Fig. 7. Representation of the manner in which a nerve is seen to terminate in a muscle, such ending being one form of "nerve-ending" termed a "muscle plate." It tends to emphasize the close relationship existing between muscle and nerve, and to justify the expression "neuro-muscular mechanism," the nervous system being as important for movements as the muscles. (Schäfer's Histology.)
Complicated as is the nervous system, modern advances in the sciences of anatomy and physiology have made the comprehension of the subject easier. It is now known that the nervous system, in spite of its wide ramifications, is also made up of cells which are structurally and functionally related to each other, and make connection with every part of the whole community, the body. A nerve-cell, or neurone, may be very complicated in its structure because of its many branches or extensions from the main body of the cell.
Fig. 8. Three muscle-fibres lying beside each other, with the small blood-vessels (capillaries) around and between them. Such are the appearances presented under the microscope by skeletal or striped muscles such as those of the larynx. (Schäfer.)
It may be said, in general terms, that the nervous centres, the brain and the spinal cord, which are parts of one anatomical whole, are characterized by the presence of the cell-bodies as well as their extensions, while nerves consist only of the extensions or arms of the cell-bodies. The nerve-cell whose body is in the top of the brain may have an extension or arm which may reach practically to the end of the spinal cord, and there make communication with another cell whose arm, in turn, may reach as far as the toe. Such nerve arms or extensions constitute the nerve-fibres, and bundles of these nerves, or nerve-trunks.
Usually nerve-fibres make connection with the cells of an organ by a special modification of structure known as a nerve-ending. A nervous message or influence (nerve-impulse) may pass either to the centre—i.e., toward a cell-body—or from it; in other words, a nervous impulse may originate in the centre or in some organ more or less distant from it; a nervous impulse may be central or peripheral. Nearly all central impulses, we now know, arise because of the peripheral ones. One may illustrate this important relation by a telegraph system. The message a railroad operator sends out—e.g., that which determines whether a train is to be held at a certain station or sent on—might depend wholly on information received from another office. The extra flow of blood to the stomach when food enters it is owing to such a relation of things. The food acts as a stimulus to the ends of the nerve-fibres, and, in consequence, there is an ingoing (afferent) message or impulse, and, by reason of this, an outgoing (efferent) one to the muscle-cells of the small blood-vessels, owing to which they contract less strongly and the calibre of these vessels is increased; hence more blood reaches the smallest vessels of all (capillaries.) Such a physiological relation of things is termed reflex action. For such reflex action there are required structurally at least two neurones or nerve-cells, and functionally a stimulus of a certain strength and quality. Of course, if more blood passes to the stomach there must be less somewhere else, as the total volume of the blood is limited. The value of the knowledge of such a fact is obvious. It must be unwise to exercise vigorously immediately after meals, for this determines blood to the muscles which would serve a better purpose in the digestive organs. For a like reason the singer who would do his best before the public will refrain from taking a large meal before appearing.
As this subject of reflex action is of the highest importance, the reader is advised to make himself thoroughly familiar with the principles involved before perusing the future chapters of this work. [Fig. 16] shows the structural relations for reflex action. It also indicates how such nervous relations may be complicated by other connections of the nerve-cells involved in the reflex action. It will be seen that they make many upward connections with the brain, in consequence of which consciousness may be involved. Ordinarily one is more or less conscious of reflex action, though the will is not involved; in fact, a willed or voluntary action is usually considered the reverse of a reflex or involuntary action. But for a reflex action the brain is not essential. As is well known, a snake's hinder part will move in response to a touch when completely severed from the head end; and movements of considerable complexity can be evoked in a headless frog.
Herein, then, lies the solution of the problem. This is Nature's way of bringing one part into harmonious relations with another. As by a telegraphic system the most distant parts of a vast railway system may be brought into harmonious working, so is it with the body by means of the nervous system. The nerve-centres correspond to the heads of the railway system, or, perhaps more correctly, to the various officials resident in some large city who from this centre regulate the affairs of the whole line.
The muscular system is made up of cells of two kinds, those characteristic of the muscles used in ordinary movements, and those employed for the movements of the internal organs. The muscles of the limbs are made up of striped muscle-cells; those of the stomach, etc., of unstriped cells. These latter are slower to act when stimulated, contract more slowly, and cease to function more tardily when the stimulus is withdrawn.
The muscular mechanisms used by the singer and speaker are of the skeletal variety.