Freiburger historische Bestände - digital

160

Cunningham Memoirs.

23. The angle of the Sylvian fissure is more acute in the human infant
and child, and also in the ape, than in the human adult; it is also, in man,
more acute in the right hemisphere than in the left. It is apparently little
influenced by the form of the head.

24. At the time when the posterior limb of the Sylvian fissure is first
formed by the apposition of the lips of the upper and lower opercula of the
Sylvian region, it is placed far above the squamous suture. The relative
distance between the fissure and suture is greater at this period than at any
other stage of growth.

25. From this period to the end of the fifth year the fissure and suture
rapidly approach each other; but after this date the process goes on more
slowly, and the permanent condition is gained about the ninth year. Very
frequently the process is retarded or accelerated.

26. This change in relative position is brought about—(a) by a rise of
the sutural line, or in other words, of the upper margin of the squamous
bone; (b) by a descent of the posterior limb of the Sylvian fissure on the
outer surface of the hemisphere.

27. The descent of the Sylvian fissure on the outer surface of the
hemisphere is not produced by a portion of the outer surface slipping round
so as to take up a position upon the broadening floor of the young cranium,
but by a growth antagonism between the fronto-parietal and temporal
opercula, in which the former gains the victory by pushing down the
latter.

28. In cebus the Sylvian fissure lies above the level of the squamous
suture; in the macaque, hamadryas, and orang, it lies immediately subjacent
to the fore-part of the suture; in Cynocephalus anubis and the
chimpanzee the fissure is situated in its fore-part below the level of the
front part of the suture.

29. The relative depth of the parietal and temporal lobes in the lower
apes resembles that in the human child; in the anthropoid ape the relative
parietal depth of the hemisphere exceeds that in the human adult.

30. The topography of the two anterior limbs of the fissure of Sylvius
is subject to a considerable amount of variability.