NORMAL KNEE-JERK. 9
system. Not a few of these changes have their ori- gin in the influence exerted by the different parts of the central nervous system on each other, and there can be but little doubt that the mutual dependence of the cerebro-spinal centers is much greater than has generally been supposed. Indeed, it would almost seem as if the nervous connections were so intimate that a change in the activity of any one of these centers would make itself felt in all the rest, as if, to speak figuratively, there were a balancing of nervous tension throughout the nervous system, so that a change in any one part must be felt in all other parts. Thus, though an increase in pressure, due to a sudden production of nerve force, might, perhaps, encounter less resistance, and so make itself felt chiefly in certain directions, it would produce a slight effect throughout the whole system. The picture represents a condition of things similar to that existing in the circulatory system, where a change of pressure brought about at any part tends to be transmitted to all the rest. Far be it from the writer to offer or support a theory of the action of what we call nerve force. The line of thought has been suggested, however, by the results of his own and similar experiments, which have shown that a strong sensory irritation, a voluntary action, or even an emotion, is sufficient to influence the extent of the knee-jerk. It' has long been known that the nervous system binds the many organs of the body into a whole, and that through it the condition of every part is made to have its influence on all the rest, but the closeness of this union has never been illustrated with such startling distinctness as it is in the inces- sant variations of the knee-jerk.
