Reflex Action. — Now nerve-cells may be stimulated directly, that is, the stimulus may be applied to the nerves themselves. Usually, however, the stimulus is not applied to the cells directly but is conveyed to them along a nerve. The nerve conducts the impression, which it has received, to the nerve-cells; and they, in turn, are stimulated, and discharge their energy along other nerves to muscles, glands, or other structures, as the case may be. There is thus a chain of events following the irritation of the nerve, and to the completed process a term of great importance in nervous physiology is applied, namely, reflex action.
Fig. 82 will render the meaning of this phrase more easily understood. In the figure, B is a nerve-cell. Leading to it is a nerve A coming from (1) some sensitive surface, say the skin. Connected with the nerve-cell is another nerve c, which passes to a muscle (2). Suppose some thing (a prick, sting, &c.) irritates the surface (1), immediately an impression is transmitted along the nerve A to the nerve-cell B. The cell receives the impression, is stimulated by it, and is thus caused to dis charge its activity along the nerve c to the muscle (2), leading the muscle to contract. This is a simple reflex action, but there may be many much more complicated, in which there may be involved many cells and nerves, and many muscles or blood-vessels or glands, &c. The nerve A leading inwards to the cell is called a sensory nerve because it conveys the irritation that has been made on the surface. But sensory is not a good word, because no sensation or feeling need result. A better word is, therefore afferent (ad, to, and fero, I carry), meaning carrying to the centre. Similarly the nerve c may be called efferent (e or ex, from, and fero, I carry), carrying from the centre. Many of the movements and actions of our daily life are nothing more than compli cated reflex actions. It is to be noted that such actions may occur without effort of will, and even without our being conscious of them. For example, tickle the sole of the foot of a sleeping man. He withdraws his foot, being at the time sound asleep and unconscious. This is a reflex act. The tickling has irritated some
nerves of the skin ; an impression has been transmitted along nerves upwards to nerve-cells in the spinal marrow.' These nerve-cells, being stimulated, have discharged their energy along nerves proceeding to the muscles of the man's leg; the muscles have responded by contracting; and the leg has been moved. All this may Gave happened without sensation on the man's part, the nerve-centres involved in the action being in the spinal man-ow. It is only when the impression reaches the brain by travelling up the spinal marrow that sensation or feeling can arise.
In warm-blooded animals, such as man, a nervous impression has been calculated to pass along a nerve at the rate of about 200 feet per second.
is the energy or activity al ready alluded to as the chief object of the nerve cell to create, and the nerve-fibre to conduct. Nerve-energy is one and the same thing in whatever part of the body produced. Ideas are as much the expressions of nerve-energy as the contraction of a muscle or the activity of a gland. But how nerve-energy is transformed into thought, or is the agent in the production of thought, or what is the kind of intermediate apparatus it stimulates to produce thought, all this we do not understand—perhaps never will. Nevertheless, a man whose nerve-energy is exhausted is as incapable of good intellectual activity as of good physical labour. Nerve energy is produced by nerve-cells, and the nerve-cells manufacture it, so to speak, from the blood. In other words, their activity depends on the blood-supply they receive, and, naturally, not only on the quantity but also on the quality of the blood. Now the blood is nourished by the food taken. Other things being equal, by a proper quantity of food of the right sort, taken at proper intervals, the blood will be maintained in strength and purity, and will, therefore, be fit nourishment for the nerve cells, as for other tissues of the body. It is necessary that this nourishment should be con stantly renewed, because nerve-energy as well as the other forces of the body are being con tinually used up by the daily work of life.