116 



TOPOGRAPHICAL ANATOMY OF THE 



be expressed as beiug in the proportion of 5 : 4'5 : 4'25 : 8. Though 

 the curves of the cornea and sclera are not the same in every direction 

 or in every part, the difference between them may be indicated by 

 saying that, on the average, the radius of the curvature of the cornea is 

 about 17 mm., while that of the sclera is about 25 mm. Where the 

 cornea and sclera meet there is a shallow sulcus ^clenv on the surface 

 of the eyeball. 



For descriptive purposes it is found convenient to employ certain 

 terms, the anterior and posierior poles of the eyeball (polus anterior : 

 polus posterior) are the central points of the anterior and posterior 

 curvatures respectively. They are joined by an imaginary line, the 

 optic axis (axis optica), the shortest distance between the two poles, 



Ciliary processe 

 Ciliary 



Cornea. - 



M. rectus superior. 



1 M. retractor oculi. 



Papilla of optic nerve. 



M. rectus inferior. M. retractor oculi. 



Fig. 48. — Sagittal section of the eyeball, 

 and by meridians (meridiani) that lie evenly on the surface of the 

 eyeball and cut the equator at right angles. The equator (equator) is 

 a line drawn on the surface equidistant between the two poles. The 

 optic axes of the two eyes diverge at an angle of about 187 degrees. 



The eyeball is composed of three concentric tunics enclosing three 

 refracting media. The tunics are: — (1) An outer, dense, fibrous coat 

 (tunica fibrosa oculi) consisting of the opaque sclera posteriorly and the 

 transparent cornea anteriorly ; (2) a middle vascular and pigmented 

 envelope (tunica vasculosa oculi) comprising a posterior and more 

 extensive portion, the choroid (chorioidea), an intermediate ciliary body 

 (corpus ciliare), and an anterior diaphragm or iris ; and (3) an internal 

 and thin nervous tunic, the retina. The refracting media are : — 

 (1) The most posterior and most voluminous, the vitreous body (corpus 

 vitreum), of jelly-like consistence ; (2) the crystalline lens (lens 

 crystallina), the most dense of the media ; and (3) an aqueous humour 

 that occupies the most anterior part of the eyeball. 



