the muscles in the body, giving what is called a convulsion. There are some other poisons which may act similarly. Convulsions are not at all uncommon in young children. The way in which a convulsion is produced is by the spreading of the disturbance all over from a single sense organ. What we have, then, in the nervous system is an arrangement whereby under special conditions a nervous disturbance can pass from any sense organ to any or all of the muscles, but under ordinary conditions the disturbance spreads to a particular muscle or group of muscles only. It is evident that the nervous system would be of no use at all, if this latter arrangement did not exist. In order for our muscles to serve us, they must act in obedience to information brought in by the sense organs, and this can happen only when certain groups of muscles work in accordance with the information brought in by certain sense organs or groups of sense organs. We explain the behavior of the central nervous system by saying that there are preferred pathways through it, or, to put it in a slightly different way, when a nervous disturbance spreads over any nerve cell it extends equally over all parts of it, but does not pass with equal ease over all the nerve junctions. Some of the nerve junctions allow the disturbance to pass more readily than others, and it is this difference in the ease of passing the nerve junctions that determines which pathway the disturbance shall follow. We know no other means by which this picking out of particular paths from the huge number of possible paths could be accomplished. What we are describing now is the simple foundation on which all our nervous activities rest. For that reason we are not taking up at this point the working of the brain, but only the direct connections between sense organs and muscles. In some very low animals the whole nervous system is made up of such simple connections. [Illustration: DIAGRAM ILLUSTRATING A REFLEX An object _O_ suddenly appears in front of the eye. Its image, formed on the retina _R_, a sense organ, starts impulses along the fibers of the sensory nerve cell _s_, which in turn stimulate the motor nerve cells _m_. These in turn stimulate the appropriate muscles of the eyelid, compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)]