STRUCTURE OF THE EYE. 147 



Removing these parts, we come next to a membrane lying 

 in its folded state, Fig. 4-, along the upper and fore edge of the 

 eye, but capable of being stretched out so as entirely to cover 

 the external surface of the organ. Fig. 3 represents this organ, 

 the Membrana nictitans, which is formed by a reduplication of 

 the tunica conjunctiva, and is frequently designated by authors 

 as a third eyelid. The lower eyelid turned down is seen at a ; 

 the posterior bony edge of the orbit at h, the eyeball covered 

 by the conjunctiva at c, the cornea, through which are seen 

 the iris and pupil, at d. The nictitant membrane, e f g, is 

 drawn over two-thirds of the eye. It is seen to advance back- 

 wards in an oblique manner, with a semicircular edge, and 

 having at its lower part a very slender tendon, which passes 

 over the edge of the eyeball, in a sheath. But to understand 

 this mechanism, we must remove the eye, carefully separating 

 its muscles. In Fig. 6 is seen a small triangular muscle, «, 

 named the pyramidalis, arising from the lower and fore part 

 of the back of the eyeball, and of which the tendon curves over 

 the optic nerve, and passing downwards over the edge of the 

 eyeball, ascends in front, and is found to be that of the nicti- 

 tant membrane, wdiich we were tracing. This position of the 

 pyramidalis is perhaps rendered necessary by the great extent 

 of the movement of the nictitant membrane, the tendon of 

 wdiich, unless curved, would be too long to be conveniently 

 disposed of. But the optic nerve, being immediately below 

 the curve of the tendon, would be liable to be injured by it, 

 were it not for a peculiar contrivance. A broad thin muscle, 

 b, of a somewhat square shape, and therefore named quadratus^ 

 arises from the upper part of the posterior surface of the eye- 

 ball, proceeds downwards and backwards, and terminates in a 

 thin edge, of ligamentous tissue, in which is a sheath, for the 

 reception of the tendon of the pyramidalis. The quadratus 

 muscle acting simultaneously with the pyramidalis, the tendon 

 of the latter is carried in an arch quite clear of the optic nerve. 

 The nictitant membrane being attached by its upper and ante- 

 rior margin to the eyeball, and folding up in consequence chiefly 

 of its elasticity, and partly by the action of some muscular fibres, 

 forms no impediment to the motions of the eyeball. To bring 



