IX.] PROPERTIES OF CONTRACTILE TISSUE. 91 



Take a pair of electrodes which have their pla- 

 tinum points exposed on one side only, and 

 connect them with two Darnell's cells arranged 

 in series (Le. with the copper of one joined by 

 means of a wire with the zinc of the other) in- 

 terposing a key 1 to short circuit the current. 

 Instead of Daniell's cells any other cells of cor- 

 responding strength may be used. 

 The key being shut 2 , place the electrodes under 

 the nerve so that the nerve, and the nerve only, 

 touches the platinum points, then open the key, 

 so as to allow the current to pass into the nerve, 

 and after a few seconds shut it again. A move- 

 ment in the leg caused by contractions in the 

 muscles to which the nerve is distributed, will 

 be seen when the key is either opened or shut, 

 or on both occasions. Save in exceptional cases, 

 there is no movement during the passage of the 

 current through the nerve, but only when the cur- 

 rent is thrown into or shut off from it (the current 

 being supposed to be tolerably constant). Do 

 not repeat this experiment more than once or 

 twice on the same nerve, lest it be too much ex- 

 hausted for the succeeding observations. 



1 Cp. Appendix for a description of the instruments and for the 

 methods of using them. 



8 For convenience of description the terms ' shut ' and ' open ' 

 which apply }o a Du Bois Reymond key are used, but if the student 

 remembers that to shut the key is to let the current pass through it, 

 and to open the key is to break the current passing through it, he will 

 have no difficulty in using a Morse or other key in the place of that 

 of Du Bois Reymond. 



