516 THE NERVOUS SYSTEM. 
the fronto-parietal operculum proves the victor. The contiguous lips of the two opercula become, 
in the first instance, tightly pressed together, and then, as the upper operculum proves the stronger 
and the more vigorous in its growth, the posterior limb of the Sylvian fissure becomes avadually 
depressed until it assumes the inclination characteristic of the adult. It would appear that the 
opercular growth-antagonism which produces this effect in the human brain does not occur to 
the same extent, if indeed it occurs at all, in the ape. This is evident from the oblique direction 
of the posterior limb of the Sylvian fissure in the simian brain. The greater growth energy 
of the fronto-parietal operculum in the human brain is not confined to the foetal stage of 
development, but is carried into the earlier stages of infantile growth, and it is probable “that 
it is due to an extension of that district of the cortex in which the centres for the skilled 
movements of the upper limbs reside, and also to an extension of Flechsig’s parieto-occipital 
association area. 
The orbital and frontal opercula are late in appearing and very tardy in their growth. 
Indeed, it is only during the course of the first year of infantile life that they come into apposition 
with each other and with the other two opercula, so as to close in the forepart of the Sylvian fossa 
and form the anterior limbs of the Sylvian fissure. They do not begin to take shape until more 
than half of the Sylvian fossa has already been closed by the fronto-parietal and the temporal 
opercula. The orbital operculum appears first and is much more constant in its growth than 
the frontal operculum, which indeed frequently fails altogether, and, even when present, shows 
the greatest amount of variability in the degree to which it is developed. 
Variations in the degree of development of the frontal operculum influence greatly the form 
presented by the two anterior limbs of the Sylvian fissure, between which it hes. When strongly 
developed, it separates the two Sylvian limbs from each other to such an extent that they assume 
the appearance of the letter U ; when the frontal operculum is less strongly marked, the anterior 
Sylvian limbs may assume a V form or a Y form. In the latter case, the orbital and the fronto- 
parietal opercula meet below the frontal operculum to form the stem of the Y. In those cases 
where the frontal operculuin is absent altogether, a single anterior limb of the Sylvian fissure is 
the result. 
The late appearance, the slow growth, the variability of these two opercula, and also the 
tendency to abortive growth or complete suppression of the frontal operculum, all bespeak the 
fact that, from a phylogenetic point of view, the frontal and orbital opercula are, comparatively 
speaking, recent productions in the evolution of the human brain. In the anthropoid ape they 
are absent, and, consequently, the forepart of the, island of Reil is exposed on the surface of the 
simian brain. The same condition is not at all an uncommon occurrence in the brain of the 
microcephalic idiot. 
Fissure of Rolando (sulcus centralis)—The fissure of Rolando takes an oblique 
course across the outer convex surface of the cerebral hemisphere and intervenes 
between the frontal and parietal lobes. 
‘ge = Its upper end cuts the supero-mesial 
ws border of the hemisphere a short distance 
behind the mid-point between the frontal 
and occipital poles, whilst its lower end 
terminates above the middle of the pos- 
terior horizontal limb of the fissure of 
Interlocking Sylvius. Its superior extremity, as a 
ake rule, turns round the supero-mesial border 
Ascending Of the hemisphere, and is then continued 
yee backwards for a short distance on the 
mesial surface. Although, in its general 
direction, the fissure is oblique, it is 
very far from being straight. It takes 
a sinuous course across the hemisphere 
and makes two bends, which are usually 
fairly conspicuous, and which are termed 
the genua. The superior genu is placed 
between the upper and middle thirds of 
Fic. 384.—FissuRE OF ROLANDO FULLY OPENED UP, the fissure, and its concavity is directed 
so as to Exaibw phepinier locate gyri and deep forwards: the inferior genu is situated 
annectant gyrus within it. E } 2. yrs 4 
in front of the superior bend and a_ 
short distance lower down. Its convexity is directed forwards. The short portion 
of the fissure between the two genua is not infrequently very nearly horizontal 
in direction. The angle which the general direction of the fissure of Rolando 
makes with the mesial plane is termed the Rolandic angle. In the adult brain 
the average Rolandic angle is 71° 7’, and the limits of variation would appear to be 
69° and 74°. 
Ascending { 
parietal—-—— 
convolution \ 
