IRRITABILITY. 



451 



gram is being made. Such an one is the electric signal of Deprez 

 (Fig. 258), which is an electromagnet whose circuit is closed and 

 opened by the second pendulum of a clock or a metronome. 

 Or for this purpose a tuning-fork may be used which vibrates 



FIG. 257. Time-marker ; arrangement for marking two intervals: D, seconds 

 pendulum, with platinum point, E, soldered on ; A, mercury trough, into which E 

 dips at the end of its swing; B, Daniell cell; (7, electromagnets, which draw down 

 writing-lever F when the current is closed by E dipping into A ; G, spring (or piece 

 of India rubber), which raises .Fas soon as current is broken (Stewart). 



FIG. 258. Electromagnetic time-marker connected with metronome. The pen- 

 dulum of the metronome carries a wire which closes the circuit when it dips into 

 either of the mercury cups, Hg (Stewart). 



one hundred times in a second, the writing-point being attached 

 to one of its prongs. 



Moist Chamber. In order to preserve the preparation in as 

 normal condition as possible during an experiment, it is enclosed 

 in a chamber the air of which is kept moist. 



Simple Muscular Contraction. When the muscle-nerve prep- 

 aration is stimulated by a single induction shock, a single con- 

 traction of the muscle results ; this is a twitch or a simple muscular 



