h are considered to
subdivide the hemisphere into lobes, whilst others separate the gyri
in each lobe from each other. In each hemisphere of the human brain
five lobes are recognized: the temporo-sphenoidal, frontal, parietal,
occipital, and the central lobe or Island of Reil; it should, however,
be realized that these lobes do not exactly correspond to the outlines
of the bones after which they are named. Passing obliquely on the
outer face of the hemisphere from before, upward and backward, is the
well marked _Sylvian fissure_ (fig. 9, s), which is the first to
appear in the development of the hemisphere. Below it lies the
temporo-sphenoidal lobe, and above and in front of it, the parietal
and frontal lobes. As soon as it appears on the external surface of
the brain the fissure divides into three limbs, anterior horizontal
(s^1), ascending (s^2), and posterior horizontal (s^3), the latter
being by far the longest. The place whence these diverge is the
Sylvian point and corresponds to the pterion on the surface of the
skull (see ANATOMY: _Superficial and Artistic_). Between these three
limbs and the vallecula or main stem of the fissure are four
triangular tongues or opercula; these are named, according to their
position, orbital (fig. 9, C), frontal (pars triangularis) (B),
fronto-parietal (pars basilaris) (A) and temporal. The frontal lobe is
separated from the parietal by the _fissure of Rolando_ (fig. 9, r)
which extends on the outer face of the hemisphere from the
longitudinal fissure obliquely downward and forward towards the
Sylvian fissure. About 2 in. from the hinder end of the hemisphere is
the _parieto-occipital fissure_, which, commencing at the longitudinal
fissure, passes down the inner surface of the hemisphere, and
transversely outwards for a short distance on the outer surface of the
hemisphere; it separates the parietal and occipital lobes from each
other.

[Illustration: From Cunningham, _Text-book of Anatomy_.

FIG. 9.--Gyri and Sulci, on the outer surface of the Cerebral
Hemisphere.

f^1, Sulcus frontalis superior.
f^2, Sulcus frontalis inferior.
f.m, Sulcus frontalis medius.
p.m, Sulcus paramedialis.
A, Pars basilaris.
B, Pars triangularis.
C, Pars orbitalis.
S, Sylvian fissure.
s^1, Anterior horizontal limb (Sylvian fissure).
s^2, Ascending limb (Sylvian fissure).
s^3. Posterior ho