This cross section through the 
occipital lobe was made by 
cutting out a piece as shown in 
the previous photograph. It is 
what we would see if we were to 
walk into the groove and look to 
the left. The letter a 
corresponds to a point halfway 
between X and the arrowhead. 
The Nissl stain shows cell 
bodies only; these are too small 
to make out except as dots. The 
darker part of the top and the 
mushroom-shaped part just 
below are striate cortex. The 
three letter ds mark the border 
between areas 17 and 18.

    To see what the cortex looks 
like in cross section, we have 
cut a chunk from the visual 
cortex on the right side of the 
previous photograph. The 
resulting cross section, as in 
the photomicrograph to the 
left, is stained with cresyl 
violet, a dye that colors the cell 
bodies dark blue but does not 
stain axons or dendrites. With 
the photomicrograph taken at 
this low power, we cannot 
distinguish individual cells, 
but we can see dark layers of 
densely aggregated cells and 
lighter layers of more thinly 
scattered ones. Beneath the 
exposed part of the cortex, we 
see a mushroom-shaped, buried 
part that is folded under in a 
complicated way, but these two 
parts are actually continuous. 
The lightly stained substance is 
white matter; it lies under the 
part of the cortex that is 
exposed to the surface, 
separating it from the buried 
fold of cortex, and consists 
mainly of myelinated nerve 
fibers, which do not stain. The 
cortex, containing nerve-cell 
bodies, axons, dendrites, and 
synapses, is an example of gray 
matter.

    For anatomical richness, in 
its complexity of layering, area 
17 exceeds every other part of 
the cortex. You can see an 
indication of this complexity 
even in this low-magnification 
cross section when you 
compare area 17 with its next-
door neighbor, area 18, 
bordering area 17 at d. What is 
more, as we look along the cross 
section from the region marked 
a, which is a few degrees from 
the foveal projection to the 
cortex, toward the region 
marked b, 6 degrees out, or 
toward c, 80 to 90 degrees out, 
we see very little change in the 
thickness or the layering 
pattern. This uniformity turns 
out to be important, and I will 
return to it in Chapter 6.