XXIV INTRODUCTION. 



nal roots." These stomachs may admit fluids only, or 

 they may be large enough to receive considerable mass- 

 es of solid aliment. In the latter case exists the neces- 

 sity of teeth, or some mechanical means of triturating 

 the solid food into such fine pieces, as will admit of its 

 being exposed by an extensive surface to the action of 

 the stomach. But as much of the matter thus carried 

 in is unfit for assimilation, and there may be even more 

 of it than is required, an intestinal canal is provided, 

 by which it is carried oat again. Here then commence 

 the phenomena of a true digestion, with all its modifi- 

 cations and stages. 



The very simple structure of a plant, and its perma- 

 nent locality is attended with a circulation of its juices 

 equally simple ; which is performed and maintained by 

 the capillary attraction of its pores, and by evaporation 

 from its higher and more exposed parts. This circulation 

 is the more rapid as the evaporation becomes greater : 

 but the latter may become changed into absorption by 

 the humidity of the atmosphere ; and the circulation be as 

 a consequence reversed from the branches to the roots. 

 But it is evident that such animals as possess extensively 

 the powers of locomotion, besides having organs more 

 numerous and more complex than the parallel fibrillse of 

 vegetables, will frequently find themselves in such con- 

 ditions of temperature and locality, that a similar circu- 

 lation of the nutritious fiuid in them could not be main- 

 tained. Hence it is necessary to have more powerful 

 and regular agents for carrying on the circulation. 

 They, therefore, are furnished with innumerable blood 

 vessels, called arteries and veins, which have a common 

 centre, the heart, for propelling through them the blood 

 or nutritive fluid to all parts of the system. From the 

 heart being furnished with valves, which are all in one 

 direction, the blood can flow only in a corresponding 

 course; thus it is forced by the heart into the arteries, 



