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CHAPTER III. 
Plate IV. — the lachrymal apparatus and nictitating 
MEMBRANE. 
Fig. 1. a, is the organ which supplies this fluid, called the lachry¬ 
mal 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 con¬ 
vey 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 su¬ 
perfluous 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 comer of the eye, with its loose edge nearly verti¬ 
cal, 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 quad - 
rains, has its origin from the upper and back part of the sclerotica; 
its fibres descend towards the optic nerve, and terminate in a cur¬ 
ved 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 mus¬ 
cles detached from the eye. 
