CHAPTER XXIII 

 THE ORGANS OF SPECIAL SENSE 



The Organ of Vision 



The eyeball and optic nerve constitute the organ of vision. To 

 be described in connection with them are the eyelid and the lacrymal 

 apparatus. 



The Eyeball or Bulbus OcuU.- — This is almost spherical, although 

 slightly flattened antero-posteriorly. It consists of a wall enclosing 

 a cavity filled with fluid. 



The wall of the eyeball consists of three coats: (a) An external 

 fibrous coat — the sclera and cornea; (b) a middle vascular — the cho- 

 rioid; and (c) an internal nervous — the retina (Fig. 366). 



The Sclera (Figs. 366 and 367).- — This consists of dense fibrous 

 tissue with some elastic fibres. The fibres run both meridionally 

 and equatorially, the tendons of the straight muscles of the eyeball 

 being continuous with the meridional fibres, those of the oblique 

 muscles with the equatorial fibres. The few cells of the sclera lie 

 in distinct, very irregular cell spaces, and frequently contain pigment 

 granules. Pigmented cells in considerable numbers are regularly 

 present near the corneal junction, at the entrance of the optic nerve, 

 and on the inner surface of the sclera. Where the optic nerve pierces 

 the sclera, the continuity of the latter is broken by the entering nerve 

 fibres, forming the lamina cribosa (Fig. 375). The pigmented layer 

 of the sclera next the chorioid is known as the lamina fusca, and is 

 lined internally by a single layer of flat non-pigmented endothehum. 

 Anteriorly a loose connective tissue attaches the sclera to the scleral 

 conjunctiva. 



The Cornea (Figs. 368 and 371). — This is the anterior continua- 

 tion of the sclera so modified as readily to allow the light to pass 

 through it. It is about i mm. thick and consists of five layers, which 

 from before backward are as follows (Fig. 368) : 

 (i) Anterior epithelium. 



(2) Anterior elastic membrane or membrane of Bowman. 



(3) Substantia propria corneae. 



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