162 JMEDICINAL AND POISONOUS SUBSTANCES. 



the compounds thus formed. The second division, 

 consisting of the essential oils, camphor, empyreumatic 

 substances, and antiseptics, &c., possesses the property 

 of impeding or retarding those kinds of transformation 

 to which certain very complex organic molecules are 

 liable ; transformations which, when they take place 

 out of the body, are usually designated by the names 

 of fermentation and putrefaction. 



The third division of medicinal substances is composed 

 of bodies, the elements of which take a direct share in 

 the changes going on in the animal body. When intro- 

 duced into the system, they augment the energy of the 

 vital activity of one or more organs ; they excite mor- 

 bid phenomena in the healthy body. All of them pro- 

 duce a marked effect in a comparatively small dose, and 

 many are poisonous when administered in larger quan- 

 tity. None of the substances in this class can be said to 

 take a decided share in the nutritive process, or to be 

 employed by the organism in the production of blood ; 

 partly, because their composition is different from that 

 of blood, and, partly, because the proportion in which 

 they must be given, to exert their influence, is as noth- 

 ing, compared with the mass of the blood. 



These substances, when taken into the circulation, 

 alter, as is commonly said, the quality of the blood, and 

 in order that they may pass from the stomach into the 

 circulation with their entire efficacy, we must assume 

 that their composition is not affected by the organic in- 

 fluence of the stomach. If insoluble when given, they 

 are rendered soluble in that organ, but they are not de- 



