rst levels taken
together constitute the so-called "voluntary circuit" (see Fig. 2).

In addition to this general demarcation of functions as higher and
lower--first, second, and third level--in their anatomical seat, many
interesting discoveries have been made in the localization of the
simpler functions in the cortex itself. The accompanying figures
(Figs. 3 and 4) will show the principle centres which have been
determined; and it is not necessary to dwell upon additional details
which are still under discussion. The areas marked out are in general
the same on both hemispheres, and that is to say that most of the
centres are duplicated. The speech centres, however, are on one side
only. And in certain cases the nervous fibres which connect the cortex
with the body-organs cross below the brain to the opposite side of the
body. This is always true in cases of muscular movement; the movements
of the right side of the body are controlled by the left hemisphere,
and _vice versa_. 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. 3 contains a number of
areas which give, when stimulated with electricity, very definite and
regular movements of certain muscles on the opposite side of the body.
By careful exploration of these areas the principal muscular
combinations--those for facial movements, neck movements, movements of
the arm, trunk, legs, tail, etc.--have been very precisely
ascertained. It was concluded from these facts that these areas were
respectively the centres