SECT. XX. i. OF VERTIGO. 169 



our own. This circumftance not having been at- 

 tended to by the writers on vifion, the difeafe call- 

 ed vertigo or dizzinefs has been little underftood. 



When any perfon lofes the power of mufcular 

 a&ion, whether he is ereft or in a fitting pofture, he 

 finks down upon the ground : as is feen in fainting 

 fits, and other inftarices of great debility. Hence 

 it follows, that fome exertion of mufcular power is 

 neceffary to preferve our perpendicular attitude. 

 This is performed by proportionally exerting the 

 antagonist mufeles of the trunk, neck, and limbs ; 

 and if at any time in our locomotions we find our- 

 felves inclining to one fide, we either reflore our 

 equilibrium by the efforts of the mufeles on the 

 other fide, or by moving one of our feet extend 

 the bafe, which we reft upon, to the new center of 

 gravity. 



But the moft eafy and habitual manner of deter- 

 mining our want of perpendicularity, is by attend- 

 ing to the apparent motion of the objecls within 

 the fphere of diftincl: vifion ; for this apparent mo- 

 tion of objecls, when we incline from our perpen- 

 dicularity, or begin to fall, is as much greater than 

 the real motion of the eye, as the diameter of the 

 fphere of diftinct vifion is to our perpendicular 

 height. 



Hence no one, who is hood-winked, can walk 

 in a (traight line for a hundred fleps together ; for 

 he inclines fo greatly, before he is warned of his 

 want of perpendicularity by the fenfe of touch, not 

 having the apparent motions of ambient objects to 

 meafure this inclination by, that he is necefiitated 

 to move one of his feet outwards, to the right or 

 to the left, to fupport the new centre of gravity, and 

 thus errs from the line he endeavours to proceed 

 in. 



For the fame reafon many people become dizzy, 

 when they look from the fumrmt of a tower, which 



T 2, i* 



