ear, the tongue, without which it is quite
impossible for the mind to come into relation with outside things.
Moreover, disease or injury, especially to the organs of sense or to the
brain, weakens or destroys mental power. The size of the brain, also, is
found to bear a certain relation to mental capacity; the weight of the
average brain being about 48 ounces, while the brain of an idiot often
weighs only from 20 to 30 ounces.

THE NERVOUS SYSTEM

[Illustration: Brain and Spinal Cord]

=Divisions of Nervous System.=--This intimate connection between mind
and body is provided for through the existence of that part of the
bodily organism known as the nervous system, and it is this part,
together with its associated organs of sense, that chiefly interests the
student of psychology. A study of the character and functions of the
various parts of the nervous system, and of the nervous substance of
which these parts are composed, belongs to physiology rather than to
psychology. As the student-teacher is given a general knowledge of the
structure of the nervous system in his study of physiology, a brief
description will suffice for the present purpose. The nervous system
consists of two parts, (1) the central part, or cerebro-spinal centre,
and (2) an outer part--the spinal nerves. The central part, or
cerebro-spinal centre, includes the spinal cord, passing upward through
the vertebrae of the spinal column and the brain. The brain consists of
three parts: The cerebrum, or great brain, consisting of two
hemispheres, which, though connected, are divided in great part by a
longitudinal fissure; the cerebellum, or little brain; and the medulla
oblongata, or bulb. The spinal nerves consist of thirty-one pairs, which
branch out from the spinal cord. Each pair of nerves contains a right
and left member, distributed to the right and the left side of the body
respectively. These nerves are of two kinds, sensory, or afferent,
(in-carrying) nerves, which carry inward impressions from the outside
world, and motor, or efferent, (out-carrying) nerves, which convey
impulses outward to the muscles and cause them to contract. There are
also twelve pairs of nerves connected with the eye, ear, nose, tongue,
and face, which, instead of projecting from the spinal cord, proceed at
once from the brain through openings in the cranium. These are,
therefore, known as cerebral nerves. In their general character,
however, they do not differ from t