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especial muscles, which do not exist in other quadrupeds. 

 One of these seems to draw the nictitating membrane over 

 the eye-ball, and arises from the lower margin of the orbit, 

 towards the outer canthus, while the other, which is the an- 

 tagonist, draws it back again towards the inner angle ;' and 

 again, ' the fibres of the nictitator muscle pass in a regular 

 curve over the base of the membrane, but afterwards deviate 

 from the curve, and form an angle to include the extremity 

 of the nictitating cartilage, which consequently moves in the 

 diagonal of the contracting forces, so as to be drawn outwards 

 over the front of the eye-ball.' The following statement 

 will, I think, be found a more correct description of the ap- 

 paratus, the dissection of which I shall now exhibit and 

 demonstrate to this meeting. 



" The nictitating membrane itself is a large semilunar 

 fold of the conjunctiva, not at all fleshy, but containing a 

 true cartilage, moulded in a peculiar form ; one portion is 

 broad and leaf-like, and very elastic, and extends forward and 

 outward into this conjunctival fold, while the other portion, 

 the stem or pedicle of the cartilage, is thick and round, above 

 an inch in length, and passes inwards and backwards along 

 the inner wall of the orbit ; to its extremity is attached a 

 strong elastic tissue, which extends backwards into the orbit, 

 and is continuous with that which surrounds the muscles and 

 the optic nerve. From the surface of the stem, a little ex- 

 ternal to its extremity, arise two very powerful muscles, 

 which curve outwards to the margins of the orbit ; these are 

 the two proper nictitating muscles. The superior extends 

 from the pedicle of the cartilage, upwards and outwards, and 

 is inserted into the fibrous tissue along the upper margin of 

 the orbit, internal to its centre. The inferior passes obliquely 

 outwards and downwards, and is inserted into the lower mar- 

 gin of the orbit, near its centre ; both these muscles are in 

 close connexion with the fibres of the orbicularis palpebrarum, 

 which latter is very powerful, especially its inferior palpebral 



