228 PHYSIOLOGY 



differs also according to the nature of the tissue which is being stimu- 

 lated, and will be shorter in the case of the frog's nerve than in the 

 case of the frog's muscle. The reason for this we shall have to 

 consider later. 



TETANUS 

 If a muscle be stimulated so many times in a second (e.g. with 



the interrupted current of an ordinary induction-coil) that it has no 



time to relax between each stimulus, 

 we get a prolonged steady contraction, 

 which in a loaded muscle is much stronger 

 than the maximal muscle-twitch, owing 

 to the summation of the rapidly follow- 

 ing stimuli. This condition is called 

 tetanus. 



The rapidity of stimulation needed to 

 produce an unbroken tetanus depends on 

 the duration of a single muscle-twitch, 

 and varies therefore according to the kind 

 and condition of the muscle. Thus the 

 rapidity need only be small in the case 

 of cooled and tired muscles, or of the 

 red muscles of the rabbit and tortoise. 

 The rate varies from about 15 in the 

 case of red muscles to 30 or 40 for 

 white muscles. For the much more 



highly differentiated muscles of insects the rate is probably very 



much greater. 



FIG. 63. Curves showing forma- 

 tion of tetanus (from frog's 

 gastrocnemius). a. Six sti- 

 muli per sec. 6. Ten stimuli 

 per sec. c. Thirty stimuli 

 per sec. 



CHANGES IN THE MUSCLE ACCOMPANYING ACTIVITY 

 EXTENSIBILITY. Besides the change of form, we find changes 

 in the elasticity and extensibility of muscle taking place during 

 contraction. 



Living muscle in a perfectly normal condition is distinguished by 

 its slight but perfect elasticity ; that is to say, it is considerably 

 stretched by a slight force (in the longitudinal direction), but returns 

 to its original length when the extending weight is removed. The 

 length to which muscle is stretched is not proportional to the weight 

 used, but any given increment of weight gives rise to less elongation 

 the more the muscle is already stretched. The accompanying curves 

 show diagrammatically the elongation of muscle as compared with a 

 piece of india-rubber when the weight on it is uniformly increased. 



Dead muscle is less extensible and its elasticity is less perfect. 

 A given weight applied to a dead muxjle will not stretch it so much as 



