FRONTAL LOBE. 519 
The inferior preecentral sulcus consists of a vertical and a horizontal part, and, 
when developed in a typical manner, it presents a figure like the letter T or F. 
The vertical portion lies in front of the lower part of the fissure of Rolando, whilst 
the horizontal part extends obliquely forwards and upwards into the middle frontal 
convolution. 
The superior precentral sulcus is a short vertical sulcus which hes at a higher 
level than the inferior precentral furrow, in front of the upper part of the fissure 
of Rolando. It is almost invariably connecte? with the hinder end of the superior 
frontal sulcus. 
The anterior central convolution (ascending frontal gyrus) is a long continuous 
gyrus, which is limited in front by the two precentral furrows and behind by 
the fissure of Rolando. It extends obliquely across the hemisphere from the 
supero-mesial margin above to the posterior horizontal limb of the Sylvian fissure 
below. 
The superior frontal sulcus extends forwards in a more or less horizontal direc- 
tion from the sulcus preecentralis superior. 
The gyrus frontalis superior is the narrow convolution between the supero-mesial 
border of the hemisphere and the superior frontal sulcus. It takes a horizontal 
course to the frontal pole. 
The inferior frontal sulcus occupies a lower level than the superior frontal furrow. 
Its hinder end is placed in the angle between the vertical and horizontal parts of the 
inferior preecentral sulcus, and is not infrequently confluent with one or other of 
these. It proceeds forwards towards the superciliary margin of the hemisphere and 
ends a short distance from this in a terminal bifurcation. 
The gyrus frontalis medius is the name given to the broad convolution which 
hes between the superior and inferior frontal sulci. 
The gyrus frontalis inferior is that portion of the outer surface of the frontal 
lobe which is placed in front of the inferior preecentral sulcus and below the inferior 
frontal sulcus. The inferior frontal convolution is cut into three pieces by two 
anterior limbs of the Sylvian fissure. These are termed the pars basilaris, the pars 
triangularis, and the pars orbitalis. 
The pars basilaris is that part which lies between the vertical limb of the inferior 
preecentral sulcus and the ascending limb of the Sylvian fissure. It forms the anterior 
part of the fronto-parietal operculum, and it is traversed in an oblique direction by 
a shallow but constant furrow, termed the suleus diagonalis. The pars triangularis is 
simply another name for the frontal operculum. It is triangular in form, and lies 
between the anterior ascending and the anterior horizontal limbs of the Sylvian 
fissure. The pars orbitalis is placed below the anterior horizontal limb of the 
Sylvian fissure. 
The inferior frontal convolution possesses a special interest on account of the localisation 
within it, on the left side, of the speech centre. From it, also, the front part of the fronto-parietal 
and the whole of the frontal operculum are developed. This opercular development in connexion 
with the inferior frontal gyrus constitutes a leading point of difference between the brain of man 
and that of the ape. Even in the highest ape the inferior frontal convolution is not opercular. 
The frontal operculum is not present and the anterior part of the insula is exposed on the surface. 
Probably the excess of growth which determines the formation of the frontal operculum in man 
has some connexion with the development of the speech centre. 
The sulcus paramedialis is the term applied to a series of short irregular depres- 
sions or furrows, arranged longitudinally, close to the supero-mesial border of the 
hemisphere. These rudimentary sulci partially subdivide the superior frontal con- 
volution into an upper and a lower part, and they are of interest in so far that they 
are never developed in the ape, and are deeper and better marked in the higher 
types of human brain. 
The sulcus frontalis medius (Eberstaller) proceeds horizontally forwards in the 
forepart of the middle frontal convolution, so as to subdivide it into an upper anda 
lower part. When the furrow reaches the superciliary margin of the hemisphere 
it bifurcates, and its terminal branches spread out widely and constitute a trans- 
verse furrow, called the fronto-marginal sulcus. The sulcus frontalis medius is only 
found in man and the anthropoid apes. It is net present in any of the lower apes. 
