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144

Cunningham Memoirs.

lines present to the subjacent cerebrum are much, more fixed than the relations
of the external sutural lines.

To a large extent, then, the difference which is noted in the relative
position of the Sylvian fissure with reference to the external squamous
suture may be accounted for by a gradual rise of the sutural line on the
outer surface of the cranium. As we shall see, however, this only accounts
for the change in part.

XIV. Relative position of the Sylvian Fissure with reference to
the mesial and infero-lateral borders of the cerebral hemisphere
at different periods of growth.—Great as the growth-change which we
have just indicated in connexion with the squamous sutural line is, it is
nearly equalled by that in the position of the Sylvian fissure on the surface
of the hemisphere. The fissure descends rapidly during infancy and early
childhood, so as to increase the depth of the fronto-parietal region and
diminish that of the temporal lobe. In the following Table this change is
brought out. In preparing the indices which indicate the parietal depth
and the temporal depth, the entire breadth of the hemisphere from the
mesial to the infero-lateral border is reckoned as being equal to 100 :—

Belative Position of the Sylvian Fissure on the Surface of the Cerebral

Hemisphere.

Ternporo-parietal Depth = 100.

Period of Growth.

Number
of hemispheres
.
Examined.

Parietal
Depth.

Temporal
Depth.

Eight.

Left.

Parietal
Depth.

Temporal
Depth.

Parietal
Depth.

Temporal
Depth.

7^ to 8^ foetuses,

11

61

39

60-6

39-4

61-4

38-6

Fall-time foetuses,

11

61-6

38-4

61

39

62-4

37-6

First year of life,

9

63-9

36-1

64-6

35-5

62-5

37-5

4 to 5 years,

11

65-7

34-3

65-9

34-1

65-4

34-6

11 to 15 years,

10

69-1

30-9

69-7

30-3

68-8

31-5

Adult, ....

68

69-1

30-9

69-7

30-3

68-5

31-5