THE EVOLUTION OF THE NERVOUS SYSTEM
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.