24 
APPLICATION OF THE ARGUMENT. 
tinually passing over it. Can any pipe or outlet for cariy- 
ing off the waste liquor from a dye-house or a distillery, be 
more mechanical than this is? It is easily perceived, that 
the eye must want moisture: but could the want of the eye 
generate the gland which produces the tear, or bore the hole 
by which it is discharged,—a hole through a bone? 
It is observable, that this provision is not found in fish; 
the element in which they live supplying a constant lotion 
to the eye. 
It were, however, injustice to dismiss the eye as a piece 
of mechanism, without noticing that most exquisite of all 
contrivances, the nictitating membrane, which is found in 
the eyes of birds and of many quadrupeds. [Plate IV. fig. 
2.] Its use is to sweep the eye, which it does in an in¬ 
stant; to spread over it the lachrymal humour; to defend 
it also from sudden injuries: yet not totally, when drawn 
upon the pupil, to shut out the light. The commodious¬ 
ness with which it lies folded up in the inner corner of 
the eye, ready for use and action, and the quickness with 
which it executes its purpose, are properties known and 
obvious to every observer: but what is equally admirable, 
though not quite so obvious, is the combination of two 
different kinds of substance, muscular and elastic, and of 
two different kinds of action, by which the motion of this 
membrane is performed. It is not, as in ordinary cases, 
by the action of two antagonist muscles, one pulling for¬ 
ward and the other backward, that a reciprocal change is 
effected; but it is thus: The membrane itself is an elastic 
substance, capable of being drawn out by force like a piece 
of elastic gum, and by its own elasticity returning, when 
the force is removed, to its former position. Such being 
its nature, in order to fit it up for its office, it is connected 
by a tendon or thread with a muscle in the back part of 
the eye: this tendon or thread, though strong, is so fine 
as not to obstruct the sight, even when it passes across it; 
and the muscle itself, being placed in the bach part of the 
eye, [Plate IV. fig. 3, 4, and 5,] derives from its situation 
the advantage, not only of being secure, but of being out 
of the way; which it would hardly have been in any posi¬ 
tion that could be assigned to it in the anterior part of the 
orb, where its function lies. When the muscle behind tho 
eye contracts, the membrane, by means of the communi¬ 
cating thread, is instantly drawn over the fore-part of it. 
When the muscular contraction (which is a positive, and, 
most probably, a voluntary effort,) ceases to be exerted, 
the elasticity alone of the membrane brings it back again 
