THE MAMMALIAN BRAIN 
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continues posteriorly across the temporal lobe, forming the lateral 
boundary of the hippocampal gyrus; the olfactory lobes, and the 
olfactory tracts, which may be seen as whitish streaks diverging 
as they run posteriorly from the olfactory lobes. In the dien¬ 
cephalon note the optic chiasma in which the optic nerves come 
together and cross before continuing as the optic tracts which 
disappear from view laterally beneath the overhanging edges of 
the temporal lobes of the hemispheres; immediately posterior to 
the chiasma, the cut surface of the hollow stalk of the hypophysis; 
more posteriorly, a rounded elevation, the mammillary body. 
In the mesencephalon two large bundles of fibers, the peduncles 
of the cerebrum, may be seen to diverge from each other and 
disappear under the temporal lobes of the hemispheres. 
2. Rhombencephalon (metencephalon and myelencephalon). 
Dorsal Aspect. —Note the thickened roof of the metenceph¬ 
alon, the cerebellum, consisting of a median lobe, or vermis, and 
two lateral lobes, all with the surface much convoluted. Extend¬ 
ing posteriorly from beneath the cerebellum is the myelencepha¬ 
lon, or the medulla oblongata, the thin vascular roof of which 
(the tela chorioidea posterior) may be easily removed, exposing 
the interior of the fourth ventricle of the brain; note that the two 
narrow fiber tracts, the dorsal funiculi, which run longitudinally 
on each side of the dorsal median fissure of the posterior part of the 
medulla, and the two larger, laterally situated tracts, the lateral 
funiculi, are directly continuous posteriorly with the correspond¬ 
ing funiculi of the spinal cord. 
Ventral Aspect. —Note the floccular lobes of the cerebellum 
upon each side; the transverse band of fibers, the pons (Varolii), 
which connects the two sides of the cerebellum and thus forms the 
floor of the metencephalon; posterior to the pons, in the myelen¬ 
cephalon, a trapezoid body may be seen on each side; while on 
each side of the midline between the trapezoid bodies and 
extending posteriorly the whole length of the medulla are the two 
pyramids, continuations of the two ventral funiculi of the spinal 
cord and separated by the ventral longitudinal fissure; note that 
in the posterior region, bundles of fibers cross from each pyramid 
to the other (decussation of the pyramids); posterior to the 
