CHAPTER III. 



Tab. IV. — THE LACHRYMAL APPARATUS AXD NICTITATUTG 

 MEMBRANE. 



Fig. 1. a, the lachrymal gland, the source of the tears ; &, its sev- 

 eral ducts, diffusing this fluid over the eye ; c, c, the puncta lachry- 

 malia, which convey the tears into the lachrymal sac, terminating 

 in the nostril. 



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 muscles of the nictitating membrane are very singular 

 in their form and action, they are attached to the back of the sclerotic; 

 one of them, a, which from its shape is called quadratus, has its ori- 

 gin from the upper and back part of the sclerotic ; its fibres descend 

 towards the optic nerve, and terminate in a curved margin with a cy- 

 lindrical canal in it. The other muscle, h, which is called pyramida- 

 lis, arises from the lower and back part of the sclerotic. 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. 



