Classical Texts in Psychology
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Principles of Physiological Psychology
Wilhelm Wundt (1902)
Translated by Edward Bradford Titchener (1904)
The Bodily Substrate of the Mental Life
The Organic Evolution of Mental Function
§1. The Criteria of Mind and the Range of the Mental Life
THE mental functions form a part of the phenomena of life. Wherever we observe them, they are accompanied by the processes of nutrition and reproduction. On the other hand, the general phenomena of life may be manifested in cases where we have no reason for supposing the presence of a mind. Hence the first question that arises, in an inquiry concerning the bodily substrate of mentality, is this: What are the characteristics that justify our attributing mental functions to a living body, an object in the domain of animate nature?
Here, upon the very threshold of physiological psychology, we are confronted with unusual difficulties. The distinguishing characteristics of mind are of a subjective sort; we know them only from the contents of our own consciousness. But the question calls for objective criteria, from which we shall be able to argue to the presence of a consciousness. Now the only possible criteria of the kind consist in certain bodily movements, which carry with them an indication of their origin in psychical processes. But where are we justified in referring the movements of a living creature to conscious conditions? How uncertain the answer to this question is, especially when metaphysical prejudice has a part to play in it, may be seen at once by an appeal to history. Hylozoism inclines to regard every movement, even the fall of a stone, as a mental action; Cartesian spiritualism recognises no expression of mental life beyond the voluntary movements of man. These are extreme views. The first is beyond all verification; the second is correct only upon the one point that the manifestations of our own conscious life must always furnish the standard of reference in our judgments of similar indications in other creatures. Hence we must not begin our search for mental function among the lower types of organised nature, where its modes of expression are least perfect. It is only by working our way downwards, from man to the animals, that we shall find the point at which mental life begins.
[p. 28] Now, there are a very large number of bodily movements, having their source in our nervous system, that do not possess the character of conscious actions. Not only are the normal movements of heart, respiratory muscles, blood vessels and intestines for the most part unaccompanied by any sort of conscious affection; we find also that the muscles subserving change of position at the periphery of the body often react to stimuli in a purely mechanical and automatic way. To regard these movement-processes as mental functions would be every whit as arbitrary as to ascribe sensation to the falling stone. When, however, we rule out all the movements that may possibly go on without the participation of consciousness, there remains but one class that bears upon it the constant and unmistakable signs of an expression of the mental life, -- the class of external voluntary actions. The subjective criterion of the external voluntary action, as directly given in introspection, is that it is preceded by feelings and ideas which we take to be the conditions of the movement. Hence a movement that we observe objectively may also be regarded as dependent on the will if it points to similar mental processes as its conditions.
But the discovery of this criterion does not by any means remove the practical difficulties of our diagnosis of mind. It is not possible to distinguish certainly in every case between a purely mechanical reflex -- or even, in the lowest organisms, a movement due to external physical causes, such as the imbibition of tumescent bodies, the change of volume from fluctuations of temperature, etc. -- and a voluntary action. We have to note; in particular, that while there are characters by which we can argue with absolute confidence to the existence of a voluntary action, the absence of these characters does not always necessarily imply the absence of such action, still less the absence of psychical functions at large. Hence all that our inquiry can hope to accomplish is the determination of the lower limit at which a mental life is demonstrably present. Whether it does not, in actual fact, begin at a still lower level must remain a matter of speculation only.
The generally accepted objective criterion of an external voluntary action is the reference of movement to the universal animal impulses, the nutritive and the sexual. It is only as a result of sensory excitations that these impulses can lead the animal to a change of place that shows the marks of a voluntary action; and the special character that prompts us to refer such sensorily stimulated movements to a process in consciousness is their variability. They do not appear with mechanical regularity in response to a given external stimulus, but are varied to suit varying conditions, and brought into connexion with sense-impressions previously secured. Judgment on the ground of these criteria may, in the individual case, remain doubtful; since all vital processes, even those that are entirely [p. 29] automatic and unconscious, evince a certain adaptation to ends, and a certain consequence in their successive stages. But sustained and attentive observation of living creatures will as a general rule, enable us to decide with certainty whether any particular manifestation of life is intelligible only from that continuity of internal states which we name consciousness, or whether it may possibly have arisen in the absence of mind. That consciousness, in this sense, is an universal possession of living organisms, from man down to the protozoa, is beyond the reach of doubt. At the lowest levels of this developmental series the processes of consciousness are, of course, confined within extremely narrow limits, and the will is determined by the universal organic impulses only in the very simplest way. Nevertheless, the manifestations of life, even among the lowest protozoa, are explicable only upon the hypothesis that they possess a mind. Thus the amoeba, which is to be regarded morphologically as a naked cell (see Fig. 2, p. 33) will sometimes return after a short interval to the starch grains that it has come upon in the course of its wanderings, and will incept a new portion as nutritive material in the soft protoplasm of its body. Many of the ciliated infusoria pursue others, which they kill and devour. These are all phenomena that point towards continuity of mental processes, though in all probability to a continuity that extends only over a very short space of time. They point also, at all events in the case of the Ciliata, to a variation in the choice of means, for the satisfaction of the organic impulses, that would be unintelligible as a merely mechanical result of external influences.
We enter, of course, upon much less certain ground when we ask, further, whether the mental life really makes its first appearance at that point upon the scale of organised existence at which we notice the external voluntary action, or whether its beginnings do not reach back to a still lower level of life. Wherever living protoplasm occurs, it possesses the property of contractility. Contractile movements arise, sometimes at the instigation of external stimuli but sometimes also in the absence of any apparent external influence. They resemble the voluntary actions of the lowest protozoa, and are not explicable in terms of external physical affection, but only as the results of forces resident in the contractile substance itself.[p. 30] They cease at once with the cessation of life. We find them evinced both by the protoplasmic contents of young plant-cells and by the free protoplasm occurring throughout the animal and vegetable kingdoms. Indeed, it is probably that all elementary organisms, whether they enjoy an independent existence or form part of a compound organism, possess the property of contractility at least during a certain period of their development. Consider, e.g., the lymph corpuscles, which are found in the blood and lymph of animals, and in pus, and which occur as migratory elements in the tissues. They are not only entirely similar in bodily
configuration to certain of the lowest protozoa, but they also undergo changes of form which, in outward appearance, are indistinguishable from the movements of these unicellular organisms (Fig. 1). Only, the voluntary character of these movements is beyond the reach of demonstration. It is true that similar structures -- particularly the colourless blood-corpuscles of invertebrates -- have been seen to take up solid substances, and that this action may be interpreted as an inception of food. It is true, also, that movements in response to stimulus accompany the exercise of the digestive functions in certain plants. But in neither case is there any definite indication of a true impulse, i.e. an impulse determined by sensation, toward the food-stuff, or of any sort of psychological middle term between stimulus and movement. The same thing holds of the movements of the lower forms of algae, fungi and swarm-spores, produced by a variable distribution of water and carbon dioxide, or by different kinds of light rays. On the other hand, the movements of certain bacteria are so suddenly affected by light ad by the gases of respiration, that they at once suggest an origin in sensations, But, here again, we cannot he sure that the changes are not simply physical effects, as is undoubtedly the case with the movements evolved by hygrometric changes in the environment.[p. 31]
We must, however, always remember, in passing judgment upon this whole group of observations, that the demonstration of physical conditions, to which the phenomena of protoplasmic contraction and of the movement of elementary organisms may be referred, is by no means incompatible with the hypothesis of concomitant psychical processes. Physiology seeks to derive the processes in our own nervous system from general physical forces, without considering whether these processes are or are not accompanied by processes of consciousness. We are bidden to believe, both by theory of knowledge and by the philosophy of nature, that all manifestations of life, on the physical side, are referable to natural laws of universal validity. And physiology, acting in accordance with this requirement, has found it justified in every instance in which she has succeeded in reaching a solution of her problems. It follows, then, that the existence of mental functions can never be inferred from the physical nature of organic movements, but only from certain special conditions attending their performance. On the other hand, observation shows that the chemical and physiological properties of living protoplasm are essentially the same, whether we can prove that it manifests a mental life or whether we cannot. This holds, in particular, of the attributes of contractility and irritability. In physical regard, therefore, protoplasm maintains its identity throughout. If we add to this the fact that it is impossible to draw a hard and fast line at the point where protoplasmic movements first begin to take on a psychological character, -- that there is a gradual transition from the walled-in protoplasm of the plant-cell on through the migratory lymph-corpuscles of animals and the free-living monera and rhizopods, to the more motile ciliated and mouth-bearing infusoria -- we cannot resist the conjecture that psychical life and the capacity of giving expression to it are universally represented in contractile substance.
From the standpoint of observation, then, we must regard it as a highly probable hypothesis that the beginnings of the mental life date from as far back as the beginnings of life at large. The question of the origin of mental development thus resolves itself into the question of the origin of life. Further, if physiology is obliged, by the uniformity of interaction of physical forces throughout the universe, to accept the postulate that the processes of life have their ultimate basis in the general properties of matter, psychology finds it no less obligatory to assume, in this same matter, the universal substrate of natural phenomena, the presence of conditions which attain to expression as the psychical aspect of vital phenomena. But this latter statement must not mislead us. The latent life of inorganic matter must not be confused, as hylozoism confuses it, with real life and actual conscious-[p. 32] ness; nor must it be considered, with materialism, as a function of matter. The former interpretation is wrong, because it assumes the existence of vital phenomena at a point where not these phenomena themselves are given, but only the common ground upon which they rest and whereby they become possible; the second is wrong, because it posits a one-sided dependence, where in reality we find an interrelation of simultaneously presented but incommensurable processes. We employ the concept of material substance to denote the ground of all objective phenomena. Hence it is the office of this concept to make intelligible all the various form of physical occurrence, including the physical manifestations of life. Now among these manifestations we find movements which indicate the presence of a consciousness. Our postulates concerning matter will then, explain the physical causation of such movements, but can never account for the concomitant psychical functions. To explain these, we must make appeal to our own consciousness.
We cannot, of course, here at the very outset of our psychology, return any final answer to the question of the ultimate objective criteria of the mental life. All that we can do, at the present stage, is to indicate in brief the position to be taken up in psychological practice. It is however, easy to see that the wide divergence of opinion on the subject is mainly due to the intermixture of science with philosophy, or to a fixity of judgment that has its source in philosophical theory. Only in this way can we account for the fact that there may still be found, in works upon the scope of the mental life, views that range between the two extremes current in DESCARTES' day. One author will assert that the animals, if not without exception, at least as far up the scale as the higher invertebrates and the lower vertebrates, are mere reflex machines; another looks upon life and mind as convertible terms, and accordingly endows plants as well as animals with consciousness. The former view is evidently influenced, to some extent, by the idea that psychical and physical are antithetical terms. The alternative (physical or psychical) is often presented as if the one concept necessarily excluded the other, -- as, indeed it did, in the metaphysical dualism of DESCARTES. But this is misleading. The close interconnection of the phenomena of the physical life and the processes of consciousness makes the relation 'physical and psychical' on the face of it, much more probable. We should, as a matter of fact, admit at once that, e.g., a sensation is a psychical quality, without meaning to deny that it is accompanied by a physical process in the sense-organ and the sense-centre. And such a coexistence of the two kinds of vital processes is, in many crises, beyond all dispute. How far it extends, over the phenomena of life at large, is again a question that, naturally, cannot be answered at the outset of our psychological investigations. But, at all events, we should be merely obscuring the facts, if we made our first approach to them with the alternative 'physical or psychical' in our hands. And the danger of misinter- [p. 33] pretation is, at best, grave enough. Many movements, that may in all probability be regarded as purely automatic, are, as we said above, purposive in character; and many of them, again, are self-regulating. It is, therefore, very difficult to draw the line of division in the concrete case.
We may say, then, that the mechanistic explanation of the movements of the lower animals is not the outcome of impartial and unprejudiced observation. But the rival theory, which ascribes mind and consciousness to the plant-world, is in no better case. Fechner, the chief representative of this theory, himself expressly declares that be derived it from considerations of general philosophy: he further attributes consciousness to the earth and the other heavenly bodies, making this cosmic consciousness the whole, of which the individual forms of consciousness in plant and animal are parts. Hypotheses of this sort have, no doubt, a certain justification. They emphasise the intrinsic impossibility of the view that mental life may suddenly appear, at some point of time and space, as a new thing; that we need not seek for its general conditions in the universal substrate of the vital processes. When, however, we ask how we should conceive of these conditions, we raise a metaphysical question, -- a question that lies well beyond the reach of psychology and its empirical problems.
§ 2. The Differentiation of Mental Functions and of their Physical Substrate
The organic cell in the earliest stages of its development, consists either of a naked mass of protoplasm, contractile throughout its substance, or of a denser and immotile cortex within which motile protoplasm is contained.
And the same two forms are evinced by the lowest independent organisms in which we can observe movement-processes indicative of psychical conditions (Fig. 2). The substrate of the elementary mental functions is here entirely homogeneous, and coextensive with the whole mass of the [p. 34] body. The only sense that is plainly functioning is the sense of touch. An impression made upon any portion of the contractile protoplasm first of all releases a movement at the place of direct impact, which may then extend to purposively co-ordinated motion of the entire body. The beginnings of a differentiation of mental function can, however, be found even in the protozoa, wherever the cortical layer surrounding be contractile body-substance has developed special organs of movement, cilia and flagella (Fig. 3). Oftentimes this development goes hand in hand with a differentiation of the nutritive functions, An [sic] oral aperture and digestive cavity are found, and in many instances a system of open canals appears, whose fluid contents are kept in motion by a contractile vessicle. The cilia with which these infusoria are furnished render them incomparably more motile than the organisms lying at the very lowest point of the organic scale, the monera and rhizopods, which consist merely of a viscous body-mass. They are, however, more than organs of locomotion; they function as organs of touch, and sometimes appear to be sensitive to light as well. The spot of red pigment noticed in many of the infusoria may also have some connexion with light-sensation; but we have as yet no certain ground for regarding it as a primitive organ of vision.
In the compound organisms we observe a more radical differentiation of mental function and its bodily substrate. The metazoan germ-cell divides into a number of cells. These seem to be originally of the same kind, so that not infrequently all like manifest the primitive contractility of protoplasm in course of time, however, they become modified in matter and form; the tissues of the plant and animal body are derived from them and from the products of their growth, and the structural changes are accompanied by a more and more complete specialisation of function. The conditions which govern this process of differentiation, to which the whole of organic nature is subject, are still wrapped in obscurity. Our knowledge halts abruptly at the changes of outward form in which the internal development finds its expression.
In the plant-world we see the nutritive functions attain such a degree of elaboration that the organism (and this is true more especially of the higher plants) has, so to say, , no other concern than to increase its present stock of organic substance. In the animal world, on the other hand, the process of evolution is characterised by the progressive discrimination of the animal and vegetative functions, and a consequent differentiation of these two great provinces into their separate departments. The cell-mass of the yolk, originally homogeneous, divides up first of all into a peripheral and a central layer of different structural character (Figg. 4 and 5); while the cleavage cavity gradually widens out to form the future body-cavity.[p. 35] At this stage, sensation and movement appear to reside exclusively in the outer cell-layer, the ectoderm, while the nutritive functions are discharged by the inner layer or entoderm. At a higher level of evolution a third
layer of cells, the mesoderm, forms between the two. The initial stages of development are thus identical over the whole series of forms from coelenterates to vertebrates, the differentiation of organs beginning always with the distinction of three germinal layers. The outermost layer is the source of the nervous system and sense-organs, as well as of the muscular system; the innermost furnishes the organs of nutrition; and the intermediate layer, the vascular system. In the vertebrates, the skeleton is also derived from the ectoderm.
This discrimination of organs is accompanied by a differentiation of the elementary constituents of the tissues. When the separation of ectoderm and entoderm is first accomplished, the cells of the former discharge the combined function of sensation and movement. The initial step toward a separation of these two cardinal functions is apparently taken in the hydridae and medusae, where the ectoderm cells send out contractile processes into the interior of the body. The sensory and motor functions are here still united in a single cell, but are distributed over different portions of it (Fig. 6). In the next stage, the properties of sensation and contractility pass to special and spatially separated cells, while connective elements [p.36] develop, to mediate the functional interconnexion of the different structures. There thus arises a third class of cells, lying in the paths of connexion between sensory and muscular cells, and acting probably as organs for the reception and transmission of stimuli. The sensory cells now become external organs, devoted to the reception of physical stimuli. At the same time, they undergo a differentiation, which fits them for excitation by various forms of movement-process in the outside world. Similarly, the contractile cells become organs for receiving and converting into external movements the excitations transmitted to them. But the psychical functions par excellence are discharged by the cells of the third class, the nerve-cells, which are connected by their processes with both the sensory and the muscular cells, and, as we have said, mediate the functional interconnexion of the two group's of organs. Hence the simplest scheme of a nervous system is given with a centrally situated nerve-cell connected on the one hand with a sense-cell and on the other hand with a contractile muscle-cell both directed towards the external world, but mediating the one the reception of sense-stimuli and the other the motor reaction upon them.
It is, however, quite certain that this simplest scheme never actually occurs. As soon as special nerve-cells are formed at all they are formed in numbers, joined together in longitudinal and transverse series, so that a great many of them are connected only by way of others of their kind with the peripheral structures. This multiplication of the central elements means, of course, that the process of differentiation extends to the nerve-cells themselves. They assume various functions, according to the connexions in which they stand with one another and with the peripheral organs. Those lying in the neighbourhood of the terminal organs are employed in functions, auxiliary to the strictly psycho-physical processes, which run their after course without the participation of consciousness. Others enter into intimate relation with the mechanisms of nutrition; they sustain and regulate the physiological processes of secretion and circulation. They thus lose their place among the immediate bodily conditions of the mental [p. 37] life, and exert only an indirect influence upon mind. This progressive differentiation of functions and of their substrate within the nervous system finds its expression in the relative increase of the mass of the nervous elements, and in the elaboration of special nerve-centres, compact bodies of nerve-cells and their fibrillar processes. We have an instance of such centres in the ganglia of the invertebrates, which appear at the most various stages of development, from the comparatively simple nerve-rings of the coelenterates and the lower worms and molluscs, up to the brain-like ganglionic masses of the anthropods and higher molluscs (Fig. 7).
Finally, among the vertebrates, the importance of the nerve-centres for the whole organisation of the animal is shown, from the first, in their relation to the external bodily form and to the development of the various systems of organs.
Immediately after the separation of the formative materials into the two layers of the germ-primule, there appears in the ectoderm a groove, open above, at the bottom of which is a streak of darker tissue. This is the primitive streak, whose direction corresponds with the future longitudinal axis of the embryo (Fig. 8). Presently, the groove closes and becomes the neural tube, the primule of the myel (spinal cord) and its sheaths. The anterior portion of this; tube gives rise, by expansion, to the primule of the brain. Concomitantly with the closure of the neural tube begins the differentiation of the germinating cells into nerve-cells. They increase in size, and send out runners, which become transformed into the various cell-processes (Fig. 9).
At this point there begins a serial differentiation of function and its physical substrate, whose investigation will form the subject of the following Chapters. We shall set out with a consideration of the structural elements of the nervous system in their morphological and chemical characters. We shall next raise the question of the nature of the processes at work within those elements; in other words, we shall attack the problem of a physiological mechanics of nervous substance. This discussion will be followed by a brief description of the structural development of the nervous centres, with especial reference to the morphology of the human brain. We shall then be prepared to approach the two main problems that are presented by the co-ordination of functions in the nervous system. The first of these is the determination of the course of the paths of nervous conduction, as conditioned by the individual connexions of the nervous elements; and the second is the problem of the physiological functions of the central parts, -- the last and most important question for the relation of nervous process to the processes of the psychical life.
 ROMANES, Animal Intelligence, 4th ed., 1886, 18 ff.; Mental Evolution in Animals, 1885, 18, 55; MAX VERWORN, Psychophysiologische Protisten-Studien,1889, 146 ff. VERWORN'S statement that voluntary actions appear for the first time in the Ciliata, and that all movements made in response to stimulus by the non-ciliated protozoa, so far as they are not of purely mechanical or chemical origin, should be interpreted as reflexes, is evidently a result not of observation, but rather of a forgone theoretical conviction that voluntary actions must have developed out of reflexes. On this theory see Part iv. below.
 M. SCHULTZE, Das Protoplasma der Rhizopoden, 1863. ENGELMANN, Beiträge zur Physiologie des Protoplasmas, ii., 1869. VERWORN, Die Bewegung der lebendigen Substanz, 1892, 51 ff.; Allgemeine Physiologie, 1901, 363 ff. (General Physiology, 1899, 146 ff., 527)
 T. W. ENGELMANN, in PFLÜGER'S Archiv. f. d. ges. Physiologie, xxvi. 537; xxix. 415; xxx. 95. PFEFFER, Untersuchungen aus d. botan. Institut zu Tübingen, i. 363, 483; ii. 582. For further details, see Ch. vii. § 3, below. On the physical causes of proto-plasmic movement, cf. BÜTSCHLI, Untersuchungen über mikroskopische Schäume und das Protoplasma. 1892, 172.
 A. BETHE, Dürfen wir den Ameisen und Bienen psychische Qualitäten zuschreiben? In PFLÜGER'S Arch. f. d. ges. Physiol., 1xx. 1898, 15 ff. Cf. the critical remarks of WASSMANN, Die psychischen Fähigkeiten der Ameisen, 1899, and Biol. Centralblatt, xviii. 1898, 578.
 The relations of the various cavities, three or four in number, are in reality much more complicated. It would be more nearly true to say that, where the change indicated in the text takes place, the body-cavity gradually replaces the cleavage-cavity. Cf. MINOT, Embryology, 1897, ch. ix. -- TRANSLATOR.
 The author gives no references here. The mesoderm is now divided, by the best writers, into mesothlium, the source of the muscles and mesodermic glands, and mesenchyma, the source of connective and skeletal tissue. The derivation of the mesenchyma itself is still an open question. -- TRANSLATOR.
 KLEINENBERG, Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung 1872, 21 ff. O and R. HERTWIG, Das Nervensystem und die Sinnesorgane der Medusen, 1878, 157. [The cells from the epithelial layer of Hydra shown in Fig. 6 (KLEINENBERG'S 'neuromuscular' cells) are now to be regarded as muscle-cells. Later Note, by AUTHOR.]