What causes sea sickness

What causes sea sicknessIn walking on any deck of a ship at sea, inasmuch as the plane of such deck is constantly changing its relation to the vertical, no step can be taken simply by a repetition of the adjustment for preceding step. It is as if going up or down stairs, the steps of which are no two alike in height. The labor of new adjustments probably taxes the mind and tends to fatigue. Even then no adjustment seems adequate, for by the time the foot reaches the point to which the adjustment was made to carry it, that point of the deck has changed position. The consequence is a series of shocks, such as have been designated on a preceding page by the more general term violence.

Again, it is our constitutional habit to maintain a certain relation, when standing or sitting, between our long axis and the vertical; and it is contrary to our habitual expectation that any objective force interfere with such relation. But as at every instant the lines of the ship's deck change in respect of the vertical, we are with every such change thrown out of relation to the vertical, and suffer a disturbance of our muscular adjustment and slight shock. At every moment we readjust, and are as often thrown out of adjustment. This being contrary to an habitual expectation, there results a disappointment from every such disturbance; and it is the accumulated sum of the effects of the separate shocks that constitutes the illness attending such experience. So far I have considered only the effect, on the new passenger, of the changes of position of ships' decks in respect of the vertical.

While the complex cause of sea-sickness embraces several lesser factors, the chief one is motion. The motions that make us sick are a class that are distinguished very clearly from those that do not make us sick. I have arranged all motions to which the body can be subjected into three classes. It has not been necessary to make the class distinctions, but only to define them as they occur in nature. There is in this classification, therefore, nothing arbitrary. A movement may be active or passive in respect of mind as certainly as it is so in respect of body. My classification is based on this fact, and accordingly

Motions of class first are active mentally and active bodily;

Motions of class second are active mentally and passive bodily;

Motions of class third are passive mentally and passive bodily.

In class first both the mental determination to move and the motive power are subjective. A typical example is walking on a level or up hill. In class second there is either a subjective determination to move or a consent to the movement, but the motive power is objective. A type of class second is a ride on a horse, whose motions are well known and therefore comprehended by the rider. Any passive motion to which the body is. adjusted by nervous effort, volitionally and consciously, or automatically and unconsciously, belongs to class second. In class third both the determining cause and the motive power are objective. As typical of this class we may take any case in which a person is subjected to heterogeneous passive motion, no detail of which he is aware of nor understands beforehand, to which, on this account, he is utterly unable to adjust himself.