SEC. 6. THE PROTECTIVE MECHANISMS OF THE EYE. 



The eyeball is protected by the eyelids, which are capable of 

 movements called respectively opening and shutting the eye. The 

 eye is shut by the contraction of the orbicularis muscle, carried out 

 either as a reflex or voluntary act, by means of the facial nerve. 

 The eye is opened chiefly by the raising of the upper eyelid, 

 through the contraction of the levator palpebrae carried out by 

 means of the third nerve. The upper eyelid is also raised and the 

 lower depressed, the eye being thus opened, by means of plain 

 muscular fibres existing in the two eyelids and governed by the 

 cervical sympathetic. The shutting of the eye as in winking is in 

 general effected more rapidly than the opening. 



The eye is kept continually moist partly by the secretion of the 

 glands in the conjunctiva, and of the Meibomian glands, but chiefly 

 by the secretion of the lachrymal gland. Under ordinary circum- 

 stances the fluid thus formed is carried away by the lachrymal 

 canals into the nasal sac and thus into the cavity of the nose. 

 When the secretion becomes too abundant to escape in this way it 

 overflows on to the cheeks in the form of tears. 



If a quantity of tears be collected, they are found to form 

 a clear faintly alkaline fluid, in many respects like saliva, contain- 

 ing about 1 p.c. of solids, of which a small part is proteid in nature. 

 Among the salts present sodium chloride is conspicuous. 



The nervous mechanism of the secretion of tears, in many 

 respects, resembles that of the secretion of saliva. A flow is usually 

 brought about either in a reflex manner by stimuli applied to the 



