158 HUMAN PHYSIOLOGY. 



The retina is remarkably complex, consisting of ten distinct layers, from 

 within outward, supported by connective tissue. These are as follows, 

 viz.: i. Membrana limitans interna. 2. Fibres of optic nerve. 3. 

 Layers of ganglionic corpuscles, 4. Molecular layer. 5. Internal granu- 

 lar layer. 6. Molecular layer. 7. External granular layer. 8. Mem- 

 brana 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 Jacobson's membrane. They 

 consist of an external portion which is clear, homogeneous and highly re- 

 fracting, and of an internal portion which is slightly granular and less 

 refractive; the outer end of each rod is in direct contact with the pig- 

 mentary 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 membrane ; 

 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 propor- 

 tion of rods to cones varies in different parts of the retina, though there are 

 on the 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 

 fibre 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 reduced to a minimum. The optic nerve after 

 passing forward from the brain penetrates in succession the sclerotic, 

 choroid, and retina ; the nerve fibres 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 fibres in the retina is estimated to be about 

 800,000, and for each fibre 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 pig- 

 mentary layer. They, with the pigment layer, are the elements interme- 

 diating the change of the ethereal vibrations 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 re- 

 fracting media; it is globular in form and constitutes about four-fifths of the 

 ball ; it is hollowed out anteriorly for the reception of the crystalline lens. 



