The Physical Basis of Mind / Being the Second Series of Problems of Life and Mind.

by George Henry Lewes

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 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.

173. A constant pressure on the tympanum excites no sound; only a rhythmic alternation of pressures will excite the sensation. A constant temperature is not felt; only changes in temperature. If Light and Sound were as uniform as the circulation of the blood, or the pressure of the atmosphere, we should be seldom conscious of the existence of these stimuli. But because the changes are varied and marked, our attention is necessarily arrested by them. The changes going on within the tissues are too graduated to fix the attention; it is only by considering their cumulative effects that we become impressed with their importance. For example, the development of the sexual glands determines conspicuous physical and moral results—we note consequent effects on voice, hair, horns, structure of the skull and size of the muscles, no less than the rise of new feelings, desires, instincts, ideas. Any organic interference with the activity of the ovaries will alter the moral disposition of the animal: suppression of this organic process means non-development of the feelings of maternity; the moral superstructure is absent because its physical basis is wanting.

174. Blood supplies the tissues with their plasmodes; a constant supply of oxygenated blood is therefore necessary to the vitality of the tissues. But it is an error to suppose that oxygen is the special stimulus of nerve-centres, or that their activity depends on their oxidation; on the contrary, the deficiency of oxygen or surplus of carbonic acid is that which stimulates. When saturated with oxygen, the blood paralyzes respiration; when some of the oxygen is withdrawn, respiration revives. Here—as in all other cases—we have to remember that differences in degree readily pass into differences in kind, so that an excess of a stimulus produces a reversal of the effect; thus although surplus of carbonic acid excites respiratory movements, excess of carbonic acid causes Asphyxia. Abundance of blood is requisite for the continuous activity of nerve-centres; but while a temporary deficiency of blood renders them more excitable, too great a deficiency paralyzes them. Anæmia, which causes great excitability, and convulsions (so that nerves when dying are most irritable), may easily become the cause of the death of the tissue. There are substances which can only be dissolved by a given quantity of liquid; if this quantity be in excess, they are precipitated from the solution. There are vibrations of a given order which cause each string to respond; change the special order, and the string returns to its repose.

In the stillness and darkness of the night we are excluded from most of the external stimuli, yet a massive stream of systemic sensations keeps the sensitive mechanism active, and in sleep directs the dreams. The cramps and epileptiform attacks which occur during sleep are most probably due to the over-excitability produced by surplus carbonic acid. To temporary anæmia may be assigned the strange exaggeration of our sensations during the moments which precede awakening; and the greater vividness of dream-images.

It is only needful to mention in passing the varied stimuli by which cerebral changes act upon the organism. The mention of a name will cause a blush, a brightening of the eye, a quickening of the pulse. The thought of her absent infant will cause a flow of milk in the mother’s breast.

175. We may formulate the foregoing considerations in another law: