86 OF STIMULUS AND EXERTION. SECT.' X II. 3. 



and introduces a new link between them ; whence 

 every repetition ftrengthens this new affociation or 

 catenation, and the ilimulus may be gradually de- 

 creafed, or be nearly withdrawn, and yet the ef- 

 fect mail c ntinue ; becaufe the ienforial power of 

 aifociation or catenation being united with the fti- 

 inulus, increafes in energy with every repetition 

 of the catenated circle ; and it is by thefe means 

 that all the irritative affociations of motions are 

 originally produced. 



2. When a ftimulus is repeated at fuch diftant 

 intervals of time, that the natural quantity of fen- 

 ibrial power becomes completely reftored in the 

 acting fibres, it will acl with the fame energy as 

 when firft applied. Hence thofe who have lately 

 accuftomed ihemfelves to large dofes of opium by 

 beginning with fmall ones, and gradually increaf- 

 ingthem, and repeating them frequently, as men- 

 tioned in the preceding paragraph ; if they inter- 

 mit the ufe of it for a few days only, mult begin 

 again with as fmall dofes as they took at firft, other- 

 wife they will experience the inconveniences of in- 

 toxication. 



On this circumftance depend the conftant un- 

 failing effects of the various kinds of flimulus, 

 which excite into aclion all the vafcular fyilems in 

 the body ; the arterial, venous, abforbent, and 

 glandular veflcls, are brought into perpetual un- 

 wearied aftion by the fluids, which are adapted to 

 IHmuIate them ; but thefe have the fenforial power 

 of affociation added to that of irritation, and even 

 in foine degree that of fenfation, and even of voli- 

 tion, as will be fpoken of in their places ; and life 

 itfelf is thus carried on by the production of fenfo- 

 rial power being equal to its wafte or expenditure 

 in the perpetual movement of the vafcular organi- 

 zation. 



7. When 



