bs 
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440 THE NERVOUS SYSTEM. 
are finally jomed together by a short commissural portion, termed the optic chiasma. 
The optic nerve is continued forwards and outwards, on either side, from the chiasma 
and tract. 
The crura cerebri, the optic tract, and the optic chiasma enclose a deep 
rhomboidal or lozenge-shaped interval on the base of the brain, which is termed 
the interpeduncular space. Within the limits of this area the following parts may 
be seen as we pass from behind forwards: (1) the locus perforatus posticus ; (2) the 
corpora mammillaria; (3) the tuber cinereum and the stalk of the pituitary body. 
At its posterior angle, immediately in front of the pons Varolii, the inter- 
peduncular space is very deep and is floored by a layer of gray matter, which is 
perforated by numerous small apertures. This is the locus perforatus posticus. 
Through the apertures which are dotted over its surface the small postero-mesial 
basal branches of the posterior cerebral artery enter the brain. 
The corpora mammillaria are two small white pea-like eminences placed side by 
side in front of the locus perforatus posticus. 
The tuber cinereum is a slightly raised field of gray matter, which occupies the 
interval between the anterior portions of the optic tracts in front of the corpora 
mammillaria. Springing from the fore-part of the tuber cinereum, immediately 
behind the optic chiasma, is the infundibulum, or the stalk which connects the 
pituitary body with the base of the brain. 
Outside the limits of the fore-part of the interpeduncular space there is on 
either side a small depressed triangular field of gray matter, which leads outwards 
into the Sylvian fissure. It is perforated by the antero- mesial and the antero- 
lateral groups of basal arteries, and receives the name of the locus perforatus 
anticus. 
General Connexions of the Several Parts of the Brain.— The medulla 
oblongata, the pons Varolii, and the cerebellum occupy the posterior cranial fossa, 
and they are separated from the 
cerebral hemispheres which le 
above them by a partition of dura 
mater, termed the tentorium cere- 
belli. Further, they surround a 
cavity, a portion of the primitive 
cavity of the early neural tube, 
which is termed the fourth ventricle 
of the brain, and they all stand in 
intimate connexion with each other. 
The medulla is for the most part 
carried upwards into the pons 
Varolii; but at the same time two 
large strands from its dorsal aspect, 
termed the restiform bodies, are 
prolonged into the cerebellum, and 
constitute its inferior peduncles, or 
the chief bonds of union between 
the medulla and the cerebellum. 
The pons Varolii has large numbers 
of transverse fibres entering into 
its composition, and the great majority of these are gathered together on “either 
side in the form of a large rope-like strand. This plunges into the corresponding 
hemisphere of the cerebellum, and constitutes its middle peduncle. 
The cerebrum, which forms the great mass of the brain, occupies the anterior 
and middle cranial fossee, and extends backwards into the occipital region above the 
tentorium and the cerebellum. The greater part of the cerebrum is formed by the 
cerebral hemispheres, which are separated from each other in the mesial plane by 
the great longitudinal fissure. At the bottom of this fissure is the corpus callosum, 
a broad commissural band which connects the two hemispheres with each other. 
Each hemisphere is hollow, the cavity in its interior being termed the lateral 
ventricle of the brain. Between and below the cerebral hemispheres, and almost 
Mesencephalon 
Superior cerebellar peduncle 
Middle cerebellar peduncle 
Inferior cerebellar peduncle 
Fria. 326.—ScHEMA, showing the connexions of the several 
parts of the brain. 
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