FUNCTIONS OF NERVE CELLS AND FIBRES.
The functions of nerve cells and fibres are different. The cell is a highly active mass of living material; and it draws nourishment from the blood, supplied to it in abundance. That nourishment not only keeps the cell living, but is to be considered as raw material which the cell works up so as to derive from it force or energy. The cell is thus a little manufac tory, deriving its raw material from the blood, and developing from it nervous energy. The nerve -fibre, again, is the conductor of the nervous activity ; it affords the pathway along which the energy generated by the cell may be discharged. Of course the nerve-fibre, con sisting, as it does, of living nervous substance, is also capable of generating energy, but its special function is to conduct influences to or from the cells.
Nervous Action. — It has been seen that nerve-fibres are in connection with nerve-cells, and that the fibres conduct the energy developed in the cell ; and so the question arises, "Whither is the energy conducted, to what place does it proceed?" Well, in highly developed creatures like the higher animals and man, nerves proceed to nearly every tissue and organ of the body. They go to muscles and enter even into their very fibres ; they are found in the coats of blood vessels ; they have recently been shown to be intimately connected with the cells of glands; they ramify through the skin ; they are among the chief parts of the structure of the organs of sense. The activity sent to a muscle reveals itself in the contraction of the muscle, in move ment; the activity sent to a gland produces increased action of the gland, increased flow of the gland secretion, the fluid prepared by the gland. Thus it is the organ at the end of the nerve—the terminal organ, as it is called,— and not the nerve-cell or the nerve-fibre, that determines the mode in which the nerve-im pulse shall display itself. Nerves that proceed to muscles are called excito-motor, because the activity they conduct leads to motion ; those that go to glands are called excito-secretory, because they excite to secretion ; those that are found in the walls of blood- vessels are called vasomotor, because they excite changes in the capacity of the vessels, make them wide or narrow, by causing the muscular tissue in their walls to contract or become relaxed; and there are others that are called sensory, for a reason that will be explained immediately.
These names, as we have seen, are founded on a mistake, because it is not the fibre that de termines motion, or secretion, or sensation, but the organ in which it ends, the terminal organ.
Another question arises, namely, When or why does nervous activity display itself ? what causes a nerve-cell or a group of them to dis charge its energy along a nerve, so that move ment or some other effect is produced? The cause is called a stimulus, or an excitation. For example, a barrel of gunpowder is a store of energy, but the energy is quiet, latent, confined, and cannot reveal itself until it has been liber ated. Apply a lighted match to the barrel : the match excites the powder, causing it at once to liberate and discharge its energy. The lighted match was the stimulus to the gunpowder. So nerve-cells and fibres require a stimulus before they will discharge or transmit their energy. Nerve structures may be stimulated mechani cally, by pinching or pricking. They may be stimulated chemically ; for example, something sour taken into the mouth causes at once a great flow of fluid —saliva—into the mouth, because the nerve supplying the salivary glands has been stimulated by the acid substance. Again, nervous structures may be stimulated electrically. Every one knows that if he takes into his hands the handles of a moderately strong electrical coil, his fingers close over the handles of the instrument, and though he desire it ever so much he cannot let go. That is be cause the electricity has stimulated the muscles, directly no doubt, but also through their nerves, to contract, and, so long as the stimulus con tinues, his muscles remain contracted, thus keep ing his hands closed. Heat also will stimulate nerves.