388 THE PROPERTIES OF STRIPED MUSCLE. 



instrument for investigating muscular fatigue in man has been employed by 

 Dr. Waller ; the essential difference between the two methods being, that in 

 the one the muscle contracts isotonically, in the other isometrically. 



The exhausting effect of continuous voluntary contraction was investigated 

 by Donders, in man, with special reference to the case in which a weight is 

 sustained by the hand with the forearm supine and horizontal. The method 

 is analogous to that previously described. A cord is attached to the weight, 

 by the severing of which the forearm is liberated. Instantly the hand rises, 

 hastening to assume a position which corresponds to the equilibrium length 

 of the acting muscle, the biceps. This length can be calculated with 

 approximate accuracy from the angular measurement of the movement exe- 

 cuted by the forearm in assuming its new position. The experiment affords 

 direct proof that the effect of muscular fatigue is, as has been already 

 explained, to increase the equilibrium length of the excited muscle ; 

 whence it follows that the longer the effort lasts and the oftener it is 

 repeated, the stronger is the influence which must be exercised on the 

 muscle by the central nervous system. The sensation of fatigue which by a 

 conscious effort we strive to overcome, in keeping up the weight as long as 

 possible, is probably a fairly true index of the gradual intensification of the 

 excitatory process in the motor cell and motor nerve. 



Effect of exhaustion on the duration of the response to single excitations. 

 The observation was originally made by Helmholtz, that when a muscle 



FIG. 216. Muscle curves showing the effect of fatigue. After Waller. 



is excited repeatedly by an instantaneous stimulus, the time occupied by 

 the response increases at the same time that the height of the curve 

 diminishes. This was subsequently shown in a striking way by Wundt, 

 who so arranged a muscle that each time it relaxed after excitation, it 

 re-established an exciting contact which, in contracting, it immediately 

 broke. The diminishing frequency of the contractions thus automat- 

 ically produced, showed that each lasts a little longer than its prede- 

 cessor. The character of the change of form of the isotonic curve which 

 is thus produced, is well seen in Fig. 216, which represents a succession of 

 such curves with their starting-points coincident. Their heights at first 

 increase, then diminish as the spans of the curves become greater. It 

 has been stated that the increase of duration is due to failure in the pro- 

 cess of relaxation, but of this such curves can afford no evidence. All 

 that they show is that the state of excitation and the contractile 

 stress by which it expresses itself, both increase and diminish more 

 slowly. 



To understand the prolongation of the mechanical response in a tired 

 muscle, we have only to consider that the response itself is a " wave of 

 contraction " which is normally propagated at a certain rate ; and if 

 we assume that the wave length remains the same, and that the propa- 

 gation rate is diminished, the duration of the effect at any point over 

 which the wave passes must increase proportionally to that diminution. 



