1 8 OPHTHALMOLOGY FOR VETERINARIANS 



It is composed principally of nerve elements and is 

 practically an expansion of the optic nerve. Its function 

 is to receive the image focused by the refractive media, 

 which is conveyed through the optic nerve to the sight 

 center in the occipital lobes. The retina of the living 

 eye is transparent, while that of the dead eye is opaque. 

 It also loses its transparency in portions undergoing 

 pathologic changes. 



It extends posteriorly from the optic nerve, and ap- 

 parently terminates anteriorly at an irregular Hne, 

 known as the ora serrata, posterior to the apex of the 

 cihary body, but ''the microscope shows that under 

 a similar form it extends still farther, even up to the 

 edge of the pupil. It, therefore. Hues the inner surface 

 of the cihary body and the posterior surface of the 

 iris'^ (Fuchs). 



Corresponding with the central vision at the poste- 

 rior pole hes a small yellowish spot, called the macula 

 lutea, a slight depression in the center of which is the 

 fovea centralis. 



Histologically, the retina is a very complicated 

 structure, and consists from without inward of ten 

 layers: i, A pigment epithelial layer; 2, a layer of rods 

 and cones; 3, an external limiting membrane; 4, an 

 external molecular layer; 5, an external granular layer; 

 6, an internal molecular layer; 7, an internal granular 

 layer; 8, a layer of granular cells; 9, a layer of nerve- 

 fibers; 10, an internal limiting layer or membrane. 



