20 OPHTHALMOLOGY FOR VETERINARIANS 



as the cones only exist in the fovea, while the rods 

 become more abundant from the border of the macula 

 lutea to the ora serrata, while the cones diminish. 



The retinal vessels enter through the center of the 

 optic nerve, expanding and dividing, much Hke the twigs 

 of a tree, through the retinal tissue to the ora serrata 

 without anastomosing. These afford nourishment to 

 the inner layers, while the outer layers are dependent 

 upon the small vessels of the inner layer of the chorioid 

 for nourishment. The retinal vessels can be distinctly 

 seen by the aid of the ophthalmoscope. 



The cornea comprises about one-sixth of the ex- 

 ternal anterior portion of the eyeball. It is a perfectly 

 transparent body, and one of the refractive media next 

 in importance to the lens. The anterior surface is con- 

 vex and the posterior is concave. 



It is composed of five layers. From without inward, 

 they are: The epithelial layer, Bowman's membrane, 

 the stroma or cornea proper, Descemet's membrane, 

 and the endothelium. The epithelium is composed of 

 columna, polyhedral and squamous cells, and is con- 

 tinuous with the conjunctiva. Bowman's membrane 

 is devoid of cells, and is described as an elastic homo- 

 geneous membrane. It is strongly adherent to the 

 stroma. The stroma is the thickest layer, and con- 

 sists of numerous fibrous layers enclosing corpuscles 

 similar in structure closely connected by a cement 

 substance. The corneal cells proper are fixed non- 



