The membrana nictitans and third eyelids.— In the quadrumana, as in man, the conjunc tiva forms at the nasal canthus a simple semi lunar fold, but larger. In the other mammi fera, including the herbivorous cetacea, and ex cepting only the true cetacea, this fold is deve loped into the membrane 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 membrane 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 membrane 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 membrane 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 membrane nictitans outwards over the front of the eyeball. I think I have observed in the rabbit that the mem brane nictitans receives part of the expansion of the levator of the upper 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 membrane nictitans in quadruped mem 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 fibro-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. This muscular apparatus
is supplied by the nervus abducens.
The quadrates 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.
Pyramidatis muscle.—The fleshy part of his muscle is comparatively smaller. It arises )3r a broad curved base from the lower part of he eyeball, opposite the preceding. In its scent towards the optic nerve, the muscle be omes contracted, and at last ends in a slender 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 pe 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.