THE SENSES. 



197 



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

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

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

 conjunctiva moist. It passes out 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 pain- 

 ful emotion overflows the lower lid in the form of 

 tears. 



The eyelids are closed by the contraction of a 

 sphincter muscle (orUcularis) , supplied by the 

 Facial nerve; the upper lid is raised by the Levator 

 palpebrcB superioris, which is supplied by the 

 Third nerve. 



THE EYEBALL. 



The eyeball or the organ of vision (Fig. 365) 

 consists of a variety of structures which may be 

 thus enumerated: 



The sclerotic, or outermost coat, envelopes 

 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, 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 delicate sub-epitheiiai plexus, 



and sending up fine twigs be- 



and cornea is a ciliary muscle, the function of twee . n the epithelial ceils to 



J end in a second plexus on the 



which is to adapt the eye for seeing objects at 

 various distances. 



Structure of Sclerotic. The sclerotic coat 

 is composed of connective tissue, arranged in vari- 

 ously disposed and inter-communicating layers. It 

 is strong, tough, and opaque, and not very elastic. 



Structure of Cornea. The cornea is a transparent membrane which 

 forms a segment of a smaller sphere than the rest of the eyeball, and is 

 let in, as it were, into the sclerotic with which it is continuous all round. 

 It is coated with a laminated anterior epithelium (a, Fig. 367) consisting 

 of seven or eight layers of cells, of which the superficial ones are flattened 



FIG. 366. Vertical sectionof 

 rabbit's cornea, stained with 

 gold chloride, e, Laminated 

 anterior epithelium. Imme- 

 diately beneath this is the an- 

 terior elastic lamina of Bow- 

 man. n, Nerves forming a 



free surface; d, Descemefs 

 membrane, consisting of a 

 fine elastic layer, and a single 

 layer of epithelial cells; the 

 substance of the cornea, /, is 

 seen to befibrillated, and con- 

 tains many layers of branched 

 corpuscles, arranged parallel 

 to the free surface, and here 

 seen edgewise. (Schofield.) 



