XXXI.] ELECTRICAL STIMULATION. 1 85 



2. Interrupted Current or Repeated Shocks. 



(a.) Arrange the induction machine so as to cause Neef s hammer 

 to vibrate as directed in Lesson XXVIII. 2. On applying the 

 electrodes to the sciatic nerve or gaetrocnemius muscle, at once 

 the muscle is thrown into a state of rigid spasm or continuous 

 contraction, called tetanus, this condition lasting as long as the 

 nerve or muscle is stimulated, or until exhaustion occurs. 



3. Constant Current. —Apparatus. — Daniell's cells, Du Bois 

 key (or, preferably, a simple make and break key), four wires, 

 electrodes, forceps, and nerve-muscle preparation, or simply expose 

 the sciatic nerve in situ. 



(a.) Use two .Daniell's cells. If two or more Daniell's cells be 

 used, always connect them in series, i.e., the + pole of one cell 

 with the - pole of the next. Connect two wires, as in fig. no, 

 to the free + and - poles of the battery 

 B, and introduce a Du Bois key (K') to 

 short-circuit the battery circuit. Fix two 

 shielded electrodes in the other binding- 

 screws of the Du Bois key, and having 

 prepared a nerve-muscle preparation, lay 

 the divided sciatic nerve (N) across them, 

 as shown in fig. no. A simple key to 

 make or break the current is preferable 

 to the short-circuiting key, as the latter 

 allows polarisation currents to pass when it 



is closed. ^iq. no.-Scheme of Cou- 



(h.) Make and break the current, and a stant current, i^. Battery; 



• ,' 1 1. .• i -i 1 • li'. Short-cuciiitiiig key; 



Smgle muscular contraction or twitch is JV. Nerve; J/. Muscle. 



obtained, either at making or breaking, or 



both at making and breaking. Notice that if the key be raised 



to allow the current to flow continuously through the nerve, no 



contraction occurs, provided there be no variation in the intensity 



of the current. The electrodes may also be applied to the muscle 



directly. 



(c.) Rapidly make and break the current by opening and 

 closing the key ; a more or less perfect Manns is produced. ' 



{(l.) If it be desired to test the effect of a constant current on 

 muscle alone, then the terminations of the motor nerves in the 

 muscle must have been paralysed previously by curare, so that 

 in this case the electrodes must be applied directly to the 

 muscle. 



4. Muscle on Mercury. — Lay the muscle of a nerve-muscle 

 preparation on the surface of mercury. Stimulate the nerve, tha 



