OBJECTS OF SUPPLY OP FOOD. 117 



this tissuo. So, in the heart -wood of a Plant, the circulation 

 almost ceases ; for so long as no air or moisture from without, 

 find access to the interior of the stem, this part retains its firm- 

 ness unchanged, and its particles require no renewal. But the 

 soft tissues of an Animal are largely supplied with blood, and also 

 with absorbing vessels ; and the greatest part of the food taken 

 in by one which has attained its full growth, is devoted to the 

 maintenance of these parts in their right condition, which is 

 essential for the proper performance of their functions. Of all 

 these soft tissues, that which forms the brain and nerves is sup- 

 plied with the largest proportion of blood, and undergoes the 

 most rapid changes during life; and it is this which most 

 speedily decomposes after death. The soft tissues of Plants do 

 not so quickly decay as those of animals, and the circulation of 

 nutritious sap through them is less active ; but still a movement 

 of fluid takes place, and the same object is attained by it. 



163. Thus we perceive t\i&tfood has for its object, in the 

 first place, to supply the materials of the growth and extension 

 of the system, enabling the minute germ containing the seed to 

 develop itself in time into a lofty tree ; next, to maintain the 

 parts so formed, in their healthy state ; by affording the mate- 

 rials by which those that have begun to decay may be replaced ; 

 further, to provide a store capable of supplying the occasional 

 extraordinary demand for reparation, which disease or injury 

 .may produce ; and, lastly, to enable the being to develop the 

 germs of new individuals ; and to supply them with a store of 

 nutriment, on which they may live, until able to provide for 

 themselves. Now of the 55 simple substances into which the 

 solids, fluids, and gases, of the inorganic world may be separated, 

 Vegetables are principally made up of four ; and of these only 

 three exist in any large proportion. These three are carbon, 

 oxygen, and hydrogen ; and the fourth is nitrogen. It appears, 

 from recent inquiries, that the organised tissues of Plants, that 

 is, their cells, fibres, vessels, &c., freed from their contents- are 

 composed of a substance, which everywhere possesses the same 

 composition ; and that this consists of 24 Carbon, 20 Hydrogen, 

 and 10 Oxygen, without any Nitrogen. The composition of 



