40 KATURAL THEOLOGY. 



In the. configuration of the muscle which, though 

 placed behind the eye, draws the nictitating mem- 

 brane over the eye, there is, what the authors just 

 now quoted deservedly call a marvellous mecha- 

 nism. I suppose this structure to be found in other 

 animals ; but, in the memoirs from which this ac- 

 count is taken, it is anatomically demonstrated 

 only in the cassowary. The muscle is passed 

 through a loop formed by another rauscle ; and is 

 there inflected as if it were round a pully. This 

 is a peculiarity, and observe the advantage of it. 

 A single muscle \\'ith a straight tendon, which is 

 the common muscular form, would have been suf- 

 ficient, if it had had power to draw far enough. But 

 the contraction necessary to draw the membrane 

 over the wiiole eye, required a longer muscle than 

 could lie straight at the bottom of the eye. There- 

 fore, in order to have a greater length in a less 

 compass, the chord of the main muscle makes an 

 angle. This so fai' answers the end; but still fur- 

 ther, it makes an angle, not round a fixed pivot, 

 but round a loop formed by another muscle, w hich 

 second muscle, whenever it contracts, of course 

 twitches the first muscle at the point of inflection, 

 and thereby assists the action designed by both.'^ 



^^There is one effect, however, of this apparatus, which our author 

 has omitted to notice — that is, the rapidity of motion in tlie mein- 

 brana nictitans, produced by the obhque direction and junction of 

 the tendons of these muscles, this will be illustrated hereafter. 



The mtmhrana nictitans is peculiar to birds : the term is not ap- 

 plicable to the corresponding structure in quadrupeds, the object 



