THE EVOLUTION OF THE NERVOUS SYSTEM

The easiest way to grasp the organisation of our complex nervousrnstructure is to study its evolution from its humble beginnings in thernsimplest forms of life.rnrnIn single-celled animalculæ, the most primitive type of livingrncreatures, a single cell performs all the functions of life; it moves,rnbreathes, assimilates, excretes, and feels. With the development ofrnmulti-cellular organisms, however, different cells are given differentrnwork to do, and made to do that, and nothing else.rnrnIt then becomes necessary that co-ordination should be maintainedrnbetween the sense organs that perceive the prey and the muscles thatrnmove to its capture, and for this purpose other cells are told off tornspecialise in communication.rnrnThus it will be seen that the functional unit of the nervous systemrnis not the nerve cell, but what is called the SENSORI-MOTOR ARC,rnconsisting of a nerve carrying the incoming sensation from a sensernorgan and making contact with another nerve which carries the outgoingrnimpulse to a muscle or organ.rnrnWhen a multiplicity of muscles becomes available for movement, it isrnnecessary to further link up the sensori-motor arcs, so that otherrnparts of the structure may be brought into play, and the responsernnot be confined to one muscle alone; so nerve cells form loops uponrnthe arcs, and loops upon the loops, with further intercommunicationsrnamong themselves, the organisation becoming more and more elaborate,rnadmitting of more and more complex reactions to stimulus, till finallyrnthe wonderful complications of the human brain are achieved.