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TEXT-BOOK OF PHYSIOLOGY. 



Identity of Efferent and Afferent Nerves and Nerve Impulses. 

 Notwithstanding the classification of nerve-fibers based on differ- 

 ences of physiologic actions, there are no characters, either histologic 

 or chemic, which serve to distinguish them from one another. More- 

 over, 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 character 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 physio- 

 logically 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./^ nerve is merely the transmitter^ , 

 of the nerve impulse, which if conducted to a\ fj 

 muscle excites contraction; to a gland, secretion; I-/ 

 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 preparation (the gastrocnemius 

 muscle and sciatic nerve) and to use as a stimulus 

 the induced electric current. (See Fig. 49.) 

 When kept moist, this preparation is extremely 

 sensitive to either the galvanic or the induced 

 current. 



Though the development and conduction of a 

 nerve impulse may be demonstrated by the de- 

 flection of the galvanometer needle or the move- 

 ment 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 enclosed 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. 



A single shock of an induced current develops, it is believed, 

 a single nerve impulse followed by a single muscle contraction. A 

 minimal contraction following a minimal electric stimulus presupposes 

 the development of a nerve impulse of low intensity. Within certain 

 limits a maximal contraction following a maximal electric stimulus 

 presupposes the development of a nerve impulse of high intensity. 



FIG. 49. NERVE- 

 MUSCLE PREP- 

 ARATION OF A 

 FROG. F. Fe- 

 mur. S. Sciatic 

 nerve. I. Ten- 

 do Achillis. 



