32 NATURAL THEOLOGY 



what is equally admirable, though not quite so obvious, is 

 the combination of two kmds 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 ordi- 

 nary cases, by the action of two antagonist muscles — the one 

 pulling forward 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 be- 

 ing its nature, in order to fit it up for its office,' it is connect- 

 ed, by a tendon or thread, with a muscle in the back part of 

 the eye : this tendon or thread, though strong, is so fine a? 

 not to obstruct the sight even when it passes across it ; and 

 the muscle itself being placed in the hack part of the eye, 

 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 position that could be assigned to it in the 

 anterior part of the orb, where its function lies. "When the 

 muscle behind the eye contracts, the membrane by means 

 of the communicating thread is instantly drawn over the 

 fore part of it. "VYhen the m.uscular 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 to its position.^ Does not this, if any thing can 

 do it, bespeak an artist, master of his work, acquainted with 

 his materials ? " Of a thousand other things," say the French 

 academicians, " we perceive not the contrivance, because wo 

 understand them only by their effects, of which we know not 

 the causes ; but we here treat of a machine, all the parts 

 whereof are visible, and which need only be looked upon to 

 discover the reasons of its motion and action."! 



* Philosophical Transactions, 1796. 



\ Memoirs for a Natural History of Animals, by the Royal Acad- 

 emy cf Sciences at Paris, done into English by order of the E,oyaJ So 

 aieiy, 1701, p. 249. 



