370 PRACTICE OF PHYSIC. 



of tone, and weaker action in the muscular fibres of the stom- 

 ach : and I conclude, therefore, that this imbecility may be 

 considered as the proximate cause of the disease I am to treat 

 of under the name of Dyspepsia. 



" This perhaps should be demonstrated here ; and in former 

 courses, especially when I first began to teach, I thought it ne- 

 cessary, as I found imperfect and false theories prevailing here. 

 But I hope it is not so necessary now, and I shall not formally 

 enter upon it : I shall therefore here treat only of one symp- 

 tom, vomiting, because that leads to a doctrine which I have 

 first started ; and, as I judge, is to be applied in many parts of 

 the system. 



" Vomiting, as an increased action, has been commonly sup- 

 posed to be always occasioned by a direct stimulus applied to 

 the fibres of the stomach, and it may often be so ; but I main- 

 tain, that it is more frequently occasioned by a disagreeable 

 sensation exciting such motions as are proper to throw off the 

 cause. Such sensations often excite motions without any mat- 

 ter applied to the part moved, at least, without any matter 

 applied, that has any direct tendency to excite motion, but 

 does it only in consequence of a sensation carried to the brain, 

 and from thence exciting the motion. But as a motion is thus 

 excited, we name the power of such a sensation a stimulus; 

 but to distinguish it from the proper direct stimulus, we call it 

 an indirect stimulus. The operations of such indirect stimuli 

 are frequent in the system, both in health and sickness ; and as 

 they are often exerted for salutary purposes, they are consider- 

 ed as evidences of a vis medicatrix naturae. 



" The operation of this vis medicatrix is often mysterious, 

 but not more so than the whole human system. Sensations are 

 agreeable or disagreeable without our being able to say why 

 any substance is the one or the other, more than we can say 

 why the particular refrangibility of certain parts of light gives 

 the sensation of red, and that of others that of blue. We 

 can say only that such is the institution of nature. But sensa- 

 tions, as being agreeable or disagreeable, produce desire or aver- 

 sion, and these again produce certain motions, such as vomit- 

 ing ; but in all this, the nature of the operation is a mystery 

 absolutely unknown to us. But the motions produced by de- 



