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82

Cunningham Memoirs.

therefore, is not proportionate with that of the hemisphere : it is very much
more rapid. But the figures in the Table further show that this excess of
growth is altogether in a backward direction. The fronto-central and
occipito-central indices prove this. Its anterior border retains very nearly
the same position with reference to the anterior end of the cerebrum
throughout, but its posterior end steadily approaches more and more nearly
to the occipital pole. Thus the occipito-central index in the fourth month
is 49*9, and in the eighth month, 44*8.

So much for the Sylvian fossa. We must next study the manner in
which the area becomes covered in by the growth of the opercula. Each
of the four subdivisions of the boundary wall acts as an independent line
of growth. In the course of time, therefore, there are formed four distinct
opercula, viz. a fronto-parietal (the operculum of Burdach), a temporal, a
frontal, and an orbital. The temporal operculum, an offshoot from the
subjacent temporal lobe, first becomes apparent (fig. 25, i., t.). In the first
instance the growth affects only the anterior half of the temporal wall, but
it gradually extends backwards to the posterior angle of the fossa. Before
it reaches this, however, the fronto-parietal operculum becomes evident in
the anterior two-thirds of this subdivision of the bounding wall (fig. 25,
i., f.p.), and it gradually extends backwards to meet the posterior part of
the temporal operculum at the posterior angle of the fossa. These changes
occur in the latter half of the fifth month. As the temporal operculum
creeps upwards over the lower part of the floor of the fossa, and the
fronto-parietal operculum grows downwards over the upper part of the
area, it becomes apparent that the growth energy of the former is more
intense than that of the latter. It therefore follows that when the two
opercula meet in the sixth month, and close in the posterior part of the
space, there is more of the Sylvian area covered by the temporal than by
the parieto-frontal operculum. During this time there is no appearance
of either the frontal or the orbital operculum. The portions of the
bounding wall from which these proceed are becoming more salient and
projecting, but it is not until the temporal and parieto-frontal opercula
have met over rather more than the posterior half of the fossa that they
begin to show signs of active growth. This stage is reached about the