Freiburger historische Bestände - digital

148

Cunningham Memoirs.

In the cebus the Sylvian fissure lies 7 mm. above the mid-point of the
squamous suture. This gives a Sylvio-squamous index of 12 T. In the
baboon the Sylvian fissure crosses under the squamous suture a short
distance in front of its mid-point, and at this very spot it runs into the
parallel fissure. The disposition of the fissure in the chimpanzee is not in
every respect similar. It crosses under the suture at a point which is
situated further forwards than in the baboon, so that only a very small
part of the front portion of the fissure lies under cover of the squamous
bone.

The disposition in the orang undoubtedly approaches most closely to
that exhibited in the adult human head. F£re' has already drawn attention
to this, and has pointed out that the fixed relationship between the
squamous suture and the Sylvian fissure appears to be attained at an
earlier date in the orang than in the human child. Both of his specimens
were young, whilst mine was also very far from being full grown. I estimated
its age at six years. In comparing the cranio-cerebral relations in
the orang and the chimpanzee we must not lose sight of the fact that in
the former there is a spheno-parietal suture as in man, whilst in the chimpanzee
the squamosal bone reaches forwards so as to articulate with the
frontal bone and shut off the parietal from the sphenoid. The commencement
of the Sylvian fissure in the orang is placed at a level slightly below
that of the spheno-parietal suture, and it is only as it passes backwards that
it comes to lie immediately subjacent to the fore-part of the squamous
suture.

In the orang the so-called anterior limb of the Sylvian fissure was placed
subjacent to the fronto-sphenoid suture ; in the chimpanzee it ascended
so as to cross very obliquely the line of the fronto-squamous suture.

But it is impossible to appreciate fully the relations which the fissures
of the ape-cerebrum present to the sutural lines unless we determine at the
same time the precise position of the various fissures on the surface of the
cerebrum. We have already done this in the case of the Sylvian fissure in
the human brain {vide p. 144), and we have constructed indices which
give an approximate idea of the relative depth of the portions of brain
surface which lie above and below the Sylvian fissure. It is necessary to