308 THE SENSES 



tecting it from dust. The lids are closed by the orbiculares pal- 

 pebrarum and opened by the levatores palpebrarum superiores. 

 In the ordinary closing of the lids only the upper one is moved, 

 but the lower one is raised in forcible contraction of the orbicu- 

 laris. Intervention of the will is not necessary to the action of 

 these muscles, though they are striated. Except during fatigue, 

 the eyes are kept open involuntarily, but when the cornea is 

 touched no effort of the will can prevent contraction of the or icu- 

 laris palpebrarum. During sleep the globes are rotated upward. 



The Lachrymal Apparatus. This consists of the lachry- 

 mal glands, canal, duct and sac, and the nasal duct. The 

 secretion of the lachrymal gland keeps the cornea and conjunc- 

 tiva constantly bathed in a thin fluid. It is situated in the orbital 

 cavity at its upper and outer portion. Its secretion is discharged 

 upon the conjunctiva by several little ducts. The excess of se- 

 cretion is carried into the riose through the nasal duct. Near 

 the inner canthus is a small opening in each lid; these openings 

 are the orifices of the lachrymal canals, which canals join at the 

 inner angle of the eye to form the lachrymal sac ; the sac is con- 

 tinued below as the nasal duct, opening into the inferior meatus 

 of the nose. The secretion of tears is much diminished during 

 sleep. The influence of the nervous system on lachrymal secre- 

 tion is well known. Emotional disturbances operate through 

 the sympathetic to increase the flow. Irritation of the mucous 

 membrane of the nose or eye is followed by a like result. 



Movements of the Ball. The capsule of Tenon, a fibrous 

 membrane outside the sclerotic, holds the ball loosely in place. 

 A small amount of adipose tissue behind the globe is never ab- 

 sent. Movements of the ball are effected through the action 

 of the internal and external recti, the superior and inferior 

 recti, and the superior and inferior oblique ; of these, all but 

 the two last named arise from the apex of the orbital cavity. 

 The recti are inserted into the sclerotic just back of the cornea. 

 The superior oblique runs along the inner aspect of the orbital 



