274 OF VERTIGO. SECT. XX. 6, 



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 volition, may be immediately flopped by 

 the exertion of the fame power on the antagonifl 

 mufcles ; otherwife thefe with all the other clafles 

 of motion continue to go on, fome time after they 

 are excited, as the palpitation of the heart conti- 

 nues after the objeft of fear, which occafioned it, 

 is removed. But this circijmftance is in no clafs 

 of motions more remarkable than in thofe depen- 

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

 -and then covers his eyes witji 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 form- 

 ed on the retina, but is extinguifhed by the perpe^ 

 tual change of the motions of this nerve in our 

 attention to other objects. To this mult be added, 

 that the longer time any movements have continu- 

 ed to be excited without fatigue to the organ, the 

 longer will they continue fpontaneoufly, after the 

 excitement is withdrawn : as the tafte of tobacco 

 in the mouth after a perfon has been fmpaking it. 

 This tafte remains To ftrong, that if a perfon con- 

 tinues to draw air through a tobacco pipe lr\ the 

 dark, after having been fmoking fome time, he 

 cannot diftinguifh whether his pipe be lighted op 

 not. 



From thefe two confederations it appears, that 

 the dizzinefs felt in the head, after feeing objects 

 in unufuai motion, is no other than a continuation 

 pf the motions of {he optic nerve excited 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 faller} 

 pn the ground ; and the longer he has continued 



to 



