74 



PHYSIO LOGIC A L PHYSICS. 



[Chap. VII. 



Fig. 41. Heidenhain's Tetanometer. 



on a brass column by the axle . The electro-magnet 

 is connected with the brass support of the arm k. 

 which can be caused to make or break contact with 

 the screw z by moving it with the insulated handle A. 

 The lever has s on its upper surface a steel spring i, 



bearing a small 

 platinum plate which 

 presses against the 

 platinum point of 

 the screw s y , of the 

 brass column s. The 

 screw s" regulates 

 the pressure of the 

 platinum plate 

 against the platinum 

 point. The other 

 end of the lever 

 carries a wedge- 

 shaped piece of 

 ivory /<-, with the thin edge downwards, suspended 

 above a little ivory support t, which has a deep 

 groove. This ivory support can be raised up to the 

 lever or lowered from it by the screw sc. To 

 use the apparatus, the limb of a frog is taken, the 

 sciatic nerve is dissected out as long as possible, 

 and laid over the gastrocnemius. The muscles of the 

 thigh are then cleared away, and the femur snipped 

 through below the head. The limb is fixed by the 

 femur in a pair of forceps ; a fine silk thread is tied to 

 the end of the nerve, and by its means the nerve is 

 laid through the notches h' across the groove of 

 the ivory support t, and attached to the ivory axle A. 

 By turning this axle the nerve can be pulled through 

 the notches so as to bring a fresh piece across the 

 groove. One pole of an element is connected to the 

 screw s tl and the other to z. The current passes up s 

 to the screw point s t along the lever down the column 



