CH. X.] VOLUNTARY TETANUS 123 



disc on the surface of its membrane, the air within it is set into 

 vibrations of the same rate as those occurring in the thumb muscles ; 

 and these are propagated to the recording tambour and are written 

 in a magnified form by the end of the lever on a recording travelling 

 surface. 



The tracing obtained is very like that in fig. 151 ; it is an incom- 

 plete tetanus, which by a time marker can be seen to be made up of 

 10 to 12 vibrations a second. 



In some diseases these tremors are much increased, as in the 

 clonic convulsions of epilepsy, or those produced by strychnine 

 poisoning, but the rate is the same. 



Similar tracings can be obtained in animals by strapping the 

 receiving tambour on the surface of a muscle, and causing it to 

 contract by stimulating the brain or spinal cord. The rate of stimu- 

 lation makes no difference ; however slow or fast the stimuli occur, 

 the nerve-cells of the central nervous system give out impulses at 

 their own normal rate. 



The same is seen in a reflex action. If a tracing is taken from a 

 frog's gastrocnemius, the muscle being left in connection with the 

 rest of the body, its tendon only being severed and tied to a lever, 

 and if the sciatic nerve of the other leg is cut through, and the end 

 attached to the spinal cord is stimulated, an impulse passes up to the 

 cells of the cord, and is then reflected down to the gastrocnemius, 

 under observation. The impulse has thus to traverse nerve-cells; 

 the rate of simultation then makes no difference ; the reflex contrac- 

 tion occurs at the same rate, 10 or 12 per second. 



But now a difficulty arises ; if a twitch only occupies T V of a 

 second, there would be time for ten complete twitches in a second ; 

 they would not fuse to form even an incomplete tetanus. There must 

 be some means by which each individual contraction can be lengthened 

 till it fuses with the next contraction ; or, in other words, our results 

 of electrical stimulation of excised muscles must not be applied 

 without reserve to the contraction of the intact muscles in the living 

 body in response to the will. Eecent experiments made by Sir J. 

 Burdon Sanderson on the electrical variation that accompanies 

 voluntary movements, have shown that this is the case : each com- 

 ponent of the so-called voluntary tetanus is a somewhat prolonged 

 single contraction; a condition which closely resembles the tonic 

 contraction of involuntary muscle. 



Lever Systems. The arrangement of the muscles, tendons, and 

 bones presents examples of the three systems of levers which will be 

 known to anyone who has studied mechanics ; the student of anatomy 

 will have no difficulty in finding examples of all three systems in 

 the body. What is most striking is that the majority of cases are 

 levers of the third kind, in which there is a loss of the mechanical 



