LACRYMAL ORGANS. 



97 



cepting only the true cetacea, this fold is deve- 

 loped into the membrana nictitans, which is 

 disposed vertically within the horizontal eyelids 

 at the nasal canthus, and is capable of being 

 pushed more or less towards the temporal can- 

 thus, over the front of the eyeball. 



The membrana nictitans derives firmness 

 from a thin plate of cartilage, which has some- 

 times a sort of pedicle passing backwards by 

 the inner side of the eyeball. In the sheep, for 

 example, the cartilage of the membrana nicti- 

 tans is of the shape of the letter T. The cross 

 top forms the margin of the membrane; and the 

 leg, closely embraced by the glandule of Har- 

 der, extends backwards between the eyeball 

 and inner wall of the orbit. The cartilage of 

 the membrana nictitans, unlike the tarsal car- 

 tilages, is true cartilage, with nucleated cor- 

 puscles. 



In the elephant it is said there is a muscular 

 arrangement for carrying the membrana nictitans 

 outwards over the front of the eyeball. I think 

 I have observed in the rabbit that the mem- 

 brana nictitans receives part of the expansion of 

 the levator of the tipper eyelid. The membrana 

 nictitans has not, however, like the third eyelid 

 of birds, any proper muscular apparatus; and 

 when it is moveable, the motion is produced by 

 the eyeball, on its being retracted deeper into 

 the orbit by the retractor muscle, which dis- 

 places and presses forwards the cartilaginous 

 pedicle above described. 



The structure called semilunar fold in man, 

 and membrana nictitans in quadruped mam- 

 mifera, has attained its greatest development 

 in birds, in which it is called the third 

 eyelid. It is transparent and capable of 

 covering the whole front of the eyeball. En- 

 closed in the conjunctival reduplication there is 

 a nbro-cartilaginous structure, very thin and 

 membranous. Of a triangular form, the third 

 eyelid has its free margin oblique from above 

 downwards, and from without inwards. In the 

 state of repose it is retracted and folded verti- 

 cally in the nasal angle of the eye. The third 

 eyelid is drawn over the front of the eye by a 

 very peculiar mechanism consisting of two 

 muscles, the slender tendon of one of which 

 runs through an elongated loop in the broad 

 free end of the other. T?his muscular apparatus 

 is supplied by the nervus abducens. 



The quadrutus is a broad thin trapezoidal 

 muscle. It arises from the upper and posterior 

 part of the eyeball, behind the prominence of 

 its largest circumference. From this point its 

 fibres, which form a thin but broad fleshy belly, 

 descend towards the optic nerve, converging 

 somewhat. It then terminates abruptly in a 

 free tendinous margin, close to the upper part 

 of the optic nerve. In this free tendinous mar- 

 gin, which is considerably narrower than the 

 origin, there is an elongated loop or canal, in 

 which the tendon of the other muscle plays. 



Pyramidalis muscle. The fleshy part of 

 this muscle is comparatively smaller. It arises 

 by a broad curved base from the lower part of 

 the eyeball, opposite the preceding. In its 

 ascent towards the optic nerve, the muscle be- 

 comes contracted, and at last ends in a slender 



VOL. III. 



tendon on the nasal side of the optic nerve- 

 The tendon immediately enters the pulley-canal 

 in the extreme margin of the quadratus, and in 

 traversing it turns round the upper part of the 

 optic nerve. Thus changing its original direc- 

 tion, it passes downwards on the temporal side 

 of the optic nerve to the lower part of the eye- 

 ball, round the prominent circumference of 

 which it turns to get to the front of it, 

 when it immediately enters the lower angle 

 of the third eyelid. Having entered, it di- 

 vides into two parts, one of which expands 

 and runs between the layers of conjunctiva, 

 forming the third eyelid, to the nasal angle of 

 the eye ; the other passes along and forms the 

 pretty firm free margin of the eyelid in question. 

 Having; traversed the whole margin of this and 

 arrived at the upper part of the eyeball, it is 

 inserted into the sclerotica just at the middle of 

 the line whence the quadratus derives its origin. 

 It is by this arrangement that the superior angle 

 of the third eyelid is attached to the sclerotica, 

 and consequently rendered immoveable. 



By its own elasticity the third eyelid remains 

 retracted at the nasal angle of the eye. In this 

 state its tendinous margin is relaxed ; but when 

 the two muscles just described contract, the 

 tendon of the pyramidalis is drawn straight, 

 and the third eyelid is thus stretched over the 

 front of the eye. 



" From this curious disposition of the 

 muscles," says Dr. Porterfield,* "it is easy to 

 conceive, how this internal eyelid is extended 

 over the cornea far enough to cover all the 

 pupil, though the muscles themselves are con- 

 tained in a small space. Every body knows, 

 that the contraction of all muscles is only in a 

 certain given proportion to their length ; and 

 therefore that the eyelid might be drawn far 

 enough over the cornea, nature was obliged to 

 make use of a long muscle, which could not 

 be contained in so small a place as the orbit, 

 without being bent or inflected ; and therefore 

 the one muscle is bent upwards near the optic 

 nerve, making an acute angle, where it passes 

 through the perforated end of the other 

 muscle, by which means its action is greatly 

 increased. But its action is yet more increased 

 by the contraction of the square muscle itself, 

 which must draw the cord or tendon of the py- 

 ramidal muscle which passes through it, through 

 a space double of what it moves itself; and 

 thus the membrana nictitans is extended far 

 enough to cover the whole cornea though its 

 muscles are contained in a small space." 



In Owls Nitzsch discovered a small bone on 

 the lower surface of the bony ring of the 

 sclerotica, ossiculum tubercula?*e,for the support 

 of the long tendon of the pyramidalis. In par- 

 rots the third eyelid is small. 



Among reptiles there is in chelonia and 

 lizards a third eyelid much the same as in 

 birds, but smaller and less moveable. It is 

 moved only by a single muscle analogous to 

 the pyramidalis of birds. "An allied struc- 

 ture," says Miiller, " is a spectacle-like trans- 

 parent part in the lower eyelid of some lizards, 



* On the Eye, vol. i. p. 34. Edinburgh, 1759. 



ii 



