14 THE VITAL FUNCTIONS. 



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 consti- 

 tuting fermentation and putrefaction, which occur when their 

 vitality is extinct, and when they are consequently aban- 

 doned to the uncontrolled operation of their natural chemi- 

 cal 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 unfitting 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 properties, 

 are, in their turn, discarded and replaced by others. Hence, 

 the continued interchange and renewal of particles which 

 take place in the more active organs of the system, especial- 

 ly in the higher classes of animals. In the fabric of those 

 animals which possess an extensive system of circulating 

 and absorbing vessels, the changes that are effected are so 

 considerable and so rapid, that even in the densest textures, 

 such as the bones, scarcely any portion of the substance 

 which originally composed them is permanently retained in 

 their structure. To so great an extent is this renovation of 

 materials carried on in the human system, that doubts may 

 very reasonably be entertained as to the identity of any por- 

 tion of the body after the lapse of a certain time. The pe-. 

 riod assigned by the ancients for this entire change of the 

 substance of the body, was seven or eight years: but modern 

 inquiries, which show us the rapid reparation that takes 

 place in injured parts, and the quick renewal of the bones 

 themselves, tend to prove that even a shorter time than this 



