6, PRELIMINARY CONSIDERATIONS 



then, we obtain evidence of the operation of external 

 agents in the phenomena of vital motion, and learn at 

 the same time the mode of action, so far at least as 

 that force is concerned which is most intimately 

 wedded to motion namely, heat. 



This result is in harmony with that which 



marks the operation of the same agent in inanimate 



bodies, for these contract or expand according to the 



presence or absence of heat: and yet there is an 



important difference in the midst of this seeming 



analogy. It is found, indeed, that there is a much 



greater alteration of volume in organic solids, under 



trifling changes of temperature, than in any other 



bodies. Under no higher degree of heat than what 



is expressed by the term warmth, the fibrils of the 



areolar covering of the body are greatly elongated, 



provided they were previously in a contracted state; 



and in the fibres of the dartos and erectile tissue, the 



former length may be doubled. In inorganic solids, on 



the contrary, tlie change, under these circumstances, 



would be almost inappreciable; and even in fluids, 



which are apt to undergo greater changes than solids, 



and are very sensible to the motive power of heat, 



as may be seen in the mounting or falling of the 



liquid in the tube of the thermometer, we are 



acquainted with no instances where the change is in 



