454 



THE SPECIAL SENSES 



empties it into the nasal passages. Not only does this 

 fluid aid in lubricating the surface of the ball and eyelid, 

 but in passing continuously across the exposed part of the 

 eyeball it removes from the surface dust or other foreign 

 particles which may tend to collect there. Sometimes this 

 fluid is poured forth very copiously, as when the membrane 

 covering the eyeball is irritated, or the person is under strong 

 emotion. Under such circumstances the ducts cannot 

 drain it off fast enough, and it accumulates and rolls over 

 the edges of the lids down the cheeks. Under ordinary 

 conditions of flow, the oily secretion at the edge of the lids 

 is sufficient to prevent its escape in this way. 



Back of the lids the eyeball itself rests upon a fatty layer 

 which lines the orbit. Connected to it is the optic nerve 

 which enters the eyeball from the back through an opening 

 in the socket. This nerve carries the visual fibers to their 

 centers in the brain. The eyeball is also held in place and 

 moved by six muscles. These muscles are controlled by 

 motor impulses sent from the brain along the fibers of the 

 third, fourth, and sixth cranial nerves. The attachment of 



these muscles shows an ingenious 

 mechanical adaptation to the 

 structure of the orbit. (See Fig. 

 214.) Four (rectus muscles) are 

 straight muscles attached at one 

 end to the ej'-eball and at the 

 other to the bottom of the eye 

 socket. Two (the internal and 

 external rectus) are attached to the 

 sides of the eyeball, and, by contracting and relaxing alter- 

 nately, turn the eye toward or away from the nose. Two 

 (the inferior and superior rectus) are attached to the lower 



Fie. 214 — Muscles of the eye- 

 ball. 



