4. The mesencephalon (p. 351, and Figs. 141 and 142). Study it first in
a preparation. Then study the floor on your own specimen; origin of
third nerves.
5. The diencephalon (Figs. 141 and 142). Study the roof and thalami and
the pineal body on a preparation and on a longitudinal section; the
floor on your specimen.
6. The telencephalon (p. 357). (Note that only _one_ side of this is to
be dissected.)
_a._ Study it externally; sulci and gyri (Figs. 145 and 146).
_b._ Examine a preparation showing the corpus callosum (Fig. 147). Then
slice away with a very sharp scalpel the top of _one_ hemisphere nearly
to the corpus callosum (see the preparation). Expose the corpus callosum
on this side to its cranial and caudal borders, by _tearing_ away the
brain-substance at its side and above it.
_c._ Raise the corpus callosum at the side and remove it, thus exposing
the lateral ventricle in which note the septum pellucidum and fornix,
the corpus striatum, and choroid plexus of the lateral ventricle (Fig.
148). (These are to be exposed on _one_ side only, the other being left
intact.)
_d._ Expose the anterior and inferior horns of the ventricle and find
the hippocampus, the fimbria, caudal part of the fornix, the foramen of
Monroe, the anterior commissure. See all these also on a preparation
(Fig. 148).
_e._ Remove the occipital and parietal portions of the cerebrum, on the
side already dissected, so as to expose the roof of the third ventricle
and the midbrain in your specimen, and note the pineal body, choroid
plexus of third ventricle, and structures on the roof of the midbrain
(Fig. 141).
_f._ Remove the choroid plexus or roof of the third ventricle and study
again the thalami (Fig. 141).
_g._ Make a longitudinal section of the brain, in the following manner:
Use a very sharp large scalpel, or a razor. Have this wet with the
alcohol mixture at the time of using. Place the brain ventral surface
down on a sheet of cork or a block of soft wood, the long axis of the
brain coinciding with the direction of grain of the wood. Holding the
brain firmly with one hand, place the wet knife between the hemispheres
with its edge resting on the corpus callosum. See that it is in the
median plane and parallel with the long axis of the brain. See also that
it is not inclined to one side or the other, so that it will make on
cutting a median section throughout. The point of the knife should just
reach the cork or wood between the olfactory bulbs. Now draw the knife
caudad, keeping its point against the cork: the brain will thus be
divided.
If the section is not exactly median, observe the amount of divergence
by placing the two halves together and finding the median ventral line.
Then on the half that has _too much_ slice away thin shavings until the
cavities are exposed, showing the section to be median. Compare with a
demonstration section or Fig. 143. Draw the section and compare with a
section of shark’s brain (see Fig. 143).
_h._ Study a series of transverse sections, identifying parts. Observe
especially in these sections the fornix, corpus callosum, and
ventricles, and the distribution of white and gray matter (see Figs.
149-153).