DEAD AND LIVING BODIES. 5 



this they require nothing more than the presence of oxygen, 

 the existence of moisture, and a moderate degree of warmth, 

 to insure their decomposition. These conditions, though essen- 

 tial, are so universally present, that animal and vegetable mat- 

 ters are generally considered as liable to decay "of themselves." 

 If, however, such substances be deprived of access of air, or be 

 frozen, or have their water driven off by desiccation, they are 

 capable of retaining their chemical composition for an appa- 

 rently indefinite period of time ; and one or other of these con- 

 ditions is carried out in all processes which have as their end 

 the preservation unchanged of the organic substances which 

 form the bodies of animals and plants. 



3. DIFFERENCES BETWEEN DEAD AND LIVING BODIES. 



Whilst all living bodies, whether animal or vegetable, are 

 composed essentially of organic substances, there are never- 

 theless associated with the living organism larger or smaller 

 amounts of matter which is practically dead. On the other 

 hand there are numerous secondary organic products which at 

 no time enter into the composition of living bodies, and which 

 are therefore just as much "dead" substances as the genuine 

 inorganic substances. 



The general distinctions between dead and living matter are 

 the following : 



a. Mode of Increase. Living bodies possess the power of 

 taking into their interior certain materials (food), foreign to 

 those composing their own substance, and of converting these 

 into the materials of which they are themselves built up. This 

 process is known as " assimilation," and it is in virtue of this 

 that living bodies grow. The growth of the organism, there- 

 fore, and its increase in size, is not effected by the mere 

 addition of matter from the outside, but by the taking of 

 matter into the interior of the body, and its modification 

 there. 



On the other hand, when dead bodies increase in size (as 

 crystals do in supersaturated solutions), this is effected simply 

 by the addition of particles from the outside, or, as it is 

 technically called, by the " accretion," instead of by the 

 " intussusception," of matter. The newly "added particles 

 undergo no change from their previous constitution, and the 

 essential element of "assimilation" is thus wanting, so that 

 the process is in no sense one of " growth " properly so called. 



