PHYSIOLOGICAL AND ELECTRICAL STIMULI. 535 



As a general rule, the stimulating solutions must be more concentrated when applied 

 to a nerve than to muscle, in order that a contraction may be produced. 



[Methods. If a nerve-muscle preparation of a frog's limb be made, and a straw flag (p. 472) 

 attached to the toes while the femur is fixed in a clamp, and its nerve be then dipped in a 

 saturated solution of common salt, the toes soon begin to twitch, and by and by the whole limb 

 becomes tetanic, and thus keeps the straw flag extended. The effect of fluid en a muscle or 

 nerve is easily tested by fixing the muscle in a clamp, while a drop of the fluid is placed on a 

 greased surface, which gives it a convex form. The end of the muscle or nerve is then brought 

 into contact with the cupola of the drop (Kilhne).] 



4. The Physiological or normal stimulus excites the nerves in the normal 

 intact body. Its nature is entirely unknown. The " nerve-motion " thereby set 

 up travels either in a " centrifugal" or outgoing direction from the central nervous 

 system, giving rise to motion, inhibition of motion, or secretion ; or in a " centri- 

 petal " or ingoing direction from the specific end-organs of the nerves of the special 

 senses or the sensory nerves. In the latter case, the impulse reaches the central 

 organs, where it may excite sensation or perception, or it may be transferred to the 

 motor areas and be conducted in a centrifugal direction, constituting a " reflex " 

 stimulation ( 360). A single physiological nerve-impulse travels more slowly than 

 that excited by the momentary application of an induction shock (Lovhi, v. Kries). 

 It is not a uniform process excited by varying intensity and greater or less frequency 

 of stimulation, but it is essentially a process varying considerably in duration, and 

 it may even last as long as J second (v. Kries). 



5. Electrical Stimuli. [The following forms of electrical stimuli may be 

 used : 



(1) A constant current, which may be made or broken ( 328). 



(2) Induction shocks, either make or break shocks ( 329). 



(3) An interrupted current ( 329). 



The electrical current acts most powerfully upon the nerves at the moment when 

 it is applied, and at the moment when it ceases ( 336) ; in a similar way, any 

 increase or decrease in the strength of a constant current acts as a stimulus. If an 

 electrical current be applied to a nerve, and its strength be very gradually increased 

 or diminished, then the visible signs of stimulation of the nerve are very slight. 

 As a general rule, the stimulation is more energetic the more rapid the variations of 

 the strength of the current applied to the nerve, i.e., the more suddenly the 

 intensity of the stimulating current is increased or diminished (du Bois-Reymond). 



An electrical current must have a certain strength or liminal intensity before it 

 is effective. By uniformly increasing the strength of the current, the size of the 

 contraction increases rapidly at first, then more slowly {Tiger stedt and Willhard). 



An electrical current, in order to stimulate a nerve, must have a certain duration, 

 it must act at least during 0*0015 second (Ficlc, 1863) ; even with currents of 

 slightly longer duration, the opening shock may have no effect. If the duration of 

 the closing shock of a constant current be so arranged that it is just too short to 

 be active, then it merely requires to last 1*3 to 2 times longer to produce the most 

 complete effect (Grunhagen). 



The electrical current is most active when it flows in the long axis of the nerve ; 

 it is inactive when applied vertically to the axis of the nerve (Galvani). Similarly, 

 muscles are incomparably less excited by transverse than by longitudinal currents 

 (Gmffrh). 



The greater the length of nerve traversed by the current, the less the stimulus 

 that is required (Pfaff). 



Constant Current. If the constant current be used as a nervous stimulus, the 

 stimulating effect on the sensory nerves is most marked at the moment of making 

 and breaking the current ; during the time the current passes, only slight excite- 

 ment is perceived, but, even under these circumstances, very strong currents may 

 cause very considerable, and even, unbearable, sensations. If a constant current be 



