CHAPTEE LVIII 



THE EYE AND VISION 



THE eyeball is contained in the cavity of the skull called the orbit ; 

 here also are vessels and nerves for the supply of the eyeball, 

 muscles to move it, and a quantity of adipose tissue. In the front 

 of the eyeball are the lids and lacrimal apparatus. 



The eyelids consist of two movable folds of skin, each of which is 

 kept in shape by a thin plate of fibrous tissue called the tarsus. 

 Along their free edges are inserted a number of curved hairs (eye- 

 lashes), 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. Imbedded in the tarsus are a number of long 

 sebaceous glands (Meibomian), the ducts of which open near the free 

 edge of the lid. In the loose connective tissue in front of the 

 tarsus, the bundles of the orbicularis muscle are situated. 



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 continuous with the mucous 

 membrane of the nose. 



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 superioris, supplied by the third nerve. 



The lacrimal gland, composed of 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, 



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