iv GENEEAL PHYSIOLOGY OF NERVOUS SYSTEM 217 



electrical variation can be manifested after the excitability of the 

 nerve has entirely disappeared. 



V. The term stimulus, as applied to nerve, covers every agent 

 capable of translating its excitability into action as directly 

 expressed in the external sign of the current of action, by which 

 the physical change of the nerve is manifested. The indirect 

 subjective proof of nerve excitation is sensation, when the stimulus 

 acts upon our sense organs : consciousness of the voluntary 

 impulse when it proceeds from the higher centres. The indirect 

 objective proof is a simple muscular contraction when the stimulus 

 acts upon the motor nerves, a reflex muscular contraction when 

 it acts on the sensory nerves. In most of the work done upon 

 nerve the reaction of the muscle has been taken as the index of 

 activity, so that the results for the most part apply only to motor 

 nerves. 



We must distinguish between natural and artificial nerve 

 stimuli. Nerve, like muscle, is excitable at every point of its 

 course by a great number of stimulating agents of varying 

 character (chemical, thermal, mechanical, electrical). Normally, 

 however, sensory nerves and afferent nerves in general are always 

 excited from the sense-organs with which their peripheral termina- 

 tion is in relation ; and motor nerves and efferent nerves in 

 general are always excited from the central organ from which they 

 take origin. Moreover, the peripheral organ of sensory nerves is 

 normally excited exclusively by external stimuli of a definite 

 character, which are therefore known as specific stimuli. As we 

 shall see later in describing the sense-organs, their nerve-endings 

 are so constituted that they are highly susceptible to the influence 

 of stimuli which would be powerless to excite the nerves themselves 

 at the different points of their course. 



For this reason the natural stimuli for the respective sense- 

 organs are also termed adequate stimuli ; they are adapted to the 

 specific constitution of the sensory nerve-endings which they 

 stimulate. The adequate stimulus for the optic nerve is light, 

 which alone can excite retinal nerve-endings; the adequate 

 stimulus for the auditory nerve is sound, which alone can excite 

 the nerve-endings of the organ of Corti, etc. 



Motor nerves, again, are normally excited by specific stimuli, 

 produced by the (reflex or automatic) activity of the ganglion cells 

 of the central organ from which they originate, and on which 

 they are morphologically and functionally dependent. 



The fact that naturally every nerve is excitable only at one 

 of its ends (peripheral or central), and only to a definite kind of 

 stimulus, is one of the most admirable adaptations of the animal 

 organisation, and prevents that chaotic disorder in the activity of 

 the whole system which would occur if the nerves were excitable 

 at every point of their course by different external and internal 



