As in the heart we find two nervous agencies at work, the one exerting an exciting and the other a restraining influence, so it is in respect to all our functions and organs. There is a system of action and counter-action, and in a state of health there is a proper relative balance maintained between the two. There is interaction of nervous force, and no function or organ in normal life has full and unrestricted play, for its activity is modified by the interferences of nervous impulses arriving from other parts. Some of these impulses are of an inhibitory nature, and the amount of response to stimulation depends not only on the strength of the stimulus, but also on that of the counteracting inhibitory impulse. For example, a person receives a violent blow, and a nervous impulse of great force and tension is sent from the seat of injury to the brain centres. The natural response to the stimulus will be a correspondingly strong discharge of energy from the receiving centres along the motor tracts, and vigorous muscular contraction taking the form of retaliatory violence will result; this action may be called cerebro-reflex, and the less the injured person is controlled by reason, the simpler will be the mechanism of the act.

But in educated and disciplined communities, cerebro-reflex movements are not allowed their full play, for the functions of the highest controlling centres come into action; and in addition to excito-motor impulses sent from the motor centres, there are other impulses discharged from the centres of the highest level. These latter will be of an inhibitory nature, and will modify those emanating from the lower levels. If the assaulted person has so developed his faculties of restraint and judgment that they have become the dominant features of his character, he will reflect that, instead of returning the blow, it may be better to either take no notice of it.

or proceed by legal action. In either case a strong inhibitory impulse from the highest centres will neutralize the cerebro-reflex one. If the injury is severe, or the power of restraint not fully developed, the action of the highest level may not be sufficient to entirely neutralize the reflex response, and the fist may be clenched or a cry uttered. It is evident, then, that not only function, but conduct also, depends largely on inhibition.

The controlling action of the highest centres in inhibiting cerebro-reflex action is seen in the exaggerated start a person makes on receiving a sensory impression when his mind is not ready to receive it; for instance, the touch of a friend's hand on the shoulder will cause one to almost jump from the chair, if the faculties of attention are off guard, or if one is in a state of reverie. Similarly, in hypnosis (inhibition of the cells of the highest cortical centres) reflex actions are generally exaggerated.

Inhibition may be either very extended in its action or extremely circumscribed. An example of the former is seen in the tremendous effect which follows irritation of the small mass of grey matter (the vital knot) situated at the upper part of the medulla oblongata. An intense inhibitory impulse is discharged, and the result is instant death from fatal interference with the functions of the heart and lungs: general inhibition is produced. Its confinement to a very small area is seen when the auriculo-cervical nerve in a rabbit is irritated. Paralysis from inhibition of the vaso-motor nerves of the ear is produced with immediate flushing of the part. Brown-Sequard has obtained some typical illustrations of inhibitory action. By passing carbonic acid through the upper part of the larynx of a dog, and so irritating the peripheral endings of the laryngeal nerves, respiratory movements were at once checked, though communication between the lungs and the external air was freely maintained by means of a tube, and though the gas was prevented acting on the bronchial mucous membrane by division of the trachea.

On the other hand, when he irritated the pharyngeal mucous membrane by directing the vapour of chloroform upon it, he found that the contrary effect was produced, and respiratory movements became greatly exaggerated and accelerated, though the quantity applied was very small, and it was prevented reaching the lungs. *

In the case of general inhibition of vital functions following irritation of the vital knot, we have seen the most intense and widespread action of this force, and in the different degrees of cardiac interference produced by stimulation of the pneumogastric we have an example of a more localized but still intense form of inhibition. As Nasmyth's hammer can either wield a gigantic bar of steel or crack a walnut, so inhibition has all grades of action, and its effects may be extremely circumscribed or widespread.

For example: No function of organic life is more influenced by mental states than that of salivation. The thought of savoury food when one is hungry is sufficient to make the salivary glands secrete and the mouth water, while fear or disgust will have an exactly opposite effect, and the mouth becomes dry and parched. †

* Brown-Sequard has pointed out how in the oncoming of sleep inhibitory impulses affect different muscles in order: the eyelids become heavy from inhibitory action on the levatores palpebrarum, the head tends to fall from inhibition of the muscles which support it. At the same time an inhibitory impulse affects the heart and lungs, and the action of these becomes slower. Consentaneously the mental faculties become dulled and inactive from their inhibition. In using hypnotism one endeavours by word-painting to figure in the subject's mind an image of the natural processes, relying on suggestion being followed by realization of the conditions suggested.