The brain is an intricate structure that looks like a boxing glove placed over a spiral of sea creatures. The outer layer of the brain is called the cortex, or the neocortex. It is a wrinkled surface that covers the top and sides of the brain. This is the part that looks like a boxing glove, albeit a wrinkled one. Underneath the cortex are the subcortical regions: the cerebellum and brain stem at the very base of the brain, the thalamus and related regions toward the middle of the brain, and the limbic system, which wraps around the thalamus. The basal ganglia are also in the middle of the brain, close to the thalamus.

The distinction between the cortical and subcortical regions of the brain is an important one. The cortex is a relatively recent evolutionary achievement and the cortical structures are much more developed in humans than they are in more primitive animals. Most of the more complex psychological processes, the ones we think of as uniquely human, such as language, abstract thought, and reading, are controlled by the cortex. The subcortical regions process more fundamental psychological and even physiological functions. The lowest parts of the brain, closest to the spinal cord, are the oldest parts and regulate physiological processes we share with more primitive animals, such as breathing, heartbeat, and digestion.

Why is the cortex so wrinkled?

The surface of the cortex is covered with folds and looks somewhat like a walnut. These folds are referred to as convolutions. The rounded parts of the convolutions are called gyri, gyrus in the singular. The grooves between the gyri are called sulci (sulcus in the singular). These extra folds allow for much greater surface area, which in turn greatly increases the number of neurons that can fit into the relatively small space of the human skull. The more neurons we have, the more powerfully we can process information. To illustrate this efficient use of space, imagine an accordion or pleated paper fan, first folded up and then stretched out from end to end.