OH. LV.] THE EYEBALL. 74* 



hairs (eyelashes), which, when the lids are half closed, serve to 

 protect the eye from dust and other foreign bodies : the tactile 

 sensibility of the lids is very delicate. On the inner surface 

 of the tarsus are disposed a number of small racemose glands 

 (Mriljomiaii), the ducts of which open near the free edge of 

 the lid. 



The orbital surface of each lid is lined by a delicate, highly 

 sensitive mucous membrane (conjunctiva), which is continuous 

 with the skin at the free edge of each lid, and after lining the 

 inner surface of the eyelid is reflected on to the eyeball, being 

 somewhat loosely adherent to the sclerotic coat. Its epithelium, 

 which is columnar, is continued over the cornea as its anterior 

 epithelium, where it becomes stratified. At the inner edge of 

 the eye the conjunctiva becomes continuous with the mucous 

 lining of the lacrimal sac and duct, which again is con- 

 tinuous with the mucous membrane of the inferior meatus of 

 the nose. 



The lacrimal gland, composed of several lobules made up of 

 acini resembling the serous salivary glands, 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 lacrimalia) near 

 the inner angle of the eye, one in each lid, into the lacrimal sac, 

 and thence along the nasal duct into the inferior meatus of the 

 nose. The excessive secretion poured out under the influence 

 of an irritating vapour or painful emotion overflows the lower 

 lid in the form of tears. The secretory nerves are stated to be 

 contained in the lacrinml and subcutaneous molar branches of the 

 fifth nerve, and in the cervical sympathetic. 



The eyelids are closed by the contraction of a sphincter muscle 

 (orbicularis), supplied by the facial nerve ; the upper lid is raised 

 by the levator palpebrae superioris, supplied by the third nerve. 



The Eyeball. 



The eyeball or the organ of vision (fig. 559) 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 filled by the aqueous and vitreous humours 

 and the crystalline lens; but, also, there is suspended iu the 



