XVI INTRODUCTION. 



This completes a view of the animal world, beginning with 

 man, the most perfect member of it, and descending to those 

 obscure and minute creatures which are scarcely visible ex- 

 cept with the assistance of the microscope. It will be observed 

 that one common plan pervades the whole, that the same 

 general objects are had in view, in the structure of every class, 

 and that there is a general analogy in the methods employed 

 for effecting these objects, although there is a great variety in 

 the details ; that there is a grand simplicity in the design, 

 though a great diversity in the means. In short, not only in 

 the structure of each individual animal, but in the wonderful 

 manner in which that structure is varied to correspond to the 

 nature, habits, and wants of the different classes, may be per- 

 ceived the wisdom, the power, and the benevolence of that great 

 Creator, who has devised and formed, and who continues to 

 uphold, the myriads of animated beings with which the earth 

 is filled. 



NATURAL CLASSIFICATION OF MEDICINAL AGENTS. 



The substances employed as medicines are found, in com- 

 mon with the other objects of nature, everywhere surround- 

 ing us. They are prescribed either in their natural condition, 

 that is, as they are found on the surface of the earth or be- 

 neath it ; or as they are artificially prepared, that is, changed 

 from their natural condition, either by the abstraction of some 

 of their parts or by the addition of new parts. The natural 

 substances consist of both simple and compound bodies, de- 

 rived from the organic and inorganic kingdoms of nature. 

 The artificial are the productions of the pharmaceutical art; 

 thence the study of the nature of medicinal agents implies 

 some acquaintance with Natural History and Chemical Sci- 

 ence. 



The medicinal agents which are the products of organiza- 

 tion are of animal and of vegetable origin ; the inorganic sub- 

 stances are minerals. The medicinal properties of organic 

 bodies (whether animal or vegetable) vary at different periods 

 of their existence, and owing to changes of a functional kind. 

 In animal bodies they are most active during the exercise of 

 some peculiar function. In vegetables they often remain la- 

 tent or are not formed until the plants have attained their full 

 growth and are capable of exercising their reproductive facul- 

 ties. The medicinal properties of mineral substances (inor- 



