The easiest way to grasp the organisation of our complex nervous structure is to study its evolution from its humble beginnings in the simplest forms of life. In single-celled animalculæ, the most primitive type of living creatures, a single cell performs all the functions of life; it moves, breathes, assimilates, excretes, and feels. With the development of multi-cellular organisms, however, different cells are given different work to do, and made to do that, and nothing else. It then becomes necessary that co-ordination should be maintained between the sense organs that perceive the prey and the muscles that move to its capture, and for this purpose other cells are told off to specialise in communication. Thus it will be seen that the functional unit of the nervous system is not the nerve cell, but what is called the SENSORI-MOTOR ARC, consisting of a nerve carrying the incoming sensation from a sense organ and making contact with another nerve which carries the outgoing impulse to a muscle or organ. When a multiplicity of muscles becomes available for movement, it is necessary to further link up the sensori-motor arcs, so that other parts of the structure may be brought into play, and the response not be confined to one muscle alone; so nerve cells form loops upon the arcs, and loops upon the loops, with further intercommunications among themselves, the organisation becoming more and more elaborate, admitting of more and more complex reactions to stimulus, till finally the wonderful complications of the human brain are achieved. CHAPTER III HOW AN IDEA ENTERS THE MIND When an impression is made on a sense organ, the sensation derived from it is telegraphed up the connecting nerve fibre to the brain, and there translated, by a process of which we know nothing, from a sensation to a thought. We believe that the mind learns by experience to associate certain kinds of sensation with certain objects or conditions in the environment, and when it feels these particular sensations, deduces that certain objects are present, and forms mental images, or thought models, intended to represent these objects. The truth of our percepts is determined by the closeness with which our thought model corresponds to its original. An exact copy is a true concept, an imperfect copy an inaccurate concept. We “recognise” an object by a process of classification, noting its likeness or unlikeness to other objects already known. When an unfamiliar object attracts our attention, we put it through a process of comparison until we find to which compartment in our concept-pictures it should be assigned, and if we cannot find a perfect match, we put it in the most suitable compartment we can discover, and then partition off a little subclass for it, thus admitting its identity in essentials, but its difference in details from the other occupants of that compartment.