INTRODUCTION. S5 



permitted to re-assume its original state, for the pur- 

 pose of undergoing a fresh alteration. 



The external organs of sense may be compared 

 to sieves, which permit nothing to pass through, 

 and act on the corresponding nerve, but that which 

 possesses a capacity of exciting that nerve, and that 

 alone. Thus the tongue is provided with spongy 

 papillae, which imbibe saiine solutions ; the ear, 

 with a gelatinous pulp, agitated by the vibrations of 

 the atmosphere; the eye, with transparent lenses, 

 permeable only by light. 



The external irritating causes which occasion a 

 contraction of the fibres, in all probability exercise 

 this action by producing the same effect upon the 

 fibre through the nerves, that volition does ; name- 

 ly, by altering the nervous fluid, so as to cause a 

 change in the dimensions of that fibre upon which 

 its influence is exerted. With this latter kind of 

 operation the will has nothing to do, it is frequently 

 carried on without even awakening the conscious- 

 ness of the individual ; and when actually separated 

 from the body, the muscles are still susceptible of 

 irritation, so long as the nerve remaining with them 

 possesses its exciting power ; a phenomenon most 

 evidently unconnected with any thing like volition. 



The nervous fluid undergoes alteration from [re- 

 peated] muscular irritation, as well as from sensibi- 

 lity and voluntary motion, and the same necessity 

 then exists as in the latter cases, of re-invigorating 

 the power of this fluid, by restoring it to its quies- 



D 2 



