By the end of the preschool period, the brain's has grown significantly because it has become considerably thicker in connecting the hemispheres.

corpus callosum.

Explanation:

The corpus callosum is a thick band of nerve fibers that divides the cerebral cortex lobes into left and right hemispheres. It connects the left and right sides of the brain, allowing for communication between both hemispheres. The corpus callosum transfers motor, sensory, and cognitive information between the brain hemispheres.

Functions:

The corpus callosum is the largest fiber bundle in the brain, containing nearly 200 million axons. It is composed of white matter fiber tracts known as commissural fibers. It is involved in several functions of the body including:

Communication between brain hemispheres

Eye movement and vision

Maintaining the balance of arousal and attention

Tactile localization