In the Reptilia the medulla has a marked flexure with a ventral
convexity, and an undoubted cerebral cortex for the first time makes
its appearance. The mesial wall of the cerebral hemisphere is divided
into a large dorsal hippocampal area (fig. 18, _Hip._) and a smaller
ventral olfactory tubercle. Between these two a narrow area of
ganglionic matter runs forward from the side of the _lamina
terminalis_ and is known as the paraterminal or precommissural area
(Elliot Smith, _Journ. Anat. and Phys._ vol. xxxii. p. 411). To the
upper lateral part of the hemisphere Elliot Smith has given the name
of _neopallium_, while the lower lateral part, imperfectly separated
from it, is called the _pyriform lobe_. In the Lacertilia the pineal
eye, if it be an eye, is better developed than in any existing
vertebrate, though even in them there is no evidence of its being used
for sight. Behind the so-called pineal eye and its stalk is the
_epiphysis_ or pineal body, and sometimes there is a dorsal sac
between them (see fig. 18).[1] The middle or soft commissure appears
in certain reptiles (_Crocodilia_ and _Chelonia_), as does also the
_corpus mammillare_ (Edinger, Senckenberg, _Naturf. Gesell._ Bd. xix.,
1896, and Bd. xxii., 1899; Haller, _Morph. Jahrb._ Bd. xxviii., 1900,
p. 252). Among the birds there is great unity of type, the cerebellum
is large and, by its forward projection, presses the optic lobes down
toward the ventro-lateral part of the brain. The cerebral hemispheres
are also large, owing chiefly to the great size of the _corpora
striata_, which already show a differentiation into caudate nucleus,
putamen and globus pallidus. The pallium is reptilian in character,
though its cortical area is more extensive. The geniculate bodies are
very large (Bumm, _Zeits. wiss. Zool._ Bd. xxxviii., 1883, p. 430;
Brandis, _Arch. mikr. Anat._ Bd. xli., 1893, p. 623, and xliii., 1894,
p. 96, and xliv., 1895, p. 534; Boyce and Warrington, _Phil. Trans._
vol. cxci., 1899, p. 293).
Among the Mammalia the Monotremata have a cerebellum which shows, in
addition to the central lobe of the lower vertebrates, a flocculus on
each side, and the two halves of the cerebellum are united by a
ventral commissure, the _pons varolii_. The pallium is reptilian in
its arrangement, but that part of it which Elliot Smith has named the
neopallium is very large, both in the Ornithorynchus and Echidna, a
fact very difficult to account for. In the latter animal the cortical
area is so extensive as to be thrown into many and deep sulci, and yet
the Echidna is one of the lowliest of mammals in other respects. A
well-marked rhinal fissure separates the pyriform lobe from the
neopallium, while, on the mesial surface, the hippocampal fissure
separates the neopallium from the hippocampal area. Just below the
hippocampal fissure a specially coloured tract indicates the first
appearance of the fascia dentata (see fig. 20). The anterior
commissure is divided, as in reptiles, into dorsal and ventral parts,
of which the latter is the larger (fig. 20, _Comm. V. and D_.), while
just behind the dorsal part is the first appearance of the fimbria or
fornix. In addition to the two fissures already named, there is, in
the Echidna, one which in position and mode of formation corresponds
with the Sylvian fissure of higher mammals. Elliot Smith, however,
wisely refuses to homologize it absolutely with that fissure, and
proposes the name of pseudosylvian for it. The pineal body is
rudimentary, and the optic lobes are now, and throughout the Mammalia,
subdivided into four _corpora quadrigemina_.
[Illustration: From _Cat. R.C.S. England_.