124 ELECTRICAL EXCITATION. 



the same side. The upper platinum-tipped screw is withdrawn and the under 

 platinum tip brought into contact with the vibrating hammer, at the moment 

 that it is drawn down by the temporary magnet. Compare the effects with 

 those previously felt, particularly when the primary coil is covered by the 

 secondary. It is convenient in all cases to interpose a key in the primary 

 circuit. 



2. The Single Induction Shock. Connect the primary coil of the 

 Induction apparatus with a Daniell cell, interposing a key. Arrange a rheo- 

 scopic preparation of the lower limbs of a pithed frog, by placing two slender 

 glass rods under the sacral plexuses, having first opened the visceral cavity and, 

 removed the viscera. Connect the electrodes with the terminals of the 

 secondary coil, and place them underneath the nerves thus separated from 

 other structures. The preparation should be supported in the vertical position 

 by a clamp. Remove the secondary from the primary coil until no response 

 occurs on closing or opening the key. Then bring the secondary coil gradually 

 nearer and observe that at a certain distance the preparation responds only to 

 the "break" shock, afterwards to both "make" and "break," but more 

 strongly to the latter, and finally with equal vigour to both. 



3- The Extra Current. To demonstrate Faraday's " extra current " 

 physiologically, introduce the exciting electrodes into the primary circuit of the 

 induction apparatus, removing the secondary coil. Connect the electrodes by a 

 couple of copper wires whose ends are united by a key, so that when the key is 

 closed the electrodes are in metallic connection by the wires and key as well as 

 by the coil and battery. Place the electrodes under the sacral plexus in the 

 preparation used in the previous experiment ; and observe that when the key 

 is opened the rheoscopic limbs respond strongly ; this is due to the extra 

 current, that is, to the induction current which is produced in the primary 

 coil in the same direction with the battery current, immediately after the 

 sudden diminution produced by the opening of the key. 



4- Unipolar Excitation. Connect one electrode with one terminal 

 of the induction coil and place it under the sacral nerves of the same pre- 

 paration, which for this purpose must be on a glass plate. No response takes 

 place either on making or breaking. Touch the preparation or otherwise 

 connect it with the earth, and it will be observed that it responds at break. 



5- Faradization. Arrange the coil for faradization and place the 

 electrodes under the sacral plexus in a similar preparation. The limbs are 

 extended and the muscles rigid. The spasm so produced persists, though 

 with gradually diminishing intensity, so long as the primary circuit remains 

 closed. This condition of tonic contraction is designated l^etanus. 



6. Galvani's Experiment. Take a clean bit of zinc wire, and coil 

 round one end of it a copper wire of the same length, so as to make a fork. 

 Pith a frog and lay it on its belly. Remove the skin from the back of the thigh, 

 separate with the finder and remove the narrow biceps femoris. Separate the 

 sciatic nerve which is thus brought into view, from the surrounding struc- 

 tures, and touch the nerve first with the copper wire, then with the zinc. It 

 will be observed that on closing the circuit thus formed, the gastrocnemitis muscle 

 contracts and the foot is extended. If it is moderately excitable, the same thing 

 happens also on opening it. 



