Part 106

3. Demonstrate the arachnoid by pulling it off with forceps.

4. Reflect the pia mater in the same way as the dura mater.

5. Study the fissures and grooves of the cord.

6. Cut across the cord with fine scissors at the point where it is freed from its membranes and examine the section. Note the arrangement of gray and white matter and the fissures and grooves, particularly the anterior or ventral. Demonstrate the central canal with the blowpipe.

7. Study the origin of the spinal nerves (p. 337). Count them. Direction of exit? Carefully clean one in the thoracic region from dura mater and connective tissue, with fine scissors, and study dorsal and ventral roots and ganglion (see Fig. 135). Then follow it out and find its dorsal ramus and ventral ramus and the communicating branch of the latter with the sympathetic system. Do not trace the peripheral branches of the nerve at present.

II. THE BRAIN (p. 339).

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.

3. The pons (p. 347).

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.