(From consuesco, to be accustomed to.) Custom. Custom and habit are two terms often used synonymously, and indeed the former is often confounded with the latter. By custom is meant a frequent repetition of the same act; by habit, the effect that custom has on the mind or body; so that the former is the cause of the latter.

It has been often alleged, and with truth, that we are creatures of habit. Custom produces a regularity in all our returning wants; and the hour of dinner, of exercise, or sleep, brings on the feeling of want, independent of any real demand. In general, the frequent and regular repetition of small impressions produces habit, and their influence is soon unperceived: violent impressions never become habitual; for, when repeated, the body or mind would sink under their force. The influence of custom on our sensations is singular. Accustomed sensations, as we have just remarked, are soon unnoticed; and it requires a little increased action to render them the objects of our attention. Yet, when the energy of the mind is excited by the sensation, custom adds to the power of discrimination. The shepherd will distinguish every individual sheep of a large flock; the painter discover beauties and faults in-visible to the common eye; and the musician feel with pain the minutest deviation from tune. Custom, therefore, which blunts bodily sensations, renders the mental 3Q ones more acute. Volition is not an exertion of mind, but apparently a simple impulse, directed almost necessarily to an end; and it is affected by custom, nearly like the organs of the body. Thus a sensation, which excited a perceptible exertion of volution, will, in time, produce it and the correspondent action, without our being sensible of its interference; and so rapid is this progress, that we seem to will two ends or objects at the same time, though they are evidently, when examined, distinct operations. But though by custom we are no longer sensible of bodily impressions, or the exercise of volition, yet the corporeal organs in their several functions acquire, like those of the mind, peculiar accuracy of discrimination. The musician is not, for instance, sensible of his willing any one motion; yet, with the most exquisite nicety he touches a particular part of the string, and executes a variety of the nicest and most complicated motions with the most delicate precision. Indeed, it appears to be a general rule in the animal economy, that if an idea has frequently produced a motion, its power is increased; but if the motion connected with the idea has been prevented, the power is diminished or lost.

It has been supposed that the will, by custom and exercise, may acquire a power over motions in the body not originally subject to it; and we think we have observed some instances of this power in a slight degree on the motions of the alimentary canal. The most striking instance of this kind, however, was the power which colonel Townsend obtained over the heart and arteries; and we know an eminent professor who, in his youth, could, and is perhaps still able to, produce a considerable effect on his pulse. But this effect of custom is very limited. Its power of increasing the force and facility of action of the moving fibres is sufficiently well known. In the action of medicines on the moving fibres we find some variety. Moderate power, by custom, loses its peculiar effect. Thus, the dose of emetics and laxatives, when repeated, must be increased; and the dram drinker gradually requires additional quantities, or augmented strength, of his liquor: but if the power be very active, the repetition gives greater facility of motion, as, by the repetition, the mobility of the moving fibres is increased. Another effect of custom on the moving fibres resembles the association of ideas. If two muscles, or the different parts of one muscle, have been used to act together, exciting the action of one will produce that of the other. If, however, this kind of association is prevented by a strong effort of volition, and strengthened by a different habit, they are induced to act separately, with the greatest precision. This power is constantly attained by musicians.

A singular effect has been attributed to custom, which may perhaps be more satisfactorily explained on other principles. As we usually feel only in the sentient extremities of the nerves, it has been supposed, that from custom we refer every affection of the nerve, in its course, to the extremities; and thus the sailor, who had lost his leg many years before, feels a pain in the toe. This, however, certainly does not depend on custom; for a disease of the origin, or in the course of a nerve, is at once referred to its extremity, though the sensation was never before experienced.

Custom, we find, regulates the degree of tension necessary to produce sensation. The sailor used to the sound of cannon can hear a person speak in a common tone during their loudest roar; and a deaf person will hear more easily while a drum is beating, or in a carriage. It equally regulates the degree of tension necessary to the action of muscular fibres, as the musician experiences by the degree of pressure suitable to the production of a given sound, from a piano-forte or an organ; and it associates motions with sensations not otherwise connected. There is no necessary connection, for instance, between a particular figure of a country dance and a given tune, since many different figures may be suited to it; but when the music begins, the accustomed movements, without any apparent exertion of volition, follow. Custom also associates different motions, though not necessarily connected; and from the habit established they cannot be performed separately. It determines the degree of force and velocity with which motions can be performed, and which, after the habit is established, cannot be violated: a blacksmith can never become a watchmaker. Custom establishes also the order in which certain sensations and motions return. An infant can soon be brought to feed only at regular hours; and those who retire regularly to the garden immediately after breakfast, will feel little or no inconvenience should their breakfast be delayed for an hour or two. The same call will also regularly return with almost every change of the constitution in other respects.

These motions, established and associated by custom, are sometimes broken with difficulty, and occasionally with injury to the constitution. If the supply of food, or the discharge of the excretions, be not obeyed, the call will often not again recur till the next period, and the feelings be uncomfortable in the interval. Indeed, the deprivation of the most trifling accustomed gratification frequently injures the health, while the most discordant noises, the most offensive smells, or the most disgusting objects, lose every unpleasing effect from habit. It was this which occasioned the ancient sage to remark, "Optimum vitae genus eligito, nam consuetudo faciet jucundissimum."choose the best occupation, for custom will make it the pleasantest. On the other hand, these accustomed associated motions constitute the most obstinate diseases. Intermittent fevers, epilepsies, co-mata, etc. when the periodical return is established, are most difficult of cure. In such circumstances we cannot often succeed without stopping all motion, to begin again a new and more salutary series.