THE CORPUS CALLOSUM
The corpus callosum is a thick, 
bent plate of axons near the 
center of this brain section, 
made by cutting apart the 
human cerebral hemispheres 
and looking at the cut surface.

    The corpus callosum (Latin 
for "tough body") is by far the 
largest bundle of nerve fibers in 
the entire nervous system. Its 
population has been estimated 
at 200 million axons--the true 
number is probably higher, as 
this estimate was based on light 
microscopy rather than on 
electron microscopy--a number 
to be contrasted to 1.5 million 
for each optic nerve and 32,000 
for the auditory nerve. Its 
cross-sectional area is about 
700 square millimeters, 
compared with a few square 
millimeters for the optic nerve. 
It joins the two cerebral 
hemispheres, along with a 
relatively tiny fascicle of fibers 
called the anterior 
commissure, as shown in this 
illustration and the next. The 
word commissure signifies a set 
of fibers connecting two 
homologous neural structures 
on opposite sides of the brain or 
spinal cord; thus the corpus 
callosum is sometimes called 
the great cerebral commissure.