128 MUSCULAR CONTRACTION. [CH. X. 



the kind of tracings one obtains. The number of contractions 

 corresponds to the number of stimulations ; the condition of pro- 

 longed contraction so produced, the muscle never relaxing com- 

 pletely between the individual contractions of which it is made 

 up, is called tetanus : incomplete tetanus, or clonus, when the 

 individual contractions are discernible (fig. 151); complete tetanus, 

 as in fig. 152, when the contractions are completely fused to 

 form a continuous line without waves. 



The rate of faradisation necessary to cause complete tetanus 

 varies a good deal; for frog's muscle it averages 15 to 20 per 

 second; for the pale muscles of the rabbit, 20 per second: for the 

 more slowly contracting red muscles of the same animal, 10 per 

 second ; and for the extremely slowly contracting muscles of the 

 tortoise 2 per second is enough. With fatigue, the rate necessary 

 to produce complete tetanus is diminished. 



Voluntary Tetanus. 



We have seen that voluntary muscles under the influence of 

 artificial stimuli may be made to contract in two ways ; a single 

 excitation causes a single contraction ; a rapid series of excitations 

 causes a series of contractions which fuse to form tetanus. 



We now come to the important question, in which of these two 

 ways does voluntary muscle ordinarily contract in the body 1 The 

 answer to this is, that voluntary contraction resembles, though it 

 is not absolutely identical with, tetanus artificially produced. It 

 is certainly never a twitch. The nerve-cells from which the motor 

 fibres originate do not possess the power of sending isolated 

 impulses to the muscles ; they send a series of impulses which 

 result in a muscular tetanus,* or voluntary tetanus, as it may 

 conveniently be termed. 



If a stethoscope is placed over any contracting muscle of the 

 human body, such as the biceps, a low sound is heard. The tone 

 of this sound, which was investigated by Wollaston, and later by 

 Helmholtz, corresponds to thirty-six vibrations per second ; this 

 was regarded as the first overtone of a note of eighteen vibra- 

 tions per second, and for a long time 1 8 per second was believed 

 to be the rate of voluntary tetanus. 



The so-called " muscle sound " is, however, no indication of the 

 rate of muscular vibration. Any irregular sound of low intensity 

 will produce the same note ; it is, in fact, the natural resonance- 

 tone of the membrana tympani of the ear, and, therefore, selected 



* The use of the word tetanus in physiology must not be confounded with 

 the disease known by the same name, in which the most marked symptom is 

 an intense condition of muscular tetanus or cramp. 



