together, and a 
moisture, which is forced out and congeals on the surface, causes 
that glazy opacity of the cornea which always occurs. 
The aqueous humour preserves the convexity of the cornea, and 
admits the iris to float freely in it; it is also supposed to assist in 
vision, by refracting the rays of light passing through the cor¬ 
nea. The iris regulates the quantity of light necessary in order 
to the formation of a distinct picture on the retina, by contract¬ 
ing or dilating the pupil, as occasion may require. The corpora 
nigra seems to act as an internal curtain, not only stopping, 
but by their pigment absorbing, the rays of light which fall too 
obliquely to be refracted to a distinct and perfect focus. In the 
foetal state there is a membrane extending from one side of the 
iris to the other, called the membrana pupillaris; but, on the 
animal coming into the world, the stimulus of light on the iris, 
which was before motionless, causes it to contract and dilate; 
the membrane thus becomes broken, and disappears by absorp¬ 
tion. The iris ever after is indebted to light for its motion and 
the performance of its natural functions. It is a new stimulus, 
and to the powder of which it responds, as the lungs of the new¬ 
born infant, also previously motionless, begin to obey the in¬ 
fluence of an unaccustomed agent—atmospheric air. The ciliary 
processes discharge a function of a somewhat similar character. 
The crystaline lens, being of a double convex form, is the grand 
refracting power of the eye; it causes the rays which pass 
through it to converge, and would bring them to a focus imme¬ 
diately behind it. The vitreous humour, which is of less density 
than the lens, while it fills up and maintains the globular form 
of the eye, modifies the conveyance of the rays, so that the 
focus is actually within the vitreous humour, and a little anterior 
to the retina. Then the rays decussate, they diverge and fall 
upon the retina, so as to produce an image in an inverted po¬ 
sition. 
It was Dr. Young’s opinion, that the crystaline lens was mus¬ 
cular and fibrous in its structure; and that it had the power of 
altering its degree of convexity, and accommodating itself to 
near or distant objects. Sir Everard Home and .lohn Hunter 
agree with him. 1 confess that I do not coincide witli this doc- 
