The Sylvian fissure is a deep, lateral indention that divides the lobes of the human brain. In essence, it divides the top of the brain from its bottom. It is the deepest and most easily discernible of the many fissures in the human brain. Other than serving as a major landmark of the brain's landscape, no purpose or specific function is known to be attributable to the Sylvian fissure.
In the front portion of the brain, the Sylvian fissure divides the frontal lobe — which controls decision-making, problem-solving, and emotion — from the temporal lobe — which regulates memory, language, and learning functions. In the rear portion of the brain, it divides the temporal lobe from the parietal lobe, which processes sensory input received from the body. The frontal and parietal lobes fall above the fissure, while the temporal lobe falls below it.
This fissure was referred to in early anatomical documents as the anfractuosa fissura. The Sylvian fissure is so-named because it is thought to have been discovered by Franciscus Sylvius, a professor of medicine at Leiden University. Modern science also calls this brain feature the lateral sulcus, the lateral fissure, or the fissure of Sylvius.
A Sylvian fissure begins at the roots of the eyes, passes across the temples, and ends near the roots of the brain stem. It does not completely encircle the brain. It occurs unilaterally, meaning that it appears across both hemispheres of the brain. It does not divide the brain into two roughly equal portions — as does the medial longitudinal fissure, which divides the brain into the left and right hemispheres. The portion of the brain above the Sylvian fissure is distinctly larger than the amount below it.
Injuries occurring near this feature may cause language impairment and result in difficulty learning new things or retaining and recalling information already learned. Such damage may also impair the brain's ability to process stimuli from the body, resulting in impairment in any of the five senses. Emotional instability or lack of emotional control may also characterize these injuries.
This particular fissure is one of the first fissures of the brain to develop in fetuses. It appears at about week 14 of the gestational period, at roughly the same time that the eyelids, fingernails, and reproductive organs develop. Most other brain fissures do not develop until week 20, and some develop as late as 33 weeks.