THE SPECIAL SENSES 



345 



FIG. 221. 



A Front View of the Left Eye, with 

 the Eyelids partially dissected 

 to show Lachrymal Gland, L. G., 

 and Lachrymal Duct, L.D. t to 

 Nose. 



623. The Tear Gland is located above the eyeball, be- 

 tween the ball and the bony arch on the side toward the 

 temple. It is flattened and oval in shape, about three quar- 

 ters of an inch in length. About ten small ducts lead from 

 it and open on the under side of the 

 upper lid. The secretion that it fur- 

 nishes to the conjunctiva is formed 

 continually. The tears pass across 

 the eye and flow into two small ducts, 

 the openings into which can be seen 

 on the borders of each eyelid near 

 the inner angle of the eye. They 

 lead to a canal which empties into 

 the nasal passage (Fig. 166). When 

 one weeps, why is it necessary to 

 blow the nose frequently ? At the ordinary rate of sup- 

 ply, the tears do not overflow, as there is a waxy secretion 

 along the edge of the eyelid that turns them toward the 

 ducts. When have you noticed a waxy 

 secretion in the corner of the eye ? 



624. The Need of Two Eyes. We 

 judge the distance of objects by the 

 lines of convergence of the two eyes. 

 A boy with one eye has difficulty in 

 knowing when a ball thrown will 

 reach his hand. When we look at 

 FIG. 222.-Diagram of a a solid object, each eye sees a little 

 stereoscope. more of the object on its side than 



Two photographs, A and 5, are doCS the Other. Thus tWO CVCS make 

 seen combined at C. The rays * 



of light from A and B are re- it easier to distinguish solid bodies. 



fracted by the prisms into the ._ . . 



eyes so that they appear to By taking tWO photographs Of 3. SCQUQ 



from slightly different points and 



arranging them so that the eyes look at the pictures 

 separately at the same time, the idea of solidity is given. 

 The stereoscope (Fig. 222) secures this arrangement. 



