muscles are convulsed; but if the nerves of one limb be divided before the shock, the muscles of that limb will not be convulsed. 171. We are apt to regard the discharge on the moving organs as if that were the sole response of a stimulation; but although the most conspicuous, it is by no means the most important effect. Besides exciting the muscles, more or less, every neural process has its influence on the organic processes of secretion, and effects thermal and electrical changes. Schiff has demonstrated that every sensation raises the temperature of the brain; Nothnagel, that irritation of a sensory nerve causes constriction of the cerebral arteries, and hence cerebral anæmia. Brown Séquard and Lombard find the temperature of a limb raised when its skin is pinched, and lowered when the skin _elsewhere_ is pinched. Georges Pouchet has shown that fishes change color according to the brightness or darkness of the ground over which they remain; and these changes are dependent on nervous stimulation, mainly through the eye, division of the optic nerves preventing the change. These are so many _a posteriori_ confirmations of what _a priori_ may be foreseen. They are cited here merely to enforce the consideration, seldom adequately kept before the mind, that every neural process is a change which causes other changes in the whole organism. STIMULI. 172. Stimuli are classed as external and internal, or physical and physiological. The one class comprises all the agencies in the External Medium which _appreciably_ affect the organism; the other class all the changes _in_ the organism which appreciably disturb the equilibrium of any organ. Although the pressure of the atmosphere, for example, unquestionably affects the organism, and determines organic processes, it is not reckoned as a stimulus unless the effect become appreciable under sudden variations of the pressure. In like manner the blood is not reckoned among the internal stimuli, except when sudden variations in its composition, or its circulation, determine appreciable changes. Because the external stimuli, and the so-called Senses which respond to them, are more conspicuous than the internal stimuli and the Systemic Senses, they have unfortunately usurped too much attention. The massive influence of the Systemic Sensations in determining the desires, volitions, and conceptions of mankind has not been adequately recognized. Yet every one knows the effect of impure air, or a congested liver, in swaying the mental mood; and how a heavy meal interferes with muscular and mental exertion.[192] What is conspicuous in such marked effects, is less conspicuously, but not less necessarily, present in slighter stimuli.