LOBES AND GYRI OF THE DORSO-LATERAL SURFACE.
Turning again to the basal aspect of the hemisphere, the vallecula Sylvii (fossa cerebri lateralis) appears as a deep cleft, lateral to the substantia perforata anterior, that separates the basis cerebri into an anterior and posterior division. From the valley the fissura cerebri lateralis, or Sylvian fissure, extends outward, at first as the truncus fissurae lateralis, toward the dorso-lateral surface of the hemisphere. On reaching the latter, the fissure divides into three branches : (1) the short ramus anterior horizontalis, running horizontally forward, (2) the ramus anterior ascenders, also short and directed almost vertically upward, and (3) the long ramus posterior, which continues the direction of the anterior horizontal limb backward and somewhat obliquely upward and at its end usually divides in a Y-like manner into a ramus ascendens and a ramus descenders. Approxi mately from the middle of the dorsal border of the hemisphere, the sulcus centralis or fissure of Roland, runs obliquely downward and forward toward the posterior ramus of the fissura cerebri lateralis. As a rule, this furrow exhibits two knee-like bends, one at the junction of the upper and middle thirds, the other at the transition of the second and lower thirds ; the fissure, moreover, usually crosses the upper border of the hemisphere.
Lobus frontalis. The frontal lobe lies above the fissure cerebri lateralis and in front of the central fissure, and presents the following fissures and convolutions. The sulcus praecentralis superior begins somewhat below the upper border of the hemisphere and runs more or less parallel with the sulcus centralis. Somewhat lower, the sulcus praecentralis inferior continues in the same direction and below penetrates between the ramus anterior ascendens fissurae cerebri lateralis and the lower end of the sulcus cen tralis. The upper end of the inferior precentral sulcus almost constantly lies in advance of the lower end of the superior fissure. As variations, the precentral fissures may con nect with the central fissure and the lower precentral may join the fissura cerebri lateralis.
The sulcus frontalis superior extends forward from the superior precentral sulcus, approaching the upper border of the hemisphere in front. At times the fissure cuts
through the sulcus praecentralis superior towards the central fissure, thereby producing the cruciform type of precentral furrow. In many cases the superior frontal sulcus is interrupted by two or three annectant convolutions. The fissure may also be doubled.
The sulcus frontalis inferior likewise extends forward, from the inferior precentral fissure, but more arched and downward. The fissure is usually clearly marked, but it may present very variable forms and be interrupted by deep or superficial annectant convolutions. Ordinarily a short furrow, the sulcus radiatus, extends downward from the inferior frontal between the anterior horizontal and ascending rami of the fissura cerebri lateralis.
The small sulcus frontalis medius is generally to be seen between the superior and inferior frontal fissures. This sulcus is often readily identified, but it may exhibit the most diverse forms, since it may be displaced or effaced by annectant convolutions. At times the fissure is clearly recognizable as a continuous and deep furrow.
The foregoing fissures bound the following convolutions. The gyrus centralis anterior lies between the superior and inferior precental fissures in front and the central fissure behind. The gyms frontalis superior is bounded by the superior frontal fissure below and the superior prefrontal fissure behind. Between the superior and inferior frontal fissures extends the gyrus frontalis medius, which is subdivided by the median frontal sulcus into a pars superior and a pars inferior. The gyros iron/a/is inferior lies below the inferior frontal fissure. This convolution, also known as Broca' s convolution on the left side, includes three subdivisions:— The pars opercularis, between the lower end of the inferior precentral fissure and the anterior ascending ramus of the fissura cerebri lateralis; The pars triangularis, between the anterior ascending and horizontal rami of the lateral fissure ; and The pars orbitalis, between the anterior horizontal ramus and the trunk of the lateral fissure.