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



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 an anaesthetised animal 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 stimulation makes no difference; however slow or fast the 

 stimuli occur, the nerve-colls 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 stimulation then makes no 

 difference; the reflex contraction 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 bo lengthened 

 till it fuses with the next contraction, or else each component part 

 of the contraction is itself composite ; recent experiments by Piper 

 indicate that the latter explanation is the true one; he found 

 that each wave of the curve obtained by the graphic method is 

 really itself due to fusion of contractions occurring at a more rapid 

 rate. The method he employed was to count the number of electrical 

 variations which accompany a voluntary contraction, on the assump- 

 tion that each fundamental unit of the contraction has an electrical 

 change as its concomitant. This can be accomplished by the use of a 

 very delicate galvanometer (the string galvanometer, p. 121), the 

 movements of which can be photographed on a rapidly travelling 

 plate. The number of electrical variations is then found to be a 

 ttxed one for each muscle, but to vary in different muscles. Various 

 spinal and cranial motor centres have thus different rhythms, and 

 of those hitherto studied the cells of the motor fibres of the fifth 

 cranial nerve have the highest rate of discharge, 86 to 100 per 

 second. In muscles supplied by spinal nerves the rate is lower, 

 40 to 60. 



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 



