XXXI.] ELECTRICAL STIMULATION. 183 



Note that although ammonia applied directly- to a motor nerve does not 

 cause contraction of the corresponding muscle, yet when it is applied to 

 the central end of the divided vagus of a rabbit it causes marked reflex 

 movements of the respiratory muscles. 



7. Drying. If the nerve be allowed to hang freely in the air 

 for some time, it gradually dies, and the muscles twitch irregularly, 

 as when a nerve is stimulated chemically. Moisten the nerve 

 with normal saline and the twitching may cease. It may be 

 that glycerin acts as a stimulus through absorbing water. 



LESSON XXXI. 



SINGLE AND INTERRUPTED INDUCTION SHOCKS 

 TETANUS -CONSTANT CURRENT. 



1. Electrical Stimulation Single Induction Shocks. Appa- 

 ratus. Darnell's cell, induction machine, two Du Bois keys (or 

 one spring key or mercury key and one Du Bois key), five wires, 

 electrodes. 



(a.) Arrange a cell and induction machine for single induction 

 shocks as in fig. 109. A spring contact-key or Hg-key is more 



FIG. 109. Scheme for Single Induction Shocks. B. Battery; K, K'. Keys; P. Primary, 

 and S. Secondary coil of the induction machine ; N. Nerve ; M. Muscle. 



convenient in the primary circuit. Electrodes are fixed to the 

 short-circuiting key (K') in the secondary circuit, and over them 

 the nerve is to be placed. 



(b.) Expose the sciatic nerve in a pithed frog, place it on electrodes 

 preferably a pair fixed in ebonite, and so shielded that only 

 one surface of their platinum terminals is exposed under it. Or 

 use the simple shielded electrodes described in Lesson XXVII. 6. 

 Pull the secondary coil (S) far away from the primary (P), raise 

 the short-circuiting key (K'), make and break the primary circuit 



