THE SENSE OF SIGHT. 241 



6. Molecular layer. 



7. External granular layer. 



8. Membrana limitans externa. 



9. Jacobson's membrane, or layer of rods and cones. 

 10. The layer of pigment cells. 



The most important of these, however, is the layer of rods and 

 cones in the external portion of the retina. The rods are straight, 

 elongated cylinders extending through the entire thickness of Jacob- 

 son's membrane. They consist of an external portion, which is 

 clear, homogeneous, and highly refracting, and of an internal por- 

 tion, which is slightly granular and less refractive ; the outer end 

 of each rod is in direct contact with the pigment epithelium lining 

 the choroid, while the inner end, tapering to a fine thread, pierces 

 the external limiting membrane and passes into the external granular 

 layer. The cones consist also of two portions, the inner of which 

 is somewhat thicker than the rod and rests upon the limiting mem- 

 brane ; the outer portion tapers to a fine point, which is known as the 

 cone-style. The cones, as a rule, are somewhat shorter than the 

 rods. The proportion of rods to cones varies in different parts of 

 the retina, though there are on an average about fourteen rods to one 

 cone. In the macula lutea, where vision is most acute, the rods 

 are almost entirely absent, cones alone being present. All the 

 retinal elements at this point are changed. The nerve-fiber layer 

 is absent, the axis-cylinders radiating in such a manner as to leave 

 the spot free from their covering. The remaining layers are all thinned 

 and the stroma is reduced to a minimum. The optic nerve, after 

 passing forward from the brain, penetrates in succession the sclerotic, 

 choroid, and retina ; the nerve-fibers then spread out over the anterior 

 surface of the retina and become connected with the large ganglionic 

 cells, the third layer of the retina. 



The number of optic nerve-fibers in the retina is estimated to be 

 about 800,000, and for each fiber there are about seven cones, one 

 hundred rods, and seven pigment cells. The points of the rods and 

 cones are directed toward the choroid, or away from the entering 

 light, and dip into the pigment layer. They, with the pigment layer, 

 are the intermediating elements in the change of the ethereal vibra- 

 tions into nerve force ; out of these nerve vibrations the brain 

 fashions the sensations of light, form, and color. 



The vitreous humor, which supports the retina, is the largest of the 

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