Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/123

Rh nerves, and with them escape through the posterior lacerated foramen. Trace them to the muscles of the neck.

The twelfth nerves (hypoglossal) leave the medulla behind the first ten cranial nerves, escape from the skull through the hypoglossal canals, and may be traced easily to the ventral part of the tongue.

Exercise XXVIII. Draw a ventral view of the brain, showing the origins of the cranial nerves.

Using a very sharp razor cut the brain in two along a plane parallel to, and a millimeter or two at the left of, the longitudinal fissure. Place the larger piece in water and cautiously dissect away the remnants of the left half of the brain, exposing the structures described below.

The corpus callosum is the thick commissure connecting the cerebral hemispheres. Posteriorly it takes the form of an oval mass (splenium); anteriorly it bends downward (genu), forming the rostrum. This section exposes a region of the brain not observed on the dorsal side of the organ—the thalamencephalon, or diencephalon. The diencephalon of the dogfish, frog and other lower vertebrates is visible dorsally, but in mammals the backward extension of the cerebral hemispheres, which is in general coincident with the development of higher intelligence, conceals the diencephalon.

The third ventricle is the cavity within the diencephalon. The fornix is dorsal to this ventricle. From its union, posteriorly, with the corpus callosum, the fornix runs forward and downward anterior to the third ventricle. Observe the anterior commissure at the lower end of the fornix. The lamina terminalis is the anterior boundary of the third ventricle. The conspicuous intermediate mass, or middle commissure, extends from one lateral wall to the diencephalon to the other through the third ventricle. The in-