THE SENSES. 585 



comes continuous with the mucous lining of the lachrymal sac and duct, 

 which again is continuous with the mucous membrane of the inferior 

 meatus of the nose. 



The lachrymal gland is lodged in the upper and outer angle of the 

 orbit. Its secretion, which issues from several ducts on the inner sur- 

 face of the upper lid, under ordinary circumstances just suffices to keep 

 the conjunctiva moist. It passes oat through two small openings (puncta 

 lachrymalia) near the inner angle of the eye, one in each lid, into the 

 lachrymal sac, and thence along the nasal duct into the inferior meatus 

 of the nose. The excessive secretion poured out under the influence of 

 any irritating vapor or painful emotion overflows the lower lid in the 

 form of tears. 



The eyelids are closed by the contraction of a sphincter muscle 

 (orbicular is), supplied by the Facial nerve; the upper lid is raised by the 

 Levator palpebrce superioris, which is supplied by the Third nerve. 



The Eyeball. 



The eyeball or the organ of vision (Fig. 390) consists of a variety of 

 structures which may be thus enumerated: 



The sclerotic, or outermost coat, envelops about five-sixths of the 

 eyeball: continuous with it, in front, and occupying the remaining sixth, 

 is the cornea. Immediately within the sclerotic is the choroid coat, and 

 within the choroid is the retina. The interior of the eyeball is well-nigh 

 filled by the aqueous and vitreous humors and the crystalline lens; but, 



Ciliary muscle- 

 Ciliary process- 

 Canal of Petit- 

 Cornea 

 Anterior chamber- 



Ciliary 



Ciliary muscle 



FIG. 390 



also, there is suspended in the interior a contractile and perforated cur- 

 tain the iris, for regulating the admission of light, and behind the 

 junction of the sclerotic and cornea is the ciliary muscle, the function 

 of which is to adapt the eye for seeing objects at various distances. 



