SENSE OF SIGHT. 867 



over it. This would appear to indicate such an adjustment of the con- 

 stituents of the retina as to- avoid the slightest interference with rays 

 striking on this point, since we have seen that the optic nerve-fibres are 

 not sensitive. 



Again, in the yellow spot the cones of the retina are closely packed 

 together, and in addition numerous ganglionic cells are found, while the 

 other layers of the retina are fainter than elsewhere. The yellow color 

 is due to the deposit of numerous yellow pigment cells. 



If it be admitted in the first place that the optic nerve-fibres are not 

 sensitive to light, and in the second place that the layer of rods and 

 cones represents the sentient surface for light-waves, a connection must 

 evidently exist between the receiving surface and the nerve-fibres to 

 admit of the impression becoming a sensation and being perceived by 

 the brain. Microscopical examination will succeed in tracing a connec- 

 tion by means of fine filaments penetrating all the layers of the retina 

 and connecting the nerve-fibres with the ganglionic cells, the granular 

 layers, and finally with the rods and cones. It is through this path that 

 the irritation caused by the rays of light and received in the meshes of 

 the rods and cones passes to the optic nerve-fibres, and thence to the 

 brain, there creating the sensation of light. 



Like other nerves of special sense, the fibres of the optic nerve, 

 though insensitive to light, respond to other irritants, and then the brain 

 perceives the sensation peculiar to that special nerve and recognizes 

 that an irritant has acted upon the nerve of vision, and a flash of light 

 is the result. Thus, in cases of section of the optic nerve a flash of 

 light is experienced, and then total darkness follows; so, also, if the 

 optic nerve be stimulated by electricit}^ a sensation of light is the 

 result. All the nerves of special sense to this extent agree in their 

 nature, and the optic nerve no more conveys sound-waves to the brain 

 than does the auditory nerve waves of light ; but both nerves at their 

 terminations are supplied with special forms of apparatus, the so-called 

 special sense organs, which only admit of being excited by appropriate 

 stimuli; thus the terminal fibres of the optic nerve are especially 

 adapted for receiving impressions of waves of light, the auditory nerve 

 waves of sound, the difference lying in the different impressions made 

 upon' different special centres in the brain. 



As to the mode in which rays of light call into action the specific 

 functions of the layer of rods and cones, but little is definitely known. 

 Recent investigations appear to point to a chemical decomposition being 

 concerned in this process. It is well-known that rays of light produce 

 decomposition in many substances, which are then spoken of as sensitive 

 to light. That a ray of light shall produce such decomposition, it must 

 be absorbed, We therefore, perhaps, see the explanation of the invariable 



