LESSON II 



MUSCLE AND NERVE (Continued) 

 MYOGRAPHY 



1. The Construction and Action of the Dry Cell. The electric 

 method of stimulation is employed most frequently in the laboratory 

 because it is the most convenient. The electric energy is derived, as a 

 rule, from a Voltaic cell. As a generator may be employed a Daniell, 

 Grove, or Leclanche cell. The moist cells, however, have been dis- 

 placed in the course of time by the so-called dry cells which give off no 

 fumes and acids, need no refilling, and give, as a rule, good service at 

 slighter cost. The dry cell commonly used 

 is a modification of the Leclanche cell. It 

 consists of a jacket of zinc lined with plaster 

 of Paris and saturated with ammonium 

 chlorid. Its inner space is taken up by a 

 carbon plate which is surrounded by black 

 oxid of manganese. The plate of carbon 

 projecting from this mixture forms the posi- 

 tive pole or anode, whereas the negative pole 

 or cathode is represented by the zinc. 



Electricity "flows" from a place of high 

 potential to a place of low potential. Hence, 

 if the carbon and zinc of the battery are con- 

 nected by means of a conductor, say, a copper 

 wire, a current is set up which leaves the 

 generator at the former pole and enters it 

 at the latter. Inside the cell the current 

 flows from the zinc to the carbon to complete 

 the circuit (Fig. 2). 



The difference in the potential between 

 the two poles of a battery constitutes the 

 electromotive force. It is maintained by the 

 interaction of the chemical substances con- 

 tained in the battery. The latter, therefore, 

 corresponds to a reservoir of electricity which remains filled as 

 long as there is enough material present to yield chemical energy. 

 When this material has been used up, the difference in the electric po- 

 tential disappears and the current ceases. 



In its passage through wires the electric current loses a certain 

 amount of its initial energy, owing to the resistance which it must first 

 overcome. Consequently, the strength of a current or the rate of flow 



25 



FIG. 2. DIAGRAM OF 



DANIELL CELL. 

 Cu, Copper plate (+); Z, 

 zinc plate ( ). The direc- 

 tion of the current is indi- 

 cated by the arrows. 



