18 METHODOLOGICAL INTRODUCTION. 



physiological laws, or to identify it with certain physical forces or 

 imponderable fluids, all physiological experiments indicate that it is 

 always followed by a chemical reaction, and that the nervous system 

 experiences chemical changes by and through its own activity. It 

 must, indeed, be admitted that any actual proof of such chemical 

 metamorphoses is at present perfectly unattainable, and that our 

 chemical methods would here afford us no higher aid than that 

 which the scalpel yields to the pathological anatomist. But 

 ought we to despair of attaining our object, because we do not as 

 yet clearly perceive the direction we are to follow ? Weariness of 

 the senses is the diminished impressibility of the nerves of sense, 

 but its cause cannot reasonably be sought for in any other than a 

 chemical change, experienced by the conducting substance of the 

 nerves. Such a chemical metamorphosis of the nerves of sense 

 from external impressions can no longer greatly excite our 

 astonishment, since we have witnessed the unexpected pheno- 

 menon of a picture produced suddenly, and as it were by magic, 

 from the chemical changes effected by the rays of light on an 

 iodised silver plate. Should we not be equally justified in saying 

 that the iodised plate, which after being exposed for a few seconds 

 to a strong light gives only faint and half effaced images, is 

 wearied like the retina, when after repeated and continuous per- 

 ception of an image, it gives back only the faint outlines of the 

 object? We may rest assured that the nervous system is not 

 exempt from chemical action ; and if the nervous system itself 

 must fall within the domain of chemical contemplation, and a 

 chemical expression remains to be found for its action, no less 

 than for that of digestion and for the formation of blood, it is 

 scarcely necessary to offer further proof of the fact that chemistry 

 is destined to play the most important part in physiology and 

 medicine. However much we may endeavour to exclude chemistry 

 from certain physiological investigations, we shall always find that 

 it involuntarily forces itself upon our notice ; for without it we 

 shall be unable to find a physiological equation or a philosophical 

 expression for a process. In a scientific point of view chemistry must, 

 therefore, be regarded as an invaluable acquisition to physiology. We 

 have, then, little cause to dread that Cicero's observation " Suo 

 quisque studio delectatus alterum contemnit" will be applied to our- 

 selves, when we assert that physiological chemistry is the crowning 

 point of every physiological enquiry. 



When we turn to practical physiology, to pathology, and 

 therapeutics, we are again reminded that chemistry is indis- 



