Popular Science Monthly/Volume 23/July 1883/The Cause of Sea-Sickness
WHEN such an apparently simple disorder as sea-sickness exists in the midst of mankind for at least two thousand years? claiming yearly more victims, and all in spite of the best efforts of medical mankind to overcome it, it becomes of interest to inquire whether this is because its true nature has never been understood, or because it is essentially incurable.
The phenomena of sea-sickness are too well known to need detailed description. Violent and persistent vomiting is associated with it in most minds, and is the prominent symptom in most cases; but there are also a cold, clammy skin, headache, continuous nausea, great prostration, and indifference, the whole being accompanied by nervous irritability, and, in most cases, intense mental depression.
Plutarch was, perhaps, the first theorist on the subject. He thought that sea-sickness was caused by the smell of the salt-water; and, following him, men have propounded theory after theory, only to leave us of to-day with a large stock of theories, and but few good results to show for them.
Perhaps the most acceptable theory to-day is the one which places the origin of the trouble in the inner ear. The ear consists of three parts: the outer of these runs in as far as the drum; the middle part is inside of the drum, and contains the chain of ear-bones; while the inner ear is a complicated affair, containing the essential organ of hearing.
As far as we are concerned, the inner ear is a membranous bag filled with fluid, and situated in the solid bone. From the back part of this bag run out three semicircular tubes communicating at both their ends with the bag or vestibule. These run in three different planes, and are lined with hair-like nerve-filaments, which are much more abundant and more sensitive at the anterior part of the tubes. The tubes are filled with liquid in which float little calcareous particles, the otoliths. These tubes are known as the semicircular canals. It was difficult to see what connection with the sense of hearing these canals could possibly have, and some time ago it was noticed that injuries to these impaired the sense of hearing in no way, but caused most curious effects in the loss of equilibrium.
For instance, in pigeons, when the vertical canal was cut, the bird turned a series of back somersaults; and, when the horizontal canal was cut, the pigeon whirled around in an horizontal plane, in every case tending to rotate in the plane of the canal which was cut. And what is especially interesting to us is, that in these and other experiments Moreover, in some recent inquiries addressed to deaf-mutes upon another subject, the fact was noticed that all who were insusceptible to dizziness on account of the impairment of their semicircular canals by disease reported themselves also exempt from sea-sickness.or injury to these canals was almost invariably followed by vomiting. Experiments, by Professor Ferrier and others, point to a very close relation between these canals and the sense of equilibrium, and an especially strong suggestion is given by the disease known as labyrinthine vertigo, or Ménière's disease. This is characterized by an irritated and congested condition of these semicircular canals, due to some internal cause, and its symptoms are the same throbbing in the head, the pale, cold skin, and vomiting, as in sea-sickness, and in addition the patient reels and staggers, being unable to keep his balance. In this disease we seem to have the exact reverse of sea-sickness, the irritated condition of the canals causing the unusual movements of the body; whereas in sea-sickness we have the unusual movements of the body which result from the pitching of the ship, causing the irritation of the canals, and in both cases the irritated state of the semicircular canals is accompanied by vomiting. The mechanical explanation of why such irregular motion should cause irritation of the canals seems simple. By the pitching movements of the ship, which are by far the worst, the head is carried backward and forward through a long arc. At the end of the descent the head stops, but by its inertia the fluid in the canals rushes on and washes the otoliths up against the nerve-filaments at the front of the canals. These are extremely sensitive, and the repetition of this process a few times serves to establish an excessive irritation which is expressed by giddiness and vomiting. Why such gradual motions should cause sea-sickness, while much more violent ones, such as horseback-riding, do not, can only be explained by saying that in the more violent ones the individual has a stimulus to adapt his positions to the motion, which he has not in the often unnoticed pitching of a ship. At any rate, this theory explains why lying down should afford relief, as the otoliths then rest at the back and less sensitive part of the canals; and it also explains why riding backward should cause nausea and giddiness, as here, of course, the otoliths drag behind and irritate the anterior parts of the canals.
But the semicircular canals must not receive all the credit. The viscera of the abdomen are very full of blood, and irritated in seasickness, and this condition will cause vomiting, as shown by very many experiments on animals. The intestines are attached loosely to the backbone by a fold of membrane containing some very large blood-vessels. Ordinarily the intestines are held up and supported in place by the muscles of the abdomen, and consequently do not drag too heavily on their attachment. But in sea-sickness, it is said, either on account of confused messages sent to them from the irritated semicircular canals, or because of the novelty and uncertainty of the motions of the ship, these muscles are unable to tell when to contract and when to relax, thus affording but poor support to the intestines. Consequently by their inertia the intestines bulge forward at the end of each descent of the ship, thereby stretching and irritating their attachment, and in consequence the abdominal blood-vessels are engorged with blood, and this condition is expressed by vomiting, which is merely Nature's effort to equalize the circulation. Force is lent to this view of sea-sickness by the fact that jumping from a great height causes fearful nausea on reaching the ground in this case also the intestines pushing forward the abdominal wall and stretching their attachment.
There is probably a minor kind of sea-sickness, caused by the mere churning about of food in the stomach, irritating the nerves there as they would be irritated by a dose of mustard. This is often the sort experienced in small boats, and is at once relieved by vomiting.
The power of the imagination as one of the causes of sea-sickness ought not to go without some mention. Whether or not it is more powerful here than in other diseases it would be hard to say, but so prominent is the mental effort that Mr. Bache some years ago wrote a very interesting article on the subject, in which he maintained that sea-sickness was wholly of mental origin; that the idea of motion was the result of the concurrent testimony of the senses; and that in a new motion, where there was a conflict of impressions, the brain was disturbed. He said that motion caused nausea in two cases—1. When the motion of the observer's body is in doubt; 2. When the motion is acknowledged by the mind but the motion is not felt. But, however attractive this may be, it offers us little that is tangible. Of the very many other causes suggested, it seems only necessary to name the prominent ones. Naylor suggested spasm of the capillaries of the brain. Barris attributed it to the instability of surrounding objects. Stocker thinks it largely due to a partial vacuum in the lungs. Wollaston believed it was caused by the rise and fall of blood in the brain, as the mercury would rise and fall in a barometer, under like conditions; and Dr. Barker considers it the result of the sudden changes in the relations of the fluids and solids in the body.
Whether or not the individual is to be sick, and the duration and extent of his sickness, seem to depend to a certain extent on the general condition of the system, and also somewhat upon wholly unknown conditions, in many cases the most robust yielding the first. In this connection it is an interesting fact that children under three or four years are almost invariably exempt from sea-sickness, although ordinarily they vomit so much more readily than adults.
This is no place for the discussion of remedies. Bromide of sodium is the prominent one just at present, and probably does lessen the nervous susceptibility somewhat; but let its advocates read the glowing testimonials in favor of Chapman's ice-bags for the spine, nitrite of amyl, champagne, chloral, and all the rest. The belt to support the abdomen seems a rational remedy, but it was first proposed for that use in 1814, and but few to-day have even heard of it, and it seems fair to assume that suffering mankind would not have discarded a really efficient remedy.
In conclusion, what I have tried to show is, that the stomach is not the cause of the disorder, although generally the seat of it; that the organs irritated seem to be the semicircular canals of the ear, or the abdominal viscera, or both, which become full of blood and cause vomiting, which seems rather an effort of Nature to equalize the circulation than any desire on the part of the stomach to rid itself of its contents.