46 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



Ill a like manner if a nerve-muscle preparation he isolated, as shown in 

 Fig. 22, and a needle, held in the hand or connected with a large metallic 

 conductor or a condenser, he brought in contact with some point of the nerve, 

 the excitation which occurs on the opening and, with a strong current, on the 

 closing of the primary circuit will be strictly limited to the part of the nerve 

 touched by the needle. This method can be used to advantage in studying 

 the rate of conduction in nerves or any problem which requires strict local- 

 ization of electric excitation. 



(d) Effect of the Duration of the Electric Current on its Power to Irritate 

 Nerves and Muscles. — As we have seen, a constant battery current, when flow- 

 ing uninterruptedly through a motor nerve, does not ordinarily excite it; very 

 slow variations in the strength of the current also fail to irritate; but rapid 

 alterations in the strength, whether in the direction of increase or decrease, act 

 as vigorous stimuli. For example, medullated nerves are irritated more vigor- 

 ously by the rapid changes of intensity of induced currents than by the some- 

 what slower changes occurring at the make and break of battery currents. 

 Within certain limits, at least, the more rapidly the intensity of the current 

 changes, the greater the irritating effect upon nerves. That there is a limit 

 even for the rapidly reacting protoplasm of medullated nerves is shown by 

 the fact that by unipolar excitation the charging and discharging of the con- 

 densers through a nerve is the more effective the greater the capacity of the 

 condensers. The process is more prolonged if the condenser is large, and 

 the effect is greater. 1 Not all nerves are equally susceptible to rapid altera- 

 tions of the intensity of the current. Xon-medullated nerves do not appear 

 to react as readily as medullated to electric currents of short duration. For 

 instance, the nerves of the claw muscles of the crab are not readily excited 

 by induced currents, and respond better to the more prolonged influence of 

 the closing and opening of battery currents. 2 



The question now arises, Is the reaction of muscle to electric currents the 

 same as that of nerves? Experiment shows that muscles which have been 

 removed from the action of nerves, by means of curare, differ from medullated 

 nerves in that they are excited more vigorously by the opening and closing of 

 battery currents; less vigorously by making and breaking induction currents. 

 This latter fact is well seen in experiments in which two gastrocnemius 

 muscles from the same frog, one of which has been curarized and the other 

 not, are connected with an induction apparatus in series, so that the cur- 

 rent shall flow through them both in the same direction. If the primary 

 current be made and broken, the non-curarized muscle will respond to a 

 weaker induction shock than the curarized. By the curarized muscles the max- 

 imal contraction got on opening and closing a battery current is both higher 

 and more prolonged than that to be obtained with a single induction shock. 

 Unstriated muscles exhibit this difference to a still greater degree than 

 striated muscle ; they react well to the closing of battery currents of medium 



1 Hermann: Handbuch der Physiologie, Bd. ii. Theil 1, S. 88. 



2 Biedermann: Elektrophysiologie, 1S95, Bd. ii. S. 546. 



