[Illustration: FIG. 14--PYRAMIDAL CELL OF CEREBRAL CORTEX OF MOUSE (after Ramon y Cajal).] _Voluntary Neuron Motion_.--This theory of Duval's supposes that to some extent the neurons or nerve cells are possessed of voluntary movement. At least during certain states of the mind, they are moved and seem to have an inherent, if not quite voluntary, power {120} of motion. There are many objections urged against the theory because of this neuronic motion. It has been said that the movement of neurons has been observed in certain of the _Medusae_. The observation has been doubted and it lacks confirmation. In higher animals, of course, the observation is impossible because an investigation of the nervous system for this purpose would necessarily bring about the death of the animal and the cessation of spontaneous mobility. Whether it occurs or not, therefore, is a theoretic problem. So many objections tell against Duval's theory that it is now only discussed because of its subjective value. Neuroglia Theory.--Ramon y Cajal elaborated a second theory of explanation for the mechanism of the nervous system that has seemed to many authorities in brain physiology much more satisfactory than Duval's theory of the actual motion of the neurons themselves. The Spanish nervous histologist had made a special study of the neuroglia or connective tissue cells in the central nervous system. These are very small in size but very numerous. Ramon y Cajal suggested that it was because the terminal filaments of these neuroglia cells inserted themselves between the neuronic filaments, thus insulating one from another, somewhat as if an insulating plug were inserted between two portions of an electric circuit, that the interruption of nervous currents took place. This explanation is free from many of the objections urged against Duval's theory. The small size of the neuroglia cells makes it easy to understand how movement may take place in them sufficient to bring about separation of neurons. It would not be surprising if they should be more or less actively contractile. Whenever they contract, neuronic filaments which they have been holding apart, come together so as to permit the passage of nervous impulses, if any are flowing at the time. When the neuroglia cells become fatigued or seriously disturbed, they refuse any longer to obey the will in any way, or at least gradually get beyond control, and in their relaxation becoming prolonged, push neurons apart. When a man is very tired it gradually becomes impossible for him to keep awake. This is partly because poisons, produced in the course of fatigue, exhaust the vitality of the neuroglia cells and also of the neurons, so that less energy is required to push these latter apart.