ADYNAMIA. 371 



sire and aversion are manifestly operations of a vis medicatrix 

 naturae as much as any concurring in diseases. 



" There should therefore be no difficulty in admitting a vis 

 medicatrix naturae ; and I think there would be none, if its 

 operations were always in consequence of evident sensations* 

 and especially, of sensations of impression, the only sensations 

 almost taken notice of by philosophers. 



" But they are more commonly the effects of sensations of 

 consciousness; that is, agreeable or disagreeable sensations from 

 the state of the body itself, particularly the state of its motions, 

 more weak or vigorous, more free or interrupted, not with 

 any distinct perception of these states, but merely of some- 

 thing pleasant or uneasy. 



" It is by these that the motions of the internal parts are 

 regulated, and often without the concurrence of any volition, 

 and often without the sensations being perceived. 



" Whoever considers all this, will in general find no mystery 

 in the vis medicatrix ; though we have often spoke of it as a 

 mystery, because we did not think it proper to enter into this 

 explanation. Now, when I have given it, I hope it will be un- 

 derstood what I mean by an indirect stimulus, and their exis- 

 tence and operation will also be understood. 



" To apply this to the simple case of vomiting, the simple 

 theory of this is a supposition of the stimulus, as we have said. 



" But in many cases no such stimulus is applied. The 

 fainting after blood-letting is attended with vomiting: this, 

 therefore, is produced by debility alone, for it may be prevented 

 entirely by the horizontal posture : and the same doctrine is 

 confirmed, so far as the same attends other cases of fainting. 



" That vomiting may be excited by an operation in the brain, 

 cannot be doubted, and the effects of fractures shew it, but es- 

 pecially the operation of sea-sickness, or that produced by the 

 motion of a carriage, or merely by looking at a stream. All 

 these operate by vertigo, carrying off the energy of the brain: 

 from the stomach. 



" Vomiting is often excited by a disagreeable sensation not 

 applied to the stomach, but to the taste or smell, or even by the 

 imagination renewing these. It is not to be denied that emetics 

 are sometimes direct stimulants, but many are otherwise ; as 



2 A 2 



