16 PHYSIOLOGY. 



to themselves new materials by which they are nourished. Plants 

 derive their nourishment from the inorganic world, from the excre- 

 tions of animals, and from decaying organic matter. Animals, on 

 the other hand, can be nourished only by organic matter, either 

 animal or vegetable. Both possess the power of rearranging the 

 constituents of these substances into forms identical with those of 

 the elements of their various tissues. 



But whilst new matter is being deposited, the old particles must 

 be thrown off, otherwise the growth would be unlimited ; and if ex- 

 cretion alone went on, disorganization would very soon occur. In 

 both operations, new combinations are continually taking place as it 

 were, in opposite directions; in the one, from the simple to the com- 

 plex organized parts; in the other, from the complex constituents of 

 the textures, to the simple organic or inorganic compounds. 



From the deco7npositio7i of animal and vegetable matters, a great 

 variety of products results. This decomposition is of two kinds, 

 distinguished by the names o^ fermentatio?i and putrefaction ; the 

 former term has been limited by Liebig to the decomposition of sub- 

 stances devoid of nitrogen, and the latter to that of azotized sub- 

 stances. The products of this decomposition are nitrogen and 

 hydrogen, (which partly escape in a free state), water, carbonic 

 acid, carburetted hydrogen, olefiant gas, ammonia, cyanogen, prus- 

 sic acid, phosphuretted hydrogen, and hydrosulphuric acid; while in 

 some cases the elements reunite in different proportions, so as to form 

 a new organic compound, as in the production of sugar from starch in 

 the saccharine fermentation. Sometimes from one organic substance 

 two new compounds are generated, one organic, the other inorganic; 

 as in vinous fermentation, during which carbonic acid and alcohol 

 are formed from sugar. 



The conditions more or less necessary for the spontaneous decom- 

 position of organic matter, are moisture, the access of atmospheric 

 air, and a certain temperature. The first and last are always ne- 

 cessary, the second not invariably. 



The gaseous products of the decomposition of animal matter, and 

 of the human body in particular, are carbonic acid, sometimes nitro- 

 gen, hydrogen, sulphuretted hydrogen, phosphuretted hydrogen and 

 ammonia. x\cetic acid is also formed, and sometimes nitric acid. 



By contrasting the characters of organized bodies as described, 

 with those of the inorganic world, the distinction between the two 

 may be seen. Inorganic bodies are either aeriform, liquid, or solid; 

 they are irregular in shape, and generally angular. No distinction 

 of parts or organs is to be found in the mineral substance ; its mi- 

 nutest fragment is in every respect of the same nature as the largest 

 mass. Inorganic bodies are unlimited in size and duration, continu- 

 ing for ages without augmentation or waste, provided no mechanical 

 or chemical agent be brought to act upon them. There is in them 



