54 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



traction will be absent. It has been ascertained that the direction in which 

 the current is flowing through the nerve determines which of these con- 

 tractions shall cease to appear. The cause of this will be explained a little 

 later. 



(6) Effect of Strength of Itritant. As a rule, the stronger an electric current 

 the greater its irritating effect. This can be readily tested upon a nerve with 

 the induction current, the strength of which can be varied at pleasure. The 

 strength jjg the induced current obtained from a given apparatus depends 

 upon the s'trength of the current in the primary coil, and on the distance of 

 the secondary from the primary coil. In ordinary induction machines (see Fig. 

 10, p. 48) the secondary coil is arranged to slide in a groove, and can be easily 

 approached to or removed from the primary coil, thus placing the coils of wire 

 of the secondary coil more or less under the influence of the magnetic field 

 about the primary coil. This permits the strength of the current to be graded 

 at will. The strength of the induced current does not increase, however, in 

 direct proportion to the nearness of the coils. As the secondary approaches 

 the primary coil, the induced current increases in strength at first very slowly, 

 and later more and more rapidly, reaching its greatest intensity when the 

 secondary coil has been pushed over the primary. 



The relation of the strength of a current to the irritating effect upon a nerve 

 can be readily tested with such an induction apparatus. The secondary coils 

 can be connected with a pair of electrodes on which the nerve of a nerve- 

 muscle preparation rests (as in Fig. 11, page 48), and the muscle can be 

 arranged to record the height of its contractions (as in Fig. 13, p. 49). 

 The experiment can be begun by placing the secondary coil at such a dis- 

 tance from the primary that the making and breaking shocks are too feeble to 

 have any effect upon the nerve. Then the secondary coil can be gradually 

 approached to the primary, the primary current being made and broken at 

 regular intervals. At a certain point the breaking shock will excite a very 

 feeble contraction, the making shock producing no effect. If this contraction 

 is barely sufficient to be recognized, we call it the minimal breaking contraction 

 (see Fig. 19, a). In seeking the minimal contraction care must be taken not 

 to excite the preparation at too short intervals of time, for, as we shall see, 

 an irritation too slight to excite even a minimal contraction may, if repeated 

 at short intervals, increase the irritability of the preparation and so become 

 effective. By using a short-circuiting key in the secondary circuit we can 

 cut out the making shocks, and test the effect of a further increase in the 

 strength of the current by the response of the muscle to the breaking shocks. 

 As the contractions become larger, care must be taken not to irritate the muscle 

 too frequently, lest it be fatigued and so fail to give the normal response. As 

 the current is strengthened the breaking contractions will become higher and 

 higher until a point is reached beyond which the strength of the current may 

 be increased to a considerable extent without any further heightening effect (Fig. 

 19, 6). If the current be still further increased, this first maximum is suc- 

 ceeded by a still further growth in the height of the contractions, until finally 



