CH. X.] TETANUS 121 



remains at a maximum contraction, till it begins to relax from 

 fatigue. 



A succession of stimuli may be sent into the nerve of a nerve- 

 muscle preparation by means of the Wagner's hammer of a coil, or 

 the vibrating reed previously mentioned (p. 111). This method of 

 stimulation is called faradisation. Figs. 151 and 152 show the kind 

 of tracings one obtains. The number of contractions corresponds to 

 the number of stimulations ; the condition of prolonged contraction 

 so produced, the muscle never relaxing completely 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 con- 

 tractions are so rapid as to be 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 



FIG. 152. Curve of complete tetanus, from a series of very rapid shocks from a magnetic interrupter. 

 (Tracing to be read right to left.) 



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 ? 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 



