240 PRACTICAL PHYSIOLOGY. [XLVIL 



Make a transverse cut across the triceps, and so arrange the 

 nerve that its cut end rests on the transverse section of the 

 muscle, and its longitudinal surface on the longitudinal surface of 

 the muscle. As soon as this interval is bridged over, the leg muscles 

 contract. 



There is a large difference in potential between the transversely- 

 cut muscle and its longitudinal surface — there is a " muscle-current " 

 in the muscle from the artificial transverse section to the longitudinal 

 surface, so when the nerve bridges over these surfaces, there is an 

 external derivation-current passing in the nerve, whereby the latter 

 is stimulated. 



Thus the "physiological ''heoscope" is used to show the 

 presence of electrical currents in muscle under certain conditions. 



3. Secondary Contraction or Twitch and Secondary Tetanus. 



(a.) Arrange an induction coil for single make and break shocks. 

 Make two nerve-muscle preparations. 



{/j.) Place the left sciatic nerve (A) over the right gastrocnemius 

 (B) or thigh muscles, and the right sciatic nerve over the electrodes 

 (E) (fig. 164). 



('•.) Stimulate the nerve of B with single induction shocks — the 

 muscles of both B and A contract. The contraction in A is called 



a secondary contraction. A is the 

 rheoscopic limb as by its contraction it 

 shows the existence of an electrical 

 current in B. When B contracts, 

 there is a sudden diminution of its 

 muscle-current, which circulates in the 

 nerve of A. This sudden diminution 

 — negative variation — is tantamount 

 to a stimulus, and so the nerve of A 

 is stimulated. 



(d.) Arrange the induction coil for 

 repeated shocks, and stimulate the 

 nerve of B. B is tetanised, and so is 

 A simultaneously. This is secondary 

 f la. i64.-Secondary Contraction, tetanus. The nerve of A is stimulated 

 by the sudden series of negative varia- 

 tions of the muscle- current during the contraction of B. So that 

 the electrical change during tetanus is interrupted and not con- 

 tinuous like the change in form of the muscle, and with 50 shocks 

 per second each electrica' chang? must reach its maximum and 

 subside in y^". 



(e.) Ligature the nerve of A near the muscle, stimulate the nerve 

 of B ; there is no contraction of A although B contracts. 



