Elements of physiological psychology : a treatise of the activities and nature of the mind from the physical and experimental point of view

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Preface

-«3

There can be no doubt that an important movement in psychology has arisen in recent times through the effort to approach the phenomena of mind from the experimental and physiological point of view. Different students of psychological science will estimate differently both the net result already reached by this effort and the promise of further additions to the sum of our knowledge from continued investigation of the same kind. Some writers have certainly indulged in extravagant claims as to the past triumphs of so-called Physiological Psychology, and in equally extravagant expectations as to its future discoveries. On the other hand, a larger number, perhaps, have been inclined either to fear or to depreciate every attempt to mingle the methods, laws, and speculations of the physical sciences with the study of the human soul. These latter apparently anticipate that some discovery in the localization of cerebral function, or in psychometry, may jeopard the birthright of man as a spiritual and rational being. Or possibly they wish to regard the soul as separated, by nature and with respect to its modes of action, from the material body in such a way as to render it impossible to understand more of the one by learning more about the other.

As a result of some years of study of the general subject, I express with considerable confidence the opinion that there is no grormd for extravagant claims or expectations, and still less ground for any fear «f of consequences. In all cases of new and somewhat rankly growing

^ scientific enterprises, it is much the better way to waive the discus-

g sion of actual or possible achievements, as well as of welcomed or

;* dreaded revelations of new truth, and proceed at once to the busi-

^ ness on hand. It is proposed in this book to follow this better way.

It will be the task of the book itself to set forth the assured or alleged results of Physiological Psychology ; and this will be done at

Ul Q

352441 iv PEEFACE.

every step with such degree of assurance as belongs to the evidence hitherto attainable upon the particular subject discussed. With declamation, either in attack or defence of the " old psychology," of the " introspective method," etc., one may dispense without serious loss.

The study of the phenomena of consciousness by the method here proposed necessarilj' requires some acquaintance with a considerable circuit of sciences which are not usually all alike closely allied. The number of scholars who can form opinions with equal freedom and confidence in all of these sciences is very small. Moreover, since all psycho-physical laws ai'e supposed — as the very term indicates— to govern the correlations of phenomena of consciousness with phenomena of the nervous sj'stem, a peculiar mystery belongs to much of the domain within which psycho-physical science is compelled to move. These facts may fitly, on the one hand, excite caution in the writer ; and, on the other hand, excuse him for many inevitable failures to set forth with perfect definiteness and confidence the conclusions he has to propose. Much will be said that must be accepted as provisional, as only probably true. Much room must also be made for conjecture and speculation. What is most important, however, is that conjecture should not be put forth as ascertained fact, or speculation as unquestioned law.

It would have been a great assistance to me if I had had more predecessors in the path which I am to take. But with the exception of Wundt's masterly work (Grundziige der physiologischen Psychologic, second edition in 1880), no one book has attempted to cover, even in a summary way, the entire ground. The number of monographs, however, which have dealt with individual questions subordinate to, or part of, the main inquiry is very great. These two facts also render the attempt at a general survey of Physiological Psychology for readers of English both peculiarly attractive and peculiai-ly difiicult. I can only indulge the hope that I have done something toward breaking this path and rendering it easier and more secure, both for mj'self and for others, in the future.

The investigatox'S and authors to whom I am under obligations for material upon the various questions discussed, or statements made, in this book are by no means all mentioned by name. Of course, much of what is said on the structure of the nervous system, and on the phenomena of sensation and perception, has already become part of that general fund of facts and laws which belongs alike to all students of the subject. But by quoting certain author-

PEEFACE. V

ities in the text, and by a few (in comparison with the number which might have been cited) references in foot-notes, I have connected some of the discoveries and views of modern psycho-physical science with their authors. These may serve somewhat as guide to those persons who wish to pursue such studies still further.

I am under particular obligations to Dr. James K. Thacher, Professor of Physiology in the Yale Medical School, for valuable assistance in that description of the Nervous Mechanism, its structure and functions, which- the First Part of the book contains. If I have escaped the mistake of assuming to teach more than is really known upon this subject, it has been in large measure due to his friendly and skilful guidance. Valuable assistance has also been received from Kussell H. Chittenden, Professor of Physiological Chemistry, and Charles S. Hastings, Professor of Physics — both of the Sheffield Scientific School.

The method and arrangement of the book have been chosen so as to fit it for use, both as a text-book by special students of the subjects of which it treats, and also by the general reader who is interested in knowing what results have been reached by the more modern — and even the latest — psycho-physical researches.

George T. Ladd.

Table Of Contents

PAGE

PART EIRST.

THE NERVOUS MECHANISM.

CHAPTER I. The Elements of the Nervous System 17-55

§§ 1-4, General Function of the Nervous System. — §§ 5-16, Chemical Constitution of the Nervous Elements. — §§ 17-30, Structural Form of the Nervous Elements. — §§ 31-36, Common Properties of the Nervous Elements.

CHAPTER n.

Combination of the Nervous Elements into a System 56-101

§§ 1-3, Threefold Plan of the Nervous System.— § 4, The Sympathetic and Cerebro-spinal Systems. — § 5, Membranes of Brain and Spinal Cord.— §§ 6-13, Structure of the Spinal Cord.— §§ 13-14, General Arrangement of the Encephalon. — § 15, Structure of the Medulla Oblongata.— §16, Structure of the Cerebellum.— § 17, Structure of the Pons Varolii.— §§ 18-24, Structure of the Cerebrum. — §§ 25-27, Cortex of the Cerebral Hemispheres.— §§ 28-29, Arrangement of the Nerve-Tracts. — § 30, The Cranial and Spinal Nerves.

CHAPTER IIL

§§ 1-3, General Oflce of the Nerves.— § 4, The Nerve-Muscle Machine.—§§ 5-8, The Conditions of Neural Action.— §§ 9-19, Phenomena induced in the Nerves by different Stimuli.— §§ 20-23, Electrical and other Processes in the excited Nerve-Stretch.— §§ 24-26, Laws

Viii TABLE OF CONTENTS.

of the Nerve-Commotion. — § 27, Speed of the Nerve-Commotion. — § 28, Effect of Section.— § 29, Nervous Conduction in the Central Organs.—i;§ 30-32, Paths of Conduction in the Spinal Cord.— §§ 33-35, Paths of Conduction in the Brain.

CHAPTER IV.

PAGB

Automatic AND Reflex Functions of the Central Organs.. 130-162

§§ 1-2, Nature and Kinds of Reflex Action.— §§ 3-5, TheSpinal Cord as a Central Organ. — ^§ 6-9, Laws of Spinal Reflexes.— § 10, Irregular Automatism of the Cord. — § 11, Centres of the Cord. — t^ 12, Excitability of the Cord.— § 13, Inhibition of the Cord. — § 14, The Brain as a Central Organ. — §§ 15-16, Functions of the Medulla Oblongata—-§§ 17-19, Centres of the Medulla Oblongata.— § 20, Influence of the Cerebral Lobes, — § 21, Functions of the Cerebellum. — g§ 22-27, Functions of the Basal Ganglia.— § 28, Gray Matter of the Third Ventricle.

CHAPTER V.

End-Organs of the Nervous System 163-197

§y 1-2, Characteristics of the End-Organs. — § 3, The Kinds of End- Organs.— §^ 4-5, The End-Organs of Smell.— §§ 6-7, The End-Organs of Taste.— §§ 8-10, The End-Organs of Touch.— § 11, The End- Organ of Sight. — §§ 12-16, Tunics, Media, and Appendages of the Eye. — g 17, The Mechanism of Accommodation. — §§ 18-21, Structure and Functions of the Retina — § 22, Photo-Chemistry of Vision. — §§ 23-26, External and Middle Ear.— § 27, Structure of the Labyrinth. ■ — § 28, End-Apparatus of the Vestibule. — § 29, The Organ of Corti. — §§"30-31, Problem solved by the Labyrinth.— § 32, End-Organs of Motion.

CHAPTER VI.

The Development of the Nervous Mechanism 198-21 S

§ 1, Nature of Embryonic Life. — §§ 2-5, Earliest Development of the Ovum. — §§ 6-8, Blastodermic Layers and their Differentia. — §§ 9- 11, Head-Fold and Brain-Vesicles. — § 12, Development of Cranial and Spinal Nerves. — §§ 13-15, Subsequent Development of the Brain. — §§ 16-17, Development of Eye and Ear. — § 18, Histogenetic Changes in the Embryo. — § 19, Conclusions.

CHAPTER VII.

Mechanical Theory of the Nervous System 214-23(1

§ 1, Machine-like Nature of the Body.— §§ 2-6, The Nervous System as a Mechanism. — § 7, Summation and Interference in Nerves. — i^ 8, Evidence from the Electrical Phenomena. — i^ 9, Theory of du Bois-Roymond. — §^5 10-11, Theory of Hermann. — S^i5 12-13, Theory of Wundt. — §§ 14-15, General Conclusions as to a Mechanical Theory.

TABLE OF CONTENTS. IX

Part Second. Correlations Of The Nervous Mech- Anism And The Mind

CHAPTER I.

PAGE

The Localization of Cerebral Function 239-263

§§ 1-3, Proofs of the Brain's Special Significance. — §§ 4-7, The Brain as a Measure of Intelligence. — § 8, Special Significance of the Cerebral Hemispheres. — ^§ 9-10, The Question of Localization. — §§ 11-13, The History of Discovery.— §g 14-16, The Evidence from Experiment.— § 17, The Evidence from Pathology.— § 18, The Evidence from Anatomy. — § 19, True Method of Investigation.

CHAPTER IL

The Localization of Cerebral Function [Continued] 263-802

§§ 1-4, Difliculties from Negative Cases. — §§ 5-6, Experiments in Stimulation. — ^ 7, Experiments in Extirpation. — ^§ 8-9, Nature of so-called Motor Centres. — §§ 10-15, Method and Results of Bxner. — § 16, Confirmatory Conclusions from other Sources. — § 17, The Evidence of Histology. — § 18, Relation of Motion and Sensibility. — §§ 19- 21, Visual and Auditory Centres of Ferrier and Munk. — ^ 22, Exner's Cerebral Field of Vision. — g 23, Relations between the Retinas and the Cerebrum. — § 24, Localization of Smell and Taste. — §§ 25- 27, The Phenomena of Aphasia. — § 28, Cerebral Lesions in Aphasia. — § 29, Conjectures as to the Frontal Lobes. — § 30, Negative Conclusions of Goltz. — § 31, Conclusion as to three leading Principles.

CHAPTER III. The Quality op Sensations 303-324

§§ 1-5, Sensations and Things. — § 6, The Subjects investigated. — § 7, Specific Energy of the Nerves. — §§ 8-11, Sensations of Smell. — §§ 12-15, Sensations of Taste.— §§ 16-17, The Varieties of Sound. — §§ 18-20, The Pitch of Tones. -^§§ 21-22, The Composition of Clangs. — § 23, Analysis of Sounds by the Ear.

CHAPTER IV. The Quality of Sensations [Continued] 325-355

§ 1, Analysis of Sensations of Sight. — §§ 2-3, The Stimulus of Sight. — § 4, Relation of Quality and Quantity. — §§ 5-8, The Different Color-

TABLE OF CONTENTS.

Tones. — § 9, The Complementary Colors. — §§ 10-13, Conditions of Changes in Coloi-. — § 14, Phenomena of Contrast. — §§15-17, Theories of Visual Sensations. — § 18, Symbolism of Visual Sensations. — § 19, Sensations of the Skin. — § 30, The Muscular Sensations. — § 21, Sensations of Pressure. — §§ 22-24, Sensations of Temperature. — § 25, Specific Energy of the Nerves.

CHAPTEK V.

FAOg

The QuAwriTY of Sensations 356-381

§§ 1-3, Distinction of Variations in Quantity. — §§ 4^5, The Measurement of Sensations. — § 6, Nature of the Least Observable Difference.— § 7, The Determining of the Limits. — § 8, Methods of Experiment.— § 9, Statement of Weber's Law. — § 10, Measurement of Sensations of Pressure. — § 11, Measurement of Sensations of Temperature.—§§ 12-15, The Intensity of Sounds.— §§ 16-18, The Intensity of Visual Sensations. — ^§ 19-21, Measurement of Taste and Smell.— §§ 32-23, Value of Weber's Law.

CHAPTBE VI.

The Presentations of Sense 382-419

§§ 1-2, Sensations and Things. — § 3, General Nature of the Presentations of Sense. — §§ 4-5, Laws of the Synthesis of Sensations. — §§ 6-7, Nativistic and Empiristic Theories. — §§ 8-11, Nature of the Spatial Series.— §§ 12-15, The Theory of Local Signs.— § 16, The Stages of Perception. — § 17, Perceptions of Smell. — § 18, Perceptions of Taste.— §§ 19-20, Perceptions of Hearing.— §§ 21-22, Sense of Locality by the Skin.— §§ 23-25, Weber's Sensation-Circles.— § 26, The Discernment of Motion. — § 27, Localizing of Temperature-Sensations.— § 28, Localizing of Muscular Sensations. — §§ 29-30, Construction of the Field of Touch. — § 31, Feelings of Double Contact.

CHAPTEK Vn.

The Presentations of Sense [Continued] 420-46']

§ 1, General Principles ai)plied to the Eye. — § 3, Data or Motifs of Vision. — §§ 3-6, Nature of the Primary Retinal Field. — § 7, Value of the Retinal Elements. —ij^ 8-9, Motions of the Eye.— § 10, The Law of Listing. — § 11, Meridians of the Field of Vision. — § 12, Effect of Accommodation. — i:§ 13-16, Single and Double Images.— § 17, The Fixation of Attention.— §§ 18-20, Stereoscopic Vision and Vision of Perspective.- §§ 21-23, The Use of Secondary Helps.— §§ 24-25, General Office of Experience. — §§ 26-28, Judgment of Spatial Extension and Relations.— i^ 29, Visual Perception of Motion.— §§ 30-34, Errors of Sense.— § 35, Development of Visual Percei:)tiou.

TABLE OF CONTENTS. XI

CHAPTER Vm.

PAGE TrME-REIiATIONS OF MENTAL PHENOMENA 468-497

§§ 1-3, Time-Form as belonging to Mental Phenomena. — i^§ 3-4, Elements of Physiological and Psycho-physical Time. — §§ 5-7, Effect of the Inertia of the Nervous System. — §§ 8-12, Nature and Length of Simple Reaction-Time. — §§ 13-15, Methods for discovering Apperception-Time.— §§ 16-18, Length of Apperception-Time. — §§ 19- 30, Nature of Will-Time.— §§ 21-23, Subjective Estimate of Time.— §§ 24-85, Reaction-Time of complex Mental Processes. — § 86, The Circuit of Consciousness. — § 37, Various Influences upon Psycho-physical Time. — § 88, Conclusions.

CHAPTER IX.

Feelings and Motions 498-581

§ 1, Nature of the Inquiry. — §§ 3-4, Physiological Theory of the Nature of Feeling. — §§ 5-7, Psychological Theory of the Nature of Feeling. — §^ 8-9, Classification of the Feelings. — §§ 10-11, Characteristics of all Feeling. — § 13, Physical Apparatus of Feeling. — §§ 13- 14, Nature of Common Feeling. — §§ 15-18, Feelings of Sensation. — §§ 19-31, The Emotions.— § 33, Mental Moods.— §§ 33-34, The Esthetic Feelings. — § 35, The Intellectual Feelings.— § 36, The Feeling of Effort. — §§ 27-31, Voluntary and Involuntary Movements.

CHAPTER X.

Physical Basis of the Higher Faculties 533-559

§§ 1-3, The Method of Investigation. —§§ 4-6, Physiological Basis of Acts of Will.— §§ 7-9, The Will in Attention.— §§ 10-11, Cerebral Processes in Attention. — § 13, Freedom of Will. — § 13, Physical Basis of Consciousness. — §§ 13-14, Physiological Basis of Memory. — §§ 15- 17, Memory as Retentive. — §§ 18-19, Memory as Reproductive. — §§ 30-31, Organic Memory. — § 33, Memory as a Psychical Process. — § 33, Physical Basis of Conception.

CHAPTER XI.

Certain Statical Relations of the Body and Mental Phenomena 560-583

§§ 1-3, Popular Estimate of the Relations of Body and Mind. — §§ 3-9, Relations Dependent on Age and Development. — §§ 10-18, Relations Dependent on Sex. — § 13, Characteristics of different Races. — § 14, The Theory of Temperament. — §§ 15-17, Kinds of Temperament.— § 18, Physical Basis of Temperament. — § 19, General Correlations of Body and Miud.

XU. TABLE OF CONTENTS.

Part Third. The Nature Of The Mind

CHAPTEE I.

PAGE

The Faculties of the Mind, and its Unity 585 -613

§§ 1-4, The Method of Investigation. — §§ 5-6, Mental Phenomena and Cerebral Changes. — §§ 7-9, Physical Theory of the Cause of Psychical States.— § 10, The Subject of Psychical States.— g§ 11-13, Variety of the Phenomena of Consciousness — ^§ 14-16, Classification of Psychical States.— § 17, Nature of the Mental Faculties.— §§ 18-23, The Unity of Consciousness.

CHAPTER 11.

The Development of the Mind 614-632

§§ 1-2, Genetic Study of the Mind.— §§ 3-5, Reality of Mental Development. — §§ 6-8, Stages of Mental Development.— §§ 9-10, Dependence of Mental on Physical Growth.— § 11, Psychical Factors in Development.— §§ 12-16, The Theory of Mental Evolution.

CHAPTER III.

Real Connection of Brain and Mind 633-667

§ 1, General Question of a Connection of Brain and Mind. — §§ 2-6, The Brain as the " Seat" of the Mind.— §§ 7-9, The Brain as the "Organ" of the Mind. — § 10, Special "Bond" between Brain and Mind. — § 11, Figurative Connection of Brain with Mind. — § 12, Causal Relation of Brain and Mind. — §§ 13-14, Occasionalism and Pre-established Harmony. — § 15, Positivism and Monism. — § 16, The Position of Dualism. — § 17, Conservation and Correlation of Energy. — §§ 18- 24, The Causal Nexus declared Valid.

CHAPTER IV.

The Mind as Real Being 668-688

§ 1, The Metaphysical Treatment of Mind.— §§ 2-8, The Mind as a Real Being.— §§ 9-10, The Spirituality of Mind.— §§ 11-15, The Unity of Mind.— § 16, First and Last Things of the Mind.

PHYSIOLOGICAL PSYCHOLOGY.

nSTTEODUOTIOH.

§ 1. A CLEAR conception oi Physiological Psychology requires some special knowledge of the nature and methods of those two sciences, the results of whose investigation it endeavors to combine. These sciences are, of course, Psychology and Physiology — the latter being understood in a broad way as including also various apphcations of the general theory of physics to the functions of the animal organism. But as the form taken by this compound term would itself seem to indicate, the two do not stand upon precisely the same level in effecting this combination, whether we consider the end that the one science into which both enter desires to reach, or the means that it employs to reach the end. For the noun ("psychology") in the compound term may be said more particularly to define the end desired; the adjective ("physiological") the character of the means which it is proposed especially to employ. Hence "Physiological Psychology" can scarcely claim to be an independent science, or even a definite branch of the science of psychology in general. It is rather to be regarded simply as psychology approached and studied from a certain — the so-called " physiological " — side or point of view. It is necessary, then, in the first place, to define what we understand by the science of psychology, and how it is proposed to treat this science as subject to the physiological method, and as approached by means of physiological experimentation and researches.

§ 2. Perhaps the most common definition of psychology, up to the present time, has regarded it as " the science of the human soul." If this definition had always been given, on beginning the pursuit of the science, only in a provisional way, and with the implied or open confession that it is the business of psychology itself to demonstrate the existence of a particular entity called " the soul," and to show how this entity is needed to explain the phenomena of consciousness, then httle valid objection could have been made to it. But such has by no means been the case. For example, one writer on the subject (Drbal), at the very commencement of his treatise, asserts that "psychology is the science of the human soul as the real foundation of the spiritual life ; " and another (Erdmann) declares that " the subject-matter of psychology is the subjective spirit," meaning by this term the human soul. Objections have, therefore, been more or less fitly and forcefully urged agninst this definition as ordinarily employed. It has been said that clearly ■we have no right to assume any such entity as the soul ; and even that a careful study of all the phenomena — especially by the experimental and physiological method — does not compel or induce us to conclude that such entity exists. It has been claimed, especially of late, that there may be a " psychology without a soul," and, indeed, that this kind of psychology is alone worthy of being considered truly scientific. Further objection to the same definition has been made in other quarters, because it seems to regard the question as settled, whether man has not moi-e than one subject (or "ground") of the manifold phenomena called psychical ; whether, in fact, he may not be the fortunate possessor of both an " animal " and a "rational" soul, etc. It would be aside from the course of our inquiries to consider these objections in detail at this time ; or to state at any length how far we are inclined to agree with them and how far to express dissent. They may all be, for the present, set aside by stating the course of procedure which the study of psychology from the physiological point of view seems to us plainly to recommend.

The satisfactory definition of any science is often one of the latest and most difficult achievements of that science. When such definition is placed at the beginning of an investigation, it must often really include results reached only by going carefully and repeatedly over the entire ground of the science. In all such cases the learner of the science is quite unable fully to comprehend the definition, or to understand the positions upon various disputed questions which it may really involve. In general, then, it is better that the earliest so-called definition should be simply a description of that class of phenomena which it is proposed, as far as possible, to isolate for purposes of inquiry. This remark applies with peculiar force to psychology, both on account of such objections as those mentioned above, and also on account of certain difficulties inherent in the subject itself. Accordingly, it will serve our purpose best to "define " this science simply by ascribing to it a certain more or less definite sphere of plienomena. Thus we shall consider psychology as that science which has for its primary subject of investigation all the phenomena of human consciousness, or of the sentient life of man. If the term " sentience " be employed as preferable to consciousness, it must be understood as equivalent to consciousness in the broader sense of the latter word. This definition, or rather description, plainly implies an acquaintance experimentally with certain phenomena that cannot, strictly speaking, be defined. These are the phenomena of consciousness ; and one result of all our subsequent investigations will be to show us that consciousness and its primary phenomena can never be defined. The definition of psychology need not, however, be understood to imply the real existence of any one entity such as a soul.

Nevertheless it would be very inconvenient, not to say impossible, to begin and continue the investigation of psychical phenomena, using only roundabout phrases through fear of implying the real existence of some spiritual entity called the Soul or the Mind. In some sort there cannot be any description, much less any scientific study, of the phenomena of consciousness without implying somewhat which requires us to use a word like these. In all languages, and in the constant everyday use of them all, men in stating and describing the phenomena of their own sentient life employ such terms as "I" and "me," and place in a kind of contrast with them such other terms as " thou " and " he " or "it." Inasmuch as recollection, and the assumption of some kind of continuous personal identity, enter into all their experience, and underlie all their relations with each other and with the physical world which surrounds them, they are compelled to use language implying a permanent subject of the phenomena of consciousness. No one doubts as to his right to ascribe to himself the phenomena of his own consciousness ; and as well to ascribe certain other phenomena, which are not attributed to himself as their subject, to other subjects (so-called " persons "), which he supposes to have, each one, a consciousness of his own. No one doubts that this subject is in every case somehow the same with itself from hour to hour and day to day, and even from year to year. In all the earlier part of this treatise the word "mind" will be employed simply as the equivalent of the subject (which all language as expressive of universal experience necessarily recognizes) of the phenomena of consciousness. In other words, whatever all men inevitably mean by the word "I" (the empirical ego of philosophy), whenever they say / think, or feel, or intend this or that ; and whatever they understand others to mean by using similar language — thus much, and no more, we propose at first to include under the term "mind." This term is preferred to the word "soul," in part out of concession to the prejudices to which allusion has already been made, and in part because it seems to admit of the handling which it is proposed to give to it subsequently, with more freedom from entangling alliances with ethical, social, aad religious ideas. In other words, we wish to begin and continue as far as possible upon purely scientific grounds. And when, subsequently, these grounds are in part abandoned for certain fields of rational speculation, we wish to have the connection between the two kept open and unimpeded.

§ 3. In accordance with what has already been said concerning the nature of psychology, Ave may define Physiological Psychology as the science which investigates the phenomena of human consciousness from the " physiological " point of view or method of approach. Remembering the cautions which have akeady been expressed, we may also say that it is the science of the human mind as investigated by means of its relations to the human physical organism. A more accurate definition, however, requires that something further should be said concerning the nature and method of that science which furnishes the adjective to our compound term. Human Physiology is the science of the functions (or modes of the behavior in its correlated action) of the human physical organism. As studied at present it implies an acquaintance with the fields of gross and special microscopic anatomy (histology), of embryology and the general doctrine of development, of biology, — including the allied phenomena of plant life, — of molecular physics and chemistry as related to the structure and action of the bodily tissues, and of other forms of kindred knowledge. It is only a relatively small part of this vast domain, however, with which Physiological Psychology has directly to deal ; for it is only a part of the human organism which has any direct relation to the phenomena of consciousness. As will appear subsequently, it is with the nervous system alone, that our science has its chief immediate concern. Indeed it might be described — though in a stiU somewhat indefinite, but more full and complete, way — as the science which investigates the correlations that exist between the structure and functions of the human nervous mechanism and the phenomena of consciousness, and which derives therefi'om conclusions as to the laws and nature of the mind.

§ 4. Physiology is compelled, from its very nature as a physical science, to regard the nervous system as a mechanism. Physiological Psychology, inasmuch as it relies so largely upon physiology for its data and method and points of view, is also required to consider

IlSrTRODTJCTION". 5

this system in the same way. Those unique relations in which the structure and functions of the nervous substance of the body stand to the phenomena of the mind cannot deter the investigator from ass)iming toward it the so-called mechanical point of view. Physiology presents psychology with a description of this nervous substance as a vast and complex system of material molecules, which are acted upon by different forms of the energy of nature outside (external stimuli), and by intimate changes in the contiguous molecules of the other substances of the body (internal stimuli) ; and which behave as they do on account of the influences thus received, as well as on account of their own molecular constitution and arrangement. But all this is the description of a material mechanism. The word "mechanism "is preferable to the word "machine" for describing such a system of interacting molecules as constitute the living nervous substance, because we attach to the latter word the mental picture of something which has a certain magnitude and rigidity of parts that act and react upon each other in a palpable way under the ordinary laws of mechanics. A steam-engine is a machine whose parts may be seen to push and pull and turn each other after the ordinary fashion of all levers and wheels. But the molecules of the steam, from the activity of which all the motion of the rigid and ponderous parts of this machine is derived, are no less material and governed by physical law in their changing relations to each other than are the masses of the machine itself. The interaction of the minute particles of the steam falls more fitly, however, under the conception of mechanism. Indeed, it is only as falling under this general conception that these molecules admit of any scientific treatment at all. Now it is not our purpose to begin the consideration of the human nervous system by debating the question, how completely it falls under the conception of mechanism, and whether some other conception be not needed to supplement this when the unique relations of this system to the phenomena of the mind are taken into account. Whatever is to be said upon such a question must appear in its proper place in the order adopted for the discussion of the general subject. Physiological Psychology, however, can scarcely establish itself at all unless it be willing to receive from the proper one of the two sciences which enter into it that conception of the nervous system at which this science has arrived as the result of the most successful modern researches. As far as the nervous system admits of being subjected at all to scientific treatment, for the purpose of attaining a more complete knowledge of the nature of its functions, it is necessarily considered as a complex molecular mechanism. We shall, then, receive, in a grateful