Dr. Wollaston on Sea-Sickness. 7 
could primarily affect the system, was by its influence om the 
motion of the blood; for, at the same instant that the chest is 
dilated for the reception of air, its vessels become also more 
open to the reception of the blood, so that the return of blood 
from the head is more free than at any other period of a com- 
plete respiration. On the contrary, by the act of expelling air 
from the lungs, the ingress of blood is so far obstructed, that, 
when the surface of the brain is exposed by the trepan, a suc- 
cessive turgescence 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 relief is obtained by occasional complete inspi- 
rations. 
In sea-sickness also the act of inspiration will have some 
tendency to relieve, if regulated so as to counteract any tem- 
porary 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, (with- 
out being giddy ) will agree that the principal uneasiness fs 
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 evi- 
dent 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 force sufficient to resist the pressure of a column of 
blood, of between five and six feet from the head downwards. 
If the deck were by any means, suddenly and entirely 
