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118

Cunningham Memoirs.

posterior ends of the hemisphere, and noted marked differences in this
respect in the brain of man, the anthropoid apes, and the lower apes. There
is an equally sharp distinction to be observed when we study its position
with reference to the cranial wall.

In the adult human brain the insula lies under cover of the frontal,
parietal, squamo-zygomatic, and alisphenoid bones. If we take the line of
the coronal suture we find that 13 per cent, of its long axis lies in front of
this, and 87 per cent, behind. Very different is the condition in the ape.
In cebus the insula is placed entirely under cover of the anterior and lower
part of the parietal bone. The coronal suture coincides with its anterior
extremity. In the chimpanzee (PI. iv., fig. 9, the dotted line indicates the
position of the coronal suture) and all the low old world apes (PI. iv., fig. 8,
the dotted line indicates the position of the coronal suture) which I have
examined the submerged portion of the insula is also for the most part
covered by the lower and fore-part of the parietal bone, but it also extends
downwards so as to lie partially under cover of the squamous bone. As
in cebus the coronal suture corresponds more or less accurately with
the anterior limit of the insula. The orang approaches most nearly to
man in the cranial relations of the island of Reil. The upper and anterior
corner of the insula projects very slightly beyond the coronal suture (PL
iv., fig. 11), but with this exception the remainder of the area is placed
under cover of the parietal and squamous bones.

But, as we might expect from what we have learned in regard to the
manner in which the insula extends backwards in the foetal brain, we find
that during the early stages of the growth of the brain the cranial relations
of the island of Eeil undergo very striking changes. In its very early
condition there is a greater part of the Sylvian fossa under cover of the
frontal bone than under cover of the parietal bone. Its relations to the
cranial wall at successive periods will be better understood if we take the
long axis of the area as being equal to 100 and divide this into two parts—
an anterior and a posterior—by the line of the coronal suture drawn
downwards so as to cross it. The following Table gives the results obtained
in this way :—