Comparative Physiology. 463 



tables the absorption of old and deposition of new particles take 

 place so slowly in the natural condition as to be scarcely per- 

 ceptible ; but when disease or injury calls the actions of repa- 

 ration into play, they are effected with a rapidity and certainty 

 not surpassed in any other parts of the system. The constant 

 movements of the body produce a waste or wearing away of the 

 material, both of its harder and softer structures ; hence arises 

 one cause of the increased demand for nutriment from continued 

 muscular exertion." 



Vitality has been generally supposed to exert a resisting 

 force, preserving the materials endowed with vitality from che- 

 mical decomposition. Some, however, have carried the opposite 

 doctrine so far as to maintain that matter endowed with life is 

 more susceptible of change. Undoubtedly, where life is most 

 active in plants, the parts are more susceptible of decomposition 

 when vitality is paralysed, than in the more solid tissues ; but 

 the preserving power of life is perhaps only the more dis- 

 played in preserving parts so susceptible of change from de- 

 composition. Liebig seems to consider life as exercising a 

 resisting power ; the more that nitrogen abounds in the tissue, 

 he considers, the greater is the susceptibility of change. The 

 actions of the mind waste the nervous system, he says, and those 

 of the body the muscular, producing inert dead matter, which 

 is removed by the oxygen inhaled and made use of in the form- 

 ation of bile and urine. The vital force he describes as pro- 

 duced by the oxidation of carbon and hydrogen, especially the 

 former ; and the source of carbon as confined to the waste or 

 decayed particles of the body and the non-azotised constituents 

 of the food, as starch, sugar, &c., which, he says, may be called 

 the food of the liver. When these are exhausted the fat of 

 the body is next consumed ; and starvation ends in a defi- 

 ciency of vital force being generated from a want of carbon, 

 and the tissues thus deprived of vitality becoming subject to 

 chemical decomposition. Mliller seems to entertain similar 

 views of the resistance opposed by vitality to chemical agency, 

 and it seems to be the most general opinion. 



" One of the most striking differences between animals and 

 vegetables is to be found in the aliments on which they are 

 respectively supported, and the mode of their ingestion or intro- 

 duction into the system. The essential nutriment of plants 

 appears to be supplied by the inorganic world, chiefly toater 

 with saline impregnations, and carbon ; the water partly from 

 the soil and partly fi-om the moisture of the atmosphere ; the 

 carbon principally from the carbonic acid of the air ; but most 

 plants require for their healthy growth that it be introduced by 

 the roots also. It appears that the organic matter which rich 

 soils contain is itself applied to the nutrition of the plant, by 



H H 2 



