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.
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
