For convenience sake, the behaviorist calls whatever happens to the victim of an experiment the “stimulus” or “situation,” and whatever the victim does in consequence the “response.” So we will imagine that a stimulus of some sort, say the spectacle of Eliza crossing the ice, impinges upon a receptor--in this case the retina. Physically, this is a light stimulus, and for a brief instant Eliza may be said to be all in your eye. But a conductor (the optic nerve) takes the matter up and transmits an impulse (in the form of a “wave of chemical decomposition”) to the brain. Here it encounters a lot of cells, called neurones. If J. B. Eggen be right in saying that “there is nothing inside the brain but a lot of neurones,” it could not well encounter anything else. A neurone under the microscope looks somewhat like a bush which has been pulled up by the roots. The ball of earth adhering to it represents the nucleus, or brain-cell proper. The branches are called dendrites, and in the human body they extend from sometimes less than a millimeter to sometimes more than a yard in length--in the latter case becoming what the layman usually means when he says “nerves.” Each nucleus possesses several, often a great number, and it is through these dendrites that it receives the neural, or nervous, impulse which it passes on through its axone to the next cell. There should be a small tap-root to our bush, terminating in brush-like fibres, to represent this axone. Usually there is but one to a cell, though it may send off collaterals. We have many millions of these neurones, each one complicated beyond belief if examined in detail. But let us not venture too far into that awful tangle of vaguely definable structures where even the neurologist gropes with difficulty, and about which he is constrained to make his tentative conclusions with bated breath. But it may be well to note that neurones are of three sorts. Let us call them receiving neurones, association neurones, and sending neurones; and let us liken them to three lines of soldiers, the receivers busily getting information from No Man’s Land, the associators passing it on to the senders operating the artillery. Receptors, conductors and effectors again. But out of fairness to Dr. Watson it should be said that he calls them nothing of the sort. In fact, he thinks that “entirely too much” has been made of the association neurone, and “of the whole localization of function.” The brain is not popular with behaviorists anywhere, for sometimes it shows a tendency to upset their most cherished theories.