Inhibition. - Definition. - Dynamogenesis. - Theories of Brown-Sequard. - Lauder Brunton. - Interference. - Experiments.

I propose in the following chapter to consider briefly the subject of inhibition, as it seems to me to be the key to the proper comprehension of hypnotic phenomena. The subject is a difficult one, and I would refer the student desirous of fully studying it to consult the works of Lauder Brunton, * Brown-Sequard,+ and Heidenhain .‡

Inhibition is defined as being 'the more or less complete arrest of present functional activity of a structure or organ by a restraining influence exerted over it through a nerve centre,' § or more fully by Lauder Brunton as 'the arrest of the functions of a structure or organ by the action upon it of another, while its powers to execute those functions is still retained and can be manifested as soon as the restraining power is removed.'

It will be seen from these definitions that inhibition is quite a different thing to the loss of function which occurs in paralysis, and the cessation of function which follows exhaustion. There are two characteristics which are of great importance in the consideration of the relationship between inhibition and hypnotism: the suddenness of its induction and removal, and the want of apparent proportion between the cause and effect - i.e., a relatively small excitation being capable of determining most extensive results. Under ordinary circumstances stimulation of a motor nerve by an electric current produces in response the contraction of the muscles to which the nerve is distributed, and irritation of a sensory nerve produces a sensory reaction; e.g., stimulation of the gustatory nerve arouses taste sensations, of the optic nerve visual sensations, etc. But stimulation of a nerve is not always followed by functional activity. On the contrary, it is sometimes followed by cessation of activity. ' Afferent impulses reaching a nervous centre may, instead of stimulating it to activity, stop or inhibit an activity previously going on.

Whenever in any tissue energy is being set free, nervous energy brought to bear on the tissue may affect the rate or amount of energy set free in two different ways: on the one hand, it may increase or quicken the setting free of energy; on the other hand, it may slacken, hinder, or inhibit the setting free of energy' (Foster's ' Physiology,' p. 185).

* ' On the Nature of Inhibition and the Action of Drugs upon it' (reprinted from Nature), London, 1883. ' On Inhibition, Peripheral and Central,' West Riding Reports, 1874.

† ' Sur l'Inhibition et la Dynamogenesie,' Gazette Hebdominaire de Me'decine, etc., 1882.

‡ 'Hypnotism, or Animal Magnetism,' London, 1888.

§ ' Lexicon of Medical Terms,' New Sydenham Society.

The recognition of the part inhibition plays in vital phenomena is undoubtedly one of the most important discoveries which have been made in physiology since Harvey discovered the circulation of the blood. It throws light upon an immense number of phenomena previously inexplicable, and enables us to form theories of a satisfactory nature about many vital problems. It offers an explanation of the nature of hypnotic states, which is at least as satisfactory as that we have of the action of many drugs.

The nervous mechanism of the heart affords the best and most commonly cited example of inhibitory action, and here it was first studied by Weber and Claude Bernard in 1848. The cardiac ganglia derived from the sympathetic preside over the movements of the organ, and in response to the stimulus of the intraventricular blood-pressure cause rhythmical contraction of the cavities. Their action is, however, controlled by the pneumogastric nerve, through which impulses of an inhibitory nature are constantly travelling and acting as a restraining force. Paralysis of the pneumogastric, of course, does away with its action, and hence we have, among other symptoms of this condition, increased rapidity of the contractions of the heart from withdrawal of the inhibitory influence. This is demonstrated by dividing the pneumogastric in animals, and it has also been seen in cases where it has been possible to exert pressure on the nerve in men. Thus, the late Professor Czermack had a small glandular tumour in close contact with the right pneumogastric nerve, and he was able by pressure on this to compress the nerve to any extent he wished, and either to completely stop the heart or simply to retard it (Lauder Brunton, 'Inhibition,' West Riding Reports, 1874).

If when the heart is beating regularly, the branches of the pneumogastric going to it are stimulated by the passage through them of an interrupted electric current, the heart will stop beating, and it will be found flaccid and in a state of muscular relaxation. This happens when the current is of sufficient intensity; but it may be insufficiently powerful to stop the action altogether, and in that case we shall see all degrees of interference, from mere slight retardation to complete arrest.

But the action of the heart may be inhibited by influencing it through the pneumogastric in other ways. Golz's tapping experiment illustrates this. He exposed the heart and vessels, and also the intestines, in a frog, and he found that tapping on the intestines with the scalpel handle had the effect of stopping the heart. The practical bearing of this experiment is seen in cases where sudden death follows a blow on the stomach or abdomen. The heart is stopped by an intense stimulation of the pneumogastric through its gastric branches.

These examples show the effect of inhibition of mechanical origin, but we constantly see how the heart may be inhibited by influences of psychical origin. If the stimulation is intense, and the organ weak, absolute stoppage of the heart may ensue; and as it is with the interrupted current, so it is with the effects of emotion and shock. The heart may either stop or its action may simply be interfered with.