182 MODE OF ACTION OF VEGETABLE 



in such a condition, the health may be benefited by 

 the use of compounds which are capable of sup- 

 plying the place of the nitrogenised product pro- 

 duced in the healthy state of the body, and essen- 

 tial to the production of an important element of 

 respiration. In a chemical sense — and it is this 

 alone which the preceding remarks are intended to 

 shew — caffeine or theine, asparagine, and theobro- 

 mine, are, in virtue of their composition, better 

 adapted to this purpose than all other nitrogen- 

 ised vegetable principles. The action of these sub- 

 stances, in ordinary circumstances, is not obvious, 

 but it unquestionably exists. 



89. With respect to the action of the other nitro- 

 genised vegetable principles, such as quinine, or the 

 alkaloids of opium, &c., which manifests itself, not in 

 the processes of secretion, but in phenomena of an- 

 other kind, physiologists and pathologists entertain 

 no doubt that it is exerted chiefly on the brain and 

 nerves. This action is commonly said to be dyna- 

 mic — that is, it accelerates, or retards, or alters in 

 some way the phenomena of motion in animal life. 

 If we reflect that this action is exerted by sub- 

 stances which are material, tangible and ponder- 

 able ; that they disappear in the organism ; that a 

 double dose acts more powerfully than a single one ; 

 that, after a time, a fresh dose must be given, if we 

 wish to produce the action a second time ; all these 

 considerations, viewed chemically, permit only one 

 form of explanation ; the supposition, namely, that 



