THE PHYSIOLOGY OF THE NERVE-FIBRE 



581 



corresponding half of the muscle to contract ; but excitation at 6 or 6', 

 where the nerves lie, will cause both halves to contract. The " action 

 current " spreads along a nerve in both directions. 



The rate of conduction in nerve is modified by various factors. 

 Cooling decreases, gentle warmth raises, the rate. Exact measure- 

 ments of the relation of temperature to conductivity do not afford 

 proof that the transmission of nervous energy is a chemical rather than 



FIG. 320. VELOCITY OF MOTOR IMPULSE ix HUMAN NERVE. (Waller.) 



a physical process. Its nature is unknown. Chloroform and many 

 other poisons diminish and then abolish the conductivity. 



Unlike muscle, the excised nerve, as the result of activity, shows 

 no measurable mechanical, thermal, or chemical changes. Like 

 muscle, it shows electrical phenomena the current of injury, the 

 current of action (negative variation), and electrotonic currents (see 

 beJow). ft 



Fia. 321. DIAGRAM OF THE 

 SARTORIUS EXPERIMENT TO SHOW 

 THE TRANSMISSION OF A NERVOUS 

 IMPULSE IN BOTH DIRECTIONS. 



FIG. 322. DIAGRAM OF THE 

 GRACILIS EXPERIMENT TO SHOW 

 THE TRANSMISSION OF A NERVOUS 

 IMPULSE IN BOTH DIRECTIONS. 



The Effects of a Constant Current. To study these effects, a sciatic- 

 gastro:nemius preparation and non-polarizable electrodes are used. 

 Both make and break of a constant current excite the nerve, and cause 

 a contraction of the muscle; no excitation occurs while the "current is 

 flowing. The effect of the make and break stimuli vary according to 

 the strength of the current, and according to its direction. If it be 



