THE MUSCLE-NERVE PREPARATION AS A MACHINE. 107 



supplying a muscle. If the intra-muscular nerves be still in good condition, 

 the muscle, as a whole, responds readily to single induction-shocks, because 

 these can act upon the intra-muscular nerves. If these nerves, on the other 

 hand, have lost their irritability, the muscle does not respond readily to 

 single induction-shocks,, or to the interrupted current, but can still easily be 

 thrown into contraction by the constant current. 



In the second place, while in a nerve no impulse is, as a rule, generated 

 during the passage of a constant current, between the break and the make, 

 provided that it is not too strong, and that it remains uniform in strength, 

 in a urarized muscle, on the other hand, even with moderate and perfectly 

 uniform currents, a kind of tetanus or apparently a series of rhythmically 

 repeated contractions is very frequently witnessed during the passage of the 

 current. The exact nature and cause of these phenomena in muscle, we 

 must not, however, discuss here. 



THE MUSCLE-NERVE PREPARATION AS A MACHINE. 



77. The facts described in the foregoing sections show that a muscle 

 with its nerve may be justly regarded as a machine which, when stimulated, 

 will do a certain amount of work. But the actual amount of work which a 

 muscle-nerve preparation will do is found to depend on a large number of 

 circumstances, and consequently to vary within very wide limits. These 

 variations will be largely determined by the condition of the muscle and 

 nerve in respect to their nutrition ; in other words, by the degree of irrita- 

 bility manifested by the muscle or by the nerve, or by both. But quite 

 apart from the general influences affecting its nutrition and thus its irrita- 

 bility, a muscle-nerve preparation is affected as regards the amount of its 

 work by a variety of other circumstances, which we may briefly consider 

 here, reserving to a succeeding section the study of variations in irritability. 



The influence of the nature and mode of application of the stimulus. When 

 we apply a weak stimulus a weak induction-shock to a nerve we get a 

 small contraction, a slight shortening of the muscle ; when we apply a 

 stronger stimulus a stronger induction-shock we get a larger contraction, 

 a greater shortening of the muscle. We take, other things being equal, the 

 amount of contraction of the muscle as a measure of the nervous impulse, 

 and say that in the former case a weak or slight, in the latter case a stronger 

 or larger, nervous impulse has been generated. Now, the muscle of the 

 muscle-nerve preparation consists of many muscular fibres, and the nerve of 

 many nerve fibres ; and we may fairly suppose that in two experiments we 

 may in the one experiment bring the induction-shock or other stimulus to 

 bear on a few fibres only, and in the other experiment on many or even all 

 the fibres of the nerve. In the former case, only those muscular fibres in 

 which the few nerve fibres stimulated end will be thrown into contraction, 

 the others remaining quiet, and the shortening of the muscle, as a whole, 

 since only a few fibres take part in it, will necessarily be less than when all 

 the fibres of the nerve are stimulated and all the fibres of the muscles con- 

 tract. That is to say, the amount of contraction will depend on the number 

 of fibres stimulated. For simplicity's sake, however, we will in what fol- 

 lows, except when otherwise indicated, suppose that when a nerve is stimu- 

 lated, all the fibres are stimulated and all the muscular fibres contract. 



This being premised, we may say that, other things being equal, the mag- 

 nitude of a nervous impulse, and so the magnitude of the ensuing contraction, 

 is directly dependent on what we may call the strength of the stimulus. 

 Tim* taking a single induction-shock as the most manageable stimulus, we 

 find that if, before we begin, we place the secondary coil (Fig. 14, sec. c.) a 



