i88 ELEMENTARY LESSONS ON [CHAP. in. 



points along its length with a morsel of muscle cut from 

 a living frog ; and that a conductor of one metal when 

 joining a nerve to a muscle also sufficed to cause con- 

 traction in the latter. Galvani and Aldini regarded 

 these facts as a disproof of Volta's contact -theory. 

 Volta regarded them as proving that the contact 

 between nerve and muscle itself produced (as in the 

 case of two dissimilar metals) opposite electrical con- 

 ditions. Nobili, later, showed that when the nerve and 

 the muscle of the frog were respectively connected by a 

 water- contact with the terminals of a delicate galvan- 

 ometer, a current is produced which lasts several hours : 

 he even arranged a number of frogs' legs in series, 

 like the cells of a battery, and thus increased the current. 

 Matteucci showed that through the muscle alone there is 

 an electromotive-force. Du Bois Reymond has shown 

 that if the end of a muscle be cut across, the ends of the 

 muscular fibres of the transverse section are negative, 

 and the sides of the muscular fibres are positive, and 

 that this difference of potential will produce a current 

 even while the muscle is at rest. To demonstrate this 

 he employed a fine astatic galvanometer with 20,000 

 turns of wire in its coils ; and to obviate errors arising 

 from the contact of the ends of the wires with the tissues 

 ^tnpolar^sable electrodes were used, made by plunging 

 terminal zinc points into a saturated solution of sulphate 

 of zinc, contained in a fine glass tube, the end of which 

 was stopped with a porous plug of moistened china clay. 

 The contraction of muscles also produces currents. 

 These Du Bois Reymond obtained from his own muscles 

 by dipping the tips of his fore -fingers into two cups 

 of salt water communicating with the galvanometer 

 terminals. A sudden contraction of the muscles of 

 either arm produced a current from the contracted 

 toward the uncontracted muscles. Dewar has shown 

 that when light falls upon the retina of the eye an 

 electric current is set up in the optic nerve. 



