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]).
III. PERIPHERAL NERVOUS SYSTEM.
(There are some advantages in dissecting the eye with its muscles before dissecting the nerves, as a knowledge of the eye-muscles is presupposed for dissecting some of the cranial nerves. For directions on the eye, see [p. 469].)
A new specimen should be used, if possible, for the peripheral nervous system, though that used for the blood-vessels can be employed, at considerable disadvantage.
Prepare as for the blood-vessels. The arteries should be injected with red starch, to aid in tracing the nerves.
1. The Cranial Nerves ([p. 369]) and Sympathetic System ([p. 404]).
1. Reflect the skin covering the sternomastoid muscle, and make a longitudinal incision of the muscle so as to expose the carotid artery. Lying along the artery find the combined trunk of the sympathetic and vagus nerves. Follow the vagus ([p. 378]) first craniad; transect the muscles as necessity arises, and find its ganglion nodosum and at the same time locate the superior cervical ganglion of the sympathetic nerve ([p. 404], and [Fig. 156]). Then find the hypoglossal nerve ([Fig. 156], b), passing outside of the carotid artery to the tongue, and the accessory ([Fig. 156], c), passing to the trapezius. Cut and reflect the digastric muscle and find the small glossopharyngeal nerve ([Fig. 156], a), passing to the surface of the bulla and then beneath the carotid artery.
2. Follow the vagus ([p. 378]) caudad to its termination. To do this it is necessary to remove one side of the thorax, as in dissecting the blood-vessels. Do not injure the nerves of the axilla, nor the phrenic or sympathetic nerves. For the vagus in the thorax, compare [Fig. 157]. Find the branches of the nerve; in dissecting them, pull on them to make them tense. They are then more easily visible. To dissect the abdominal portion of the vagus, open the abdominal cavity, and compare [Fig. 164] ([p. 407]).