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Plate 4: The Lachrymal Apparatus and Nictitating Membrane

Fig. 1. a, is the organ which supplies this fluid, called the lachrymal gland, it is situated at the outer and upper part of the orbit of the eye. This is the gland which secretes or separates the tears from the blood. There are five or six ducts or tubes, b, which convey this fluid to the globe of the eye, for the purpose of keeping it moist, and for facilitating its movements; the motion of the eyelid diffuses the tears, and c, c, the puncta lachrymalia, take up the superfluous moisture, which passes through d, the lachrymal sac and duct into the nostril at e.

Fig. 2. The nictitating membrane, or third eyelid; it is a thin semi-transparent fold of the conjunctive, which, in a state of rest, lies in the inner corner of the eye, with its loose edge nearly vertical, but can be drawn out so as to cover the whole front of the globe. In this figure it is represented in the act of being drawn over the eye. By means of this membrane, according to Cavier, the eagle is enabled to look at the sun.

Fig. 3. The two muscles of the nictitating membrane are very singular in their form and action; they are attached to the back of the sclerotica; one of them, a, which from its shape is called quadratus, has its origin from the upper and back part of the sclerotica; its fibres descend towards the optic nerve, and terminate in a curved margin with a cylindrical canal in it. The other muscle, b, which is called pyramidalis, arises from the lower and back part of the sclerotica. It has a long tendinous chord, c, which passes through the canal of the quadratus, a, as a pulley, and having arrived at the lower and exterior part of the eye-ball, is inserted into the loose edge of the nictitating membrane. This description refers also to Fig. 4, a profile of the eye, and Fig. 5, the membrane and its muscles detached from the eye.