Page:Popular Science Monthly Volume 27.djvu/79

 striatum. The fissure of Rolando separates the frontal lobe, F, from the rest of the brain. It begins at the great longitudinal division between the hemispheres, and pursues an uninterrupted course to within a short distance of the horizontal branch of the Sylvian fissure. Back of the fissure of Rolando is the external perpendicular fissure (E); it

 (Wundt.) Left side. S, Sylvian fissure—s, perpendicular, s'. horizontal, branches of this fissure; R, fissure of Rolando; E, external perpendicular fissure; F3, third frontal convolution or convolution of Broca; AF, ascending frontal convolution; AP, ascending parietal convolution; AG, angular gyrus or pli courbe; F, frontal lobe; P, parietal lobe; T.S, temporo-sphenoidal lobe; O, occipital lobe.

appears as a simple notch on the upper edge of the hemisphere. It is a prolongation, on the convex or lateral surface of the brain, of the deep fissure of the internal zone. This fissure marks the rear limit of the parietal lobe (P), which therefore lies between the fissure of Rolando and this furrow. Back of the parietal lobe is the occipital lobe (O). This region is less exactly defined; an ideal prolongation of the external perpendicular fissure would determine its anterior and inferior limits. The temporo-sphenoidal lobe (T) has already been noticed as lying below the fissure of Sylvius. Among the various convolutions formed by these fissures there are three or four which must be named, because it is with them that the experiments in brain-functions are chiefly concerned. In the frontal lobe there are two of these convolutions (F 3), the third frontal convolution, or the convolution of Broca, and (A F) the fourth frontal convolution or ascending frontal fold. Broca's convolution has somewhat the shape of a horseshoe, and is formed around the ascending branch of the Sylvian fissure. The ascending frontal fold lies directly to the left of the fissure of Rolando, which it follows throughout. In the parietal lobe we notice (A P) the ascending parietal convolution immediately to the right of the fissure of Rolando, and (A G) the angular gyrus or pli courbe. This latter convolution is very complex in man.