By means of the nervous system, an intimate relation is maintained between mind and body, for nervous energy superintends the functions of both. The fibres of nervous matter are universally present in the organization, uniting the physical and spiritual elements of man's being. Even the minutest nerve-rootlets convey impressions to the dome of thought and influence the intellectual faculties. We recognize muscular force, the strength of the body, molecular force, molecules in motion, as heat, light, chemical force, electricity, and nervous force, a certain influence which reacts between the animal functions and the cerebrum, thus connecting the conditions of the body with those of the mind. We cannot speak of the effects of mind or body separately, but we must consider their action and reaction upon each other, for they are always associated. There are many difficulties in understanding this relationship, some of which may be obviated by a study of the development of nervous matter, and its functions in the lower orders of organization.
Within the plant-cells is found a vital, vegetable substance termed bioplasm, or protoplasm; which furnishes the same nutritive power as the tissues of the polyp and jelly fish. Many families of animals have pulpy bodies, and slight instinctive motion and sensibility, and in proportion as the nervous system is developed, both of these powers are unfolded. Plants have a low degree of sensibility, limited motion, respiratory and circulatory organs. Animals possess quicker perceptions and sensibilities, the power of voluntary motion, and, likewise a rudimental nervous system. Some articulates have no bony skeleton, their muscles being attached to the skin which constitutes a soft contracting envelope. One of the simplest forms of animal life in which a nervous system is found, is the five-rayed star-fish. In each ray there are filaments which connect with similar nerve-filaments from other rays, and form a circle around the digestive cavity. It probably has no conscious perception, and its movements do not necessarily indicate sensation or volition. In some worms a rudimentary nervous system is sparingly distributed to the cavities of the thorax and abdomen, and, as in the star-fish, the largest nerve-filament is found around the esophagus, presiding over nutrition.
A higher grade of organization requires a more complete arrangement of nervous substance. Stimulus applied to one organ is readily communicated to, and excites activity in another.
The nervous system of some insects consists of two long, white cords, which run longitudinally through the abdomen, and are dilated at intervals into knots, consisting of collections of nerve-cells, called ganglia. They are really nerve-centers, which receive and transmit impulses, originate and impart nervous influence according to the nature of their organic surroundings. The ganglia situated over the esophagus of insects correspond to the medulla oblongata in man, in which originate the spinal accessory, glosso-pharyngeal, and pneumogastric nerves. The latter possess double endowments, and not only participate in the operations of deglutition, digestion, circulation, and respiration, but are also nerves of sensation and instinctive motion. The suspension of respiration produces suffocation. In insects, these ganglia are scarcely any larger than those distributed within the abdomen, with which they connect by means of minute, nervous filaments. Insects are nimble in their movements, and manifest instinct, corresponding to the perfection of their muscular and nervous systems. When we ascend to vertebrates, those animals having a backbone, the amount of the nervous substance is greater, the organic functions are more complex, and the actions begin to display intelligence.
Man possesses not only a complete sympathetic system, the rudiments of which are found in worms and insects, and a complete spinal system, less perfectly displayed in fishes, birds, and quadrupeds, but, superadded to all these is a magnificent cerebrum, and, as we have seen, all parts of the body are connected by the nervous system. The subtle play of sensory and motor impulses, of sentient and spiritual forces, indicates a perfection of nervous endowments nowhere paralleled, and barely approached by inferior animals. This meager reference to brainless animals, whoso knots of ganglia throughout their bodies act automatically as little brains, shows that instinct arises simultaneously with the development of the functions over which it presides. Here begins rudimentary, unreasoning intelligence. It originates within the body as an inward, vital impulse, is manifested in an undeviating manner, and therefore displays no intention or discretion. While Dr. Carpenter likens the human organism "to a keyed instrument, from which any music it is capable of producing can be called forth at the will of the performer," he compares "a bee or any other insect to a barrel organ, which plays with the greatest exactness a certain number of tunes that are set upon it, but can do nothing else." Instinct cannot learn from experience, or improve by practice; but it seems to be the prophetic germ of a higher intelligence. It is nearly as difficult to draw the dividing line between instinct and a low grade of intelligence, as it is to distinguish between the psychical and psychological[4] functions of the brain.
The intimate relation of instinct to intelligence is admirably illustrated in the working honey-bee. With forethought it selects a habitation, constructs comb, collects honey, provides a cell for the ova, covers the chrysalis, for which it deposits special nourishment, and is disposed to defend its possessions. It is a social insect, lives in colonies, chastises trespassers, fights its enemies, and defends its home. It manifests a degree of intelligence, but its sagacity is instinctive. Reason, though not so acute as instinct, becomes, by education, discerning and keenly penetrative, and reveals the very secrets of profound thought. We recall the aptness of Prof. Agassiz's remark: "There is even a certain antagonism between instinct and intelligence, so that instinct loses its force and peculiar characteristics, whenever intelligence becomes developed." Animals having larger reasoning powers manifest less instinct, and some, as the leopard, exercise both in a limited degree. This double endowment with instinct and low reasoning intelligence, is indicated by his lying in ambush awaiting his prey, the hiding-place being selected near the haunt of other animals, where nature offers some allurement to gratify the appetite.
Simple reflex action is an instinctive expression, manifesting an intuitive perception, almost intelligent, as shown by the contraction of the stomach upon the food, simply because it impinges upon the inner coats, and thus excites them to action. A better illustration, because it displays sympathy, is when the skin, disabled by cold, cannot act, and its duties are largely performed by the kidneys. Though reflex action is easily traced in the lower organic processes, some writers have placed it on a level with rational deliberation. Undoubtedly, all animals having perception have also what perception implies—consciousness—and this indicates the possession, in some degree, of reason. Compound reflex action extends into the domain of thought. Simple reflex action, or instinct, answers to the animal faculties, such as acquisitiveness, secretiveness, selfishness, reproductiveness, etc., and accomplishes two important purposes; self-preservation and the reproduction of the specie. With many persons, these appear to be the chief ends of life!
The psychical functions connect, not only with animal propensities, but also with the highest psychological faculties. Instinct is the representative of animal conditions, just as the highest spiritual faculties are indicative of qualities and principles. The consistent mean of conduct is an equilibrium between these ultimate tendencies of our being. The psychological functions render the animal nature subservient to the rule of purity and holiness, and deeply influence it by the essential elements of spiritual existence. The psychical organs sustain an intermediate relation, receiving the impressions of the bodily propensities, and, likewise, of the highest emotions. Obviously, these extreme influences, the one growing out of animal conditions, the other, the result of spiritual relations, pass into the psychical medium and are refracted by it, or made equivalent to one force. The body requires the qualifying influences of mind. The tendencies of the animal faculties are selfish and limiting, those of the emotive, general, universal. The propensities, like gravity, expend their force upon matter; the emotions pour forth torrents of feeling, and produce rhapsodies of sentiment. The propensities naturally restrict their expression to a specific object of sense; the emotions respond to immaterial being. The tendencies of the former are acquisitive, selfish, gratifying; of the latter, bestowing, expanding, diffusing. The one class is restricted to the orbits of time and matter, the other flows on through the limitless cycles of infinity and immortality. The former is satiated in animal gratification, the latter in spiritual beatification. The one culminates in animal enjoyment, the other expands to its ultimate conceptions in the perfections of Divine Love.