THE SCLERA AND CORNEA 



1091 



follow the direction of the axes of the orbits and are therefore not parallel ; each 

 nerve either leaves or connects with its eyeball about 1 mm. below and 3 mm, 

 to the inner or nasal side of the posterior pole (Fig. 796). The eyeball measures 

 rather more in its transverse and antero-posterior diameters than in its vertical 

 diameter, the former amounting to about 24 mm. (nearly an inch), the latter to 

 about 23.5 mm. (nine-tenths of an inch). The diameters in the female are some- 

 what less than in the male. At birth the eyeball has a transverse diameter of 

 about 17.5 mm., while at puberty it measures from 20 to 21 mm. 



The eyeball is composed of three investing tunics and of three main refracting 

 media, 



POSTERIOR CHAMBER 



FIG. 797. Diagram of a horizontal section of the right eye, showing the upper surface of the lower 



segment. (Testut.) 



THE TUNICS OF THE EYE. 



From without inward the three tunics are: 



I. Sclera and Cornea. 

 II. Choroid, Ciliary Body, and Iris. 

 III. Retina, 



I. The Sclera and Cornea (Tunica Fibrosa Oculi). 



The sclera and cornea (Figs. 797 and 798) form the external tunic of the eyeball; 

 they are essentially fibrous in structure, the sclera being opaque, and forming 

 the posterior five-sixths of the globe; the cornea, which forms the remaining sixth, 

 is transparent. 



