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AN AMKIUCAN TEXT-BOOK OF PHYSIOLOGY. 



fall of the density of the current in the secondary coil is very rapid, and this rapid double change 

 in density of the current causes the induction shock to be a very effective irritant. The break- 

 ing induction shock, as we call that which is produced by breaking the primary current, is 

 found to act more vigorously than the making shock, which is the reverse of what is found 

 with direct battery currents. The cause of this lies in the nature of the apparatus. At the 

 moment that the current begins to flow into the primary coil, it induces not only a current 

 in the secondary coil, but also currents in the coils of wire of the primary coil. These 

 extra induced currents in the primary coil have the opposite direction to the battery cur- 

 rent and tend to oppose its entrance, and thereby to prevent it from immediately gain- 

 ing its full intensity. This delay affects the development of the induced current in the 

 secondary coil, causing it to be weaker and to have a slower rise and fall of intensity than 

 would otherwise be the case. When the primary current is broken, on the other hand, 

 there is no opposition to its cessation, and the current induced in the secondary coil is 

 intense and has a rapid rise and fall. These differences are illustrated in Figure 12. 



Myogram. — To accurately test the effect of the making and breaking 

 induction shocks, it is necessary to record the reaction of the nerve; this can 

 be done by recording the extent to which the corresponding muscle contracts 

 in response to the stimulus which it receives from the nerve. In such an 

 experiment it is customary to use what is known as a nerve-muscle prepara- 

 tion. The gastrocnemius muscle and sciatic nerve of a frog, for instance, are 

 carefully dissected out, the attachment of the muscle to the femur being pre- 

 served, and the bone being cut through at such a point that a sufficiently long 

 piece of it shall be left to fasten in a clamp, and so support the muscle (see 

 Fig. 13). 



Fig. 13.— Method of recording muscular contraction. 



The simplest method of recording the extent of the muscular contraction 

 is to connect the muscle by means of a fine thread with a light lever, and let 

 the point of the lever rest against a smooth surface covered with soot, so that 

 when the muscle contracts it shall draw up the lever and trace a line of cor- 

 responding length upon the blackened surface. The combination of instru- 



