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ADDITIONAL NOTE. VII. 

 OLD AGE AND DEATH. 



The age-worn fibres goaded to contract 



By repetition palsied, cease to act. CANTO II. 1. 4. 



I. Effects of Age. 



THE immediate cause of the infirmities of age, or of the progress of 

 life to death, has not yet been well ascertained. The answer to the 

 question, why animals become feeble and diseased after a time, though 

 nourished with the same food which increased their growth from in- 

 fancy, and afterwards supported them for many years in unimpaired 

 health and strength, must be sought for from the laws of animal 

 excitability, which, though at first increased, is afterwards diminished 

 by frequent repetitions of its adapted stimulus, and at length ceases 

 to obey it. 



1. There are four kinds of stimulus which induce the fibres to con- 

 tract, which constitute the muscles or the organs of sense; as, first, The 

 application of external bodies, Avhich excites into action the sensorial 

 power of irritation; 2dly, Pleasure and pain, which excite into action 

 the sensorial power of sensation; 3dly, Desire and aversion, which 

 excite into action the power of volition ; and lastly, The fibrous con- 

 tractions, which precede association, which is another sensorial power; 

 see Zoonomia, Vol. I. Sect. II. 13. 



Many of the motions of the organic system, which are necessary to 

 life, are excited by more than one of these stimuli at the same time, 

 arid some of them occasionally by them all. Thus respiration is 

 generally caused by the stimulus of blood in the lungs, or by the sen- 

 sation of the want of oxygen; but is also occasionally voluntary. The 

 actions of the heart also, though generally owing to the stimulus of 

 the blood, are also inflamed by the association of its motions with 

 those of the stomach, whence sometimes arises an inequality of the 

 pulse, and with other parts of the system, as with the capillaries, 

 whence heat of the skin in fevers with a feeble pulse, see Zoonomia. 



