512 THE NERVOUS SYSTEM. 
connects them together. The upper surface of the corpus callosum can be 
displayed by gently drawing asunder the contiguous mesial surfaces of the 
cerebral hemispheres. The creat longitudinal fissure is occupied by a mesial 
fold of dura mater, termed the falx cerebri, which partially subdivides the part 
of the cranial cavity allotted to the cerebrum into a right and left chamber. 
External Configuration of each Cerebral Hemisphere.—EKach cerebral hemi- 
sphere presents an external, an internal, and an inferior surface. The external 
surface 18 convex and is adapted accurately to the deep surface of the cranial 
vault. The internal or mesial surface is flat and perpendicular, and bounds the 
great longitudinal fissure. In great part it is in contact with the falx cerebri; and 
where that partition is deficient, it is applied to the corresponding portion of the 
internal surface of the opposite hemisphere. The inferior surface is irregular and 
is adapted to the cranial floor, where it forms the anterior and middle cranial 
fossee, and, behind these, to the upper surface of the tentorium cerebell. Traversing 
this surface in a transverse direction, nearer the anterior end of the hemisphere 
than the posterior end, is the stem of the Sylvian fissure. This deep cleft divides 
the inferior surface into an anterior or orbital area, which rests on the orbital plate 
of the frontal bone, and is consequently concave from side to side, and a more 
extensive posterior or ¢entorial area, which lies on the floor of the lateral part of the 
middle cranial fossa and upon the upper surface of the tentorium cerebelli. This 
surface is arched from before backwards, and looks inwards as well as downwards. 
In its hinder two-thirds it lies above the cerebellum, from which it is separated by 
the tentorium cerebelli. 
The borders which intervene between these surfaces are the supero-mesial, the 
superciliary, the infero-lateral, and the internal occipital. The swpero-mesial 
border, convex from before backwards, intervenes between the convex external 
surface and the flat internal surface of the hemisphere. The swperciliary border is 
highly arched and separates the orbital surface from the external surface. The 
infero-lateral border marks off the tentorial surface from the external surface. The 
internal occipital border can only be seen in cases where the brain has been hardened 
in situ and faithfully retains the natural form. It extends from the posterior end 
of the hemisphere towards the hinder extremity of the corpus callosum, and inter- 
venes between the mesial and tentorial surfaces. It is the border which les along 
the straight blood sinus, and it therefore occupies the angle which is formed by the _ 
attachment of the posterior part of the falx cerebri to the upper surface of the 
tentorium cerebelli. 
The most projecting part of ane anterior end of the cerebral hemisphere is called 
the frontal pole, whilst the most projecting part of the hinder end is termed the 
occipital pole. On the under surface of the hemisphere the prominent point of 
cerebral substance which extends forwards below the Sylvian fissure receives the 
name of the temporal pole. In a well-hardened brain a broad groove is usually 
present on the inner and lower aspect of the occipital pole of the right hemisphere. 
This corresponds to the commencement of the right lateral venous sinus. A less 
distinct groove on the occipital pole of the left hemisphere frequently indicates the 
commencement of the left lateral sinus. On the tentorial surface, a short distance 
behind the temporal pole, a well-marked depression is always visible. This corre- 
sponds to the high elevation on the anterior surface of the petrous portion of the 
temporal bone over the superior semicircular canal. 
Cerebral Gyri and Sulci.—The surface of the cerebral hemispheres are rendered 
highly irregular by the presence of convolutions or gyri, separated from each other 
by. intervening furrows of very varying depth, termed sulci or fissures. The surface 
pattern which is presented by these gyri and sulci is, in its general features, the 
same in all normal human brains; but when the comparison is pushed into minute 
detail many differences become manifest, not only in the brains of different indi- 
viduals, but also in the two cerebral hemispheres of the same individual. 
There are two varieties of furrows, viz. complete and incomplete. The complete 
fissures are few in number in the adult brain, and are formed by inwardly-directed 
infoldings involving the entire thickness of the cerebral wall. They consequently 
show in the interior of the cerebral cavity or lateral ventricle in the form of internal 
