=Elasticity of Muscles.=—Muscles are very extensible, and after stretching return to their original length. Their elasticity is a quality of great practical importance. It enables them to 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 thumbnail 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 observations 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, burning with a heated needle, touching with a drop of glycerin or a strong solution of salt. But of all methods of stimulation, the best is the current from an induction coil. Since it does not injure the nerve, it can be applied as often as may be desired. The amateur provided with an induction coil is in a position to study the relation