THE ORGAN OF SIGHT. 



413 



Through the cornea can be seen the yellow curtain-like iris, 

 with an opening in its centre, the pupil. 



Coats of the Eye (Fig. 167). The outer coat of the eye- 

 ball is formed by the sclerotic (or sclera) (b) and the cornea (c). 

 The sclerotic is the whitish opaque portion covering the caudal 

 three-fourths of the eye. It forms a tough sac serving as a 

 protection for the structures within. The cornea (c] is the cir- 

 cular transparent portion of the outer coat, covering the free 

 surface of the eye; it is a direct continuation of the sclerotic. 

 Just before passing into the cornea the sclerotic is much thick- 

 ened, forming a broad white zone (^') about the eyeball near 

 its equator. To this zone, at its caudal margin, are attached 

 the rectus muscles of the eyeball. The sclerotic has many 

 pigment-cells on its inner surface, giving it a brownish appear- 

 ance and forming the lamina fusca. 



The cornea (c) is transparent and more strongly cuived 

 than the sclerotic. It is thicker than 

 most of the sclerotic coat, but not so 

 thick as the white zone of the latter. 



Within the sclerotic is the second 

 coat of the eye, formed by the choroid 

 (d) and the iris (e). The choroid (d) 

 is a vascular layer, and contains much 

 pigment, giving it a dark color. It 

 lines the inner surface of the sclerotic, 

 but does not cover the inner surface of 

 the cornea. At the white zone of the 

 sclerotic it is thrown into a large num- 

 ber (about seventy) of meridional folds, (*' thickened white zone"); 



c, cornea; a, choroid; e, ins; 

 the pliC35 ciliares or Ciliary folds, which /, ciliary body; g, retina; A, 



together constitute the so-called ciliary crvstal ! ine le "?5 '' >nula cili- 



* ans; j, anterior chamber of 

 t>0dy (COrpUS Ciliare) (/}. This forms eye; A, vitreous humor. 



a circular zone on the inner surface of the white zone of the 

 sclerotic, and is continuous with the iris (e}. The choroid coat 

 is furnished in certain parts of its area with a layer of cells 

 containing crystals, which give it a brilliant color, forming the 

 tapetum. The color of the tapetum is a metallic yellowish 



FIG. 167. DIAGRAM OF A 



SECTION OF THE EYE. 

 rt, optic nerve; b, sclerotic 



