VISION. BIT 
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 
effected in the retina as sensory impressions or impulses, which, 
when completed by corresponding changes in the brain, develop 
into sensations, which are represented psychically by percep- 
tions ; hence, though all these have a natural connection, they 
may for the moment be considered separately. It is as yet 
beyond our power to explain how they are related to each 
other except in the most general way, and the manner in 
which a.mental perception grows out of a physical alteration 
in the molecules of the brain is at present entirely beyond 
human comprehension. 
Fia. 419. Fie. 420. 
Fig. 419.—Verticat section of retina (after H. Miiller). 1, layer of rods and cones ; 2, rods; 
3, cones ; 4, 5, 6, external granule layer ; 7, internal granule layer ; 9, 10, finely "granular 
gra: st lay er: 11, layer of nerve-cells ; 12, 14, ‘fibers of optic nerve ; 13, membrana limitans. 
Fia. onnection of rods and cones ‘of ‘retina wit! nervous ‘elements (after Sap ppey). 
1, 2, 8, rods and cones seen from in front; 4, 5, 6, side view. The rest will be clear from': 
the’ preceding figure. 
37 
