GENERAL PHYSIOLOGY OF NERVE-TISSUE 103 



which it speedily-declines and disappears. It may, therefore, be graphically 

 represented as a wave-like movement with a definite length and time dura- 

 tion. (See page 106.) Under strictly physiologic 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. Experimen- 

 tally, however, it can be demonstrated 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 Conduction of the Nerve Impulse. The passage of a 

 nerve impulse, either from the brain to the periphery or in the reverse direc- 

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

 impulse travels in human sensory nerves has been estimated at about 50 

 meters a second, and for motor nerves at from 28 to 33 meters a second. The 

 rate of movement 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 n 

 meters a second, for sensory impulses 12 meters, and for tactile impulses 40 

 meters a second. 



Nerve Fatigue. Inasmuch as nerves are parts of living cells, the seat 

 of nutritive changes, it might be supposed that the passage of nerve impulses 

 would be attended by the disruption of energy-holding compounds, the pro- 

 duction of waste products, the liberation of heat, and in time by the phenom- 

 ena of fatigue. Though it is probable that changes of this character occur, 

 yet no reliable experimental data have been obtained which afford a clue as 

 to the nature or extent of any such changes. Stimulation of motor nerves 

 with the induced electric current for four hours appears to be without 

 influence either on the intensity of the nerve impulse or the rate of its 

 conduction. 



Identity of Efferent and Afferent Nerves and Nerve Impulses. 

 Notwithstanding the classification of nerve-fibers based on differences of 

 physiologic actions, there are no characters, either histologic or chemic, 

 which serve to distinguish them from one another. Moreover, as the nerve 

 impulse is conducted through a nerve-fiber equally well in both directions, 

 as determined by experiments, it is probable that it does not differ in char- 

 acter in the two classes of nerves. That the efferent fibers conduct the 

 nerve impulses from the nerve-centers to the periphery, and the afferent 

 nerves from the periphery to the centers, is because of the fact that they 

 receive their stimulus physiologically only in the centers or at the periphery. 

 The fundamental reason for difference of effects produced by stimulation 

 of different nerves is the character of the organ to which the nerve impulse 

 is conducted. A nerve is merely the transmitter of the nerve impulse, 

 which if conducted to a muscle excites contraction; to a gland, secretion; 

 to a blood-vessel, variation in caliber; to special areas in the brain, sensa- 

 tions of light, sound, pain, etc. 



Electric Excitation of Nerves. For the purpose of studying the 

 physiologic activities of nerves it has been found convenient to employ the 



