GENERAL PHYSIOLOGY OF NERVE-TISSUE. 101 



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 influ- 

 ence 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 pro- 

 duced 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, sensations 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 nerve-muscle prepara- 

 tion (the gastrocnemius muscle and sciatic nerve) and to 

 use as a stimulus the induced electric current. (See 

 Fig. 50.) When kept moist, this preparation is ex- Fl6 - 5- NERVE, 

 tremely sensitive to either the galvanic or the induced TION OF A FROG AR p\ 



Current. P>mur. S. Sciatic 



Though the development and conduction of a nerve " e c r ^j is ^_ 7 L ^ f s 

 impulse may be demonstrated by the deflection of the and Stirling.) 

 galvanometer needle or the movement of the mercury in 

 the capillary electrometer, it is more conveniently demonstrated by the con- 

 traction of a muscle, the vigor of which, within limits, may be taken as a 

 measure of the intensity of the impulse. The preparation should be en- 

 closed in a moist chamber and the nerve connected with the inductorium 

 through the intervention of non-polarizable electrodes. The muscle may 

 be attached to the muscle-lever and its contractions recorded. 



