CHAP, ii.] THE CONTRACTILE TISSUES. 87 



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

 nerve being more irritable. It is possible that the irritability 

 of a nerve may vary considerably at different points of its course. 



We have in a preceding section discussed at length the manner 

 in which a stimulus repeated sufficiently rapidly produces a com- 

 plete and uniform tetanus, during which the constituent single 

 contractions cannot be recognized either by the appearance of the 

 muscle itself or by any features in the curve which it may be 

 made to describe, though the 'muscular sound' shews that the 

 muscle is really in a state of vibration. If the frequency of 

 the stimulus be reduced the tetanus becomes incomplete and 

 a flickering of the muscle becomes obvious, and upon further 

 reduction of the frequency the flickering gives place to a rhythmic 

 series of single contractions. Since the height to which the lever 

 is raised, i.e. the amount of total shortening resulting from 

 any second contraction, is greater when that contraction starts 

 from the summit of the preceding curve than when it starts from 

 the decline, it is obvious that the amount of total contraction will 

 up to a certain limit increase with the frequency of repetition 

 of the stimulus. Thus a stimulus repeated rapidly will produce a 

 tetanus, shortening the muscle and raising the weight to a greater 

 extent than will the same stimulus less rapidly repeated. The 

 exact frequency of repetition required to produce complete tetanus 

 varies according to the condition of the muscle and is not the same 

 for all muscles, being dependent on the rapidity with which 

 the muscle executes each single contraction. In those animals 

 which possess two kinds of skeletal muscles, red and pale, the 

 red muscles (the single contractions of which are slow and long- 

 drawn) 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. 



When the stimulus is repeated more frequently than is 

 required to bring about a complete tetanus the constituent 

 contractions are still proportionately increased in frequency. This 

 is shewn by the increased pitch of the muscular sound. How 

 far the increase in the frequency of the constituent contractions 

 can be carried by increasing the frequency of the stimulus is 

 a question which presents considerable difficulties, and cannot be 

 discussed here. 



The value of the muscle as a machine is also in part dependent 

 on the Load. It might be imagined that a muscle, which, when 

 loaded with a given weight, and stimulated by a current of a given 

 intensity, had contracted to a certain extent, would only contract 

 to half that extent when loaded with twice the weight and stimu- 

 lated with the same stimulus. Such however is not the case; the 

 height to which the weight is raised may be in the second instance 



