38 NATURAL THEOLOGY. 



It were, however, injustice to dismiss the eye 

 as a piece of mechanism, witliout noticing that 

 most exquisite of all contrivances, the nictitating 

 memhrane^ which is found in the eyes of birds, 

 and of many quadrupeds. Its use is to sweep the 

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

 the lacrymal humour ; to defend it also from sud- 

 den injuries ; yet not totally, when drawn upon the 

 pupil, to shut out the light. The commodiousness 

 with which it lies folded up in the upper corner 

 of the e3'e, ready for use and action, and the 

 quickness with which it executes its purpose, are 

 properties known and obvious to every observer ; 

 but vv^hat is equally admirable, though not quite so 

 obvious, is the combination of two 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, the one pulling 

 forward, and the other backward, that a recipro- 

 cal change is effected ; but it is thus : the mem- 

 brane itself is an elastic substance, capable of 

 being drav;n 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 



ratus for the presen-ation of the eye, in the Appendix, there being 

 a crrcat deal that is curious in it hitherto unnoticed. It will be there 

 found that, ahhough the eye of the fish has no eyelid, yet it has the 

 rapid motion of the eye-ball, which, under water, must serve to free 

 it from any impurity. Some curious instances are, at the same 

 time, aiforded, of a still more artificial mode, in the lobster and 

 crab, of removing whatever obstructs the sight. 



