This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
From what has been stated in the previous paragraphs it is clear that we may distinctly separate the nerve-trunks from the nerve-centres. The fibres serve simply to convey impulses either from without to a centre or in the opposite direction, while the centres conduct and do much more. They take heed, some consciously and some unconsciously, of the impulses carried to them by the ingoing nerve-fibres, and then send out impulses along outgoing nerve-fibres; these impulses call into action the proper organs for the safety and well-being of the body in general. The centres do not merely transmit and reflect, they also co-ordinate.
The nerve-fibres of the body fall into two great groups corresponding to those which carry impulses to the centres and those which carry them out from the centres. The former are called afferent or sensory fibres and the latter efferent or motor.
For what is the spinal cord a centre? What else does it do? How does histology support the belief that the spinal cord is both a nerve-centre and a conductor of nervous impulses?
What is the function of nerve-fibres? What is done by nerve-eentres in addition to conducting nerve-impulses?
Into what main groups may nerve-fibres be classified?
The posterior roots of the spinal nerves contain only afferent, the anterior only efferent, nerve-fibres.
Nerve-centres are of three kinds: (1) Automatic centres, which, without being excited by the action of any sensory nerve or by the Will, originate in themselves stimuli for efferent nerves. (2) Reflex centres, which act quite independently of the Will and of consciousness, but are aroused by the action of a nervous impulse conveyed to them by a sensory nerve, and in turn excite one or more efferent nerves. (3) Conscious or psychic centres, whose activity is accompanied by some kind of mental action; as feeling, or willing, or reasoning.