22 THE PHILOSOPHY 



with fenfatiorijwhich implies a diftincl perception of pleafure^ 

 and pain. 



There is an inferior fpecies of fenfatlon, which is diftin- 

 guifhed by the term irritability. This term denotes that pow- 

 er by which mufcular fibres, even after they are detached 

 from the body, contract upon the appUcation of any flimulat- 

 ing fnbftance, whether folid or fluid. The heart of a frog 

 when pricked with the point of a pin continues to beat, or 

 t-o contra(St and dilate, for feveral hours after it has been 

 cut out of the animal's body. The heart of a viper or of a turtle 

 beats diftinctly from twenty to thirty hours after the death 

 of thefe animals. The periftaltic motion of the inteflines is 

 produced by their irritability. Yv^hen the inteftines of a dog, 

 ©r any other quadruped, are fuddenly cut into different por- 

 tions, ail thefe portions crawl about like worms, and contraft 

 upon the flighted touch. Though irritability be unqueflion- 

 ably a vital principle, yet it is equally certain that mufcular 

 fibres, when feparated from the body to which they belong, 

 have no diflincl perception of plealure or pain. Their re- 

 gular contracSlion and dilatation are evident fymptoms of life, 

 which, in many cafes, may lead us to attribute living powers 

 to ,fubflances that enjoy neither Ufe nor fenfation, Henccj, 

 though all plants were irritable, this circumflance would not 

 prove that they are poffeiTed of life. The contradtion and 

 dilatation of the fenfitive plants, and the various motions of 

 the leaves, branches, flowers,-and roots of vegetables formerly 

 mentioned, feem to indicate that moft plants are endowed 

 with irritability. Perhaps all vegetables have more or lefs of 

 this quality. The heart, inteflines, and diaphragm, are the 

 moft irritable parts of animal bodies ; and, to difcover whe- 

 ther this quality refides in all plants, experiments fhould be 

 made chiefly on their leaves, flowers, buds, and the tender 

 Sbres of the roots. 



