PHYSIOLOGY OF NERVE TISSUE. 75 



disturbance, a combination of physical and chemical processes attended by 

 the liberation of energy, which propagates itself from molecule to molecule. 

 Judging from the deflections of the galvanometer needle it is probable that 

 when the nerve impulse makes its appearance at any given point it is at first 

 feeble but soon reaches a maximum development after which it speedily 

 declines and disappears. It may, therefore, be graphically represented as a 

 wave-like movement with a definite length and time duration. Under 

 strictly physiological conditions the nerve impulse passes in one direction 

 only; in efferent nerves from the center to the periphery, in afferent nerves 

 from the periphery to the center. Experimentally, however, it can be dem- 

 onstrated that when a nerve impulse is aroused in the course of a nerve by 

 an adequate stimulus it travels equally well in both directions from the point 

 of stimulation. When once started the impulse is confined to the single 

 fiber and does not diffuse itself to fibers adjacent to it in the same nerve trunk. 



Rapidity of Transmission of Nerve Force. The passage of a nervous 

 impulse, either from the brain to the periphery or in the reverse direction, 

 requires an appreciable period of time. The velocity with which the impulse 

 travels in human sensor nerves has been estimated at about 190 feet a second, 

 and for motor nerves at from 100 to 200 feet a second. The rate of move- 

 ment is, however, somewhat modified by temperature, cold lessening and 

 heat increasing the rapidity; it is also modified by electric conditions, by the 

 action of drugs, the strength of the stimulus, etc. The rate of transmission 

 through the spinal cord is considerably slower than in nerves, the average 

 velocity for voluntary motor impulses being only 33 feet a second, for sensi- 

 tive impressions 40 feet, and for tactile impressions 140 feet a second. 



Electric Currents in Muscles and Nerves. If a muscle or nerve be 

 divided and non-polarizable electrodes be placed upon the natural longitu- 

 dinal surface at the equator, and upon the transverse section, electric currents 

 are observed with the aid of a delicate glavanometer. The direction of 

 the current is always from the positive equatorial surface to the negative 

 transverse surface. The strength of the current increases or diminishes 

 according as the positive electrode is moved toward or from the equator. 

 When the electrodes are placed on the two transverse ends of a nerve,, an 

 axial current will be observed the direction of which is opposite to that of 

 the normal impulse of the nerve. 



The electromotive force of the strongest nerve-current has been estimated 

 to be equal to the 0.026 of a Daniell battery; the force of the current of the 

 frog muscle, about 0.05 to 0.08 of a Daniell. 



Negative Variation of Currents in Muscles and Nerves. If a muscle 

 or nerve be thrown into a condition of tetanus, it will be observed that the 



