The brain will usually be found to be in an entirely satisfactory
condition for study in any specimen injected with five per cent.
formalin or the glycerine and formalin mixture. The brain is a little
swollen, but all parts are well preserved, and the white and gray matter
are clearly marked off from each other. Either the specimen used for the
muscles or that employed for the blood-vessels may therefore be
used,--or if the brain was removed from the specimen employed for the
viscera, that will be satisfactory.
The following directions for removing the brain are designed for
specimens preserved as above. For removing the _fresh_ brain the process
is essentially similar, but as the brain is then very soft, care should
be taken not to tear it. The fresh brain should be preserved in the
alcohol-formalin mixture given below, and should be allowed to rest only
on some soft substance, as absorbent cotton.
Remove the head from the body by cutting through the neck a little
craniad of the first rib if this has not already been done. Remove all
skin, muscles, and other soft parts from the head and cervical vertebræ,
as far as possible. Remove the structures in the orbit by cutting
through the zygomatic arch at each end, and removing it. The lower jaw
should also be removed, if this has not already been done. (If a fresh
specimen is used, and the head is to be employed for other purposes, the
brain can be removed without separating the head from the body, and
without taking away the lower jaw and other structures on the ventral
surface of the skull.)
Have at hand dissecting-instruments and a dish containing alcohol and
formalin in the following proportions (Parker and Floyd’s mixture):
95 per cent. alcohol 6 parts
2 per cent. formalin 2 parts
In the bottom of the dish should be placed a little absorbent cotton, to
support the brain.
In removing the brain have at hand entire and dissected skulls and note
the relations of parts on these as far as necessary before cutting the
specimen.
With bone-forceps make a small opening in the parietal bone so as to
expose the dura mater, but do not cut through the dura mater. With some
blunt instrument free the dura mater from the bone about the opening,
and continue to cut away the bone until the dorsal and lateral faces of
the cerebrum are fully exposed craniad of the tentorium. The olfactory
bulbs (Fig. 137, _I_) should be exposed carefully and as fully as
possible. Cut away the dorsal arch of the atlas and carefully insert the
forceps in the foramen magnum and, working as before, remove the
squamous portion of the occipital and the parietal bones as far as the
tentorium and as far ventrad as possible. Leave the dura mater intact if
possible. Free the surface of the tentorium from the dura mater,
carefully separate slightly the cerebellum and cerebrum; insert the
bone-forceps (not too far) with the blades inclined from without
ventromediad, and cut the tentorium on each side. Remove it slowly,
cutting adhesions to the dura mater. That part of the dura mater which
dips between the cerebral hemispheres is the falx cerebri. Cut the dura
mater along both sides of the falx cerebri and remove it by turning it
down at the sides and cutting it at the level of the cut edge of the
bone. Remove it also from the cerebellum and notice how it dips down on
both sides of the tentorium and in close contact with it. Cut the falx
at the cranial end between the olfactory bulbs and cut the tentorial
dura (cut its adhesions, but do not remove with it the pineal body). The
falx and tentorial dura may then be removed.
Allow the head to hang sideways over the dish of alcohol-formalin in
such a way that the brain will tend to fall out of the cranium. Free the
olfactory bulbs from the bone. Then begin at the caudal end and tilt the
brain out with the handle of a scalpel. In doing this note carefully and
cut the cranial nerves. They should be left with central ends as long as
possible, and those on the side which is uppermost should be cut first.
In doing this refer to the foramina in the base of the skull and to Fig.
138. Take especial pains also not to break off the hypophysis, which is
lodged in the sella turcica.
The brain falls out and rests with its dorsal surface on the cotton. Now
remove the remainder of the dura mater, carefully cutting all adhesions
to nerves. Remove also the pia mater, as far as that can be done without
pulling off at the same time parts of the brain-substance. Preserve the
brain in the alcohol-formalin mixture.
_Study of the Brain._--In the study of the brain demonstration specimens
are to be used as much as or more than your own specimen. See everything
on a demonstration preparation before attempting to expose it in your
own specimen.
I. Examine the brain of a shark or of a frog. Cranial nerves may be
neglected, but the divisions of the brain should be recognized in dorsal
and ventral views and in longitudinal sections, and sketched.
II. Read the general description of the cat’s brain (pp. 339-343), using
your own specimen and a longitudinal section. Cut nothing on your own
specimen except when especially directed to do so. Study the cavities on
a preparation. Compare the diagrams (Figs. 139 and 140) and the figures
of the brain.
III. Study the individual parts as follows. To avoid errors make
constant reference to preparations and figures.
1. The medulla (p. 344 and Figs. 138 and 141). Use your own specimen
and a preparation and dissect out carefully the cranial nerves on your
own specimen.
2. The cerebellum (p. 347). Study it entire, then to expose the fourth
ventricle (p. 349) slice away with a very sharp scalpel one-half of the
cerebellum by making a median longitudinal incision and then horizontal
incisions.