THE ACTIVE FORCES OF LIVING ORGANISMS 47 



pressure ; but as the conditions under which they 

 have been going on are such as lead chiefly to con- 

 traction, or chiefly to expansion, so does their general 

 character incline more in this or that direction. What 

 those conditions are has already been described in 

 dealing with physiological reaction. Irrespective of 

 the influence of respiratory movements on blood- 

 pressure, vaso-motor changes, when excessive and 

 frequent, are probably one of the chief causes of 

 an ill-balanced state of metabolism, although the 

 anatomical relations of the vaso-motor centre to the 

 rest of the nervous system, and especially to the 

 brain, are but little known. 



What is the nature of the force transmitted by a The nature 



■' of the 



nerve-fibre? Is it molecular? is it chemical or atomic? nervous 



impulse. 



is it electrical ? There are many who deny that the 

 nerve impulse is of an electrical order ; but the state- 

 ments on this point are not absolutely conclusive, 

 although in some measure doubtless true. The facts 

 on which they rest are briefly as follows : The 

 velocity of an electrical current in a nerve-trunk is 

 far greater than that of a nerve impulse passing 

 along the same fibres. A nervous impulse is not 

 transmissible by an electric conductor — as, for instance, 

 by a copper wire. If you ligature a nerve, it will no 

 longer transmit nervous impulses, but will still act as 

 a conducting medium to an electrical current. A 

 crushed motor fibre which has lost its functional 

 power will generate electricity for a time. 



With respect to the arrest of a nerve impulse by a 



