OF HEALTH AND HUMAN NATURE. 43 



panied by a perception in the mind, as in seeing, hearing, tasting, 

 smelling, feeling : animal contractility is excited by the volition 

 of the mind conveyed to the voluntary muscles by means of the 

 nerves. Organic sensibility is attended by no perception, and is fol- 

 lowed by contraction totally independent of the will : — the heart 

 feels, if we may so speak, (physiology has no proper term for 

 the idea) the stimulus of the blood, and, without our influence, 

 forthwith contracts ; the lacteals feel the stimulus of the chyle 

 without our knowledge, and propel it without our assistance. 

 But although we never acquire the least direct voluntary power 

 over the actions of organic contractility, — over the peristaltic 

 motion of the intestines or the contractions of the blood vessels, 



" Tut, man I one fire puts out another's burning, 

 " One pain is lessen'd by another's anguish ; 



" Turn giddy, and be holp by backward turning; 

 " One desperate grief cures with another's languish ; 



" Take thou some new infection to thy eye, 



" And the rank poison of the old will die." 



Shakspeare. Romeo, Act. i. Sc. ii. 



The effect of vicissitudes of temperature, and a large number of other pa- 

 thological phenomena, are principally explicable on the derangement of the 

 balance of excitability and for the most part consequently of circulation. 

 (Sect. XX. B) 



Notwithstanding it is a general law (53) that the effects of a stimulus dimi- 

 nish the more frequently it is applied, yet if it is applied so energetically as to 

 leave the sensibility heightened, especially if to the point of inflammation, its 

 subsequent power is greatly increased. Immense potations of spirituous liquors 

 may gradually be borne, but if the increase is too sudden, the sensibility of the 

 stomach may become such that a single glass will prove violently irritating. 



The specific action of one agent frequently prevents that of another ; — small- 

 pox and measles very rarely occur together, and the former is often prevented 

 for ever by the cow-pock. 



While moderate excitement is necessary to maintain action, violent wears out 

 the power, and very violent may suddenly destroy life altogether : according 

 to the verses, 



Nutritur ventis, ventis extinguitur ignis, 

 Lcnis alit ftammas, grandior aura nccat. 



