3$ SENSORIAL Sect. XL 2. 1. 



the bladder, and laftly every gland of the body appears to be in- 

 dued with a kind of tafte, by which it felects or forms each its 

 peculiar fluid from the blood ; and by which it is irritated into 

 activity. 



Many of thefe external properties of bodies, which ftimulate 

 our organs of fenfe, do not feem to effect this by a fingle im- 

 pulfe, but by repeated impulfes ; as the nerve of the ear is prob- 

 ably not excitable by a fingle vibration of air, nor the optic 

 nerve by a fingle particle of light ♦, which circumftance produ- 

 ces fome analogy between thofe two fenfes, at the fame time 

 the folidity of bodies is perceived by a fingle application of a 

 folid body to the nerves of touch, and that even through the cu- 

 ticle ; and we are probably poflefled of a peculiar fenfe to diftin- 

 guifh the nice degrees of heat and cold. 



The fenfes of touch and of hearing acquaint us with the me- 

 chanical impact and vibration of bodies, thofe of fmell and tafte 

 feem to acquaint us with fome of their chemical properties, 

 while the fenfes of vifion and of heat acquaint us with the ex- 

 iftence of their peculiar fluids. 



Senfation and Volition, 



II. Many rriotions are produced by pleafure or pain, and that 

 even in contradiction to the power of volition, as in laughing, 

 or in the ftrangury ; but as no name has been given to pleafure 

 or pain, at the time it is exerted fo as to caufe fibrous motions, 

 we have ufed the term fenfation for this purpofe ; and mean it 

 to bear the fame analogy to pleafure and pain, that the word vo- 

 lition does to defire and averfion. 



1. It was mentioned in the fifth Section, that, what we have 

 termed fenfation is a motion of the central parts, or of the whole 

 fenforium, beginning at fome of the extremities of it. This ap- 

 pears firft, becaufe our pains and pleafures are always caufed by 

 o ur ideas or mufcular motions, which are the motions of the extrem- 

 ities of the fenforium. And, fecondly, becaufe the fenfation of 

 pleafure or pain frequently continues fome time after the ideas 

 or mufcular motions which excited it have ceafed : for we often 

 feel a glow of pleafure from an agreeable reverie, for many min- 

 utes after the ideas, that were the fubject of it, have efcaped 

 cur memory ; and frequently experience a dejection of fpirits 

 without being able to aflign the caufe of it but by much recol- 

 lection. 



When the fenforial faculty of defire or averfion is excited (o 

 as to caufe fibrous motions, it is termed volition ; which is faid 

 in Sect. V. to be a motion of the central parts, or of the whole 



fenforium, 



