A STUDY OF THE SENSES 299 



the eyelid moves automatically at frequent intervals over 

 the surface of the eye, the exposed portion of the eyeball is 

 kept from drying. 



In the lower, inner corner of each eye one can detect a 

 small elevation or papilla, in the center of which is an open- 

 ing. This is the beginning of a tear 

 duct which drains off into the nose 

 cavity the excess of tears. An upper 

 branch of this duct is shown in 

 Fig. 135. If these tubes become 

 stopped, the tears drop down upon 

 the cheeks. This is also true when 

 an increased quantity of liquid is FlG . 135. _ Front View of 

 secreted in weeping. Left Eye, with Eyelid 



Sebaceous Glands. Along the partly removed. 



edge of both lids are numerous oil ^D = tear duct. 



LG = tear gland, 

 glands. Under ordinary conditions 



their fatty secretions prevent the tears from flowing out of 

 the eyes. At times these sebaceous glands send out an 

 abnormal amount of secretion. If this dries, it forms a yel- 

 lowish rim on the edge of each lid, which may temporarily 

 glue the closed eyelids together. 



Movements of the Eyes. When I hold my head still and 

 look at the middle region of the opposite wall of the room, I 

 find I can, by moving my eyes, look at the ceiling, at the 

 floor, out of a window on either of the side walls ; I can, 

 also, look to each of the four corners of the walls in front of 

 me. All these movements of the eyeballs are accomplished 

 by the separate or combined action of the six muscles which 

 extend from each eyeball to the walls of the eye socket. 



Experience tells me that it is impossible for me to look 

 in one direction with my right eye and in the opposite direc- 

 tion with my left. The two eyes, therefore, work together 

 as one, and all the motions we have been describing are con- 

 trolled by impulses sent out from the brain along the third, 

 fourth, and sixth pairs of nerves (see p. 279). 



