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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 75

15. Certain fissures, therefore, which are complete and temporary in
the human brain are complete and permanent in the ape's brain.

1 6. The complete permanent fissures in the human brain are :—(a) the
hinder part of the fissura arcuata ; (b) the fore-part of the calcarine; (c) in
many cases the parietooccipital; and (d) in some cases the mid-collateral.
The Sylvian fissure is not a complete fissure.

17. Of the fissura arcuata the hinder part alone is preserved as the
fissura hippocampi. The fore-part, which is generally supposed to be
retained as the callosal fissure, is in reality obliterated.

18. Synchronous with the appearance of the radial transitory fissures
on the mesial face of the hemisphere two fissures appear, which lie in
series with the former, and occupy the ground afterwards held by the
parietooccipital and calcarine fissures. They may be termed the precursors
of these fissures.

19. The precursor of the parietooccipital fissure sometimes shows an
unbroken continuity of existence with the parietooccipital fissure of the
adult brain. In other cases it is obliterated, and its place is afterwards
taken by a secondary sulcus, which attains, however, a very great depth.

20. In the adult brain the parietooccipital fissure, even in its complete
form, does not form any eminence on the inner wall of the posterior horn
of the ventricle, because it does not extend downwards as far as the cavity.
Above its lower end the hemisphere is solid.

21. The posterior part of the calcarine precursor is in every case
obliterated, and the anterior part retained. The extent of the part
obliterated varies considerably in different brains.

22. The anterior preserved portion of the calcarine fissure forms the
"stem" of the —^-shaped fissural- arrangement on this part of the
hemisphere, and its hinder part corresponds to the calcar avis.

23. In the place of the hinder portion of the calcarine precursor, which
is obliterated, a secondary furrow appears. This may be termed the
posterior calcarine sulcus.

24. The posterior calcarine sulcus is formed in two j)ieces which run
together, and also join the " stem." In this way the entire length of what,
in ordinary anatomical language, is called the calcarine fissure is formed.

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