38 INTRODUCTION. 



This mutual influence of the nerves and the fibres, 

 both in the intestinal and in the arterial systems, is 

 the true mainspring of vegetative life in the animal 

 creation. 



As each external sense is permeable only by cer- 

 tain sensible substances, so each internal organ is 

 accessible only to certain agents of irritation. Thus 

 mercury irritates the salivary glands ; cantharides, 

 the bladder, fyc. Such agents are usually termed 

 specifics. 



The nervous system being homogeneous, and 

 continuously connected through all its parts, local 

 sensations and irritations are found to produce 

 general debility ; and each particular function, when 

 exercised overmuch, may weaken the rest : thus too 

 much food enfeebles the thinking faculty; protracted 

 meditation proves injurious to digestion, &c. ; ex- 

 cessive local irritation has further the power of 

 enfeebling the entire body, as if all the forces of life 

 were embodied in a single point. 



When irritation exists in one place, it may be 

 counteracted and diminished by producing an oppo- 

 site irritation in another ; such is the mode in. which 

 purgatives and many other specific remedies are 

 found to operate. 



Rapid as the sketch which we have given on this 

 subject necessarily is, it may suffice to establish the 

 possibility of accounting for all the phenomena of 

 physical life, by the admission of such a fluid as we 

 Jiave supposed the nervous fluid to be, from a con- 



