EYEBALL 



533 



General Structure of the Eyeball. The eyeball consists 

 of three concentrically arranged coats enclosing a cavity in 

 wHich three refracting media are placed. 



The coats or tunics are: (i) an external fibrous envelope 

 composed of a posterior, opaque part, called the sclera, and 

 an anterior, clear transparent portion, called the cornea ; (2) 

 a middle vascular envelope, known as the uveal tract, in 

 which three subdivisions are recognised, viz., a posterior part 

 called the chorioid, an anterior pcrtion termed the iris, which 



. Pupil 



Anterior chamber L ens 

 Iris ' ', . 



Posterior chamber 



Ciliary body 

 Corona ciliaris 



Cornea 



Zonula ciliaris 

 / Venous sinus of sclera 

 Conjunctiva 



Vitreous body 



Sclers 



Hyaloid canal 



/ / I-/ ,- . . 



I i <ff Central fovea 



tmj Optic nerve 

 FIG. 226. Diagrammatic section of Eyeball. 



lies posterior to the cornea, and an intervening ciliary body ; 

 (3) the nervous internal tunic or retina, in which the fibres 

 of the optic nerve are outspread. 



The refracting media are: (i) the cornea; (2) posterior 

 to the cornea a watery fluid called the aqueous humour, con- 

 tained in a space partially subdivided by the iris into the 

 anterior and posterior chambers of the eye; (3) the crystalline 

 lens, behind the posterior chamber; and (4) the vitreous body, 

 which occupies the posterior part of the interior of the eyeball. 



Dissection. The superficial surface of the sclera and of the 

 cornea should now be examined ; but to complete the study of 



