12 EXCRETION. DECOMPOSITION. [INTROD. 



power of rearranging the constituents of these substances into 

 forms identical with those of the elements of their various tissues 

 and of thus making them part and parcel of themselves. 



Together with a process of supply, there is one of waste con- 

 tinually in operation. Animals and plants are ever throwing off 

 effete particles from their organisms. These, under the name of 

 excretions, appear in various forms either as inorganic compounds, 

 or as secondary organic products. Thus carbonic acid is given 

 off in large quantities from animals ; water, likewise, forms a con- 

 siderable portion of their excreted matter, and serves to hold in 

 solution salts, and secondary organic compounds, which result from 

 the waste of the tissues. In this way, also, urea, lithic acid, and 

 biliary matters are excreted. In plants, water is excreted from 

 the leaves, a phenomenon which has been compared to the perspira- 

 tion of animals ; and various other excretions, which are sometimes 

 made to serve an additional purpose in the ceconomy of the vege- 

 table, besides that of getting rid of superfluous matter, are doubtless 

 formed by the secondary combinations of the effete particles of 

 their textures. 



These two processes, excretion, or the expulsion of effete particles, 

 and assimilation of substances from without, are necessarily mu- 

 tually dependent. As long as new matter is being appropriated, 

 old particles must be thrown off, otherwise growth would be un- 

 limited and were excretion alone to go on, the destruction of the 

 organism must speedily ensue, by the gradual waste of the tissues, 

 to which no new supply was afforded. In both processes new 

 combinations are taking place, as it were, in opposite directions ; in 

 the one from the simple to the complex to form organized parts, 

 in the other, from the complex constituents of the textures to the 

 simple organic, or inorganic compounds. 



As each texture of the organism has this tendency to change 

 during life, so, the whole organism tends to decomposition, when 

 death puts a stop to all further absorption of nutritive matters. 

 Dead organized matter is speedily dissipated under certain con- 

 ditions. These are the presence of air, moisture, and a certain 

 temperature, or contact with an organic substance which is itself 

 undergoing decomposition. The affinity which held together the 

 elements of the organic substances is destroyed by the cause which 

 occasioned their death, and they are set free to obey new affinities 

 and form new compounds. 



When we consider the large number of equivalents which 

 enter into the formation of each molecule of organic compounds, 



