THE MUSCLE-NERVE PREPARATION AS A MACHINE. 109 



contraction when it is sent along the nerve than when it is sent across the 

 nerve ; indeed it is maintained that a current which passes through a nerve 

 in an absolutely transverse direction is powerless to generate impulses. 



It would also appear, at all events up to certain limits, that the longer 

 the piece of nerve through which the current passes, the greater is the effect 

 of the stimulus. 



When two pairs of electrodes are placed on the nerve of a long and per- 

 fectly fresh and successful nerve-preparation, one near to the cut end, and 

 the other nearer the muscle, it is found that the same stimulus produces a 

 greater contraction when applied through the former pair of electrodes than 

 through the latter. This has been interpreted as meaning that the impulse 

 started at the further electrodes gathers strength, like an avalanche, in its 

 progress to the muscle. It is more probable, however, that the larger con- 

 traction produced by stimulation of the part of the nerve near the cut end 

 is due to the stimulus setting free a larger impulse, i. e., to this part of the 

 nerve being more irritable. The mere section, possibly by developing nerve 

 currents, increases for a time the irritability at the cut end. A similar 

 greater irritability may, however, also be observed in the part of the nerve 

 nearer the spinal cord while it is still in connection with the spinal cord ; 

 and it is possible that the irritability of a nerve may vary considerably at 

 different points of its course. 



78. We have seen that when single stimuli are repeated with sufficient 

 frequency, the individual contractions are fused into tetanus ; as the fre- 

 quency of the repetition is increased, the individual contractions are less 

 obvious on the curve, until at last we get a curve on which they seem to be 

 entirely lost and which we may speak of as a complete tetanus. By such a 

 tetanus a much greater contraction, a much greater shortening of the muscle, 

 is of course obtained than by single contractions. 



The exact frequency of repetition required to produce complete tetanus 

 will depend chiefly on the length of the individual contractions, and this 

 varies in different animals, in different muscles of the same animal, and in 

 the same muscle under different conditions. Jn a cold-blooded animal, a 

 single contraction is as a rule more prolonged than in a warm-blooded ani- 

 mal, and tetanus is consequently produced in the former by a less frequent 

 repetition of the stimulus. A tired muscle has a longer contraction than a 

 fresh muscle, and hence in many tetanus curves the individual contractions, 

 easily recognized at first, disappear later on, owing to the individual contrac- 

 tions being lengthened out by the exhaustion caused by the tetanus itself. 

 In many animals, e. g., the rabbit, some muscles (such as the adductor mag- 

 nus femoris) are pale, while others (such as the semitendinosus) are red. 

 The red muscles are not only more richly supplied with bloodvessels, but the 

 muscle substance of the fibres contains more haemoglobin than the pale, and 

 there are other structural differences. Now the single contraction of one of 

 these red muscles is more prolonged than a single contraction of one of the 

 pale muscles produced by the same stimulus. Hence the red muscles are 

 thrown into complete tetanus with a repetition of much less frequency than 

 that required for the pale muscles. Thus, ten stimuli in a second are quite 

 sufficient to throw the red muscles of the rabbit into complete tetanus, while 

 the pale muscles require at least twenty stimuli in a second. 



So long as signs of the individual contractions are visible on the curve of 

 tetanus it is easy to recognize that each stimulation produces one of the con- 

 stituent single contractions, and that the number so to speak of the vibra- 

 tions of the muscle making up the tetanus corresponds to the number of 

 stimulations ; but the question whether, when we increase the number of 

 stimulations beyond that necessary to produce a complete tetanus, we still 



