CHAPTER IX 

 Fatigue of Muscle and Nerve 



Effects of fatigue on muscle : (a) On the form of the muscle 

 curve. Take a nerve-muscle preparation and fit it up as for recording 

 the simple muscle curve (p. 29). Make an abscissa, and mark, as 

 usual, upon it the point of stimulation. Take a normal curve with 

 the muscle free-weighted. Remove the writing point from the drum, 

 which is then allowed to revolve continuously and to stimulate the 

 muscle at every revolution. After fifty of such excitations without 

 record, apply the lever point again to the drum (making use, of course, 

 of the stop), and let the muscle describe another curve at the same 

 place as the first. Remove the writing point again for the duration 

 of fifty excitations, and repeat the above procedure, and so on a 

 number of times until the fatigue curves are pronounced. Notice 

 the effects of fatigue upon muscle, in prolonging the latency period, 

 diminishing the amount and slowing the course of its contraction, 

 and greatly delaying, and at length even preventing, its relaxation. 



A fatigue curve or series of curves can also be obtained by allowing 

 the lever point to remain in contact with the cylinder during the 

 whole of the experiment, and thus recording every contraction ; but 

 the individual curves in a tracing so obtained are very numerous, and 

 tend to obscure one another. 



(6) On the extent of contraction. The effect of fatigue upon the 

 extent of contraction is best recorded upon a stationary drum, moved 

 by hand about half a millimetre after each excitation ; or on a very 

 slowly moving drum : the extent of the contraction is shown by 

 the ordinates described by the lever. If the slowest rate of movement 

 of the drum (1 mm. per second or less is used), arrange to make and 

 break the primary circuit about every half-second. This can be done 

 either by closing and opening a key by the hand, or by allowing 

 a metallic bridge, actuated mechanically (e.g., by a metronome), to 

 close and open a gap between two mercury cups in the circuit. Use 

 maximal stimuli. Keep the point of the lever which must be free- 

 weighted against the smoked paper, and record every contraction. 

 In this way a continuous fatigue curve is obtained, exhibiting the 



45 



