The human brain is effectively divided into the left and right hemispheres, or sides. The body is wired in a seemingly peculiar way, in that the right side of your body is wired into the left side of your brain and vise versa. These two hemispheres are connected by a structure called the corpus callosum.
The corpus callosum is a dense group of fibers that run from the left hemisphere of your brain into the right side. These fibers constitute the largest connective pathway in the human brain, consisting of over 200 million nerve fibers, according to the National Organization for Disorders of the Corpus Callosum (NODCC). Since each side of your brain controls, and receives sensory input from, one side of your body, tasks that require coordination between the two sides of your body require coordination between the two sides of your brain. Facilitating this communication is what the corpus callosum does.
The corpus callosum begins to form in the brain at the tail end of the first trimester of pregnancy, or 12 to 16 weeks after conception. When an infant is born, the entirety of the structure is developed, but its effectiveness develops from birth until adolescence. By the time a child enters his teenage years, the raw functioning of the corpus callosum will be fully developed, and the brain will begin getting better at using it to coordinate the two sides of the brain. To use an automotive metaphor, if the corpus callosum is a car then adolescence marks the point at which the car is fully constructed. The teen then begins to learn how to use the car to its full potential.
Some individuals have disorders that result in their corpus callosum not fully developing. Others have their corpus callosum surgically severed, as a treatment for severe seizures. The absence of a normal corpus callosum in children can lead to delays in certain developmental milestones, such as walking or talking, reports the National Institute of Neurological Disorders and Stroke (NIND). When children reach their teenage years, those with disorders of the corpus callosum may begin to lag behind their peers in understanding and navigating complex social situations. Because the corpus callosum facilitates coordination between the two sides of the body, individuals with disorders of the corpus callosum or those with a severed corpus callosum may have some generalized "clumsiness" and greater difficulty with tasks that require coordinated movements of both sides of the body.
While the corpus callosum is not the only pathway between the two sides of the brain, it is by and far the most significant. The presence of these other pathways is what allows an individual whose corpus callosum has been severed to continue to function normally. However, these other connections are diffuse and small in comparison. For example, the largest of the other pathways is the anterior commissure. According to the NODCC, it has about 50,000 nerve fibers, or 0.00025 percent of the nerve fibers in the corpus callosum.