MEDICINAL AND POISONOUS SUBSTANCES. 161 



the food, take a decided share in the formation of the 

 secretions, and particularly of the bile, then it is plain, 

 that the organism must possess the power of causing 

 foreign matters, which are neither parts nor constituents 

 of the organs in which vital activity resides, to serve 

 for certain vital processes. All nitrogenized substances 

 capable of being rendered soluble, without exception, 

 when introduced into the organs of circulation or of di- 

 gestion, must, if their composition be adapted for such 

 purposes, be employed by the organism in the same 

 manner as the nitrogenized products which are formed 

 in the act of metamorphosis of tissues. 



We are acquainted with a multitude of substances, 

 which exercise a most marked influence on the act of 

 transformation, as well as on the nutritive process, 

 while their elements take no share in the resulting 

 changes. These are uniformly substances, the particles 

 of which are in a certain state of motion or of decompo- 

 sition, which state is communicated to all such parts of 

 the organism as are capable of undergoing a similar 

 transformation. 



74. Medicinal and poisonous substances form a sec- 

 ond and most extensive class of compounds, the ele- 

 ments of which are capable of taking a direct or an 

 indirect share in the processes of secretion and of 

 transformation. These may be subdivided into three 

 great orders ; the first (which includes the metallic 

 poisons) consists of substances which enter into chemi- 

 cal combination with certain parts or constituents of 

 the body, while the vital force is insufficient to destroy 

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