14 PEACTICAL PHYSIOLOGY 



Helmholtz showed that it is possible to equalise these Faradic shocks 

 by short-circuiting, instead of completely breaking, the battery-current, 

 and for the reason already explained. For this purpose (Fig. 19) a 

 stout wire, W, connects the binding-screws 7 and 1, Sj_ is screwed up 

 out of reach of the spring, and S 2 is screwed up. Follow the circuit 

 of the current which passes from binding-screws 7 to 1 by the 

 side-wire, and so to the primary coil, back to the electro-magnet E, 

 to binding-screw 6 and to the battery. When, however, the current 

 reaches E, it becomes a magnet, and pulls down the spring into 

 contact with S 2 . This short-circuits the battery-current out of the 

 coil, for the current will now pass from the pillar A, by way of H, 

 to the pillar B, and so back to the battery. There is still left the 

 circuit 7 W, 1, PC, E, H, A, 7, in which the break extra current 

 can run and reduce the strength of the current induced in the 

 secondary coil at break. 



Determine the distance between the coils at which the shocks are 

 now just felt on the tongue ; it will be found to be reduced, showing 

 that the break-shock which was alone felt before has been reduced 

 down to or even below the strength of the make-shock. 



CHAPTER II. 



THE GRAPHIC METHOD. MAXIMAL AND MINIMAL STIMULI. 

 UNIPOLAR EXCITATION. 



THE graphic method is applied to muscle in order to obtain a 

 permanent magnified record of the change in form of a muscle during 

 contraction, and further, to investigate the time-relations of the con- 

 traction. For this purpose it is necessary to describe the method of 

 preparing the muscle and then three special pieces of apparatus : 

 (1) a magnifying lever, the muscle lever, or myograph, which can write 

 on (2) a surface either stationary or moving at a uniform rate, the 

 drum, and (3) an instrument for recording time on the drum, the 

 chronograph, which will be described in Chapter III. 



The Muscle- and Nerve-Preparation. The quickest way to kill a 

 frog is to "pith" it. The articulation between the skull and the 

 vertebral column can be felt with the tip of the finger ; it is severed 

 by a transverse cut with a pair of scissors, and then a probe or blanket- 

 pin is inserted into the skull to destroy the brain. The spinal cord is 

 destroyed in a similar way, and this final stimulation of the nerve-cells 



