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130

Cunningham Memoirs.

and the angular gyri of the parietal lobe with the two upper convolutions
of the temporal lobe. These arching gyri bear a very close connexion
with the transverse convolutions of Heschl which appear on the deep
surface of the temporal operculum. Eberstaller has very properly called
attention to this. He says: " Just as the inferior parietal lobule in the
lower apes presents a single weak arching convolution, the gyrus angularis,
bent around the upper end of the parallel fissure, whilst the gyrus supra-
marginalis which closes the end of the Sylvian fissure still lies concealed
as a deep gyrus within it, only to come to the surface in the higher apes,
so there is also found in these higher apes and in man, going hand in hand
with the development of the temporal lobe, several hidden gyri in the
Sylvian fissure, which shorten the Sylvian fissure and increase the cortical
district of that region in which we have to seek the sensible Sprach-
centrumP

In well-marked cases the transverse gyri of Heschl are arranged in the
human brain one behind the other in the form of a series of steps and stairs
which gradually lead to the surface. Further, it is no uncommon thing to
find that the hindmost of these gyri has completely reached the surface and
has ranged itself in front of the supramarginal convolution. To one who
studies the development of the Sylvian region the explanation of this is
simple. In its early condition the Sylvian fossa is surrounded by steep
perpendicular banks which ultimately form the lips of the different
opercula. The posterior rounded angle of the region, however, ascends
with a gradual and easy inclination towards the surface of the cerebral
mantle, so that in many cases it is exceedingly difficult to fix accurately
at this stage the posterior end of the insula (PI. n., figs. 12 to 15). It is
upon the surface of this inclined plane that the transverse gyri of Heschl
are formed, but the process is hidden from view by the meeting of the
parietal and temporal opercula over them. The anterior gyrus of Heschl
appears first (about the beginning of the seventh month, PI. n., figs. 20 and
21, h)} and the others assume shape in regular order from before backwards.

Sometimes there seems to be a secondary forward growth of this part
of the cortex which leads to a very decided shortening of the Sylvian
fissure. In fact, in these cases the fissure may fall short of the posterior