486 THE HUMAN BODY. 



measures about 22.5 millimeters (^ inch), and from side 

 to side about 25 millimeters (1 inch). Except when looking 

 at nea/ objects, the antero-posterior axes of the eyeballs are 

 nearly parallel, but the optic nerves diverge considerably 

 (Fig. 123); each joins its eyeball, not at the centre, but about 

 2.5 mm. ( T V inch) on the nasal side of the posterior end of 

 its antero-posterior axis. In general terms the eyeball may 

 be described as consisting of three coats and three refract- 

 /- ing media. 



f\J The outer coat^ I and 3. Fig. 124, consists of the sclerotic 

 I Vand the cornea, the latter being transparent and situated in 

 front; the former is opaque and white and covers the back 

 and sides of the globe and part of the front, where it is seen 

 between the eyelids as the white of the eye. Both are 

 tough and strong, being composed of dense connective tis- 

 sue. The white of the eye and the cornea are also covered 

 over by a thin layer of the conjunctiva, 4 and 5. Behind 

 the proper connective-tissue layer, 3, of the cornea is a thin 

 structureless membrane, G, lined inside by a single layer of 

 epithelial cells; it is called the membrane of Descemet, or 

 posterior elastic layer. 



I ty 7 The second coat consists of the choroid, 9, 10, the ciliary 

 \^/ processes, 11, 13, and the iris. 14. The choroid consists 

 mainly of blood-vessels supported by loose connective tissue 

 containing numerous corpuscles^vTrich in its inner layers are 

 richly filled with dark brown or black pigment granules. 

 Towards the front of the eyeball, where it begins to dimin- 

 ish in diameter, the choroid is thrown into plaits, the ciliary 

 processes, 11, 12, 13. Beyond these it continues as the_mSj_ 

 which forms the colored part of the eye which is ..seen 

 through the cornea; and in the centre of this is. a circular 

 X?^ aperture, t\\Q pupil: so the second coat does not, like the 

 outer one, completely envelop the ball. In the iris are 

 two sets of plain muscular fibres; a circular around the 

 margin of the pupil and narrowing it when they contract ; 

 the other set radiate from the inner to the outer margin of 

 the iris and by their contraction dilate the pupil. The 

 pigment in the iris is yellow, or of lighter or darker 

 brown, according to the color of the eye, and more or less 



