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FISSURE OF SYLVIUS. 513 
elevations on its wall. The complete fissures are the following: (1) the dentate or 
hippocampal fissure; (2) the anterior part of the calcarine fissure; and (3) a 
portion of the collateral fissure. ‘The incomplete fissures are merely surface furrows 
of very varying depth, which do not produce any effect on the inner surface of the 
ventricular wall. 
General Structure of the Cerebral Hemispheres.—Each cerebral hemisphere 
is composed of an outside coating of gray matter, spread in a continuous and un- 
interrupted layer over its surface, and an internal mass of white matter, which forms 
for the most part the immediate wall of the ventricular cavity. The gray coating 
is termed the cerebral cortex, and the internal white matter is called the medullary 
centre. Each convolution shows a corresponding structure. On transverse section 
it is seen to present an external covering of gray cortex, supported by a central core 
of white matter. 
But, in addition to the gray matter on the outside, there are certain large 
deposits of gray matter embedded in the basal part of each cerebral hemisphere. 
These cerebral nuclei constitute the corpus striatum, and, although to some extent 
isolated from the gray matter on the surface, it can be easily shown that at certain 
points they are directly continuous with it. 
By means of the convolutions and sulci, the gray matter on the surface of the 
hemisphere is enormously increased in quantity without unduly adding to the bulk 
of the organ; and, further, the vascular pia mater, which dips into every fissure, 1s 
increased in extent to a like degree. Opportunity is, therefore, afforded to the 
cortical vessels of breaking up into twigs of exceeding fineness before they enter the 
substance of the hemisphere. The distribution of blood to the gray cortex is, in this 
way, equalised and rendered uniform. 
Cerebral Lobes and Interlobar Fissures.— Certain of the fissures which 
traverse the surface of the cerebrum are arbitrarily chosen for subdividing the 
surface into districts or areas, which are termed lobes. ‘These fissures are termed 
interlobar, and are the following: (1) the fissure of Sylvius; (2) the fissure little 
Rolando; (3) the parieto-occipital; (4) the calloso-marginal; (5) the collateral; 
and (6) the limiting sulcus of Reil. 
The lobes which are mapped out by these fissures are: (1) the frontal; (2) the 
parietal; (3) the occipital; (4) the temporal; (5) the insula, or the Island of Reil; 
(6) the limbic. To these may be added a seventh lobe, in no way related to the 
interlobar fissures, viz. the olfactory lobe. With the exception of the occipital and 
olfactory lobes and the insula, this subdivision of the hemisphere presents little 
morphological or physiological value, and is chiefly adopted for topographical 
purposes. 
Fissure of Sylvius (fissura cerebri lateralis)—This is the most conspicuous 
fissure on the surface of the cerebral hemisphere. It is composed of a short main 
stem, from the outer extremity of which three branches or limbs radiate. The 
stem of the Sylvian fissure is placed on the inferior surface of the hemisphere. It 
begins at the locus perforatus anticus in a depression termed the vallecula Sylvii. 
From this it passes horizontally outwards, forming a deep cleft between the 
temporal pole and the orbital surface of the frontal lobe. Appearing on the 
outer surface of the hemisphere at a point called the Sylvian point, the Sylvian 
fissure immediately divides into three radiating branches. These are: (1) the 
ramus horizontalis posterior; (2) the ramus horizontalis anterior; (3) the ramus 
anterior ascendens. ' 
The posterior horizontal limb is the longest and best marked of the three limbs. 
It extends backwards, with a slight inclination upwards on the outer surface of the 
hemisphere for a distance which may vary from about two to three inches. It 
intervenes between the frontal and parietal lobes which lie above it and the 
temporal lobe which lies below it, and it finally ends in the region subjacent to the 
parietal eminence of the cranial wall by turning upwards into the parietal lobe in 
the form of an ascending terminal piece. 
The anterior horizontal limb extends horizontally forwards in the frontal lobe for 
a distance of about three-quarters of an inch immediately above and parallel to the 
posterior part of the superciliary margin of the hemisphere. 
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