268 



THE HUMAN BODY 



excitation of 6, of a body on the line 6 B or its prolongation. It is 

 only other conflicting experiences, as that with the eyes closed 



FIG. 91. Diagram illustrating the points at which incident rays meet the 

 retina, xx, optic axis; k, first nodal point; k' , second nodal point; b, point where 

 the image of B would be formed, were the eye properly accommodated for it; 

 a, the retinal point where the image of A would be formed. 



external bodies do not excite visual sensations, and the constant 

 connection of the pressure felt on the eyelid with the visual sen- 



FIG. 92. Diagrammatic section through the eyeball, xx, optic axis; k, nodal 

 point. 



sation, that enable us when we press the eyeball to conclude that, 

 in spite of what we seem to see, the luminous sensation is not due 

 to objective light from outside the eye. 



The Excitation of the Visual Apparatus by Light. Light only 

 excites the retina when it reaches its nerve end organs, the rods 

 and cones. The proofs of this are several. 



1 . Light does not arouse visual sensations when it falls directly on 

 the fibers of the optic nerve. Where this nerve enters there is a 

 retinal part possessing only nerve-fibers, and this part is blind. 

 Close the left eye and look steadily with the right at the cross in 



