GENERAL PHYSIOLOGY. 31 



suppose that the central nerve globules, by the simple effect 

 of their nutrition, and without excitation coming from out- 

 side the body, are capable of setting free forces which act 

 upon the fibres; this property has been called automatism 

 of the nerve centres (will, muscular tone ?) 



5. Excitability of the Nerve Elements. The excitabil- 

 ity of the nerve elements, especially if a nerve used for 

 experimental researches, may vary under many circum- 

 stances. Heat increases this up to a certain point: cold 

 diminishes it. Certain medicinal agents, as, for instance, 

 strychnine, have the power of exciting the reflex properties 

 of the nervous centres ; others, like the bromide of potas- 

 sium, enfeeble these properties. Woorara (curare), on the 

 other hand, seems to act especially upon the motory termi- 

 nations of the nerves, and there to arrest the power of 

 transmission, for it is hardly reasonable to suppose that it 

 would act upon the motory nerves, and not upon the sensory 

 nerves; this would show that these two kinds of nerves have 

 no different characters. 



Electricity acts at the same time both as an excitant and 

 as a modifying agent of excitability to a nerve; in fact, 

 when a current is applied to a nerve, excitability is increased 

 at the negative pole, and diminished at the positive pole, 

 a phenomenon more especially described under the head 

 of electro-tonus. 



But the excitability of a nerve is especially dependent on 

 its nutrition. Every nerve tube separated from a central 

 living organ undergoes fatty degeneration and ceases to 

 be excitable at the end of a few days. Absolute rest pro- 

 duces the same effect, for the function is necessary to the 

 maintenance of life and of nutrition; per 'contra, the exag- 

 gerated excitations produce momentarily the weakening of a 

 nerve, which must needs recover its strength by rest, and 

 we have remarked that excitation of the nerve modifies 

 temporarily the phenomena of nutrition. 



II. GENERAL PHYSIOLOGY OF THE NERVE CENTRES. 



For a long time the point of departure of the nervous sys- 

 tem was a matter of ignorance : the size and position of the 

 brain led the ancient physiologists to consider that as the 

 principal centre of the nerve-substance : the spinal cord was 

 to them but a collection of nerves ending at the brain. The 

 minute study (histology) of the gray axis of the spinal cord 



