reaches back over the rest of
the fore-brain, and also over the mid-brain, and hides these and the
pineal gland in the dorsal view of the brain (Figure 2). The hollow of the
hemisphere on either side communicates with the third ventricle, the
original cavity of the fore-brain (1 in Figure 5), by an aperture (the
foramen of Monro), indicated by a black arrow (f.M.). Besides their
original communication through the intermediation of the fore-brain,
the hemispheres are also united above its roof by a broad bridge of
fibre, the corpus callosum (c.c.), which is distinctive of the
mammalian animals. The original fore-brain vesicle has its lateral
walls thickened to form the optic thalami (o.th.), between which a
middle commissure, (m.c.), absent in lower types, stretches like a
great beam across the third ventricle. The original fore-brain is often
called the thalamencephalon, the hemisphere, the prosencephalon,
the olfactory lobes, the rhinencephalon.

Section 121. The parts of mid-brain (mesencephalon) will be easily
recognised. Its cavity is in the adult mammal called the iter; its floor is
differentiated into bundles of fibres, the crura cerebri (c.cb.), figured
also in Figure 4.

Section 122. The cerebellum (metencephalon) consists of a central
mass, the vermis (v.cbm.), and it also has lateral lobes (l.l.),
prolonged into flocculi (f.cbm.), which lastare -em-bedded in pits, [in]
the periotic bone, and on that account render the extraction of the
brain from the cranium far more difficult than it would otherwise be.
The roof of the hind-brain, before and behind the cerebellum,
consists of extremely thin plates of nervous matter. Its floor is greatly
thickened to form the mass of the medulla, and in front a great
transverse track of fibres is specialized, the pons Varolii (p.V.). Its
cavity is called, the fourth ventricle.

Section 123. Figure 2 gives a dorsal view of the rabbit's brain; a
horizontal slice has been taken at the level of the corpus callosum.
The lateral ventricle (i.e., the hollows of the hemisphere) is not yet
opened. A lower cut (Figure 3) exposes this (V.L.). The level of these
slices is approximately indicated in Figure 1 by the lines A and B.
This latter figure will repay careful examination. The arrow, ar.,
plunges into the third ventricle, behind the great middle commissure
(m.c.), and the barb is supposed to lie under the roof of the
mid-brain, the corpora quadrigemina (c.q.). The position