THE SENSE OF SIGHT 403 



presents the appearance of a foramen. About T V 

 inch to the inner side of the yellow spot is the poms 

 options, at which point the optic nerve enters, the 

 nervous matter being heaped up here so as to form 

 the colliculus. 



Observed under the microscope the retina consists 

 of cell elements arranged in layers as follows: 



1. The layer of pigment cells. 



2. The layer of rods and cones, or Jacobsen's layer. 



3. The external limiting membrane. 



4. The outer nuclear or granular layer. 



5. The outer molecular or reticular layer. 



6. The inner nuclear or granular layer. 



7. The inner molecular or reticular layer. 



8. The layer of ganglion cells. 



9. The layer of nerve fibers. 



The layers in the retina are held together, except 

 the layer of rods, and cones, by a fine net-work of 

 neuroglia, called the fibers of Miiller. 



The Function of the Retina. The rods and cones 

 are the most important layer of cell elements in the 

 retina as necessary to vision. This layer possesses 

 the property of receiving light and color stimuli which 

 are transformed into energy that arouses nerve 

 impulses in the fibers of the optic nerve and the latter 

 convey them to the centre of vision in the cerebrum 

 (cuneus) and we are conscious of external objects 

 with their combined colors, etc., through the sense 

 of sight. A ray of light passes through the pupil, 

 crystalline lens, and vitreous humor, to be thrown upon 

 the retina, when it passes through all the layers of 

 the retina and is stopped only upon reaching the 

 pigmentary epithelium in which the rods and cones 

 are embedded. Just how light and color stimuli 

 create nerve impulses is not exactly understood. It 

 is supposed to be due to ether vibrations being trans- 

 formed into heat, which excites the rods and cones, 



