32 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



primary current. When do you perceive the stimuli? Which stimula- 

 tion is the stronger, the making or the breaking induction shock? 



Annotation. If the direction of the induced current is determined by means of 

 a galvanometer, it will be found that the making shock is opposed to the primary 

 current, while the breaking shock passes in the same direction as the primary cur- 

 rent. Secondly, the make induction develops more slowly than the break induction, 

 because the entering primary current must first overcome the self-induction in the 

 primary coil before it can produce a similar effect in the secondary coil. As it passes 

 from one turn of the primary wire to another, an induced current is momentarily 

 set up in the secondary coil which possesses a direction opposite to the primary. 

 Its strength is thereby diminished. On the break, this impediment is not present, 

 and hence, the induction in the secondary coil is enabled to reach its maximal value 

 with much greater rapidity. 



Quickly Repeated Induction Shocks or Tetanic Current. Connect 

 in series two dry cells, a key, and posts 1 and 3 of the inductorium. 

 This connection brings Neef s hammer into the circuit and allows us to 

 make and break the primary current in rapid succession, thereby pro- 

 ducing an entire series of inductions. Compare the stimulating value 

 of the quickly interrupted or tetanic current with that of single induc- 

 tion shocks. 



7. Changes in the Strength of the Current. Use single induction 

 shocks. Push the secondary coil of the inductorium over the primary. 

 Stimulate. Gradually increase the distance between the secondary and 

 primary coils and repeat the stimulation. What difference do you no- 

 tice? Explain. 



Annotation. The strength of the induction shocks depends first of all upon the 

 electromotive force of the primary current. Secondly, it is proportional to the separa- 

 tion of the secondary coil from the primary, becoming the weaker the greater the 

 distance between them. The strength of the induction current may be indicated 

 approximately by giving the strength of the battery and the distance between the 

 coils in centimeters. The latter may be read off directly from the scale inscribed 

 upon the base of the inductorium. 



8. Direct and Indirect Stimulation. Suspend the leg of a frog from 

 the clamp in the manner described previously (p. 23). Place the 

 sciatic nerve upon the electrodes. Moisten the preparation repeatedly 

 with normal saline to prevent its drying. Stimulate it first with the 

 constant and then with the interrupted current. Do the results agree 

 with those obtained by stimulating your tongue or finger? 



Hold the electrodes firmly against the body of the gastrocnemius 

 muscle. Repeat the stimulation with the same strength of current 

 and compare the effects of stimulating the muscle directly and indi- 

 rectly through its nerve. Which is the more irritable tissue of the two, 

 as betrayed by the threshold value of the current required to activate it? 



9. Myography. The registration of the contraction of muscle neces- 

 sitates a means of holding the muscle, a writing; lever, and a surface 

 upon which the record may be made. The writing lever is fastened to 

 the stand below the muscle clamp. The tendo achillis of the gastroc- 



