222 PHYSIOLOGY 



CHAP. 



produced a form of stimulation very similar to faradisation, with 

 the advantage of eliminating all the errors due to spread of the 

 stimulus to other parts of the nerve. 



A highly special form of stimulation is obtained by rapidly 

 removing the compression applied to the nerve. But in this case 

 it is not certain that there is true mechanical excitation ; more 

 probably the muscular reaction depends on the recovery by the 

 nerve of its normal fluid content, which had been altered by the 

 previous compression ; this gives rise to a demarcation current 

 which excites the nerve. 



(d) The best excitant of nerve, as of muscle, with the strongest 

 analogy to physiological excitants is undoubtedly the electrical 

 current, of which the efficacy as a stimulus was demonstrated by 

 Galvani and Volta. 



r 



FIG. 143. Daniell cell and Du Bois-Reymond key. 



The electrical stimulus most employed is the induced current, 

 generated in a secondary circuit by the make and break of the 

 current which passes through the primary circuit of an induction 

 coil (Fig. 142). It can be perfectly graduated, is capable of 

 yielding a comparatively high electromotive force, is of brief 

 duration, and develops very rapidly. The regular series of make 

 and break shocks, or of alternating break and make shocks, from 

 Du Bois-Eeymond's sliding induction-coil is generally known in 

 the laboratory as the tetanising current. 



The direct application of the constant galvanic current from 

 a cell (Fig. 143) has the disadvantage, owing to its prolonged 

 passage, of producing electrochemical changes in the tissues greater 

 than those due to other methods of stimulation. This inconveni- 

 ence can be reduced to a minimum by employing very brief 

 currents in alternating directions. It is also easy by means of a 

 rheochord to regulate the intensity and exactly measure the 

 electromotive force of the current employed as stimulus. Later on 



