of
the receptive organs with the efferent nerves that it is not improbable
that, through the central nervous organ, each receptive organ is
connected with every motor nerve of the whole nervous system,--the facts
of strychnine poisoning show that if this is not literally true it is at
least approximately so. Hence one of the goals to which each afferent
fibre from a receptive organ leads is a number of motor nerves. Their
conducting paths must, therefore, converge in passing to the
starting-points of the motor nerves; because these latter are
instruments common to the use of a number of different receptive organs
in so far as they excite reflex actions. On the other hand those of
their conducting paths which are concerned in the genesis of sensation,
instead of converging, diverge, at least as far as the _cortex cerebri_,
or if not divergent, remain separate. These considerations would make it
appear likely that the conducting path from each receptive organ divides
in the central nervous system into two main lines, one of which goes off
to its own particular region of the _cortex cerebri_ whither run
conductors only of similar sensual species to itself, while the other
main line passes with many others to a great motor station where, as at
a telephone exchange, coordinate use of the outgoing lines is assured to
them all. Now there is in fact a portion of the cortex in mammals the
functions of which are so pre-eminently motor, as judged by our present
methods, that it is commonly designated the _motor cortex_ (see fig.
24). This region of the cortex occupies in the Primates, including Man,
the pre-central gyrus. Among the items of evidence which reveal its
motor capabilities are the following.

[Illustration: FIG. 24.--Diagram of the Topography of the Main Groups of
Foci in the Motor Field of Chimpanzee.]

_The Precentral or Motor Region of the Cortex._--The application to it
of electric currents excites movements in the skeletal muscles. The
movements occur in the half of the body of the side crossed from that of
the hemisphere excited. The "motor representation," as it is termed, is
in the cortex better described as a representation of definite actions
than of particular muscles. The actions "represented" in the top part of
the gyrus, namely next the great longitudinal fissure, move the leg;
those in the lowest part of the gyrus belong to the tongue and mouth.
The topical distribution along the length of the gyrus may