The lateral geniculate body 
seems to be two organs in one. 
With some justification we can 
consider the ventral, or bottom, 
two layers (ventral means 
"belly") as an entity because 
the cells they contain are 
different from the cells in the 
other four layers: they are 
bigger and respond differently 
to visual stimuli. We should 
also consider the four dorsal, or 
upper, layers (dorsal means 
"back" as opposed to "belly") 
as a separate structure because 
they are histologically and 
physiologically so similar to 
each other. Because of the 
different sizes of their cells, 
these two sets of layers are 
called magnocellular (ventral) 
and parvocellular (dorsal).

    Fibers from the six layers 
combine in a broad band called 
the optic radiations, which 
ascends to the primary visual 
cortex (see the illustration of 
the visual pathway seen 
earlier). There, the fibers fan 
out in a regular way and 
distribute themselves so as to 
make a single orderly map, just 
as the optic nerve did on 
reaching the geniculate. This 
brings us, finally, to the 
cortex.