The paths they follow must be the most open in the nervous system. Receptors and effectors must, in an electrician’s phrase, be incessantly switched on; or, to express the analogy more accurately, the flexor and extensor tone mechanisms are incessantly and reciprocally switching each other on and off. It must be confessed that it is very difficult to explain the knee-jerk if it be not a reflex action, but, as has been supposed, a direct response of the thigh muscles to their own stretching. The latter hypothesis does not appear to be reconcilable with its dependence upon the maintenance of the nervous connection of the muscles with the spinal cord. It cannot be elicited unless the “spinal arc” is intact. It ceases after the severance of either sensory or motor roots. Nor will it occur if the supply of blood to the lower end of the spinal cord has been cut off. Still more difficult is it to explain its extraordinary sympathy with everything that happens in the whole nervous system, if the impulses which cause it do not pass through the spinal cord. By a very simple mechanical arrangement it is possible to record the amplitude of the knee-jerk. The foot moves a lever which writes on a travelling surface. The jerk is elicited by the hammer of a clock strapped to the shin. In this way it is possible to extend the period of observation over several consecutive hours, the subject becoming completely oblivious of the movement his foot is making once a second, if it be screened from his view. In deep sleep the jerks stop; but the subject may doze, and still jerk follows tap. And the record made by his foot mirrors all the changes in his nervous system. If he clench his fist, the movement is reinforced, as it is when a child cries, a lamp is lighted, his ear itches. There is music in an adjoining room. His foot is the baton which beats _fortissimo_ to Wagner, and is lulled to _piano_ by the “Lieder ohne Wörte.” On a bright day this spinal pulse throbs gaily. It is indolent in dull, depressing weather. The knee-jerk is the physician’s guide to the condition of the nervous system.