BY DR. BURDON-SANDERSON. 323 



vidod beyond. In the dog or cat the mode of preparation is 

 very much the same as in the rabbit. In the cat, the compara- 

 tive thickness of the nerve facilitates the manipulation. 



In exciting the superior la^ngeal, the great source of 

 difficulty is the proximity of the vagus and the consequent 

 liability of that nerve to be acted on by the induced current 

 in a unipolar way. This accident, which is of course fatal to 

 the success of the investigation, the functions of the two 

 nerves being opposite, is to be avoided, not by the use of 

 complicated arrangements for the insulation of the nerve, but 

 by placing it in such a way on the ordinary copper points that 

 the part acted on is separated by a considerable air space from 

 the surrounding tissues. Before beginning the excitation, the 

 secondary coil must be shifted to a distance from the primary, 

 and the primary current divided by means of Helmholtz's side 

 wire into two branches, one of which only passes through the 

 breaker. The other is led directly from the battery to the coil, 

 so that the primary current is never entirely opened. In this 

 way the opening induction shock, which, in the ordinary 

 arrangement of the induction apparatus, possesses a much 

 greater tension than that of the closing shock, is so reduced 

 that the two become nearly equal to each other. 1 Conse- 

 quently, as the risk of unipolar action varies with the maxi- 

 mum intensity of the current, it is very much diminished by 

 this contrivance so much so, indeed, that if care is taken to 

 prepare the nerve properly, even moderately strong currents 

 may be used without any effects referable to unipolar excita- 

 tion of the vagus manifesting themselves. Excitation of the 

 central end of the superior laryngeal produces, according to 

 the strength of the current used, either diminution of fre- 

 quency of the respiratory movements or complete relaxation 

 of the muscles of inspiration. The most advantageous way 

 of judging of its effect on the diaphragm, is to expose that 

 muscle in the way directed in 91. It is then seen that that 

 muscle becomes absolutely flaccid during excitation of the 

 nerve, and it is drawn up by the elastic contraction of the 

 lungs, so as to assume its highest possible position. When 

 the excitation is discontinued, the relaxation either gives way 

 to natural breathing or is immediately succeeded by one or 

 two vigorous inspirations. If the current is so feeble that it 

 merelj r diminishes the frequency of the respirations, without 

 arresting them, the tracings show that there is no diminution 

 of the duration of the inspiratory acts, and that the slowing 

 is entirely due to a prolongation of the intervals, i. e., of the 



1 For a fuller explanation of the difference between the two induced 

 currents and of the effect of Helmholtz's modification, see Rosenthal, 

 "Electricitatslehre," p. 120. 



