360 GENERAL PROPERTIES OF MUSCLE AT REST. 



It is often advisable to use a key both between the battery 

 and the induction coil, and between the secondary coil and the 

 electrodes. 



Current with Definite Interruptions by Means of 

 the Metronome. Arrange as for a single induction shock, 

 except that, in place of the key 6, insert the electrical metro- 

 nome, an instrument which may be described as a key which 

 is opened and closed by clock-work at regular intervals of 

 time, the length of interval being varied at will. The key a 

 may be dispensed with, as, unless a special provision be made, 

 the shocks given will be both opening and closing. 



Current interrupted by Means of an Oscillating 

 Rod. Bring one wire straight from the battery to the pri- 

 mary coil, connect the other wire with a slip of thin elastic 

 steel (the length will be determined by the rapidity of inter- 

 ruption required), one end of which is made fast, while at the 

 other a needle, at right angles to, but in electrical continuity 

 with, the steel slip, hangs over a mercury cup at such a dis- 

 tance, that when the steel slip is at rest, the needle is quite 

 clear of the mercury, but that when the slip is made to oscil- 

 late, with each oscillation the needle dips in and out of the 

 mercury. Connect the mercury of the cup with the other 

 binding screw of the primary coil. At each oscillation of the 

 slip, the current will accordingly be made and broken. 



CHAPTER XX. 

 GENERAL PROPERTIES OF MUSCLE AT REST. 



I. Elasticity. Obs. I. Take a gastrocnemius, prepared 

 as directed in Chap. XIX., sec. I., except that the nerve may 

 be neglected whollj- ; fasten the femur in the clamp of the 

 moist chamber, and attach the mus'cle to the lever, as directed 

 in sec. XI. Let the lever be perfectly horizontal. 



Draw on the recording surface a straight line, on which 

 jnake a mark for zero, and mark off abscissae in the proportion 

 of 10, 30, 50, 100, 150, 200, 300, 400, etc. 



Disregarding the weight of the lever (or of the pan, etc., 

 when Helmholtz's arrangement is used), the muscle may be 

 supposed to have its natural length when no weight is brought 

 to bear upon it. This may be indicated by bringing the re- 

 cording point of the lever to touch the zero point on the re- 

 cording surface. Next shift the recording surface until the 

 point of the lever touches the point corresponding to 10. 



