364 DIGESTION, Sect. XXXVII. 4. t ; 



thus be compared to the erection of the penis, or of the nipples 

 ot the breafts of women ; while new particles become added at 

 the fame time ; as in the procefs of nutrition above defcribed. 



When only the natural grawth of the various parts of the 

 body is produced, a plealurable fenfation attends it, as in youth, 

 and perhaps in thole, who are in the progrefs of becoming fat. 

 "When an unnatural growth is the confequence, as in inflam- 

 matory dileaies, a painful fenfation attends the enlargement of 

 the fyitem. 



IV. This appoHtion of new parts, as the old ones difappear, 

 felecled from the aliment we take, firfl enlarges and ftrength- 

 ens our bodies for twenty years ; for another twenty years it 

 keeps us in health and vigour, and adds ftrength and folidity to 

 the fytlem, and then gradually ceafes to nouriih us properly ; 

 and for another twenty years we gradually fink into decay, and 

 finally ceafe to act, and to exift. 



On confidering this fubjecr. one mould have imagined at firfl 

 view, that it might have been eafier for nature to have fup- 

 ported her progeny for ever in health and life, than to have per- 

 petually reproduced them by the wonderful and myfterious 

 procefs of generation. But it feems our bodies by long habit 

 ceafe to obey the ftimulus of the aliment, which {hould fupport 

 us. After we have acquired our height and folidity we make 

 no more new parts, and the fyilem obeys the irritations, fen- 

 fations, volitions, and afTociations, 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 mentioned. 1. If a ftimulus be greater 

 than natural, it produces too great an exertion of the ftimulated 

 organ, and in confequence exhaufts the fpirit of animation ; and 

 the moving organ ceafes to acl:, even though the ftimulus be 

 continued. And though reft will recruit this exhauftion, yet 

 fome degree of permanent injury remains, as is evident after ex- 

 pofing the eyes 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 fmall dofes of aloes or rhubarb are ex- 

 hibited,) they may be gradually increafed, without exciting the 

 organ into aclion ; which wili thus acquire a habit of difobedi- 

 ence to the ftimulus ; thus by increasing the dofe by degrees, 

 great quantities of opium or wine may be taken without intoxi- 

 cation. See Seel, XII. 3. 1. 



3. Another mode, by which life is gradually undermined, is 

 when irritative motions continue to be produced in confequence 

 of ftimulus, but are not fucceeded by fenfation ; hence the 

 ftimulus of contagious matter is not capable of producing fever 



