Physiologists remark several varieties of motion which may be grouped in two—voluntary and involuntary movements. Among the involuntary movements, which are also called automatic, some result from the impression produced by an idea, a passion, or a scene, gay or sad, or by a movement identical with that which is produced. Such are laughing, and the motions of the face expressing sadness, anger, fear, and other moral or physical impressions; trembling of the limbs in consequence of deep emotion, yawning, and so forth. Others proceed from the excitement of the sensitive nerves, such as sneezing, coughing, winking, chattering of the teeth, or shivering after a cold bath.
In certain cases, in fact, impressions transmitted from the organs to the brain, either directly by the nerves of sensation or indirectly by the spinal cord, without any sensation taking place, or what amounts to the same thing, without our being conscious of any, occasion an excitement which is transmitted to the motor-nerves, and causes movements in which the will has no part. These movements are generally executed by the muscles of organic life, which are not under the control of the will; but they can also be made by those which are under its control. These are called reflex movements. Some of them are undoubtedly automatic; as for the others, it has not been demonstrated that a sensation and an act of the will do not precede the muscular contraction. Thus sneezing and coughing are independent of the will, it can neither prevent them nor arrest their development It is the same with the chattering of the teeth, with shivering and winking of the eyelids on contact with the air, with the tears, or only when a foreign body menaces the eye. In this last case, although the muscular contraction is instantaneous and seems to be involuntary, it is evidently preceded by a sensation which the eye has transmitted to the brain. This may even be considered as voluntary, since if the attention is excited, the will can oppose the instinctive movement.
Other movements take place in the organism of which we have more or less perception; when a carriage threatens to overset, for example, we throw ourselves to the side opposite the inclination; or when suddenly coming on the edge of a precipice the body stiffens or is thrown backward; and when a player at ball or billiards inclines or turns in the same direction in which he wishes to direct his ball. Analogous phenomena are caused by a sort of attraction or instinctive imitation. When, for example, the eyes follow the movement of a great waterfall into a gulf, the body very soon follows the oscillations, though we do not perceive them until the motion becomes so great as to threaten to drag us into the abyss. We are indebted to M. Chevreul for the observation and explanation of effects of this nature, of which charlatanism avails itself in table-turning. If the elbow be placed upon a table, and a pendulum formed of a string and a ring be held in the hand, and we fix the eyes on the ring, we shall soon see it begin to oscillate, although the arm apparently remains immovable; the oscillations may keep the same direction, or change according to the mental desire, and this without any bad faith or a consenting movement on the part of the person holding the pendulum. But if we place a support under the hand near the end of the fingers, or a bandage over the eyes of the experimenter, the oscillations cease. They were caused by an almost imperceptible and involuntary movement of the fore-arm and hand, under the influence of the eyes looking at the ring and the direction that it takes. And again it is a similar movement or series of movements which gives the impulse to the turning table; in the unconsciousness of the muscular contraction lies all the merit of this phenomenon, which loses its marvellous character as soon as you show the too credulous performers that they themselves are the involuntary movers.
Voluntary movements, as their name indicates, are produced under the influence of the will, but not under its direct or immediate action. The volition of the movements of locomotion, for example, emanate from a certain part of the brain, from the cerebral lobes; but in order that the movement may be executed, the muscles must contract, and this muscular contraction owes its origin to a force which emanates from the occipital protuberance, a different part of the brain from that which gives birth to thought. Observations on paralytics prove that the will is insufficient to produce movement.
In order that movements may be executed in the order and with the unity necessary to the accomplishment of the will, they must be co-ordinate. Several physiologists, and especially Flourens, have regarded the cerebellum as the organ essential to this co-ordination of movement A lesion of this part of the brain produces a disturbance in locomotion similar to that caused by drunkenness; but pathological observation has demonstrated that there is sometimes absence of co-ordination when the cerebellum is not affected.
The muscles unite in the voluntary movements, some as motors, and others, antagonistic to these, as the moderators of movement M. Duchenne (of Boulogne) has shown that in the voluntary movements of the limbs and the trunk these two systems of muscles, impulsive and modifying, are simultaneously contracted by a double nervous excitation; one to produce the movement and the other to modify it. Without this unity of intention between the antagonistic muscles, the movements would lose their precision and their certainty.