MUSCLE 277 



meet sudden resistance without rupture, as when a man 

 alights from a height. At the moment when the feet touch 

 ground elasticity dissipates the shock. The stretching of the 

 muscles^then leads reflexly to the increase of their tone. Here 

 we see an advantage in the short reaction-time of the knee- 

 jerk. Tone comes into play long before impulses generated 

 by contact of the sole of the foot with the ground have had time 

 to reach the'brain, or even to induce reflex contraction through 

 the spinal cord. The elasticity of the muscles is also of use 

 in the performance of certain sudden actions. A pea is flicked 

 across the room by pressing the thumb-nail against the pad of a 

 finger, or a finger against the thumb, and releasing it with a jerk. 

 An electrical change accompanies an impulse in its passage 

 down a nerve, and a wave of contraction in its passage along 

 a muscle. In 1788 Galvani observed that the hind-limbs of a 

 frog, suspended by a metal hook to metal railings, twitched 

 when the wind blew them against the bars. The hook passed 

 through the lumbar plexus of nerves. He recognized that the 

 cause of the twitch was the closing of a circuit. The birth of 

 dynamic or galvanic electricity dates from this observation ; 

 and ever since this phenomenon was first observed the electric 

 changes in nerve-muscle preparations made from frogs' legs 

 have been favourite subjects of research. Many obvervations 

 with regard to nerve-conduction and muscle-contraction may 

 be made, and many experiments performed, without special 

 apparatus. A frog having been killed by cutting off its head, 

 or by placing it beneath a tumbler with a wad of cotton-wool 

 soaked in chloroform, the skin of the leg is removed, displaying 

 the khaki-coloured muscles, bluish tendons, and bright white 

 threads of nerve. A stretch of the largest nerve of the back of 

 the thigh, the sciatic, is isolated. All the muscles of the thigh 

 are then cut away and the bone nipped across just above the 

 knee. The bones below the knee are removed, the superficial 

 muscle of the calf, the gastrocnemius, being allowed to hang 

 free, its bifid end attached to the fragment of thigh-bone. Its 

 lower end terminating in the tendo Achillis, with its insertion 

 into the prominence of the heel, is left intact. The bone is 

 fixed in a clamp. A light lever made from a wooden spill 

 is suspended from the tendo Achillis. The nerve may then 

 be stimulated in various ways : by crushing in a pair of forceps, 



