IRRITABILITY. 29 



recombine them; ascertain the necessary proportions 

 in which two or more bodies unite, whence shall 

 result a product totally differing from each com- 

 ponent in character and properties : all this and 

 more is within its power ; but it cannot recompose 

 a single fluid, nor a single solid of any organic 

 body. Here its penetration is baffled, and its art 

 at fault. If, then, chemistry cannot ascertain the 

 true nature of the constituents of organic bodies, 

 neither can it discover the means by which they 

 maintain their specific differences, not only as it 

 regards the form, but the quality of their substance ; 

 quality, which widely differs even where the same 

 nutriment is in each instance received. The same 

 soil, the same water, the same air, may nourish 

 the vine and the aconite, the rose and the hemlock, 

 the nutritious plant and the deadly poison; their 

 very roots and leaves may be intertwined ; but 

 their identity and their specific properties will con- 

 tinue unaltered. 



It is evident, however, that the great Creator has 

 endowed organic nature with a remarkable pro- 

 perty, which the tubes of all having life possess, 

 and upon which their action depends, termed by 

 physiologists irritability. It denotes a power 

 residing in the fibrous parts of organic bodies, of 

 responding to the action of various and appropriate 

 stimuli, external or internal ; and exhibited in 

 contractile or expansive movements. It renders the 

 tubes or vessels agents in the propulsion of the 



