i GENEKAL PHYSIOLOGY OF MUSCLE 19 



contractile protoplasm has suffered. The degree of this residual 

 shortening is, of course, in definite relation with the duration of 

 the tetanus. 



It follows from the theory of summation of contractions that 

 in tetanus the shortening of the muscle, and thus the work it is 

 able to accomplish, is greater than in a simple contraction. To 

 this rule there is, however, one exception ; according to von Frey 

 a lightly loaded muscle contracts equally both to a single shock, 

 and to a tetanising current. 



The degree of shortening and the yield of work from the 

 tetanised muscle depend on the frequency and strength of the 

 tetanising current. When the stimulation frequency exceeds 

 300 per second, and the current is sufficiently weak, no tetanus 

 results (Harless, Heidenhain), or at most a single initial twitch . 

 (Bernstein). In order to produce tetanus the current must be 

 strengthened ; this is due to the fact that after the first stimulation 

 the excitability of the muscle drops, and consequently it no longer 

 reacts to subsequent stimuli so long as these remain minimal. 

 According to Salomonson (1904), on the contrary, this is merely [ 

 a physical phenomenon. 



The upper limit of the stimulation frequency that can produce 

 tetanus has not yet been ascertained. Bernstein obtained it with 

 an acoustic interrupter that sent 2000-3000 induction shocks into 

 the muscle per second : Kronecker with 20,000 shocks per second. 



Tesla and d' Arson val discovered that high frequency alternating 

 currents sufficiently intense to render a carbon filament in- 

 candescent fail to excite a muscle or nerve. While a constant 

 current of 5 milliamperes excites both at break and at make 

 of the circuit, an alternating current of 5 amperes, of high 

 frequency (about one million per second), produces no effect, motor 

 or sensory. By a special contrivance this current can be passed 

 through one or more persons and at the same time through a 

 series of incandescent lamps ; the lamps light up, while the 

 individuals included in the circuit feel neither sensation nor 

 motion. Einthoven (1900) subsequently demonstrated that it is 

 possible to evoke muscular contractions by means of indirect 

 stimulation, even with alternating currents of the highest frequency 

 (up to a million per second), provided the strength of the current 

 is enormously increased in proportion with its frequency. 



Comparison of the rate of the muscular contractions produced 

 by an instantaneous shock from an induced current with the slow 

 persistent contractions by which the skeletal muscles are usually 

 thrown into voluntary contraction, has led to the conclusion that 

 the latter are tetanic in character, i.e. are the effects of a series of 

 impulses from the nerve centres. To support this theory of the 

 discontinuity of excitation in voluntary contraction, Wollaston 

 (1810) adduced the sound developed by the muscle in contracting, 



