July, 1913. 



KNOWLEDGE. 



255 



sense of sight that sea-sickness is produced, the 

 sight of the waves, the motion of the ship's masts, 

 and other objects seen at sea being conveyed through 

 the optic nerve to the brain. The effect of atropine 

 on some cases is pointed out as confirmatory evidence 

 of this theory 



In considering the action of the sea upon special 

 senses, and through them upon the brain, we are 

 coming into the region of scientific reasoning and 

 leaving that of empiricism. It is, however, to 

 another special sense we must look for a true 

 explanation. There are some cases in which the 

 sense of sight plays an important part ; but there are 

 great difficulties in the way of accepting disturbance 

 of this sense as the cause in all cases, or, indeed, as 

 the true cause in any, the effect of atropine notwith- 

 standing. The cases benefited by this drug are those 

 in which physical fear is a prominent feature. The 

 temporary impairment of sight by atropine — a 

 bandage would have the same effect — calms the 

 nervous system by preventing the patient seeing 

 the motion of the water and the ship. 



Thus it is a well-known fact, and one in my 

 own experience, that a traveller will be made ill 

 by observing the motion of a passing ship, who 

 does not become so by feeling that of the one on 

 which he is travelling. Nor does the optic nerve 

 theory explain the cases, with which all are familiar, 

 where the patient, having been asleep in fine weather, 

 or when the ship is in dock, wakes up violently sick 

 when the motion of a rough sea ensues. 



It is to the organ which is closely allied to the 

 sense of hearing, which has to do with equilibrium 

 and the indication of the perpendicular position,- 

 that we must turn for a satisfactory explanation 

 of the many phenomena of sea-sickness. 



It will be well, before proceeding to indicate 

 the way in which the effects are produced, to 

 review briefly the anatomy and physiology of the 

 part. 



The internal ear is divided into the cochlea (or 

 special organ of hearing), the vestibule, and the 

 semicircular canals. It is the last that claim our 

 attention. These canals are hollowed out of the 

 petrous portion of the temporal bone : thev are 

 three in number, each one being placed at "right 

 angles to the other two. Each osseous canal 

 contains a membraneous one, the lining epithelium 

 of which secretes a fluid called endolymph. 



The auditory nerve divides into two at the bottom 

 of the internal auditory meatus : the anterior branch 

 goes to the cochlea and special organ of hearing ; 

 the posterior is distributed to the semicircular 

 canals. We have between this nerve and the vagus 

 nerve a distinct connection by means of the facial 

 nerve, along which impulses can be conveyed to the 

 stomach from the semicircular canals. 



In so far as the vomiting of sea-sickness is a reflex 

 act it is possible that the sympathetic nerve has also 

 something to do with the causation of sickness, 



when the terminal branches of the posterior division 

 of the auditory nerve are irritated. 



The anatomical direction of the canals themselves 

 should be remembered — i.e., two vertical and one 

 horizontal. 



The semicircular canals have two functions: 

 (1) that directly connected with hearing, by collect- 

 ing in their fluid contents sonorous undulations from 

 the bones of the cranium ; (2) that indirectly 

 connected with the sense of hearing, by informing 

 us as to our equilibrium, by means of the constant 

 alterations in pressure of the fluid within the 

 canals. 



This is closely associated with coordination of 

 muscular movements. It has been found that when 

 the horizontal canal is divided in a pigeon a constant 

 movement of the head from side to side occurs. 

 When one of the vertical canals is divided, up-and- 

 down movements are produced. These movements 

 are associated with loss of coordination, as after the 

 experiment the bird is unable to fly in a proper 

 manner. 



Having regard to these anatomical and physio- 

 logical facts, and in view of the proofs derived from 

 observation of the signs and symptoms of sea-sick- 

 ness, there can be little doubt — 



That the true cause of sea-sickness is irritation of 

 the terminal fibres of the auditory nerve dis- 

 tributed to the membranous labyrinth ; 

 That the irritation is conveyed, through the nerve 

 connections mentioned above, to the vagus, 

 and possibly to the sympathetic, and thence 

 to the walls of the stomach ; 

 That it is primarily an irritation of cranial nerves; 

 That this irritation is caused by the motion of a 

 ship on rough water, but not by this alone. 

 Any motion which is contrary to that usually 

 experienced by the fluid contained in the 

 semicircular canals will cause a set of 

 symptoms exactly similar to those of sea- 

 sickness. 

 Our semicircular canals constitute a sort of human 

 spirit-level. It is not difficult to conceive that any 

 motion which will more or less violently throw the 

 fluid against its containing walls, richly supplied 

 with delicate nerve-endings in direct communication 

 with brain and stomach, will produce symptoms 

 referable first to our equilibrium, then to the cere- 

 brum, and finally to the stomach. This is what 

 occurs in sea-sickness. 



In searching for the true pathological cause of any 

 condition it is a usual and proper rule to find out 

 under what varying conditions similar symptoms are 

 produced. Such a survey has not been accorded to 

 sea-sickness. It was concluded that the person 

 affected was suffering from a sort of bilious attack, 

 produced by the unusual motion, combined, perhaps, 

 with the smell of oil, and so on, on a steamboat. 

 Such a lax way of viewing causation would have been 

 condemned in any other disease, but a good-humoured 

 laugh at the sufferer, combined with advice as to 



