a ae 
2 
INSULAR OPERCULA. 515 
The frontal operculum is the small triangular piece of cerebral substance which 
intervenes between the ascending and anterior horizontal limbs of the Sylvian 
fissure. It covers over a small part of the anterior portion of the insula, and is 
sometimes termed the pars triangularis. 
The orbital operculum is, for the most part, on the under surface of the hemi- 
sphere. It lies below and to the inner side of the anterior horizontal hmb of the 
Sylvian fissure, and proceeds backwards from the orbital aspect of the frontal lobe 
over the forepart of the insula. 
Development of the Sylvian Fissure and of the Insular District of the Cerebral Hemi- 
sphere.—It is only during the latter half of the intrauterine period of development that the 
opercula take shape and grow over the insula, so as to shut it out from the surface. In its early 
condition the insula presents the form of a depressed area on the side of the cerebral hemisphere, 
surrounded by a dis- 
tinct boundary wall 
formed by the sur- 
rounding more elevated 
surface of the hemi- 
sphere. After a time 
this depressed area, 
which is ¢alled the 
Sylvian fossa, assumes 
a triangular outline, 
and then the bounding 
wall is observed to be 
composed of three dis- 
tinct parts, viz: an 
upper or fronto- 
parietal, a lower or Fic. 382.—THRiKE STAGES IN THE DEVELOPMENT OF THE INSULA AND THE 
temporal, and an an- INSULAR OPERCULA. 
terior or orbital part. 
The rounded angle, 
LO LE 
A, Right cerebral hemisphere from a foetus in the latter part of the fourth month 
of development ; B, Right cerebral hemisphere from a foetus in the fifth 
formed by the meeting month of development ; C, Right cerebral hemisphere from a foetus in the 
of the upper and an- latter part of the eighth month of development. 
terior portions of the [ ¢ the temporal operculum has been removed, and thus a large part of the 
boundary, now becomes insula is exposed. 
flattened, and a short » p, Fronto-parietal operculum. —F, Frontal operculum. 0, Orbital operculum. 
oblique part of the 
limiting wall, termed the frontal portion, assumes shape in this position. Each of these four 
portions of the bounding wall of the Sylvian fossa becomes a line of growth, from which an 
operculum takes origin, and by the approximation of these opercula, as they grow over the 
surface of the Sylvian fossa, the insula becomes closed in and the limbs of the Sylvian fissure 
are formed. 
The temporal and fronto-parietal opercula make their appearance somewhere about the end 
of the fifth month of foetal development, long before the other two opercula show any indication 
of growth. The temporal operculum grows more rapidly than the fronto-parietal ; so that, when 
Fic. 383.—DIaAGRAM TO ILLUSTRATE THE DEVELOPMENT OF THE OPERCULA WHICH COVER THE INSULA. 
A, Sylvian fossa before opercula begin to form ; B, Fronto-parietal and temporal opercula well advanced ; 
C, All the four opercula developed but not in apposition. 
F.P. Fronto-parietal operculum. O.R. Orbital wall of fossa. s*. Anterior horizontal limb of Sylvian fissure. 
T. Temporal operculum, F. Frontal operculum. s°, Ascending limb. 
F.R. Frontal wall of fossa. O. Orbital operculum, s*, Posterior horizontal limb. 
_ the margins of these two opercula come together to form the posterior limb of the Sylvian fissure, 
there is a greater extent of the Sylvian fossa covered by the temporal operculum than by the fronto- 
parietal operculum. This accounts for the more oblique direction of the Sylvian fissure in the foetal 
brain. But at this stage a growth-antagonism between the two opercula takes place, and in this 
