Popular Science Monthly/Volume 75/October 1909/The Origin of the Nervous System and its Appropriation of Effectors IV
|THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS|
By G. H. PARKER
PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY
IV. The Appropriation of Effectors
IN the preceding articles in this series the origin and development of the neuromuscular mechanism has been broadly sketched in a succession of representative stages. The first stage was that of the independent effector, the muscle which was brought into action by the direct influence of environmental changes as seen in the pore sphincters of sponges. The second stage was that of the combined receptor and effector in which the receptors, in the form of diffuse sensory epithelia or specialized sense-organs, served as delicate triggers to set the muscles in action and thereby render the effectors responsive to a wider range of stimuli than they would be under independent stimulation. Finally, the third stage is seen in the complete neuromuscular mechanism in which a central nervous organ or adjustor has developed between the receptors and the effectors. This adjustor serves as a switchboard for nervous transmission and a repository for the effects of nervous activities.
This line of progressive differentiation from the muscle to the complete nervous system is complicated by the fact that in the more complete examples of the third stage the nervous system is found connected not only with such effectors as muscles, but with electric organs, chromatophores, glands, luminous organs, etc. If the history of the growth of the neuromuscular mechanism as it has been sketched in these articles is a correct one, the effectors just named must be regarded in the light of relatively recent acquisitions and in my opinion they illustrate an invasion and appropriation on the part of the nervous system of territory that was not originally under its control. This principle of appropriation results not only in the acquisition of totally new forms of effectors such as glands, etc., but also in gaining control over independently and newly developed muscles. Examples of this kind will be taken up first in discussing this question of nervous appropriation.
The differentiation of the central nervous organs is in large part a process that goes on hand in hand with the differentiation of the muscles. This is well seen not only in the higher invertebrates, but also in the vertebrates. The differentiation of a single muscle into a group of muscles and the consequent and corresponding changes in the nervous relations, both central and peripheral, are too well known to require comment. To this process must be added, I believe, the appropriation of totally new muscles. There is good reason to assume that the heartbeat in tunicates is of myogenic origin and the fact that the embryonic vertebrate heart pulses before it contains any nervous elements is strong evidence in favor of the view that the cardiac muscle of the primitive vertebrate was a muscle developed independently of nervous control. That that muscle in modern adult vertebrates is under a certain amount of nervous control is unquestionable, but this control is not of the kind usually seen in other neuromuscular combinations. The nerves that enter the heart are probably not ordinarily directly concerned with its beat, for, as already pointed out, this continues after they are cut. The function of these nerves seems to be that of modifying this beat and in this respect two classes of fibers may be. distinguished: augmentors which increase the beat, and inhibitors which retard or even check it. This whole nervous mechanism has the appearance of having been superimposed upon a muscle that was originally non-nervously active, and I therefore regard the vertebrate heart as an example of an originally independent muscle secondarily brought under the influence of central nervous organs. Many other muscles, like the sphincter pupillæ, etc., have doubtless had a like history, but as they have not been investigated from this standpoint, the question of their exact relations to nervous control must remain for the present somewhat open.
In the vertebrates at least, nervous effectors include not only muscles, but also electric organs. These organs occur not infrequently among the fishes. They are best represented in the South American electric eel, the electric catfish of Africa and the torpedoes of the Mediterranean Sea and the Atlantic and Indian Oceans. They also occur less fully developed in certain skates, mormyres and the star-gazer. These organs are usually imbedded in a mass of the fish's muscle or they occupy such positions that they clearly replace muscles. Their histogenesis, as worked out particularly in the skates by Ewart (1888), shows conclusively that each electric plate is a modified muscle-fiber and in fact there seems to be good reason to conclude that all known electric organs, excepting possibly those of the electric catfish, are modified muscles. This is entirely consistent with what is known of the physiology of these two kinds of effectors, for muscles not only move parts, but generate through their activity a certain amount of electricity, while the electric organs have lost the power of producing molar movements and have enormously increased that of producing electricity. Electric organs, though often described as a special class of effectors, are in reality merely modified muscles and therefore can not be regarded properly as a new appropriation of the nervous system. The chromatophores, on the other hand, are effectors which are in no sense derived from muscles. These organs enable many animals to make relatively sudden changes in their external coloration, and though they are present in many animals, they are most perfectly developed in the arthropods, mollusks and vertebrates. They are also present in the more complex types of eyes, where their movements serve to protect the receptive elements from exposure to excessive light or to open them to the full effects of dim light. The investigation of these organs dates from comparatively recent times and van Rynberk (1906), who has recently summarized our information about them, has shown that the accounts already given are in many respects contradictory. Hence what I shall have to say I shall draw mostly from those fields with which I am somewhat acquainted at first hand.
That some chromatophores are completely independent of nervous control even though they are most intimately associated with nervous mechanisms is well attested. The deeper part of the compound eye in the shrimp, Palæmonetes, contains a layer of cells, the retinular cells (Fig. 1), which though they carry rhabdomes and end proximally in nerve-fibers and are therefore unquestionably sensory cells, contain many dark pigment-granules which change positions in accordance with the illumination. From this standpoint these cells are true chromatophores. In an eye exposed to the light the pigment-granules occupy distal positions in these cells; in one in the dark they come to lie in proximal positions. The place occupied by the pigment in a given eye is entirely determined by the presence or absence of light in that eye, for the two eyes have no sympathetic relations. Moreover if a persistent shadow is cast on part of one eye, the condition characteristic for the dark is assumed by that part even though the pigment in the rest of the eye is in the position characteristic for light. These observations show the physiological independence of the chromatophores in different parts of the eye. These organs, though connected by nerve-fibers with the central nervous organs, are also in their action independent of such parts, for the movements of their pigment from the dark to the light position and the reverse go on in an essentially normal way even after these connections have been cut. Chromatophores then may carry out under direct stimulation somewhat complicated pigment-migrations in intimate relations to the successful action of such an organ as an eye, and yet with complete independence of central-nervous control. Other chromatophores, like those in the skin of lizards, can be as clearly demonstrated to be under the control of nerves as those in the eyes of Palæmonetes have been shown to be free from this control. The integumentary color changes in lizards are often extremely complicated processes, especially in such forms as the chameleon, but they include as a fundamental principle the inward and outward migration of dark pigment-granules within certain large unicellular chromatophores (Fig. 2). When these pigment-granules pass out into the processes of the chromatophores, they give to the surface of the lizard a dark or even black aspect. When they migrate inward to the body of the chromatophore,
Fig. 2. Two Chromatophores from the Skin of a Lizard, showing the condition due to the dark (A) and to the light (B). c, chromatophore; d, derma; e, epidermis; g, irregular masses of ground color.
which is often hidden in pigment masses of some particular color, they thus allow the ground-color behind them to assert itself. By this simple inward and outward migration of the pigment, the chief change in the color differences of the lizard's skin is accomplished. The question that we have to consider is to what extent these changes are controlled by the central nervous organs.
The inward and outward migration of the pigment of the chromatophores is well seen in the skin of the so-called Florida chameleon, Anolis. According to Carlton (1903), who has studied this animal with care, the passive state in its chromatophores is that in which their pigment is gathered together in the cell-bodies. This state is brought about when the lizard is removed from the stimulating effect of light, when the blood and nerve supply of a given region are cut off, when the animal is etherized, or when it dies. In fact any change that might be expected to interfere with nervous activity calls forth this condition. Since nicotine is a poison for the sympathetic nervous system, rendering it temporarily inactive, and since the inward migration of the chromatophoral pigment is immediately produced on injecting a very small amount of nicotine into the Anolis, it is probable that the reverse process, the outward migration, is dependent upon the normal action of these poisoned parts, the sympathetic nerves. For these reasons I believe that in Anolis the inward migration is a process which is ordinarily under the control of the chromatophore itself and that the outward migration, which takes place all over the animal when even only a small spot in the skin is illuminated (Parker and Starratt, 1905), is dependent upon the action of sympathetic nerves.
In the true Chameleon, as Brücke (1852) and many others have demonstrated, precisely the reverse is true; the outward migration is independent of nerves and the inward migration is produced by them. Moreover, judging from the results of experiments on the spinal cord, the nerves which in Chameleon are concerned with these changes are not sympathetic nerves, but spinal nerves.
These differences between Anolis and Chameleon I believe to be well founded. In my opinion both animals have descended from a stock in which the chromatophores were entirely independent of nervous control and in the process of descent the chromatophores of different lines became separately appropriated as effectors of the nervous system. In the ancestors of Anolis the sympathetic nervous system became related to the outward migration of pigment; and in those of the Chameleon the spinal system associated itself with the inward migration. The fact that Chameleon and Anolis belong not only to separate families, but to separate suborders of lizards, rather emphasizes this view than otherwise.
Such instances as the independent retinal chromatophores of Palæmonetes and the nervously dependent chromatophores of Chameleon and Anolis lead me to believe that chromatophores are effectors evolved independently of nervous control, but in some cases secondarily appropriated as nervous end-organs.
What has been said of chromatophores so far as their relation to nerves is concerned is probably also true of glands. The majority of glands are unquestionably independent of direct nervous control. In almost all instances a blood supply is essential to the action of a gland, and as this can be controlled by nerves there is thus an indirect influence of the nervous system on the action of the gland, but this nervous control over the blood supply is very different from a direct nervous control over secretion. I know of no good reason to assume that nerves have any direct influence on the secretions of the kidneys, the liver or even the pancreas. The pancreatic juice which appears with such precision on the arrival of food in the small intestine has been shown by Bayliss and Starling (1904) to be secreted not through the action of nerves on the gland, but through the action of a substance, secretin, produced by the food in the intestine and carried by the blood to the gland. If into the blood of a fasting animal whose nerves to the pancreas have been cut a small amount of secretin is injected, the pancreas will begin to produce its characteristic secretion.
Although most glands are not under direct nervous control, some are as completely under this control as the majority of muscles are. The best examples of this condition are the sweat glands and the salivary glands. The fact that when the nerves supplied to the salivary gland are stimulated, secretion may take place at a pressure higher than that of the blood supplied to the gland shows conclusively that the production of saliva is not a simple organic filtration process, but is dependent upon action called forth in the secretory cells by a nervous impulse. This view gains additional support from the fact that in the salivary glands nerve fibers have been found to end in connection with the secretory cells. There is therefore every reason to believe that the salivary glands, and the same may be said of the sweat glands, are organs whose secretions are directly controlled by nerves.
As these several examples show, some glands are completely under the control of nerves and others are not. In my opinion the latter represent the primitive state of this form of effector and the former the condition after such organs have been appropriated by the developing nervous system.
Luminous or phosphorescent organs afford another class of effectors which have probably originated independently and fallen secondarily under the influence of the nervous system. These organs, however, have been studied so imperfectly that it is at present difficult if not impossible to get satisfactory evidence as to their exact condition. Some animals have been supposed to possess phosphorescent organs when in reality their luminosity was due entirely to reflection; others like certain earthworms were found to be phosphorescent because their slime contained photogenic bacteria. But aside from these spurious cases there is an abundant range of truly phosphorescent animals, examples of which occur from protozoans to vertebrates. One peculiarity in their distribution is that true phosphorescent animals are not found in fresh water; they are either marine or air-inhabiting.
In all cases where animal phosphorescence has been examined with care, it seems to be dependent upon the production of a special substance by the light-producing cells. This substance is not in the nature of a living, structurally organized material like muscle, for it can be crushed into a paste and still show light. Moreover, Bongardt (1903) dried the phosphorescent organ of a common firefly over calcium chloride and then kept it in a sealed tube from July 16, 1901, till August 3, 1902, a period of over a year. After this the tube was opened and the organ wet with distilled water; in twelve minutes it glowed so that it could be seen at a distance of two meters. Evidence of this kind supports the view that the phosphorescent substance is not living but rather formed material, such as a secretion, and resembles in this respect pepsin or trysin.
If phosphorescent organs produce a substance essentially a secretion to which their characteristic activity is due, they might without impropriety he classed as glands, but if they are thus classed, it must he remembered that the majority of them are so placed that they have no access to cavities or the exterior; hence they would be in the nature of ductless glands. In one respect, however, they differ even from ductless glands; the substance that they produce is not carried away from them even by the blood-stream hut is used locally for the production of light. Hence though phosphorescent organs may be in many important respects like glands, they differ in certain ways from all ordinary glands.
Whether phosphorescent organs are under the control of nerves or not is a question of some uncertainty. The fact that many highly specialized phosphorescent organs have a rich innervation indicates that they are under nervous influence, but even this may be of the indirect kind such as has already been indicated for glands and not a direct control. In ctenophores Peters (1905) has shown that a few paddle-plates will glow on mechanical stimulation precisely as the rows of plates in the normal animals do. He has also shown that the primitive nervous system of these animals plays no direct part in this phosphorescence. This instance seems to me to be a perfectly clear case of phosphorescence not under the control of nerves, though in an animal with a nervous mechanism.
In the common firefly the relations are not so well understood. Thus Bongardt (1903), though he describes an intimate nervous plexus in the luminous organ of this animal, believes that its rhythmic photogenic activity is not under even indirect nervous control. He maintains, on what, however, is not really strong experimental evidence, that the firefly can not extinguish its light through nervous action and he believes that the phosphorescent rhythm is due to totally different factors. This case merely shows the fragmentary nature of our knowledge of this phenomenon even in so well-known an example as the firefly.
As a good instance of nervous control over phosphorescence the brittlestar, Ophiopsila, recently studied by Mangold (1907), may be quoted. On mechanical stimulation the ventral surfaces of the arms of this animal glow for a short time. The phosphorescence begins in the stimulated part and, if this be an arm, it may spread over this arm to the disk and thence to the other arms. The course that it follows is that of the radial and circular nerve-strands. If any of these are interrupted by being cut, the phosphorescence does not pass beyond the cut, thus showing that it is probably controlled by the nerve.
These instances, few and confessedly fragmentary as they are, indicate that phosphorescent organs, though in many important respects like glands, are in reality a separate class of effectors and that in some instances their action is independent of nervous control, while in others it is under this control. In my opinion the instances of independent action represent a primitive state; the others a condition brought about through the appropriation of these organs as end-organs by a developing nervous system.
If what has been stated in this article is correct, we must picture to ourselves as steps in the evolution of the nervous system not only the independent origin of muscle around which the nervous organs subsequently develop, but also the independent origin of other effectors such as chromatophores, glands and phosphorescent organs and the secondary appropriation of many of these by a developing nervous system. This principle of appropriation I believe to be as significant in elucidating the present condition of the nervous system and its appendages, as the principle of evolutionary sequence of parts, muscle, sense organ, and central nervous organ, as given in the first three articles.
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