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.
In dissecting the body numerous white cords are found which at first eight might be taken for tendons. That they are something else soon becomes clear, since a great many of them have no connection with muscles, and those which have, usually enter near the middle of the belly of the muscle, instead of being fixed to its ends as most tendons are. These cords are nerve-trunks: followed from the middle line of the body each (Fig. 79) will be found to break up into finer and finer branches, until the subdivisions become too small to be followed without the aid of a microscope. Traced towards the middle of the body the trunk will, in most cases, be found to increase by the union of others with it, and ultimately to join a much larger mass of different structure, from which other similar trunks spring. This mass is nerve-centre. The end of a nerve attached to the centre is, naturally, its central, and the other its distal or peripheral end.
What is it in the body which represents the master builder? What are the relations between the nervous system and the other organs of the body ? Is the control of the nervous system always consciously exercised?
How may we recognize that certain white cords found on dissecting the body are not tendons? What are they? What is found when nerve-trunks are traced towards the outer parts of the body? What when traced in the opposite direction? What is a nerve-centre? What is meant by the peripheral end of a nerve?
Fig. 79. Diagram Illustrating the general arrangement of the nervous system.
Nerve-centres give origin to nerve-trunks; these radiate all over the body, branching and becoming smaller and smaller as they proceed from the centre ; finally they end in or among the cells and fibres of the various organs. The general arrangement of the nerve-centres and of the larger nerve-trunks of the body is shown in Fig. 79.