Popular Science Monthly/Volume 84/February 1914/The Origin and Evolution of the Nervous System
|THE ORIGIN AND EVOLUTION OF THE NERVOUS SYSTEM|
TO the ancients what we designate as personality was a more or less general attribute of the human body rather than an aggregate of functions having a strictly nervous source. In fact Aristotle, who was such an accurate observer and profound thinker in so many fields of biology, denied positively that the brain was in any direct way concerned with sensation and declared the heart to be the sensorium commune for the whole body. To Galen is ascribed the belief that the brain was the seat of the rational soul, the heart the location of courage and fear, and the liver that of love. This distribution of the element of personality over the physical body finds its expression in the common speech of to-day, particularly in relation to the heart, which is widely accepted by the popular mind as the source of the more tender emotions. It was chiefly through the anatomists and physiologists of the early Renaissance that the modern movement, which has tended to limit personality to the nervous system, was seriously begun, a movement which, with the increase of knowledge, has gained support to such an extent that it can now be maintained beyond any reasonable doubt. Human personality is in no true sense the outcome of the non-nervous organs, such as the digestive or the circulatory organs, but is the direct product of the nervous system. This system, to be sure, is embedded among the other organs of the body and the environment thus provided influences profoundly its condition and action, but what is meant by individual personality, acuteness or dullness of sense, quickness or slowness of action, temperamental traits, such as a gloomy or bright disposition, incapacity, shiftlessness, honesty, thriftiness or sweetness, are all, strictly speaking, functions of the nervous organs. Although only the higher animals can be said to possess personality in this sense, traces of it occur in the lower forms and its evolution is indissolubly connected with that of the nervous system. It is the object of this paper to trace in broad outlines the development of those organs which in the higher animals come to be the seat of personality.
The nervous organs of the higher animals, including man, consist of enormously intricate systems of interwoven nerve cells or neurones whose unique character was first fully grasped some twenty years ago by Waldeyer. These neurones, like other cells, possess a nucleated cell-body, the ganglion cell of the older neurologists, from which extremely attenuated processes, the nerve fibers, reach out to the most distant parts of the animal. These processes are the most characteristic parts of the neurone. Extending as they do in the largest animals for some meters from their cell bodies, they afford an example of a cell process such as is seen in no other histological unit. Not only are the nerve cells or neurones thus highly specialized in their structure, but they also exhibit profound physiological differentiation. Thus among the primary sensory neurones each one is connected, as a rule, with a particular portion of the animal for which no other neurone is responsible, and among the motor neurones each one controls a group of muscle fibers not called into action by any other neurone. Hence functional specialization among these elements has come to be so extreme that the nervous system may be described as one in which differentiation has reached to its very cells, a condition that is shown in no other elements of the body except possibly in the reproductive cells.
Notwithstanding the high degree of differentiation exhibited by the neurones of the higher animals, these elements may be easily grouped into relatively few classes distinguishable through their connections. These classes are three in number: first, the afferent, or as they are commonly called, the sensory neurones extending in general from the surface of the animal to the central organs and transmitting sensory impulses; secondly, the efferent neurones connecting the central organs with the muscles, glands, etc., and transmitting efferent impulses; and finally, what may be called the association neurones, to extend to the whole nervous system, a term used by Flechsig for elements in a limited part of the brain, or those neurones which lie entirely within the central organ and connect one part of this organ with another. Although the nervous organs of the higher animals are composed of an abundance of all three classes of neurones, the association neurones in all probability far outnumber those of the other two classes and constitute the chief mass of these organs.
Almost all nervous operations in the higher animals involve all three classes of neurones. The typical nervous reaction of these animals consists of a sensory stimulation followed by a motor response. This operation has been called a reflex, to use that term in its widest sense, that is, irrespective of the association of the action with voluntary or conscious operations. Such a reflex takes place over an arc of neurones, the sensory members transmitting to the association elements, and these in turn to the motor elements, but in describing the reflex its parts are not conveniently dealt with from the standpoint of the neurone. The reflex, as ordinarily understood, begins with the activity of a sense organ or receptor, from which a sensory impulse passes to the central nervous system or adjustor, whence the nervous disturbance makes its way to the third element or effector, usually a muscle. The sense organs or receptors are, for the most part, the distal ends of sensory neurones. The central organs or adjusters include the proximal ends of these elements, all the association neurones, and the proximal ends of the efferent neurones. The effectors are not neurones at all, but muscle fibers, gland cells or other types of cells under the control of nerves. Thus the ordinary reflex may be said to involve in sequence the activity of a receptor, adjustor and an effector, to use modern terminology, and these three elements are recognizable in every complete reflex arc.
Our own reflexes are sometimes associated with consciousness and sometimes not. When we pass from a region of dim light to one of bright light the pupils of our eyes contract without our being conscious of the fact. In a similar way, when food is introduced into the digestive tract, a whole succession of reflex movements is called forth without any direct relation to our consciousness. On the other hand, if we burn a finger, it is usually withdrawn with full recognition of the sensation and the response. Thus a reflex may or may not be association with a conscious state.
From this standpoint, what is the condition in the lower animals? Have they nervous systems composed of neurones and exhibiting reflexes which in some instances are associated with consciousness, and in others not? In other words, what have been the steps by which has developed that mechanism which serves us at once as the means of our simplest reflexes and the material basis for our intellectual life?
As an example of the lower animals whose nervous activities are worthy of consideration we may take the earthworm. This animal has at its anterior end a small brain from which a ventral ganglionic chain extends posteriorly through the rest of its body. It possesses sensory neurones which extend from the skin into the central nervous organ and motor neurones reaching from the central organs to the muscles. The central organ itself contains association neurones. Thus the three classes of nervous cells which occur in man are also represented in the earthworm but with this difference. The association neurones, which in man are relatively very numerous, are in the earthworm comparatively few. Otherwise the essential composition of the nervous organs in these two forms has much in common.
Not only is the nervous system of the earthworm composed of elements essentially similar to those of the higher animals, but it exhibits similar functional relations. The earthworm responds to a large range of stimuli by appropriate and characteristic reactions, and its movements justify the conclusion that its reflex arcs, like those of the higher animals, involve receptors, an adjustor, and effectors.
Whether certain of the reflexes of the earthworm are associated with consciousness or not is a question that can not be answered definitely, since no absolute criterion for consciousness in any organism other than one's self can be given. Earthworms, however, apparently possess some capacity to profit by experience. Within the past year Yerkes has reported on the training of an earthworm which in a surprisingly short time acquired the habit of escaping successfully from a very simple maze. These results, should they prove true for other individuals, suggest a certain degree of consciousness in these creatures as a basis of their ability to learn. It is, therefore, not impossible that certain of the reflexes of earthworms may be associated with conscious states, even though these states may be of a very low order.
But, though the reflexes of the lower animals show some features that suggest consciousness, it is not probable that this state is anything like as characteristic of these simple forms as of the more complex ones. Certainly some of the performances of these more primitive beings have every mark of the unconscious reflexes of our own bodies. Thus bees that have been artificially hatched and have never seen the colony at work make as perfect comb as though they had learned the art by having been co-workers in an established hive. Such bees, moreover, will not only build comb such as they themselves were hatched from, but will shape a queen cell, a form with which they have had absolutely not the least acquaintance in the past. Thus the very complex operation of comb-building in the bee resembles our own unconscious inborn reflexes, such as the constriction of the pupil and the movements of the digestive tube, rather than our voluntary operations, and this is probably true of many of the activities of the lower animals. In fact, it seems fair to conclude that, though such animals as the insects, crabs, and even the worms, possess a nervous system composed of elements similar to those in the higher forms, their reflexes are much more mechanical and less associated with anything that can be called a conscious state than are those of the higher forms. In other words, these lower animals are more in the nature of reflex machines than are the higher forms, though they are not, as some investigators would have us believe, exclusively so.
But if the nervous system in many of the lower animals is composed of elements similar to those in the higher forms, and exhibits activities not unlike our own, are there not still more primitive animals in which this system shows a real reduction and exhibits a condition which marks the actual beginnings of nervous organization? Such primitive forms have long been supposed to exist among the cœlenterates and are well represented by the sea-anemones.
Sea-anemones are sack-like animals with a single opening leading into the digestive cavity and serving both as mouth and anus. This opening is usually surrounded by a cluster of tentacles. The living body of the sea-anemone consists of the thin membranous wall that separates the digestive cavity from the outer sea-water, and that is drawn out in processes to form the hollow tentacles. In no part of its structure is the sea-anemone massive, as is the case in most higher forms, where the muscles, skeleton and so forth usually give rise to a considerable thickness of tissue; in fact, the animal exhibits no welldefined organs except the digestive organs, and may be described as a membranous digestive sac.
Although the body of the sea-anemone is really nothing more than membranous walls, these walls have long been known to contain both nerve and muscle. These two tissues occur over almost the whole animal. According to the Hertwigs, the nervous tissue is more abundant in the neighborhood of the mouth than elsewhere, and this region has been regarded by some investigators as a central nervous organ. But the studies of Jordan and others have shown conclusively that this opinion is not correct, and that the removal of this region interferes in no serious way with the reactions of the animal. Apparently each part of the sea-anemone carries with it its own neuromuscular mechanism, a condition well illustrated by the tentacles. These organs are chiefly concerned with appropriating the food and are stimulated by the dissolved materials in the food. A tentacle when cut off from a sea-anemone and held in sea-water can still be stimulated by food and will exhibit almost exactly the same kind of movements when thus isolated that it did when a part of the whole animal, thus demonstrating the completeness and independence of its own neuromuscular mechanism. Nervous transmission can be accomplished from almost any part of the sea-anemone to almost any other part, but as such experiments as those with the tentacles indicate, no one part of the animal's nervous organization seems to be more important than any other part. In other words, the nervous system in the sea-anemone is diffuse rather than centralized.
When the minute organization of the nervous system of these animals is studied, it is found to consist of a vast number of sensory neurones which connect the surface of the animal with the underlying muscles and which form there what appears to be an intricate nervous network. This nervous mechanism is concerned primarily with the reception of stimuli and the immediate excitation of the muscles. The nervous mechanism is a receptor mechanism that acts as a trigger for setting off the muscle. The whole neuromuscular apparatus seems to be made up of those two elements which in the higher animals were designated receptors and effectors and without the intervention of an adjustor or central nervous organ. Viewed from the standpoint of development, this condition points indubitably to the conclusion that the central nervous organs were evolved only after the appearance of sense organs and muscle, and that such animals as the sea-anemone may well be taken to represent this step in the evolution of the nervous system. This general view of the origin of the central nervous organs was advanced as early as 1886 by Kleinenberg and was reaffirmed ten years later by Rakowitza.
The evolution of nerve and muscle, so far as this problem can be attacked in such lowly form as the sea-anemone and other cœlenterates, is a question about which there has been much difference of opinion. As early as 1872 Kleinenberg showed that in the fresh-water coelenterate, Hydra, there were certain peculiar T-shaped cells that he called neuromuscular cells and that he believed to represent both nerve and muscle. In these cells the arm of the T reached the surface of the animal and was thought by Kleinenberg to act as a nervous receptor; the cross-piece being contractile was known to be muscle. Kleinenberg assumed that the division of such cells and the differentiation of their parts were the processes which gave rise to the nervous and muscular tissues of the higher animal. In 1879 the Hertwigs in their account of the structure of sea-anemones showed that the so-called neuromuscular cells of Kleinenberg were in reality simply epithelio-muscle cells and were without nervous significance. These investigators, in opposition to Kleinenberg, advanced the view that nerve and muscle, though simultaneously differentiated, were derived from different groups of cells. According to both Kleinenberg and the Hertwigs nerve and muscle were simultaneously evolved, but Kleinenberg maintained that these tissues came from a single form of cell, the Hertwigs that they arose from separate kinds of cells.
My own studies on the origin of nerve and muscle have led to rather different conclusions from those summarized in the last paragraph. In studying the reactions of one of our common sponges, Stylotella, I was impressed with the extreme slowness with which the animal responded to a stimulus. The oscula of this sponge can be made to close by the application of several kinds of stimuli. The closure of these openings is accomplished by the contraction of the ring of muscular tissue surrounding them. This response occurs some minutes after the stimulus has been applied, a condition in strong contrast with the quick reactions of such animals as sea-anemones. These forms respond to most stimuli within a second or so, the sponges only within minutes. Moreover, in sponges transmission from the place where the stimulus is applied to the responding muscle is possible only over very short distances and is carried on at a very slow rate. Transmission in Stylotella resembles very closely the kind of transmission seen in ciliated epithelium. The successive beat of the cilia is dependent upon an impulse which progresses from cell to cell in the epithelium at a relatively slow rate and is neither purely mechanical nor nervous in its method of propagation. It probably represents a primitive form of protoplasmic transmission, a forerunner of the true nervous impulse, and as such gives us some insight into the nature of the non-nervous transmission in sponges. The results of my studies on Stylotella support the conclusions of most biologists who have worked upon sponges, that these animals probably possess no true nervous tissue. Their muscles, in my opinion, are brought into action almost entirely by the direct effect of the stimulus rather than through nerves, and this accounts, I believe, for their very slow response to external disturbances. It is possible that in certain sponges some form of nervous tissue may be demonstrated eventually, or that such organs as those described by von Lendenfeldt as synocils may be shown to have a sensory significance, but such cases, if they do occur, will probably remain exceptional, for as a whole sponges seem to be a group of animals almost if not quite devoid of true nervous tissue. Granting this conclusion, it must be evident that the condition in sponges throws a very important light on the question of the origin of nerve and muscle. Their state suggests at once that nerve and muscle have not been differentiated simultaneously, as maintained by Kleinenberg, the Hertwigs, and others, but that muscle preceded nerve in its evolution and that sponges represent animals with effectors but without differentiated receptors. If then it may be claimed that phylogenetically the sense organ preceded the central nervous organ, it may also be maintained that muscles preceded sense organs. Thus the three elements of the reflex arc of the higher animals were probably evolved separately and in the order, effector, receptor, adjustor.
If muscle originated before nerve and was brought into action at first by direct stimulation, it is natural to expect that examples of this form of response might still be found among the higher animals. And such seems to be the case. Thus the sphincter of the iris in the lower vertebrates, though well known to be under the influence of nerves, was shown by Steinach some time ago to be directly stimulated by light, a condition which, judging from the more recent work of Hertel, probably applies even to the human eye. This muscle then exhibits a certain capacity for normal direct stimulation. Another example of the same kind is seen in the embryonic vertebrate heart. Though the beat of the adult heart may be a matter of controversy from the standpoint of the myogenic and neurogenic theories, there can be no doubt that the muscle of the embryonic heart beats, as shown by His, before it has become invaded by nerves. And this view is supported by Barrow's recent discovery that the isolated cells of the heart-muscle will contract rhythmically under conditions where not the least vestige of a nerve can influence them. Thus the embryonic heart-muscle and the sphincter of the iris are muscles whose activity may be normally called forth by direct stimulation, a condition which reproduces, so far as independence is concerned, the state met with in the muscles of the sponges. These examples then show that even in the higher animals certain muscles respond normally to direct stimulation and thus exhibit a form of activity which is believed to be generally characteristic of sponges.
In my opinion the simultaneous origin of nerve and muscle can no longer be maintained. Muscle arose first and the simple effectors thus produced were the first element of the neuromuscular mechanism. These effectors were directly stimulated and consequently slow in action. They afforded centers around which nervous tissue first differentiated in the form of sense organs or receptors whose function it was to serve as triggers to initiate muscle action quickly. As these receptors became more highly developed, a third element, the central nervous organ, arose from the nervous elements between the receptor and the effector. This organ, the adjuster, served as a means of conducting and modifying the sensory impulses on their way from the receptor to the effector and ultimately it also served as a storehouse for the nervous experience of the individual and as the seat of its intellectual life. It is interesting to observe that this view of the origin of the nervous system is in accord with the philosophical speculations of Bergson according to whom the nervous system has been evolved primarily as an organ for animal response and only secondarily as one concerned with intellectual activities.
But if we picture the nervous system as having arisen as an appendage to the musculature and as having grown in complication as the musculature became differentiated, we are still far from an adequate view of even the more obvious aspects of its evolution. The nervous system controls many more kinds of effectors than muscle and its sensory elements are vastly more complex than is implied in the preceding sketch. To gain a more comprehensive view of the evolution of these organs, it is necessary to consider a subsidiary but important process, the appropriation of effectors and receptors.
The nervous system of many of the higher animals not only acts upon the musculature; it may also control electric organs, luminous organs, chromatophores, glands, etc. Not all such organs are under the influence of the nervous system, but it is not difficult to find for each group of effectors animals in which the given type of organs is under the influence of nerves. The electric organs and the chromatophores of fishes, are of this kind as well as the salivary glands of the mammals and the luminous organs of the brittle stars.
How has the nervous system gained control over these organs? Except the electric organs, which are probably modified muscle, all these organs have arisen in my opinion as independent effectors. Most of them can be identified as such in one group of animals or another. Thus among the glands the pancreas in the higher vertebrates has been shown to be in its action essentially non-nervous. Such highly differentiated, but independent effectors have, I believe, been appropriated from time to time by the nervous system in that during ontogeny certain motor fibers, instead of becoming attached to their appropriate muscles, have wandered to new effectors which have been sufficiently responsive to their stimuli to give a basis for a permanent attachment. Thus the nervous system, once established around muscles, has widened its influence in that it has appropriated other types of independent effectors, which upon application were found to be responsive to its stimulus.
But the differentiated nervous system has not only extended itself on the side of its effectors, it has probably also made receptor appropriations. This is well illustrated by several groups of related sense organs such as the organs of touch and hearing in the vertebrates or those of the chemical senses in the same animals. The latter may serve as an example.
The chemical sense organs in vertebrates include not only those of smell and of taste, but also the organs of the common chemical sense such as are concerned with the chemical irritability of the skin of the frog or of the exposed or semi-exposed mucous surfaces of man. All these chemical receptors are stimulated by solutions. In taste the stimuli are the dissolved materials in the food; in smell they are the solutions formed on the moist olfactory surface from the materials wafted in the air to the nose.
The neurones concerned with the reception of these stimuli exhibit interesting relations. The olfactory neurones, as is well known, have their cell bodies in the olfactory epithelium, whence their neurites extend into the central olfactory apparatus. They reproduce in a most striking way the type of primary sensory neurone common to the invertebrates, and in this respect they represent the most primitive type of sensory neurone in the body of vertebrates. The neurones concerned with the common chemical sense are like those of the olfactory sense except that their cell bodies have migrated centrally and constitute a part of one of the cerebro-spinal ganglia. As a result the distal ends of these neurones are represented as free-nerve terminations in the epithelium of the moist parts of the vertebrate skin. The gustatory neurones reproduce almost exactly the condition of those of the common chemical sense, except that their distal free terminations are around taste buds instead of being in an ordinary epithelium.
The conditions shown by these three types of receptor mechanisms suggest at once a genetic connection. The olfactory type is undoubtedly the most primitive, and stimulation in this instance is initiated by the chemical action of the superimposed solution on the hairs of the olfactory cells. The neurone for the common chemical sense has probably been derived from one of the olfactory type by a proximal migration of its cell body. The stimulation of its free-nerve terminals may be conceived to take place, as Botezat has recently pointed out, through the secretory activity of the surrounding epithelial cells as a result of their contact with the stimulating solution, rather than from the direct action of this solution on the nerve endings themselves. From this standpoint the epithelium comes to be an essential element in the stimulation of the neurone and affords, so to speak, a favorable sensory environment for the real nerve-endings. Finally, the gustatory neurones may be said to have appropriated certain of these epithelial cells which have become differentiated into taste buds and whose activity, probably secretory in character, to follow Botezat, is called forth by the superimposed solution and is essential to the stimulation of the nerve endings. Thus in the evolution of the chemical sense organs of vertebrates certain integumentary cells originally quite independent of the receptors came to be involved with these and were eventually appropriated by them as essential parts of the gustatory apparatus. This process of appropriation is not unlike that seen among the effectors and represents one of the important steps by which the nervous system in the course of its evolution has added to its complexity. Although the nervous system probably arose in a scattered way at spots where the primitive multicellular animal had developed muscle, it became unified through the need for general transmission tracts, and, by increasing its own elements as well as by appropriating additional effectors and receptors, it has impressed upon the higher animals, including ourselves, a unity so profound that it includes everything that we mean by personality.