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GENEflAL PHYSIOLOGY 



passes through the spring into the screw T, through the coil P, 

 about the iron rod, into the second smaller brass column, and then 

 returns to the element. The result is that the soft iron rod becomes 

 magnetic and attracts the hammer that is suspended above it. 

 Thereby thecontact of the spring with the screw T is broken. By the 

 breaking of this contact, however, the current is broken, the mag- 

 netism in the soft iron rod consequently ceases, and the hammer 

 springs up by reason of the tension of the spring. As a result of 

 this the spring again touches the screw T, and the current is made 

 again. By this ingenious contrivance, as long as the element 

 remains in the circuit, the current is continually made and broken 

 in rapid rhythmic succession. In du Bois-Reymond's sledge- 



FlG. 196. — Du Bois-Reymond's sledge-inductorium. 



apparatus (Fig. 196) such a hammer is inserted into the primary 

 circuit, and by its play there occurs in the secondary circuit for 

 every opening and everj' closing an induction-shock ; thus a rapid 

 succession of shocks takes place. When sent through living sub- 

 stance, these act as rapid intermittent stimuli and produce a tetanus. 



For the construction of most of these pieces of apparatus we 

 are indebted to the inventive genius of du Bois-Reymond alone, 

 who has created a method that has become, and will remain, in- 

 dispensable in many fields of physiology. 



We will now pass from this excursus regarding the technique 

 of galvanic stimulation to the effects that the galvanic stimulus 

 exerts upon living substance. 



a. The Phenomena of Excitation 



It is a noteworthy fact that, although electrical stimulation is 

 onu of the most common methods in use in physiology, it has 

 been employed almost exclusively upon the nerve- and the 

 muscle-fibre, and only occasionally upon plant-cells and unicel- 



