ORGANS OF PERCEPTION. 185 
of the sclerotic coat is to preserve the form of the eye, to 
furnish points of insertion or attachment, for the muscles 
which move the eye-ball, and to support the delicate mem- 
branes which line its central surface. At the same time, the 
action of the straight muscles on this organ, when flexible, 
must have a tendency to alter the form of the contained 
humours, and in this manner part of the desired effect may 
be produced. 
The cornea has likewise been considered as the instru- 
ment, by which the eye is able to exercise the power of 
seeing: objects at different distances. This effect has been 
supposed to take place by the elasticity of the lamine of 
the cornea, acted upon by the straight muscles, with which 
it is so intimately united. But the effect produced in this 
manner, is considered by many so small as to be incapable 
of accounting for the display of the power we are now con- 
sidering. More recently, Mr Cramrron has demonstrat- 
ed the existence of a muscle in birds, capable of changing 
the form of the cornea, and which he considers as the organ 
employed to alter the convexity of the eye *. 
When we look first at a near and then at a distant ob- 
ject, or the reverse, we feel that a muscular effort is requir- 
ed for the adjustment of the eye, to the change in the dis- 
tance of the object to which it is directed. This exertion 
becomes very evident, when we look at a spot on one of the 
panes of glass in a window, and then look through the glass 
in the direction of the spot to some distant object, as a tree 
or house. What are those muscles which are called into 
exercise ? The manner in which the effort is made, leads 
irresistibly to the belief, that the straight muscles are ex- 
cited to action, and may either act upon the sclerotica or 
cornea, according to the conditions of these objects, or upon 
* Annals of Philosophy, 1. p. 173. 
