VISION. 



539 



an object. It is true that an image of the object is formed on 

 the retina, but it would be a very crude conception of nervous 

 processes, indeed, to assume that anything resembling that 

 image were formed on the cells of the brain, not to speak of 

 the superposition of images inconsistent with that clear mem- 

 ory of objects we retain. Before an object is " seen," not only 

 must there be a clear image formed on the retina, but impulses 

 generated in that nerve expansion must be conducted to the 

 brain, and rouse in certain cells there peculiar molecular condi- 

 tions, upon which the perception finally depends. 



For the sake of clearness, we may speak of the changes 



Fig. 387. Fig. 388. 



Fio. 387.— Vertical section of retina (after H. MflUerJ. 1, layer of rods and cones; 8, 

 rods; 3, cones; 4, 5, 6, external granule layer; 7, internal granule layer; 9, 10, fine- 

 ly granular gray layer; 11, layer of nerve-cells; IS, 14, fibers of optic nerve; 13, 

 membrana limitans. 



Fig. 388. — Connection of rods and cones of retina with nervous elements (after Sap- 

 pey). 1, 8, 3, rods and cones seen from in front; 4, 5, 6, side view. The rest wul 

 be clear from the preceding figure. 



