130 MUSCULAR CONTRACTION. [OH. x. 



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 gastroc- 

 nemius, 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 ^ 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. Recent 

 experiments made by Sir J. Burdon Sanderson on the electrical 

 variation that accompanies voluntary movements, have shown 

 that this is the case : each component of the so-called voluntary 

 tetanus is a much-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 hav6 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 power of a lever, though a gain in the rapidity 

 and extent of the movement. 



Most muscular acts involve the action of several muscles, often 

 of many muscles. The acts of walking and running are examples 

 of very complicated muscular actions in which it is necessary not 

 only that many muscles should take part, but also must do so in 

 their proper order and in due relation to the action of auxiliary 

 and antagonistic muscles. This harmony in a complicated mus- 

 cular action is called co-ordination. 



By the device of taking instantaneous photographs at rapidly 

 repeated intervals during a muscular act, the details of different 

 modes of locomotion in man and other animals have been very 

 thoroughly worked out. With this branch of research the name 

 of Prof. Marey is intimately associated. 



