IN THE ANIMAL ORGANISM. 189 



The contact of the living tissues with the elements of 

 nutrition is determined in the animal body by a me- 

 chanical force produced within the body, which gives to 

 certain organs the power of causing change of place, 

 of producing motion, and of overcoming mechanical 

 resistance. 



We may communicate motion to a body at rest by 

 means of a number of forces, very different in their 

 manifestations. Thus, a time-piece may be set in 

 motion by a falling weight (gravitation), or by a bent 

 spring (elasticity) . Every kind of motion may be pro- 

 duced by the electric or magnetic force, as well as by 

 chemical attraction ; while we cannot say, as long as 

 we only consider the manifestation of these forces in the 

 phenomenon or result produced, which of these various 

 causes of change of place has set the body in motion. 



In the animal organism we are acquainted with only 

 one cause of motion ; and. this is the same cause which 

 determines the growth of living tissues, and gives them 

 the power of resistance to external agencies ; it is the 

 vital force. 



In order to attain a clear conception of these mani- 

 festations of the vital force, so different in form, we 

 must bear in mind, that every known force is recognised 

 by two conditions of activity, entirely different in the 

 phenomena they offer to the attention of the observer. 



The force of gravitation inherent in the particles of a 

 stone, gives to them a continual tendency to move 

 towards the centre of the earth. 



This effect of gravitation becomes inappreciable to 



