for impressions from the outside world. The nerve-cells carrying
messages from the various parts of the body terminate in particular
areas. Thus an area in the back part of the brain receives messages
from the eyes; another area near the top of the brain receives messages
from the skin. These areas are quite clearly marked out and may be
studied in detail by means of the accompanying diagram.

There is another large group of nerve-cells which, when traced out, are
found to have one terminal in the brain and the other in the muscles
throughout the body. The area in the brain, where these neurones
emerge, is near the top of the brain in the area marked _Motor_ on the
diagram. From here the fibers travel down through the spinal cord and
out to the muscles. The nerve-cells in this group are called motor
neurones and their function is to carry messages from the brain out to
the muscles, for a muscle ordinarily does not act without a nervous
current to set it off.

So far we have seen that the brain has the two functions of receiving
impressions from the sense-organs and of sending out orders to the
muscles. There is a further mechanism that must now be described. When
messages are received in the sensory areas, it is necessary that there
be some means within the brain of transmitting them over to the motor
area so that they may be acted upon. Such an arrangement is provided by
another group of nerve-cells in the brain, having as their function the
transmission of the nervous current from one area to another. They are
called association neurones and transmit the nervous current from
sensory areas to motor areas or from one sensory area to another. For
example, suppose you see a brick falling from above and you dodge
quickly back. The neural action accompanying this occurrence consists
of an impression upon the nerve-cells in the eye, the conduction of the