THE PROBLEM The actual anatomic foundations of the concept of the reflex arc were laid by Ramon y Cajal between 1887 and 1892, when he carried out a systematic study of the spinal cord with the method of Golgi. Cajal's findings were soon confirmed by the best neuro-anatomists (van Gehuchten, von Lenhossek, Held, Retzius, von Kolliker and others).1A short time later, when Cajal and van Gehuchten developed the concept of axonal polarization, the elementary reflex arc acquired a more general significance; it became the expression of the fundamental plan of structure and function of the nervous system. Figure 1 reproduces the original diagram of Cajal, showing the reflex arc composed of the afferent sensory neuron, the efferent motoneuron and a connecting link that may either be only a collateral from the posterior root or also include one or more intercalated (internuncial) neurons. When the internuncial neurons lie outside