The conditions which determine the lines of least resistance are manifold and variable. The nervous system is a continuous whole, each part of which is connected with diverse organs; but in spite of this anatomical diversity, the deeper uniformity causes the activity of each part to depend on and involve the activity of every other, more or less. By “more or less” is meant, that although the excitation of one part necessarily affects the state of all the others, because of their structural community, so that _each sensation and each motion really represents a change in the whole organism_, yet the responsive discharge determined in each organ by this change, depends on the tension of the organ and its centre at that moment. A bad harvest really affects the whole nation; but its effect is conspicuous on the welfare of the poor rather than of the rich, although the price of bread is the same to rich and poor. Nervous centres, and muscular or glandular organs, differ in their excitability; one condition of this greater excitability being the greater frequency with which they are called into activity. The medulla oblongata is normally more excitable than the medulla spinalis; the heart more than the limbs. Hence a stimulus which will increase the respiration and the pulse may have no _appreciable_ effect on the limbs; but some effect it must have. 170. Imagine all the nerve-centres to be a connected group of bells varying in size. Every agitation of the connecting wire will more or less agitate all the bells; but since some are heavier than others, and some of the cranks less movable, there will be many vibrations of the wire which will cause some bells to sound, others simply to oscillate without sounding, and others not sensibly to oscillate. Even some of the lighter bells will not ring if any external pressure arrests them; or if they are already ringing, the added impulses, not being rhythmically timed, will _arrest_ the ringing. So the stimulus of a sensory nerve agitates its centre, and through it the whole system; usually the stimulation is mainly reflected on the group of muscles innervated from that centre, because this is the readiest path of discharge; but it sometimes does not mainly discharge along this path, the line of least resistance lying in another direction; and the discharge never takes this path without also irradiating upwards and downwards through the central tissue. Thus irradiated, it falls into the general stream of neural processes; and according to the state in which the various centres are at the moment it modifies their activity. A nervous shock--physical or mental--sensibly affects all the organs. A severe wound paralyzes, for a time, parts far removed from the wounded spot. A blow on the stomach will arrest the heart; a fright will do the same. Terror relaxes the limbs, or sets them trembling; so does a concussion: if a frog be thrown violently on the ground, all its