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 laminae 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 CRAMPTON 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 

 [ercise ? The manner in which the effort is made, leads 



sistibly to the belief, that the straight muscles are ex- 

 ited f o action, and may either act upon the sclerotica or 



ica, .^cording to the conditions of these objects, or upoiv 



Annals of Philosophy, i. p. 1 73. 



