THE WORK OF THE SENSES.--And what a problem the senses have to solve! On the one hand the great universe of sights and sounds, of tastes and smells, of contacts and temperatures, and whatever else may belong to the material world in which we live; and on the other hand the little shapeless mass of gray and white pulpy matter called the brain, incapable of sustaining its own shape, shut away in the darkness of a bony case with no possibility of contact with the outside world, and possessing no means of communicating with it except through the senses. And yet this universe of external things must be brought into communication with the seemingly insignificant but really wonderful brain, else the mind could never be. Here we discover, then, the two great factors which first require our study if we would understand the growth of the mind--_the material world without, and the brain within_. For it is the action and interaction of these which lie at the bottom of the mind's development. Let us first look a little more closely at the brain and the accompanying nervous system. 3. STRUCTURAL ELEMENTS OF THE NERVOUS SYSTEM It will help in understanding both the structure and the working of the nervous system to keep in mind that it contains _but one fundamental unit of structure_. This is the neurone. Just as the house is built up by adding brick upon brick, so brain, cord, nerves and organs of sense are formed by the union of numberless neurones. [Illustration: FIG. 6.--Neurones in different stages of development, from _a_ to _e_. In _a_, the elementary cell body alone is present; in _c_, a dendrite is shown projecting upward and an axon downward.--After DONALDSON.] THE NEURONE.--What, then, is a neurone? What is its structure, its function, how does it act? A neurone is _a protoplasmic cell, with its outgrowing fibers_. The cell part of the neurone is of a variety of shapes, triangular, pyramidal, cylindrical, and irregular. The cells vary in size from 1/250 to 1/3500 of an inch in diameter. In general the function of the cell is thought to be to generate the nervous energy responsible for our consciousness--sensation, memory, reasoning, feeling and all the rest, and for our movements. The cell also provides for the nutrition of the fibers. [Illustration: FIG. 7.--Longitudinal (a) and Transverse (b) section of nerve fiber. The heavy border represents the medullary, or enveloping sheath, which becomes thicker in the larger fibers.--After DONALDSON.]