The stimulations coming in from the body to the brain generally travel on the same side, although in certain cases parallel impulses are also sent over to the other hemisphere as well.
For example, the very important optic nerve, which is necessary to vision, comes from each eye separately in a large bunch of fibres, and divides at the base of the brain, so that each eye sends impulses directly to the visual centres of both hemispheres.
[Illustration: FIG.

3 .-- Outer surface of left hemisphere of the brain (modified from Exner): _a_, fissure of Rolando; _b_, fissure of Sylvius.] [Illustration: FIG.

4 .-- Inner (mesial) surface of the right hemisphere of the brain (modified from Schaefer and Horsley).

In both figures the shaded area is the motor zone.] Of all the special questions which have arisen about the localization of functions in the nervous system, that of the function of certain areas known as "motor centres" has been eagerly discussed.

The region on both sides of the fissure of Rolando in Fig.