LOBES AND GYRI OF THE CONVEX SURFACE i. The frontal lobe (lobus frontalis) comprises the anterior polar region of the hemisphere and forms a part of all three sur faces (Figs. 26, 31 and 34). On the convex surface, it extends as far back as the central sulcus and the lateral cerebral fissure; on the basal surface, it is bounded behind by the stem of the lateral cerebral fissure and the anterior perforated spot; and it is limited posteriorly by the sulcus cinguli and sulcus parol factorius anterior on the medial surface of the cerebral hemisphere.
On the convex surface, the frontal lobe has the following sulci and gyri (Figs. 27 and 28): Superior precentral (s. prmcentralis superior) . Inferior precentral (s. prmcentralis inferior) Silk' Superior frontal (s. frontalis superior) Inferior frontal (s. frontalis inferior) Middle frontal (s. frontalis medius) Paramedial (s. paramedialis).
Anterior central (g. centralis anterior) Superior frontal (g. frontalis superior) Gyri .
Middle frontal (g. frontalis medius) Inferior frontal (g. frontalis inferior).
The precentral sulci (Fig. 27) are parallel with the central sulcus and are located about a half inch in front of it, the lower end of the inferior precentral being insinuated between the central sulcus and the ascending ramus of the lateral fissure of the cerebrum. They form the anterior boundary of the anterior central gyrus. The superior frontal sulcus and the inferior frontal sulcus are respectively continuous with the corresponding precentral sulcus from which they trend downward and forward parallel with the supero-medial border of the hemisphere. They separate from each other three gyri of nearly equal width, viz., the superior, middle and inferior frontal gyri (Fig. 24).
The superior frontal gyrus is incompletely divided in the human brain by an interrupted sulcus, called the sulcus para medialis (Fig. 27) which is located near the supero-medial border of the hemisphere and is said by Cunningham to be better devel oped in the higher types of the human race and to be rare in the higher apes.
A series of shallow furrows, described by Eberstaller as the middle frontal sulcus (s. frontalis medics, Fig. 27) partially sub
divides the middle frontal.gyrus into an upper and a lower part. The middle frontal sulcus, not found below the anthropoid apes (Cunningham), is best marked anteriorly and, at the super ciliary border of the hemisphere, bifurcates and forms a hori zontal furrow, the fronto-marginal sulcus. The posterior end, the foot, of the middle frontal gyrus, like that of the superior and inferior frontal, lies in the psychic-motor zone of the brain. It contains the writing center (Gordinier) in the left hemisphere of right-handed people.
The inferior frontal gyrus is highly developed in the human brain, especially in the left hemisphere of right-handed people. It is deeply cleft along its lower border by the anterior ascending and anterior horizontal rami of the lateral fissure of the cerebrum and is thus divided into a pars orbitalis, situated below the anterior horizontal ramus, a pars triangularis, inclosed between the anterior horizontal and ascending rami, and a pars basilaris, located between the anterior ascending ramus of the lateral fissure and the inferior precentral sulcus. The pars basilaris constitutes the foot of the inferior frontal gyrus and is continu ous with the gyrus centralis anterior; on the left side it contains the speech center. The pars basilaris is often divided into an anterior and posterior part by the sulcus diagonalis; the pars triangularis is deeply indented from above by a branch of the inferior frontal sulcus, called the sulcus radiatus.
The anterior portions of the superior middle and inferior frontal gyri comprise a psychic center, center of attention, volition, inhibition, etc., "of abstract concept" (Mills).
The anterior central gyrus (g. centralis anterior) lies between the precentral sulci and the central sulcus. It is joined to the posterior central gyrus by the paracentral lobule, above the central sulcus, and by the fronto-parietal part of the operculum, below it. The anterior central gyrus, together with the para central lobule constitutes the emissive motor zone of the human cerebrum.