NEUROLOGY. 113 
siderable depth, called the jisswre of Sylvius, is the posterior limit of each 
anterior segment. ‘The fissure of Sylvius corresponds on each side to the 
posterior concave edge of the ala of the sphenoid bone, which is received 
within it. Commencing from within, at a triangular flat surface (locus per- 
foratus anticus), at the posterior extremity of each olfactory process, it pro- 
ceeds outwards and curves backwards, its convexity directed forwards to 
the lateral surface of the brain. It is of considerable depth, especially at its 
internal extremity, and like all fissures of the brain, is lined by pia mater. 
The island of Reil is found at the bottom of this fissure, projecting from its 
floor’ as a small insulated lobe, bounded by a bifurcation of the fissure. 
The middle segment, which lies immediately behind the fissure of Sylvius, 
is on a plane much lower than the anterior, and corresponds on each side 
to the deep and hollow median fossa of the cranium. It consists of two 
lateral, very convex lobes, known as the middle lobes of the brain, which are 
separated from each other by a deep depression. These lobes, which are 
distinctly defined in front by the fissure, have no exact Boundary behind, 
but pass off very gradually into the posterior lobes of the hemispheres, as may 
be seen by raising up the cerebellum. 
Commencing at the anterior fissure and passing backwards we shall per- 
ceive the following parts. This fissure is limited by the anterior fold or 
reflexion of the corpus callosum; behind this is a thin layer of a greyish’ 
color which principally constitutes the floor of the third ventricle, and is 
known as the tuber cinerewm. The pituitarg process is continuous with it, 
being probably its extension, and is a hollow process of nervous matter 
above the sella turcica by which the brain is in a measure tied to the pitui- 
tary body. : 
~ Immediately in front of the pituitary process, the union of two white 
bands, which form lateral boundaries to a large portion of the tuber cine- 
reum, the optic tracts, takes place along the median line. ‘This forms the 
commissure of the optic nerves from which these nerves diverge. Behind 
the pituitary process, the tuber cinereum extends back to two small and 
very white bodies called corpora mammillaria or albicantia. 
Behind the mammillary bodies, we find a deep depression, lying between 
two thick processes of fibrous matter, which pass from below upwards and 
outwards, expanding as they advance, and upon which each hemisphere is 
sustained like a mushroom on ‘its tale These are the crura cerébri, the 
peduncles of the cerebral hemispheres. The depression: separating them is 
the intercrural space. The nervous matter forming the floor of this space 
is of a greyish color and known as the pons Tarini. The third pair of 
nerves emerges from the interpeduncular space. 
~ he inner margin of each middle lobe of the brain is separated from the 
corresponding crus cerebri by a fissure which passes from behind forwards, 
and terminates in the fissure of Sylvius. Backwards this fissure is continu- 
ous with the transverse fissure separating the cerebrum from the cerebellum, 
and corresponding to the posterior edge of the corpus callosum. The con- 
tinuity thus established between these three series of fissures constitutes the 
great cerebral or transverse fissure. 
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