NATURAL THEOLOGY. 135 



eye, which it does in an instant ; to spread over it 

 the lachrymal humor ; to defend it also from sudden 

 injuries ; yet not totally, when drawn over the pupil, 

 to shut out the light. The commodiousness with 

 which it lies folded up in one 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, is the manner in which it performs its 

 office. It is an elastic substance, like Indian rubber, 

 and when drawn out, returns of its own accord, thus 

 saving the necessity of a muscle for this purpose. 



We are next to notice what the French Academi- 

 cians call the marvellous mechanism, by which this 

 membrane is drawn over the eye. There is a string 

 or tendon attached to the edge of the curtain, and 

 which is connected with a muscle in the back part of 

 the eye. When the muscle exerts itself, the mem- 

 brane, by means of the communicating thread, is 

 instantly drawn over the eye ball. But what is so 

 deservedly called marvellous in this construction, is 

 this. " The muscle which pulls jthe membrane, is 

 , passed through a loop fyrmed by another muscle ; 

 and is there inflected, as if it were tound a pulley. 

 This is a peculiarity, and observe the advantage of 

 it. A single muscle with a straight tendon, which is 

 the common muscular form, would have been suffi- 

 cient, if it had the power to draw far enough. But 

 the contraction necessary to draw the membrane over 

 the whole eye, required a longer muscle than could 

 lie straight at the bottom of the eye. Therefore, in 



