Remarkable evidence of the existence of muscle-tone is afforded by the =knee-jerk=. Place a person on an upright chair, with his legs crossed, muscles lax, foot hanging free. With a paper-knife or the end of a stethoscope, or even the hand used edgewise, tap the ligament which connects his knee-cap with his shin. The tap is instantly followed by a jerking forward of the foot. The deep muscles of the thigh, vastus, and crureus, have contracted. This phenomenon is easy to account for. When we are standing upright, the trunk is supported on three joints, of which one—the hip—is a perfect ball and socket, and the other two—knee and ankle—are of the same order so far as the absence of any provision for locking them is concerned. If the muscles on the front and the back of the leg did not constantly adjust our balance, by swaying the trunk forward when it falls back, and pulling it back when it sways forward, the joints of the leg would double up beneath us. A photographer knows how little confidence is to be placed in a man’s assertion that he is able to stand still. This see-saw of alternate contraction and relaxation is kept up by means of nerve-impulses which ascend from the nerve-endings surrounding the separate bundles of tendons, or from the Pacinian bodies which are found in abundance in the neighbourhood of tendons and ligaments, or from the elaborately twisted nerve-fibres found in muscle-spindles, or possibly from all three classes. Muscle and tendon are richly supplied with sense-organs susceptible to pressure and stretching. There is an abundance of nerve-endings to choose from. The slightest change in their tension, whether due to the muscle’s own contraction or to the action upon it of other muscles or weights, is recorded not only in the spinal cord, but also in the cortex of the cerebellum, and, if the contraction is an act of volition, in the cortex of the great brain. Although it was skin which was tapped, skin-nerves have nothing to do with the jerk. It was the result of the slight sudden stretching. In short, the tone-mechanism has been fooled. Notice the position of the leg. The knee is semiflexed; the foot is hanging free. There is nothing for the extensor muscles of the thigh to do. Now, if ever, they are justified in dozing. It is not to be wondered at that the sudden stretching of the ligament takes them off their guard, or that on waking they give a quite unreasonable start. The phenomenon is, as we asserted, easy to account for. It would also be easy to explain, if it were not for the extreme rapidity with which the jerk follows the tap. The interval is about one-hundredth of a second. This is thought to be too short to allow an impulse to ascend a sensory nerve, pass through the cord, and descend a motor nerve. It is true that these reflexes of adjustment must stand on a different level to other reflexes. The tone-impulses which cause them are incessantly patrolling to and fro from sense-organs to nerve-endings.