OF VITAL MOTION. 69 



are seen in the coats of vessels. There is no evidence 

 as yet of the introduction of any new and mystical 

 agent in these movements, and the eflfects of the 

 nervous influence, so far as we may at present see, 

 are strictly analogous to those which mark the opera- 

 tion of the forces, which have their origin in the 

 various molecular changes of the organism. 



2. Of extra-organic force as the agent in the move- 

 ments under consideration. 



In the rudimentary forms of organic muscle which 

 occur in the coats of bloodvessels, facts have already 

 been stated which show that the state of contraction 

 is consequent upon the abstraction of heat. In the 

 higher forms of these structures, however, the influ- 

 ence of external heat is greatly masked by the opera- 

 tion of the organic forces ; but at the same time there 

 are facts which show that contraction is produced here, 

 as in the simpler tissues, by the operation of external 

 cold. Such a fact is the action of the intestines, which 

 is very apt to happen in delicate persons on a draught 

 of cold water. 



The influence of light, as a special agent, is 

 well seen in the iris, which may be said to represent 

 an intermediate structure, connecting the simpler 

 variety of organic muscle found in the coats of vessels 

 with that more perfect form, which occurs in the 



