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portion, and which portion is chiefly employed in winking. 

 The action of these nictitating muscles is very obvious ; if the 

 orbicularis contract, they may act at the same time, and so 

 the front of the eye will be completely covered ; or if they 

 act independently of it, as no doubt they do, it is plain that 

 the two muscles will draw the middle eye-lid directly out- 

 wards, that is, in the diagonal of the two lines which they 

 respectively take, and if, at the same moment, the eye be 

 slightly adducted, the greater portion of the forepart of the 

 globe will be swept over by the membrane ; it is also obvious 

 that if these muscles act singly or alternately, the nictitating 

 membrane will be moved obliquely, and in varied directions, 

 according as the condition of the cornea may require. When 

 the nictitating muscles cease to act, the strong elastic tissue 

 attached to the end of the pedicle will immediately retract 

 the eye-lid, by drawing the cartilage inwards and backwards. 

 The mechanism, therefore, of this apparatus, consists of 

 two muscles and an elastic ligament, the antagonist to both. 

 There would be no use in a retractor muscle, as has been 

 erroneously described, nor is there, indeed, sufficient space for 

 such a muscle to contract and shorten itself to the requisite 

 degree. I do not find in any animal with a membrana nic- 

 titans that there is a retractor muscle, and this fact led me 

 to doubt the accuracy of the descriptions in the different 

 works already alluded to : elasticity is not only sufficient, but 

 is actually superior for this purpose, inasmuch as it is a force 

 in constant operation, thereby retaining the lid in its retracted 

 position, except at the moment when the nictitating muscles 

 are in action, and when this fold forms a transient covering 

 to the cornea, almost as perfect as the membrana nictitans 

 in the bird. In the latter, however, a different, though a 

 beautiful, mechanical arrangement has been adopted ; the 

 membrane, which is to serve as an occasional screen to the 

 eye, as well as a rapid nictitating membrane, is delicate and 

 semitransparent, and therefore devoid of cartilage ; it is moved 



VOL. IV. o 



