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LECTURE IV. 



ON DIGESTION. 



Those bodies which we call living, are 

 chiefly characterised by iheir powers of con- 

 verting surrounding substances into their 

 own nature ; of building up the structure 

 of their own bodies, and repairing the in- 

 juries they may accidentally sustain. They 

 not only diflfer considerably from each other 

 in their size and external form, but also 

 in their internal structure. Vegetables im- 

 bibe watery fluids from the earth, in which 

 saline and other matters are dissolved or 

 suspended, and these, in the very vessels 

 of the vegetable, become the sap or nutri- 

 tive fluid of the plant. This ascends with 

 celerity and force ; forms the leaves, flow- 

 ers, and fruit, in which it is more particu- 

 larly exposed to the influence of light, 

 heat, and air ; then returns and augments 

 the woody stems and trunk, impregnates 

 them with juices which concrete ; and even- 



