The frontal lobe has a superior, an inferior, and a precentral sulcus. The first two divide the anterior portion into the superior, middle, and inferior frontal convolutions. That portion of the inferior or third left frontal convolution which surrounds the ascending limb of the fissure of Sylvius is called Broca's convolution, and is the centre for speech. Posterior to these and running upward and backward, forming the anterior wall of the central fissure, is the precentral or ascending frontal convolution.

Fig. 41.   Fissures, sulci, and gyri (convolutions) of the lateral surface of the cerebral hemisphere.

Fig. 41. - Fissures, sulci, and gyri (convolutions) of the lateral surface of the cerebral hemisphere.

The convolution forming the anterior extremity of the parietal lobe and the posterior wall of the central sulcus or fissure of Rolando is called the postcentral or ascending parietal convolution. Immediately behind it is the postcentral or interparietal sulcus. The upper portion of this sulcus divides, one branch going upward and one backward. Immediately above the posterior branch is the superior parietal gyrus or lobule, and below it and surrounding the posterior extremity of the fissure of Sylvius is the supramarginal gyrus. Posterior to the supramarginal gyrus and surrounding the posterior extremity of the superior temporal, or temporosphe-noidal sulcus is the convolution known as the angular gyrus.

The occipital lobe on its convex surface is divided into superior and inferior occipital convolutions by the lateral occipital sulcus.

The temporal or temporosphenoidal lobe is also divided into superior, middle, and inferior, or first, second, and third temporal convolutions by the superior, or parallel, and middle fissures. On the under surface is a fourth temporal convolution, separated from the third by the inferior temporal fissure. These fissures may not be distinct.