The brain is divided into two hemispheres, left and right. Each hemisphere has 43 billion neurons. Their axons subdivide, like branches of a tree, to allow trillions of links among them. But, in contrast to the rich interconnections within a hemisphere, the bond between hemispheres is a tiny cable, the corpus callosum, with just over 200 million axons—roughly one axon between hemispheres for every two hundred within a hemisphere.
