104 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. IX. 



so produced, the muscle never relaxing completely between the 

 individual contractions of which it is made up, is called tetanus: 

 incomplete tetanus, when the individual contractions are discernible 

 (fig. 124, 0, D, and E) ; complete tetanus, as in fig. 124, F, when the 

 contractions are so jrapid as to be completely fused to form a con- 

 tinuous 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 iraiscles of the tortoise 2 per second 

 is enough. With fatigue as the period of relaxation becomes pro- 

 longed, 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 ? 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 vibrations per 

 second, and for a long time 18 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 meinbrana tympani of the ear, and, therefore, selected by the 

 organ of hearing when we listen to any irregular mixture of faint, 

 low-pitched tones and noises. 



A much more certain indication of the rate of voluntary tetanus 

 is obtained by the graphic method. The myographs hitherto de- 

 scribed are obviously inapplicable to the investigation of such a 



