694 HANDBOOK OF PHYSIOLOGY. 



to protect the eye from dust and other foreign bodies: their tactile sen- 

 sibility is also very delicate. 



On the inner surface of the elastic tissue are disposed a number of 

 small racemose glands (Meibomian), whose ducts 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. The epithelial layer is continued over the cornea at its 

 anterior epithelium. At the inner edge of the eye the conjunctiva 

 becomes 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, 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 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 secretions 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 

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

 levator palpebrce super ioris, which is supplied by the third nerve. 



The Eyeball. 



The eyeball or the organ of vision (fig. 406) 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, also, there is suspended in the interior a contractile and per- 

 forated curtain, the iris, for regulating the admission of light, and 

 behind at 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. 



Structure of the Sclerotic Coat. The sclerotic coat is composed of 



