On Sea- Sickness, 443 



brain is exposed by the trepan, a successive tur^escence 

 and subsidence of the brain is seen, in alternate motion 

 with the different states of the chest. It is probably from 

 this cause that, in severe head- aches, a degree of temporary 

 rehei IS obtained by occasional complete inspirations. 



Tn sea-sickness also the act of Inspiration will have some 

 tendency to relieve, if regulated so as to counteract any 

 temporary pressure of blood upon the brain; but the cause 

 of such pressure requires first to be investigated. 



All those who have ever suffered from sea-sickness 

 (without being giddy) will agree that the principal uneasi- 

 ness is felt during the subsidence of the vessel by the 

 sinking of the wave on which it rests. It is during this 

 subsidence that the blood has a tendency to press with 

 unusual force upon the brain. 



If a person be supposed standing erect upon deck, it is 

 evident that the brain, which is uppermost, then sustains 

 no pressure from the mere weight of the blood, and that 

 the vessels of the feet and lower parts of the body must 

 contract, with a ibrce sufficient to resist the pressure of a 

 column of blood, of between five and $ix feet from the head 

 downwards. 



If the deck were by any means suddenly and entirely 

 removed, the blood would be no longer supported by its 

 vessels ; but both would fall together with the same velo- 

 city by the free action of gravity ; and the same contraction 

 of the vessels which before supported the weight of the 

 blood would now occasion it to press upon the brain, with 

 a force proportional to its former altitude. 



In the same manner, and for the same reason, during a 

 more gradual subsidence of the deck, and partial removal 

 of support, there must be a partial diminution of the pres- 

 sure of the blood upon its vessels, and consequently a 

 partial reaction upon the brain, which would be directly 

 counteracted by a full inspiration. 



The consequence of external motion upon the blood will 

 be best elucidated by what may be seen to occur in a co- 

 lumn of mercury similarly circumstanced. 



A barometer, when carried out to sea in a calm, rests at 

 the same height at which it would stand on shore; but, 

 when the ship falls by subsidence of the wave, the mercury 

 is seen apparently to rise in the tube that contains it, be- 

 cause a portion of its gravity is then employed in occasion- 

 ing its descent along with the vessel ; and accordingly, 

 if It were confined in a tube closed at bottom, it would no 

 Jonger press with its whole weight upon the lower end. In 



the 



