Popular Science Monthly/Volume 75/September 1909/The Origin of the Nervous System and its Appropriation of Effectors III
|THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS|
PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY
III. Central Nervous Organs
IN dealing with the differentiation of nervous organs, the earthworm affords a good example of a simple type of well-centralized nervous system. The central nervous organs in this animal (Fig. 1) consist of a brain or cerebral ganglion situated anteriorly and dorsal to the buccal cavity, right and left œsophageal connectives extending from the brain ventrally to the ventral nerve-cord which stretches as a segmented organ from near the anterior end of the worm over its ventral line posteriorly to the tail. The segments in the ventral cord agree in
number and position with those of the worm's body and from each segment three pairs of nerves pass out to the integument and muscles of the adjacent region.
The essential nervous elements of the ventral cord can be made out in transverse sections (Fig. 2). In such sections the integument will be seen to be filled with sense-cells, each of which ends peripherally in a sensory bristle and gives rise centrally, in addition to a few sub-epithelial processes, to a single nerve-fiber which passes inward between the muscles and enters the ventral ganglion by one of its three nerves; finally this fiber spreads out in the fibrillar substance or neuropile of the ganglion. This cell-body in the integument with its processes including the nerve-fiber constitutes a primary sensory neurone. These neurones usually do not spread beyond the ganglion with which they are directly connected, but in exceptional cases they may extend into the ganglion anterior or posterior to this one.
In the ventral and lateral portions of each ganglion are numerous large nerve-cells from which coarse processes extend through the pile, fibrillating as they pass to terminate as motor nerve-fibers in the muscles of the adjacent part of the body. These cells with their processes constitute the primary motor neurones of the earthworm and, like the sensory neurones, they may be present in any one of the three nerves of a segment. Their longitudinal extent is probably not much beyond a single segment.
The primary sensory and motor neurones not only give rise to the nerves of the earthworm, but they contribute a larger part of the substance
of each ganglion. As stated in the first article, they form when together the necessary elements for the simplest, conventional reflex-arc. How they are related to one another in the neuropile is not conclusively settled, but, judging from the work of Apáthy (1897) and others, the connection here as in the nervous net is one of direct continuity.
Besides the motor and sensory neurones, the central nervous organs of the earthworm contain a considerable number of so-called association neurones. These are nerve-cells with longer or shorter processes that connect parts within the same ganglion or run from one ganglion to another. They give rise to no fibers that extend into the nerves and hence they are strictly limited to the central nervous organs. Their longitudinal extent is seldom over more than one or two segments.
Since the sensory, motor, and association neurones thus far described make up the bulk of the ventral nerve-cord of the earthworm and since none of these have a longitudinal extent of more than a few segments, it follows that the cord must be conceived as made up of an immense number of overlapping short neurones which in this collective way stretch over its hundred and twenty or more segments. But the nerve cord of the earthworm is not composed exclusively of short neurones. In its dorsal portion are three giant fibers which, though their nature has been even recently disputed, are without much doubt nervous organs. The middle and largest of these fibers extends almost the whole length of the ventral cord and, according to Friedländer (1894), has unquestionable connections with ganglion-cells. The two lateral fibers, though smaller, have much the same extent as the median one and are also directly connected with cells. Both sets of fibers connect by branches with the neuropile of the successive segments. Thus the ventral cord of the earthworm may be described as composed of three long neurones and an immense number of overlapping short neurones.
This peculiarity in the structure of the cord makes itself manifest in the movements of the worm. Undoubtedly the slow waves of muscular activity that move over the worm from head to tail as it creeps along are dependent upon the interlocked short neurones, whereas the
Fig. 8. Transverse Section of the Ventral Nervous Cord of Segalion (modified from Hatschek). bm, basement membrane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropile; s, space occupied by vacuolated supporting tissue.
sudden drawing together of the worm as a whole, when it is vigorously stimulated, is very probably the result of impulses spread through the long neurones.
The absence of degenerated fiber-tracts in the ventral cords of earthworms that have been cut in two and the rapidity with which nervous regeneration takes place in these worms are conditions that very likely depend upon the almost entire formation of the cord from systems of short neurones.
At first sight the central nervous apparatus of the earthworm seems to be widely different from the neuromuscular mechanism of the cœlenterates, but the difference in reality is not so pronounced. To begin with, the whole nervous mechanism of the cœlenterate is within an epithelial layer, whereas the central nervous organs of the earthworm are solid masses of nerve-cells, fibers, and neuropile entirely distinct from any epithelium. But this condition is apparently a recent acquisition on the part of the earthworm, for in another annelid, Sagalion, the ventral cord (Fig. 3) and the brain are still a part of the superficial
Fig. 4. Transverse Section of the Ventral Nervous Cord of an Earthworm, showing the ganglion-cells on the ventral side (v) and the nerve-fibers and neuropile on the dorsal side (d).
ectoderm and differ from the condition in the cœlenterates only in that they represent a concentration of nervous elements in certain regions instead of a diffuse condition as in the sea-anemones, etc. In Nereis the brain is epithelial, but the cord by a process of delamination has broken away from the integument, and in the earthworm the whole central nervous system, brain as well as cord, has delaminated. It is chiefly this concentration and separation of the nervous organs from the skin that justifies, in my opinion, the statement that an earthworm has central nervous organs and a sea-anemone has not.
The fact, however, that the central nervous system of the earthworm has developed on the lines of the cœlenterate, has left its mark in the distribution of nervous materials in the ventral cord of this animal. In the ectoderm of the cœlenterate the cell-bodies of the nervous mechanism are nearer the exterior of the animal than are their processes, the fibrillar mass, and the same is true in the ventral ganglia of the earthworm (Fig. 4); here the cell-bodies are on the ventral side of the ganglion, i. e., next the integumentary epithelium, and the neuropile and nerve-fibers are on the opposite or dorsal side of the ganglion. This peculiarity in the distribution of nervous materials is apparently true for most higher metazoans.
Another point of comparison between the nervous mechanism in cœlenterates and in the earthworm is the presence of nerves in the latter and their absence in the former. As already pointed out, the nerves in the earthworm are bundles of independent fibers which course more or less together between their end-organs and the central apparatus. The fibers in a nerve have no necessary functional relations one with another, but are brought together chiefly by convenience of passage. They are characteristic of those animals in which sense organs and muscles have become well differentiated and widely separated from the central organs, and are not to be confused with elongated bundles of nervous elements such as are to be met with in some cœlenterates and many echinoderms, for though these may represent early steps in the evolution of nerves, they still retain so many evidences of functional interrelation among their elements that they are to be classed rather with nervous nets than with nerves.
The differentiation of nerves as thus defined implies an increased interrelation of neurones in the central apparatus as compared with the condition in the more primitive nervous net. The nature of this growing interrelation has been well expressed by Sherrington (1906) in his principle of the common path. This principle implies that each sense organ may be connected through the central organ with every effector and conversely any effector may receive through the central organ impulses from any sense-organ. In consequence the central organ must contain many common paths which are momentarily used, now for this, now for that combination of particular receptors and effectors. This condition without doubt obtains in earthworms as it does in higher animals, and is a feature that can hardly be said to exist in the nervous nets of the cœlenterates.
It is also probable that the nervous mechanism in cœlenterates differs from that in the earthworm in its capacity as a nervous transmitter. Attention has already been called to the fact that transmission in the nervous net of a cœlenterate may occur in almost any direction and that in the central nervous organs of vertebrates it is very definitely limited and may in fact flow in only one of two apparently possible directions. So definite a restriction can not be asserted for the earthworm but, as Norman (1900) has shown, significant differences do obtain. If an earthworm that is creeping forward over a smooth surface is suddenly cut in two near the middle, the anterior portion will move onward without much disturbance whereas the posterior part will wriggle as though in convulsions. This reaction, which can be repeatedly obtained on even fragments of worms, shows that a single cut involves a stimulation which in a posterior direction gives rise to a wholly different form of response to what it does anteriorly; in other words, transmission in the nerve-cord of the worm is specialized as compared with transmission in the nervous net of the cœlenterate.
There is good reason to believe that the cerebral ganglion or brain of the earthworm is in a measure degenerate. Certainly if we turn to such an annelid as Nereis we find in place of the small mass of ganglionic cells and fibers that represent the brain in the earthworm a much more extensive organ connected with a considerable number of sense organs none of which are present in the earthworm. Eight peristomial tentacles, a pair of palps, a pair of antennæ and, two pairs of eyes are found connected by nerves with the brain of Nereis and represent a condition in strong contrast with the unspecialized state in the earthworm. Yet both the earthworm and Nereis show much the same traits when deprived of their brains (Loeb, 1894). Each worm is immensely reduced in activity somewhat as a jellyfish is after the removal of its sense-bodies, and one is justified in concluding that the head of even the earthworm is an especially sensitive region through which many slight environmental influences that might not be able to affect other parts of the body gain access at this point to the neuromuscular mechanism. That such a condition should obtain at the anterior end of a bilateral animal has long been recognized as appropriate, for this is the part of the animal that in normal locomotion first reaches the new environment. But I am not acquainted with any discussion as to the mutual relations of the nervous parts at the anterior end of an animal so far as their origins are concerned. If what has been said in these lectures is true, namely, that sense-organs in general precede central nervous organs in evolution, then the brain of the worm has developed at its anterior end because the chief sense-organs were originally there, and not vice versa, a statement that I believe to hold for the growth of the brain in all animals. Intricate and marvelous as the brain of the higher animals is, it is, in my opinion, the product of a group of sense-organs that in evolution preceded it in point of time.
The annelids then possess a neuromuscular mechanism in which there are not only primary organs such as muscles, and secondary organs, the sense-organs, but also tertiary organs, central nervous organs. These central organs intervene in position between the receptors and effectors and in the annelids are composed almost exclusively of short overlapping neurones. It is probable that in the sea-anemone these neurones are represented by the so-called ganglion-cells of the nervous layer, but I would not go as far as Havet (1901) and designate these cells in cœlenterates as motor cells, for though some of them undoubtedly connect with the muscle-fibers, others may be purely association neurones. I believe further that in the sea-anemones the fibrils from many sense-cells connect directly with muscle-fibers without the intervention of ganglion-cells.
As an example of a central nervous system built upon the annelid type but with increased complication, we may turn to the arthropods. The central nervous system of these animals, like that of the annelids, consists of a dorsal brain, œsophageal connectives, and a ventral, segmented cord. These organs have been formed by a process of delamination as in the earthworm and exhibit the same fundamental arrangement of cellular elements as is seen in this animal, i. e., the ganglion-cells are on the side of the cord next the exterior, and the neuropile and nerve-fibers next the interior.
The chief fundamental point of difference in the nervous systems of the annelids and arthropods consists in the great number of long neurones in the latter as compared with the former. In the crab, as demonstrated by Bethe (1897), many of the primary sensory neurones extend over half the length of the ventral cord instead of being limited to a few segments as in the earthworm, and the same is true of the primary motor neurones. Moreover, the association neurones have shown an extensive growth. Although in the crab there are some neurones limited to one or two segments, as is the rule in the earthworm, the great majority extend over many segments and often through the whole length of the nervous system. In this way the central nervous organs of these animals are locked together much more closely than are those in the worm and exhibit consequently in their physiology a unity that the worms do not possess. This nervous unity, moreover, has developed to such a degree in the higher arthropods that we may with reason ascribe to such animals as the insects a primitive form of intellectual life not unlike that found in the vertebrates. The structural basis for this seems to me to be foreshadowed in the few long neurones of the worm which, as I have just pointed out, come to be the common type in the arthropods. The type of central nervous system with long neurones also characterizes the other higher invertebrates such as the mollusks, etc.
The central nervous system of the vertebrates and of certain other closely allied forms like the tunicates, is usually put in strong contrast with that of the higher invertebrates. The most striking feature in this contrast is the fact that the vertebrate nervous system is tubular and the invertebrate solid. As is well known, the central nervous organs in vertebrates develop from an ectodermic tube that has been infolded from the median dorsal surface of the animal. This simple nerve-tube with nervous connections, but otherwise almost unmodified, exists to-day in that primitive vertebrate amphioxus. In the higher vertebrates the posterior portion of this tube becomes uniformly thickened and forms the spinal cord, the central canal of which gives evidence of its tubular nature. The anterior portion undergoes still more profound changes than the posterior part in that its wall thickens very differently in different regions and expands in several lobe-like outgrowths, giving rise thus to the brain whose ventricles represent the original cavity of the nerve-tube.
Notwithstanding the striking difference between the central nervous organs of vertebrates and invertebrates, they show certain fundamental similarities and the first of these has to do with the distribution of nervous materials. Since the nerve-tube from which the central nervous organs in vertebrates are developed is infolded ectoderm, it follows that the inner surface of the tube represents a portion of the outer surface of the animal. This inner surface even in the adult central nervous system is always covered by an epithelium as the exterior of the animal is, and the nervous materials which surround it are related to this epithelium in a characteristic way. This relation can be most easily seen in any transverse section of the spinal cord. Beginning at the central canal of such a section (Fig. 5) and proceeding
Fig. 5. Transverse Section of the Spinal Cord of a Vertebrate (Salamander), c, central canal; e, epidermis; g, gray substance composed of ganglion cells and neuropile; w, white substance or nerve-fibers.
through the substance of the cord to the opposite face, one passes first an epithelial layer, then gray substances composed of nerve-cells, neuropile, etc., and finally white substance made up of nerve-fibers. Precisely this sequence is met with in the central nervous system of any primitive invertebrate such as Segalion, where, as already pointed out, in passing through the thickness of the central nervous organ from the exterior to the interior one meets first external epithelium, then ganglion-cells and fibrillæ corresponding to the gray substance of vertebrates, and finally nerve-fibers corresponding to the white substance of these animals. Thus the nervous materials of the vertebrate spinal cord are distributed through that structure on a plan similar to that found in invertebrates, and this plan, though considerably modified, also holds good for the vertebrate brain. So far as these particulars are concerned, the vertebrate central nervous system differs from that of the higher invertebrates chiefly in that in separating from the integument it has carried with it its epithelial mother-tissue instead of leaving this tissue behind.
Not only are the materials of the vertebrate central organs distributed on a plan that is best understood from the standpoint of the invertebrates, but the primary neurones of vertebrates are also most clearly interpreted from this point of view. The primary motor neurones of vertebrates (Fig. 6) resemble very closely those of invertebrates, for their cell-bodies are within the central nervous mass and their neurites extend as motor nerve-fibers to the skeletal muscles. The primary sensory neurones also agree with those of the invertebrates except that their cell-bodies instead of being in or near the integument, as in most invertebrates, have migrated centrally and thus form the dorsal ganglia. At least this appears to have occurred in all vertebrate sensory nerves except the olfactory, which still retains the usual invertebrate condition.
Fig. 6. Diagram of the Primary Neurones of the Vertebrate Nervous System as seen in Transverse Section, c, spinal cord; dg, dorsal ganglion; i, Integument; m, muscle; mn, motor neurone; sn, sensory neurone.
Association neurones, which were met with in the invertebrates, are abundantly present in the vertebrates.
How the neurones in vertebrates are related to one another has been a matter of much dispute. Whether the gray substance of the central organs in these animals contains a true nervous net as seems to be the case in many invertebrates or whether their neurones retain greater individuality and are related morphologically only through contact, is not yet settled. That many embryonic neurones, or neurocytes, are in the beginning widely separated from others with which they are ultimately closely related is true and gives color to the belief that they may never fuse anatomically, though physiologically they do become continuous. The fact that nervous transmission through central organs in adult vertebrates is slow, open to exhaustion, and restricted to one direction as contrasted with transmission through nerve-fibers, is strong physiological evidence of a special central mechanism of interrelation between neurones such as Sherrington (1906) has pictured in the synapse. That no special anatomical condition has thus far been discovered that answers to this physiological requirement can in no sense be taken as an objection to it. That the vertebrate central nervous system is in many of its parts a synaptic organ can not be doubted, but that all its parts are synaptic is not yet proved. Possibly this is a feature characteristic of only the more specialized parts of the vertebrate central organs and entirely absent from the invertebrate, but whether this difference really exists or not must remain for future investigation.
Although it can not be said at present that a synaptic nervous system is the peculiar possession of the vertebrates, there are two important features in which the central organs of these animals differ from those of the invertebrates. In the first place, the central organs of vertebrates exhibit a largeof long neurones over short ones, and in the second place, they show an enormous increase in the number of association neurones. In an earthworm there are only three long neurones and the rest are short ones; in a crab the long and short neurones are perhaps about equally abundant; but in a vertebrate the long neurones certainly far outnumber the short ones. In any transverse section of the spinal cord of one of the higher animals almost all of the white substance in view excepting a thin layer surrounding the ventral horn is made up of systems of long neurones. In this respect the condition in the vertebrates seems to be almost the reverse of that in worms and in consequence transection of their central nervous organs results in profound and extensive degeneration such as is never met with in animals like worms. For this reason the central nervous system of the vertebrate, though giving much evidence of segmentation in its early stages of growth, is finally a physiological unit such as is realized in no other group of animals, a condition well evidenced by the fact that some of its most recent phylogenetic acquisitions, like the pyramidal tracts of the mammals, may consist of neurones that reach almost from one end of the system to the other.
The second feature that distinguishes the central nervous organs of vertebrates from those of invertebrates is the enormous development of association neurones. These neurones are present in worms, are numerous in arthropods, but are overwhelmingly abundant in vertebrates. Of the white substance seen in the transverse section of the spinal cord almost all except the dorsal columns represent association neurones. Judged from this standpoint there are certainly many more association neurones in the cord than all other kinds taken together. But the association neurones are not only the most numerous in the vertebrates; they also constitute the basis of the most significant evolution. The central nervous organs that show the most conspicuous progressive changes in the vertebrates are the cerebellum and the cerebrum, particularly their cortical portions, and when it is remembered that few or no primary sensory or motor neurones contribute to these two organs, but that they are made up of association neurones almost exclusively, it will be seen how enormously important these neurones become. The association neurones in the vertebrates are not only the organs of intricate nervous exchange, but in the region of the cerebral cortex they afford the material basis of the intellectual life. Thus in the vertebrates the primary sensory and motor neurones in number and importance are outstripped by the association neurones.
As thus sketched the development of the adjuster or central nervous element of the neuromuscular mechanism takes place in the region between the receptors and the effectors and in time after these two sets of organs have appeared. Its primary function is undoubtedly transmission involving the principle of the common path; secondarily it comes to be a repository of the effects of nervous stimulation whereby its principal function as a modifier of impulses is made possible.
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