ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



17 



skin, the secretions of which quickly injure it. (c) When the nerve is 

 on the electrodes it must be kept moist by normal tap-water saline 

 solution ('70 per cent, sodium chloride in tap-water) upon a piece of 

 filter-paper, but care must be taken that the current from the electrodes 

 is not short-circuited thereby, (d) The nerve itself should not be 

 picked up by forceps, but should be lifted by the pieces of the 

 vertebral column. Consequently the whole length of the nerve 

 should always be dissected out; as a rule it should not be cut 

 across in the thigh nor simply exposed in the thigh and two 

 electrodes pushed under it. 



The Muscle-lever takes one of two chief forms : 



(a) The crank-lever (Fig. 24) consists of an L-shaped piece of 

 metal, the horizontal arm of which is long and carries the writing 



FIG. 25. The simple lever with after-loading screw. F, clamp ; L, lever ; M, muscle. 



point, whilst the vertical arm is short and to this the thread round 

 the tendo-Achillis is firmly tied. The muscle rests, in the same 

 straight line as the lever, on the muscle-board, a horizontal piece of 

 wood covered with cork. The whole is carried on a vertical stand 

 (Fig. 24), the arm of which is movable on the base, so that the 

 writing point of the myograph can be swung towards and away from 

 the drum without altering the position of the base of the stand. 

 When the thread has been tied to the lever, a pin is pushed through 

 the lower end of the femur into the cork; this gives the muscle a 

 fixed point from which to pull. It is necessary to see that, when 

 the muscle is at rest, the thread attached to the lever is taut, and 

 that there is no "slack" to be taken in when contraction begins; 

 further, the writing arm should be horizontal. 



In this form of lever the movement of the writing point is at right 



