SECT. XXXVII. 4. DIGESTION, SECRETION, &c. $31 



On confidering this fubject one ftiould have ima- 

 gined at firfl view, that it might have been eafier 

 for nature to have fupportcd her progeny for ever 

 in health and life, than to have perpetually repro- 

 duced them by the wonderful and myfterious pro- 

 cefs of generation. But it feems our bodies by 

 long habit ceafe to obey the ftimulus of the aliment, 

 which fhould fupport us. After we have acquired 

 our height and folidity we make no more new 

 parts, and the fyftem obeys the irritations, fenfati- 

 ons, volitions, and aflbciations, with lefs and lefs 

 energy, till the whole finks into inaction. 



Three caufes may confpire to render our nerves 

 lefs excitable, which have been already mentione<J. 

 I. If a flimulus be greater than natural, it produces 

 too great an exertion of the ilimulated organ, and 

 in confequence exhaufts the fpirit of animation ; 

 and the moving organ ceafes to act, even though 

 the ftimulus be continued. And though reft will 

 recruit this exhauflion, yet fome degree of perma- 

 nent injury remains, as is evident after expofing the 

 jeyes long to too ftrong a light. 2. If excitations 

 weaker than natural be applied, fo as not to excite 

 the organ into action, (as when frnall dofes of aloe 

 pr rhubarb are exhibited,) they may be gradually 

 increafed, without exciting the organ into action ; 

 which will thus acquire a habit of difobedience to 

 the ftimulus ; thus by increafing the dofe by de- 

 grees, great quantities of opium or wine may be 

 taken without intoxication. See SJfcSt. XII. 3. T. 



3. Another mode, by which life is gradually un- 

 dermined, is when irritative motions continue to 

 be produced in confequence of ftiinulus, but are 

 not fucceeded by fenfation ; hence the ftimulus of 

 contagious matter is not capable of producing fever 

 a fecond time, becaufe it is not fucceeded by fen- 

 fation. See Sect. XII. 3.6. And hence, owing to 

 the want of the general pleafurable fenfation, which 



ought 



