What part of the brain accounts for two-thirds of the total mass?

The cerebrum is the largest brain structure in humans and accounts for about two-thirds of the brain’s mass. It is also the most highly developed part of the human brain.

The cerebrum consists of two large masses, called cerebral hemispheres, which are almost mirror images of each other. A bridge of nerve fibers connects them. Each hemisphere is in turn divided into four lobes. The cerebrum functions as the center of higher brain functions, interpreting sensory impulses and initiating muscle movements. It stores information and uses it to process reasoning, thinking and perception. It also functions in determining intelligence and personality.