Sect. XX. 6. OF VERTIGO. 179 



and particularly when we lie down in bed, and compofe our- 

 felves to fleep ; in this cafe it is obfervable, that the rattling noife 

 of the coach, as well as the undulatory motion, haunts us. The 

 drunken vertigo, and the vulgar cuftom of rocking children, will 

 be confidered in the next Section. 



6. The motions, which are produced by the power of voli- 

 tion, may be immediately flopped by the exertion of the fame 

 power on the antagonifl mufcles ; otherwife thefe with all the 

 other clafTes of motion continue to go on, fome time after they 

 are excited, as the palpitation of the heart continues after the 

 object of fear, which occafioned it, is removed. But this cir- 

 cumftance is in no clafs of motions more remarkable than in 

 thofe dependent on irritation ; thus if any one looks at the fun, 

 and then covers his eyes with his hand, he will for many feconds 

 of time perceive the image of the fun marked on his retina : a 

 fimilar image of all other vifible objects would remain fome time 

 formed on the retina, but is extinguifhed by the perpetual change 

 of the motions of this nerve in our attention to other objects. To 

 this muft be added, that the longer time any movements have 

 continued to be excited without fatigue to the organ,* the longer 

 will they continue fpontaneoufly, after the excitement is with- 

 drawn : as the tafte of tobacco in the mouth after a perfon has 

 been fmoking it. This tafte remains fo flrong, that if a perfon 

 continues to draw air through a tobacco pipe in the darjc, after 

 having been fmoking fome time, he cannot diftinguifh whether 

 his pipe be lighted or not. 



From thefe two confederations it appears, that the dizzinefs 

 felt in the head, after feeing objects in unufual. motion, is no 

 other than a continuation of the motions of the optic nerve ex- 

 cited by thofe objects, and which engage our attention. Thus 

 on turning round on one foot, the vertigo continues for fome 

 feconds of time after the perfon is fallen on the ground ; and 

 the longer he has continued to revolve, the longer will continue 

 thefe fucceflive motions of the parts of the optic nerve. 



After revolving with your eyes open till you become vertig- 

 inous, as foon as you ceafe to revolve, not only the circumambi- 

 ent objects appear to circulate round you in a direction contrary 

 to that, in which you have been turning, but you are liable to 

 roll your eyes forwards and backwards ; as is well obferved, 

 and ingeniously demonftrated 'by Dr. Wells in a late publication 

 on vifion. The fame occurs, if you revolve with your eyes 

 clofed, and open them immediately at the time of your ceafing 

 to turn ; and even during the whole time of revolving, as may 

 be felt by your hand prefTed lightly on your clofed eyelids. To 

 thefe movements of the eyes-, of which he fuppoies the obferver 



to 



