Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

52 ANATOMY.
Special emptied of blood, weigh in the adult from two pounds
Anatomy, five ounces to three pounds three ounces, and at an ave-
'~'*’~v~N“y,rage about three pounds; and of this the brain alone
weighs two pounds. The statement of Haller, that the
brain weighs five pounds, is incredible, unless it be under¬
stood of Troy weight, in which case even it seems exag¬
gerated, since of more than 200 brains weighed by Soem¬
mering, not one amounted to four pounds. The statement,
that the brain of Cromwell weighed six pounds and one
fourth, seems equally incredible. The specific gravity of
the adult brain is to water as 1031 to 1000. This, how¬
ever, varies with age. The cerebellum weighs about five
or six ounces, and is therefore about the seventh part only
of the weight of the brain.
General The brain (cerebrum) is divided above and before into
d>-ion of tw0 lateral halves, named hemispheres (hemispfucria),
the brain. rjg|it and jept) separated by a deep furrow, in which the
vertical, crescentic, or dichotomous portion (falx) of the
hard membrane is received. Each hemisphere is bounded
by a superior or convex, an inner or plane, and an inferior
convex and concave surface. The lower surface of each
hemisphere, also, anatomists distinguish into three lobes,
an anterior, posterior, and middle.
The cerebellum is also divided into two hemispheres,
separated by a middle furrow of less depth, receiving, as
that of the brain, a crescentic production, smaller in size,
from the hard membrane.
Convolut- The exterior surface of the convoluted division is form-
s^r*ace ed into eminences longitudinal and rounded, but directed
braird vari°us ways> named convolutions or circumvolutions
(gyri, Soemmering, Wenzel), and separated from each
other by deep hollows (sulci). To see this surface, which
is termed the convoluted, the vascular membrane termed
pia mater (meninx tenuis) must be removed.
The convoluted surface of each hemisphere may be
divided into five regions : 1. The commutual or dichoto¬
mous ; 2. the lateral-superior or convex; 3. the antero¬
inferior or frontal; 4. the medio-inferior or spheno-tem-
poral; and 5. the posterior or cerebellic region.
1. The commutual, plane, of a shape nearly semicircular,
forms the mesial boundary of each hemisphere, and cor¬
responds to the falciform or dichotomous portion of the
hard membrane (yr\\iy\ 6%Kr\oa, meninx dura), by which it
is separated from the similar surface of the opposite he¬
misphere. Before and behind it extends from the supe¬
rior to the inferior surface of the brain; but a consider¬
able portion of its middle is terminated by the upper sur¬
face of the middle band (mesolobe, corpus callosum), which
lies between the two hemispheres. It is contained be¬
tween the semicircular and the rectilinear margins.
2. The convex region occupies the anterior, upper,
lateral, and posterior parts of the hemisphere, from their
anterior to their posterior extremity, and from the semi¬
circular margin to a line which extends between these
extremities along the lateral borders of the organ.
3. The antero-inferior ox frontal rests on the horizontal
part of the frontal and ethmoid bones, commencing before
with a curved outline, bounded behind by the curvilinear
hollow named the fissure of Sylvius, and at its inner or
mesial margin by the great fissure which separates the
hemispheres. This inner margin presents one convolution,
consisting of a longitudinal eminence, extending in the
adult brain about inch from the posterior towards the
anterior end of the notch. The outer furrow contains the
cerebral portion of the first pair or olfacient nerves. (1,1.)
4. The medio-inferior or spheno-temporal is situate im¬
mediately behind this region, from which it is separated
by the curvilinear hollow (fossa Sylvii). In the ordi¬
nary descriptions this forms the middle lobe; while the
posterior part, corresponding to the cerebellum, though dis- Special
tinguished by no evident limit, is with equal impropriety Anatomy,
named \he posterior lobe. The whole region, from the cur-''J^~v~v“//
vilinear hollow to the posterior tip of the hemisphere,
may, however, be subdivided into two, the medio-inferior
and postero-inferior regions of the convoluted surface, ac¬
cording as they correspond to different containing parts.
5. The posterior cerebellic region of the convoluted sur¬
face, which is plane, corresponds to the horizontal or ce¬
rebellic part of the hard membrane.
The convoluted surface is formed of cerebral matter, of
a gray or dirty wax colour, the surface of which is smooth
and polished where it has not been rent by the removal
of the membranes and their attachments. In the fur¬
rows are many minute orifices, into which the soft mem¬
brane (XeTry meninx tenuis, pia mater) transmits
filamentous bodies, containing minute blood-vessels.
Neither the eminences nor the hollows are uniform in
number or distribution ; and in no two brains is it possible
to trace any similarity in their figure, presence, or direc¬
tion, in the upper, lateral, and posterior part of the con¬
voluted surface, unless where it approaches the central
or figurate surface, where a number of important objects
are presented.
The convoluted surface communicates with another
interior surface at two parts; ls£, on the middle plane,
under the posterior end of the middle band or mesolobe
(corpus callosum) ; 2ri, on each side of the middle plane,
at the outer margin of the fluted masses termed limbs of
the brain (crura cerebri), between these limbs and the
posterior end of the optic chamber or couch (thalamus
opticus). This surface of the organ may be termed the
central or figurate.
The exterior surface of the cerebellum consists of thin Laminated
portions of cerebral substance named plates (lamince), or surface of
leaves (folia), placed contiguously, either parallel or con- ^ cere‘
centric, and separated by furrows of various depth. Thisbe um*
surface, which may be named the laminar ox foliated sur¬
face of the small brain, communicates also with the figu¬
rate surface,—ls£, above on the middle plane, between
the semilunar notch behind, and the white cerebral plate
termed Vieussenian valve before; 2d, at its inferior sur¬
face, between the almonds or spinal lobules above, and the
upper end (medulla oblongata) of the spinal chord below.
The outline of each hemispherical surface of the cerebel¬
lum describes three fourths of a circle; and as the seg¬
ments mutually meet towards the mesial plane, the mode of
union varies according to the figure of the objects to which
they are adapted. 1st, The hemispherical border, ap¬
proaching the anterior part of the organ, is suddenly in¬
terrupted where the cerebellic peduncles (crura cerebelli)
are connected with the protuberance ; and, pursuing a
retrograde direction on each side towards the mesial plane,
forms a re-entrant curvature or notch—the semilunar—Semilunar
corresponding to the lower pair of the bigeminous bodies. llotch-
2d, The hemispherical borders, approaching the posterior
part of the cerebellum, proceed, near the mesial plane,
by an acute circular turn, almost straight backwards,
and form, at the posterior edge of the organ, a deep rect¬
angular notch, H, not unlike the figure of the ancient Purse-like
lyre, named the perpendicular fissure of Malacarne, or notch.
the purse-like fissure of Reil, and containing the cerebellic
vertical portion of the hard membrane (falx cerebelli).
Between these two boundaries the cerebellic plates, of
which the hemispheres consist, are united in the middle
by an interlacement, named suture (raphe), of the cere¬
bellum. A large hollow between the hemispheres, ex¬
tending backwards from the semilunar to the purse-like
fissure, is the small valley (vallecula) of Haller.