8 THE VITAL FUNCTIONS. 



to react upon one another, and to produce, from 

 slight variations of circumstances, a totally new 

 order of combinations. Thus a degree of heat, 

 which would occasion no change in most mineral 

 substances, will at once effect the complete dis- 

 union of the elements of an animal or vegetable 

 body. Organic substances are, in like manner, 

 unable to resist the slower, but equally destruc- 

 tive agency of water and atmospheric air ; and 

 they are also liable to various spontaneous 

 changes, such as those constituting fermentation 

 and putrefaction, which occur when their vitality 

 is extinct, and when they are consequently 

 abandoned to the uncontrolled operation of their 

 natural chemical affinities. This tendency to 

 decomposition may, indeed, be regarded as 

 inherent in all organized substances, and as 

 requiring for its counteraction, in the living 

 system, that perpetual renovation of materials 

 which is supplied by the powers of nutrition. 



It would appear that during the continuance 

 of life, the progress of decay is arrested at its 

 very commencement ; and that the particles, 

 which first undergo changes imfitting them for the 

 exercise of their functions, and which, if suffered 

 to remain, would accelerate the destruction of 

 the adjoining parts, are immediately removed, 

 and their place supplied by particles that have 

 been modified for that purpose, and which, 

 when they afterwards lose these salutary pro- 



