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Physiological Psychology 















There can be no doubt that an important movement in psychol- 
ogy 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 cer- 
tainly indulged in extravagant claims as to the past triumphs of so- 
called Physiological Psychology, and in equally extravagant expec- 
tations 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 al- 
leged results of Physiological Psychology ; and this will be done at 




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 seri- 
ous loss. 

The study of the phenomena of consciousness by the method here 
proposed necessarilj' requires some acquaintance with a consider- 
able 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 indi- 
cates — 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 com- 
pelled 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 confi- 
dence 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 ex- 
ception 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 Physiolog- 
ical 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- 


ities in the text, and by a few (in comparison with the number 
which might have been cited) references in foot-notes, I have con- 
nected 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, Pro- 
fessor of Physiology in the Yale Medical School, for valuable as- 
sistance 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. 

Yale Univeksity, New Haven, February, 1887. 



Introduction 1-14 



The Elements of the Nervous System 17-55 

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


Combination of the Nervous Elements into a System 56-101 

§§ 1-3, Threefold Plan of the Nervous System.— § 4, The Sympa- 
thetic 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, Struct- 
ure of the Pons Varolii.— §§ 18-24, Structure of the Cerebrum. — 
§§ 25-27, Cortex of the Cerebral Hemispheres.— §§ 28-29, Arrange- 
ment of the Nerve-Tracts. — § 30, The Cranial and Spinal Nerves. 


The Nerves as Conductors 102-129 

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


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



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 Ob- 
longata— -§§ 17-19, Centres of the Medulla Oblongata.— § 20, Influ- 
ence of the Cerebral Lobes, — § 21, Functions of the Cerebellum. — 
g§ 22-27, Functions of the Basal Ganglia.— § 28, Gray Matter of the 
Third Ventricle. 


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-Or- 
gans 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 


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. 


Mechanical Theory of the Nervous System 214-23(1 

§ 1, Machine-like Nature of the Body.— §§ 2-6, The Nervous Sys- 
tem 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. 






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 Evi- 
dence from Anatomy. — § 19, True Method of Investigation. 


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 Evi- 
dence of Histology. — § 18, Relation of Motion and Sensibility. — §§ 19- 
21, Visual and Auditory Centres of Ferrier and Munk. — ^ 22, Ex- 
ner'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 Conclu- 
sions of Goltz. — § 31, Conclusion as to three leading Principles. 


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. 


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- 


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, Sen- 
sations of Pressure. — §§ 22-24, Sensations of Temperature. — § 25, 
Specific Energy of the Nerves. 



The QuAwriTY of Sensations 356-381 

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


The Presentations of Sense 382-419 

§§ 1-2, Sensations and Things. — § 3, General Nature of the Pres- 
entations 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 Lo- 
cality by the Skin.— §§ 23-25, Weber's Sensation-Circles.— § 26, The 
Discernment of Motion. — § 27, Localizing of Temperature-Sensa- 
tions. — § 28, Localizing of Muscular Sensations. — §§ 29-30, Construc- 
tion of the Field of Touch. — § 31, Feelings of Double Contact. 


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 Exten- 
sion and Relations.— i^ 29, Visual Perception of Motion.— §§ 30-34, 
Errors of Sense.— § 35, Development of Visual Percei:)tiou. 




§§ 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 Ap- 
perception-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-phys- 
ical Time. — § 88, Conclusions. 


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, Charac- 
teristics 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 Es- 
thetic Feelings. — § 35, The Intellectual Feelings.— § 36, The Feeling 
of Effort. — §§ 27-31, Voluntary and Involuntary Movements. 


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. 


Certain Statical Relations of the Body and Mental Phe- 
nomena 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 Tempera- 
ment. — § 18, Physical Basis of Temperament. — § 19, General Corre- 
lations of Body and Miud. 





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. 


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. 


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-estab- 
lished Harmony. — § 15, Positivism and Monism. — § 16, The Position 
of Dualism. — § 17, Conservation and Correlation of Energy. — §§ 18- 
24, The Causal Nexus declared Valid. 


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. 



§ 1. A CLEAR conception oi Physiological Psychology requires some 
special knowledge of the nature and methods of those two sci- 
ences, 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 apphca- 
tions 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 particu- 
larly to define the end desired; the adjective ("physiological") 
the character of the means which it is proposed especially to em- 
ploy. 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 psychol- 
ogy approached and studied from a certain — the so-called " physi- 
ological " — 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 physi- 
ological method, and as approached by means of physiological ex- 
perimentation 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 de- 
monstrate the existence of a particular entity called " the soul," and 


to show how this entity is needed to explain the phenomena of con- 
sciousness, 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 (Erd- 
mann) declares that " the subject-matter of psychology is the sub- 
jective 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, espe- 
cially of late, that there may be a " psychology without a soul," and, 
indeed, that this kind of psychology is alone worthy of being con- 
sidered 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 psy- 
chology from the physiological point of view seems to us plainly to 

The satisfactory definition of any science is often one of the latest 
and most difficult achievements of that science. When such defini- 
tion 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 "de- 
fine " 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 con- 
sciousness, it must be understood as equivalent to consciousness in 
the broader sense of the latter word. This definition, or rather de- 
scription, plainly implies an acquaintance experimentally with cer- 
tain phenomena that cannot, strictly speaking, be defined. These 
are the phenomena of consciousness ; and one result of all our sub- 
sequent 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 impos- 
sible, to begin and continue the investigation of psychical phenom- 
ena, 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 scien- 
tific study, of the phenomena of consciousness without implying 
somewhat which requires us to use a word like these. In all lan- 
guages, 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 phenom- 
ena, 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 equiva- 
lent of the subject (which all language as expressive of universal 
experience necessarily recognizes) of the phenomena of conscious- 
ness. 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 under- 
stand 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 alli- 
ances 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 aban- 
doned 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 con- 
sciousness 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 or- 
ganism. A more accurate definition, however, requires that some- 
thing 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 correla- 
tions 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 

§ 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 


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 as- 
s)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 be- 
have 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 ner- 
vous substance, because we attach to the latter word the mental pict- 
ure 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 mate- 
rial and governed by physical law in their changing relations to 
each other than are the masses of the machine itself. The inter- 
action of the minute particles of the steam falls more fitly, how- 
ever, 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 mechan- 
ism, and whether some other conception be not needed to supple- 
ment 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 com- 
plex molecular mechanism. We shall, then, receive, in a grateful 


and docile manner, all tliat the noble science of human physiology 
has to teach us, under the guidance of the conception of a mech- 
anism, both directly concerning the manner in which the nervous 
matter of the human body performs its wonderful functions, and 
more indirectly concerning the relations in which these functions 
stand to the phenomena of consciousness. 

§ 5. Physiological Psychology — it is by this time apparent — par- 
takes of the nature and methods of two sciences that differ widely 
from each other. One is a science which involves introspection ; 
for it is only by the method of introspection that the real and pres-^ 
ent facts of human consciousness can be reached. The other is a 
physical science, and involves external observation to determine the 
external facts of the structure, development, and functions of a 
physical mechanism. Two sets of phenomena must then be exam- 
ined in their relations to each other, and, so far as possible, the 
laws (or permanent modes) of these relations pointed out. It is 
due to this fact, in part, that both the peculiar difficulties and the 
peculiar interest and value of psycho-physical researches are so 

In every science a beginning is first made by ascertaining and 
comparing together all the important phenomena ; the laws, or 
regular modes of the occurrence of the phenomena in relation to 
each other, are then investigated ; and, finally, certain conclusions 
are drawn concerning the nature and significance of those real be- 
ings which reason compels us to assume as permanent subjects of 
the different classes of phenomena. In its effort to establish itself 
upon a scientific basis. Physiological Psychology has no choice but 
to follow essentially the sarce method of procedure. In its case, 
however, as has already been remarked, the phenomena which are 
to be ascertained and compan d belong to two orders that obviously 
differ greatly from each other ; and the laws which it is sought to 
discover ai'e laws which maintain themselves between these two or- 
ders of phenomena. The phenomena of the nervous system, like 
all physical phenomena, consist in changes in the constitution and 
mutual relation of material masses and molecules. They are, then, 
of a kind to be related to each other, under the conception of mech- 
anism, inside of the nervous system and of the entire human body ; 
and also, outside of the body, to the various forms of physical energy 
in nature which act upon these masses and molecules. But the 
psychical jDhenomena are states of consciousness, constantly shifting 
modes of the behavior of that subject which Ave have agreed — as 
much as possible without involving any j^remature assumptions — to 
call the Mind. Still the above-mentioned two orders of phenomena 


are obviously to a large extent related to each other ; they may, in 
fact, be said to be correlated in a unique manner. The constant 
forms of this correlation constitute the laws for the discovery of 
which Physiological Psychology undertakes its special researches. 
It endeavors to bring the two orders of phenomena face to face, to 
look at them as they stand thus related to each other, and, as far as 
possible, to unite them in terms of a uniform character, under law. 
It might seem that simjDly to attempt the accomplishment of the 
task just described should satisfy all legitimate demands. And, 
indeed, no little protest has of late been made against any attempt 
on the part of scientific psychology (and how much more when 
studied from the physiological and experimental point of view) to 
proceed further than this. All inquirers have been warned, not only 
against introducing metaphysical assumptions into the beginnings 
of psychology, but also against allowing any admixtui'e of the two 
during the investigations pursued by the latter. We have, indeed, 
just agreed that metaphysical assumptions as to the natui-e of mind 
should prejudice as Httle as possible our statement of psychological 
facts and laws. But if the warning against so-called " metaphysics " 
be understood to mean that inquiry must be stopped when the 
phenomena and their uniform modes of relation have been enume- 
rated, and that no venture must be made upon any discussions or 
conclusions regarding the real nature of the subject of them all 
(the mind), such warning may very weU be quietly disregarded. 
What we are chiefly interested in, on undertaking aU josychological 
investigation, is the real nature — the permanent characteristics, the 
claims to be a substantial existence, a spiritual unity — and the ori- 
gin and destiny of the mind. To assume as little as possible con- 
cerning all this, at the first, is simply a matter of wise reserve and 
self-control in the interests of scientific investigation. We feel no 
hesitancy, however, in announcing our intention, ultimately, to 
draw whatever conclusions seem to us legitimate and desirable con- 
cerning many of these so-caUed "metaphysical" inquiries. Psy- 
chology — no less truly when studied from the physiological and ex- 
perimental point of view — has the undoubted right, and is under 
the obhgation, to contribute as much as possible toward the solu- 
tion of these inquiries. Nor do observation and wide reading 
show that the advocates of " psychology without a soul," and freed 
from all metaphysics, are at all certain to avoid drawing con- 
clusions, not to say introducing illegitimate assumptions, upon 
these very same inquiries. In brief, Physiological Psychology has 
the right, which belongs to it as a science, to introduce whatever 
conclusions as to the natiire of mind follow legitimately from its 


discussions of plienomena and laws. It has even a right to in. 
dulge in well-founded and reasonable speculation. Such things are 
not necessarily objectionable when indulged in by any of the more 
purely physical sciences. Indeed, there is not one of these sciences 
which would not look comparatively bare and unattractive if wholly 
stripped of its more or less questionable inferences, its metaphysi- 
cal assumptions, its guessings, and speculations. 

§ 6. The remai'ks immediately foregoing serve to indicate what 
are the principal Divisions of this work. The Fii'st Part will 
consist of a description of the structure and functions of the Ner- 
vous System. This system will there be considered tmder the 
conception of a mechanism, and as far as possible without any 
direct or indirect reference to the phenomena of consciousness as 
determined by introspection. The Second Part will describe the 
various classes of correlations which exist between the phenomena 
of the nei'vous mechanism and mental phenomena. It will also 
attempt to state what is known of the laws which maintain 
themselves over these various classes. No attempt will be made, 
however, to describe and discuss any of the phenomena which may 
be classed as abnormal, or as consisting (so far as they are psychical) 
in so-called " disturbances of consciousness," except when reference 
to such abnormal phenomena is necessary in order to explain those 
which are called ordinary or normal. The phenomena of insanity, 
delirium, hypnotism, somnambulism, ecstasy, mind-reading, spir- 
itualism, and even of sleep and dreaming, will therefore be defi- 
nitely excluded. The chief reason for such exclusion is to be found 
in a lack of space, it being difl&cult even to bring within the limits 
of a single volume a sufficiently thorough discussion of the more 
ordinary phenomena with which Physiological Psychology is caUed 
upon to deal. 

The various correlations of the mind and the nervous mechanism 
(of which the Second Part treats) may be conveniently consid- 
ered under several principal groups or classes. The first of these 
includes more particularly such relations as can be established be- 
tween the condition and activity of the supreme nei-vous centres 
and the phenomena of conscious sensation and voUtion. Most of 
what can be said at present upon this point may be summed up in 
the discussion of the localization of cerebral function, as taken in 
connection with the description of the automatic and reflex action of 
these centres considered as parts of the nervous mechanism. The 
second class of these correlations covers all the phenomena with 
whifh psycho-physics (in the more precise use of the term) attempts 
to deal. It- discusses the relations which exist between the quality, 


quantity, combination, and order of succession in time, of the vari- 
ous stimuli which act upon the nervous system, and the kind, mag- 
nitude, composite result, and time-relations of the mental phe- 
nomena. Hence the significance of the term psycho-physics. As 
Physiological Psychology is ordinarily and legitimately treated, 
it includes these more specially psycho-physical researches. An- 
other class of these correlations covers certain related phenomena 
of mind and body as dependent upon age, sex, race, etc. 

Besides the foregoing groups, or classes, certain observations 
which have more or less of scientific confirmation and value, 
may be made regarding the physical basis of the feelings and voli- 
tions controlling the bodily members, and of the higher faculties of 
memory, association of ideas, etc. The Third Part will fitly intro- 
duce, at the close of the psycho-physical researches, the presenta- 
tion of such conclusions as may be legitimately gathered, or more 
speculatively inferred, concerning the nature (considered as a real 
being) of the human mind. The justification of the order and ex- 
tent of the entire discussion, and especially of the Third Part as a 
whole, has already been given to some extent ; the rest must be 
left to the progress and result of the discussion itself. 

§ 7. It has already been said that the peculiarity of Physiological 
Psychology, considered as a branch of the general science of mind, 
consists largely in the method of its approach to its subject. At- 
tention must now be more specifically called to this method as 
necessarily partaking of the methods of the two sciences whose 
researches it undertakes to combine. The method of physiology, 
which is in general that of external observation as employed in all 
the physical sciences, should be applied only when supplemented by 
the many delicate and accurate instruments of observation now at 
command, and guarded and checked by that accumulation of expe- 
rience concerning the best ways of studying nature and concern- 
ing her ways of working which the whole body of such sciences 
has made. On the other hand, the method of psychology has or- 
dinarily been defined as solely the method of introspection or self- 
consciousness. These two methods are obviously very different. 
It would not be strange, then, if the science which finds it neces- 
sary to combine the two should experience some special difficulty. 
This difficulty has, however, more often been exaggerated than ex- 
plained and (what is quite possible) for the most part removed. 

Our present purpose does not require that we should examine at 
length the question whether the introspective method is the only 
one possible in psychology. Scarcely moi-e is necessary than the 
statement of the bearing of this question upon the inquiries it 


is proposed to make. There should in general be no mystery or 
arrogant assumption' about the use of such words as "science" and 
"scientific method." Science is nothing but knowledge — real, veri- 
fiable, and systematic. Scientific method is nothing but the way 
of arriving at such knowledge. Now, although Physiological Psy- 
chology brings the investigator face to face with some of the most 
interesting and distinctive mysteries, it is not, as a science, to be 
regarded as especially mysterious. Inasmuch as its specific busi- 
ness is to ascertain and combine, under definite laws, two widely 
differing classes of facts (facts of the human nervous mechanism and 
facts of human consciousness) it is, of course, compelled, first of all, 
to ascertain both kinds of facts. The phenomena of consciousness, 
as primary facts, can be ascerlained in no other way than in and by 
consciousness itself. Whatever fault may be found with the so- 
called introspective method in psychology, on account of its alleged 
inaccuracy, lack of scientific and progressive quality, etc, from the 
very natui'e of the case no other way of ascertaining what the phe- 
nomena of consciousness in themselves are can ever take the place 
of the dii'ect examination of consciousness. And there is no way of 
directly examining consciousness but the way of being conscious 
one's self. On the other hand, it is perfectly obvious to students 
of psychology and of its history (on grounds which need not be 
stated here) that the scientific treatment of the facts of conscious- 
ness can never be, to any satisfactory extent, accomplished by in- 
trospection alone. For psychology, in order to make valid its claim 
to be a science, must not merely display the alleged facts of individ- 
ual mental expex'ience ; it must treat these facts analytically, must 
resolve them into their ultimate factors, and trace the stages of 
their development from what is simpler to what is more complex ; 
it must also show on all sides their connections and causes, thus 
placing the phenomena of the mind as much as possible in interac- 
tion with the rest of the world. It is because human physiology can 
contribute largely to such scientific treatment (as distinguished from 
the mere observation, grouping, and cataloguing) of the phenomena 
of the mind that it is entitled to be considered as furnishing one 
distinctive and fruitful branch of psychological researches. 

§ 8. The following statements will, accordingly, be found to hold 
good concerning the method of Physiological Psychology. It must 
employ faithfully the methods distinctive of both the two sciences 
which it endeavors to combine. Facts as to the structure and 
functions of the nervous mechanism, and as to the effect upon it of 
various kinds of physical energy acting as stimuli, must be ascer- 
tained by external observation. In general they must be accepted 


by us as contributed from the modern science of human physiology. 
The primary facts of consciousness must be ascertained from con- 
sciousness itself ; or, since they have already been for a long time 
subjected to this form of observation, and tabulated, compared, and 
classified, they may be accepted from the science of introsjjective 
psychology. Care must be taken, however, to make sure that all 
alleged psychical facts are really facts ; but iipon this point, again, 
there is no other way of making sure than in and through conscious- 
ness. The principal laws and inferences also of introspective psy- 
chology may be accepted (at least in a provisional way) on begin- 
ning the study of Physiological Psychology. The final result of 
such study will doubtless be, not only to sujDplement and explain, 
but also to modify and correct, the statement of these laws and in- 
ferences. But here, as in other scientific research, we are obliged 
to work our way through many mistakes, obscurities, and other ob- 
stacles, progressively nearer the complete and verifiable knowledge 
of the truth. 

Furthermore, from the nature of the case. Physiological Psychol- 
ogy takes its point of starting from the facts and laws of physiology 
as reached by the method of external observation. This follows 
necessarily from the relation in which the two sciences of physiol- 
ogy and psychology stand as entering into the proposed combina- 
tion. The enlargement of our knowledge of the latter is the 
end to be reached ; but the former is to give us the way by which, 
and the guidance under which, the approach to this end must be 

It will also become evident, in the course of the following inves- 
tigation, that we are seldom or never able to proceed directly with 
the work of comparing the immediate physical antecedents or con- 
sequents of the mental phenomena with these phenomena them- 
selves, and so of drawing conclusions at once as to the laws by 
which the two classes of facts are connected. Such immediate an- 
tecedents and consequents are hid in the inexplorable recesses of 
the living and molecularly active brain. It is seldom, indeed, that 
our direct observation can approach within the tenth, or it may be 
within the hundredth, remove of what goes on in these recesses. We 
are obliged to examine the physical phenomena from a greater dis- 
tance and in a more indirect way. For example, physics can inform 
us what combinations of what wave-lengths of the vibration of ether 
fall on the eye when a certain form of conscious sensation, which 
we call " yellow " or " red " or " blue " arises ; physiology can lo- 
cate the nervous elements of the retina upon which the waves fall, 
can conjecture something as to the chemical changes there produced, 


and trace doubtfully the paths along which the resulting nervous 
impulses rise to the brain and diffuse themselves over certain of 
its areas ; psycho-physics can tell approximately the relatious in 
which the varying quantities of the stimulus stand to the resulting 
degrees of the sensations. But in all this we are still at a great dis- 
tance from the enjoyment of those opportunities which would seem 
necessaiy to make the science of Physiological Psychology as com- 
prehensive and exact as could readily be wished. As a rule, certain 
kinds and amounts of physical energy, more or less definitely meas- 
urable, are known to be acting on the peripheral parts of the body, 
and the next series of observed facts is the emergence in conscious- 
ness of a psychical experience quite unlike all kinds of physical en- 
ei'gy. To be sure, Fechner's ' conception of psycho-physics is that 
it treats those "physical activities which are the bearers {Trdger) or 
conditions of the psychical, and accordingly stand in direct func- 
tional relation with them ;" or again, "psycho-physics is an exact 
doctrine of the relations of function or dependence between body 
and soul — of the universals that lie between the bodily and spirit- 
ual, the physical and psychical world." But it will be seen that of 
such physical activities we have little or no assured knowledge ; al- 
though we have the best of grounds for believing that such activities 
exist, and that they stand in important relations under law with the 
facts of the conscious psychical life. 

It follows, then, that Physiological Psychology is, pre-eminently, 
first experimental and then speculative ; it can never become 
strictly demonstrative, or even deductive to any considerable ex- 
tent. That a strictly demonstrative science of the relations be- 
tween the structure and functions of the nervous mechanism and 
the phenomena of consciousness is impossible, we might argue from 
the most ordinary experience. To infer from certain movements 
of material molecules that certain facts of consciousness munt take 
place, under the most universal laws of all Being, involves a kind 
and amount of knowledge of which we cannot even clearly conceive. 

In brief, our proper course will be, first, to explain, as completely 
as possible, the structure and functions of the nervous mechanism ; 
and then to set forth, as fully as the present means at disposal will 
permit, the various relations in which its action under stimuli 
stand to the phenomena of the mind. In attempting the latter 
problem we shall come upon a few, but only a few, general state- 
ments of fact which deserve to be s^ooken of as laws in any strict 
meaning of the word. 

1 Elemente d. Psychophysik, pp. 8 and 10. Leipzig, 1860. 


§ 9. If the correctness of the remarks last made be admitted, the 
inquiry may be raised : What justification has this so-called sci- 
ence of Physiological Psychology for the large claims which it has 
made of late ; and, indeed, what right has it to exist as a special 
discipline at all? The full answer to the call for self-justification 
must be made by the actual achievements of the science itself. It 
will be better, then, to leave it to the convictions of the reader when 
the presentation of these achievements shall have been made. But 
even at this point an appeal may be taken to certain facts. We 
have already repeatedly conceded the fact that we are to investigate 
the phenomena of consciousness (that is, study psychology) by a 
special method rather than try to establish an independent science 
or even separate branch of the general science of mind. The de- 
mand for a justification is then reduced to this — Is there valid 
reason for studying psychology in this particular way ; for approach- 
ing its domain through the researches and conclusions of physi- 
ology ? To such a question there can be but one intelligent answer. 
There is an abundance of valid reason. 

The history of modern investigation, and the conclusions of the 
modern science of man, both physical and psychological, emphasize 
the necessity of studying his nature and development as that of a 
living unity. Such science shows man to be at the head of a series 
of physical and psychical existences ; he cannot be understood as 
he is, in his whole nature and in his place within nature at large, 
without taking both sides of this living unity into account. For 
man is known to himself as body and mind — and not as bodiless 
spirit or a mindless congeries of moving molecules. That the struct- 
ure and functions of the body, especially of the nervous mechan- 
ism, and the activities of the mind, are extensively and intimately 
correlated, is a fact beyond all doubt. It is the particular task of 
Physiological Psychology to show in what manner, and to what ex- 
tent, such correlation exists. Moreover, there are few questions 
more interesting, from a philosophical and an ethical point of view, 
than such as the following : What is the nature of mind, considered 
in the light of its correlations with the body ? and. Do the so-called 
physiological and the so-called psychical phenomena belong to one 
subject, or to more than one? But these and similar questions 
can be scientifically answered only by giving a speculative treat- 
ment to the conclusions of psycho-physical investigation. 

In brief, it may be said that introspective psychology, important 
as its results have been, and indispensable as its method is, has 
shown its incompetency to deal with many of the most interesting 
inquiries which it has itself raised. On the other hand, psychology 


as pursued by tlie experimental and physiological method has al- 
ready thi'own a flood of fresh light upon many of these inquiries. 
We may affirm with Wundt,' without fear of successful contradic- 
tion : "Psychology is compelled to make use of objective changes 
in order, by means of the influences which they exert on our con- 
sciousness, to establish the subjective properties and laws of that 
consciousness." On this fact and on th§ real achievements of the 
method we confidently rest its claims to serious and permanent con- 

' Art. "Ueber psychophysiken Methoden," Philosophisclie Studien, 1881, 
heft 1, p. 4. 




§ 1. In all forms of animal life, except the very lowest, the pres- 
ence and activity of a nervous system constitutes the chief charac- 
teristic of their difference from all the more nearly corresponding 
forms of plant life. Both animals and plants are organisms, and 
their structure — regarded as a whole composed of an indefinite 
number of material masses or particles, which move with reference 
to each other for the accomplishment of a common piece of work 
— may be considered as a "natural mechanism." Both have mate- 
rial parts of superior firmness, adapted to divide off and to support 
their softer parts. Plants, as well as animals, are possessed of liv- 
ing, and, more especially, of contractile tissue ; they are therefore 
capable of the functions of nutrition, of propagation, and of that 
so-called automatic motion which is thought to be a fundamental 
property of protoplasm. As is well known, science is not yet able 
always to distinguish between the lowest forms of animal and the 
lowest forms of plant life. But nervous tissue and its functions 
do not belong to the vegetable kingdom ; on the contrary, the pos- 
session and use of at least a rudimentary mechanism of nerve-fibres 
and nerve-cells are found in most members of the animal kingdom. 

It is true that, even in the case of animals which do possess 
a nervous system, the structure and functions of the nervous tissue 
are very closely related to those of the merely contractile tissue. 
Thus the muscular tissue of the animal might seem to be a connect- 
ing-link between its own nervous tissue and the contractile tissue 
of the plant. For the motor nerves, at least, are anatomically con- 
nected by means of their end-plates with the contractile substance 
of the muscular fibre, and the result of modern experimentation, 
with both muscles and nerves, has been to make clear many feat- 
ures of resemblance between them. On the other hand, even the 
isolated nervous elements, when subjected to the same exjDeri- 
mental tests as those which are used to determine the funda- 
mental properties of contractile tissue, exhibit certain marked 
differences of behavior ; while the functions of such elements, 


when combined into a very simple nervous system, are alto- 
gether unique. Moreover, as the nervous system of the animal 
becomes more elaborate and complex, and especially as its central 
organs — spinal cord and brain — are relatively developed, other new 
and wonderful functions are seen to be connected with it. In the 
case of the superior vertebrate animals, and especially of man, the 
significance of this particular form of a physical mechanism be- 
comes, therefore, vastly increased. Thus the minute structure of 
the nervous mechanism invites the student of chemistry, molecular 
physics, and histology, to investigations of the greatest interest and 
yet of extreme difficulty ; while the functions of this mechanism 
are so curiously and intimately connected with the phenomena, not 
merely of all higher animal life, but also of human consciousness, 
that inquiry into them is, among all physical inquiries, the one of 
unparalleled intellectual interest and importance. 

§ 2. It will be the work of this entire treatise to set forth in some 
detail the unique functions of the human nervous mechanism, to 
which allusion has just been made. For the present a very gen- 
eral and somewhat indefinite statement of these functions must suf- 
fice. In general, and somewhat indefinitely, it may be said, then, 
that the one great function of the nervous system is to concatenate (or 
link together into a whole) the manifold elements, both physical and 
psycho-physical, which enter into the twofold life of man. Differ- 
ent and distant parts of the body, whether they belong to the same 
or to different so-called systems (as, for example, the circulatory, 
the secretory, the digestive, the muscular), are bound together, and 
made to exercise their functions in reciprocal dependence and for 
common ends, by the nervous mechanism. The whole body is also 
linked to the external world, and kept in either unconscious or con- 
scious adjustment to the changeful play of its forces, by the same 
mechanism. And further, the development of the mental life, at 
least in all its more primitive and fundamental factors, is mediated 
by the nervous system. For it is certainly in connection with the 
exercise of nervous functions that sensation takes place ; and, by 
development and combination of the sensations, all our perceptions 
of the so-called " Things " of the external world. It is the nervous 
mechanism which uiiites the entire body with the physical stimuli 
of tlie external world, on the one hand, and, on the other, with the 
primitive activities of mind. What relation the nervous functions 
have, and whether they have any direct relation at all, to memory, 
judgment, and the higher activities of mind in general, we do not 
now even inquire. 

The significance of the above-mentioned function of " concatena- 


tion," so far as it concerns the different and distant parts of the 
body, might be illustrated in many ways. Inasmuch as the plant 
is an organism, there is a reciprocal dependence of the structure 
and action of all its parts. But each part of the plant acts directly 
and slowly on only contiguous parts in effecting the distribution 
of the fluids, upon the spread of which the life and growth of the 
plant depend. In the case of the animal, however, an effect pro- 
duced in one part of the body may quickly spread to other distant 
parts by the mediation of the nervous system. The circulation of 
the blood is made to affect, and to be affected by, the state of the 
skin and the muscles, the state of the respiratory organs, or the 
state of the mind's feeling as determined by the ideas before the 
mind. A draught of cold air, for example, strikes some peripheral 
portion of the body ; the heart and lungs modify their activities, 
the muscles contract, and a shudder runs through the physical 
framework ; the secretions are disturbed, and the mind is, perhaps, 
seized with a vague feeling of fear. Such a complex effect of the 
stimulus of cold on some region of the skin has been brought about 
by the action of the nervous system, with its peripheral end-organs, 
conducting nerve-fibres, and nervous centres. Or, again, the seeing 
of some sight or the hearing of some sound is followed by ideas and 
emotions of shame, or of fear, or of joy. A complex co-ordination 
of the muscles then takes place, so as to move the limbs in running, 
to give or ward off a blow, to extend the hand in greeting, to lift 
up or bow down the head. In this case, also, the action of heart 
and lungs and secretory organs is greatly modified ; the capillary 
circulation is altered, and the cheeks are blanched or reddened ; 
the pupils and lachrymal ducts of the eyes are moved ; the very 
hair of the head seems to sympathize with the state of the mind. 
Thus, changes which involve the functions of almost all the tissues 
and organs of the body are accomplished by the mediation of the 
nervous mechanism. Unlike the modifications in expression of 
function which take place in the plant, they are accomplished with 
what seems a practical instantaneousness. The complexity of the 
reciprocal changes which characterize the life of the higher animals 
is due to the general functions of the nervous system ; the speed 
with which the changes are accomplished is dependent upon the 
laws of the propagation of nervous impulses within that system. 

Further illustration of this general office of the mechanism of 
nerve-fibres and nerve-cells in " concatenating " the manifold ele- 
ments of physical and psycho-physical life may well be left to the 
progress of our examination. 

§ 3. The application of the term " mechanism " to the nervous sys- 


tern of man has already (see p. 4 £f.) been partially explained and 
justified. We now describe the elementary parts of such a system 
as considered from the same general point of view which induces 
us to aj^ply this term to the structure and functions of the entire 
system. In order to do this, it is necessary to speak, first, of the 
structure, and, second, of the function of these parts, regarded as the 
fundamental and distinguishing factors of a complex mechanism. 
That is to say, two inquiries must be made : What is the composi- 
tion and form of those ultimate structures called nervous elements, 
into which microscopic anatomy analyzes the nervous system ? and, 
W^hat can such stnictures do which fits them to act as parts of a 
" mechanism " like that of the nervous system ? It is obvious that 
the answers to these inquiries lie at the very entrance upon the 
way toward a complete science of the nervous mechanism. But 
even if the fullest imaginable answers were already attained, much 
would remain to be done in order to perfect the science. Histology 
would still have to inform us precisely how the elements are com- 
bined into the manifold organs of a system. Physiology would 
have to discover the laws according to which the functions of the 
elements are modified, when they act as thus combined. Of course, 
to know completely the nature, number, and properties of all the 
individual factors of a mechanical system, and to know also pre- 
cisely how those factors are combined into the system, as well as 
how their modes of behavior are affected by such a combination, 
would be to have a complete science. of such system. 

A strictly deductive science of the molecular motion, and con- 
sequent function of the elements of the nervous mechanism, is, in- 
deed, a conceivable attainment. But it need scarcely be said that 
we are indefinitely far from, not only the attainment, but even the 
reasonable px'ospect of such a complete physical science of the ner- 
vous system. None of the questions raised respecting the struct- 
ure and functions of its elements, whether considered apart or in 
combination, can be answered with complete satisfaction. More- 
over, the scientific study and description of the neiwous mechanism 
is compelled from the first to pui'sue a somewhat different path 
from that open to many forms of physical science. The direct 
path to the complete science of the subject is impassable ; it is ren- 
dered impassable by the most fundamental and universal of our 
experiences respecting the nature of the phenomena of the nervous 
system. The immediate effects of the molecular changes which 
take place in the nervous elements, even when isolated as much as 
possiV^le, can only with difficulty be made the subject of direct obser- 
vation. Histology has enormous difficulties to overcome in its effort 


to describe how these elements are combined in the living human 
body, and physiology has like difficulties in the way of its effort to 
determine the functions of those organs which are constructed by 
means of such combination. Only the beginning of a theory which 
shall correlate that mode of molecular motion which is peculiar to 
nervous matter with other modes of the motion of matter has yet 
been made. 

In spite of the foregoing concessions, a careful study of the ele- 
ments of the nervous system is the indispensable mode of approach 
to the subject of physiological psychology. It is these elements 
which, when variously combined, constitute all the organs of the 
system ; it is they which, when acting in combination, do all the 
woi'k of the system. 

§ 4. The Elements of the Nervous System of Man, as elements, do 
not differ in any essential known respect from those of other verte- 
brate animals. Upon this subject, then, histology with its micro- 
scope, and physiology with its experimentation, can describe the 
nerve-fibres and nerve-cells of other animals, and then safely draw 
certain inferences from them which will apply to the case of man. 
It is, however, the development of enlarged or of new organs by 
the combination of these elements, and the development and elab- 
oration of function as dependent upon such organs, which consti- 
tute the difference between the nervous system of man and that of 
the lower animals. It is here that histology meets with its supreme 
difficulties and its most interesting problems ; it is here that physi- 
ology is most insecure when trying to carry over to the structure and 
functions of the human nervous mechanism the conclusions which 
have been reached by experiments upon the lower animals. On 
the contrary, the nerve-fibres and nerve-cells of these animals are, 
in most respects, perfectly competent to tell us all we need to know 
regarding the nerve-fibres and nerve-cells of man. In describing 
the constitution, structure, and function of the nervous elements, 
therefore, it will not generally be necessary to pay attention to the 
specific animal form from which the description is taken. In other 
wordSj the discussion of the nervous elements belongs to the most 
general histology and physiology of the nervous system. 

§ 5. The elements of the nervous mechanism require to be con- 
sidered in three ways : (1) as respects their chemical constitution ; 
(2) as respects their formal structure ; (3) as resjDects their general 
physiological function. 

§ 6. The Chemistry of the Nervous System is of necessity in an 
exceedingly unsatisfactory condition. The facts concerning which 
perfect certainty is attainable are very few in number ; the bearing 


of those facts on our theory of nerve-function is both slight and 
disputable. Physiological chemistry is in general encompassed 
with many difficulties. These difficulties are not due simply to the 
complex constitution of most of the substances with which it has 
to deal. They are also very largely due to the fact that these sub- 
stances are products of life ; and living tissue cannot be at the 
same time kept in normal condition and subjected to the handling- 
necessary for chemical analysis. As soon as it is no longer alive, 
or at any rate long before any chemical analysis can be completed, 
the constitution of such tissue is changed. However carefully the 
chemical elements, the constituents, which enter into the ner- 
vous substance may be preserved, their constitution, their chemical 
arrangement and behavior, cannot be preserved. It is impossible 
— for example — for the chemist even to determine the specific 
gravity of uncoagulated blood, " except by operating with extreme 
expedition and at temperatures below 0° C." 

Moreovei', the difficulties which belong to the chemistry of all 
living tissue are especially great in the case of the nervous tissues. 
In their natui'al state the proximate principles which compose these 
tissues are very complex and unstable compounds. To obtain spe- 
cific portions or kinds of nervous substance free from foreign ingre- 
dients — as, for example, the axis-cylinder of the nerves, or the rods 
and cones of the retina — is by no means always easy. The analysis 
of such substance, when once the substance is obtained, is often ex- 
tremely tedious in respect to process, and doubtful in respect to 
result. Nevertheless, the principal conclusions, which may be ac- 
cepted with considerable confidence in their correctness, are as 
follows : 

§ 7. Nervous Matter is of two kinds,. called white or fibrous, and 
gray or vesicular, which differ not only in color and microscopic 
structure, but also in specific gravity and chemical constitution. 
The specific gravity of the white nervous matter is greater than that 
of the gray. Danilewski ' found the specific gravity of the gray 
matter in man to vary from 1.02927 to 1.03854 ; that of the white 
matter from 1.03902 to 1.04334. Others (as Bastian, W. Krause, 
and L. Fischer) calculate the mean specific gravity of the gray mat- 
ter at about 1.031, of the white at 1.036-1.040. This difference in 
the weight of the two is chiefly due to the difference in the relative 
amount of water and of solids which they contain. Of 100 parts of 
each, from the brain of the ox, the gray matter was found to be 

' See Med. Centralbl , xviii., p. 241, as cited by Dreclisel, with apparent 
confidence, in Hermann's Handbuch der Physioiogie, V., i. , p. 577. Leipzig, 



composed of 81.60 parts of water and 18.40 of solids ; the white, 
of 68.35 of water and 81.65 of solids. The amount of water is also 
larger in the brain of the young animal than in that of the adult. 
The brain of the foetus was found by Weisbach to consist of from 
87.9 to 92.6 parts of water. The amount of water entering into the 
composition of the different parts of the central nervous system is 
unequal. The following is a tabulated statement * of the facts to 
which allusion has just been made : 

Pkopoktion of Water in One Hundred Parts. 

Age, 20 to 30. 

Age, 30 to 50. 

Age, 70 to 94. 

White substance of the brain 






Gray substance of the brain 



Pons Varolii 


Medulla oblongata 


The amount of water varies in the different regions of the spinal 
cord. Bernhardt found a smaller proportion of water in the cervi- 
cal (73.05 per cent.) than in the lumbar (76.04) region of the cord. 
The gray matter also contains more of albumen, lecithin, and lactic 
acid than the white, and less of cholesterin, fat, and protagon. 

§ 8. Of the solids contained in the matter of the nerve-centres, 
more than one-half in the gray, and about one-quarter in the white, 
consist of certain proteid or albuminous substances. Bodies of 
this general class are the only ones never absent from the active 
living cells ; they therefore exist in the primordial structures of 
all vegetable and animal organisms, and occupy a peculiar place 
among organic proximate principles. Of these proteid substances 
found in the nerve-centres very little is as yet known. Gamgee ' 
mentions three such substances — one soluble in water and probably 
derived from the gray matter, another a globulin-like body which 
is dissolved by a ten per cent, solution of common salt, still another 
a myosin-like body which remains in solution when a ten per cent, 
salt solution of brain is boiled. 

§ 9. Three other non-phosphorized bodies of different classes 
from that above mentioned are found in nervous tissues : these are 
Cholesterin, Neurokeratin, and, more doubtfully, Cerebrin. Cho- 

' Derived from Weisbach's observations, and found in Gamgee, Physiologi- 
cal Chemistry of the Animal Body, i. , p. 445. London, 1880. 

"^ Physiological Chemistry, i., p. 433 ; see, also, the article of D. Petrowsky, 
*' Ziisammensetzung der grauen und der weissen Substanz des Gehirns," Pflii- 
ger" s Archiv, vii., p. 367. 


lesterin is among the most abundant of the constituents of the ner- 
vous tissues — especially of tbe white matter of the cerebro-spinal 
axis and of the nerves. It is a " monad alcohol," the only alcohol 
which occurs in the human body in a free state. On account of its 
solubility in ether, cold or hot, and in warm alcohol, cholesterin 
finds its way into both ethereal and alcoholic extracts of the ner- 
vous tissues. It is a non-nitrogenous body, crystallizing in beau- 
tiful white crystals, which, when separated pure from its solutions 
in ether or chloroform, takes the shape of fine needles, and when 
sejDarated from alcohol takes the shape of rhombic tables. It is sup- 
posed to exist preformed in the brain. Its formula is C^gH^^O + H^O- 

Neurokeratin is most easily derived by treating the meduUated 
nerve-fibres with boiling alcohol and ether, so as to extract the fatty 
matters of the medullary sheath ; in the place of this sheath there 
is left, as a kind of irregular framework, a highly refractile sub- 
stance which resembles the horny matter of epidermis in its power 
of resistance to chemical agents. This substance is also found in 
the gray matter of the nerve-centres, and in the retinal epithelial 
cells and pigment cells of the choroid ; but not in the non-medul- 
lated nerve-fibres. It contains nitrogen, 2.93 per cent, of sulphur, 
and leaves 1.6 per cent, of ash. 

Cerebrin was announced by Milller, in 1858, as a non-phosphorized 
nitrogenous body, obtained from a precipitate from the brain when 
pounded up with baryta water to the consistence of thin milk and 
then boiled. He described it as a loose, Avhite, very light powder, 
destitute of smell and taste, soluble in boiling alcohol and ether, but 
insoluble in water. He gave to it the formula Cj^H.^^NO^. Thudi- 
chum believes that brain matter contains a class of nitrogenous bod- 
ies free from phosphorus, to which he gives the name of "cerebrins." 
Gamgee, however, thinks it very unlikely that a body produced, 
like Mailer's cerebrin, "by the prolonged action of a solution of 
boiling barium hydrate on so complex an organic mixture as brain 
should be a definite proximate principle of the unaltered organ ; " ' 
but the same authority admits " that the precipitate which sepa- 
rates itself from an alcoholic solution of brain contains, besides 
cholesterin, pi-otagon, and the so-called lecithins, " a body for 
which we may retain the name of cerebrin." 

Nuclein was discovered by Miescher in the nuclei of pus-corpus- 
cles and in the yellow corpuscles of yolk of egg. Other observers 
subsequently obtained it from various other substances, especially 
from the nuclei of the red blood-corpuscles of birds and amphibia. 

' Physiological Chemistry, i. , p. 439. 
'Ibid., i.,p. 433. 


Von Jaksch ' thinks lie has discovered nucleiu in the human brain. 
His claim seems to be credited by Drechsel.^ Its formula is given 
as C2gH^;,N^P30j2- But the veiy existence of nuclein, as a definite 
body, has been denied by chemists Hke Worm-Miiller and Gam- 
gee ; and the analyses of Von Jaksch do not agree with those ob- 
tained from other sources than the substance of the human brain. 
The whole question of nuclein must then be left in doubt. 

§ 10. No other substances found in the nervous system are, how- 
ever, both so interesting and so difficult to consider, from the mixed 
chemical and psycho-physical point of view, as certain complex phos- 
phorized fats. The entire progress of our inquiry will make it obvi- 
ous how inadequate and misleading is the use often made of such 
off-hand remarks as the celebrated dictum : " No thought without 
phosphorus." Yet it is doubtless true that the highly elaborate and 
unstable compounds containing phosphorus, which enter into the 
composition of nervous matter, have a significance for physiological 
and psychological researches such as belongs to no other material 
bodies. These comj)lex bodies are especially characteristic of the 
centres of the nervous system. The strife of discovery and of con- 
firmatory experiment has been chiefly carried on over the following 
three : Protagon, Lecithin, and Cerebrin. Of these three, however, 
probably only the two former are phosphorized bodies. The main 
question involved in controversy concerns the relation in which leci- 
thin and cerebrin stand to protagon. Is protagon a definite prox- 
imate principle of the brain, and are lecithin and cerebrin bodies of 
ill-defined properties and doubtful claim to existence as proximate 
principles of the brain ? or, are lecithin and cerebrin definite prox- 
imate principles, and is protagon a mechanical admixture of the 
two ? The latter view of protagon has been held by Diaconow, 
Hoppe-Seyler, and Thudichum ; on the contrary, its claims to the 
position of the " only well-chax'acterized phosphorized proximate 
principle " of the brain as yet discovered have been defended (and, 
it may be said, apparently established) by the researches of Gam- 
gee and others. 

Protagon was discovered, as a new proximate principle that -can 
be separated from the brain, in 1864, by Dr. Oscar Liebreich ; his 
discovery was announced in a paper ^ published in 1865. This in- 
vestigator gave to this substance the name which it still bears, as 

' See article " Ueber das Vorkommen von Nuclein im Menscliengeliirn," 
Pfliiger's Archiv, xiii., p. 469. 

''^ In Hermann's Handb. d. Physiol., V., 1., p. 578. 

' 'Ueber die cbemisclie Bescbaflfenheit der Geliirnsubstanz." Annalen der 
Chemie und Pbarmacie, cxxxiv. , pp. 29-44. 


in his opinion the first to be definitely ascertained among the spe- 
cific constituents of the brain (Trpwrayo?, leading the van). He as- 
signed to it the formula Cj^^H^^jN^O^^P. In spite of subsequent 
denials and disproofs of its existence, the extremely careful and 
often-repeated researches of Gamgee ' and Blankenhorn have cor- 
roborated the discovery of Liebreich. The process by which pro- 
tagon is obtained from the brain may be thus briefly described (the 
description will serve to illustrate in general the processes of physi- 
ological chemistry) : Perfectly fresh ox's brains are freed from the 
blood and membranes, and are then digested for about a day in 
eighty-five per cent, alcohol ; from this fluid, when filtered, a quan- 
tity of white flocculent precipitate is obtained, and the cholesterin 
and other bodies soluble in ether are dissolved out ; from the sub- 
stance left undissolved, when dried and reduced to powder and 
digested for many hours with alcohol, and then filtered and cooled, 
microscopic crj- stals separate themselves, arranged for the most part 
in rosettes. The substance thus crystallized is protagon. It is con- 
sidered by some chemists the easiest to obtain, and indeed the only 
very well-established phosphorized proximate principle of the brain. 
Such a material substance is indeed a long way removed from the 
living nervous mass, with its capacity for exercising such marvel- 
lous physiological and psycho-j)hysical functions. But it is the best 
representative that chemistry can as yet present of a scientific result 
upon which to base any attempt to point out definite relations be- 
tween psychical activities and the chemical constitution of those 
complex phosphorized fats which exist in the central nervous 
mechanism. The empirical formula of protagon, as given by Gam- 
gee, is C|j„H3„„Nj.P03j. It has been made highly probable that pro- 
tagon cannot be a compound or mixture of cerebrin and lecithin ; it 
may, then, be announced as a proximate principle of the brain. 

Lecithin is an organic phosphorized compound which exists in 
large quantities in ova, spermatozoa, etc., as well as in the nervous 
tissues. It is described as a yellowish-white, waxy, very hygro- 
scopic solid, which in thin layers shines with a silky lustre. It is 
soluble in ether and alcohol ; on being stirred with water it forms a 
starch-like solution difficult to filter. Diaconow assigns to it the for- 
mula C.^H^^NPOj, -f H^O. Gamgee supposes that the lecithin of Dia- 
conow is only one of a group of similar bodies which possess a higher 
percentage of phosphorus than protagon, and the "general smeary 
characters " of lecithin. We may, then, speak of " the lecithins " as 
highly phosphorized compounds existing in the matter of the brain. 

' See his Physiological Chemistry, i. , pp. 425-429 ; and article in the Jour 
nal of Physiology, ii., pp. 113-131. 

nsro:EGA]sric bodies iisr beaiis'. 


The various products of the decomposition of protagon and leci- 
thin it is not necessary to describe. Neurin is the only one of 
these products which deserves for our purpose even to be named. 
It may be obtained from either protagon or lecithin. Dr. Thudi- 
chum's elaborate " Eesearches on the Chemical Constitution of the 
Brain " ^ conclude that at least three well-defined groups of phos- 
phorized bodies may be separated from the brain ; these are dis- 
tinguished as (1) kephalins, (2) myelins, (3) lecithins. The exist- 
ence of a group of bodies which may be termed "lecithins " has just 
above been declared probable. Thudichum thinks that all these 
bodies contain phosphoric acid combined proximately with glyce- 
rin, but " differ in the manner in which they contain the nitrogen 
and the acid radicles which constitute the great bulk of their sub-' 
stance." The researches of Dr. Thudichum still await confirmation. 

§ 11. In addition to the substances already mentioned, the brain 
contains certain extractive matters which are found also in other 
tissues, especially in muscle. Among these bodies are creatin, 
inosite, xanthin, and lactic acids. 

§ 12. The brain also contains an extremely small amount of inor- 
ganic matters— so small, indeed, that few facts can be relied on 
concerning it. The estimates of this amount vary from 0.1 to 1 
per cent, of the fresh brain. Among such inorganic matters are 
alkaline phosphates and sulphates, chalk, magnesia, oxide of iron, 
etc. It is said that the ash of the gray matter has an alkaline reac- 
tion, that of the white matter an acid reaction,^ and that the reac- 
tion of the former during life is acid, while that of the latter is 
neutral or weak alkaline. 

§ 13. All quantitative analyses of the brain are exceedingly doubt- 
ful ; the older results are wholly worthless. The following table of 
Petrowsky,' which gives the chief organic constituents of the brain 
of the ox, is an object of interest rather than of complete confidence : 


Gray Matter. 

White Matter, 

Albumen and gelatin 



18.68 + 

6.71 + 

1.45 + 

per cent. 

24.72 + 


per cent. 



Cholesterin and fats 

Substances insoluble in 


' Reports of Medical Officer of the Privy Council and Local Government 
Board, 1874, pp. 113 ff. 

2 See Gamgee, Physiological Chemistry,!., p. 445; Hermann, Handb. d. 
Physiol., v., i., p. 577. 

^ " Zusammensetzung der grauen und der weissen Substanz des Gehirns," 
Pfliiger's Archiv, vii., p. 367. 


§ 14. The specific chemistry of the elements of the nervous sys. 
tem, or of the various parts of such elements which histological 
science reveals, is yet more meagre and doubtful than its general 
chemistry. The micro-chemistry of the nerve-cells tells us simply 
that they are in the main protoplasmic, and therefore rich in pro- 
teid substances ; and since an analysis of the two kinds of nervous 
matter shows that the gray is much the poorer in complex phos- 
phorized constituents and in cholesterin, we conclude that the cells 
which enter so largely into the gray matter are also poor in the 
same substances. The different structural parts of the nerve-fibres 
have doubtless a different chemical constitution. This is proved 
by the difference in their appearance under the microscope, by the 
different action of reagents upon them, and, to some extent, by 
chemical analysis. The neurilemma, or membranous envelope of 
the nerve-fibres, like the sarcolemma, on prolonged boiling, yields 
gelatin. The axis-cylinder appears to be a mixture of proteid with 
complex fat-like bodies. The white substance of Schwann is rich 
in complex phosphorized fats, in cholesterin, and in the so-called 

The researches of Kiihne ' and others — for the most part his 
pupils — have developed certain interesting results with respect to 
the chemical constitution and chemicals change of the nervous tis- 
sue of the eye. Many of the various non-nervous parts of the ear 
and the eye have been carefully analyzed. The extremely small 
amount of such material which is obtainable for chemical analysis 
is one reason why so little can be known concerning the chemical 
constitution of the substance of the retina. It is said to have an 
acid reaction. It is a fair surmise, on general grounds, that it con- 
tains the same bodies as the central nervous system. The two seg- 
ments into which the rods and cones break up exhibit marked dif- 
ferences in their chemical as well as optical characters. The inner 
segments are composed of protoplasm of "a marvellous transpar- 
ency." The outer limbs of the rods have an external envelope 
which agrees in its physical characters with neurokeratin. This 
envelope encloses a mixture of proteid bodies and of other sub- 
stances similar to those found in the other nervous tissues. 

§ 15. If knowledge of the chemical constitution of the nervous 
system is so far behind what Ave could wish, knowledge of the 
chemical proce ses and chemical changes which are connected with 
the physiological functions of this system must be declared to be 

' For a list of papers by Kuhne and liis pupils on this subject, see Gamgee, 
Physiological Chemistry, i., p. 462 f. ; and for an account by him of his re- 
searches and their results, see Hermann, Handb. d. Physiol., I., i., pp. 235 ff. 


almost wholly wanting. Even the beginnings of scientific general 
statements, ox' laws, respecting the relations between the chemical 
constitution of the nervous system and its various physiological 
activities have yet to be made. Different investigators will doubt- 
less differ as to the prospect for such discovery in the future. 
When chemistry can deduce the molecular behavior of the most 
highly complex chemical compound from the nature and number 
of its component chemical elements, and physiology can definitely 
connect all the physiological functions of nervous matter with the 
molecular motions of its chemical constituents, we shall have the 
means for a strictly scientific solution of such problems. 

§ 16. It need scarcely be said that we have little knowledge re- 
specting the relation which exists between the chemical constitu- 
tion and chemical processes of the nervous system, on the one hand, 
and, on the other, the jDhenomena of so-called mind. 

Nevertheless, certain important general relations may be point- 
ed out between the chemical nature of the nervous mechanism 
and its psycho-physical functions. The extremely high organiza- 
tion and chemically sensitive constitution of this mechanism are 
bej'ond doubt related to all its distinctive functions. Like every 
other natural material structure, the nervous system is obviously 
adapted to its peculiar kind of work. Chemically considered, it 
appears as composed of a number of extremely complex and highly 
unstable compounds. It therefore holds in its chemical consti- 
tution a large amount of disposable energy ; this energy it yields 
readily when the equilibrium of its molecules is in any way dis- 
turbed. Within certain limits, it explodes with increasing surren- 
der of its disposable energy as the number and intensity of the 
demands upon it are increased — very much as would a gun which 
should be arranged so as to go off with greater energy as the press- 
ure of the finger on its trigger is repeated or increased. 

It is probable that the substance of the nerves is the seat of a 
chemical synthesis, as the result of which still more complex bodies 
are constructed from the already complex alimentary material fur- 
nished by the blood ; such bodies have a high value as combus- 
tibles, and thus, as has been said, possess a significant amount of 
disposable energy. The relation of a supply of oxygen to the 
nerve-centres is also important to notice. The nexwe-fibres require 
comparatively a small amount of oxygen. It may be conjectux*ed 
that in their case, as in the case of muscle-fibre, intra-molecular 
oxygen is of some use in preparing explosive materials. But at 
present we must be satisfied Avith conjecture on this point. On the 
contrary, the vascular xiature of the central organs creates a pre- 


sumption that the chemical processes which have their seat in them 
require an abundance of oxygen. Experience confirms this pre- 
sumption. The respiratory centre in the medulla oblongata is 
chiefly controlled in its action by the amount of oxygen which 
reaches it in the blood. The phenomena of consciousness vanish 
when the supply of oxygenated blood is cut off from the brain. 

Although we are still in the dark as to the precise significance of 
the visual purple, the phenomena which the study of it has brought 
to light are suggestive of unseen chemical processes that are set up 
in the retina, and so serve as stimulus for the fibrils of the optic 
nerve. In general we know that certain sensations are dependent 
upon the chemical constitution and activity of the various end- 
organs of sense. 

Further researches will undoubtedly greatly enlarge our knowl- 
edge of those facts of relation which exist between the chemical 
constitution and changes of the nervous mechanism and the phe- 
nomena of psychical life. Perhaps such particular .statements of 
fact may be gathered into such more general statements of fact, 
verifiable by experiment, as we consider sufficient to constitute 
scientifically established laws. But lohy certain chemical constitu- 
ents, when combined and changed in definite fashion, should be 
specifically connected with certain conscious experiences will always 
remain an unanswerable inquiry. 

§ 17. From the chemical constitution of the elements of the ner- 
vous system we now pass to their Structural Form. The science 
which must be our guide is no longer chemistry, but microscopic 
anatomy, or histology; this science furnishes us with a large amount 
of trustworthy information mingled with a still larger amount of 
conjecture and doubt. Even the number of those elements upon 
which histology is entitled to focus its microscope and declare that 
such, and no others, are capable of performing distinctively ner- 
vous functions can scarcely be said, as yet, to be placed beyond all 
doubt ; neither can it be claimed that the microscope has yet dem- 
onstrated the ultimate structure of those two species of such ele- 
ments the reality of whose nervous functions is beyond doubt. 

It is customary to speak of nerve-fibres and ganglion-cells as the 
only structural elements of the nervous system. If, however, by the 
term " ganglion-ceU " we intend only such bodies as histology usu- 
ally describes under this type (for example, the so-called motor 
ganglion-ceiis of the spinal cord) the limitation is without sufficient 
warrant. For there are many cells, which almost certainly have 
some significance as parts of the nervous system, that differ in 
form very widely from the typical ganglion-cell. Moreover, by such 


an off-hand twofold division the important question is often silently 
passed by : What is the significance for the nervous functions of that 
diffuse, finely granular substance, found in considerable quantity in 
the great nerve-centres, and called neuroglia, or nerve-cement {Ner- 
ven-kitt ; Kittsubstanz) ? This substance is most frequently classed 
with the connective tissue; but, according to Henle,* "it is at all 
events to be distinguished from connective tissue on account of its 
chemical properties." That certain microscopic forms of so-called 
"neuroglia" are largely unlike other forms recognized as being 
nerve-cells beyond doubt cannot be argued in proof of its ina- 
bility to perform any of the strictly nervous functions, except upon 
the basis of the assumption that we already know beyond reasonable 
question what are all the elementary structural forms of true ner- 
vous matter. But, assays Eckhard,^ "if we start the inquiry, what 
formal elements of the brain and spinal cord take part in the activi- 
ties of these (the nervous) organs, and in what way do they take 
part, we are able to give to it only a very unsatisfactory answer." 
We are not in a position, then, either to affirm or to deny abso- 
lutely the claim sometimes set up for the neuroglia, that it con- 
tains true nervous elements. 

It is best to recur to the facts which microscopic anatomy dis- 
closes as a basis for classifying the different structural elements of 
the nervous system. These may be briefly described as follows : ' 
As to the true nervous character of fibres of various kinds, not only 
as conducting bands between the nervous centres and the peripheral 
parts of the body, but also within the substance of these centres, 
there is no dispute. Nerve-fibres undoubtedly constitute one of 
the structural elements of the nervous mechanism. Besides the 
nerve-fibres, histology distinguishes in the gray substance of the 
nervous centres — where all the structural elements of the nervous 
system are to be found in greatest abundance and variety — three 
other species of structural form. Such are (a) certain cells, known 
more particularly as the " ganglionic nerve-cells." These cells (to 
be described more minutely hereafter) are irregular magses of finely 
granular protoplasm, possessed of a clear nucleus and one or more 
nucleoli, and sending off one or more processes. 

(6) Corpuscles, consisting either of naked nuclei or of nuclei 
with only a small amount of surrounding protoplasm, and having 
various shapes sometimes difficult to make out, are also found 
abundantly in the gray matter of certain nervous centres. Such 

* Anatomie des Mensclien. Text, p. 306. Braunschweig, 1880. 

^ Hermann, Handb. d. Physiol , II., ii.,p. 15. 

^ Comp. Henle, Anatomie des Menschen. Text, p. 306. 


bodies are usually much smaller than the cells of undoubted ner- 
vous character described above, many of them being scarcely more 
than ■j-g'iro - -gVo 0"' ^^' ^ven -g-uVoj ^^ ^^ inch in diameter. Some of 
them, like the typical ganglionic cell, give off processes which are 
thought to be continuous with nerve-fibres. It is altogether prob- 
able that these cells of the second class differ only in their dimen- 
sions from the cells of the first class. In some places (for example, 
in the cortex of the cerebrum, or large brain) they appear to have 
the characteristics of transitional forms between the undeveloped 
gi'anules of the same class and the more highly developed ganglion- 
cells. In other places (as in the cerebellum) they form indeiDend- 
ent layers. They may be described as nuclei "invested by only 
a small quantity of cell-substance." ' Some are multipolar, some 
bipolar, some unipolar. Admitting, as we seem compelled to do on 
account of their resemblance to the typical form of the ganglionic 
nerve-cell, that some of these cells are true nervous elements, it is 
impossible for histology to draw the line through the entire class, 
and so to affirm beyond doubt that any of them are without claim 
to be counted among such elements. 

(c) The diffuse, finely granular substance, already referred to as 
so-called "neuroglia," which fills in the gaps between the nerve- 
fibres and the cells of the two preceding classes, constitutes the other 
form of matter observed in the nervous centres. It exists in quan- 
tity large enough to form an essential constituent of some locali- 
ties of the brain and spinal cord. It is not always clear, however, 
to what this appearance of granular or molecular matter, in which 
the nerve-cells seem embedded, is due. According to some author- 
ities, it may result from a confused interlacement of fine nerve- 
fibrils and fine ramifications of nerve-cells ; or from the crushing 
of such nervous matter in the process of examination." The neu- 
roglia itself is described as a delicate reticulum, or network, in 
which certain small cells (neuroglia-cells) supposed to belong to 
the sustentacular tissue, and other more conspicuous cells, usually) 
stellate in section (" cells of Deiters "), are found. 

§ 18. Of the three foregoing kinds of structural forms found in 
the gray nervous matter, it is perhaps safest to class the first two 
together under the term "nerve-cells." We should then have to 
remember how greatly these vary in size and formation — all the 
way from the naked, or almost naked, nucleus to the large ganglion- 
cell with its many processes and complex microscopic structure. 

' See Max Schultze in Strieker, Human and Comparative Histology, i., p 
183. London, 1870. 

'■'See Quain's Elements of Anatomy, ii., p 149. London, 1882. 



The last of the three (neuroglia) may then be regarded as a susten- 
tacular tissue ; though with the confession that in the brain and 
spinal cord it is by no means always easy to distinguish susten- 
tacular from true nervous tissue.' 

Of the structures known as nerve-fibres and nerve-cells, his- 
tology enables us to give a further more minute, if not a com- 
plete, description ; it also excites our interest by making it possi- 
ble to conjecture what is the regular anatomical relation between 
the two. When combined wdth physiological researches, histology 
also enables us to make considerable progress toward distinguish- 
ing the separate as well as the combined functions of these ele- 
ments. We consider, then, with particular detail, the structure and 
functions of nerve-fibres and ganglionic nerve-cells. 

§ 19. What is ordinarily called a nerve appears to the naked eye, 
when dissected from an animal, as a cord of a whitish or grayish 

Fig. 1. — Cross-section of the Sciatic Nerve of Man. ^/,. (After Key and Retzius.) The left lower 
half is schematic, n, n. Bundles of nerve-fibres, surrounded by pn, pn, the perineurium : be- 
tween them appears the connective tissue, epineuruim (ep, ep), and adipose substance (ad). 

color, and of uniform stnicture. The nerve is really, however, one 
or more bundles, or fascicles, of a larger or smaller size, bound to- 
gether by connective tissue. Accordingly, on following it toward 
its peripheral termination we find that it divides and subdivides 
until its subdivisions consist of a sinaie nervous element called a 

' Comp. Ranvier, Traite Tecliuique d'Histologie, i. , p. 717. Paris, 1875. 


Nerve-fibre. The bundles have a special sheath formed of con- 
nective tissue {neurilemma, or perineurium), which in the finest 
branches becomes reduced to a single layer of cells cemented to- 
gether edge to edge, and is called the "sheath of Henle." On fol- 
lowing the fibres backward again toward the central organs, it is 
found that several of them are bound together to form a nerve- 
fascicle ; a small amount of fibrillar connective tissue appears be- 
tween the several fibres within the same sheath ; the character of 
the sheath itself is changed, and it becomes attached to surround- 
ing structures by a layer of connective tissue. It is the fibres into 
which the nerves break up on being followed toward their periph- 
eral terminations, or by junction of which, successively, they are 
composed on being followed toward their central termination, that 
are to be considered as the true elements of the nervous system. 

§ 20. Such nerve-fibres as compose the nerves which stretch from 
the central organs to the peripheral parts of vertebrate animals 
may readily be divided into two classes : one called meduUated 
fibres or nerve-tubes, and the other non-medullated fibres or fibres 
of-Remak. Nerves in which there is a large proportion of medul- 
lated fibres have a characteristic white or watery appearance ; those 
in which only non-medullated fibres, or only a few medullated fibres, 
exist are grayish and slightly translucent. Vertebrates alone have 
the former. The medullated nerve-tubes belong particularly to 
the cerebro-spinal system, and are, therefore, of prime interest in 
psycho-physical researches ; the fibres of Remak are very abundant 
in all the nerves belonging to the sympathetic system. This two- 
fold division of nerve-fibres, while admitting of easy application to 
the constituent elements of the nerve-trunks, becomes more diffi- 
cult when we attempt to carry it out within the complex nervous 
matter of the central organs. Here Max Schultze ' points out sev- 
eral varieties of nerve-fibres. There are, first, those "very fine 
threads which lie on the extreme verge of microscopic mensura- 
tion," and which require an enlargement of from 500 to 800 diame- 
ters in order to be made visible. In such fibres no internal struct- 
ure can be detected by the microscope. To these Schultze gives 
the name of " primitive nerve-fibrils." Second : certain very deli- 
cate transparent fibres of albuminous composition, and distinguished 
from the foregoing by their greater size and their manifest fibrillar 
structure, are found in the central organs. These are the so-called 
naked axis-cylinders. Both the foregoing, when invested with a 
medullary sheath, become converted into the third, or medullated, 
form of nerve-fibre. These fibres in the nerves, while running be- 
' See Strieker's Human and Comparative Histology, i. , pp. , 147 £E. 


tween the central orgaus and the end-organs, become invested with 
a delicate membrane, and are thus converted into nerve-tubes of 
the well-known threefold structure. A fourth form of nerve-fibre 
occurs in the peripheral nerves, and is distinguished from the fore- 
going by the absence of the medullary sheath. This is the pe- 
ripheral non-medullated fibre, or fibre of Eemak, already alluded to. 
As they appear to the mici'oscopist, then, on an examination of all 
the kinds of nerve-fibres which are found in all the different parts 
of the nervous system, the following table of varieties is proposed 
by Schultze : 

I. Non-medullated (J" P^iiiiitive fibrils. 

oi -< 2. Fascicuh of primitive fibrils. 

I 3. These last, with a sheath of Schwann. 

r 1. Primitive fibrils with medullary sheath. 

II. MeduUated fibres. J ^- ^^f^^'^^^ «f primitive fibrils with such 
j sheath. 

I 3. These last, with a sheath of Schwann. 

The exposition of Schultze, although of value in setting forth 
the variety of forms in which the nerve-fibre is actually found by 
the histologist, does not constitute an objection to the twofold di- 
vision first proj)osed. On the contrary, it leads directly to such a 
division. For it will be noticed that both of the chief classes of 
fibres are regarded as composed of a number of primitive fibrils ; 
both are also regarded as becoming invested in their peripheral 
course with an outside membrane. The two classes, however, are 
really derived upon the basis of the fact that some of the primitive 
fibrils, whether they have already become invested with this mem- 
brane or not, possess a medullary sheath, and others do not. It is 
the presence or absence of this medullary sheath which constitutes 
the one important difference between the different classes of nerve- 

§ 21. MeduUated nerve-fibres, or nerve-tubes, have a threefold 
structure. Such fibres, when separated by teasing with needles 
from the fascicle of nerve-fibres and examined under the microscope 
while still fresh, appear pellucid, with a central part and a border 
on each side, like a translucent liquid in a tube of translucent 
walls. The lines of this double contour are at first comparatively 
sharp and regular ; lengthening the focus of the instrument ob- 
scures slightly the central part, and causes the parts on the border 
to appear brighter. Little by little the appearance of the fibres 
changes. The contours become irregular, and the substance (myelin) 



composing the borders becomes folded, striated, and granulated in 
appearance. The myelin wells over the sides of the ends of the 
fibres in irregular globular or contorted masses. Occasionally a 
pale filament may be seen projecting beyond the torn end of a 
fibre. Owing to the fact that various solutions have different effects 
upon the different parts of the nerve-fibres, it is 
possible to prepare specimens which shall exhibit 
clearly their threefold structure. Thus, for ex- 
ample, a solution of picrocarminate of ammonia 
colors the central part of the fibre, or axis-cylin- 
der, but not the myelin ; whereas osmic acid 
stains the axis-cylinder slightly, the myelin thor- 
oughly, but not the substance of the annular 
rings. By use, then, of various reagents, to color 
the nerve-fibres, and by numerous observations 
of them under various circumstances, their three- 
fold nature in a living state is thought to be dem- 
onstrated. We distinguish, then, in the medul- 
lated fibres : (1) An outer membrane, extremely 
thin, pellucid, with nuclei in it, and called the 
primitive sheath or sheath of Schioann ; (2) an in- 
terior layer of dimly granular, white, and highly 
Fig. 2.— Medulla ted refracting substaucc, semi-liquid during life, and 
i^Tmrtaeguiar contour after death undergoing a jorocess resembling co- 
sbowing. (schwaibe.) agulatiou— Called the medullanj sheath or loMte 
substance of Schwann; and (3) a cylindrical band of albuminous 
material, transparent, but with marks of fibrillation — called the 
axis-cylinder. Only the last is supposed to constitute the essen- 
tial nervous structure ; for, as we have already seen, many nerve- 
fibres are simj^le naked axis-cylinders, and all fibres for a certain 
distance from the cells in which they originate are devoid of the 
medullary sheath. There is considerable evidence that the presence 
of this sheath depends upon the need of insulation only. 

§ 22. Besides the threefold longitudinal structure of the medul- 
lated nerve-fibre, we have to notice certain structural modifications 
that occur at intervals in its length. The peripheral nerve-tube 
does not run along as a regular cylinder, but places of constriction 
appear at certain points situated beneath the outer sheath ; these 
constrictions are made at the expense of the medullary sheath or 
myelin. They are called annular constrictions or nodes of Eaiivier , 
the portion of nerve-fibre included between two of these constric- 
tions is called an interannular segment. At the constrictions the 
ends of the segments of the outer sheath are joined together by a 

A B 




Fig. 3.— a, Medullated Nerve-fibres from the 
Sciatic of a Rabbit, stained with osmic acid, 
and dissociated in water. (Ranvier.) 

B, Single Fibre Enlarged ^""/j, a, a. An- 
nular constrictions, or nodes of Ranvier, 
nearly midway between which is n, the nu- 
cleus, with protoplasm, p, surrounding it ; 
ca, axis-cylinder. 

Fig. 4. — Medullated Nerve-fibres. (Schwalbe.) 
rt, Ajds-cylinder ; s, sheath of Schwann ; n, 
nucleus ; p, p. granular substance at the poles 
of the nucleus ; r, r, Ranvier's nodes, where 
the medullary sheath is interrupted and 
the axis-cylinder appears ; i, »", incisures of 



tbin layer of cementing substance which extends inward toward 
the axis-cyUnder. These interannular segments of the nerve-fibre 
vary greatly in length. When several nerve-fibres lie parallel with 
each other, the segments of four or five of them often seem to have 
about the same length, and then the series appears interrupted by 
some segment considerably longer or shorter than the rest. 

Each interannular segment of a nerve-fibre has a flattened ellip- 
tical nucleus, situated nearly equidistant between the two annular 
constrictions which limit the segment. This nucleus 
often has a nucleolus ; between the nucleus and the 
myelin there exists a minute mass of protoplasm which 
is spread beneath the membrane of Schwann and fixed 
to it. 

Scattered irregularly along each interannular seg- 
ment are delicate lines or fissures which seem to run 
obliquely through the medullary sheath from the mem- 
brane on the surface of the nerve-fibre to the axis-cyl- 
inder. Their significance is not yet determined ; they 
are called the " incisures of Schmidt." (See Fig. 4.) 

§ 23. The complex microscopic structure of the med- 
ullated nerve-fibre, as described above — outer sheath, 
meduUai'y sheath, axis-cylinder, interannular segments 
limited at each end by annular constrictions, nucleus 
and nucleolus, and incisures of Schmidt — cannot be 
considered as "ultimate," even in the restricted sense 
in which we use the word as applied to what the eye 
can see by the aid of optical instruments. Other still 
more minute characteristics of its structure must be 
briefly mentioned, although with the understanding 
that their interpretation, and even their existence, is 
more doubtful than are the characteristics already de- 

The fact that isolated axis-cylinders, when submitted 
to the action of picrocarminate of ammonia, are stained red along 
their median line, while an extremely thin homogeneous border is 
left comparatively uncolored, and the additional fact that minute 
flakes or scales sometimes seem to appear upon their surface, have 
led to the conjecture that the axis-cylinder has a double structure. 
The clear homogeneous border probably corresponds to the so-called 
"sheath of Mauthner." ' 

The "fibrillated" appearance of the axis-cylinder under the mi- 
croscope has already been referred to as undoubted ; but the exact 
' See Ranvier, Traite Technique d'Histologie, i., pp. 738, 742. 


Fig. 5.— Medul- 
]ated Nerve- 
tihre from the 
Sciatic of an 
Adult Kahbit. 

«»»/!• (E-an- 
vier.) ff, An- 
n u 1 a r c o n- 
Btriction ; and 
c7/, axis - cyl- 
i n d e r with 
double con- 
tour showing. 



nature and the interpretation of this appearance are still matters 
of dispute. On account of the fact that the light must be passed 
through two or perhaps three cylinders in order to reach the inte- 
rior structure of the nerve-fibre, its examination under the high 
powers of the microscope which are necessary to see this fibrillated 
structure is extremely difficult. In spite of this difficulty, however, 
Hans Schultze ' claims that the fibrils of the axis-cylinder can be 

Fig. 6.— Fibrillated Appearance of the Ajus-cylinders of Medullatecl Nerve-fibres. (Hans Schultze.) 

distinctly traced in hving fibres, when these are in process of form- 
ing and are still naked, or where they issue from the cells without 
a medullary sheath, or where they lose this sheath at the annular 
constrictions or in the peripheral end-plexuses. Various prepara- 
tions of dead nerve-fibre, treated with different reagents, seem to 
demonstrate the same fibrillated structure. Moreover, from the 
fact that the nervous substance of the fibrils takes a carmine tinge, 
while the interfibrillary nucleated substance remains stained steel- 
blue with the nitrate of silver, Schultze argues that the axis-cylinder 
consists of two chemical substances. The fibrillated appearance 
can, therefore, scarcely be considered as due to the arrangement of 
' In the Archiv f. Anat. und Physiol., 1878, Anat. Abtli., pp. 259-285. 



rows of granules in straight lines.' According to T. W. Engel- 
mann,'^ in good preparations these fibrils appear distinct, and are 
never seen to anastomose or form a plexus of fibrils. By actual count 
the number of fibrils remains the same — at any rate, between any 
two annular constrictions ; nor are they apparently interrupted in 
their course by these constrictions. The fibrils, as found in different 
nerve-fibres, seem not to differ in respect to size or closeness of 

contact, but their number 
differs in nei've-fibres of 
different sizes. Engel- 
mann counted about four 
rHlinW hundred in the thickest 

fibres taken from the mo- 
tor roots of the spinal 
cord of the frog. The 
closeness of their contact, 
and the smallness of their 
number, as compared 
with that of the fibrils into 
which the fibre breaks up 
at its peripheral termina- 
tions, make it difiicult to 
see how these subdivi- 
sions of the axis-cylinder 
can have any separate 
function as the conduc- 
tors of nervous impulses. 
Further information I'e- 
garding them must be left 
to subsequent researches. 
(See Fig. 6.) 

The strict continuity of 
the axis-cylinder through 
the annular constrictions 
maybe called in question. 
Engelmann found that, on being treated with nitrate of silver, the 
axis-cylinders, as a rule, were broken off at the annular constric- 
tions or nodes.' Out of a hundred cases of broken cylinders only 
four appeared where they had not parted in the middle of these 
constrictions. It is not to be inferred from this, however, that nor- 
mal and living nerve-fibres are interrupted by a space of even mi- 
' So article of H. D. Schmidt, in tlie Monthly Micr. Jour., 1874, vol. xii. 
' Pfluger'B Archiv, xxii. , p. 36. ^ PflUger's Arcliiv, xxii., pp. 1-24. 

Via. 7.— Fibrillated Axis-cylinders broken at the Nodes 
of Ranvier. (Kngeimann.) 



croseopic proportions at these nodes ; no such interruption appears. 
But it is by no means impossible that these fibres are to be regarded 
as composed of a number of annular segments cemented together — 
each separate fibril placed exactly end to end with its fellow in the 
adjoining segments. Such an arrangement 
would accord with the theory which regards 
the segments as elongated and developed 

§ 24. Non-meduUated nerve-fibres, or fibres 
of Be male, differ from those already described 
in that they do not possess a medullary 
sheath. They are grayish and translucent, 
longitudinally striated, with flattened elon- 
gated nuclei lying at frequent intervals upon 
their surface. When gathered together within 
a sheath of neurilemma, they are not placed 
side by side as are the medullated nerve- 
tubes ; they are rather formed in the interior 
of the nerve, where they unite and divide and 
make an intricate plexus or network of fibres. 
They are grouped in larger bundles, some- 
times alone, but more frequently in connec- 
tion with medullated fibres. Their striated 
appearance is probably due to the fact that 
they, like the axis-cylinder of the medullated 
nerve-fibres, are composed of numerous fibrils. 
As has already been said, they belong to the 
sympathetic system. 

§ 25. The size of different nerve-fibres in 
the human body varies greatly, according to 
their kind, position, and, perhaps, function. 
As a rule the non - medullated fibres are 
smaller than the medullated, the former be- 
ing from -g-oVo- to -g-oVo' ^^ ^^ inch in diam- 
eter, and the latter (in the trunk and branches fig. 8.— Fibres of Remak from 

„,, Nj, ^ , , „ .. the Pneumogastric of the 

oi the nerve) trom y^oo- to g-gVo ^i an inch. nog. "o/^. (Ranvier.) n, 

-o J. i.1 • T • J. 1 i? 11 -I T ii Nucleus with surrounding 

JtJut this rule is not always lollowed. In the protoplasm, p ,• 6, strise cor° 
white matter of the cord the medullated responding to fibrils. 
fibres range in size from y^Vo- ^^ ^ oVt ^^ ^^ inch, in parts of the 
anterior columns, and about , oVt of an inch in those regions of 
the lateral and posterior columns which are nearest the gray matter 
of the cord. In the gray matter of the cord and brain the fibres are 
much finer — being from ^qVo ^o tt^o ¥ o^ ^^ i^^h in diameter, or 


even of an almost immeasurable fineness ; they are finest of all in 
the superficial layers of the brain and in the nerves of special sense. 
In some instances the axis-cylinder may be not more than jo-oWo" 
of an inch in diameter. 

§ 26. The number of fibres which enter into the composition of 
individual nerves also varies greatly. In the common motor nerve 
of the tongue it has been estimated at about five thousand, in that 
of the eyes at fifteen thousand, in the optic nerve at one hundred 
thousand at least. 

§ 27. So-called ganglion-cells, or nerve-cells, are the second of 
the two structural elements which can be more minutely described 
as undoubtedly belonging to the nervous system. These bodies 
vary greatly in size and shape, but they all show, when subjected 
to microscopic examination, certain well-recognized common charac- 
teristics. Nerve-cells are irregular masses of protoplasm, finely 
granular and delicately striated, with a large nucleus which is well- 
defined and vesicular in appearance, and which usually contains a 
shining nucleolus ; they send off one or more processes. In the gray 
matter of the cord and brain they are embedded in the neuroglia 
or so-called " nerve-cement ; " in the smaller nervous centres, such 
as the ganglia of the sympathetic and the ganglia on the posterior 
roots of the spinal cord, they are surrounded by a capsule of con- 
nective tissue. 

§ 28. Careful microscopic investigation of the nerve-cell with high 
magnifying powers of the instrument reveals the great complexity 
of its structure. In describing this complex structure the bipolar 
ganglion-cell of the fish may be considered as a common type. Such 
a cell is caUed by Max Schultze ' a " nucleated swelling of the axis- 
cylinder." When found in the course of a nerve-fibre it appears 
at first sight as a complete interruption to the continuity of the 
fibre. Further examination is thought to show, however, that, 
when the fibre reaches the cell, the axis-cylinder loses its medul- 
lary sheath, and the fibrils which constitute the substance of the 
cylinder become dissociated, and continue their course over the 
surface of the "ganglionic globe" to its opposite pole ; here they 
reunite and form a fibre identical with the one that approached 
the nearer jDole of the cell. The "ganglionic globe " itself appears 
to be composed of granular substance. We may distinguish, then, 
in such a ganglion-cell these two parts : (1) a fibrillary covering, 
the fibrils of which are continuous with the fibrils of the axis- 
cylinder on either side of the cell ; and (2) a granular globe con- 
taining near its surface a nucleus, within which one or more nucleoli 
1 In Strieker, Human and Comparative Histology, i., p. 174. 



appear. ' A repetition of these parts of the structure of the bipolar 
cell, it is claimed, may be expected and found in ganglionic nerve- 
cells in general. 

A microscopic structure substantially like that of the bipolar 
ganglion-cell of the fish, as already described, 
is found to belong to the multipolar cells of 
the anterior horns of the spinal cord of man, 
and of the ox, or of other mammals. Among 
the many processes given out by such a cell, 
the researches of Deiters and of others have 
demonstrated that ordinarily only one be- 
comes continuous with the axis-cylinder of the 
peripherally running nerve-fibre. This one, 
called the -'prolongation (or process) of Dei- 
ters," has sometimes been distinctly seen to 
be fibrillated ; and it is supposed that its 
fibrils are, as a rule, continuous with those 
of the axis-cylinder of the nerve-fibi'e. Hence 
it is called the " axis-cylinder process." The 
other processes from the cell also seem to be 
fibrillar ; but the quantity of interfibrillar 
granular substance which they contain is 
greater than that in the axis-cylinder process. 
These fibrils ramify, anastomose with each 
other, and become lost in an intricate net- 
work of extremely minute nervous filaments. 
Over the surface and within the interior of the 
" ganglionic globe " of the multipolar cell the 
fibrils of all these processes run in every di- 
rection with bewildering complexity. Their 
relation to one another, and to the various 
parts of the substance of the cell, cannot be 
said to be determined with any degree of cer- 
tainty. Most of the fibrils appear only to tra- 
verse the ganglion-cells, but some of them, 
perhaps, originate within the cells. In the case 
of auy thus originating, it is not as yet possible to say whether 
or not they arise out of the nuclei and nucleoli, and so, whether we 
may consider these parts of the cells as the special sources or cen- 
tres of the nerve-fibrils, asHarless, Meynert, and others have done.^ 

' See Ranvier, Traite Technique d'Histologie, i. , p. 712. 

" See, on this whole subject. Max Schultze in Strieker, Human and Compara- 
tive Histology, i., jip. 172-187 ; Ranvier, Traite Technique d'Histologie, i., pp. 
710 if. ; and Hans Schultze, Archiv f. Auat, u. Physiol., 1878, pp. 259-285. 

Fig. 9.— Nerve-cell from the 
Spinal Ganglion of the 
Ray. 350/j, (Ranvier.) 
my. Medullary sheath of 
nerve-fibre, enclosing ca, 
the axis-cylinder, the fi- 
brils of vrhich (/) separate 
and run over the gangli- 
onic globe, m; n, nii- 



§ 29. The variety of shapes taken by the nerve-cells has already 
been mentioned, as well as the fact that they may be classified as 
unipolar, bipolar, and multipolar. Some are nearly round ; others 
ovoidal, caudate, stellate, or shaped like a flask or the blade of a 
paddle. Still others appear somewhat like the foot of an animal 
with claws ; while the branching processes of others give them the 
appearance of sprawling out irregularly in a half-score of different 
directions. To a certain extent the shape of the cells is character- 
istic of that region of the central nervous system where they are 

Pig. 10.— Multipolar Ganglion-cell from the Anterior Horn of the Gray Substance of the Spinal 
Cord of the Ox. (After Deiters.) 1, Nucleus ; 2, axis-cylinder process ; 3, 3, branching 

found, in most abundant numbers, embedded in the neuroglia. 
For example, large ganglion-cells of irregular shape, with branch- 
ing processes, which have been called " motor," are found in the ante- 
rior horns of the gray matter of tlie spinal cord ; pyramidal cells of 
various sizes, with i^rocesses from both base and apex, are character- 
istic of the cortex of the cerebrum ; and just at the inner edge of 
the gray cortical matter in the cerebellum appear irregular globu- 
lar or ovoidal cells, which send off one or two branching processes 
toward the surface of the cerebrum. The ganglion-cells of the sym- 
pathetic also are usually globular or ovoidal, and have one or more 
processes which pierce their capsule and become non-medullated 


nerve-fibres. Uuipolar cells are found in the spinal ganglia of tlie 
higher animals, bipolar in the spinal ganglia of fishes. 

Nerve-cells vary in size as much as in shape ; the limits may per- 
haps be given as from about ^|-q- to 3-V0 °^ ^^ inch.' No special 
physiological significance can in any case be assigned to the shape 
of the nerve-cell ; we are wholly ignorant of the meaning of such 
a variety of forms, and of the value of any particlar form in a 
given position. It is possible, however, that the large size of the 
so-called " motor-cells " of the anterior horns of the spinal cord is 
indicative of theii- special physiological function. We may also 
fairly incline to interpret the multiplication of ganglion-cells in the 
central parts of the nervous system as significant of the large 
amount and high quality of work which must be done by them 
within these centres. It is possible that the shape of the cells is 
largely due to the mechanical conditions which control their de- 
velopment within the embryo ; but upon this subject we have 
scarcely any trustworthy information. 

§ 30. The structure of the nerve-fibres and nerve-cells, and the 
nature of the histological relations which apparently exist between 
the two, have led to a captivating theory intended to reduce all the 
elements of the nervous mechanism to modifications of a single 
form. Extremely different in structure as the various parts of the 
nervous system obviously are, we are told that modern histological 
science refers them all, for their elements, to "one perfectly defi- 
nite type ; " " this ty^De is the ganglionic nerve-cell. The important 
common characteristic, that they send out prolongations which 
become nerve-fibres, is assumed to belong to all such cells. The 
fibres are, accordingly, considered to be prolongations of the cells, 
and to be formed of substance like that of the source from which 
they appear to arise. Nerve-fibres may then be described as nerve- 
cells drawn out into an extremely long peduncle, which serves to 
connect them with other similar cells and fibres, or with certain 
muscular fibres which the nervous matter commands. This mor- 
phological theory of the nervous elements rests, however, upon a 
doubtful basis, and certain strong objections may be brought against 
it. We are probably warranted simply in asserting that both classes 
of these elements, Hke the other primary structural forms of the 
body, may be regarded as differentiations of one type (the cell) 
under conditions of which we are almost wholly ignorant. 

There is accumulating evidence in favor of the view that nerve- 

^ See an article of J. Hoffmann in the American Journal of Neurology and 
Psychiatry, August, 1883 pp. 432 ff. 

- Eanvier, Traite Technique d'Histologie, i. , p. 710. 


fibres are, in general, connected, both histologically and physiologi- 
cally, with the nerve-cells. One of the processes of each cell may, 
therefore, as a rule, be regarded as continuous with the axis-cylin- 
der of a nerve-fibre. It is true that this connection can by no 
means always be traced by the microscope. A score of years ago 
one investigator ' declared that, after having examined the gray 
matter of the spinal cord a great number of times, he had demon- 
strated this alleged connection only very rarely. Kepeated obser- 
vations since, of the improved modern kind, have not done away 
with the comparative infrequency of the desired demonstration. 
But from the very nature of the case a great number of the nerve- 
fibres must have their connection with the cells broken off by the 
treatment they receive in preparation for examination. And the 
positive cases where such connection has been traced may fairly be 
said to have indicated the rule. Moreover, the facts of physiology 
(to which reference will be made subsequently) seem to favor such 
a view of the anatomical relation of these two elements of the ner- 
vous system. 

Additional evidence upon this subject may perhaps be derived 
from the recent researches of E. A. Birge.^ This investigator under- 
took the gigantic task of counting the nervous elements in the gan- 
glia and roots of the spinal cord of a large number of frogs. He 
apparently discovered a general relation indicating some agreement 
in the number of the so-called motor-cells and the fibres alleged to 
originate from these cells. In one case (No. 42) an actual count of 
ten motor-roots gave 5,734 fibres and 5,777 cells on the right, and 
5,740 cells on the left side of the cord. Other results of counting, 
however, were by no means so favorable to the statement that the 
number of the fibres in the roots agrees exactly with the number of 
cells in the corresponding x-egion of the cord. Nor could more 
complete results of this kind form any sufficient warrant for the 
conclusion that everywhere in the nervous system the number of 
fibres corresponds with the cells, or that the nerve-fibres all spring 
from the nerve-cells ; much less, that they may be reduced to one 
form of such cells as to a perfectly definite type. 

§ 31. The discussion of the chemical constitution and structural 
form of the elements of the nervous system introduces the ques- 
tion as to the Functions of these Elements. This question must 
be answered, if at all, by the science of physiology. And in view 

' Vulpian, see Lemons sur la Pliysiologie du Systeme Nerveux, p. 318, Lect- 
ure of July 9, 1864. ' 

'Archiv f. Anat. u. Physiol., 1882, Physiolog. Abtli., pp. 435-479, espe- 
cially p. 471. 


of our ignorance of the genuine nervous character of all other 
claimants to a place among the elem'ents of the nervous system, our 
inquiry is narrowed to the following terms : What can nerve-fibres 
and ganglionic nerve-cells do ? With the activities of these ele- 
ments, as combined into the complex organs of the human nervous 
mechanism, the whole of our subsequent examination is designed 
to deal We speak here very briefly of certain fundamental prop- 
erties of the two nerve-elements already described — that is, of .the 
nerve-fibres as gathered into bundles called nerves, and of the cells 
as collected into ganglia and connected with these nerves. 

Nerves and nerve-cells have certain properties in common ; that 
is to say, within certain limits both can do the same things. Both 
are capable of becoming the subjects of a specific kind of molecu. 
lar motion which we are entitled to consider as distinctively " neu- 
ral" but about whose nature and mathematical or physical relations 
to other modes of the molecular motion of matter we are still al- 
most totally ignorant. Both are also capable of projDagating this 
distinctively " neural commotion " from one portion of their struct- 
ure to another. In a word, both nerve-fibres and nerve-cells have 
the properties of Excitability and Conductivity ; and the excitation 
and conduction of excitation which these nervous elements display 
are of a kind peculiar to themselves. It is the production, propa- 
gation, modification, and distribution of this distinctive nerve-com- 
motion which constitutes the one constant function, or projDerty, 
of the nervous elements, whether considered as isolated or as com- 
bined into organs. It is customary with some writers to speak of 
the production of psychical phenomena as the crowning function 
of the nervous system. But whatever may be the view we shall 
find ourselves compelled to take of the relations between the loca- 
tion, quantity, quality, and combinations of this neural molecular 
motion and the phenomena of self-conscious life, from our present 
point of view the utterances of such writers — if designed as anything 
other than figures of speech which need to be explained in detail 
to be even suggestive of those real facts and relations which they, in 
truth, only symbolize — are of little interest or value. We are speak- 
ing of a material structure, which, although alive and standing in 
altogether unique relations to psychical phenomena, is, nevertheless, 
in itself considered, nothing but a very complex collection of moving 
molecules. The peculiar form of molecular motion which charac- 
terizes this structure — namely, so-called " nerve-commotion " — is 
its unique function. Inasmuch as such nerve-commotion may be 
considered as originally set up in a single nervous element or group 
of elements, and then propagated from this initial point along cer- 

48 FuisrcTioisr of the itervous elements. 

tain more or less definitely marked tracts to other elements oi 
groups of elements, we may divide the one function into two — the 
function of excitation and the function of conduction. 

§ 32. Nerve-commotion, or neural molecular action, is, of course, 
never an uncaused event. It begins at certain points in the ner- 
vous elements, where it is set agoing by the application of appropri- 
ate causes of excitation. The causes of the excitation of the ner- 
vous elements are called "stimuli." Stimuli are of two general 
kinds — external and internal. External stimuli comprise all such 
modes of the motion of matter as act upon the peripheral parts of the 
nervous system, and so produce wdthin it a state of excitation or 
nerve-commotion ; among these are light, heat, chemical changes, 
etc. Internal stimuli are such as act upon the nerve-cells of the 
central organs ; they consist, in general, of changes in the blood 
produced by an increase or decrease of oxygen, the presence of 
drugs, etc. The susceptibility of a nerve to any form of external 
stimulus is called its " irritability ; " and when a nerve will no 
longer respond to such stimulus by being thrown into a condition 
of excitation, it is said to have lost its irritability. As the word is, 
generally used, then, the irritability of a nerve is its property' of 
excitability under the action of some form of external stimulus. 
When excited by such stimulus it is said to be irritated. We shall 
use both sets of words, reserving the words " pynitn.tinn " and " ex- 
citabilitv" for the gene^gLalaie and fuj^^^tJQ^ 'sf all nervous tissue 
considerecl as capable of a specific molecular commotion ' — a nerve- 

§ 33. But although all the nervous elements may be said to have 
the properties of neural excitability and conductivity, important 
difiierences arise as to the conditions under which, and as to the 
modes in which, they exercise their functions when combined into 
a complex nervous system. In the normal condition of such a sys- 
tem it is by no means all of its parts which are directly excitable, 
whether by external or by internal stimuli ; nor is the efi'ect of the 
excitation of both the elementary structural forms of such a system 
exactly the same. A single nerve may, indeed, be separated from 
the other parts of the nervous system, with a muscle attached, and 
may then be made to exercise its neural function in moving the 
muscle by being itself stimulated with dififerent kinds of stimuli at 
different points along its course. But in their normal place and 
condition nerves are never excited by the direct application of 

' It is a pity that we have in English no one word which can be nsed under 
all conditions, and compounded ad libitum^ in order to designate a property, a 
process, a state, etc., as can the German word Erregen, Erregung, etc. 


stimuli ; they are always excited indirectly by the propagation to 
them of nerve-commotion which originates in the central organs- or 
in the end-organs. The distinctive office of the nerves is, then, to 
act as conductors of molecular motion set up in themselves by the 
du-ect excitation of the nervous elements in which they either cen- 
trally or peripherally terminate. Moreover, large portions of the 
central organs do not respond to the direct application of various 
kinds of stimuli to their surface. We are obliged, then, to suppose 
that many of the nerve-cells which compose these organs are excita- 
ble only by stimulation through the nerve-fibres that run into them. 
The case of the normal nervous system, with respect to its excita- 
bility, may, then, be briefly described in the following terms : The 
end-organs of sense are directly excitable by external stimuli, and 
each specific kind of end-organ which is characteristic of a particu- 
lar sense is excitable only by the specific kind of stimuH appropri- 
ate to that sense. The afferent or centripetal nerves are excited 
only by the end-organs of sense ; their specific function is to con- 
duct the nerve-commotion, started by the external stimuli in these 
end-organs, toward the central organs. The efferent or centrifugal 
nerves are not directly excited by either internal or external stimuli, 
but only by the central organs ; their specific function is to conduct 
the nerve-commotion started in them by the central organs to the 
muscles, glands, etc.— to the peripheral parts of the body which 
are to be moved through their excitation. The central nerve-cells 
themselves are excited either through the nerve-commotion brought 
to them by the afferent nerves or by internal stimuli. Nerve- 
commotions are also said , to arise in them automatically ; but 
the facts covered by the term " automatic " require further distinc- 
tions to be made as to the functional activity of the different nerve- 

§ 34. If the distinctive normal function of the nerves is the con- 
ducting of neural molecular motion between the central organs 
and the end-organs, the function of the ganglion-cells can by no 
means be pronounced so simple. These cells are, indeed, also con- 
ductors of nerve-commotion ; within the central organs they form 
important parts of the tracts along which such commotion passes. 
They serve also as points for the division and redistribution of this 
commotion ; they may be spoken of as switching-places in the sys- 
tem or network of tracts. In these " shunting-places" of the cell 
many lines of conduction meet ; and the one of them taken by any 
impulse entering the ceil may depend upon the relative amount of 
resistance offered by these Hues. The work of the cell may then 
be considered as "re-directive." The office of the cell in distri- 

50 ruNCTioisr of the nervous eleme]S"ts. 

bution of the nerve-commotion may also be either to condense or 
to disperse it ; in the former case the distribution might be spoken 
spoken of as "associative," in the latter as "dissociative."' They 
may also intensify or diminish the nerve-commotion entering them. 
But the nerve-cells have also other functions, or forms of the one 
neural function, which have been classed as either (a) automatic, 
(6) reflex, or (c) inhibitory. 

(a.) Automatism, or the power of initiating the peculiar form of 
molecular motion known as "vital impulses," independently of 
the action of any discoverable stimulus from without, is one of the 
fundamental propei'ties of protoplasm. An amoeba, for example, is a 
minute mass of such protoplasm ; it executes movements which can- 
not be wholly explained by reference to any changes in its environ- 
ment. The difficulty of distinguishing automatic from reflex action 
in the spinal cord, and muscular from nervous automatism in the 
sporadic ganglia, need not concern us at present. According to 
Eckhard "^ two kinds of this automatic function of the ganglion-cells 
may be distinguished — viz., the automatic-tonic and the automatic- 
rhythmic. In the foi'mer case the control of the cells over the 
muscular structvires by means of the efferent nerves is irregular ; 
in the latter this control results in the nearly simultaneous contrac- 
tion of the same set of such structures, repeated at regular intervals ; 
as is the case in the movements of the heart and lungs. In neither 
case, however, can we form any clear conception of the origin within 
the cells of this neural commotion, of the nature of the forces at 
work to produce it, or of the changes in material that are involved 
in it. We can only say that as yet we know no reasons lying out- 
side of the structui-e and activities of the living nerve-cells them- 
selves which will account for the starting of the excitation. In this 
sense, at least, such neural action is "automatic." 

(6.) The reflex function of the ganglion-cells admits of a some- 
what more detailed and satisfactory statement ; but the phenomena 
and laws of reflex nervous action are properly discussed as belong- 
ing to the central organs of the nervous system. It is enough, at 
present, to note that the great changes which take place in the 
character of nervous impulses, when, after entering the central 
organs by the afferent tracts, they are, as it is said, " reflected " 
from those organs along the efferent tracts, are indubitable evi- 
dence of the specific molecular activity of the ganglion-cells. For 
the afferent impulses are, in fact, not simply reflected in these cells ; 

' See A. Hill, Tlie Plan of the Central Nervous System, p. 2. Cambridge 

'•^ Hermann's Haudb. d. Physiol., II., ii., p. 19 f. 


they are greatly modified as to their number, intensity, character, 
and distribution. This modification is proof of profound molecu- 
lar changes that are instituted in the substance of the cells them- 
selves. It is one proof, among others, that a large expenditure of 
energy in the cells accompanies the transmutation of afferent into 
efferent impulses. 

(c.) The function of inhibition, as ascribed to ganglion-cells, must 
be pronounced more doubtful in character than either of the two 
foregoing. It was found by Wundt ' that nervous impulses are 
delayed on passing through the sj^inal ganglion. Such impulses 
seem also to consume an amount of time in travelling along or 
through the cord that cannot readily be accounted for as wholly 
due to the length of the nervous tracts which they thus traverse. 
But until our information is more precise as to the microscopic 
structure of the cord, and as to the tracts within it which the ner- 
vous impulses follow, we cannot say with confidence how much of 
this delay is due to molecular changes peculiar to the cells them- 
selves. That the automatic and reflex functions of the medulla 
oblongata may be compounded, as it were, in such way as either to 
inhibit or to accelerate the action of the heart and lungs and mus- 
cular walls of the arteries, is a well-known fact. It has already 
been said that nerve-cells may diminish as well as intensify the 
nerve-commotion entering them. When afferent impulses reach the 
ganglion-cells of the centres, and find them already at work, such 
impulses result, according to circumstances, in either inhibiting or 
augmenting this activity.^ Moreover, the tone given forth by a 
muscle, when tetanized by stimulating the nerve to which the mus- 
cle is attached with repeated induction-shocks, has the same num- 
ber of vibrations per second as there are of such shocks ; but the 
tone given forth by muscle tetanized through tlie spinal cord, or 
by action of the will, has a constant number of vibrations, namely, 
about nineteen per second. It would appear from this, also, that 
the central apparatus of nerve-cells controls the impulses which 
tetanize the muscle, according to the molecular structiu-e and 
changes of those cells. In this sense, then, the cells may be said 
to exercise inhibitory functions under certain conditions. 

§ 35. A consideration of the different effects produced by the 
conduction of nervous impulses along the different nerves of the 
system would seem at first to justify the classification of the nerves 
according to the varieties of their functional activity. In this way 

' Untersuchungen zur Mechanik der Nerven, 1876, Abth. ii., pp. 45 ff. 
- Comp. Foster, A Text-book of Physiology, fourth, edition, p. 134. New 
York, 1«80. 

52 ruNCTiojsr or the neevous elements. 

we should distingiiisli the following classes : (a) nerves of motion 
controlling- the muscular apparatus, whether of smooth or of striated 
muscular fibres ; (6) nerves of inhibition ; (c) nerves of secretion ; 
(d) trophic nerves, or nerves which have a direct influence upon nu- 
trition ; (e) centripetal nei-ves that have no sensory function ; and, 
finally, (/"") sensory nerves, or those the excitation of which may 
result in conscious sensation.' 

That the irritation of different nerves may have results so differ- 
ent as are indicated by the foregoing classes must indeed be ad- 
mitted ; but it is quite another question whether this difference is 
not wholly due to the sources of origin for the nerve-commotions 
sent along them, and to the structures in which it terminates, rather 
than to any difference in the essential physiological function of the 
nerves themselves. Just as the same electrical current may pass 
along the same kind of wire, and write a message, or ring a bell, or 
move the legs of a frog ; just so the irritation of certain fibres of the 
pneumogastric nerve results in controlling the motion of the heart ; 
the ii'ritation of other nerves seems to have an immediate metabohc 
effect in directing the secretory processes ; that of still others pro- 
foundly modifies the nutrition of the portions of the body to which 
they are distributed. All these effects are in ajDpearance greatly 
riulike the movement of a muscle under stimulation fi-om a nerve. 
With regard to the influence of the nerves on nutrition (then- ^?-o- 
phic function) it is not necessary, in order to account for it, that 
some specific action of a particular kind of nerves should be as- 
sumed. We should sup230se, of course, that the chemical j)rocesses 
in which nutrition consists would be changed in character by the 
molecular changes in the tissue which irritating any of the nerve- 
fibres running into it would inevitably bring about. 

Further consideration of the six possible classes of nerves given 
above reveals the fact that they may all be reduced to two, accord- 
ing to the direction in which their function of conducting nerve- 
commotion is exercised. The first four conduct it outward from 
the nervous centres, and are therefore called "efferent;" the last 
two conduct it inward toward the nervous centres, and are there- 
fore called " afferent." Into these two kinds all nerves are custom- 
arily divided, so far as their physiological function is concerned. 

§ 36. The further question now arises, W^hether the general phy- 
siological function of these two jDrincipal classes of nerves differs in 
kind as well as in direction ; or are afferent and efferent nerves to 
be identified so far as their specific neural function is concerned ? 
Inasmuch as every nerve-fibre, in the normal condition of the ner- 

' Comp. Sigmund Mayer, in Hermauu, Handb. d. Pliysiol., II., i., pp. 200 ft 


vous system, is a stretch of nervous matter between two termina- 
tions — a point of origin and a point of issue for the state of excita- 
tion — it might, at first, seem simpler to consider it as intrinsically 
capable of propagating nerve-commotion in one direction only. It 
would be concluded, then, that the behavior of afferent and effer- 
ent nerves, when stimulated, is essentially different with respect 
to their molecular processes. Certain phenomena are sometimes 
urged in favor of such a conclusion. 

The application of heat to an efferent (or motor) nerve causes no 
contraction in the muscle which the nerve supplies ; heat does not 
appear to be a stimulus of such nerves. On the contrary, Griitzner ^ 
concluded that heating the different kinds of afferent nerves to 
from about 115° to 125° Fahr. does excite them. The passage of 
a constant current along an efferent nerve, so long as this cur- 
rent does not suddenly change in strength, does not stimulate this 
nerve so that the muscle contracts ; but such a current does excite 
nervous impulses in a sensory nerve. Moreover, certain chemical 
substances are said to act as stimuli on efferent nerves which have 
no such effect upon sensory nerves. 

On the other hand, the rate of conduction in both afferent and 
efferent fibres, under similar conditions, is about the same. The 
laws which evince the behavior of nerves under stimulation by elec- 
tricity, and which are most relied upon as a basis for a mechanical 
theory of the nervous system, are largely the same for both kinds 
of fibres. There is a large amount of scientific information, called 
" general physiology of the nerves," which looks in the direction of 
identifying the molecular processes in the two classes of nerve- 
fibres. This is true in particular of the remarkable phenomenon 
known as the "negative variation "of the nerve-current. More- 
over, the marked difference (referred to above) in the results ob- 
tained by stimulating motor nerves on the one hand, and sensory 
nerves on the othei', is plainly, to a great extent, due to the differ- 
ence in the sources of the stimulation ; the former are excited by 
the central organs, the latter by the end-organs of sense. The mo- 
lecular structure of these two sets of organs, and their consequent 
molecular motion when acted upon by the appropriate stimuli, dif- 
fer widely ; we do not, then, need to assume a specific difference 
in the function of the connecting nerve-strands in order to account 
for a marked difference in the results. Thus it may be assumed 
that molecular disturbances, which would be quite powerless to stir 
the sluggish muscle-fibres when transmitted to them by a motor 
nerve, would occasion profound changes in the more sensitive 
' Pflijger's Arcliiv, xvii., p. 215. 


structui^e of the ganglion-cells when transmitted to the latter by a 
sensory nerve. 

Various attempts have been made to demonstrate, experimen- 
tally, that motor and sensory nerves can perform each other's func- 
tions. Such experiments have not yet been altogether successful. 
They consist, in general, of attempts to unite by healing the cen- 
tral part of a divided sensory nerve and the peripheral part of a 
divided motor nerve, and then to show that the nerve thus united 
discharges certain sensory or motor functions, as the case may 
be. Philipeaux and Vulpian,' after various rather unsuccessful 
attempts of Flourens, Bidder, Schiff, and others, succeeded in 
uniting the central portion of the liugual (or sensory gustatory) 
nerve of young dogs with the peripheral end of the hypoglossal 
(motor nerve of the tongue) on the same side. Stimulation of the 
lingual nerve above the point of union then produced contractions 
in the hypoglossal of the same side, and that even when the lin- 
gual was divided high up so as to preclude any reflex action. But 
the action obtained was found to be apparently due to the dtorda 
(motor) fibres present in the lingual. In 1863 Bert succeeded in 
reversing the course of the nerve-fibres in the tail of a rat, by bend- 
ing this appendage over and implanting its end in the animal's 
back. After healing had taken place, the transplanted tail was cut 
off near its origiu, and found to be sensitive — of course, in the re- 
verse direction of the nerve-fibres from the natural one. This 
experiment would seem, then, to show that sensory nerve-fibres, 
when reversed, can transmit sensory impulses in the direction 
which was formerly centrifugal. The experiments of Kiihne '^ and 
others upon the intramuscular ramifications of the nerve-fibres in 
the sartorius muscle of the frog point in the same direction. If 
the broad end of this muscle be divided by a longitudinal slit into 
a forked shape, then stimulation of one of the two tines of the fork 
beyond their division will stimulate the fibrils of the other tine ; 
that is, the minute twigs of the motor nerve in the tine which is 
directly stimulated have acted centripetally, and the excitation has 
then descended the twigs of the other tine. 

For all the foregoing, and for other reasons, we seem warranted 
in assuming that there is no such specific difference in the func- 
tion of the two kinds of nerves as is dependent upon the peculiar 
structure or molecular processes of each kind. Both afferent and 

' See Vulpian, Legons sur la Thysiologie du Systi^me Nerveux, etc , pp. 
274 ff. ; and comp the remarks of Hermann, Handb. d. Physiol., II., 1., pp. 
lOflf., and of Foster, Text-book of Physiology, pp. 503-508. 

^ Archiv f. Auat., Physiol., etc., 1859, pp. 595 fE. 


efferent nerves are probably capable of the same kind of molecular 
commotion called nervo as excitation, and of conducting this commo- 
tion in either direction. The marked difference in the results of the 
exercise of this function in the two cases is probably due chiefly to 
the difference in the organs from which the excitation of the nerve 
starts, and into which it is discharged. With respect to neural mo- 
lecular disturbances, all nerves are excitable, conductors of excita- 
tion, and exciters of nerve-cells and muscle-fibres. And if to this 
description we add the statement that nerve-cells can, acting auto- 
matically, originate this nerve-commotion, can modify its character 
profoundly as it passes through them, and distribute it in various 
directions, we state, in the most general form, w^hat is at present 
known as to the functions of the nervous elements. 




§ 1. In the last chapter the nervous elements were considered, as 
far as possible, without reference to their combination for the ac- 
complishment of a common work. Regarded as isolated, and as 
possessed only of those properties which belong to all living mat- 
ter of the peculiar chemical constitution and structural form which 
are described by the word "nervous," these elements are of great 
interest to physiological and psycho-physical researches. But in 
their normal position and activity the nerve-fibres and nerve-cells 
are always combined into certain organs, which are then arranged 
in a symmetrical whole. Thus combined they are dependent upon 
each other for the parts which they play in the entire s^'stem. The 
condition and function of each element are thus determined by the 
condition and function of the rest. One part of this system excites 
another, or modifies the excitation received from another. We are 
unable to isolate perfectly any one of these elements, and so study 
its normal functions apart. It is, indeed, possible to dissect out a 
nerve with a muscle attached, to keejj it alive for a time, and thus 
to inquire what an isolated nerve will do. In this way many of the 
most important discoveries in the general physiology of the nerves 
have been made. But every nerve is itself a compound of nervous 
elements which have been placed for purposes of experiment under 
abnormal conditions. The action of the nerve-cells, even when 
gathered into small masses called ganglia, is not open to direct in- 
spection. Moreover, when different tracts of nerves, or different 
regions in the central organs where ganglion-cells abound, are par- 
tially isolated by being laid bare for the direct application of stimu- 
lus, just so far as they are separated from the system they are in 
abnormal condition and show abnormal results ; and just so far as 
they are normal in condition and function they are still connected 
with the system. It is the mutual condition and reciprocal action 
of the elements, when combined into this totality, which constitute 


the nervous mechanism. A brief description of the manner of this 
combination is, then, indispensable at this point. 

§ 2. It will be of great service toward understanding such a de- 
sci'iption if it is begun under the guidance of some appropriate idea. 
Nerve-fibres and nerve-cells exist in enormous numbers within the 
human nervous system, and are combined in different proportions 
to make the different organs of this system. The significance of 
the combination appears only in the light of reflection upon the 
amount and kind of work which is to be done. The office of the 
nervous mechanism has been said (p. 18 f.) to be that of " concate- 
nating " all the functions of the living body in accordance with the 
complex internal and external conditions to which it is subject. 
But in the case of any of the higher animals, and especially in the 
case of man, this one office requires the doing of a quantity and 
variety of work that are proportionate to the complexity of these 
conditions. How shall such a quantity and variety of work be done ? 
To answer this question may be said — speaking figuratively — to be 
the problem before the nervous system. The actual arrangement 
of the elements of this system, in the exercise of their reciprocally 
conditioned activities, is the solution of the problem. As in all 
very complex questions of this sort, so this particular problem is 
solved by a wise division of labor. 

The manner in which the human nervous mechanism is developed 
as a response to the before-mentioned problem is made clear by con- 
sidering, in the first place, a much simpler form of the same prob- 
lem. The simple protoplasmic speck called an amoeba may be con- 
sidered as a living molecular mechanism. It appears, even under 
the higher powers of the microscope, as almost wholly, if not quite, 
composed of undifferentiated protoplasm, in the midst of which, as 
a rule, lies a single nucleus. If differentiated at all, it may be ob- 
served to have a somewhat solid external layer, called an ectosarc, 
and a more fluid granular interior, called endosarc. But minute 
and almost structureless as it appears, the amoeba is really com- 
posed of a great number of molecules that are undergoing constant 
change ; and it is capable of exercising several wonderful functions 
that do not belong to any non-living collection of molecules. Its sub- 
stance is metabolic, respiratory, reproductive. The protoplasm of 
the amoeba is the subject of constant chemical alterations, by which 
the old protoplasm is broken up and its products cast off, while 
new protoplasm is formed. Oxygen is assumed by this substance 
and carbonic acid excreted. The unit which is constituted by the 
amoeba may, by fission (or by other means), divide into two parts, 
Bach of which becomes a fresh unit. But more important for our 


purpose is the fact that the amoeba is irritable aacl automatic. It 
is almost unceasingly in motion. It is living matter ; and when 
acted on by stimuli, it suffers an explosion of energy which gener- 
ally results in a change of place and form. Inasmuch as these pe- 
culiar " amoeboid " movements seem substantially identical with 
those which occur in a muscle and result in its contraction, the 
animalcule may be said to be contractile. But inasmuch as some 
of these movements cannot be ascribed to irritation of the external 
molecules of the amoeba by the surrounding medium, but seem 
rather to be due to energy set fi'ee in consequence of unknown in- 
ternal changes, we call it automatic. We say, " it has a will of its 
own." Thus does the molecular mechanism of this small bit of 
protoplasm, under the stimulus of changes in the pressure and 
temperature of its medium, and in accordance with the unknown 
laws of its internal self -originating changes, solve the problem pre- 
sented to it. 

Let it be supposed that the problem becomes more complicated, 
and the animal structure which is to solve it correspondingly com- 
plex. The metabolic function of the animal may then be assigned 
to a separate system of structui'es ; and the closely related secretory 
and excretory functions as well. The reproductive function may 
then also acquire its own peculiar organs. The muscles perform 
movements in masses because they retain in an eminent degree the 
" amoeboid " contractility. But the property of being irritable and 
automatic becomes the special endowment of the nervous system. 
All these different systems, in order that they may be moved in 
united masses, are then adjusted to a mechanical framework (of in- 
different value so far as really vital changes are concerned) of carti- 
lage, bone, etc. 

But the eminently irritable and automatic system of molecules 
called nervous must undergo a further differentiation of function. 
In the structureless protoplasm of the amoeba, the external mole- 
cules are, of course, the ones primarily to be affected by the exter- 
nal stimuli. It is with the intei'nal molecules, on the other hand, 
that the changes called " automatic " begin. But the continual 
flux of its protoplasmic substance indicates that, in its simplest 
form, any of the molecules of the animalcule may in turn act either 
as irritable or as automatic. The primary differentiation of this 
substance into ectosarc and endosarc points, however, to a division 
of labor. 

By this primary differentiation of the substance of the animal, 
one cell, or group of cells, becomes more eminently irritable, 
another automatic. The former has thus been fitted for the spe- 


cial work of responding to external stimuli by vital impulses ; 
the latter for that of initiating so-called automatic impulses. The 
position of the former in the animal mechanism will then natu- 
rally be at the surface, where it can be acted upon by the appro- 
priate external stimuli ; the position of the latter will naturally be 
withdrawn from the surface, where it can be protected from such 
stimuli and left undisturbed for action that is either automatic 
or excited by only internal stimuli. But if the two kinds of sub- 
stance are to perform one work, although by division of labor, 
they must be connected ; that is, the eminently irritable protojDlasm 
of the surface must be joined by irritable protoplasmic material 
with the eminently automatic protoplasm of the interior. Three 
sets of organs are then called for in this rudimentary differentiation 
of the nervous substance : (1) superficial cells susceptible to exter- 
nal stimuli ; (2) central and eminently automatic cells, also suscep- 
tible to internal stimuli ; (3) a strand of irritable protoplasm con- 
necting the two. 

Yet one more step in the distribution of functions between the 
irritable and the automatic protoplasm of the complex animal or- 
ganism must be taken, in order to reach the fundamental triple ar- 
rangement of a nervous system. The system of eminently contrac- 
tile tissue called muscular must be brought into connection with the 
parts already described. In order that the more highly organized 
animal may, like the amoeba, both have and exercise " a will of its 
own," certain of its muscle-fibres must be placed under the control 
of the central and automatic cells. In order, also, that the entire 
muscular system may feel the reflex influence of external stimuli, 
and so, by co-ordinated contractions adapt the organs of the body 
to the changes of its environment, the muscle-fibres must be indi- 
rectly connected, through the automatic cells, with such superficial 
cells as are sensitive to these stimuli. The nervous system, there- 
fore, in its most fundamental form consists of these three sets of 
contrivances with their respective functions : (A) sensitive cells 
upon the surface of the body ; (B) central cells that are both auto- 
matic and modifiers and distributers of sensory impulses; (C) con- 
necting cords, or strands, that can convey the nervous impulses 
either centripetally from A to B, or centrifugally from B to the con- 
tractile muscular tissues of the body. 

Higher developments of this triple-formed fundamental type of 
a nervous system are reached by further differentiations of A, B, 
and G. If various kinds of stimuli are to act upon this system, 
then the sensitive cells upon the surface {A) must be modified into 
various external organs of sense : and with these organs the ter- 


minations of the centripetal or sensory nei'vous strands must be 
variously connected. The terminations of the centrifugal or motor 
nervous strands may also be variously modified so as to connect 
■with and control the contractile tissue of many sets of muscles. 
The central cells may be variously grouped and arranged, with 
functions more or less localized, so as to receive, modify, and dis- 
tribute, in manifold ways, the diiferent sensory impulses ; and so 
as to co-ordinate these impulses for definite results in the periph- 
eral parts of the body. Other such central cells may become more 
particularly related to the phenomena of conscious sensation and 
volition. Such a highly developed nervous system will then con- 
sist of the following parts : (^4) End-organs of Sense, like the skin, 
the eye, and the ear ; {A') End-organs of Motion, like the so-called 
motor end-plates and terminal nerve-bulbs ; (B) Central Organs, 
like the various peripheral and sjDoradic ganglia, the spinal cord, 
and brain, in which may come to exist (6) certain portions more 
distinctively automatic, (&') certain others more concerned in re- 
ceiving and distributing reflexly the sensory impulses, and (6") still 
others more particularly connected with the phenomena of con- 
sciousness ; and (G) Conducting Nerves, which will be either (c) 
centripetal, afferent, and sensory, or (c') centrifugal, efferent, and 
motor, designed to connect the central organs and the end-organs. 
We are now to consider the details with which such a highly de- 
veloped nervous system is actually constructed in the case of man. 
Our guides will, of course, be anatomy and histology. 

§ 3. In the manner already described (Chapter I., § 19) the indi- 
vidual nerve-fibres are collected and bound together in fascicles or 
groups of fascicles, called nerves, and in larger bundles or nerve- 
trunks. The nerve-cells are grouped into minute masses of nervous 
matter, such as the sporadic ganglia found in the sinus, auricular 
walls, and auriculo-ventricular groove of the heart ; or they are 
gathered into larger bodies, intersected with most intricate ramifi- 
cations of the nerves and intersi^ersed with the finely granular sub- 
stance called neuroglia, such as constitute the various parts of the 
brain and spinal cord. 

§ 4. The nerves and ganglionic masses of nervous matter in the 
human body are arranged in two great systems, the Sympathetic 
and the Cerebro-spinal. The Sympathetic Nervous System consists 
of a pair of nervous cords, situated one on each side of the spinal 
column ; of three main plexuses, situated in the cavities of the 
thorax and abdomen ; of a great number of smaller ganglia, lying 
in relation with the viscera of the same cavities, and widely distrib- 
uted over the body, especially in connection with the vascular sys< 


tem ; and of a great multitude of fine distributory nerves. Each 
of the two cords consists of a number of gangHa united by interme- 
diate nerves. In the other regions of the spinal column the num- 
ber of these ganglia equals that of the vertebra) (sacral 5, lumbar 
5, thoracic or dorsal 12), but in the neck (cervical) there are only 
3. From this ganglia ted cord a communicating and a distribu- 
tory series of nerve-branches are derived. By the communicating 
branches — each of which contains not only non-meduUated nerve- 
fibres from the sympathetic system to the cerebro-spinal nerves, 
but also meduUated fibres from the cerebro-spinal to the sympa- 
thetic — the two systems are brought into close anatomical and 
physiological relation, and a kind of double interchange takes place 
between them. The distributory branches of nerves in the sympa- 
thetic system bring the gangiiated cord into connection with the 
blood-vessels and viscera of the body. The involuntary muscles 
in the coats of these vessels and in the walls of the viscera are 
thus bound together, and through the sympathetic fibres brought 
under the control of the cerebro-siDinal axis. The three main plex- 
uses referred to are collections of nerve-cells and a dense plexiform 
arrangement of nerve-fibres. One of them is situated at the base 
of the heart, to which it gives off branches that wind around that 
organ and penetrate its muscular substance ; another is placed at 
the upper part of the abdominal cavity, and gives origin to numei-- 
ous plexiform branches that supply the viscera of the abdomen ; 
the third is in front of the last lumbar vertebra, and supplies the 
vaso-motor nerves and nerves of the muscular coats and mucous 
membranes of the various organs in that region of the body. Fur- 
ther details in the anatomy of the sympathetic nervous system are 
of little interest to psycho-physical studies. To such studies it is 
of great interest, however, to know that this system forms a bond 
between the sensations, emotions, and ideas which have their 
physical basis in the molecular condition of the cerebro-spinal 
centres, and those various organs in the thoracic and abdominal 
regions whose condition is so closely related to such psychical 
states. The effect of certain emotions, for example, upon the con- 
dition of the circulation, digestion, etc., is too well known to re- 
quire a lengthy statement. 

§ 5. The Brain and Spinal Cord are the great centres of the cere- 
bro-spinal system. These bodies are situated in the bony cavity of 
the skull and spinal column. They have three Coverings or Mem- 
branes, the innermost one of which is directly united with the sur- 
face of the nervous substance, and sends numerous processes into 
its interior. (1) The Dura Mater, which is the membrane lying 

Fig. 11.— View of the Cerebro-spinal Axis. 
(After Bourgery.) '/j. The right half of the 
cranium and trunk has been removed, and 
the roots of the spinal nerves dissected out 
and laid on their several vertebrte. P, T, O, 
cerebrum ; C, cerebellum ; P, pons Varolii ; 
in o, medulla oblongata ; m «, m s, upper and 
lower extremities of the spinal marrow. CI. 
to CVIII. are cervical nerves ; DI. to DXII., 
dor!=al ; LI. to LV., lumbar : SI. to SV., sa- 
cral : Col., coccygeal. 



next to the wall of the bony cavity, is tough, white, fibrous, and of 
structure somewhat different in the cranial from the spinal cavity. 
In the former position it is identical with the inner periosteum of 
the bones of the skull ; on passing into the spinal column, how- 
ever, the periosteum divides into two or more lamellse, the inner- 
most of which is prolonged into the cylindrical tube that includes 
the spinal cord. Three processes of the dura mater divide — only 
incompletely — the cavity of the skull into two symmetrical halves 
and into an upper and lower space : (a) the /ate cerebri, a sickle- 
shaped process between the two hemispheres of the large brain ; 

Fro. 12. — The Cranium opened to show the Falx Cerebri and Tentorium Cerebelli, and the Places 
of Exit for the Cranial Blood-vessels. }4. (Schwalbe.) a, a, Falx ; 6, b, the tentorium : 3, 3, 
Sinus transversu.'!, and 2 to 3, Sinus rectus, receiving from in front the Vena magna Galena. 
4, internal jugular vein ; 5, superficial temporal vein ; and 6, middle temporal vein. 

(b) the falx cerebelli, a similar process between the two lateral lobes 
of the cerebellum, or small brain ; and (c) the tentorium cerebelli, 
an arched process over the cerebellum separating it fi'om the back 
portions of the large brain. The fluid necessary to fill up the gaps 
and smooth over the surfaces of the closed area made by the dura 
mater is contained in the intercommunicating spaces of the mem- 
brane lying next inward and called (2) Arachnoid ; this membrane 
is transparent and of delicate connective tissue. Toward the dura 
mater it presents a smooth, firm surface, like that of a serous mem- 
brane, and is covered by a layer of scaly endothelium ; this layer 
is reflected on to the roots of the spinal and cranial nerves, and 
becomes continuous with the lining of the dura mater when the 


nerves pierce the latter membrane. The space below this surface 
is called subarachnoid ; the subarachnoid or cerebro-spinal fluid (al- 
ready referred to as filling the intercommunicating compartments 
into which this space is divided by bundles of delicate areolar tis- 
sue) is alkaline and poor in albumen. (3) The Pia Mater is a vas- 
cular membrane, a minute network of fine branches of arteries 
and veins held together by delicate connective tissue. These rami- 
fications of the blood-vessels in the pia mater are on their way to 
or from the nervous substance of the spinal cord and brain. The 
membrane, therefore, closely invests this substance, being, how- 
ever, more intimately attached to the cord than to the brain. Un- 
like the arachnoid membrane, the pia mater dips into the fissures 
between the convolutions of the cerebrum. It also sends its pro- 
longations, not only into the fissures of the cord, but also, as slen- 
der bands (trabeculce) from its inner surface, into the columns of the 
cord. These trabeculse branch and anastomose within the white 
substance of the cord hke the midrib of a leaf. The pia mater 
is well supplied with nerves. 

By these three membranes the nervous masses of the cerebro- 
spinal system are protected, held together and in place with a soft 
and yielding but sufficiently firm pressure, and nourished by the 
blood. This great nervous system, as a whole, consists of the cen- 
tral organs — spinal cord and brain — and of various roots, divisions, 
and branches of spinal and cranial, or encephalic nerves. 

§ 6. The Spinal Cord, or Medulla Spinalis, extends in the spinal 
canal from the aj)erture in the cranial cavity [foramen magnum), 
above which it is continuous with the medulla oblongata, down- 
ward to opposite the body of the first lumbar vertebra, where, after 
tapering ofij it is s]3un out into a slender thread of gray nervous 
substance [filuni ierminale) that lies in the axis of the sacral canal. 
Its length is from fifteen to eighteen inches ; its weight, when di- 
vested of membranes and nerves, about an ounce and a half, or not 
far from one thirty-third of that of the brain. It is nearly cylin- 
drical in shape, its front and back surfaces being somewhat flat- 
tened ; it has two considerable enlargements of its girth — an up- 
per (cervical), from which arise the nerves that supply the upper 
limbs ; and a lower (lumbar), which supplies the lower limbs with 

§ 7. The external structure of the spinal cord requires us to no- 
tice (1) the Fissures which almost completely divide it for its whole 
length into right and left (lateral) halves, and are, therefore, fitly 
called " median ; " of these fissures (a) the one in front {anterior 
median) is somewhat broader than (6) the one behind (posterior 



Fig 13 —A. Anterior, anri B, Posterior. View of the 
Spinal Cord and Medulla Oblon.sata. B', the Filum 
terminals, whirl) has been cut off from A and B. I, 
Pyramids of the medulla, and 1'. their decussation. 
2 olives ; :i lateral strands of the medulla ; 4% cala- 
mus scriptorins ; 5, the funiculus gracilis; and 6, 
the funiculus cuneatus ; 7. the anterior, and 9, the 
posterior, fissures ; 8, the antero-latej-al impression ; 
10, postero-lateral groove. C, the cervical, and L, 
the lumbar, enlargements of the cord. 





median). Both fire filled to their bottom with processes of the pia 
mater ; and the sides of the posterior fissure are bound closely to- 
gether by the same membrane. 

Each of these symmetrical and nearly half-cylindrical halves of 
the cord is subdivided by the lines of the entrance of the posterior 
and anterior nerve-roots into (2) three Columns : (a) the anterior, 
which lies between the anterior median fissui'e and the anterior 
roots ; (6) the j^osterior, which lies between the posterior median 
fissure and the posterior roots ; and (c) the lateral column, which 
lies at the side of the cord between the other two columns. 

(3) The Commissures of the spinal cord are two bands of ner- 
vous matter which unite 
its halves, thus prevent- 
ing it from being com- 
pletely separated into 
two portions by the 
fissures. The one in 
front, at the bottom of 
the anterior median 
fissure, is composed of 
transverse nerve-fibres 
and is called (a) the 
anterior white commis- 
sure ; the one behind, 
at the bottom of the 
posterior fissure, is (6) 
the posterior gray com- 
missure. The gray 
commissure is nearly 

Fig. 14.— a, Anterior, and B, Lateral,View of a Portion of the twiCC aS large aS the 

Cord from the Cervical Region. Vi- (Schwalbe.) 1, Anterior ■rY|jjJ;;e CXCCpt at the 

median, and 2, posterior median, fissures. At 3 is the an- ' ^ 

terolateral impression, over which spread the anterior roots cCrvical and lumbar CU- 
(5). The posteri(jr roots (6), with tlieir ganglion ((>'), arise 

from the postero-lateral groove, and uniting with the ante- largCmentS of the COrd, 

rior roots form the compound nerve (7). , , . 

where the white is 
larger.' Along its whole length the gray commissure incloses a 
circular or elliptical canal [central canal), whose diameter is about 
one-twenty-fifth of an inch and which is lined by ciliated cells. Near 
the central canal lies ai,hin layer of gelatinous substance. The rest 
of the gray commissure consists for the most part of extremely fine 
nerve-fibres devoid of medullary sheath; while the white com- 
missure is composed of meduUated fibres. The thickness of the 

See Henle, Anatomie des Menschen. Text, p. 309. 



commissures is, as a rule, proportional to the size of the corre- 
sjDonding nerve-roots ; their form, as they pass into the lateral 
parts of the cord, varies in different sections of its length. 

§ 8. Transverse sections of the spinal cord show us that, as its 
external appearance v^'ould indicate, the substance of which it is 
composed is arranged in two symmetrical halves, almost, but not 
quite separated by the median fissures. This substance, like that 
of all the nervous centres, consists of both white and gray nervous 
matter. The former is external and composes the columns of the 
cord ; while the latter is internal and is surrounded by the white. 
The relative amount of the two kinds of nervous matter varies in the 
different parts of the cord. At its beginning from the filum termi- 
nale scarcely any white matter appears ; the amount of such matter, 
however, increases from below upward, and is largest in the cervi- 
cal part of the cord. The amount of gray matter is greatest in the 
ujDper and lower enlargements of the cord. 

The gray columns on either side of the cord, together with the 
commissures which unite them, form a figure somewhat like a large 
Eoman X, with diverging 
sides; but the lateral masses — --'''^ 

of these crescent-shaped bodies 
are narrower in the thoracic 
(or dorsal) region, and broader 
in the cervical and lumbar en- 
largements. Sometimes the 
figure is rather like that of a 
large X, or a pair of butterflies' 
wings. The two limbs of each 
side of the figure into which 
the gray columns are thus 
formed are called (4) Horns ; 
(a) the anterior horn is round- 
ed, (6) the 2^osterior long and 
narrow. The division into an- 
terior, posterior, and lateral 
columns, which is well marked 
on the external surface of the spinal cord, is gradually lost as we 
pass inward toward the central gray substance. Of the two horns 
of each side, the anterior has the appearance of " spongy sub- 
stance," the posterior of a kernel of such substance surrounded by 
gelatinous substance. 

§ 9. Careful study of the spinal cord with the higher powers 
of the microscope has enabled histologists to describe with further 

Fig. 1.5. — Transverse Section through the Spinal 
Cord. AF, antero-median, and PP, postero-median 
fissures; PC, posterior, LC, lateral, ami AC, anteri- 
or columns ; AR, anterior, and PR, posterior nerve- 
roots : C, central canal of cord, with its column,. r 
endothelial lining-. The pia mater is shown invest- 
ing the cord, sending processes into the anterior 
and posterior fissures, as well as delicate prolonga- 
tions into the columns. The crescentic arrange- 
ment of the gray matter is shown by the darker 
shaded portion. 



details the manner in which the nervous elements, both fibrillar and 
granular, are arranged within the connective substance. 

The White Substance of the spinal cord, besides connective tis- 
sue and lymph- and blood- 
vessels, is composed of 
nerve-fibres of compara- 
tively large or of medium 
size. The essential constit- 
uent of these fibres is the 
axis-cylinder, the diameter 
of which is generally one- 
third or one-fourth of their 
breadth. When fully de- 
veloped, they are rarely or 
never without a medullary 
sheath, but probably have 
no neurilemma. Their di- 
ameter is not constant ; 
the thickest fibres ( j-^'oir ^^ 
"SirV^ °^ ^^^ inch) are found 
in the outer portions of the 
anterior columns, where 
their size is tolerably uni- 
form. In the lateral col- 
umns the nerve-fibres vary 
greatly in size, the finer 
ones lying inward near ths gi'ay matter. In the posterior columns 
they increase in thickness as they approach the posterior gray com- 
missure. In the upper thoracic, and through the whole of the cer- 
vical, region, there is found a wedge-shaped bundle of fine fibres 
that is separated off from the posterior columns toward the middle 
line of the cord by a strong seiDtum ; this is called fasciculus gracilis, 
or "column of Goll." 

The direction of some of the nerve-fibres in the white substance 
of the cord is vertical, of others, horizontal, of still others, oblique. 
The vertical fibres are most abundant, are united with a parallel 
arrangement into fascicles of various sizes, and ascend toward the 
brain. Horizontal fibres in the white substance of the spinal cord 
are of two kinds — commissural fibres and fibres of the roots. The 
fibres of the white commissure run horizontally along the median 
border of the gray matter of the horns, and become interwoven with 
the vertical bundles of the anterior columns. Most of them pass 
from the substance of the anterior horn of one side across to the 

Fig. 16.— Section of Dorsal Part of the Spinal Cord phow- 
ing the Gray Matter of the Horns. ^%. (Henle.) 
Ca, anterior while, and Cr. graj' commissure ; Co, cen- 
tral canal ; v, vesicular column ; s. spongj' substance of 
the posterior horn, surrounded by g. gelatinous sub- 
stance ; Pr, reticular process ; Ti, intermedian lateral 



finterior column of the other side. The fibres of the posterior spi- 
nal roots run in a nearly horizontal direction inward ; they divide 
into anastomosing' bundles so minute and so intricately interwoven 
with the vertical fibres of the posterior column that their course 
is difficult to trace. Part of them (the lateral ones) run directly 
into the substantia gelatinosa of the posterior horns, and are, per- 
haps, continuous with the axis-cylinder processes of the nerve-cells 
of its spongy kernel ; part of them appear to enter the gray sub- 
stance of these horns only after curving and running a variable dis- 



Fig. 17 — Section of the Spinal Cord at the Level of the Eighth Pair of Dorsal Nerves, ^jy 
(Schematic, from Schwalbe.) s.a., anterior fissure; s.p., posterior septum (or Assure); c.a., 
anterior, and c.p., posterior, commi>sures ; c.c, central canal : co.a, anterior horn ; co.l., lateral 
horn ; vo.p., posterior horn ; n, anterior lateral, and 6, anterior median cells ; r, cells of the 
lateral horn ; d, columns of Clai-ke ; e, solitary cells of the posterior horn ; r.a.. the anterior^ 
and r.p,, the posterior, roots : f, bundle of fibres of the posterior horn ; and /', bundle of the 
posterior column ; /", iongirudinal fibres of the posterior horn ; s.g.R., gelatinous substance of 
Rolando; /.a., anterior, /.i., lateral, and/.p., posterior, columns. 

tance upward, or perhaps downward, in the posterior columns; 
The fibres of the anterior roots of the spinal cord traverse its white 
substance obliquely ; some of them enter the gray matter of the 
anterior horns on the same side, where they probably become con- 
tinuous with the axis-cylinder processes of its large ganglion-cells ; 
others of them pass through the anterior commissure to the other 
side of the cord ; still others pass into the lateral columns and the 
posterior horns. 

The Gray Substance of the spinal cord, in addition to the same 
constituents as those of the white substance, has numerous nerve- 


cella Its nerve-fibres, wliicli foiin the chief part of its mass, and 
are generally non-medullatecl, differ from those of the white sub- 
stance in that they frequently subdivide and thus become attenu- 
ated into extremely minute plexuses. The ganglion-cells of the 
spinal cord are multipolar, and give off two kinds of processes ; one 
an unbranched axis-cylinder process and the others branching pro- 
cesses, both being of a fibrillated character (comp. Chap. I., §§ 28 
and 29). The iinbranched processes of the ganglion-cells of the 
anterior horns are probably continuous with the axis-cylinders of 
the nerve-fibres of the anterior spinal roots. Of most of the simi- 
lar processes from cells in the posterior horns we cannot yet make 
the same afiirmation. The branching processes of the nerve-cells 
were traced by Gerlach ' until he thought himself able to affirm 
that their finest ramifications participate in those plexuses of nerve- 
fibres which he regards as an essential constituent of the gray sub- 
stance of the cord. Henle ^ and others consider the fate of these 
processes to be still unknown. 

Characteristic groups of ganglion-cells occur at various places in 
the sections of the gray matter of the spinal cord. In the anterior 
horns of the cervical and lumbar regions are three groups of large 
cells ; one of these is on the side of the horn (lateral), one farther 
to the front, one on its median border. They all coalesce in the 
anterior horns of the thoracic region. In the anterior horns also 
occur isolated nerve-cells of different sizes. The middle part of 
the gray lateral halves of the spinal cord contains, in parts of the 
cervical and thoracic regions, isolated groups of cells ; one impor- 
tant group is situated at the inner angle of the base of the posterior 
horn, and is called the "columns of Clarke." The other nerve-cells 
of the posterior horns are small, and are not collected into groups, 
but are distributed through that part of the substance of the horns 
which is also traversed by the above-mentioned fine plexuses of 
nerve-fibres (see Fig. 17). 

§ 10. By careful counting, E, A. Birge ' ascertained the number 
of the elements in the spinal cords of several frogs. From his con- 
clusions something may perhaps be gained toward forming a better 
conception of this organ. In seven cases Bu-ge found that the num- 
ber of fibres in the anterior roots varied from 5,984 in the smallest 
animal to 11,468 in the largest ; the number increasing at the i-ate of 
about one thousand four hundred and fifty motor fibres to each added 
ounce of weight (51.5 to the gram). The diameter of the fibres was 

' See in Strieker's Human and Comparative Histology, ii., pp. 352 fE. 

- Anat. des Menschen. Text, pp. 810 ff. 

3 Archiv f. Anat, u. Physiol, 1882, Pliysiolog. Abtli., pp. 435-479. 


also found to be much enlarged, according to tlie size and weight of 
the animal ; and the average diameter widely different in the different 
nerve-roots. For example, it varied from 3,550 fibres, in the sev- 
enth pair of nerves, to 14,133 in the tenth pair, for a cross-sec- 
tion one twenty-fifth of an inch square. So, too, were the so- 
called motor-cells of the anterior gray columns found to vary from 
4,871 to 11,517, according to the weight of the animal. It was 
found that the large masses of cells lie in two principal gToups, 
corresponding to the cervical and lumbar enlargements of the 

§ 11. It would be of great interest to our inquiries if it were 
possible to give a complete descrii^tion of the tracts of the nerve- 
fibres in their passage along the spinal cord ; but it is impossible 
for the microscope to unravel them, and the evidence of physiology 
is (as we shall see subsequently), somewhat doubtful and even con- 
flicting. Of late, however, certain of these paths have been traced 
with considerable certainty by combining the methods of embry- 
ological and pathological observation. In the development of the 
spinal cord, the medullary substance of the nerve-fibres along cer- 
tain tracts of the white columns is formed later, so as to render 
them distinguishable in cross-sections. Moreover, when the nerve- 
fibres are separated from their place of origin, degeneration of their 
elements takes place. The place of the degenerated nervous sub- 
stance is taken by connective tissue, which behaves differently un- 
der the influence of staining fluids. By following the course of 
this degeneration toward their periphery, the paths of conduction 
in the nerves may be traced. Some time ago, Tiirck ' attempted to 
mark out certain motor tracts in the brain by using this process of 
degeneration as his guide. Our great authority at present on the 
paths of the nerve-fibres in the sjDinal cord and brain, as ascer- 
tained chiefly by the former of these methods, is the work of 

Two tracts in the antero-lateral columns, which extend along the 
greater part of the spinal cord and into certain parts of the brain, 
are thus quite certainly made out. From their upper connections 
they have been named the i^yramidal tract (or tracts) and the direct 
lateral cerebellar tract. The former is directly traceable down from 
the anterior pyramid of the medulla oblongata. Most of the fibres 
of this tract cross over in the extreme upper part of the cord, and 
pass down it in the back part of the lateral column as a compact 

' Sitzgsb. d. Kaiserl. Acad., vi., pp. 303 ff. 

■^ Die Leitungsbaliuen im Gehirn u. Riickenmark d, Menschen. Leipzig, 



bvmdle. This crossed (or lateral) /^ari of the pyramidal tract can 
be traced as far as the third or fourth pair of the sacral nerves. 
But some of the fibres from the pyramids of the medulla do not 
cross in the upper part of the cord. These 
form the uyicrossed (or anterior) part of the 
pyramidal tract ; this part gradually dimin- 
ishes as it passes downward, and ceases in 
the dorsal region of the cord. The direct 
lateral cerebellar tract lies between the late- 
ral pyramidal tract and the outer surface of 
the cord. It disappears in the lumbar re- 
gion. It is thought that the rest of the ante- 
rior column of the cord, besides the anterior 
pyramidal tract, may be, for the most part, 
commissural in nature — that is, it serves to 
bind together the two halves of the cord on 
the same level, or somewhat obliquely those 
lying slightly below or slightly above. 

In the posterior white column a tract can 
be traced as far downward as the middle of 
the dorsal region of the cord ; this is the 
one already referred to as the "tract (or 
column) of Goll." 

§ 12. The spinal cord is, therefore, shown 
to be a mechanism composed by combining 
the nervous elements so as to serve the 
great purpose of conducting nerve-commo- 
tion and acting as a series of reflex and auto- 
matic centres. In it we find tracts of con- 
nected nervous elements for the movement 
of ascending and descending nervous im- 
pulses. It is also a column or pile of ner- 
vous centres, each one of which may have a 
particular value for particular functions ; 
but which are also all bound together, up 
and down, right and left, and obliquely, so 
as to act unitedly under a certain control 
from each other and from the central organs 
lying above. It is especially strong in nerve- 
cells, just where it needs to be so— namely, at the enlargements, 
where it sends off nerves to the upper and lower limbs. Its paths 
for the passage and diffusion of molecular disturbance are indefi- 
nitely numerous, and their intricacy extremely great. It has groups 

Fig. 18.— Sections through the 
Spinal Cord at diflferent ele- 
vations, to show the tracts of 
White Substance. /., eleva- 
tion of the sixth cervical 
nerves. //., of the third ; 
///., of the sixth ; and IV., 
of the twelfth, dorsal nerves; 
and v., of the fourth lumbar 
nerves ; pr, uncrossed (or 
anterior) pyramidal tract ; 
pn, crossed (or lateral) p.\ra- 
midal tract ; kn, direct late- 
ral cerebellar tract ; g, tract 
of Goll. 


of nerve-elements, such as belong to the central organs generally, 
of gangiion-celis embedded in neuroglia ; it has special local mech- 
anisms within, and yet connected with its general mechanism. It 
is adapted to do a large amount and variety of work through its 
pairs of nerves, without calling upon the higher nervous centres ; it 
is constructed so as to act like a system of relays, not only trans- 
mitting, but also modifying, inhibiting, enhancing, and distributing 
the impulses which it receives, both from the more central and from 
the peripheral portions of the cerebro-spinal system. 

§ 13. The same elements of nerve-fibres and nerve-cells, in con- 
junction with connective tissue and neuroglia, and enveloped in the 
three inclosing membranes (dura mater, arachnoid, and pia raater) 
already described, are combined with an increased variety and com- 
plexity of arrangement to form those intercranial central organs 
with which the upper end of the spinal cord is continuous. Here, 
too, these elements are gathered into fascicles of nerve-fibres 
which converge, or diverge, and mn their courses in various direc- 
tions, and into ganglionic masses, in which, besides the nerve-fibres, 
nerve-cells and diffused finely granular substance of a doubtful 
physiological character are found. Uniformity of elementary parts, 
together with the greatest intricacy of arrangement, prevails, above 
all other regions of the body, in the structure of the brain. The 
significance of the elements and elementary parts can, therefore, 
only be understood when they are considered in the localities and 
relations to other parts which are assigned them by this so intricate 

§ 14. The Encephalon, or Brain, in the most extended sense of 
the word, includes all that portion of the central nervous axis which 
is contained within the cavity of the skull. This grand mass of 
nervous matter may be divided into several parts, somewhat differ- 
ently marked off according to the point of view from which the 
division proceeds. The division proposed by Meynert ' — to which 
reference will be made later — is based upon the supposed physio- 
logical significance of the different parts, and upon their arrange- 
ment so as to discharge the functions of conduction and " suscep- 
tibility to impressions." For, as this authority rightly claims, " a 
purely histological description " is of comparatively little service 
in comprehending the meaning of the architecture of the brain. 
We shall, first of all, however, describe briefly the contents of the 
cranial cavity, as it appears both to the unaided eye and under the 
microscope, without reference to theory. 

' In Strieker, Human and Comparative Histology, ii. , pp. 367 S. 



On removing tlie entire brain from the skull, the following foul 
divisions of its mass engage the attention of even the inexperienced 
observer. Immediately above the section by which it has been sepa- 
rated from the spinal cord, and appearing as an enlarged prolonga- 
tion of the cord, is (I.) the Medulla Oblongata. Covering the ujojoer 
back part of this oi'gan, and extending beyond it on both sides, 
with its surface divided into small lobes by furrows, is (II.) the 
Cerebellum, or little or hinder brain. Swelling out in front of and 
above the medulla is (m.) the Pons Varolii, or so-called "bridge" 
of the brain. While in two hemispheres separated by a deep fis- 
sure, above both pons and cerebellum, and filling the larger part 
of the cranial cavity, is seen (lY.) the Cerebrum, or large brain, or 

Fig. 19. — View of the Brain in Profile, y,. (Henle.) C6, cerebrum; C5?, cerebellum ; Jfo, me- 
dulla oblongata ; P, pons Varolii, 

brain proper. These divisions are all readily distinguishable on 
the external surfaces of the Encephalon. 

On laying the encephalic mass open, however, certain bodies of 
nervous matter are disclosed that have been concealed beneath the 
cerebellum and the cerebrum, and that — although ordinarily re- 
garded as parts of the latter — are scarcely to be included in any one 
of the four main divisions of the brain. We shall describe in order 
the organs just named. 

§ 15. I. The Medulla Oblongata is somewhat pyramidal in form, 
about one and one-fourth inch in length, from three-fourths to one 
inch broad in its widest part, and one-half inch thick ; it extends 
from the spinal aperture of the cranial cavity {foramen magnum) to 
the lower border of the pons Varolii. It is continuous with the 
spinal cord, and somewhat resembles it in the divisions of its ex- 
ternal surface. Its anterior pyramids appear superficially continu- 

extekjstal aspect of the medulla. 



ous with the anterior columns of the cord ; its lateral area shows 
upon its upper end an oval-shaped elevation called the " olivary 
body ; " its posterior tracts also appear continuous with the poste- 
rior columns of the cord. Just outside the upper portion of each 
posterior tract, and behind the olive, ascends to the cerebellum a 
strong tract named 
the "restiform body." 
That portion of the 
posterior column of 
the upper cord (al- 
ready referred to, p. 
G8) which is marked 
off from the rest by a 
septum of pia mater, 
is continued up into 
the medulla oblonga- 
ta, and becomes more 
strongly marked. It 
is known as the fu- 
niculus gracilis ; and 
when traced still far- 
ther upward is seen to 
broaden out into an 
expansion called the 
clava. A prolongation 
of the posterior lateral 
column also gradually 
expands as it ascends, so that it acquires a " wedge-shape " form, 
and is accordingly known as the cuneate funiculus. 

The medulla oblongata, like the spinal cord, is composed of white 
and gray nervous matter ; it differs from the cord, howevei", in hav- 
ing its gray matter not confined to the central part, but gath- 
ered more into special masses or nuclei. A redistribution of the 
nerve-elements takes place in the medulla, and their arrangement 
becomes more complex. An important part of this redistribution 
is accomplished by the divergence of the posterior tracts and resti- 
form bodies, which opens up the central gray mass, and lets it 
come to the surface between the sides of the surrounding white 
matter. Looking at this redistribution as it appears from below, 
the elements of the cord may be said to be spread out and increased 
by the addition of new elements ; looking at it as it appears from 
above, the tvro great nerve-tracts of the cerebrum (tegmentum and 
crusta of the crus cerebri), and the tract of the cerebellum, may be 

Fig. 90. — Back View of the Medulla Oblongata, the Cerebellum 
being removed. (Henle.) Cq, corpus quadrigeminum ; Lc, 
locus ccei-uleus ; F, flocculus of the cerebellum; Ac, ala ci- 
nerea ; and Ac' Siilling's nucleus accessorius ; 01, clava ; Fc, 
funiculus cuneatus ; Fg, funiculus gracilis. 



said to be gathered up in the medulla, and compi-essed so as to 
form in the cord a continuous and symmetrical medullary invest- 
ment for its central gray matter. 

The intimate structure of this organ is exceedingly complicated ; 
much of it is doubtful, and as yet impossible to make out satis- 
factorily. The two important considerations are (1) to trace the 
nerve-fibres as they ascend through the medulla from the various 
columns of the cord, and (2) to locate the particular collections 
of gray matter, whether as continuous with those of the cord or as 
consisting of independent masses. 

The various tracts of White Matter in the medulla oblongata, 
although they supei-ficially appear to be prolongations of the col- 


Fig. 21. — Section showing the Decussation of the Pyramids at the point ■where the Spinal Cord 
passes into the Meriulla Oblongata. "/, . (Sohwalbe. ) f.l.a., longitudinal anterior fissure, 
through which the bundles of pyramidal fibres (py, lyy'), are crossing over at d ; V, anterior, 
and S. lateral pyramids: C.a., anterior hnrn with srroups of ganglion-cells, a and t> : cc. cen- 
tral canal; /.»■., formatio reticularis; ce, the nick, and fir, the head, of the posterior horn; 
n.c, nucleus of the funiculus cuneatus ; and w.fif., of the funiculus gracilis ; B', funiculus gra- 
cilis ; JJ ^, funiculus cuneatus ; x, group of ganglion cells. 

umns of the spinal cord, are reall}^ so to a small extent only. This 
fact is most clearly made obvious by a comparison of successive 
transverse sections. A large bundle of fibres, which in the cord 
lies in the posterior part of the lateral column (see p. 71 f.), pushes 
its way obliquely through the gray matter of the anterior horn, and 
passes in front of the central canal to the pyramid of the opposite 
side. The crossing of this bundle, as seen in the anterior median 


fissure at the lowei* part of the medulla, is called the " decussation 
of the pyramids." The abrupt passage of so many fibres through it 
breaks up the anterior horn, separates part of it from the rest, and 
pushes this sej^arated part over to one side, so that it comes to lie 
close to a part of the posterior horn. The latter also becomes gradu- 
ally shifted sidewise by an increase in the size of the posterior 
tracts, so that it comes to lie almost at right angles to the posterior 
median fissure ; its head enlarges and approaches close to the sur- 
face, where it forms a projection (funiculus of Rolando), and, higher 
up, a distinct swelling {tubercle of Rolando). Tracing the princi- 
]Dal bundles of fibres on their course from the columns of the spinal 
cord upward through the medulla oblongata, we find (in accordance 
with what has already been said) that the posterior column forms the 
substance of the three posterior funiculi of the medulla — namely, 
gracilis, cuneatus, and funiculus of Rolando : a considerable part 
of the lateral column {tlie lateral pyra-niidal tract, see p. 72) passes 
into the opposite pyramid of the medulla, and ascends in it toward 
the cerebrum in company with a small part of the anterior column 
of the same side ; while another part of the lateral column (the 
direct lateral cerebellar tract) passes at about the middle of the 
medulla obliquely backward to the restiform body, and the rest of 
it dips under the olives, and is continued toward the corpora quad- 
rigeminum and optic thalamus. Most of the anterior column dips 
under the pyramid, and passes upward toward the cerebrum, but 
part is continued into- the pyramid of the same side. 

Curved fibres may also be seen running their course in the plane 
of the different transverse sections — some superficial, some deep 
(arciform or arcuate fibres). 

As the medulla is a bilateral organ, its halves are bound together 
by commissural fibres, which run obliquely and decussate in the 
mesial plane, forming a well-marked band called raphe. In addi- 
tion to the fibres of the medulla oblongata which are continuous 
Avith those of the spinal cord, others originate within the organ 
itself. It is a centre of origin for several pairs of encephalic 

The Gray Matter of the medulla oblongata is, in part, continu- 
ous with that of the cord, and in j)art consists of independent masses. 
The former part is, as we have seen, broken up and rearranged by 
the decussation of the j^yramids. The fate of the posterior horns 
and of the central gray substance has already been described. The 
substance of the anterior horns becomes divided into many little 
masses by the nerve-fibres that traverse it, so as to form a coarse 
network of nervous matter (formatio reticularis) containing nerve- 



cells, and intersected by bundles of fibres. In the upper part of 
the organ its interior gray matter appears upon the floor of the 
fourth ventricle, into which the central canal dilates. Four special 

kernels or nuclei, of gelatinous 
appearance and containing few 
multipolar nerve-cells, are to be 
noted in each half of the me- 
dulla. These are (1) the nucle- 
us arciformis, which is situated 
just beneath the pia mater, at 
the front of the anterior pyra- 
mid ; (2) the nucleus olivaris, or 
dentate body (corpus dentatum), 
which is within the inferior 
olive, a mass of gTay matter 
folded in a zigzag or denticu- 
lated manner, forming a sort of 
capsule through the openings 
of which closely packed masses 

Fig. 22.— Section showing Gray Matter of the of fibres run into the SUrrOUnd- 

Medulla Oblongata, in the region of the upper . /o\ j.i 7 ?• 

crossing of the Pyramids. Vi- (Schwalbe.) lUg SpaCC ; [6) tlie nUCieUS OLl- 
f.l.a., anterior, and ft.lp., posterior, fissures; . - ■,■• 

n.XI. i,nd n.XII., nuclei of the vagus accessori- WTIS aCCeSSOTlUS, a Smaller gray 

us and hypoglossal nerves ; d.a.. so called upper „,„„„ Ivitio- on thp nnt«?iVlp nf 

crossing of the pyramids; py. anterior pyra- mdbb 1^ ing OH UUe OULblUe OI 

mid in which i- .*.«»• the nucleus arciformis; ^j^g dentate bodv ; and (4) the 

o, beguniing of the olivary :iucleus: oi, acces- J ^ \ I ^ 

sory olivary nucleus; /^.r. formatio reticularis; nUClcUS pyramid allS (sOmetimCS 
{7, substantia gelatinosa; /.a., /.«.', /.a.-', arci- . 

form fibres. ^-Iso Called " inner accessory nu- 

cleus " of the olive), lying on the inside of the same body. Another 
kind of collections of gray matter in the medulla consists of those 
groups of multipolar cells to which the nerves that have here their 
so-called roots of origin can be traced. These cells, resemble those 
of the gray columns of the cord — the larger ones apparently being 
connected with the roots of the motor nerves, the smaller with 
those of the sensory. It may be assumed that some of their pro- 
cesses are continuous with the axis-cylinders of the fibres of the 
nerve-roots, and that others serve to place the medulla in direct 
connection with the cerebrum ; positive demonstration of these as- 
sumptions, however, requires further histological researches. The 
nerve-nuclei in the medulla receive their name from the nerves 
whose fibres originate in them, 

§ 16. n. In the Cerebellum, or Little Brain, the general arrange- 
ment of the two kinds of nervous matter is the reverse of that of 
the spinal cord and the medulla oblongata : the gray matter is 
external, the white internal. More precisely, the cerebellum is 


a white or medullary mass rising out of three large bundles or 
stalks of nerve-fibres on each side, and enveloped with a covering 
of gray nervous matter. Like the other organs of the cerebro- 
spinal system, it is a bilateral structure. These stalks of nerves 
connect the cerebellum with three other organs, with parts of 
which they are continuous. Considered, as connections, they are 
called the " peduncles " or crura of the cerebellum. Of the three 
peduncles, (1) one {inferior peduncle) on each half of the organ is 
identical with the restiform fascicle which ascends from the me- 
dulla to the cerebellum ; (2) another {superior j^edunde), similar to 
the first in size, passes forward, over the anterior end. of the fourth 
ventricle, and. connects the cerebellum with the tegmentum of the 
crus ; (3) a third {middle peduncle) passes down on each side into 
the pons. This middle peduncle forms the larger portion of the 
white core of the organ. In addition to the fibres from these three 
sets of peduncles, this core is in part constituted by others which 
arise in the cerebellum itself ; some of the latter connect together 
the different regions of the organ lying above or below each other, 
some unite the opposite and symmetrical regions of its hemi- 

The interior relations of the fibres from the three peduncles are, 
on account of the extreme intricacy of their course, not yet fully 
made out. United in the white core of the cerebellum, they form 
a rather uniform mass, which is interrupted, however, by certain 
nuclei of a gelatinous appearance. Within either hemisphere, and 
to be disclosed by cutting through it a little to the outer side of the 
median lobe, is a mass of nervous matter arranged like the den- 
tate body of the medvilla oblongata ; it is the corpus dentatum of 
the cerebellum. Other smaller, round, or oblate masses of gray 
matter are found toward the middle of the core from the dentate 

The arrangement of the gray matter which forms the rind or 
cortex of the cerebellum is somewhat peculiar ; its characteristics 
are best seen by examining a cross-section. It is thus found that 
this cortical gray substance is arranged in thin plates, or lamellae, 
which are penetrated by prolongations of the white matter of the 
core ; these prolongations branch off into the interior of the lamel- 
lae, and give to the coi'tex the arborescent appearance known by 
the name of "arbor vitce." The primary branches of this tree-like 
prolongation of the white matter of the core within the gray mat- 
ter of the cortex stand either perpendicular or a little inclined to 
the surface of the core. The smaller branches run from one side to 
another transversely or forward in concave curves. 



The external surface of the cerebellum presents two hemispheres, 
or lateral lobes, united by a central lobe called the vermiform pro- 
cess. This central lobe on its upper (or tentorial) surface is a mere 
elevation, but the "vermiform" character of its lower (or occipital) 
surface is well defined. The process here lies at the bottom of a 
deep fossa {vallecula). From the middle peduncle of each hemi- 
sphere a large horizontal fissure extends backward along its outer 
border, and divides the hemisphere into its tentorial and occipital 

Pig. 23 — Lower Surface of Cerebellum. %. (After Sappey. ) ], in f prior vermiform process; 
2, 2, vallecula : 5, flocculus ; 6, pons Varolii ; 8, middle jieduncle of the Cerebellum : 9, medulla 
oblongata. Various pairs of nerves are seen thus: 12 and 13, roots of fifth pair ; 14, sixth 
pair; 15, facial nerve; 17, auditory; 18, glosso-pharyngeal ; 19, pneumo-gastric ; 20, spinal 
accessory ; 21, hypoglossal. 

surfaces. Each of these surfaces is divided by fissures into smaller 
lobes or lobules. 

In the gray matter of the cortex of the cerebellum three distinct 
layers of nervous substance maj' be distinguished. Of these the 
pure gray layer is the most external ; it is sometimes called the 
" molecular layer." It consists of an extremely delicate framework 
of connective tissue, in which, together with nuclei of the connec- 
tive tissue, a few roundish cells and minute fi.bres of nervous struct- 
ure appear. The middle layer is cellular and composed of a single 
irregular row of large ganglion-cells, called " cells or corpuscles of 
Purkinje." Comparatively large processes from these cells branch 
into and ramify within the outer layer. According to most observers 
(Kolliker, Deiters, and others) each of the cells sends a single me- 
dullated and unbranched process inward, which becomes continu- 
ous with the axis-cylinder of a fibre of the medullary portion of this 
organ ; but according to Stilling there are several branches from 
each, which divide to form a network in the internal layer. This 



layer is rust-colored and merges gradually into the white substance 
of the core ; it appears to contain multitudes of granules, with a 
well-defined nucleus surrounded by branching protoplasm. The 
nature of the granules is not known ; they have been considered 
by some as elements of sustentacular tissue, by others as lymph- 
corpuscles, by others as multipolar nerve-corpuscles. 

The cerebellum is thus constituted by a complex arrangement 
of the nervous elements as a kind of side mechanism of the nervous 
system, lying out of the course of its direct tracts and yet bound 
by nervous cords (the peduncles) in all directions to the other 
oi'gans of the brain. 

§ 17. in. The Pons Varolii, or Bridge of the Brain, has its princi- 
pal office in the mechanism of the central organs of the cranial 
cavity as a meeting- and switching-place of nerve-tracts between 
other organs ; but it is also itself a central organ, as well as a cen- 
tre of origin for certain nerve-fibres. The pons is really a thicken- 


Fig. 24.— Median Section through the Stem of tTie Brain. (After Reichert.) M, medulla oblon- 
gata ; of whichPa are the pyramid?, decussating atprf ; c, central canal ; pp. restiform body ; 
Pv, pons Varolii ; F4, fourth ventricle, au, arbor vitte of the cerebellum ; p, pyramid ; u, 
nvula ; re, nodule ; as, aqueduct of Sylvius ; CV, crus cerebri ; Q, corpora quadrigemina ; P, 
pineal gland ; Th, optic thalamus. Commissures : ra. the anterior; cm, the mollis ; and cp, the 
posterior. F3, the third ventricle ; /I, corpus albicans; Zc, tuber cinereum ; i, infundibulum. 

ing of the ventral wall of the fourth ventricle, composed of the mid- 
dle peduncles of the cerebellum encircling and partly blending with 
the continuation upward of the medulla oblongata. Its superficial 
fibres on the ventral surface are transverse in their general direc- 
tion ; but the middle fibres pass directly across, the lower ascend 
sUghtly, and the superior are more curved, and descend obliquely 


to reach the crus cerebelli. On removing these superficial fibres 
the prolonged fibres of the anterior pyramids are exposed to view. 
These, as they ascend through the pons, are intersected by the 
transverse fibres. At the low^er part of the organ, behind the fibres 
from the anterior pyramids, a special set of transverse fibres (tra- 
riezium) begins at a collection of gray matter [superior olivary 
nucleus) on one side, and crosses the middle line to ascend to the 
cerebellum on the other side. 

Nuclei of gray matter with small multipolar nerve-cells are found 
everywhere between the fibres of the ventral part of the pons. 
Many of its transverse fibres are probably connected with these 
cells. The posterior portion of this oi'gan is chiefly constituted by 
a continuation upward of the formatio reticularis, and of the gray 
matter of the medulla oblongata. In the reticular formation two 
or three important collections of nerve-cells lie embedded. One 
of these is the " superior olivary nucleus," which lies behind the 
outer part of the trapezium and gives origin to some of its nerve- 
fibres. Of the other nuclei in this region, one gives origin to the 
seventh or facial nerve, and others to portions of the fifth nerve. 

§ 18. IV. The Cerebrum, or Large Brain, much exceeds in size all 
the other contents of the cranial cavity ; but it surpasses them more 
especially in the variety and complexity of the arrangement here 
given to the nervous elements ; while its significance for the in- 
quiries of Physiological Psychology is altogether unique. 

As ordinarily described, this nervous mass includes a consider- 
able number of organs, which vary in structure, relations, and 
physiological functions. Besides the hemispheres of the cerebrum, 
and the great ganglia (corpora striata and optic thalami) which lie 
at their base, custom includes in this term certain bodies that ap- 
pear connected with the lower surface of the mass, viz., the corj)ora 
quadrigemina, pineal gland, crura cerebri, etc. 

§ 19. The Cerebrum is of ovoid shape and is divided — above, in 
front, and behind — into two hemispheres by a deej) median longi- 
tudinal fissure. If these hemispheres are drawn asunder by open- 
ing this fissure, they are seen to be connected at its bottom by 
a broad white band of nervous matter, the corjnis callosum. The 
outer surface of each hemisphere is convex and fitted to the con- 
cave inner side of the bones of the skull ; the inner surface along 
the median fissure is flat, and separated from the corresponding- 
surface of the other hemisphere by a process of the dura mater 
{falx cerebri) ; its under surface is separated from the cerebellum 
and the pons by another process of the same membrane [tentorium). 
From the front of the pons the large wliite nervous cords, called 


cerebral peduncles, or crura cerebri, pass upward and forward to 
connect the cereln-um with the organs lying below it. Around 
each crus winds a flat band, the optic tract ; these tracts come to- 
gether in front to form the optic commissiire from which the two 
optic nerves arise. The lozenge-shaped space enclosed by the 
crura cerebri, the optic tracts, and optic commissure, contains a 

Fig. 25 Under Aspect of the Brain. (Henle.) B, basis of the crura cerebri ; Oca, corpora al- 
bican Ha : I', olfactorv bulb; II'. optic tract; Tc, tuber cinereum ; Lpp, posterior perforated 
space : Ccl, corpus ca'llosum ; Let. lamina cinerea terminalis ; Spa, anterior perforated space ; 
T, tegmentum ; Tho. thalamus opticus; P, pons; Mo, medulla oblongata; I. to VIII., tirst to 
eigrhth pair of cranial nerves. 

gray layer {posterior perforated space), two small white bodies (cor- 
pora albicaniia), and a gray nodule {tuber cinereum) which is joined 
to a small reddish-gray oval mass {pituitary body) by a conical pro- 
cess of gray matter {ivfundibulum). In front of the optic commis- 
sure is a thin layer of gray substance (lamina cinerea) ; and on each 
side of the deep longitudinal fissure stretches the olfactory tract, 



with its bulb. The intercranial j)art of this "nerve " is now known 
really to be a projecting portion of the brain. All these structures, 
together with the cut ends of the several pairs of cranial nerves, 
may be seen upon the under surface of the cerebrum. 

§ 20. The upper surface of the cerebral hemispheres presents the 
appearance of gray nervous matter arranged in folds which are 
called " convolutions " or gyri. These convolutions are separated by 
"fissures " or sulci of varying depth, some of which are so constant 
and strongly marked that their presence is employed to divide the 
surface of the hemispheres into lobes, while others, less strongly 
marked, separate from each other the convolutions of the same 
lobe. It is the arrangement of the convolutions, with their sep- 
arating fissures, which gives the hemispheres of the brain their 
characteristic appearance, and which fits them for their unique 
functions in the economy of the nervous mechanism. 

FlQ. 26.— To show the Right Ventricle and the Left Half of the Corpus Callosum. a, transverse 
fibres, and &, longitudinal fibres of corpus callosum ; c, anterior, and d, posterior cornua of lat- 
eral ventricle ; e, septum lucidum ; /, corpus striatum ; (), ttenia semicircularis ; h, optic thala- 
mus ; k, choroid plexus ; I, teenia hippocampi ; ni, hippocampus major ; n, hippocampus 
minor ; o, eminentia coUateraLis. 

§ 21, By cutting off successive slices from the upper part of the 
hemispheres their general internal structure may be seen. It re- 



sembles the plan upon which the cerebellum is constructed. A 
core of white nervous matter is surrounded by a shell or cortex of 
gray ; the two lateral halves of the core are bound together by a 
strong band of fibres, 
usually described as 
commissural {corpus 
callosum), which is 
itself overlapped by 
one of the most 
marked convolutions 
of the brain {gyrus 
fornicatus). By cut- 
ting still deeper it is 
found that the cor- 
pus callosum forms 
the roof of a space in 
the interior of each 
hemisphere [the late- 
ral ventricles). These 
two cavities or ven- 
tricles are moistened 
by a serous fluid and 
separated by a thin 
transparent wall {sep- 
tum lucidum). The 
roof of another cav- 
ity, the third ventri- fo 
cle, is formed by an *- -■ 
expanded fold of the 
pia mater {velum in- 
terpodtum), the mar- 
gins of which are 
fringed by the so- 
called "choroid plex- 
uses ; " the latter 
contain the minute 
arteries which sup- 
ply the nervous structures of this region. Each lateral ventricle is 
divided into a central space and three curved prolongations or coi'- 
nica ; of the cornua, one (the anterior) extends forward and outward 
toward the fi"ont part of the cerebrum ; one (the posterior^) curves 
backward, outward, and then inward ; and the third (the descending) 
curves backward, outward, downward, and then forward and inward. 

Fia. 27. — Basal Ganglia of the Cerebrum seen from above. (Henle.) 
Col, genu of the corpus callosum ; Cs, corpus striatum ; Vsl, 
ventricle of the septum lucidum ; Of, column of the fornix ; St, 
stria terminalis : Tho, optic thalamus ; and Ts. its anterior tu- 
bercle ; Com, middle coiimiissure between the thalami and over 
the third ventricle ; Pv, pulviiiar ; On, conarium or pineal 
gland ; Cop, corpus quadrigeminum. 


On the floor of each lateral ventricle the exposed portions of the 
great basal ganglia of the cerebrum are visible. A large pear-shaped 
body of gray color is here seen with its broad extremity directed 
forward into the anterior cornu of the ventricle and its narrow 
end outward and backward.' This body, on account of the striped 

Fig. 28. — A Deeper DisEection of the Lateral Ventricle, and of the Velum Interpositura. a, un- 
der surface of corpus callosum, turned back ; 6, ft, posterior pillars of the fornix, turned back ; 
c, c, anterior pillars of the fornix ; d, velum interpositum and veins of Galen ; e, fifth ventricle ; 
f. f, corpus striatum ; (/, (j, taenia semicircularis ; h. h, optic thalamus ; A', choroid plexus ; /, 
taenia hippocampi ; m, hippocampus major in descending cornu; n, hippocampus minor ; o, 
eminentia collateralis. 

appearance which it presents when cut open, is called a " striate 
body" (corpus striatum). It consists of two masses, the upper one 
of which (nucleus caudatus) projects into the lateral ventricle ; the 
lower one is embedded in the white substance of the hemisphere and 
forms the principal part of the body (nucleus lenticiilaris). The two 
are separated by a layer of white matter called the " internal capsule.'' 
Between the diverging portions of the striate bodies are the obloug 

' Dalton (Topographical Anatomy of the Brain, Philadelphia, 1885, ii. , p. 
76) and others speak as though the caudate nucleus alone were to be called 
corpus striatum, the nucleus leuticularis by this name ; and the twococsidered 
as separate bodies. 


oi' somewhat ovoid masses of the " optic thalami." Each thalamus 
rests upon and partially embraces one of the crura cerebri ; its me- 
dian surface forms the side wall of the third ventricle, and upon its 
outer and back part are two small elevations, one on each side of 
the optic tract {corpora geniculata, internum and externum). In the 
depression between each striate body and the optic thalamus is a 
narrow, whitish, semitransparent band of medullary substance 
{f.amia semicircular is). Along the entire length of the floor of the 
descending cornu of the ventricle is a white eminence {jivppocampus 
major or cornu Ammonis) which is the iuuer surface of the gyrus 
fornicatus doubled upon itself like a horn. An arch-shaped band 
of nerve-fibres, consisting of two lateral halves, which, in front, 
form two pillars that descend to the base of the cerebrum and be- 
come the corpora alhicantia, and which diverge behind into two pil- 
lars that descend with the descending cornu of the ventricle and 
connect with a convolution of the brain (gyrus hippocampi), is situ- 
ated beneath the corjDus callosum ; it is called the fornix. Behind 
and between the optic thalami, and resting on the back surface of 
the crura cerebri, are four rounded eminences in two pairs, called 
corpora quadrlgemina ; the front pair are the nates, the back pair, 

§ 22. Without mentioning other more minute subdivisions, super- 
ficial or internal, in the structure of the cerebrum as seen by the 
unaided eye, we now consider the arrange- 2_ 
ment of the nerve-fibres and nerve-cells in /'''^^~^^\ 
the more important organs already named. y^ ■^- \. 

Of the fascicles of nerve-fibres belonging s^j t \ \ 

to the cerebrum, some connect it with the /^>v ; ^/^ \ 

lower organs of the encephalon ; some con- ( -p ^s^A^x^-"'- J 
nect together its hemispheres ; some join X ^ ^ /o\ ^ ^^ 
different structures in the same hemisphere ; fig- 29.— section through the 

. Mid-brain. (Schwalhe. ) aq , 

some are roots of origin for certain nerves. aqueduct of syivius; .«.?*., sub- 

_-. ^, PIT 1 • J.1 stantia nigra : p. crusta of the 

The fibres of the crura cerebri — those cms cerebri ;«, tegmentum of 

, T 1 J! ii 1 • XT i T the crus cerebri. 

strong peduncles oi the brain that ascend 

from the pons to the optic thalami and the striate bodies — are 
arranged in two groups (criista and tegmentum) separated by the 
gray matter of the substantia nigra. An important part of the 
fibres of the crusta, or front part (pes) of the crus, is continuous 
with the longitudinal fibres of the pons which come from the 
pyramids of the medulla ; it receives some fibres from the gray mat- 
ter of the substantia nigra. Many of the fibres of the crusta ter- 
minate in the nuclei of the striate bodies ; but some radiate upward 
through the internal capsule directly to the gray cortex of the 


cerebrum. (Comp. Fig. 24.) Some of the more diffused fibres of 
the tegmentum, or back and deeper part of the crus, are probably 
continued from the anterior column of the cord, and may be traced 
above to the optic thalami. Others of its fibres are collected into 
more well-defined tracts ; one of the most important of which 
comes from the superior peduncle of the cerebellum, and has al- 
ready been traced as it passes forward over the anterior end of the 
fourth ventricle (see p. 79). The formatio reticularis is continued 
into the tegmentum ; the latter, therefore, has a considerable 
amount of gray matter containing nerve-cells. Some of its fibres 
arise in these cells. The superior peduncle of the cerebellum as- 
scends, crosses over to the other side beneath the Sylvian aque- 
duct, and terminates in a collection of large pigmented cells (the 
nucleus of the tegmentum or red nudeua). 

The intimate structure of the striate bodies is not as yet entirely 
made out. On its deeper side, which is turned toward the internal 
capsule, the nucleus caudatus receives from the capsule several 
bundles of fibres. According to Meynert, some of these bundles 
serve to connect this nucleus downward with the peduncle of the 
cerebrum, some upward with its cortex ; but, according to Wer- 
nicke, it is doubtful if any of them pass to the white matter of the 
hemispheres, or come directly (that is, without traversing the len- 
ticular nucleus) from the crusta. All parts of the nucleus lenticu- 
laris are pervaded with white fibres. Some pass into its inner zone 
from the adjacent part of the internal capsule ; some connect it with 
the caudate nucleus ; some pass from it into the corona radiata, 
and then to the cerebral cortex. These nuclei appear to have a 
special connection with the frontal and parietal lobes, but also with 
some convolutions of the temporal lobe and the island of Keil. The 
gray matter of this organ is composed of delicate connective tissue, 
with "free nuclei siDaringiy distributed through it." The nerve- 
cells of the nucleus caudatus are multipolar and of two sizes ; some 
are about -g-J-y inch in diameter with many processes, but most are 
much smaller (y^'o-g- inch). Between the fibres of the gray matter 
of the nucleus lenticularis are many cells with yellow pigment in 

The three collections of gray matter — the locus niger, and the cau- 
date and lenticular nuclei of the striate body — with the nerve-fibres 
which originate in them and bind them together, have been held to 
constitute a connected chain of nervous organs, to which the name 
"ganglia of the crusta" has been given by Meynert. Eecently, 
however, this relation of the corpora striata as "basal gangHa," or 
"middle-men "between the spinal cord and the cerebral cortex, has 



been called in question by Wernicke and A. Hill. The latter argues, ' 
chiefly on morphological grounds, that the nucleus caudatus should 
be separated from the optic thalamus, and connected immediately 
with the cortex. This connection, he thinks, is favored by the na- 
ture of its development, by its minute structure, which differs from 
that of the thalamus, and by its resemblance to another nucleus 
(the amygdaloid) which has an undoubted origin from the cortex. 
23. Another chain of nervous organs, leading between the pons 

Yarohi and the hemispheres of the 
brain, consists of the tegmentum of 
the crus and its ganglia— the red 
nucleus (already described), corpus 
subthalamicon, the corpora genicu- 
lata, and the optic thalami. 

The arrangement of the nervous 
elements in the external corpus 




Iheil des 



Fig. 31. 

These and the following two FiRures show the arrangement of the white and gray substance in 
the interior of the cerebrum. (All four are from Gegenbaur.) 

Fig 30 —Horizontal Section through the Right Hemisphere. _ 
Fig 31.-Frontal Section through the Cerebrum in front of the Fornix. Posterior surface of the 

section displayed. 

geniculatum is peculiar : it consists of alternate layers of white and 
gray matter, as though occasioned by laying a lamina of the gray 
between two medullary lamina?, and then folding them in a zigzag 
manner. The nerve-cells of this organ are from ^-J-g- to gfo- of an 
inch in length, and ^ gVo »* an inch in breadth ; they are coarsely 
granular and pigmented. 

The ojMc thalamus is a mass of gray matter, with multipolar and 
fusiform cells, traversed by nerve-fibres. This gray matter is par- 
tially subdivided into two parts, an inner and an outer nucleus. 

1 The Plan of tlie Central Nervous System, pp. 35 fE. 



Its free surface (inner and upper) is covered by a layer of white 
fibres. On its outer surface is the white matter of the internal 
capsule, formed by fibres diverging from the crusta into the hemi- 
spheres. All along this surface fibres radiate from the interior of 
this organ, and mingle with those of the internal capsule on their 
way to the cerebral hemispheres. Those in front pass to the frontal 
lobe ; those in the middle pass to the back part of the same lobe 
and to the parietal lobe ; those behind to the temporo-sphenoidal 
and occipital lobes. 

"The external and under surfaces of the thalamus are not free, 
but are united with the other parts of the brain. The under sur- 
face is united with the tegmental j^art of the crus cerebri, while 
the external surface is covered by white substance, that is formed 

Fig. 32. 

FiQ. S3. 
Fig. 32.— Frontal Section through the Right Hemisphere of the Cerebrum in front of the Com- 

missura Mollis. Posterior surface of section displayed. 
Fig. 33.— Frontal Section through the Cerebrum back of the Commissura Mollis. Front surface 

of section displayed. 

of fibres of the crusta, which here diverge into the substance of the 
hemisphere and pass between the thalamus and the lenticular nu- 
cleus, forming the so-called 'internal cajDsule"" (comp. p. 87 f.). 
The cells of its substance average about yiy- inch in length and 
^jj-g in breadth ; their long axis is parallel to the course of the 
nerve-fascicles. According to a recent authority," the thalamus 
is the primary centre of the optic nerve, and is also connected with 
the olfactory nerve — originally by way of the fornix. 

The nervous substance of the corpora quadrigemina consists 
mainly of gray matter covered externally with a thin layer of 
nerve-fibres. In the interior of the upper or front pair the most 

' Quain's Anatomy, ninth edition, ii.. p. 324. 

^ A. Hill, The Plan of the Central Kervous System, pp. 20 ff. 


characteristic portion of this organ is found; it is a layer of fine 
nerve-fibres running longitudinally, between which are small, scat- 
tered nei've-cells. In the external strata of these bodies multipolar 
cells are abundant ; in their interior, at the sides of the Sylvian 
aqueduct, is a collection of gray matter which forms a continuation 
of the lining of the third ventricle. The nerve-cells in the corpora 
quadrigemina vary greatly in size. Most of those in the superficial 
strata are small ; but in the deeper strata some of them reach the 
maximum of nearly g^^g- of an inch. The centres of origin for the 
third and fourth nerves are in that nervous structure of fine fibrils 
and fusiform cells which lies along the Sylvian aqueduct. 

§ 24. The arrangement of the nervous elements in all the basal 
ganglia, as connected with the cerebral peduncles, indicates the na- 
ture of the mechanism of this region of the brain. It is constructed 
so as to co-ordinate all the nerve-tracts of motion with those of 
sense, and thus give to these ganglia important reflex and auto- 
matic powers over the sensory-motor apparatus, while subordinating 
them to the control of the nervous centres of the cerebral cortex 
that lie farther above. 

To the highest and dominating nervous centres in the cerebral 
hemispheres the paths of nervous impulse are laid from the basal 
ganglia by that blossoming out, as it were, of the nerve-fibres on 
their way to the white core of the cerebrum, which is called the 
" corona radiata." The corona is formed by the fibres that radiate 
from the striate bodies, from the optic thalami, and the internal 
capsule, into the convolutions of the lobes of the hemispheres. 

§ 25. The combination of the nervous elements into the pre- 
eminently complex mechanism of the convolutions of the human 
cerebrum may be described from two points of view ; the first is 
that from which their various external surfaces may be regarded by 
the unaided eye, the second that which histology assumes when it 
examines under the microscope various sections made from layers 
of their substance. 

The details of the external aspect of the convolutions vary so much 
in each individual, and even in the two hemispheres of the same brain, 
that the only chance of bringing order out of this apparent confusion 
is to discover what is genei'al, and for the most part constant, in the 
midst of what is particular and subject to change. In making such 
discovery the study of embryology is especially important. Certain 
sulci and their corresponding gyri appear with a marked regular- 
ity in the earlier and more fundamental stages of the development 
of the foetal brain. So, too, does the examination of the surfaces 
of the hemispheres of the adult brain show certain degi'ees of 


strength with which the sulci and gyri are distinguishable, and 
thus enable the investigator to divide them into so-called pri- 
mary, secondary, and even tertiary classes. Bischoff, Ecker, and 
others have aptly compared the primary gyri to the large mountain 
ranges whose sinuosities give to an entire region its characteristic 
features ; the secondary gyri are like those subordinate ranges 
which are brought into existence through the formation of longi- 
tudinal valleys (secondai-y sulci) in the main ranges ; while the ter- 
tiary convolutions may be compared to the small sj)urs which run 
out into the valleys between the principal ranges and from their 
sides. Only the primary gyri are, as a rule, pretty regularly dis- 

It is by means of the primary sulci that the surfaces of the hem- 
ispheres of the brain have been divided by modern anatomy into 
five territories or Lobes. ' The frontier lines of these lobes, how- 
ever, are clearly laid down only on some of the surfaces, while on 
other surfaces the lobes encroach on each other without distinct 
boundaries. The five lobes are called Frontal, Parietal, Temporo- 
sphenoidal (also Temporal or Sphenoidal), Occipital, and Central, 
or Insula, or Island of Reil ; the latter does not stand in immediate 
relation with the walls of the skull. The Frontal Lobe is divided 
from the parietal on its upper and lateral surface by the Fissure of 
Bolando (sulcus centralis) ; and on its lower surface from the tem- 
poral lobe by the horizontal branch of the Fissure of Sylvius. The 
Parietal Lobe is divided from the temporal for the greater part 
by the Fissure of Sylvius, and from the occipital — on its median 
surface completely, but on its upper surface only very incompletely 
— by the parieto-occipital fissure. The Temporo-sphenoidal lobe 
is distinctly marked off from the frontal and parietal, as already 
described ; while the boundary line between it and the occij)ital 
lobe is ill defined. The Island of Reil lies concealed between the 
frontal, parietal, and temporo-sphenoidal lobes; its surface, when 
exposed by drawing aside the margin of the Sylvian Fissure, shows 
a few short convolutions which radiate forward, upward, and back- 
ward from a central sj^ot on the lower surface. The occipital, tem- 
poro-sphenoidal, and frontal lobes, all have three principal convolu- 
tions arranged in parallel tiers (superior, middle, and inferior) ; in 
the frontal lobes these three spring from the anterior part of the 
ascending convolution just in front of the Fissure of Rolando (that 
is, from the gyrus centralis anterior) and run forward to the front 
end of the cerebrum. 

' The convolutions are here described in dependence upon the work o.t 
Ecker, The Convolutions of the Human Brain. London, 1873. 



§ 26. A few of the most important of the sulci and gyri need 
separate mention ; the accompanying diagrams will make clear 

Fig. 34. 

Fig. 35. 

Figs. 34 and 35.— Profile and Vertex Views of Cerebrum. Fr, the frontal lobe ; Par, parietal; 
Of, occipital ; Ts, temporo-sphenoidal lobe ; SS, Sylviiin fissure ; RE, fissure of Rolando ; PO, 
parieto-occipital fissure ; IP, iiitra-parietal fissure ; PP, Parallel fissure : SF and IF, supero- 
and infero-froutal fissures ; 1, 1, 1, inferior, 2, 2, 3. middle, and 3, 3, 3, superior frontal convo- 
lutions ; 4, 4, ascending frontal convolution : 5, 5, 5, ascending parietal, 5', postero-parietal, 
and 6, fi. angular convolutions ; A, supra-marginal, or convolution of the parietal eminence ; 7, 
7, superior, 8, 8, 8, middle, and 9, 9, 9, inferior temporo-sphenoidal convolutions; 10, i-uperioi, 
11, middle, and 12, inferior occipital convolutions ; a, |3, 7, 5, four annectent convolutions. 

further details (see Figs, 34, 35, and 36). Among the sulci which 
bound the main territories of the cerebral hemispheres the Fissure 


of Sylvius is much the most important. "It can," says Ecker, 
" in nowise be considered in the same category as the rest of the 
sulci on the surface of the brain." The other sulci may be re- 
garded as mere folds of the cerebral cortex ; the Fissure of Sylvius, 
on the contrary, is made by folding the entire hemisphere into an 
arch, with its concave surface downward, about the point of en- 
trance of the crus cerebii. This fissure exists in the foetal brain at 
the third month. " It arises," say Foster and Balfour,' " at the time 
when the hemispheres, owing to their growth in front of and be- 
hind the corpora striata, have assumed somewhat the form of a 
bean." The Fissure of Rolando is also always present in the human 
brain. It makes its appearance in the foetus as early as the end of 
the fifth month. It is rarely, if ever, bridged over by a secondary 
gyrus ; it, therefore, forms a point of departure in the examination 
of all the convolutions. It is botmded for its entire length by two 

Pig. 36. — Convolutions of the Inner and Tentorial Snrfaoes of the Left Hemisphere, i. i, i, cal- 
loso-marginal fissure, /, I, calcarine fissure; m,m, hippocampal fissure; n, n, collateral fis- 
sure; PO, parieco-oocipiUI fissure ; 17, 17, marginal convolution ; 18, 18, gyms fornicatus; 18', 
quadrilateral lobule ; 19, hippocampal gyrus ; 19', its recurved end ; 25, occipital lobule ; 9, 9, 
inferior teraporo-sphenoidal convolution. 

important convolutions (the anterior and posterior central or as- 
cending frontal and ascending parietal), which at both of its ends 
connect together in the form of an arch. The fissure which sepa- 
rates the parietal from the occipital lobe [parieto -occipital) and the 
one which runs from before backward through the parietal lobe 
(intra-parietal) are also to be mentioned among the more important. 
The intraparietal fissure, on the convexity of the parietal lobe, in- 
cludes, between it and the median line, the upper parietal convolu- 
tion, and embraces in its downward and outward bend the angular 
convolution. The latter convolution, and the marginal convolution 
form the inferior parietal lobule. 

> Elements of Embryology, p. 384. London, 18S3, 


Besides the superior, middle, and inferior convolutions of the 
frontal, temporo-sphenoidal, and occipital lobes, and the two cen- 
tral convolutions on each side of the Fissure of Rolando, the follow- 
ing, which belong to the median aspect of the hemispheres, ai'e 
to be noted in particular. The convolution which arches around 
the corpus callosum, and is separated from the median aspect 
of the first frontal convolution by a deep and constant fissure 
(the sulcus calloso-marginalis) is called from its shape, gyrus forni- 
catus. The back end of this convolution curves downward and then 
forward, under the name of gyrus hippocampi, to the inner tip of 
the temporal lobe. The passage of the former convolution, without 
break, into the latter, Ecker considers one of the most important 
differences between the hemispheres of the brain of man and those 
of the ape. 

§ 27. Although the general arrangement of gray nervous matter 
upon the surface, and of white matter within, is adhered to in all 
parts of the cerebral cortex, the form and disposition of the cells 
in the gray matter differ in different regions, and also in different 
layers of the same regions. But its most common form, which is 
that seen in the convolutions of the parietal lobe, corresponds to 
what Meynert ' has called " the general or five-laminated type of 
the cortex of the cerebrum." There are, as a rule, that is to say, 
five layers or laminae to be discovered in the gray matter of the 
cerebral cortex. The thickness of the entire cortex, thus com- 
posed, is, in the adult, from -^^ to \ of an inch. The first of 
the layers consists of a matrix, in which delicate nerve- fibrils run 
parallel to the surface and interlace with a few small globular or 
elongated branching nerve-cells scattered here and there. The na- 
ture of this matrix has been the subject of dispute ; by some it is 
looked upon as connective tissue (Kolliker), by others as neuroglia 
(Virchow). The second and third layers contain a large number 
of pyramidal, or spindle-shaped cells ; of these layers the third is 
the broadest, and contains the largest (but fewest) cells. The cells 
of the second layer are about -j-g^u^o of an inch in diameter, and are 
closely pressed together to form its substance ; but in the third 
layer they augment gradually in size until they reach a diameter of 
tt/oF' ^o perhaps gi^ of an inch, with their long axes perpendicu- 
lar to the cortical surface. The fourth layer contains large num- 
bers of small, globular, and irregularly-shaped and branching cells ; 
the fifth, spindle-shaped bodies with long tapering processes, and 
also a certain number of smaller irregular cells. This innermost 
layer consists chiefly of a compact accumulation of cells which give 
' In Strieker's Human and Comparative Anatomy, 11., p. 381. 



off lateral processes. Gerlach discovers here, as in the spinal cord 
(see § 9), a very minute network with which these processes are 
apparently continuous. It is also an assumption, verified by direct 
observation of some cases, and by the general analogy of the nervous 
system, that many of the extremely attenuated nerve-fibrils which 

El i ! 

,'W. ^ii ''■',.■' 


Fig. 37. — Section through the Cerebral Cortex of Man, prepared with Osmio Acid ^sy^^ 
(Schwalbe.) /, principal external, and //. internal, layer ; x, laj'er lying as a limit between the 
two ; »M, medullary substance sending out bundles of nerve-tibres into //; i, layer poor in cells, 
but with an external plexus of nerve-fibres {Id) ; ^, layer of small, and 3, of large, pyramidal 
cells ; k, inner layer of small nerve-cells. 

radiate from the white core of the convolutions are continuous with 
the basal axis-cylinder processes of the cells in the layers of gray 
substance. Small rounded corpuscles and small stellate cells, so 
pellucid that they seem to be only free nuclei, are contained in the 
neuroglia of the gray substance of the cerebral cortex. It is doubt- 


ful whether these are true nervous elements or not. The number 
of nerve-cells in the cortical substance is very great. In a portion 
of this substance, only one millimeter square and J^- millimeter 
thick, 100 to 120 have, on an average, been counted.' 

Modifications of the arrangement which prevails in most of the 
gray substance of the hemisjpheres of the brain are found in certain 
regions. In the cortex of the occipital lobe the number of layers 
is increased by the intercalation of additional granule layers to 
seven or eight. In the cortex of the Island of Eeil, and of the con- 
volutions bounding the Fissure of Sylvius, a large proportion of 
fusiform cells is found. In the fourth layer of the cerebral cortex 
of the dog, in the region which Hitzig considei'ed to be motor, 
Betz discovered certain cells lying in scattei-ed groups, with two 
large and several small processes ; these cells, on acccount of their 
great size, he called " giant-cells:" Similar cells, have been found 
by him in certain regions of the human cerebral cortex — namely, in 
the entire anterior central, and the upper end of the posterior cen- 
tral convolutions, and along the lobe which is prolonged backward 
from the two. 

§ 28. The white substance of the hemispheres of the brain may 
all be considered as originating in its cortical gray substance ; but 
the nerve-fibres of which it is composed constitute three classes, 
according to the destination of the fascicles into which the fibres are 
gathered. These three are the down-going or peduncular, the com- 
missural, and the arcuate (or fibres 2droprice). It is the business of 
the peduncular system to connect the cerebrum with the lower parts 
of the encephalon. This system, called the corona I'adiata, is nar- 
rowed into the internal capsule and continued downward to the 
crura cerebri ; its diminished size shows that a considerable por- 
tion of its fibres have entered into the optic thalami and striate 
bodies. But it is also probable that many fibres of the crusta pass 
directly into the brain's medullary centre, and through this to its 
gray cortex, without entering these ganglia. Of such tracts the 
best known is the loyramidal (probably motor). According to 
Flechsig and others, this is traceable through the internal capsule 
and corona radiata to certain frontal and parietal convolutions. 
Another tract, traceable directly to the convolutions of the cortex, 
passes from the external part of the crusta into the white matter 
of the occipital lobe (so-called direct sensory tract). The fibres 
which come from the tegmentum, and are lost, for the most pai-t, in 
the thalamus and the subthalamic region, stream outward from the 
other side of this organ, join the general system of the corona radi- 
' See Luys, The Brain and its Functions, p. 17. New York, 1883. 


ata, and diverge to nearly every part of tlie hemispheres ; but espe- 
cially to the temporo-sphenoidal and occipital lobes (probably sen- 

The commissural system of fibres has hitherto universally been 
supposed to connect the two hemispheres of the brain ; but Pro- 
fessor Hamilton, of xiberdeen, and others, have recently called this 
statement in question. The principal tract of such fibres is in the 
coi-pus callosum. Since this commissure lies in a plane above that 
of the corona radiata, the two systems of fibres intersect each other 
on their way to the convolutions of the cerebral hemispheres. A 
smaller commissure (the anterior) passes below the lenticular nuclei 
of the striate bodies and connects the convolutions around the Syl- 
vian fissure — binding together the right and left temporo-sphen- 
oidal lobes ; it also furnishes a root of origin for the olfactory 

The arcuate fibres extend over more or less territory on the 
same side, and connect the gray matter of adjacent, or more or 
less distant, convolutions in the same hemisphere — " a garland-like 
interweaving " of two convolutions around the sulcus between them. 
In certain localities, where the fascicles into which these fibres are 
gathered are strongly marked, they have received special names ; 
such are the fasciculus uncinatus which crosses the bottom of the 
Sylvian fissure and connects the convolutions of the frontal with 
those of the temj)oro-splienoidal lobe ; the fillet of the gyrus forni- 
catus, extending longitudinally in that convolution ; the longitudi- 
nal inferior fasciculus, connecting the convolutions of the occipital 
and temporo-sphenoidal lobes. Such fibi-es are sometimes called 
longitudinal or collateral fibres. It is by the commissural and arcu- 
ate fibres that the innumei-able ganglion-cells and nerve-granules of 
the cortex are bound into a unity of form and of function. The pro- 
cesses of the cells anastomose, and are thus united with immedi- 
ately adjoining cells by means of a gray fibre-plexus. The axis- 
cylinder processes become continuous with the meduUated fibres, 
which, gathered into bundles (the fasciculi of the arcuate fibres), 
line as a continuous layer the inner surface of the cortex. In this 
manner the nervous elements of that crowning mechanism, which 
is known as the chief glory of man's nervous system, are made to 
exhibit a manifoldness, and at the same time a unity of structure, 
suggestive of a common service joined with diversity of mode in 
Avhich the service is rendered. 

§ 29. The view of Meynert — to which reference has already been 
made (p. 73) — regards the gray masses and converging and diverg- 
ing tracts of the cerebro- spinal nervous mechanism as a " Projec- 



tion System " (or rather as a series of "projection systems"), wliich 
is capped and dominated by the hemispheres of the cerebrum. The 
sensory nerves may thus be figuratively described as the " feelers," 
and the motor nerves as the " arms " of its cortical gray matter. 
This matter is both a "sensory shell," upon which the centripetal 
nerve-commotions gather and dispose themselves ; aud is also the 
"motor shell" in which certain centrifugal motions originate. It 
is, therefore, an internal " Projection Field " for the muscular sys- 

Ccl' ^'"ft 

Fig. 3S. — Median Section of the Brain. A, aqueduct of Sylvius : Cba, white commissure ; Cbl, 
cerebellum ; Oca, corpus albicans ; Ccl, corpus cailosum, of which the different parts are Ccl', 
rostrum, CcP, the genu, Ccls, the body, and Ccl^, splenium ; On, conarium, or pineal gland; 
Coa, anterior. Com, middle, and Cop. posterior, commissures ; PM, foramen of Monro ; Fta, 
the anterior, and Ftii, the posterior, transverse fissures ; Let, lamina cinerea terminalis ; and 
Lq, lamina of the corpora quadrigemina : Mo, medulla oblongata : P, pons Varolii ; SI, septum 
lucidum ; SM, sulcus- of Monro ; Tc, tuber cinereum ; Vma. anterior velum medullare ; V, 
fourth ventricle ; II, optic nerve ; II', chiasm of the optic nerve. 

tern. The gray masses of the brain below its hemispheres (with 
the exception of the internal tubular mass) may — according to 
Meynert — be described as either (a) "Interruption Masses" of the 
projection system, or as belonging to (b) the " Region of Reduc- 
tion " of the mass of this system. It is in these lower gray masses 
that the great bulk of the nerve-tracts (the corona radiata) coming 
from the cortex of the cerebrum are not only broken and inter- 
rupted in their course, but are also greatly reduced in size.' The 

' For a clear and concise summary of Meynert's entire view, see Quain's 
Anatomy, ninth edition, ii., pp. 370 ff. 



functional significance of this relation in which the cerebral cortex 
stands to all the rest of the nervous mechanism will appear more 
clearl}' further on in the discussion. 

§ 30. The cerebro-spinal axis, or central nervous mechanism of 
the cavit}^ of the spinal column and skull, is connected with the 
end-organs of motion and of sense by thirty-one pairs of Spinal, 
and twelve pairs of Cranial or Encephalic Nerves. 

The thirty-oue pairs of Spinal Nerves originate in the spinal 
cord and pass out of the spinal canal through the openings called 


Fig. 39.— Posterior View of the Spinal Cord with its Nerven. }4. (After Sappey. ) I- VIII in A 
are cervical : I, II, and III in A, and IV-XII in U, dor.sal ; the last in B, and down to V in C, 
lumbar: I-V in C, Bacnil ; lu, in, origin of the posterior roots; 11, 11, posterior median tis- 
sure ; 12, 19, spinal pan^lia : i;j, 13, united i). rve ; 15, tapering of the lower end, becoming 16, 
10, the filum terininale ; IT, caudii equina. 

"intervertebral foramina." Of the entire number — enumerating 
from above — eight pairs are cervical, twelve thoracic or dorsal, 
five lumbar, five sacral, one coccygeal. Each nerve arises from the 
side of the cord by two roots, an anterior and a posterior. The 
posterior root has a swelling or ganglion upon it, the anterior has 
none ; the former is composed of sensory nerve-fibres, the latter. 


of motor nerve-fibres. The ganglion of the posterior roots contains 
unipolar nerve- cells. The roots themselves vary, in the different 
regions of the cord, both as respects direction and length. Imme- 
diately outside the ganglion the anterior root joins the posterior, 
and the united nerve — containing a mixture of motor and sensory 
fibres — soon after separates into two divisions, that are formed of 
elements from each root and that are distributed, one upon the 
back and the other upon the front and sides, to all parts of the 
trunk and limbs. 

Of the twelve pairs (adopting the Continental instead of the 
English division) of Cranial Nerves, which arise fi-om the base of 
the encephalon and pass thi'ough the openings (foramina) in the 
floor of the cranial cavity, three groups may be distinguished : (a) 
the sensor}' nerves, or nerves of special sense ; (b) the motor nerves ; 
(c) the mixed nerves, which contain both sensory and motor fibres. 
To the first grouj) belong the olfactory nerve (first pair), the optic 
(second pair), and the auditory (eighth pair) ; to the second group 
belong the nerves that supply the principal muscles of the eyeball 
(oculo-motor, third pair), the superior oblique (trochlear, foui'th 
pair), and external rectus (abducent, sixth pair), muscles of the eye, 
the muscles of facial expression (seventh pair), the muscles of the 
tongue (hypoglossal, twelfth pair), and the spinal accessory nerve 
(eleventh pair) ; to the third group belong the three nerves which 
are so widely distributed over the mucous membranes and muscles 
of the face, tongue, pharynx, and internal organs — namely, the tri- 
geminus (fifth pair), the glossoj)haryngeal (ninth pair), and the pneu- 
mogastric or vagus (tenth pair). 

§ 31. It is, then, by a process of differentiation of a few compara- 
tively simple elements, and of infinitely varied arrangement and 
combination of the elements thus differentiated, that the elaborate 
mechanism of the human nervous system is constructed, and made 
fit for the great variety of interconnected functions which it is 
called upon to jDerform. Material atoms are chemically united into 
the comjDlex and unstable molecules of which nervous matter is com- 
posed. These molecules are arranged into the structural forms of 
nerve-fibres and nerve-cells ; and the lattei", at least, are modified 
in form according to their location, and perhaps also function. 
The elements are combined into conducting cords, end-organs, and 
central organs, according to the threefold plan of a nervous sys- 
tem ; and the organs are arranged, in the case of man, with an in- 
tricacy of relations which can be only very inadequately described. 

The description of the mechanism being finished, we consider 
more in detail what it can do. 


§ 1. In that threefold economy of organs which characterizes the 
developed nervous mechanism, the office of propagating the neural 
process between the central organs and the end-organs has been 
assigned to the nerves. The power to originate this process under 
the action of external stimuli, although experiment shows that it 
belongs to the nerves, is not exercised by them while in their nor- 
mal place within the mechanism. It is the office of the end-organs 
to transmute the physical molecular processes, which are their stim- 
uli, into the physiological and neural process, and hand it over, as it 
were, to these conducting cords. But the office of the nerves as 
conductors is, of course, not like that of a tube which conducts along 
its channel some kind of fluid, nor is it like that of the wire or bell- 
metal which is thrown into vibration throughout. It is a molecular 
commotion which, when started at any point in the nerves, moves 
in both directions from point to point along its course. The 
intimate connection between the two functions of excitation and 
conduction becomes, then, at once apparent. Indeed, excitation 
may be considered as the setting uj) of the process of conduction ; 
conduction as the uninterrupted continuance, or propagation from 
point to i^oint, successively, of the process of excitation. Each 
minute subdivision of the nerve, then, must be regarded as consti- 
tuting, in some soi't, a source or centre of stimulation with respect 
to its neighboring subdivisions. If the nerve-commotion is to 
move along the nerve N, between two distant portions of its struct- 
ure, a and z, then a must act upon its neighbor 6 as a stimulus, b 
upon c, and so on successively until y is found stimulating z, and 
the process of progressive excitation or conduction is complete. 

§ 2. It follows from what has just been said that, in considering 
the nerves as conductors, the conditions and laws of the origination 
of that process of excitation which they conduct must be taken into 
account. It is neither necessary nor convenient, however, to carry 
throughout a distinction between the two functions — the excitabil- 
ity and the conductivity — of the nerves ; it is better to regard them 


as one process from somewhat different points of view. The arising 
and progressive movement of a unique molecular commotion con- 
stitutes the distinctively neural action or function of the nerves. 
And since this so-called nerve-commotion has eluded all the at- 
tempts hitherto made to discover its more intimate nature, and to 
bring it under a strict theory, we must be content with describing 
the following three classes of facts : (1) The Conditions of the pro- 
cess ; (2) the Phenomena evoked with it, or as part of it, by differ- 
ent kinds of stimuli ; (3) the Laws of its propagation. 

What is called " the genei'al physiology of nerves " attempts to 
consider their action while excluding the influence upon it from 
the central organs aud the end-organs. That is, the function of the 
nerves, as we now consider it, is exercised under ahnormal conditions. 
It has been objected to the view which regards each element of 
the nerve as the stimulus of neighboring elements, that the ef- 
fects of direct artificial stimulation must differ in important respects 
from those obtained by stimulation in the normal way. For exam- 
ple, Ziemssen and others have shown that the crushed nerve of an 
animal, or the paralyzed nerve of a man, may be made to set up a 
nerve-process hy reflex stimulation when it will no longer respond 
to stimulation applied directly to its trunk. And Grtinhagen af- 
firms that after a stretch of nerve has been reduced by the effects 
of carbonic acid to a lower degree of excitability under direct stim- 
ulation, it will still propagate through itself the excitation set 
up elsewhere with undiminished force. Such facts, however, only 
prove that the application of stimuli to the nerve for purposes of 
experiment is a very rough and ineffective way compared with nat- 
ure's method of preparing the stimuli by the modifying influence 
of the nervous tissues themselves. They do not prove that the 
neural process is not fundamentally the same in whatever way it is 
brought about. On the contrary, there is abundant evidence to 
show that this abnormal activity, when carefully studied, will give 
us the key to the normal function of the nerves. The advantages 
of simplifying the problem by experiments upon isolated nerves 
are too great, and the fund of valid information thus obtained is 
too important for us to neglect the method proposed. The science 
called " genei'al physiology of the nerves " is, indeed, very largely 
built upon experiments with the motor nerves of frogs ; and, of 
course, it may be said that frogs, with respect to their nervous sys- 
tems as otherwise, are very unlike men. But with respect to the 
character of that specific molecular process which is set up in the 
nerve when excited, frogs apjDcar to be essentially the same as men. 
At any rate, we have no physical means adequate for detecting any 


essential differences. In other words, nerves are nerves the world 
over ; and what they do as nerves simply, is essentially one thing in 
all cases. What they do in their vastly different arrangements and 
connections with central organs and end-ox"gans differs vastly in 
different cases. 

§ 3. The view that each element of every nerve, irrespective of 
its kind or specific place in the animal mechanism, can only stimu- 
late its neighbor and be stimulated by its neighbor, suggests an- 
other interesting inquiry. Is this stimulus of the nerve-elements, 
this effect in exciting contiguous elements, aniologous to any of the 
so-called external stimuli ? Or, in other words, the inquiry may be 
raised : Is the process of nerve-commotion in the nerves similar to, 
or identical with, any of those molecular processes which act as in- 
direct stimuli upon the nerves through the end organs ? In answer- 
ing this question it has long been customary to ally nerve-commo- 
tion with electricity. In a posthumous work b}'' the mathematician 
Hansen, in 1743, it was first proposed to consider the efficient 
j)rinciple of nervous action as identical with that of the electrical 
machine.' Exactly a century later (1843) du Bois-Reymond an- 
nounced the discovery of an electrical current in unexcited nerves 
(the so-called " current of rest "). Since then the phenomena of this 
current, of the negative variation of the nerve, and of electrotonus 
— all discovered almost simultaneously by the same investigator — 
have been the subject of much painstaking research. This research 
has resulted in showing that important differences exist between 
the neural process and that of the electrical current, and in making 
more and more clear the impossibility of forming a purely electri- 
cal theory of the nervous functions. On the other hand, it has also 
revealed many important similarities between the two. It is by 
experiment with the effect of electrical currents, of different kinds 
and directions, and under varying conditions, that the science of 
general physiology of the nerves has been built up. 

§ 4. In order to understand the general results of experiment 
upon the nerves, the nature and use of the so-called Nerve-muscle 
Ilachine must be understood. 

A "nerve-muscle preparation" consists of muscle freshly taken 
from the living animal with its attached nerve dissected out ; for 
example, the gastrocnemius muscle of the frog with the attached 
sciatic nerve. Such a preparation may be kept alive for some time 
in a moist chamber. By the simple contrivance of connecting the 

' See in du Bois-Reymond's great work the history of opinions on this 
point : Untersucltuiigen iiber thieriache Electricitiit, 11. . i. ,pp. 209 ff. Berlin, 


end of the muscle with a lever, arming the lever with some means 
of making a mark — either pen, or bristle, or needle — and bringing 
its point thus armed to bear on a rapidly travelling surface (plain 
paper, or smoked paper, or glass), the time and amount of the con- 
tractions of the muscle may be recorded. The most refined means 
for noting the exact instant when the stimulus is applied, and also 
the state of the effects produced at every succeeding instant of their 
duration, are of first importance. The nerve may be stimulated 
with different kinds, degrees, and directions of the electrical cur- 
rent (or with other forms of stimuli) at any points preferred in its 
stretch, and under a great variety of conditions with respect to tem- 
perature, moisture, mechanical pressure or stricture, integrity and 
vitality of its structure, etc. ; and the effects of such stimulations 
upon the contractions of the muscle may be noted and compared 
as they have been recorded. Means for testing the most delicate 
and rapid changes in the electrical or thermometric conditions of 
the nerve may be applied to it at any point of its stretch. Varia- 
tions and refinements of experiments essentially the same may be 
almost indefinitely multiplied ; the experiments may be repeated, 
and verified or corrected, by the same observer or by others. In- 
asmuch as the preparation is both muscle and nerve, an acquaint- 
ance with the behavior of the muscle, and with the laws of its con- 
traction, is necessary in order that it may be known how much of 
the complex phenomena is to be ascribed to the functional activity 
of muscle, how much to that of nerve. But into a statement of the 
general laws of contractile tissues, and of the nature and explana- 
tion of the behavior of muscle when irritated, we cannot enter.' 

Certain terms in constant use to describe the methods and results 
of experiments with the nerve-muscle machine also require a brief 
explanation. The line traced by the armed end of the lever, as it 
rises and falls with the contractions of the muscle, is known as the 
" muscle-curve." In so far as it shows changes that are due to the 
condition of the attached nerve, or to the quality, intensity, and 
order of the stimulations applied to that nerve, this curve is a 
measure of the process of neural excitation and conduction. If 
the electrical current flows with the course of the motor nerve- 
stretch — that is, from the central toward the 'oeripheral parts — it 
is called " descending," or direct ; if in the opposite direction 
" ascending," or inverse. The current to be detected in an unex- 

' For a description of the method and results of experimenting with the 
nerve-muscle preparation, more accessible to the general reader than the books 
to which reference will chiefly be made, see Foster's Text-book of Physiology, 
pp. 43 ff. 


cited nerve (a nerve, that is, whose functional activity is not at the 
time in exercise on account of the appHcation of any kind of stim- 
lus) is called a " natural current," or a " current of rest." The cur- 
rent produced by stimulating the nerve, and so calling into exer- 
cise its physiological function, is a "current of action." When 
a single induction-shock, or a number of such shocks repeated 
at sufficient intervals, is sent through a nerve-stretch, the contrac- 
tile spasm of the muscle in response to each shock shows that 
a single "nervous impulse " is passing along the nerve. When the 
single stimulations are repeated with sufficient rapidity, the single 
spasms fuse into one apparently continuous effort, known as " tet- 
anus," or "tetanic contraction." The term "tetanus" applies 
primarily to the muscle only ; but the application of rajDidly re- 
peated shocks to the nerve, such as would produce "tetanic con- 
traction " of the muscle, may be called the " tetanization of a nerve." 
The contraction which follows the closing of the current is called 
the "making contraction," or "closing contraction;" that which 
follows its opening, the "bi'eaking" or "opening" contraction. 

§ 5. Of the conditions under which alone the nerve is capable of 
exercising its function of neurility the most important are these 
three : Vitality, Oxygen, and Recovery from previous exhaustion. 

A nerve cannot act as a conductor of the neural process unless 
it is vital ; but the death of the nerve is not necessarily simultane- 
ous with that of the body from which it is taken, or of the muscle 
to which it is attached. On the contrary, by careful treatment with 
respect to moisture and temperature, and by guarding it from 
mechanical or chemical injury, it may be preserved alive for some 
time after excision. The indirect irritability of the muscle through 
the excised nerve attached to it frequently continues in warm- 
blooded animals and in high temperature not longer than about 
an hour ; in the frog and in a low tempei'ature it may last for sev- 
eral days. The nerves of the summer frog are much more perish- 
able than those of the same animal in winter. A nerve is, of course, 
alive as long as it will excite the muscle to contract. But the nerve 
is not necessarily dead when the attached muscle no longer resjDouds 
to its excitation ; the failure may be due to the death of the very 
sensitive and perishable end-organs which connect the two. Her- 
mann * considered that the existence of electrical phenomena in the 
nerves of rabbits showed the nerves to be alive for several hours 
after they would no longer stimulate the muscle, and also after the 
muscle itself could not be irritated directly. Nerves may even be 
alive after they cease to exhibit electrical phenomena that can be 
1 Handb. d. Physiol., II., i., p. 120. 


eletected by the most delicate tests available. It is possible that 
the capacity for excitation may linger after the capacity for con- 
ducting the excitation is lost. Since the nerve, unlike the muscle, 
has no death-rigor, we cannot say just when it is wholly dead. 

During the stages of dying, nerves exhibit two interesting changes 
of excitability. Immediately after it is severed from the body 
the irritability of the nerve increases temporarily, and afterward 
diminishes by successive degrees until it is wholly lost. The 
course of these changes in its irritability is found to be different 
for different parts of the same nerve-sti*etch. It was discovered by 
Valli and Ritter ' that a nerve which has once ceased to stimulate 
its attached muscle to contract will again excite muscular contrac- 
tion if the electrodes be applied farther down its stretch ; there- 
fore the lower portion of the nerve-stretch seems to preserve a 
given degree of vitality for the longest time. From this fact " Valli's 
principle " has been derived : Nerves die from the centre to the 
periphery. The temporary increase of the irritability of the ex- 
cised nerve belongs indeed to its entire stretch ; but it appears first 
in the upper part. This fact is connected with the important in- 
fluence which the cross-section of a nerve has upon its electrical 
and neural condition. As to the reason for this increase of nervous 
excitability which accompanies the first stage in the dying of the 
nerve, we are quite in the dark. 

§ 6. Closely allied to the foregoing changes are those which take 
place in the structure and functional activity of a nerve that re- 
mains in the living animal organism after having been separated 
from the central organs. Such a nerve, after a time, completely 
loses its irritability. Two investigators, Giinther and Schon," found 
this time to be, in the case of rabbits or dogs, about three or four 
days ; in a cold-blooded animal like the frog, the time may be pro- 
longed to a week, or even more. The law of increased irritability, 
produced in the entire nerve-stretch, but first manifested in the 
portion nearest the cross-section, immediately after separation from 
the central organ, holds good for most observations on nerves cut 
in situ ; its application is obvious, however, only to the case of the 
motor nerves. In 1850 Waller announced ' the discovery that the 
anatomical changes (a fatty or granular degeneration) which take 
place in the nerve-fibres after being severed from the central organs 
proceed fi.-om the place of section to the extreme peripheral portion 

1 See in du Bois-Rejinond's Untersuchungen, etc., i., pp. 321 ff. 

2 See the Archiv f. Anat. Physiol., etc., 1840, p. 270. 

^ In Philosophical Transactions, 1850, ii., p. 423; and see, also, Archiv f. 
\nat. u. Physiol., 1852, p. 392. 


of the fibre ; and that the sensory nerves do not degenerate in theii 
peripheral, but in their central portion, when the posterior roots 
are cut above the ganglion. The central portion of the nerve, when 
cut at a point lying toward the periphery from the ganglion, may 
be shown (in the case of the sensory nerves, which alone admit of 
being experimented upon for this purpose) to retain its irritability 
for a long time, although it finally loses it through lack of exercise. 
A cut nerve remaining in situ may be regenerated, and so regain its 
functional jDowers. Regeneration takes place by the axis-cylinders 
growing out from the central portion and running into and between 
the sheaths of Schwann of the peripheral portion ; it is accom- 
plished, then, by the influence of the central organs. The irrita- 
bility of the nerve returns as its structure is regenerated. Accord- 
ing to some investigators its conductivity is regained earlier than 
its power of local irritability. Duchenne ' and others claim that 
the influence of the will is the first form of stimulus to regain con- 
trol of regenerated motor fibres. 

§ 7. Oxygen, as furnished by the circulation of the arterial blood, 
is the second condition for the performance by the nerves of their 
distinctive functions. But nerves, as compared with the central 
organs or end-organs of the nervous system, or even with the mus- 
cles, are relatively independent of the presence of oxygen. Indeed, 
since the muscle is so much more sensitive to changes in the qual- 
ity of the blood, and is supplied by the same arteries that supply 
the attached nerves ; and since the irritability of the nerve is tested 
by the vital contraction of the muscle — it is difiicult to determine 
by experiment the exact effect upon the nerves of withdrawing 
from them the oxygen of the blood. The irritability of the nerves 
continues about as long in a moist vacuum, or in indifferent gases, 
as in the air. What little is known of the chemical processes 
which take place ir. the nerves confirms the view that they are rel- 
atively independent of the presence of oxygen ; and the experi- 
ments of Severini, who thinks that he has discovered a restorative 
effect of ozone (if not of ordinary oxygeu) upon these organs when 
dying, are not yet fully confirmed. It may be argued, however, 
from the marked dependence of the other forms of nervous tissue 
upon a supply of arterial blood, as well as from the general theory 
of the nervous system, that the presence of some oxygen is a nec- 
essary condition of the functional activity of the nerves. 

§ 8. Exhaustion is a condition of the nerves recovery from which 
is necessary in order that they may exercise their normal functions ; 
but exhaustion of the nerves is difiicult to distinguish experimen- 
' Traitu de I'clectrisatiou localisee, second edition. Paris, 1861. 


tally from exhaustion of the central organs or of the end-organs. 
The experiments of du Bois-Eeymond upon the negative varia- 
tion of the nerve-current under repeated irritation give us the 
first item of the desired proof. The variation under these circum- 
stances becomes constantly weaker. By ingeniously separating the 
proofs of exhaustion in the muscle from those of exhaustion in the 
nerve, Bernstein ' has shown that the latter comes on much more 
slowly than the former ; and that by far the greater amount of 
the effects attributed to exhaustion in the nerve-muscle macliine 
belong to the muscle-element of this machine. When tired, how- 
ever, the nerve recovers more slowly than the muscle. Nerve- 
cells — and therefore the central oi-gans and end-organs of the ner- 
vous mechanism — tire much more easily and quickly than nerve- 
fibres. Indeed, according to Hermann," it is conceivable that all 
the phenomena of exhaustion which take place in the normal expe- 
rience of the nervous system belong really to the organs connected 
with the nerves rather than to the nerves themselves. When we 
are tired nervously, it is not ordinarily the nerves that are tired. 
And yet the law of the exhaustion and recovery of functional ac- 
tivity doubtless belongs to normal, as it does to excised, nerve-fibres. 

§ 9. The various classes of phenomena which are evoked in con- 
nection with the starting and propagation of nerve-commotion 
along a nerve-stretch will be considered from two points of view : 
First, as regards their dependence upon the character, amount, and 
method of the application of the stimuli which are used ; and, sec- 
ond, as indicative of certain processes — chemical, thermic, electri- 
cal, etc. — ^set up in the nerves themselves. We shall thus, as far 
as possible, avoid repetition. 

§ 10. The mechanical properties of the nerves are of little inter- 
est to psycho-physical researches ; and comparatively little con- 
cerning their physiological functions has been learned by the ap- 
plication to them of mechanical stimuli. The elasticity of nerves 
in the dead body was found by Wertheim to follow the same laws 
as that of the muscle — their absolute ductility is less than that of 
muscle ; their cohesion greater. All kinds of mechanical attacks on 
the nerves excite them, and are followed by pain in the case of 
sensory nerves, contraction of the muscles in the case of motor 
nerves. By rapid shocks of this kind — for example, with a toothed 
wheel or a hammer — tetanus may be produced. A certain sudden- 
ness of influence is, in general, necessary to the effect. Yet Pon- 
tana succeeded in cutting nerves very quickly with a sharp knife 
without producing any muscular conti-action. Pi'essure of a neiwe 

'In Pfliiger's Arcliiv, xv., p. 289 f. "Handb. d. Physiol., II., i., p. 135. 


may be increased very gradually to a high degree without exciting 
it ; but its jDower of conductivity is thus temi^orarily suspended. 
Veiy moderate jDressure or slight traction of the nerve has been 
found by several investigators to increase, at least for a moment, 
the irritability of the nerve ; and perhaps, also, the speed of con- 
duction in it. All neural function is, of course, destroyed by any 
considerable mechanical injury of the nerve, such as often happens 
by stricture or pressure from a swelling. 

§ 11. IViermfc influences upon the phenomena of the neural pro- 
cess are very marked and important. On the other hand, almost 
nothing is known as to the specific heat of nerves or as to their 
power to conduct heat. Hermann thinks it probable that the 
latter is different in the two main directions of the fibres. The 
results of experiment differ as to the degree of heat which is neces- 
sary to act upon the nerves as a stimulus. Valentin, the first ob- 
server in this line, found that clipping the motor nerves of frogs 
in water heated to about 100° Fahr. (38° C.) caused contractions ; 
but Eckhard obtained such results only from temperatures above 
150.8° to 154.4°, or below 25° to 22°— that is, temperatures that 
are either deadly or permanently injurious to the nerve. Nor, 
according to the latter, is the nerve excited by changes in tempera- 
ture as it is by changes in the electrical current. Slighter changes 
near the dead-line may have an effect to excite the nerve ; but con- 
siderable changes in the medium temperatures, as a rule, have no 
such effect. It is the opinion of some, however, that such thermic 
changes, when marked and sudden, may act as a stimulus to mo- 
tor nerves. It Avas shown by E. H. Weber ' that heat and cold have 
no effect in producing sensations when applied directly to the sen- 
sory nerve-trunks of man. 

While there is little evidence, then, to show the direct excitatory 
effect of heat upon the nerves, there is no doubt whatever as to 
the importance of thermic influences upon their excitability and 
conductivity. High degrees of temperature may destroy the pow- 
er of the nei've to perform its functions, but without killing it. 
Warmth increases the immediate expenditure of energy in an ex- 
cised nerve, and so hastens its death ; cold delays this expenditure, 
and so conserves the nerve. The limit of this increased irritability 
of the nerve under the influence of heat is reached at about 122° 
Fahr. ; as the degree of heat applied rises from this point toward 
150°, its effect is rapidly felt in causing the death of the nerve. 
Sudden cooling from about 68° down to 50° may produce a tem- 

' In Wagner's Handworterb. d. Physiol., III., ii., pp. 496, 578 ; and Archiv 
f. Anat., Physiol., etc., 1847, p. 342, 1849, p. 273. 


porary rise of irritability ; but, in general, cooling below 59° di- 
minishes the irritability of nerves. The effect of temperature upon 
the speed of conduction will be referred to elsewhei-e. 

§ 12. Chemical influences have, for the most part, surprisingly 
little effect upon the irritability and conductivity of the nerves, 
especially in view of their great sensitiveness to other external in- 
fluences. Such indifference is probably due to the protection of 
the nerve by its membranes. The effect of most chemical agents, 
when long continued, is to destroy the nerve without irritating it ; 
but some agents in a concentrated form act upon it as stimuli. 
The researches of Eckhard, KoUiker, and Ktlhne have given us 
most of the information we have upon this matter. Only two 
points need mention here. First : Changes of the amount of water 
in the substance of the nerve affect its functional activity. Drying 
the nerve produces contractions ending in tetanus ; although, ac- 
cording to some authorities, these effects do not follow if the dry- 
ing be very sudden. A. slight amount of drying raises temporarily 
the irritability of the nerve. The amount of the decrease of water 
necessary to produce contractions in the attached muscle is given 
by Birkner at four to eight per cent, of the weight of the nerve ; 
irritability ceases, although the dried nerve is not dead, with a 
loss of forty per cent. Others, however, give the latter figure as 
between eight per cent, and nineteen per cent. Swelling the nerve 
in water or other indifferent fluids decreases its irritability slowly 
to the point of entire cessation. 

Second : The effect of certain acid and alkaline solutions upon 
the nerve is much hke that of drying it. Various neutral salt so- 
lutions, and free alkalies in solution, produce strong muscular con- 
tractions, ending in tetanus and death. Certain organic sub- 
stances in concentrated solutions — for example, urea, sugar, and 
glj^cerine — irritate the nerve ; so, according to most observers, does 
alcohol of from ninety per cent, to eighty per cent. The law seems 
to be, that all chemical stimulation of the nerves is closely connected 
with the destruction of the nervous tissue. 

§ 13. The phenomena evoked by applying the stimulus of elec- 
tricity to the nerve-muscle machine are very numerous and diffi- 
cult of disentanglement, since they depend upon such a variety of 
changing conditions. Following is a y&cj brief statement of some 
of the more important of such phenomena, in as far as they relate 
to the direct excitatory effect of this stimulus, and also to its effect 
in modifying the excitability of the nerve.' 

' Here, as throughout the subject of the general physiology of the nerves, 
the chief reliance has been placed upon Hermann, Handb. d. Physiol., II., i. 


The resistance which living nerves oifer to the electrical current 
does not differ much from that of living muscle ; it is given by 
most authorities as somewhat greater. According to Weber's in- 
vestigationp its resistance is about 50,000,000 times as great as 
that of copper wire. According to Harless, the conductivity of the 
nerve is on the average about 14.86 times that of distilled water. 
Hermann found the conductivity to be much greater in the longi- 
tudinal than in the transverse direction of the nerve. 

As to the direct excitatory effect upon the nerve of constant 
currents and of their variations, the main principle is that formu- 
lated by du Bois-Reymond in 1845.* This principle may be stated 
as follows : The excitatory effect of the constant current, as judged 
by the contraction-curve of the muscle, does not correspond to the 
absolute value of the intensity of the current at each moment, 
but to the change in this value from one moment to another ; and 
the effect is greater the less the time in which changes of the same 
magnitude in the current occur, or the greater their magnitude in 
the same length of time. The essential fact is that constant cur- 
rents, while they remain constant, do not irritate the nerve ; vari- 
ations in such currents do irritate it. The variation may be either 
frovi zero or to zero (the making or the breaking of the current), 
but it must have a certain degree of suddenness to be of any effect. 
Hence induction-shocks are, relatively to their actual strength, 
much more effective than the constant current in exciting the 
nerve. Great difficulties, however, stand in the way of stating 
definitely the relations that exist between variations in the strength 
of the constant current and changes in the excitation of the nerve 
produced by these variations ; Hermann, indeed, pronounces the 
difficulties "insuperable." 

It is not absolutely certain that the constant cvxrrent itself, apart 
from variations in its strength, has any excitatory effect upon the 
sensory nerves. The sensory effects produced bj such a current, — 
for example, pain in the skin, roaring in the ears, sensations of 
light and color, electrical taste, giddiness (as when the current is 
passed transversely through the head at the mastoid processes), 
etc. — are due to the end-organs and the central organs. It is per- 
haps probable that such a current itself may produce tetanus in 
certain nerves ; but the effect is very small compared with that 
produced by variations of this current, Pfluger found tetanus pro- 

' In a paper communicated to the Physiological Society in Berlin, August 
8th, of that year ; see, also, his Untersuchungen uber thierische Electricitat, 
I., p. 25». 



duced by weak currents of about tlie order of the so-called muscle- 
current ; but not by strong ones. 

§ 14. The excitatory effect of the constant current is dependent 
upon its direction. If three grades of strength are assigned to all 
sacli currents — namely, weak, medium, and strong — the results of 
all the experimenters will be found to agree as to the dependence 
of the effect of medium and strong currents upon their direction ; 
as to the case of weak currents, authorities differ. The following 
table, given by Pfliiger,' states the conclusion agreed to by the 
larger number of observers : 

Ascending Current. 

Descending Current. 














Rest or weak 


The results here tabulated are obtained by experimenting with 
the excised motor nerves of frogs. In experiments with the sen- 
sory nerves, or with any of the nerves while remaining in the liv- 
ing animal, the conditions become so complicated that satisfactory 
results in confirmation of Pfliiger's conclusions have not yet been 

§ 15. The excitatory effect of the constant current is also depend- 
ent upon its absolute strength. Du Bois-Reymond, after discovering 
his law, proceeded to raise the inquir}^ whether the height of the 
current upon which the variation is piled up, as it were, has any 
influence upon its effect. Various attempts to answer the inquiry 
have been made ; but the discovery of Pfliiger's laAv of electvotonus 
has, according to Hermann,'' changed the form of the question to 
the following : What influence upon the excitatory effect of in- 
creasing catalectrotonus and diminishing anelectrotonus does the 
absolute amount of existing electrotonus have ? In this form it will 
be referred to again. 

§ 16. The excitatory effect of the electrical current is influenced 
by the length of the nerve-stretch through which it flows. From 
the beginning of electro-physiology the opinion has prevailed that 
the excitatory effect is increased by the length of the nerve-stretch. 
This view accords theoretically with deductions from Pfluger's law 
of electrotonus. The experimental proof, however, is somewhat 
vacillating ; in part, doubtless, on account of the admixture of 

' See Untersuchungen iiber die Physiologie d. Electrotonus, p. 453. Ber- 
lin, 1859. ^ Haudb. d. PliysioL, II., i., p. 76. 


different local conditions where different considerable lengths of a 
nerve are passed through. Different investigators have found the 
increase of irritability in the nerve, as dependent upon its length, 
confined within different limits ; one has fixed the limit at from 
jig to ^ inch, another at from ^ to f inch. Willy found the rule, 
in general, to hold good only for descending currents. 

§ 17. The excitatory effect of a constant current is influenced by 
the angle between the axis of the nerve and the direction of the 
current. After considerable experimentation, Avith varying results, 
the more modern researches have, according to careful experiments 
made by Albrecht and A. Meyer, in the laboratory of Hermann,' 
confirmed the opinion of Galvani : The electrical current does not 
excite the nerve when it flows precisely at right angles to the 
nerve's axis. 

§ 18. The duration of the current also influences its effect as a 

Attention has already been called to the exhausting effects of 
long-continued stimulation of the nerve, whether by electricity or 
otherwise. But can a shock be so brief as not to stimulate the 
nerve at all? The reason why vei-y brief currents, on breaking the 
circuit, are not followed by a contraction of the muscle is obvi- 
ously to be found in the fact that the condition of anelectrotonus, 
on which the breaking conti'action depends, has not had time to 
develop itself. But J. Konig, working under Helmholtz's direc- 
tion, found that currents which would produce the making but not 
the breaking contraction, j)rovided they had sufficient duration, pro- 
duced no contractions at all if they lasted only 0.001 of a second. 
On increasing the duration of the current the strength of the con- 
tractions increased also, until at 0.017-0.018 of a second they 
reached the same height as that of the contraction produced by the 
corresponding constant ciu'rent. It may be said, then, that the 
electrical current must act uj)on a nerve for at least about 0.001^ 
of a second in order to excite it. The nerve on being cooled 
becomes more sluggish in its resj)onse to the stimulus ; at the 
freezing-point it requires a duration of nearly 0.02 of a second for 
the stimulus to start it into action. 

§ 19. Besides the direct excitatory effect upon the nerve of elec- 
trical currents, we have to consider their effect in modifying the 
action of the nerve under stimuli, whether electrical or of some 
other kind. If a nerve-stretch is under the influence of a constant 
current which is being passed through it, the effect of stimuli, 
when applied to any part of the nerve and judged by sensation or 
' See his Handb. d. Physiol,, II., i., p. 81 f. 

pfluger's law of electrotontjs. 116 

muscular contraction, is increased. This changed condition of the 
nerve with respect to its excitabihty, which the electrical current 
produces, is called " Electrotonus." The term was introduced into 
physiology by du Bois-Eeymond, who was preceded in his in- 
vestigations by Kitter, Nobili, and Matteucci, and followed by 
Valentin, Eckhard, and others. It is Pfliiger, however, who is en- 
titled to have his name permanently attached to the law of elec- 
trotonus; for it is he who most thoroughly analyzed the facts, 
separated the variables from the constants, and gave scientific form 
to the result. It is found that the modified excitability of the 
electrotonized nerve (that is, of the nerve which has been thrown 
by the passage of the electrical current into this modified condition 
of excitability) is not uniform through its entire stretch, but is 
greatest in the immediate region where the electrodes are applied. 
Moreover, it differs at the two electrodes — the condition at the 
anode (or positive pole) from that at the cathode (or negative pole). 
It differs, also, for that part of the stretch which lies between the 
electrodes as compared w^ith that which is outside of the electrodes. 
Pfluger's law states the whole case as follows : The excitability of a 
nerve under the action of the constant current is increased in the 
catelectrotonized region (that is, on both sides of the negative elec- 
trode), and diminished in the anelectrotonized region (that is, on both 
sides of the positive electrode). This law is declared by Hermann ' 
to hold good of all kinds of stimulus, and in all cases — with the only 
ajDparent exception of the suprapolar region of an ascending current. 
This electrotonic eifect of the constant current, like its direct 
excitatory effect, is influenced by the strength of the current, by its 
making and breaking, and by the length of the stretch through 
which it flows. The change in the excitability of the electrotonized 
nerve increases with the strength of the current, from the low- 
est observable point until it soon reaches a maximum ; after this 
maximum is reached, further increase of electrotonus is to be rec- 
ognized only by the expanding of this condition over the extra- 
polar parts of the nerve-stretch. Electrotonus increases also with 
the length of the nerve-stretch affected ; but this relation also 
finally reaches a maximum. Electrotonic changes in the catelec- 
trotonic region occur immediately upon making the current ; they 
then speedily but slightly increase, and more slowly diminish again. 
The anelectrotonic condition develops and extends itself compar- 
atively slowly, reaches a maximum, and then gradually falls off 
again. The immediate consequence of breaking the current is to 
increase the electrotonic condition of the nerve in the anelectrotonic 
' Handb. d. Physiol., II., i., p. 43. 


region, and very briefly to decrease it in tlie catelectrotonic region 5 
the former increase gradually vanishes ; the latter decrease is fol- 
lowed, after a few seconds, hj an increase which lasts from one- 
half a minute to fifteen minutes. 

The so-called "laws of electrotonus " are almost wholly based 
"upon esjDeriments with the motor nerves of frogs. Great and even 
insuperable difficulties stand in the way of proving experimentally 
its application to sensory nerves, or to the nerves of living and self- 
conscious man. The conditions of influence — from the central 
organs and end-organs, from sensation and will — upon the nerves 
in such cases are so complicated as to bafile all attempts to analyze 
them by means of direct experimentation. 

Further consideration of electrotonus, and of its bearing upon 
a mechanical theory of the nerves, must be for the present post- 

§ 20. The phenomena evoked in connection with the starting 
and propagating of nerve-commotion along a nerve-stretch may be 
presented — in the second place (see § 9) — as indicative of certain 
Processes set up within the nerves themselves. That the effect 
of a constant current is not exhausted in direct excitation of the 
nerve is proved by the changed condition of excitabiUty which it 
also produces. 

No mechanical process that can be made directly appreciable by 
the senses or accurately measured by mechanical means, like that 
which takes place in the contracting muscle, occurs in the nerve 
when excited to its physiological activity by means of appropriate 
stimuli. Whatever changes then take place in it are invisible and 

§ 21. Nor are we much better able, on the ground of experi- 
mental tests, to affirm the existence of any thermic process in 
connection wdth the excitation of the nerves. If any rise of tem- 
perature in the nerve is caused by the application of stimulus, it 
is exceedingly small. Helmholz,^ in connection with his investiga- 
tions into the heating of the muscle when in a state of tetanus, 
could detect no development of heat in the nerve, although his 
means would have revealed a change of only a few thousandths of 
a degree. On the other hand, Schiff and Heidenhain both de- 
tected a rise of temperature in the brain due to nervous excitation. 
But it is still a question how far this fact indicates anything more 
than change in the distribution of the arterial blood. Moreover, 
the former of the two observers failed to obtain any evidence of 
heating in the cerebellum by sensory excitation. The ease of the 
' Archiv f. Anat, Physiol., etc., 1848, p. 158. 


conducting nerve-cords and that of the cellular tissue of the cen- 
tral organs may very likely be different in this regard. 

§ 22. Nor have any chemical processes been indubitably proved 
to occur in the nerves as an accompaniment or result of the exez'- 
cise of their physiological function. The only experimental evi- 
dence of such a process is the change of reaction which some ob- 
servers have found. Funke and others have asserted that, not only 
a certain time after death, but also after exertion as caused by 
cramping produced with strychnine-poisoning, the nerves show an 
acid reaction. But Heidenhain and other observers contest this 
alleged fact. Other assertions of chemical changes set up in the 
nerves by exciting them are even more uncertain. Eanke's theory 
of a "respiration of the nerves" is quite without any sufficient ex- 
perimental proof ; and so is his claim that an absorption of the 
water of the nervous tissue results from tetanus. If any chemical 
changes are produced in the nerve by exciting it, they are like the 
thermic — exceedingly small. This fact is proved by the almost 
comjDlete independence of the nerve with respect to the oxygen 
of the arterial blood, and by the absence of any observable changes 
in its temperature when functionally active. But here again we 
must distinguish between the case of the nerves as conductors and 
that of the nervous tissue of the central organs. 

§23. Evidence of the electrical j^rocess in nerves functionally ac- 
tive is not wanting. It was not, however, until the discovery of du 
Bois Eeymond, announced in 1843, that any experimental evidence 
had been obtained to show the existence of electrical currents in 
the nerves, although it had previously been conjectured that the 
distinctively neural process is a phase of electricity. This ex- 
perimenter found that in the case of the nerve, as in that of the 
muscle, the cross- section artificially made is negative toward the 
longitudinal surface of the nerve-stretch. Weak longitudinal cur- 
rents also show themselves between the two cross-sections of a 
nerve-stretch thus prepared. The current outside the nerve-stretch 
may be considered as completed by a current in the nerve-stretch 
from its cut end to the equator. This current (called "natural 
nerve-current," or " current of rest ") is the same in the sensory, 
the motor, and the mixed nerves of the same animal ; but its elec- 
tro-motive force is greater the larger and thicker the nerve. Its 
absolute strength in the sciatic nerve of the frog is given by du 
Bois-Eeymond as 0.022 of a Daniell's cell, but by Engelmann as 
0.046. It gradually becomes extinct in the nerves of the dead 
body, but it continues for some time after their irritability is lost. 

The same discoverer, du Bois-Eeymond, found that the current 


of rest is diminished in energy by tetanizing the nerve-stretch with 
an electrical current. That is, if when one of the electrodes is placed 
at the equator, and the other at the cut end of a nerve-stretch, the 
needle of the galvanometer indicates the passage of a so-called cur- 
rent of rest, and then the muscle to which the nerve is attached be 
tetanized by passing an interrupted current through the nerve, the 
needle will swing back toward zero. This variation is called the 
"negative variation " of the nerve-current. It may be produced by 
chemical and mechanical, as well as by electrical, stimulation ; and 
when the nerve is no longer irritable the negative variation sinks 
to zero. It shows, therefore, that the electro-motive force of the 
nerve is diminished by the nerve being excited ; and the degree 
of the negative variation is a measure of this diminution, although 
it does not wholly nullify the so-called current of rest. The nega- 
tive variation of the electrical current in the nerve is closely con- 
nected with the nerve-commotion which is started and conducted 
in the nerve. Since the excitation of the nerve is known to be 
progressive, or of a wave-like character, the nature of this connec- 
tion, according to Hermann, may be more definitely stated as fol- 
lows : The electrical condition of each excited place in a nerve- 
stretch is negative toward all the places of the same nerve-fibre 
that are unexcited. Hence, between any two points in a nerve- 
fibre, while the nerve-commotion is passing over the distance, two 
phases of the current of action occur ; the first phase is in the 
same direction as the course of the wave of excitation, the second 
is in the opposite direction. 

§ 24. The Laws which are known to govern the starting and prop- 
agation of nerve-commotion along the nerves as conductors are 
few in number ; they deal chiefly with relations between the mag- 
nitude of the stimulus and the amount of the resulting impulses, 
and with the conditions for, and speed of, the unbroken propaga- 
tion of these impulses. 

The relations which exist between the magnitude of the stimulus 
applied to the nerves in their normal condition and the amount of 
resulting nervous impulse cannot be given with accuracy. For, in 
the first i^lace, there is no absolute measure for either of the two 
values which it is desired to compare. Of the various stimuli 
which act upon the nerve-fibres, electricity is the only one that ad- 
mits of even a fairly approximate measurement as an excitant of 
these fibres ; and the excitatory effect of electricity does not vary 
in direct proportion to the strength of the current, but in propor- 
tion to the changes in its strength. With reference to attaining a 
direct measurement of the amount of the process set up in the 


nerve by the stimulus, we seem to be in a yet more helpless con- 
dition. The effect of this process is almost wholly manifested- in 
the organs with which the nerve is connected rather than in the 
nerve itself. There is evidence in the case of the nerves, however, 
as in that of the muscles, that their excitation consists in the set- 
ting free, by the stimulus, of potential energy due to the molecular 
constitution of the nerve itself. But the exact nature of this en- 
ergy, and of its mathematical relations, both to the stimulus and to 
the resulting energy called forth in the organs connected with the 
nerve, we shall probably never discover. Still further, as Griitzner ' 
and others have shown, the same kind and degree of stimulus pro- 
duces different effects when applied to different nerves. 

§ 25. Allowing for the uncertain factors, however, some approxi- 
mate statement may be ventured as to the i*elation between the 
magnitude of the stimulus and that of the resulting nerve-commo- 
tion. Measuring the amount of the process in the nerve by the 
resulting contraction in the muscle, Hermann ° found that this 
amount increases, at first rapidly and then more slowly, with the 
increase of the stimulus. According to Fick,^ the height to which 
the lever is raised by the contractions of the muscle varies, within 
certain limits, in direct proportion to the amount of the stimulus. 
The last-mentioned observer also noted two remarkable phenom- 
ena : (1) On increasing the amount of the stimulus beyond the 
point necessary to produce the first maximum contraction, another 
stage is reached in which the effect further increases, in proportion 
to the stimulus, until a second maximum is gained. (2) In some 
circumstances, after reaching the first maximum, the effect dimin- 
ishes with the increase of the stimulus, then rises on further in- 
crease, until it attains a second maximum. 

The effect of several excitations may be supposed to pass along 
the nerve as separate waves of nerve-commotion ; but in order to 
keep the waves separate the interval between them must be more 
than about yro °^ ^ second (the fraction differing for different 
nerves, different animals, etc.), otherwise they fuse in the muscle 
and tetanus results. The combined effects of stimulations havine 
the requisite interval may be piled up, or summed up, in the 
nerve, and be seen in superimposed contractions of the muscle. 
Two simultaneous excitations of the same jplace of the nerve-stretch 
are thus "summed up " as long as the maximum of excitation is 
not reached ; the two are, in fact, one. If the cathodes of the two 

' Pfluger's Arcliiv, xvii., pp. 215 ff. ; and xxv., pp. 255 ff. 

2 See Archiv f. Anat. u. Physiol., 1861, p. 392. 

3 Untersuchungen iiber electrische Nerveiireizung. Braunschweig, 1864. 


exciting currents unite, the same effect takes place. A similar re- 
sult may be gained by combining the effects of two different kinds 
of stimuli — as, for example, electricity and the drying oft' of the 

§ 26. In a rough way the specific excitability also of different 
nerves, or of different localities in the same nerve, may be dis- 
covered. Harless found that the excitability of the nerves of the 
frog is twenty-two times as great in winter as in summer. In the 
cut nerve it is greater near the artificial cross-section. Many 
observers have contended that the excitability of the normal nerve 
diminishes toward its peripheral portion. Matteucci investigated 
the subject of local differences of excitability in the sensory nerves ; 
moi-e recently Eutherford ' discovered that the reflex effects of 
stimulating a sensory nerve are greater the nearer the central 
organ the stimulus is applied. Finall}', Helmholtz " and Hermann ' 
observed that the lower part of the nerve-stretch is more excitable 
under the action of an ascending, the upper under that of a de- 
scending induction-current. 

§ 27. The Speed with which the process of conduction takes 
place in the nerves has been determined with considerable accu- 
racy, under a variety of circumstances ; this, notwithstanding the 
fact that the physiologist Joh. Miiller' declared it to be forever 
impossible no longer ago than 1844. In only 1850, however, 
Helmholtz ^ announced that he had succeeded in measuring the 
speed of nervous impulses in the motor nerves of the frog. The 
rate he found to be 26.4 meters, or about 86.6 feet, per second. 
Another series of investigations, in which the pendulum-myograph 
was used, gave a result about 3 feet larger (27.25 m.). Subsequent 
investigators have substantially confirmed the figures of Helm- 
holtz. Bernstein, by a still different method of measurement, found 
that the speed of conduction in the nerves varies between 25 and 
33 meters. In the motor nerves of man the number was still later 
fixed by Helmholtz and Baxt at 33.9 meters, or about 111 feet, per 
second. Von Wittich found it to be about 98.5 feet per second. 
The complexity of the elements which enter into the measurement 
of the speed of nervous impulses in the sensorij nerves makes it neai'- 
ly impossible to obtain satisfactory results by experiment. And so 
far as the calculations take into account changes produced in the 

1 See Journal of Anat. and Physiol., 1871, v., pp. 329 ff. 

''Archiv f. Anat. u. Physiol., 1850, p. 337. 

^ Pflager's Archiv, viii., p. 261; and xvi., p. 262. 

*See his Handbuch der Physiologie, i., pp. 581 fE. Coblenz, 1844. 

""See Archiv f. Anat. u. Physiol., 1850, pp. 276-364. 


nervous centres with accompanying phenomena of sensation and 
attention, their discussion belongs elsewhere. Of the four factors 
that enter into the entire time ("reaction-time") Avhich elapses 
between the application of stimulus to a sensory nerve and the 
resulting contraction of the muscle — namely, (1) time of conduction 
in the sensory nerve ; (2) processes in the central organs ; (3) time of 
conduction in the motor nerve ; and (1) latent period of the muscle 
— it is difficult to disentangle the factor (No. (1) ) required by 
the attempt at analysis. Hirsch, by experimenting with stimuli ap- 
plied to the skin at different distances from the brain, found the 
speed of conduction in the sensory nerves of man to be about 111.5 
feet per second— a result in exceedingly close agreement with the 
figure obtained by Helmholtz for the motor nerves. Schelske 
used another method of measurement ; by applying the stimulus 
to the groin and the foot, and recording the difference of time in 
the two classes of cases, he obtained results varying between 
25.294 and 32.608 meters per second. Others have given figures 
differing more or less widely from those just stated. The general 
conclusions, however, favor numbers lying between 98 and 131 
feet per second as giving the speed of conduction in the sensory 
nerves of man. 

The speed of conduction in all nerves depends upon several va- 
rying conditions, such as their temperature, the strength of the 
stimulus, length of the nerve-stretch, and its electrical condition. 
Experiments in winter give different results from those in summer. 
In the motor nerves of man the rate can be made, by changes of 
temperature, to vary from about 98 feet to 295 feet per second. 
It has been disputed by different observers whether the speed of 
conduction is dependent in any degree uj)on the strength of the 
stimulus ; and even Hermann considers the question undecided. 
But Vintschgau ' has recently shown, as the result of a large number 
of carefully conducted experiments, that as soon as the stimulus 
rises above a certain limit of intensity, the speed of the nervous 
impulses increases with the increase of the intensity of the stimulus. 
This limit depends, however, upon the direction of the current, upon 
whether it is a making or a breaking current, upon the animal 
chosen for experiment, etc. Whether the speed of the nervous im- 
pulses is directly dependent upon the length of the nerve-stretch 
is scarcely decided beyond doubt. The effort of the science, gen- 
eral "nerve-physiology," is directed toward showing how these 
variations in speed, as experimentally determined, may be explained 
from the laws of Electrotouus. 

> In Pfliiger's Archiv, 1883, xxx., pp. 17 fE. 


§ 28. Finally, it sliould be remembered that the fact of any 
propagation of nervous impulses whatever presupposes the con- 
tinuity, integrity, and isolation of the nerve tract along which the 
impulses move. The slightest separation of the substance of the 
nerve by cross-section, even when the cut ends are left in the 
closest mechanical contact, destroys the unity of the nerve's 
phj^siological function. The ancients knew that tying the nerve 
prevented its action ; they explained the fact by saying that the 
flow of nervous fluid was thus hindered. So also does the fineness 
of the localization which belongs to the organs of motion, but 
especially to those of sense, as well as the fact that partial section 
of a nerve only lames part of the field cared for by that nerve, 
indicate the physiological isolation of the nerve-fibre during its 
course between end-organs and central organs. Since the result of 
stimulating a given nerve is in quality invariably the same, it would 
seem that the law of the " specific energy " of each nervous element 
(to which we shall leier elsewhere) is connected with the assump- 
tions necessary to explain the phenomena attendant upon the 
starting and propagating of nervous impulses in the conducting 

§ 29. Inasmuch as the central organs are to a large extent com- 
posed of nerves, a complete account of the nerves as conductors 
should include a description of the nature of that nerve-commotion 
which is propagated from point to point along the nervous ele- 
ments within these organs, and of the paths or tracts along which 
it passes. But unfortunately our knowledge upon these matters is 
exceedingly scanty and uncertain. This is in part due to the fact 
that the influence of the ganglion-cells, with which the nerve-fibres 
are mixed to form the central organs, profoundly modifies the 
neural processes of excitation and conduction. The subject be- 
longs, then, to a consideration of the functions of the central or- 
gans rather than of the nerves alone. Certain statements, how- 
ever, may most fitly be given in this connection. 

"When speaking of conduction in the spinal cord or brain we 
are not to think of a nerve-commotion as always moving along one 
fixed path, after the analogy of the far simpler case of the nerve in 
the nerve-muscle machine. It is true that the nerve-fibre in its 
normal place in the body runs insulated, as it were, between the 
spinal cord and the end-organ at the periphery. But the spinal 
cord itself does not act as a perfectly isomorphic medium. The 
very complex structure of this organ, in which nerve-fibres and 
nerve-cells are intricately interwoven, has already shown us that it 
is not adapted to act as such a medium. The case of the brain is 


even clearer. It accords, therefore, with the structure of all central 
organs, that we should find the speed of conduction slower in them 
than in the peripheral nerves. Exner ' calculated, from thie delay 
which sensory impulses experience in the cord of man, that their 
speed there is not more than about 26|- feet (8 meters) per sec- 
ond. The speed of the motor impulses in the cord he gives doubt- 
fully as varying between 36 feet and 49 feet (11 to 12 and 14 to 15 
meters). These numbers are substantially confirmed by the con- 
clusions of Burckhardt (8 to 14 meters). The latter also maintains 
that the speed of the impulses which occasion sensations of touch is 
greater than that of those which occasion pain (as 27 to 50 meters 
compared with 8 to 14). It has also been observed that, in some 
cases of persons with disease of the posterior strands of the spinal 
cord, sensations of pain arise in consciousness notably later than 
those of touch. But the interpretation of all these phenomena is 
complicated with questions of the cerebral functions ; for sensations 
of pain are pre-eminently of cerebral origin. Moreover, we can 
have but meagre confidence in our ability to tell with any pre- 
cision the length of the paths by which nervous impulses travel in 
the spinal cord of man. The fact observed by du Bois-Reymond, 
that the vibrations of the muscle tetanized through the cord are 
less than would be expected from the number of shocks given by 
the stimulus, and the fact discovered by Helmholtz, that muscle 
when tetanized by an act of will has a uniform tone indicating 
nineteen vibrations to the second (the rate of vibration into which 
the muscle is thrown by direct stimulation of the motor nerve, on 
the contrary, corresponding to the number of shocks), show the 
profound effect of the central organs over the nervous impulses. 
Although, then, the experimental evidence is not perfectly conclu- 
sive, it, on the whole, confirms what we should expect from the 
anatomical structure of the spinal cord, as to the complexity and 
relative slowness of conduction in this organ. 

§ 30. Various attempts have been made by experimental physi- 
ology to demonstrate the paths of conduction in the spinal cord. 
The evidence from histology on this difiicult subject has already 
been given (p. 71 f.). It is not always easy to make the two lines 
of evidence coincide. As to one point of experimental physiology^ 
however, no doubt has existed since the " epoch-making discov- 
ery " of Sir Charles Bell and Magendie. The sensory fibres en- 
ter the spinal cord by the posterior root, the motor fibres by the 
anterior. The demonstration of this fact is performed by dividing 
these roots, respectively, and observing the results. When a pos- 
' Pfliiger's Arcliiv, vii., pp. 632 ff. ; and compare Ibid., viii., pp 532 ff. 


terior i^oot is divided all the structures supplied by the same nerve 
lose their sensibility ; while the muscles supplied by its correspond- 
ing anterior root continue to be thrown into action by the will 
and b}^ reflex stimulation. Moreover, stimulation of the central 
end of the posterior root thus divided produces sensory effects, but 
stimulation of its peripheral end produces no motion. When an 
anterior root is divided, on the contrary, the muscles supplied by 
its nerves cannot be made to act either by volition or by reflex 
stimulation ; but no sensory paralysis is produced. Moreover, 
stimulation of the peripheral end of the nerve will now throw the 
muscles into contraction, but stimulation of the central end will 
produce no effects. An exception to the exclusively motor effects 
of the peripheral end occurs in certain cases of so-called "recurrent 
sensibility;" the sensibility shown in these cases is probably due 
to the fact that a few sensory fibres from the posterior root, after 
running a short distance in the mixed nerve, turn back and run 
upward in the anterior root. The proof is then complete, so far 
as the direct motor paths to the striated muscles, and the specifi- 
cally sensory paths which conduct impulses to the cerebral hemi- 
si^heres, are concerned. According to Sigmund Mayer ' it does 
not necessarily follow, however, that onhj centripetal impulses are 
conducted by the posterior, and only centrifugal by the anterior 

§31. The general aiTangement of the motor paths in that part 
of the spinal cord, on the same side, where they enter by the an- 
terior roots of the nerves, and of the sensory paths in the posterior 
part of the cord, is maintained throughout. In man, that is to 
say, the impulses pass up or down the cord in that region of it 
at which they leave, or by which they enter with, the anterior or 
the posterior roots. But histology shows that the two halves of the 
cord are anatomically connected by the commissures, and that every 
part of each half is bound with other parts of the same half, both 
up and down and to and fro. Physiology, too, indicates that the 
paths of sensory impulse undei'go a partial crossing from right to 
left, and from left to right. For, after complete section of one lat- 
eral half of the cord, complete loss of sensibility of either side in 
that part of the body which is supplied by those nerves of the same 
side that enter the cord below the place of section does not re- 
sult. The effects that do result depend upon the animal chosen 
for experiment, and upon the height at which the section is made. 
Experiments upon the lower animals seem also to show that in 
their case a partial crossing of the motor paths takes place in the 
' Hermann's Handb d. Physiol., II., i., p. 217. 


spinal cord ; the evidence from pathology makes it doubtful whether 
in man any crossing from side to side occurs in the voluntary 
motor paths, at least below a point very high up in the neck. All 
the evidence shows that in the lateral columns both, sensory and 
motor paths are to be found. 

§ 32. In addition to the general statement just made, experi- 
mental physiology has little to say confirming or correcting the con- 
clusions of histology (see p. 71 f.) as to the paths of neural impulses 
in the spinal cord of man.' Experiments which attempt to make 
a section, either of all the fibres in the anterior columns, leaving all 
the other fibres intact, or of all the other columns, leaving the fibres 
in the anterior columns intact, can never, indeed, be quite sure of 
their success. But, on the whole, their resvilts are confirmatory of 
the statements made in the last article. Some investigators have 
endeavored to solve the same problem by directly stimulating the 
fibres of the different columns in such manner as to confine, as far as 
possible, the influence of the excitatory current (or other stimulus) 
to certain definitely selected fibres, and so to exclude all reflex ac- 
tion. It is found that no reaction, indicative of any sensory im- 
pulses whatever, follows the stimulation of the central ends of the 
anterior white columns of the spinal cord ; but stimulation of the 
peripheral ends of these same columns may be followed by muscu- 
lar contraction, sometimes (so Longet and Kiirschner found) when 
mechanical stimuli are used, but oftener with weak electrical cur- 
rents. . Careful cutting of these columns is followed by no signs of 

On the other hand, stimulation of the central cut ends of the 
posterior columns produces signs of pain, and other sensory effects ; 
for this pui-pose Longet has used electrical, and Eigenbrodt and 
Schiff mechanical stimulation. According to Schiff and others the 
entire cord can be cut through from before back to the posterior 
columns, and if these are left the animal will retain the sense of 
feeling. As to a further differentiation of the sensory function of 
these columns, different experimenters do not agree. Some would 
confine their function to impulses that give rise to sensations of 
touch, on the ground that animals, the substance of whose cord has 
been entirely cut through with the exception of the posterior col- 
umns, retain their sensations of touch, but loose their susceptibil- 
ity to pain from impressions made on the surfaces whose nerves 
enter the cord below the place of section. Impulses which give 

' Comp. the generalizations of Eckliard in tlie chapter on " Verlauf d. mo- 
torischen u. seusiblen Innervatiouswege im Riickenmarke," Hermann, Handb. 
d. Physiol., II., ii., pp. 148 ff. 


rise to sensations of pain must therefore pass elsewhere than by 
the posterior strands ; that is, chiefly by the gray matter of the 
cord. According to others, however, these strands conduct sen- 
sory processes only in so far as they serve for the passage through 
them of the nerves from the sensory roots ; it is, then, the gray sub- 
stance of the cord which conducts these processes along upward. 
In addition to the more marked sensory effects of stimulating the 
posterior columns, some experimenters get effects which they in- 
terpret as showing the presence of motor, and even of voluntary- 
motor, paths in these columns. Stilling, for example, found that 
voluntary motions occurred after one entire anterior half of the 
cord had been cut through. But in the absence of proof that 
no motor paths in the lateral columns were left intact by his ex- 
periments, and in view of the fact that a crossing of such paths 
may take place in some of the animals, the evidence is not conclu- 
sive. Moreover, Ttirck and others have found that the posterior 
white columns may be entirely cut through without causing motor 

In the lateral columns of the cord, paths of both motor and sen- 
sory impulses are probably to be found. As to the case of motor 
paths there is, indeed, no reasonable doubt — at least there is no dis- 
pute. Ludwig and "Woroschiloff found that, in the case of the rab- 
bit, voluntary movements of the hinder extremities took place even 
after section of the anterior and posterior strands, and of the gray 
matter of the cord in the cervical region. As to the proofs of sen- 
sory paths in the lateral columns, the evidence is somewhat con- 
flicting. Longet and Stilling discovered no proof of their existence ; 
Schiff pronounces the matter doubtful ; Tiirck found that unmistak- 
able signs of pain followed the cutting of these portions of the cord. 
Experiments upon animals and pathological observation, however, 
on the whole, confirm the view that the sensory are mixed with the 
motor paths in the lateral columns. As Wundt ' expresses the ap- 
parent truth — in the side strands of the coi'd a part of the system of 
motor fibres is shoved off toward the limits of the posterior columns 
and surrounded on all sides by branches of the sensory tract. 

It must be borne in mind that the function of neither the motor 
nor the sensory tracts is such that a nerve-commotion, when started 
in one of the columns, must necessarily run its course by the short- 
est path in that one column, or else not be propagated at all to its 
destination. Both histology and physiological experiment indi- 
cate that the interlacing of the nerve-fibres, and the interruption of 
theu' course with nerve-cells, provide various secondary paths in 
'Grundziige d. pliysiolog. Psjchologie, i., p. 101, Leipzig, 1880. 


addition to that which may be called the primaiy or chief. More- 
over, the gray substance of the cord not only distributes, but also 
carries forward the nervous impulses. After entire half- section of 
the cord the sensory tracts of the other half still seem able, in a 
partially substitutionary way, to accomplish the work normal to 
both sides. And even in the case of the voluntary motor tracts in 
man's spinal cord, though such a work of substitution does not take 
place, we cannot affirm that the paths of voluntary innervation 
for a definite set of muscles are invariably the same through their 
entire length. A certain latitude of movement from the straight- 
forward course of the impulse undoubtedly exists even in such a 

§ 33. Difficult as it is for experimental physiology to deal with 
the tracing of those paths along which the sensory and motor 
impulses flow in the spinal cord, it is much more so within the 
nervous mass which fills the cranial cavity. Both the structure and 
the functions of the cerebrum, as a group of chief central organs, 
make it nearly impossible experimentally to distinguish between 
paths of voluntary and paths of merely reflex motion ; or even to 
conjecture where, within its substance, impulses that have been 
moving along some more clearly defined tract may not divide and, 
subdivide indefinitely, or — conversely — impulses that enter along- 
several converging paths be concentrated, as it were, into one or two 
that are more definitely fixed. 

The evidence by which histology has succeeded in tracing cer- 
tain tracts through the brain, from the medulla oblongata to the 
convolutions of the cerebral cortex, has been presented at sufficient 
length in the last chapter (see pp. 76 f., 87 f., and 97 f.). The fuller 
discussion of the evidence from experimental physiology concerning 
the same subject will more properly appear in subsequent chapters 
upon the automatic and reflex functions of the central organs and 
upon the localization of cerebral function. Certain tracts which pass 
directly from the crusta through the internal capsule, without en- 
tering the basal ganglia, into the frontal and parietal convolutions 
have already been referred to as probably motor. Others which 
come from the tegmentum, enter the thalamus and subthalamic 
region, and emerge after being redistributed to find their way es- 
pecially to the tempero-sphenoidal and occipital lobes, have been 
declared, in all probability, to be sensory. With this statement, 
so far as the motor tracts are concerned, we shall see that the con- 
ckisions of experimental physiology accord very well. 

§ 34. But our assured knowledge from experiment concerning the 
paths by which sensory impulses travel in the brain is exceedingly 


meagre. These paths are probably much more numerous and in- 
tricate than those along which the motor impulses are propagated. 
Moreover, we can seldom draw conclusions with safety conceriung 
the sensations of the lower animals ; we therefore largely lose our 
help fi'om experiment upon them to determine these sensory paths. 
The phenomena connected with all sensory disturbances are exceed- 
ingly complicated, and the conclusions they seem to warrant are 
often conflicting. For example, the effect of destroying a sensory 
nerve-tract in the head does not consist simply in the destruction 
or laming of some one definite function. On the contrary', if a 
sensory cranial nerve is severed, the various different functions of 
feeling pain, of pressure, and temperature, and the power of localiz- 
ing, in the region supplied by the nerve are all lost. But disease 
of the cerebro-spinal axis may impair one or more of these func- 
tions, and leave the others intact, in a given region of the periph- 
ery. Anaesthetics also may obliterate the sense of pain while leav- 
ing that of contact relatively unimpaired. 

Still more difficult of comprehension from the point of view fur- 
nished by the general physiology of the nerves are the degrees of 
tenacity with which different sensory functions, even when adminis- 
tered by the same sensory nerve, are combined. Loss of the sense 
of temperature and of the muscular sense rarely or never occur 
separately ; but muscular sense not infrequently disappears and 
the sensitiveness of the skin to pressure is retained. Upon such 
phenomena we have little clear light to throw. It can simply be said 
that the distribution of the sensory nerves within the centi'al or- 
gans must be enormously complicated, and that we have absolute- 
ly no knowledge as to any differences in the kinds, or velocity, or 
paths, of the nerve-commotions there, that will help us to account 
for the facts. Yet such differences in the sensations doubtless rest 
upon differences in the nerve-commotion that causes them, within 
that inner projection system of sensory impressions which is fur- 
nished by the cortex of the cerebrum. 

It has already been seen that the paths of sensory imjDulses cross 
over more or less completely within the spinal cord. They also, 
like the paths of motor impulses, cross in the region where the 
nerve-fibres in general decussate — namely, in the pons varolii and 
medulla oblongata. Experiment and pathology both show that 
the principal paths of sensory impulses from all the peripheral 
parts of the trunk of the body and from its mucous membrane lie 
close to those of the motor impulses in the white nervous substance 
surrounding the basal ganglia. Effusions of blood in this region 
not only cause hemiplegia, but also produce more or less impair- 


ment of the different modifications of touch, both in the skin and 
in the mucous membrane. According to some authorities, lesions 
in the same region often so impair the muscular sense that the 
contraction of the muscles which is produced by electrical stimula- 
tion is no longer felt. Veyssiere and others suppose that injuries 
to the white fibrous matter of the crura cerebri, the internal cap- 
sule, and the foot of the corona radiata, invariably produce a loss 
of sensibility on one side of the body ; while those which are more 
definitely confined to the striate body have this effect only imper- 
fectly and for a time — the amount of the effect depending upon the 
amount of the adjoining white substance which is involved in the 
injury. This view, like many others on the general subject, is 

§ 35. Attempts have been made to localize the paths of sensory 
impulses in the optic thalami and those of motor impulses in the 
striate bodies ; and in connection with this view it has been held 
that the former are chiefly concerned in the elaboration of sensory 
impulses (as sensory ganglionic centres), and the latter in the 
elaboration of motor impulses (as motor centres). This theory has 
been wrought out (with much rhetoric and conjecture) by J. 
Luys.* Luys finds in the optic thalami four centres which — lying- 
in order, one behind the other in an antero-posterior line — conduct 
and " condense " respectively the olfactory, the visual, the tact- 
ual, and the auditory impressions ; the corpora striata perform 
a similar ofiice for the motor impulses. The sensory impressions 
which come from the periphery, therefore, all run through the op- 
tic thalami, according to this theory, in order that they may be 
"intellectualized" (whatever that may mean); the motor through 
the striate bodies, in order that they may be "materialized." It 
is enough in this connection to say that no such comjjlete dis- 
tinction of function in the basal ganglia, whether as conductors 
or as central organs, has yet been made out. It is true, however, 
that the paths in the crusta and in and surrounding the striate 
bodies are probably mainly motor, while those in the tegmentum 
and in and around the optic thalami are mainly sensory. The 
tendency of the most recent investigation is toward placing more 
emphasis upon the fibrous nei-ve-matter surrounding these organs 
as furnishing paths for the conduction of both kinds of impulses. 

^ Recherches anatomiques, physiologiques, et pathologiques sur les Centres 
Nerveux, 1865 ; and The Brain and its Functions, New York, 1883. 




§ 1. When a physiological function is occasioned in a peripheral 
nerve, independently of a so-called act of will, by the stimulation 
of some other peripheral nerve, this function is said to be " reflex." 
Such a reflex function of the nerve is regularly brought about, how- 
ever, by the mediation of a collection of ganglion-cells and inter- 
lacing nerve-fibres, known as a central orgav. In other words, the 
secondary stimulation of one peripheral nerve, through a central 
organ, as a result of a primary stimulation of some other periph- 
eral nerve, is a reflex action of the nervous elements. The entire 
cerebro-spinal axis is a pile of nervous centres, increasing, on the 
whole, in complexity of structure and of function from below up- 
ward, which, with the nerve-tracts running into and out of it, con- 
stitutes a complicated mechanism capable of an indefinite variety 
of such reflex functions. But the spinal cord and the medulla 
oblongata are the special seat of many such functions. On the 
other hand, all excitations of the nervous system which originate 
in the nervous centres themselves — that is as distinguished from 
being called out there by the nerve-commotion brought to them 
through the afferent nerves — are called " automatic." The word 
automatic must doubtless often be used to conceal our ignorance 
of the real origin of a neural process. And doubtless, also, many 
processes which, on first inspection, appear to be automatic, may 
be discovered, or suspected, to be in reality reflex. But, as far as 
our information goes at present, not only movements of the mus- 
cles through the stimulating of the efferent nerves connected with 
them, but also the inhibiting of such movements, and the rise of 
sensations, must be ascribed to the automatic action of the central 
organs. Changes in the vital conditions to which these organs are 
subjected by their immediate surroundings, and especially changes 
in the condition of the blood with which they are supplied, ordina- 
rily constitute the internal stimuli to which they respond by exer- 
cising their peculiar functions. Automatic activities belong dis- 
tinctively to the central ganglia of the brain ; it is more difficult to 


vindicate tlieir existence in the spinal cord. In general, it is by no 
means easy confidently to distinguish between the purely reflex 
and the purely automatic action of particular central organs. The 
two forms of action are doubtless uniformly blended ; so that what 
is accomplished by any central organ depends both upon its own 
internal condition and molecular activity at the moment when the 
sensory impulse reaches it, and also upon the character of that im- 
pulse. Inasmuch as it is a vital molecular mechanism connected 
by an indefinite number of ties with other similar mechanisms, the 
central organ constantly acts both reflexly and automatically. 

§ 2. It follows, therefore, that several kinds of reflex action are 
theoretically supposable in the nervous system. When motor 
nerves are stimulated in a secondary way through a central organ, 
by ajDjDlying stimulus to the sensory nerve-endings, the effect may 
be called reflex-motor. If an excitation of a motor nerve were 
transferred, without action of the will, to one or more sensory paths, 
such a conversion of nervous action might be called reflex-sensory. 
In this way the attemjDt has been made to explain the feeling of 
weariness in the muscles when they have been overexerted, or the 
feeling which we describe as that of a limb being "asleep." It has 
also been pi'oposed to speak of "co-motor reflexes," in cases where 
two motor nerves are assumed to be reciprocally combined in their 
influence, through a centi'al organ; or of "co-sensory," in cases 
Avhere the same relation is sustained by two sensory nerves. As an 
example of the latter, attention has been called to the sensation 
which is felt in the nose when trying to look at the sun. Examples 
of the three last classes of alleged reflex functions of the nervous 
system are, however, for the most part very doubtful ; or they 
admit of explanation by recognized causes in another way.' It is. 
only concerning the laws of the first class of reflex actions — the 
reflex-motor or sensory-motor — that we have assured scientific 
evidence. The reflex function of a central organ may be defined, 
then, as being (at least in its simplest form) the " conversion " or 
"reflexion" of a sensory impulse into a motor excitation. We 
must guard ourselves carefully, however, against the misconception 
that lurks in these words : the eftect of the central organ is never 
that of merely converting or reflecting a nerve-commotion from one 
perfectly definite sensory path to an equally definite motor path. 
No such simple figure of speech will serve to describe its function, 

§ 3. The spinal cord — complex as its structure and functions 
are — is much the simplest and most accessible for experimental 
purposes of any of the organs of the cerebro- spinal system ; it is 
' Comp. Eckhard in Hermann's Handb. d. Physiol., II., ii., pp. 33 tL 


pre-eminently the seat of unconscious reflex-motor functions. It is 
a column or pile of centres, bound together for the reception of 
sensory impulses by its posterior roots and for redistributing them, 
as modified by its own molecular structure and condition, through 
the efferent fibres of the anterior roots. Such is its office as an 
organ of reflex action in distinction from its ofiice as an organ for 
conducting neural impulses. We consider, then, in the first place, 
the Sj)inal Cord as a Central Organ. 

§ 4. As the "nerve-muscle machine" is a preparation for testing 
experimentally the laws of the action of the nerves as conductors, 
so preparations may be made for testing the laws of the reflex and 
automatic functions of the spinal cord, by separating that organ 
from the brain by section below the medulla oblongata. For the 
purpose of experiment, the "brainless frog " is the most convenient 
of such preparations and the most fruitful of results.' If the flank 
of such a frog be lightly touched, the resulting reflex motion will 
be limited to a slight twitching of the muscles that lie immediately 
beneath the spot on the skin thus stimulated. If its legs be 
stretched out and one of them j)inched, all the segments of the 
limb thus irritated will be rapidly flexed in the definite purposeful 
way necessary to withdraw it from the irritation. If the skin of 
the region near the anal orifice be pinched, a new combination of 
muscular contractions will take place and a different form of defen- 
sive movements will result : the feet will be drawn up toward the 
spot irritated and the legs brusquely extended, as though to push 
away the irritating agent. If the stimulus applied to the skin of 
one hind leg be increased by forcibly pinching it, the resulting 
reflex motions may involve the fore leg of the same side, then the 
hind leg and fore leg of the opposide side, and finally almost all 
the muscles of the body. Moreover, changes in the chai-acter of 
these reflex motor activities take place which are plainly adapted to 
provide for changes in the animal's circumstances. For if the 
right flank of a brainless frog be irritated with a drop of acid, and 
at the same time the right leg be held (the member which, if un- 
hindered, would be, almost without exception, used in the attempt 
to remove the irritation), or the right foot cut off, the left foot 
may be used for the same purpose of defence. 

. Phenomena, similar to those obtained in the case of the frog, 
are obtained from other brainless animals. Thus the decapitated 
salamander, when the skin of one of its sides is pinched, Avill bend 
this side into concave shape in order to withdraw it. Not succeed- 

' For detailed information see Vulpian, Le90iis sur la Physiologic du Sys- 
tume Nerveux, pp. 311-465. 


ing in this way, it will make a movement with its foot as though to 
push away the cause of the irritation. In the case of the higher 
animals the reflexes of the spinal cord appear, on first inspection, 
to be comparatively feeble and lacking in purposeful character. 
The mammal, for a relatively long time after the division of the 
cord from the brain, exhibits only very imperfect reactions in parts 
of the body supplied by nerves which spring from the cord below 
the point of its section. But if the animal be kept alive for some 
time, and even without any physiological union of the severed jDarts, 
more strong, varied, and complex movements will follow upon the 
stimulation of the sensory nerves of those parts. Immediately 
after the spinal cord of a dog is divided low down in the dorsal 
region, the hind limbs hang limp and motionless ; irritating the 
skin calls forth only feeble and irregular movements, or none at all. 
But after some weeks or months have elapsed, reactions resembling 
those already described in the case of the frog (taking into account, 
of course, the difference in the structure and normal functions of the 
two animals) begin to appear. The hind limbs, instead of remain- 
ing motionless, will, when the animal is held so that they are pen- 
dent, be drawn up and let down again with a kind of regular rhythm, 
as a result of the constant stimulation of their motor nerves by the 
sensory nerves, through the spinal cord. Moreover, it is found that 
the breed, age, sex, and training of the animal determine the charac- 
ter of these brainless reflex movements. That is to say, the spinal 
cord, as a nervous mechanism, embodies in its very structure and 
functions all the peculiarities due to these causes. And when its 
activities are elicited through the stimulus which, arising in many 
various regions, flows in upon it along the sensory nerve-tracts, or 
through some stronger but limited impulse occasioned by the a]>- 
plication of stimulus to a particular spot on the skin with a definite 
degree of energy, these activities bear the character both of the 
stimulation and of the mediating central organ. 

§ 5. Little need be added to what has already been said (Chap. 
n., § 9), in description of that mechanism of the cord to which 
the foregoing remarkable functions are referred. Earlier investi- 
gators ' assumed the existence of a special sj'stem of sensory and 
motor nerve-fibres, with connecting nerve-cells, designed and apjjro- 
priated solely for executing these reflex-motor activities. That the 
motor tracts for reflex movements are to a certain extent distinct 
in the spinal cord from those devoted to sj)ecifically voluntaiy ac- 
tivities, there seems to be good reason for affirming ; but the older 

' For example, Marshall Hall, in liis New Memoir on tlie Nervous System. 
London, 1848. 


supposition, that there are double tracts, — one connected with con- 
scious and voluntary reaction upon sensation, and one connected 
with unconscious and involuntary, or merely reflex-motor, reac- 
tion, — between the spinal cord and the end-organs of sensation 
and motion, is almost certainly incorrect. It seems antecedently 
very improbable that every spot of the skin should be equij)ped 
with such a twofold outfit of both kinds of nerve-fibres. No par- 
ticular nerves which serve merely for reflex-motor functions, and 
which have no connection either with conscious sensation or with 
voluntary motion, can be pointed out. 

"What happens with respect to conscious sensation — the rise of 
it or its failure to rise — depends rather upon the effect of the 
stimulus on the end-organ, and upon the condition in which that 
stimulus finds the central organ on its arrival there. In consider- 
ing that mechanism of the spinal cord which comes into use when 
it acts as a central organ in all the reflex-motor activities belonging 
to it, the ofl&ce of the ganglion-cells is usually made prominent. 
And it can be definitely proved that these cells are an important 
part of the reflex mechanism of the cord. But the extremely del- 
icate network of interlacing nerve-fibres in which the processes 
of these cells lose themselves also bears an important part in the 
same functions. Precisely what elements of the central substance 
alone act, and precisely how the elements act that do act, it is im- 
possible to say. 

§ 6. The following laws embody the most important general re- 
sults of experiment upon the reflex-motor functions of the spinal 
cord, as applied to a variety of animals under a great number of 
chan":ing conditions and circumstances. 

The primary stimulation of the sensory nerves must have a cer- 
tain degree of strength and suddenness in order to produce a sec- 
ondary excitation of the motor nerves through the centres of the 
spinal cord. This is true of all the different kinds of stimuli by 
application of which spinal reflexes can be obtained.. Continuous 
irritation of the skin, if very slowly increased, may be carried to 
the extent necessary to destroy its sensitive surface, without giving 
rise to any reflex movements ; but a less degree of stimulus, if 
suddenly applied, will call forth such movements. Different chem- 
ical substances, when used as irritants, j)roduce effects dependent 
upon the strength of the solution. Thus a weak solution {\-\ fo) 
of sulphuric acid is recommended by some exj^erimenters ; and 
it is asserted that in this way exactly the same reflex move- 
ments, as respects kind and degree, can be repeatedly got from the 
same nerve-preparation, with a machine-like regularity. Each chem- 


ical stimulus lias its lower limit of concentration which will produce 
any reflex movement, and also its latent period. The time of the 
latent period for weak solutions of sulphuric acid is said by Baxt ' 
to increase nearly in geometrical ratio, while the concentration of 
the acid diminishes in ai'ithmetical ratio. The chemical stimulus, 
like the mechanical, can be so slowly increased in strength as to 
produce no effect. The same thing is also true of thermic stimu- 
lus. A decapitated frog may be placed in water, and the water 
gradually heated to the point at which heat- rigidity sets in, without 
showing any reflex activity. This fact, however, may be in part 
ascribed to the direct effect of the heat, diffused from the skin 
upon the central organ. The same law which renders stimulus 
inoperative, when very gradually increasing in strength, applies to 
the use of the electrical current. Repetition of the shocks is much 
more effective than a slow increase in the strength of the current. 
Single induction-currents are relatively powerless, and produce no 
effect unless they have a high degree of strength. Frequent inter- 
ruptions greatly increase the efficiency' of the constant current in 
producing reflex movements. It Avould seem, then, that a kind of 
summation of afferent impulses may take place in the spinal cord ; 
that is to say, the repeated excitation of the nervous centre starts 
a nerve-commotion in its substance, which gathers intensity until 
it breaks over, as it were, into the adjoining motor tracts. We can 
scarcely affirm, however, that such summation of many impulses is 
necessary to start off the nervous centre, as it were, since the sin- 
gle making of the constant current, or a single strong induction- 
shock, may be followed by a number of reflex movements. 

§ 7. The speed of reflex processes is apparently increased by 
increasing the strength of the stimulus. We have already spoken 
(p. 123) of the delay which the process of conduction suffers in the 
spinal cord when passing longitudinally. The time of cross-con- 
duction also in the cord seems to be a function of the strength of 
the stimulus, Exner ° calculated by an experiment, which con- 
sisted in causing one eyelid to move by stimulating the other, that 
the time consumed in the specifically central operations of the re- 
flex act can be made to vary between 0.055 and 0.047 of a second 
by increasing the strength of the stimulus. Rosenthal ^ and others 
have found that the time for any reflex act diminishes cousiderabl}'^ 
with the increase of the strength of the stimulus ; is greater in trans- 
verse than in longitudinal conduction ; and is much increased by ex- 

' Quoted in Hermann, Handb. d. Physiol., II., ii., p. 29. 

^ See Pflliger's Archiv, viii., p. 530 ff. 

3 Mooatsbericlit d. Berlin. Acad., 1873, p. 104. 


haustion of the cord. With veiy strong stimuli it becomes almost 
too brief for observation. Wundt, ' however, denies that the time of 
the reflex act is dependent upon the strength of the stimulus ; on the 
contrary, he affirms that the time is either very little or none at all 
affected by changes in strength of the stimulus, or else is even 
changed in the contrary direction to that required by the alleged 
law of Exner and Rosenthal. 

§ 8. The condUion of the sjyinal cord, at the time when it re- 
ceives the impulses of the sensory nerves, undoubtedly determines 
to a large extent the character of the resulting reflex motions. 
Lesion increases the excitability of the part below the lesion, and 
this — for example, in the case of reflex movements of the posterior 
limbs — according to the amount of the cord removed from the por- 
tion of it lying anterior to its nervous connections with these limbs.^ 
Marked effects are also produced by certain drugs, as strychnine, 
chloroform, aconite, quinine, etc. Of these drugs, some heighten 
and some depress its excitability. In an animal slightly poisoned 
with strychnine, the excitability of the cord is more or less height- 
ened ; and in cases of strong poisoning with the same drug, the 
least stimulation may call forth a condition of tetanus or convul- 
sive cramping extending to the whole body. Two ways of exjDlain- 
ing' this effect upon the mechanism of the central organ are pos- 
sible : one, that the excitability of those portions of this organ 
which mediate between the sensory and motor impulses is so much 
increased by the poison that, on being stimulated, they explode 
their molecular energy, as it were, and cause it to be diffused with 
great strength into unaccustomed paths ; the other, that the effect 
of the poison is to diminish the resistance along all the network 
of paths, both habitual and unaccustomed, in the spinal cord. 
Between these two explanations Eckhard ^ Avill not decide ; Rosen- 
thal seems to prefer the former, Foster' and others the latter. 
Chloroform and various other anaesthetics diminish the reflex ac- 
tion of the cord. As to the effect of changes in temperature, and 
in electrical condition, upon the spinal reflexes, the conclusions of 
different experimenters are somewhat divergent. This power of 
the nervous mechanism is, as we have already seen, retained longer 
in low than in high temperatures. According to Cayrade, Avhen 
the temperature of the whole cord is raised, the reflex movements, 
however produced, become more energetic and the single con- 

1 Mechanik d. Nerven, abth. ii., pp. 14 ff. Stuttgart, 1876. 

" Vulpian, Leyoiis, etc., p. 438. 

•'In Hermann, Haudb. d. Physiol., II.. ii., p. 42. 

* Text-book of Plijsiologj, p. 602. 


tractions last longer. Another observer found a temporary rise of 
excitability, followed by a depression, on beating sections of the 
cord between 75° and 158° Fabr. On the other hand, some observ- 
ers are of the opinion that cold increases the excitability of the 
cord. In experimenting with the electrical current it is very diffi- 
cult to distinguish between its effect upon the central organ as the 
mediating mechanism and the effect of the same stimulus upon 
the nerve-roots and nerve-paths between which the mediation 

§ 9. The locality to which the stimulus is applied has a marked 
influence in determining the extent and character of the resulting 
reflex movements. The most important difference of aU is that 
found by stimulating some spot of the skin, and then comparing 
the resulting reflex action with what follows upon the application 
of the same stimulus to the trunk of the nerve which is distributed 
to that region of the skin. The simple nervous impulses, which 
result from stimidating the afferent nerve-fibres directly, call forth 
irregular spasms in a few muscles only ; the complicated nervous 
impulses, which result from applying the same stimulus to the 
skin, are followed by extended movement of many muscles directed 
toward definite ends. Moreover, it is much more easy to produce 
reflex action by a shght pressure on the skin than by even strong 
induction-shocks when applied to the nerve-trunk. By separating 
a small bit of skin from that surrounding it on the back of a brain- 
less frog, while taking care not to injure the nerves that attach it 
to the body, the foregoing difference may be made strikingly clear 
in an experimental way.' What particular reflex actions will 
be evoked by the stimulus is, in each case, dependent ujDon the 
particular locahty of the skin to which the stimulus is applied. 
Such facts suggest the truth that the entu-e mechanism of the cord 
is broken up into centres of activity, which, however, are in close 
molecular relation with each other, and which are of a somewhat 
expansive nature. 

In view of the foregoing truths Pfliiger" has formulated the fol- 
lowing laws of relation between the stimulation and the resulting 
reflex action : {a) In the case of a spinal cord from which the 
medulla oblongata is wholly severed, all reflex motion confined to 
one side of the body is due to stimulation of that side, (b) Reflex 
movements of both sides never occur in a diagonal direction ; that 
is to say, stimulating one hind limb can never evoke reflex move- 

' See the article of Fick and Erlenmeyer m Pfliiger's Archiv, iii., p. 326. 
'^ In his work, Ueber d. sensorischen Functionen d. Riickenmarks. Berlin. 


merit of that limb and of the fore Hmb of the opposite side. ' (c) If 
reflex action is called out in the limbs of both sides, and such action 
is stronger on one side than on the other, then it is stronger on the 
side stimulated, (d) If the motor effects of the stimulation show 
that the excitation has been "irradiated," as it were, from one 
centre to another, then such movement of irradiation is always 
downward toward the medvdla oblongata in the brain, and upward 
in the cord toward the same organ. It is by no means certain, 
however, that these formulas (especially the second — No. b) admit 
of no exceptions which are involved in the peculiar structure and 
functions of the cords of certain animals. But the general rule 
appears to be, that the excitation of a sensory nerve witli a slight 
degree of stimulus gives rise to reflex movements which originate 
in the cord on the same side, at about the same altitude as that at 
which the sensory imjDulses enter the cord ; with an increased amount 
of stimulus, it gives rise to those also that arise in the other half of 
the cord at the same altitude ; with a stiU greater amount, to those 
which arise above and below on both sides of the cord, with the 
preference given to the same side. That is, the molecular disturb- 
ance, as it is dispersed or radiated, passes from the cells and net- 
work of fibres situated near together on the same side of the cord, 
first to those on the other side of the cord at the same altitude, and 
then diffuses itself on both sides up and down the cord." Accord- 
ingly, it is only after allowing for a difference in the obstacles to be 
overcome along the different paths anatomically open to any nerve- 
commotion in the spinal cord, that we can adopt the declaration 
of Luchsinger : ^ When an excitation is started anywhere in the 
spinal cord, it radiates from this point in all directions, but with 
diminishing intensity. Hence the title Avhich Flourens and Vul- 
pian,^ following him, have given to the spinal cord — "the organ for 
the dispersion of irritations." 

§ 10. Besides such undoubted reflex action as the foregoing, 
other cases where the spinal cord controls the muscles of the body 
are less certainly of a purely reflex character. Indeed, for some 
such cases the title of " automatic " has been employed. The cord 
is not capable of " irregular automatism " — that is, of spontaneous 
excitation like that which takes place in the higher nervous centres 

' See the observations of Luchsinger, which seem to show that in some ani- 
mals — as, e.g., the salamander, turtle, and even dogs, when under the inttu 
ence of ether — cross reflexes in violation of Pflliger's law do sometimes occur 
Pflijger's Archiv, xxii., pp. 179 fE. 

'^ Compare Wundt, Grundzijge d. physiol. Psychologie, i., pp. 103 and 109. 

^ Pflliger's Archiv, xxii., p. 178. 

^ Lec^ous sur la Physiologie, etc. , p. 404. 


on volition. If a brainless frog, for example, be kept in a condi- 
tion of perfect equilibrium with respect to stimulus, it will remain 
wholly motionless. But the cord of such an animal will continue 
to influence certain muscles of the body through the motor nerves, 
even in cases where sensory impulses are diflficult or impossible to 
trace. What is called the '• tonic action " of the cord upon the 
skeletal and sphincter muscles, or the smooth muscles of the ar- 
teries, is a chief illustration of this influence. The fact that such 
tonic action does not contract all the muscles connected with the 
cord at the same time, or any one set of them with the same en- 
ergy as any other, throws some suspicion on its alleged automatic 
character. A careful sifting of the evidence rather induces us to 
ascribe this influence to the constant reflex action of stimulus from 
subtle changes in the external circumstances in which the animal 
is placed. Moreover, the sensory nerves in the muscles and ten- 
dons, as well as in the skin and organs of special sense, may occa- 
sion the rise and continuance of such reflex action. Different in- 
vestigators, almost without exception, have failed to notice any 
lengthening of a muscle (or loss of its tone) when the nerve going 
to it is severed from the cord. That this so-called "tonic" influ- 
ence is largely reflex-motor is also shown by the fact that the tone of 
the muscles is lost when the skin covering them is removed, or when 
the posterior root which furnishes sensory impulses for the motor 
nerves connected with them is cut, Brondgeest has shown that, 
when a decapitated frog is hung up after having the sciatic plexus 
cut on one side, the leg is more flexed (that is, the muscles have 
more of tone) on the other side. But the same flaccid condition of 
the muscles can be produced by cutting only the posterior (or sen- 
sory) roots of this plexus. This observer is satisfied that the con- 
traction of. the muscles in the uninjured limb is due to stimulation 
from the nerves of the skin ; the tonic action of the cord on the 
skeletal muscles is, therefore, reflex. The only objection to consid- 
ering the tone of the sphincter muscles reflex lies in the fact that this 
tone continues to exist after all other reflex-motor action has been 
suppressed by narcotics ; but our knowledge of the nervous mechan- 
ism wliich conti'ols these muscles is not sufficiently complete to make 
it certain that we have excluded all possible forms of reflex influence. 
Of the marked influence of the nervous system upon the cali- 
bre of the arteries, and through this upon the character of the 
circulation of the blood, there is abundant evidence. Besides the 
main vaso-motor centres in the medulla oblongata, certain parts of 
the spinal cord are caj^able of actiug as such centres. Circulation 
may continue with regularity in a beheaded frog ; but the removal 


also of any considerable part of tlie cord affects the circulation 
tkrough the loss of tone in the blood-vessels which it occasions. 
The mechanisms for expanding and contracting the arteries are 
apparently interlaced with those for contracting the skeletal mus- 
cles, in all portions of the cord. But their chief work undoubt- 
edly consists in transforming afferent impulses into efferent vaso- 
motor impulses directed toward the dilatation or constriction of the 
arteries. Whether they are capable of automatic action — in the 
sense in which the medulla oblongata seems to be thus capable — 
is a question we need not discuss in detail here. 

§ 11. The facts already alluded to, and others similar, form the 
basis for the assumption of " Centres " in the spinal cord. In general, 
the application of a given amount of stimulus to a definite group of 
sensory nerves calls forth reflex-motor activities in definite groups 
of muscles by means of a certain region of the cord. What groups 
of muscles are thus moved depends upon the amount of the stimu- 
lus and the locality of its application. This fact is due to disper- 
sion of that nerve-commotion which is set up at different points 
in the course of the cord by the excitation of those points through 
the sensory nerves. That is to say, the mechanism of this central 
organ is so constructed as to connect the sensory with the motor 
tracts, more favorably in some regions than in others. Such re- 
gions are the so-called reflex centres of the spinal cord. If, how- 
ever, a more or less constant flow of motor impulses takes place 
from any region, and this flow is due to molecular activity not 
occasioned by the sensory nerve-fibres of the region, then such 
region may also be called an automatic centre. Nothing would 
seem to prevent the same region from acting as both a reflex and 
an automatic centre. The general principle may then be formulated 
as follows : " The spinal cord is the proximate centre, the proximate 
physiological hearth of excitation, for all the nerves that originate 
from it." This principle has been defended and illustrated with many 
researches by Legallois, Volkmann, Pfliiger, Goltz, Luchsinger, and 
others. In accordance with it, and especially since the "epoch- 
making " experiments of Goltz upon the spinal cord of dogs, many 
functions which were formerly ascribed to the brain have been 
shown to have their proximate centre in the spinal-cord. In ac- 
cordance with the same principle, it is discovered that different 
animals have different spinal centres varying in relation to their 
peripheral structure and their habits.' 

' Compare the results of the researches of LangendorfF in the Archiv f. 
Anat. u. Physiol., Physiolog. Abth., 1880, pp. 518 ff., and 1881, pp. 519 ff.; 
aud of Luclisiuger iu Plluger's Archiv, xxii., pp. 158 fE., and xxiii., pp. 308 fE. 


In illustration of the last point the following facts may be men- 
tioned : By the sufficiently long-continued and strong stimulation 
of any portion of the skin of a decapitated frog, reflex movements 
may be induced in all of its muscles. With rabbits, however, a reflex 
action of one hind leg can be caused by stimulating the sensory 
nerves of a fore leg, only in case a portion of the medulla oblon- 
gata (at least about one-third) be left attached to the cord. With 
the cord alone, the stimulation of one hind leg fails to excite ac- 
tion in either of the fore limbs. By using great care and artificial 
respii'ation, Luchsinger ' succeeded in obtaining what he calls a 
" trotting reflex " from the spinal cord — after being completely sev- 
ered from the medulla oblongata — of several young animals with 
which that form of movement is natural. Thus the diagonal op- 
posite extremities of goats and cats were moved in response to 
even such weak stimulation as passive motion of the fore leg, gen- 
tle pressui'e, and weak electrical currents. In general, then, it 
would seem that the spinal cord of every animal is a series of con- 
nected mechanisms, which are arranged so as to move the muscles 
of the body, either under the control of the higher nervous centres 
or in response to stimulation entering it at any point through the 
sensory peripheral nerves, in accordance with the specific structure 
and habits of the animal. 

Many of the chief special centres connected with the organic and 
vital functions are located in the medulla oblongata ; those con- 
nected with the co-ordination of impressions of the special senses 
and muscular action belong to the still superior portions of the 
cerebro-spinal system. But the spinal cord also contains mechan- 
isms which serve as centres of both these kinds." Their location, 
however, is so much a matter of the special physiology of particular 
species of animals, and is so indirectly connected with the inquiries 
of physiological psychology, that it is unnecessary to add anything 
further upon the subject. 

§ 12. The question whether the spinal cord is excitable as a 
whole, and in its several parts, by artificial stimulation, has been 
much debated. Its direct excitability as a whole is denied by 

' See Pflliger's Arcliiv, xxviii. , pp. 65 ff. 

^ Besides the vaso-motor centres already referred to, those for micturition, 
defecation, erection, parturition, etc., may also be mentioned. Goltz, ui his 
celebrated researches in 1874 (see Pfluger's Archiv, viii., pp. 474 ff. ), showed 
that normal micturition may take place in a dog in which the lumbar region 
has been completely severed from the dorsal region. The influence of the 
cerebral centres seems, however, to be necessary to cause a steady increase or 
decrease of the action of the sphincter ani. The cilio-spinal centre, located by 
Budge at the seventh and eighth cervical rootSi is more doubtful. 


SeLiff, ' who declares that the motions obtained by stimulating any 
part of the cord with electricity comprise only those muscles which 
are j)hysiologically related, to the exclusion of those which are ana- 
tomically contiguous through the stimulated part of the cord. A 
strong local stimulus, he affirms, produces just the same reflex mo- 
tions as those which are accustomed to arise on occasion of an ex- 
tended irritation of the skin at the places to which the nerves is- 
suing from this locality of the cord are distributed. It is inferred, 
then, that the resulting motions are obtained only reflexly, by in- 
volving the sensory nerve-roots. Bat that certain longitudinal parts 
of the cord can be directly stimulated seems capable of demonstra- 
tion. For Fick and Engelken'^ found that movements of the mus- 
cles were obtained when the anterior columns were isolated from 
the rest of the cord for a considerable distance and then stimulated. 
Lucbsinger's' experiments, moreover, contradict the conclusions of 
Schiff; and Mendelssohn" found that the reaction-time of the an- 
terior half, and especially of the anterior columns of the cord, was 
uniformly less than the reaction-time of its posterior columns. The 
latter also found that weaker stimuli would suffice to excite motion 
when applied to the anterior columns. But, according to Schiff ^ 
again, the cord contains no motor elements that are directly exci- 
table except the central paths of the nerve-roots. He also agrees 
with van Deen in denying that the gray matter of the cord can be 
made, by direct stimulation, to originate either motor or sensory 
impulses. It affords paths, however, for the transmission of both 
these kinds of impulse when once started by the other nervous ele- 
ments. Schiff accordingly speaks of the posterior gray columns, 
and of those parts of the posterior white columns which are not 
direct prolongations of the nerve-roots, as " cesthesodic." The corre- 
sponding parts of the anterior cord he calls "kinesodic." The sen- 
sitiveness of the posterior columns which others discover on experi- 
ment he regards as only indirect. Vulpian," on the contrary, agrees 
with Bell, Magendie, Flourens, and Longet, in holding that, while 
the gray matter is absolutely inexcitable and the posterior columns 
very excitable, the anterior columns possess only a moderate degree 
of excitability. 

' See, especially, articles in Pfliiger's Archiv, xxviii., pp. 537-555, and xxix., 
pp. 537-555. 

■-' Du Bois-Reymond's Archiv, 1867, p. 198 ; and Pfliiger's Archiv, ii., p. 414. 

•'Pfliiger's Archiv, xxii., pp. 169-176. 

■■ Archiv f. Anat. u. Physiol., 1883, Physiolog. Abth., pp. 283 fE. 

'■ Pfliiger's Archiv,, xxix. , p. 598. 

•^ Legons sur la Physiologie du Systeme nerveux, p. 362. 


By an ingenious arrangement for applying the mechanical stimu- 
lus of pricks from an extremely fine needle-point to definitely cir- 
cumscribed spots in the spinal cord of the frog, E. A. Birge ' seems 
to have demonstrated the susceptibility of the ganglion-cells to di- 
rect stimulus. Pricking these cells produces movements in defi- 
nitely located groups of muscles ; and the te tanusis invariably 
confined to the muscles of the same side as that of the cells stimu- 
lated, unless (as microscopic examination shows) the effect of the 
needle has reached certain cells on the other side. Birge also 
found that different regions of a single cross-section of the cord are 
excitable in different degrees ; the region from the posterior fissure 
to the median line of the gray matter being most inactive, and that 
of the large ganglion-cells in the anterior horn sinvariably being 
able to produce tetanus. 

In view of such conflict of testimony it can only be said that 
certain longitudinal parts of the spinal cord are plainly susceptible 
to direct stimulation, but at present it is difficult to decide which 
parts, exclusively, ai'e sensitive. 

§ 13. Thus far the spinal cord has been considered as a series of 
related centres, that act automatically or reflexly when separated 
from the brain. But in its normal condition the cord always acts, 
of course, under the influence of the brain. The brain thus exer- 
cises a profound modifying influence over the automatic and reflex 
activities of the inferior organ. The cord alone can be dej)ended 
upon, as it were, to respond with great regularity, in the form of 
definite reflex movements, to a given amount of stimulus, when 
applied at a given locality. But the action of the brain, when at- 
tached to the cord, interferes with this regularity, so that the ex- 
pected muscular movements may not result when the stimulus is 
applied. They are then said to be inhibited by the action of the 
brain. The phenomena of "inhibition," when connected with vo- 
lition, are familiar enough ; for example, one may voluntarily re- 
strain those movements of one's legs which the cord, if left to it- 
self, would produce as the result of tickling the soles of the feet. 

But the brain without conscious volition exercises the same in- 
hibitory action over the spinal cord. If a frog is suspended by the 
head, and its legs allowed to dip into a vessel of dilute acid, the in- 
terval between the contact of the acid and the withdrawal of the 
legs is considerably lengthened when the spinal cord remains un- 
divided below the medulla oblongata ; that is to say, the cord alone 
withdraws the legs quicker than the cord when influenced, or in- 
hibited, by the brain. The interval between tlje application of the 
' Arcliiv f. Anat. u. Physiol , 1882, Physiolog. Abth., pp. 481-489. 


acid and the contraction of the muscles can also be prolonged, when 
the brain is still connected with the cord, by applying chemical 
irritation at the same time to the optic lobes ; that is to say, the 
cord is hindered from performing its reflex-motor function by the 
stimulation, and consequent influence upon itself, of the higher 
nervous centre. Moreover, if at the time that one leg of a brain- 
less frog is dipped into the acid, the sciatic nerve of the other is 
strongly stimulated with an interrupted current, the same prolon- 
gation of the period of incubation will be observed ; in some cases, 
indeed, the reflex act will not take place at all. In discussing the 
reciprocal relations of the higher centres of the brain, we shall dis- 
cover many phenomena similar to the foregoing. All these centres 
may exercise this so-called " inhibitory " action upon other centres, 
according to their several physiological connections. The phenom- 
ena of inhibition are not, therefore, confined to the influence of the 
brain on the spinal cord. 

Elaborate attempts have been made to point out a special mech- 
anism of inhibition. Thus Setschenow ' has advocated the view 
that localized inhibitory centres exist in the brain, and that the de- 
pressing effect travels by certain definite tracts in the spinal cord. 
But on this subject our doubts are entitled to go even beyond the 
remark of Terrier : ^ " The nature of the inhibitory mechanism is 
exceedingly obscure." We cannot be said to have sufficient grounds 
for assuming the existence of any such specific mechanism. In 
general, nerve-commotions modify each other within the central 
organs ; they either facilitate and increase, or inhibit and diminish, 
each other's effect, according to the structure and functions of the 
organs, the amount and kind of stimulus thrown in upon them from 
without, and the exact condition in which this stimulus finds them. 
The inhibition of the cord by the brain is, then, only a special case 
under the general molecular theory of the nervous mechanism. 
The factors entering into every such case will very likely always 
prove too varied and complex to be analyzed with complete success. 

§ 14. On passing from the spinal cord into thfe brain, the diffi- 
culty of defining the specific functions — whether automatic or re- 
flex—of the different central organs becomes greatly increased. 
The phenomena are vastly more complicated, and the methods of 
analyzing them experimentally much less readily applied. The 

' TJeber d. Hemmungsmeohanismen f. d. Reflexthatigkeit im Gehirn d. 
Frosches, Berlin, 1863 ; and other papers. 

'-functions of tlie Brain, London, 1876, p. 18, where he refers to the 
elaborate paper on Inhibition in the West Riding Reports, vol. iv., by Dr. L 


most complex portions of the nervous substance, in respect both 
to structure and to function, are most completely withdrawn from 
the use of strictly scientific methods of research. "What is known, 
however, of the anatomical structure and connections of the dif- 
ferent organs of the brain, and of the paths along which the ner- 
vous impulses are propagated between them, prepares the way for 
the more specific physiology of eacli organ. The methods of such 
physiological research are in general these two : Observation of the 
results which follow the application of stimulus to each of the en- 
ceiDhalic organs, or to any definite locality in each ; and observation 
of the results which follow the total extirpation or lesion of these 
organs, or of any portion of each. Of course, both of these 
methods are almost wholly applicable only to the lower animals. 
In using the method of stimulation, the stimulus cannot be ap- 
plied to the nervous substance of the brain without a certain 
amount of injury to that substance. To stimulate any of the cranial 
organs with precision they must be exposed ; those that lie deepest 
cannot be exposed without injury to other organs and the death of 
the animal. Moreover, it is diflftcult jjrecisely to circumscribe the 
application of the stimulus. Just that form of stimulus which is 
most convenient, effective, and fruitful in results — namely, the 
electrical current — is liable to diffuse its direct effects beyond the 
region which it is desired to circumscribe. When no result follows 
the application of the current to a definite locality of the nervous 
substance, the failure may be due to the weakness of the stimulus, 
or to the fact that this particular centre is at the moment inhib- 
ited by its condition or by the activity of some connected centre. 
When a result does follow, it may be that this particular result is 
due to the direct or indirect stimulation of some other so-called 
centre, or to the stimulus hitting, by diffusion or otherwise, some 
of the contiguous sensory or motor nerve-tracts. 

Objection may also be raised against the nature of the argument 
by which an inference is drawn from the facts gained by the sec- 
ond of the above-mentioned methods. Such argument not only as- 
sumes that the activities which remain, when some of the organs 
of the brain are partially or wholly destroyed, belong to those 
organs that remain, but also that those activities whicii have dis- 
appeared belong to the organs that have disapj)eared. Both of 
these assumptions are, however, doubtful, when we come to apply 
them to the organs in their normal condition and connections 
under the action of natural stimuli ; the latter of the two is partic- 
ularly doubtful. In a word, the different mechanisms of the human 
brain, in their normal condition and relations, constitute an in- 


ter-clejoendent and intimately related system ; what each so-called 
organ or centre does, or can do, depends not only upon its own 
structure and condition at the time, but also upon the condition 
and behavior of the other organs and centres at the same time. 
Such interde23endence extends not only to those divisions which 
gross anatomy can mark off and. consider under the name "the 
organs of the brain," nor simply to those minuter subdivisions 
which histology can distinguish by aid of the microscope ; it doubt- 
less also extends to the last details of that molecular mechanism 
which the brain-substance is. These details are different for 
every individual animal, and for every individual case. Specific 
differences belonging to the different species of animal life, as well 
as those idiosyncrasies with which pathology is familiar, must alike 
be recognized. It is by no means strange, then, that the physi- 
ology of the brain is able only very slowly and imperfectly to win 
from nature the truth, and to remove the reproach of apparently 
conflicting facts. 

In spite of the above-mentioned difiiculties certain results may 
be claimed as resting upon more or less of clear evidence regard- 
ing the specific automatic and reflex-motor functions of those inter- 
cranial organs that lie inferior to the cerebral hemispheres. The 
case of these hemispheres themselves will be subsequently consid- 
ered in detail. For they are those portions of the nervous mechan- 
ism about the immediate correlation of which with the phenomena 
of consciousness there can be no doubt. Since we are now con- 
sidering the nervous system and its, central organs merely as a 
physical mechanism, we definitely rule out, as far as possible, all 
allusion to any special relation between it and the phenomena of 
self-conscious mind. 

§ 15. Besides the spinal cord, the Medulla Oblongata is the cen- 
tral organ concerning whose automatic and reflex-raotor functions 
the largest amount of precise information exists. The reflex-motor 
functions of this organ are more intricate and of a higher order 
than those belonging jjrimarily to the cord. They are especially 
such as stand related to the vital functions of the heart and blood- 
vessels ; to respiration and its allied movements of the organs in 
coughing and sneezing, etc. ; to the movements of the muscles in 
swallowing and vomiting ; to the mimetic movements of laughing, 
weeping, etc. Among the different movements in the execution of 
which the medulla oblongata is concerned, some are more purely 
reflex and some less so. Thus one cannot swallow if the sen- 
sory tracts from the throat to this central organ are broken ; but 
the movements of the heart and lungs continue after the reflex- 


motor paths to them are destroj-ed. Sensory stimulations of the 
medulla oblongata, as a rule, occasion reflex movements by second- 
ary stimulation of a number of motor tracts. Swallowing, sneezing, 
coughing, shedding of tears, changes in respiration and in the 
movements of the heart, contortions of the countenance, may all 
be occasioned, through the mediation of this organ, by one and the 
same sensory impulse. There is also a marked difference in the 
extent of the domain over which the motor results of stimulating 
the different sensory paths connected with the medulla spread 
themselves. Stimulation of the optic nerve occasions only very 
limited reflex movements, such as the winking of the eyes, the se- 
cretion of a few teai's, and a slight tendency to sneeze. Stimula- 
tion of the nerves of taste extends over a wider area of motor 
tracts ; that of the palate and lar^^nx still wider. 

§ 16. The most important reflex centres of the medulla oblon- 
gata are also automatic ; of such centres he chief ai"e those con- 
nected with breathing, the movements of the heart, and the inner- 
vation of the blood-vessels. The excitation in these cases must be 
considered as a neural process arising within the central organ 
itself. The cause of its origin is doubtless to be found in the 
changes that occur in the supply and character of the blood. Not 
only all abnormal conditions of respiration, like dyspnoea and 
apncea, but also the rhythm of normal respiration, are dej)endent 
upon the changing condition of the blood with respect to its more 
or less perfect oxidation. The stimulus to action of the respiratory 
centre in the medulla, from the condition of the blood, may be in 
part reflexly applied through the peripheral ends of the afferent 
nerves in various parts of the body ; but the main effect is doubt- 
less produced by the direct action of the blood on this centre. Its 
rhythmic nervous action may then very well be dependent upon the 
rhythmic action of the lungs, and upon the resulting periodic re- 
oxidation of the blood. For the nervous substance of the medulla 
oblongata seems to be peculiarly susceptible to the condition of 
the blood. 

§ 17. This small central organ into which the spinal cord ex- 
pands on entering the skull may then be said to be thickly 
crowded with reflex and automatic centres. To speak of the more 
important will best serve to exhibit what is known of its mech- 

The respiratory centre was first located by Flourens in that part 
of the medulla oblongata which serves as the place of origin for 
the vagus nerve, and then more definitely in the V-shaped apex 
of the fourth ventricle, or beak of the calamus scriptorius. Since 


extirpation or injury of this small portion of the nervous sub^ 
stance, when all other parts of the body are left intact, causes 
immediate and final cessation of respiration, Flourens called it the 
" vital knot " (nceud vital). Foster ' locates this centre below the 
vaso-motor centre, and between it and the calamus HcriptorUis. 
Schiff concludes that it is double, and Hes on either side in the 
region of the anterior part of the ala cinerea ; the function of each 
side, he thinks, is separate. In case of need it may be shifted 
slightly backward toward the spinal cord. The efforts of Gierke '' 
to fix it in a definite gi"oup of ganglion-cells were not successful. 
With this same centre all the modifications of respiration in sigh- 
ing, sobbing, yawning, crying, laughing, coughing, sneezing, and 
hiccoughing are connected. 

A nervous centre intimately connected with the vaso-moto?' sys- 
tem of the different parts of the body exists in the middle part of 
the medulla oblongata. Since we cannot examine experimentally 
the efi'ect upon the action of this centi-e which would be produced 
by severing all the afferent nerves that lead into it, we cannot 
demonstrate dii'ectly how much of its action is automatic, how 
much reflex. It is probably both automatic and reflex. But the 
removal of the parts in frout of the medulla, inclusive of the cor- 
pora quadrigemina, exercises no perceptible influence on the blood- 
pressure. The principal vaso-motor centres in the brain are then 
found in this portion of the medulla oblongata. Through it reflex 
motions are called forth of the most different kinds, and involving 
muscles widely separated from each other and from the region of the 
skin where the stimulus is applied. Witness the effect of a draught 
of air upon the circulation of the blood. The arteries of a i-abbit's 
ear can be made to contract by stimulating any one of more than a 
half-dozen different sensory nerves, including the sciatic plexus. In 
this same central organ must be located the so-called cardio-inhib- 
itory centre. In cases where the heart is stopped by sudden and 
great emotion, or by severe pain, the stimulus probably reaches the 
medulla from the hemispheres of the brain. 

The centre of deglutition lies in the medulla higher up than that 
of respiration. If this part of the organ be destroyed, swallowing 
is impossible. This centre has been located in the floor of the 
fourth ventricle. In the floor of the same ventricle, and in the 
adjoining region, are j^i'obably located centi-es for different secre- 
tions — as, for example, of spittle, or sweat, of tears, and possibly 
of the pancreatic and other digestive juices. The connection of 

' Text-Book of Physiology, p. 370. 
" See Ptliiger's Arcliiv, vii. , pp. 583 ff. 


various sensations and emotions with these secretions is too famil- 
iar to need description. A central mechanism for winking the 
eyes Esner would place near the beak of the calamus scriptoriun. 
The centi-al mechanism for the reflex movement of the muscles of 
the oesophagus and stomach also lies in the medulla oblongata. Of 
the centre for the production of artificial diabetes, and of other 
more conjectural centres which are packed within this small bit 
of nervous matter, scarcely more than an inch in length, we do not 
need to speak. 

§ 18. The alleged functions of the medulla oblongata in the co- 
ordination of the movements of the skeletal muscles ally this organ 
more closely with certain other inferior parts of the brain. The 
prepai\ation of a frog which has retained this organ, in addition to 
the spinal cord, although without any of the rest of the brain, will 
execute movements of the muscles that are not possible for the cord 
alone. It will not, indeed, move spontaneously ; it still requires 
external stimulation to start the mechanism of such a preparation. 
Under such stimulation, however, it will assume a j)ositiou natural 
to it in an uninjured state. When laid on its back it will make 
efforts— generally unsuccessful — to turn over. The movements of 
the limbs with which it responds to various sensory impulses are 
more complicated than those executed by the spinal cord alone ; 
they even resemble crawling motions or short leaps. Placed in the 
water, what is left of the animal will swim ; and if its motions are 
less perfect than those of the perfect frog, they are much more so 
than those of the cord alone. It is doubtful whether, when placed 
beneath the water, it will ascend to the surface to breathe, or 
make efforts to escape from water gradually heated to about 104° 
Fahr., — as will the animal that retains its cerebellum and optic 

Reflex movements of considerable complexity can also be exe- 
cuted by mammals that have been deprived of all the enceiDhalic 
centres above the medulla. Vulpian claims that a yoang rat in 
this condition will emit a cry, as of pain, when its toes are pinched. 
Such a mechanism will swallow and execute cei-taiu co-ordinated 
movements of the limbs. Infants whose nervous centres above the 
medulla are undeveloped will perform the associated movements 
of sucking when put to the breast. Moreover, the effects of le- 
sion of the centres of the medulla are very marked in respect to the 
co-ordination of motion. Eolando observed that convulsive move- 
ments followed extensive injury of this central organ. More recent 
researches seem to show that the seat of these epileptiform move- 
ments is at the place of union between the medulla and the pons ; 


it can, therefore, scarcely be located in either alone.' One-sided 
lesions are followed by certain so-called " forced " and rotai-y move- 
ments of the head, and eyes, and trunk. Such effects are most 
likel}' to be produced when the injury affects the region of the 
taherculum acusticum. In the opinion of Bechterew " the olivary 
bodies are in relation with the gray matter of the third ventricle, 
and with the semicircular canals, as central organs for the co-or- 
dination of the muscles used in balancing according to impressions 
of touch. It would then be one chief function of the medulla to 
secui'e equipoise through these sensory impressions. On the 
whole, it appears certain that considerable work in co-ordinating 
the muscular movements falls upon its mechanisms. Of such work 
it is probable that the movements concerned in articulate speech 
are a part. Any indirect relation which it may have to the produc- 
tion of those sensations and images which are woven into our 
dreams does not belong in this connection. 

§ 19. The associations among the different centres of the me- 
dulla oblongata are curious enough ; they involve an extremely 
intricate physiological apparatus. Some of these centres are in- 
directly connected with psychical activities. They are not all alike 
excitable ; they are not all voluntarily so. Thus we can volun- 
tarily control, within certain limits, the movements of the lungs, 
but not those of the heart and blood-vessels ; we can cough, but 
cannot sneeze, at wiU. Some of their functions are associated 
together regularly ; some of them seldom ; some never. Swallow- 
ing is not necessarih' connected with the activity of the other cen- 
tres, unless it be with that for the secretion of saliva ; it takes 
place, however, during arrest of respiration. The excitation of no 
other centre necessarily affects this centre. The secretion of saliva 
is constantly connected with a change in the circulation through 
the submaxillary glands. 

§ 20. An animal which possesses all, or a considerable part of 
the other nervous mechanisms of the brain that lie below the cere- 
bral hemispheres is capable of executing movements which differ 
greatly from those already described as belonging to the spinal 
cord and medulla oblongata. Very few of the movements of such 
a preparation are, indeed, even apparently spontaneous ; for al- 
most all of them a definite form and degree of stimulus acting on 
the sensory surfaces can be assigned. We are inclined, then, to sus- 
pect that those movements which are apparently spontaneous are 
really due to some stimulation from wdthout the central organs 

'See Eckliard, in Hermann, Handb. d. Physiol., II., ii., p. 98. 
'Pfluger's Archiv, xxxi., pp. 479 If., aud xxix., p. 258 f. 


which has escaped our observation. But the range of reflex-motor 
activities vrhich an animal deprived simply of its cerebral hemi- 
spheres will execute, in i-esponse to appropriate stimuli, is very 
great ; it may be said to include every form of movement possible 
for the uninjured animal. The statement is, therefore, wan-anted 
by all our knowledge of the facts, that the medulla, pons, crura 
cerebri, cerebellum, corpora quadrigemina (or optic lobes), and 
basal ganglia generally are the special mechanism for co-ordi- 
nating the movements of the muscles with the various impulses of 

A frog from which the cerebral lobes have been removed will 
respond to appropriate stimuli with all the movements of which a 
perfect frog is capable. It will swim, leap, and crawl. When 
placed on its back, it will easily and at once regain its natural posi- 
tion. When placed on a tilting board, it will constantly adjust 
the position of its body so as to maintain an equilibrium. It will 
croak with the regularity of a music-bos when its flanks are gently 
stroked. Thrown into the water it will swim with great regularity 
of motion until it is exhausted or finds something — as a small piece 
of wood placed in contact with it — upon which it can crawl. When 
submerged in the water, it will rise to the surface for air ; it wall 
not, like a mere spinal cord, remain quietly in water the temper- 
ature of which is gradually raised, but will make violent efibrts to 
escape. It is guided by the light, for it avoids objects that cast a 
strong shadow. On the other hand, it appears stupid ; it jDays no 
attention to the flies that are placed near it ; by careful exclusion 
of all stimuli it may be kept motionless for hours. We cannot 
argue from this, however, that it is without sensations, for it may 
not be hungry ; and Heubel ' asserts that a sound frog may, with 
careful manipulation, be made to lie still upon its back for a long 

Similar phenomena occur in the case of the mammal whose cere- 
bral hemispheres have been removed. The rabbit or rat thus 
operated upon will stand and run and leap. Placed on its back, 
it will regain its feet. It will follow with its head a bright light 
held in front of it ; it will start and tremble, or run, at a shrill or 
loud noise. It will utter a prolonged cry when pinched. Its mus- 
cular motions are obviously co-ordinated in response to sensory 
impulses from the organs of touch, hearing, and sight. The bird 
thus operated upon will easily regain its feet when laid upon its 
side or back, and will stand in a natural and easy posture. It will 
tuck its head under its wings, clean its feathers, and pick up corn 
' Pfluger's Archiv, xiv. , pp. 163 ff. 


or drink water presented to its beak. Thrown into the air, it will 
fly with considerable precision for some distance, and in its flight 
will guide itself, though imperfectly, so as to avoid obstacles in its 
way. It will start at sharp sounds or flashes of light. Such ani- 
mals have on the whole the appearance of being sleepy and stupid 
rather than of being deprived of any of their powers for co-ordi- 
nating sensation and motion. We conclude, then, that the organs 
which such animals possess are functionally capable of exex'cising 
all these powers of co-ordination ; we do not at present raise the 
question whether this implies the existence of psychical j)henomena 
or not. The phenomena which follow the partial loss of the cere- 
bral hemispheres in the higher mammals confirm the same conclu- 

It is much more difficult, however, to assign the special place 
which belongs to each of the organs that lie between the medulla 
oblongata and the cerebral hemispheres, under their general func- 
tion as already stated. They are all very intimately related ; act 
to a large extent dependently ; can, within certain limits, assume 
each other's functions ; and have largely the same connections with 
the peripheral organs of sense and of motion, and the same Avork 
to do as mediating between the two. 

§ 21. It is impossible to determine the special functions of the 
Cerebellum, so conflicting is the testimony of different experiment- 
ers. A high degree of probability, however, attaches itself to the 
statement that this organ is largely concerned in the co-ordination 
of motion ; although such statement cannot be held to exhaust its 
functions. The more specific theory of Wundt ' — "It is the central 
organ that brings such movements of the animal's body as are ex- 
cited by impulses from the cerebrum, into accord with its situation 
as a whole in space" — is more doubtful, precisely because it is 
more specific. Comparative anatomy seems to show that the office 
of the cerebellum in some animals differs from its office in man ; 
reasoning from the former to the latter is, therefore, especially pre- 
carious. Moreover, its functions are so closely connected anatom- 
ically with those of the pons, the crura cerebri, and the medulla, 
that it is difficult precisely to separate its work from that done by 
these organs. 

Testimony as to the result of the extirpation or lesion of the cere- 
bellum is vei-y conflicting. Apparently almost the entire length 
and breadth of its surface (in the direction of the posterior bones 
of the skull), and not only the gray matter, but also the white, as 
far as near the bifurcation of its strands, may be removed without 
'Grundzuge d. physiologische Psychologie, i., p. 301. 


any observable result.' On approacbing the middle of its thick- 
ness and removing the strands connected with the middle peduncles, 
disturbances of motion begin and increase rapidly in proportion to 
the amount of substance removed. Most of these disturbances, if 
the animal recovers w^ell, prove to be only temporary ; they are, 
therefore, probably due largely to traumatic excitation. Permanent 
disturbances, however, occur when the injuries reach the lower 
third of the organ, or when they are confined to this third. Vulpian 
accordingly concludes that the disturbance of gait which results 
from injury of the cerebellum, is due to the irritation of its more 
profound white parts or of the adjoining cerebral isthmus. But 
Sehiff believes that the mass of the organ, apart from locality, has 
a definite influence upon the co-ordination of the bodily movements ; 
though what that influence is cannot yet be clearly defined. The 
influence of locality seems to be considerable upon the effect which 
results from lesions in a given amount of the cerebellar substance ; 
but since this influence is much more marked near the connections 
of the cerebellum with other contiguous organs, some observers 
attribute it largel}- or wholly to the injury — by extension of the 
lesion, by pressure, or by inflammation — of these organs. Thus 
the place of its union with the medulla oblongata and the regions 
near the crura cerebri are especially important. But Schifi" found, 
in experimenting upon mammals, that complete vertical section of 
the cerebellum, in the exact median line of the vermiform process, 
and removal with the knife or pincers of the entire substance, with 
the exception of the flocculi and the parts external to the peduncles, 
produced no appreciable loss of the power of co-ordination. 

The efi'ect of one-sided lesions of the cerebellum in the disturb- 
ance of motion seems to be, as a rule, much more certain and 
marked than that of symmetrical lesions of both sides. Sehiff, in- 
deed, asserts that when a bilateral lesion is perfectly symmetrical 
it pi'oduces no impairment whatever of the functions of motion. 
But the entire evidence from experiment shows that sudden lesion 
of one hemisphere of this organ is almost uniformly followed by 
at least temporary impairment of the motor functions. Section of 
the middle peduncle of the cerebellum of a bird or mammal almost 
always occasions so-called " forced " movements ; the animal rolls 
around its own longitudinal axis, generally, though not invariably, 
toward the injured side. Nystagmus, or the peculiar rolling move- 
ment of the eyes suggestive of vertigo, and strabisrnus, take place 

' Compare Vulpian, Lemons sur la Physiologic, etc., pp. 603 ff.; Eckhard, in 
Hermann, Handb. d. Physiol. II., ii., pp. 102 ff.; Sehiff, m Pfluger's Archiv., 
xxxii., pp. 427 ff. ; and Ferrier, Functions of the Brain, pp. 85-123. 


in such cases. One eye may be moved inward and downward, the 
other outwai-d and upward. Hitzig ' and Ferrier ° found the same 
results to follow injm-y of the lateral lobe. The latter observed that 
strong stimulation of the cerebellar surface with the interrupted 
current causes associated movements of the eyes and head and 
hmbs, in cats and dogs and monkeys. But these effects may 
be largely due to the connection of the cerebellum with the me- 
dulla oblongata. 

The evidence from pathological cases in man conflicts, to a con- 
siderable extent, with the conclusions which we might hasten to 
derive from experiment upon the animals. According to Vulpian ' 
it is by no means rare to have unilateral lesions of the cerebellum 
followed by no paralysis of either side. In a gTcat number of such 
cases no genuine hemijDlegia results ; the resulting enfeeblement of 
motion, moreover, is as often on the same as on the opposite side. 
M. Andral is said to have made a collection of ninety-three cases 
of diseases of the cerebellum, in only one of which ataxy was ob- 
served in any marked way. In most cases where crossed hemi- 
plegia does result, Vulpian thinks it due to the destruction or 
compression of the adjacent parts, especially the roots of the cere- 
bellar peduncles. The same authority denies that the superficial 
parts of this organ are excitable, or that lesion of them is followed 
by pain or by convulsions of the body, face, or eyes. Such results 
do, however, follow excitation and lesion of its deeper parts, in 
proportion to the degree of approach to the peduncles. The dis- 
crepancy between experiment and pathology may perhaps be re- 
moved, at least in part, by remembering that the injury is sudden 
in the one case and not in the other. Moreover, few of the patho- 
logical cases are clearly enough defined to serve as a sure basis for 
conclusions. Some of them, however, would seem to warrant cer- 
tain inferences. More than fifty years since, the well-known case of 
the girl Alexandrine Labrosse was reported by Combette,'' and after- 
ward made known to students of physiology generally by Longet.^ 
This girl was found, on post mortem, to have no cerebellum ; in its 
stead was a gelatinous membrane attached to the medulla by two 
peduncles of like construction. A true pons was also wanting, but 
no loss of substance seemed to have taken place here. Yet she 
could co-ordinate all the limbs voluntarily, and had the f\ill use of 

' Untersuchungen iiber d. Gehirn, pp. 198 ff. 

^Functions of the Brain, p. 106 f. 

^ Lef;oiis sur la Physiologie, etc. , p. 607 f. 

* Revue madicale, II., p. 57 (1831). 

^ Anatomie et Physiologie du Systeme nerveux, I., p. 764 (1842). 


all the senses. She was, however, subject to falling (se laissait 
tomber souvent) and spoke imperfectly. Bouillaucl has reported 
another case of an adult whose entire cerebellum was changed into 
a brown purulent mass ; this patient could walk, tliough in a tot- 
tering and insecure way. Vulpian ' also describes a case which 
came under his own observation. A woman, dying at the age of 
sixty-nine, after twenty years in the hospital of La Salpetriere, was 
found to have suffered an entire atrophy of all the cortical gray 
substance of the cerebellum. This patient preserved great muscu- 
lar vigor, and could co-ordinate all the muscles ; but her "locomo- 
tion " wag disordered and difficult. 

On the whole, then, it must be admitted that the evidence con- 
cerning the sj)ecific functions of the cerebellum of mammals, and 
especially of man, is not such as to warrant us in making definite 
affirmations. Scarcely a single case can be adduced in which it 
is not j)0ssible to maintain that the motor disturbances which fol- 
lowed lesion or excitation of this organ should be ascribed to an 
indirect effect upon contiguous organs. Yet the coincidence of 
evidence from several different lines gives sufficient support to the 
view that the functions of the cerebellum are in some way con- 
nected with the balancing, and therefore with the precise and se- 
cure locomotion of the body in space. More definitely, with refer- 
ence to the nature of this connection, it is not possible to speak 
confidentl}'. No disturbance of the senses of hearing, of sight, or 
of muscular feeling, can be shown to follow injuries of this organ 
where other parts of the brain are not involved ; on the contrary', 
all these senses appear to have been perfect in certain cases of the 
complete absence of this organ. The only disturbance of sensi- 
bility which frequently follows affections of the cerebellum is ver- 
tigo ; the same symptom can be produced by passing a current of 
electricity through the back part of the head, or by the effusions 
of blood in this region which are sometimes occasioned by alcohol. 
Vulpian and others are, however, probably right in holding that 
the result is only indirectly to be ascribed to this organ. Indeed 
the view of Schiff has much in its favor : this view maintains that 
the aberration of motion due to lesion of the cerebellum should 
not be called a loss of co-ordination at all, since all the limbs may 
be moved in exactly the right relations necessary to carry the body 
forward or to maintain its equipoise ; bi,it the precision of the mo- 
tion is impaired, because the nervous impulses from this organ 
that innervate the neighboring groups of muscles are not rightly 
adjusted to each other in amount along the difiereut tracts. The 
' Lemons sur la Physiologic, etc., p. 629. 


balance of the innervating cells is destroyed ; and the result is a 
loss of nice adjustmeut of the amount of innervation sent to the 
particular muscles employed in equipoise and locomotion. 

It scarcely need be added that modern physiology distinctly dis- 
proves the hypothesis of Gall, who connected the sexual instinct 
Avith the cerebellum. There is no good evidence that the hinder 
brain directly participates in any way in those activities of the 
nervous system which are immediately correlated with psychical 
phenomena, whether of emotion, instinct, or intelligence. 

§ 22. The functions of only three other parts of the encephalon 
require consideration in this connection ; these are the corpora 
quadrigemina, the optic thalami, and the corpora striata. The 
crura cerebri and the pons Varolii are, as we have already seen, 
significant chiefly as organs of conduction. So far as they have 
also the intermediating functions of central organs, it is not possi- 
ble to treat of them otherwise than as concerned in that general 
reflex-motor mechanism which occupies all this region of the brain. 

§ 23. Experiments upon the Corpora Quadrigemina are rendered 
especially difficult by the small size and deep situation of these 
organs ; they cannot easily be exposed for stimulation without 
great effusion of blood, or subjected to lesion without extending the 
injury to contiguous parts. These difficulties render conclusions 
from the effect of stimulating or extirpating the corresponding 
organs (optic lobes) of the frog more than usually precarious. 
There is no doubt, however, as to some sj)ecial connection between 
the corpora quadrigemina and sensory impulses of sight ; such con- 
nection is, then, of course, to be extended to those motor activities 
that are dependent upon the sensory impulses of sight. Flourens 
and many subsequent observers have found that one-sided extirpa- 
tion of the optic lobes of birds, or of the corpora quadrigemina of 
mammals, with the cerebral hemispheres intact, produces blindness 
in the ojDj^osite eye. The amount of this blindness is different in 
different animals, as the decussation of the fibres in the optic chi- 
asm is more or less complete in different animals. In the rabbit 
such decussation appears to be complete ; in the cat and dog in- 
complete. The fact that hemianopsia in both eyes is connected 
with disease of one side of the brain is an evidence that it is incom- 
plete in man also. Moreover, when the brain is removed in front 
of the corpora quadrigemina, and these organs left intact, the ani- 
mal can still guide and co-ordinate its motions in response to visual 
impulses. (We do not in this place consider whether we are war- 
ranted in calling these impulses " sensations " — not to say "percep- 
tions" — of sight.) These organs are, then, in some sort, central 


organs of sight. Since they are connected by nerve-tracts -with the 
cortex of the cerebrum, motor innervation in response to stimulus 
from the optic nerve may arise either immediately in the corpora 
quadrigemina themselves or in the gray matter of the cortex. We 
may therefore suppose, with Wundt,' that destruction of the cere- 
bral substance abolishes only those movements of the muscles, in 
response to the stimulus of light, which involve complicated co-or- 
dinations with other excitations of sense, or with earlier established 
experience. It is scarcely allowable, however, to locate this special 
relation to visual imjDulses definitely in the substance of the corpora 
C[uadrigemiua considered as isolated from the optic thalami, the 
optic tracts, and the gray matter at the floor of the third ventricle. 
There is sound sense in Eckhard's ^ remark that the functions 
commonly attributed to these bodies should rather be ascribed to 
the region in which they lie. The nates (or anterior pair) seem to 
be more especially connected with the sensory, and the testes (or 
posterior pairj with the motor activities of sight. 

Abnormal movements of a " forced " nature, and impairment of 
the power of co-ordination, follow the injury or extii-pation of the 
corjDora quadrigemina. These phenomena may be due in part to 
the loss of guidance by visual impressions ; but they are probably 
due chiefly to the extension of the efiiects of the injury to the crui-a 
cerebri and other surrounding parts. The o'^iia lobes, according 
to Groltz, are the principal central mechanism for the croaking of 
the frog deprived of its hemispheres. Vulpian ^ makes a distinction 
between a mei^ely reflex-motor cry and the plaintive utterance of 
an animal {e.g., the rabbit) which retains these organs and the pons 
Varolii. Ferrier,^ however, was unable to make the distinction so 
clearly. The latter observer found that very marked phenomena — 
such as dilating the pupils, clenching the jaws, retraction of the 
ears and angles of the mouth, extending the legs, etc. — followed 
the stimulation of these organs with an electrical current, in the 
case of cats and dogs. But his experiments do not enable us to 
say how much of all this belongs to the specific function of the 
corpora quadrigemina as central organs, and how much to the irri- 
tation of the nerve-tracts in all the surrounding region. "While we 
seem warranted in connecting these organs with the cerebellum, 
medulla, and pons, as concerned in the co-ordination of motions 
necessary for equipoise and locomotion, it is not safe at present to 
attempt a more precise localization of function. 

1 Physiologische Psychologie, i , p. 184. 

'^ In Hermauu's Handb d. Physiol., II., ii. , p. 131. 

^ Lecous sur la Physiologie, etc., p. 541 f. 

* Functions of the Brain, p. 76. 


§ 24. The office of the so-called basal ganglia— Optic Thalami 
and Corpora Striata — in that "projection-sj^stem" which connects 
the cerebral hemispheres with the periphery of the body, has 
already been spoken of; one chief function of these ganglia has 
usually been held to be that of acting the part " of middleraen be- 
tween the cerebral convolutions and the rest of the brain." ' But 
they both have further functions as specifically central organs in co- 
ordinating the movements. of the body according to impressions of 
sense. It is difficult, if not impossible, however, to define precisely 
what these functions are. Some special relation of the optic 
thalami to impressions of sight must be admitted. The fact that 
animals deprived of the cerebral hemispheres are capable of com- 
plex co-ordination of their muscles as reflex effects of visual im- 
pressions, seems to indicate that the mechanism of the optic 
thalami is associated with that of the corpora quadrigemina in 
performing this function. In mammals complete extirpation of 
the posterior portion of one thalamus results in permanent ex- 
pansion of the pupil of the opposite eye ; and Renzi was confident 
that injury of the upper surface of the anterior portion occasioned 
bhndness. Lussana and Lemoigne found blindness in the opposite 
eye to be the invariable result of lesion of one thalamus. Cases of 
the disturbance of vision, or even of complete blindness, have been 
observed in human patients as the apparent result of disorgani- 
zation of this organ. It must be admitted, however, that the sig- 
nificance of the optic thalami for vision may be due simply to the 
fact that certain fibres of the optic nerve have their origin in it, 
and are rendered inoperative by injuring it. Experiments and 
jjathological cases connecting the optic thalami with the sensations 
of smell and taste are more doubtful and conflicting, Ferrier ^ con- 
cludes that lesions in and around this organ destroy the cutaneous 
sensation of the opposite side of the body in the monkey ; Veyssiere 
found the same thing true in dogs. But Nothnagel found that no 
effect upon sensation followed the destruction of these organs in 
the rabbit. Not a few cases of disease of the oj^tic thalami in man 
seem to point to some connection with tactile imjDressions ; other 
cases, however, are decidedly unfavorable to this view. On the 
basis of this rather meagre evidence "Wundt ^ is willing to rest the 
theory that the optic thalami are special centres for the reflex- 
motions of touch ; by the same theory he also accounts for the dis- 
turbances of motion which follow injury to these organs. He 

' See Foster, Text-Book of Physiology, p. 653. 
''■ Functions of the Brain, pp. 288 ff. 
* Physiologische Psychologic, i. , p. 188. 


thinks it probable, nevertheless, that their function is not exhausted 
by this description. "Forced " positions and movements, and various 
other marks of impaired motor activities, follow the experimental 
lesion of these organs. But such disturbances largely or wholly 
vanish after a brief time, although they can be again called out by 
stimulation. They occur, as a rule, only when the lesion affects 
the posterior part of the thalamus, or the edges of the opening 
leading from above into the third ventricle. Most of the phenom- 
ena may be explained as due to the working of a mechanism that 
has been stimulated to abnormal activity by the mechanical irrita- 
tion due to the extirpation.' We can scarcely, then, be anymore ex- 
plicit than to quote the remark of Vulpian, made some years since : " 
"We know nothing of the special functions of the optic thalami." 

§ 25. The special motor significance of the Corjjora Striata is 
undoubted ; although we cannot go to the length of holding that 
these bodies are concerned only in the elaboration and downward 
transmission of efferent impulses. Ferrier ^ and others have ob- 
served that stimulating these bodies with an interrupted current 
produces strong convulsive movements of the opposite side of the 
body ; with a very powerful stimulus the whole side is drawn 
into an arch. No such effect could be produced by stimulating 
the optic thalami. Ferrier holds ^ that "in man and the monkey 
there is little, if any, difference perceptible between the complete 
destruction of the cortical motor-centres and destruction of the 
corpus striatum." Vivisection of this organ is sometimes followed 
by hasty forward running motions. Lesions of the striate bodies, in 
the case of the animals, are usually followed by laming of the limbs 
of the opposite side ; sometimes, however, no pathological symp- 
toms result. As a rule, in the case of man, paralysis of the arms 
and legs of the opposite side follows disease of these organs. Here, 
as elsewhere in this region of the nervous system, a certain sud- 
denness of the disturbance appears necessary to secure any marked 
result. Some experiments seem to point to a difference in the 
effects of injury to the two main nuclei of the corpora striata. 
Nothnagel asserted that all mechanical injury to the nucleus len- 
ticularis of one side results in laming of the opposite side : destruc- 
tion of this nucleus on both sides brings the animal into nearly 
the same condition as the removal of the cerebral hemispheres. 
But voluntary movements persisted after complete destruction of 
both the nuclei caudati of the rabbit. 

' Comp. Eckhard in Hermann, Handb. d. Physiol., II., ii., p. 125 f. 

^ Lemons sur la Physiologie, etc., p. 659. 

3 Functions of the Brain, p. 161. "Ibid., p. 349. 


There is much evidence, then, to show that the corpora striata 
are, as compared with the optic thalami, more especially connected 
with motor activities. AVundt * considers them to be pre-eminently 
sio-nificant as ganglia for the co-ordination of those motor impulses 
which are derived from the cerebellum and the cerebrum. The ' 
relative importance which they seem to have in the higher-, as com- 
pared with the lower, animals (the monkey and man as compared 
with the rabbit, etc.) he thinks is like that of all the anterior por- 
tions of the brain ; such portions are in general, more significant 
in man than in the other animals. Wundt's view has considerable 
in its support — among other things, the fact that, in case of lesions 
of the striate bodies, voluntary motions, or those motions whose 
motor innervation originates above these organs, seem to suffer 
most. But we positively must not adopt without qualification the 
statement '^ that the corpora striata are exclusively motor, and the 
optic thalami exclusively sensory. In addition to what has already 
been said (p. 129) to caution one against this view, it may now 
be added that numerous cases are recorded where injury, appar- 
ently confined to one corpus striatum, has resulted in loss of feeling 
on the opposite side ; and other cases where disease, apparently 
confined to one optic thalamus, has caused loss of motion as well 
as of sensation. Moreover, the chief motor effects of injury to the 
striate bodies (if not all of them) may be due to the fact that the 
descending motor tracts are necessarily involved in the injury, 
rather than to any special motor function belonging to these 
bodies as a central organ. Another theory of the office of the 
striate bodies rejects entirely the view which regards them as in 
any true sense basal ganglia, with either specially motor or specially 
sensory functions ; and regards them as belonging to the cerebral 
hemisj)heres, rather than subordinate to the hemispheres in func- 
tion.^ But inasmuch as this theory has its principal support, of a 
physiological kind, from a single case of an idiot's brain, in which 
these bodies were of nearly normal size, while the cortex was defi- 
cient in the motor regions and the base of the brain in general 
small, it can scarcely be regar-ded as sufficiently confirmed. 

§ 26. The researches of the last few years have tended to show 
that some S23ecial relation exists between the nervous substance of 
the organs lying at the base of the cerebrum, and the temperature 

' Physiologische Psychologie, i., p. 193 f. 

^ As propounded by Carpenter and Todd, and apparently adopted by Fer- 
rier, Functions of the Brain, 2.52 f. 

'^ See A. Hill, The Plan of the Central Nervous System, p. 276 ; and Jour- 
nal of Anat. and Physiol., July, 1885. 


of the body. The earlier observations ' pointed out the limits be- 
tween the medulla oblongata and the pons as a region, lesion of 
which was followed by a sudden and large rise of temperature. 
Still later, other observers ascribed vaso-motor functions to the 
optic thalami,^ or asserted the existence of vaso-motor fibres in the 
crura cerebri (so Budge). In 1884, J. Ott pointed out that cutting 
the corpora striata is speedily followed by a marked rise of tem- 
perature. Yet more recently two experimenters,' working together, 
have arrived at certain conclusions based upon a large number of 
experiments, chiefly on rabbits, but also on guinea-pigs and dogs. 
They discover that, while the cortical substance can be subjected 
to the most severe and extended lesions without producing a fever- 
ish rise of temperature, puncturiDg the brain at the juncture of the 
sagittal and coronal sutures, down to the level of the striate bodies 
or deeper, invariably j^roduces a marked rise of temperature. If 
the lesion only hits the striate bodies (especially the medial side, 
near Nothnagel's nodus cursorius) the coming-on of the fever is 
slow and gradual ; but if the needle is carried further toward the 
base of the brain, the fever springs up at once and reaches a max- 
imum in two to four hours. In what way these organs act as 
" fever-centres," or precisely what nervous elements are chiefly in- 
volved in the action, has not yet been made clear. 

§ 27. Eckhard * is inclined to lay down the law that in all verte- 
brates the mechanisms for a change of place lie rather in the ante- 
rior part of this general region — corpora quadrigemina, etc.; while 
those for maintaining the upright posture and the equipoise of the 
body are localized in the region of the pons, cerebellum, and me- 
dulla oblongata. 

§ 28. It should be added that almost all observers have hitherto 
failed to attach sufficient importance to the central functions of 
the gray matter which lines the floor and walls of the third ven- 
tricle. Bechterew ^ has recently contributed the results of very 
important exiDeriments to determine the specific function of this 
central nervous substance. He finds that frogs retain the function 
of balancing even when the optic lobes are crushed, if no injury is 
done to the gray substance of the third ventricle or to the crura 

' By Tschetscliicliin, in Archiv. f. Anat. u. Physiol., 1866, pp. 151 ff. ; and 
Sclireiber, Pfliiger's Archiv., viii., pp. 576 ff. 

" Lussana and Christiani (No. 16 of the Verhandlungen d. physiolog. Gesell- 
schaft zu Berlin, 1883-84). 

^ Ed. Aronsohn and J. Sachs: See Pfliiger's Archiv., xxxvii. (1885), ppi 
232 ff. 

^In Hermann's Haudb. d. Physiol., II., ii., p. 138. 

5 Pfliiger's Archiv, xxxi. (1883), pp. 479 ff. 


cerebri ; they lose this function, however, when a section is made 
into the third ventricle. Birds (hens and pigeons), also, show the 
same loss of function when a lesion is produced by running a very 
fine needle into the cavity of this ventricle. In the case of dogs, 
Bechterew considers himself able to localize the function of equi- 
poise precisely, and to point out the special effect of injury done 
to different definitely fixed localities. For example, bilateral lesion 
of the lateral or postero-lateral parts of the wall of the third ven- 
tricle results in the impairment or loss of equipoise and co-ordi- 
nated motion on both sides of the body : the lost function is re- 
gained only after a long time, and then but partially. In none of 
these cases were any of the phenomena of motor laming of the 
extremities apparent, or any very marked disturbance of sensation. 
This gray matter of the third ventricle operates, Bechterew thinks, 
in connection with the olivary bodies for the co-ordination of motor 
impulses in response to sensations of touch, and with the semi- 
circular canals in response to sensations of sound. It is especially 
important also in equipoise through visual impulse connected Avith 
the changes in the axial direction of the eyes. Thus all the above- 
mentioned organs operate with the cerebellum as complex and 
correlated mechanisms for keeping the body balanced in response 
to changing sensory impulses. 

We stop at this point in our ascending review of the automatic 
and reflex-motor functions of the central mechanisms. For dis- 
tinctly psycho-physical and psychological questions the most im- 
portant of the activities of the nervous mechanism still await our 
examination ; these are the activities of the cerebral hemispheres. 
But nothing is known as to the molecular structure of these hemi- 
spheres, or as to their automatic and reflex-motor centres and activ- 
ities, which adds anything of importance to the description of the 
nervous system as a mechanism, or to the mechanical theory of its 
action. It is with such description and theory that we are now 
concerned. The correlations which exist between the structural 
condition, or j)hysiological function of the nervous system, and the 
phenomena of mind, are chiefly (if not wholly) capable of study as 
illustrated in the cerebral hemispheres. But the nature of the 
nervous molecular machiner}', and of its working as mere machinery, 
is understood, as far as our present information will permit, by an 
examination of the physiology of the spinal cord and of the inter- 
ci'anial ganglia lying below the hemispheres. As to the alleged 
psychical functions of these inferior organs we shall adduce further 
considerations when we come to consider such functions as belong- 
ing to the brain proper. 



§ 1. In order to understand the end-organs it is necessary to 
refer again to the place which they hold in the threefold arrange- 
ment of the nervous mechanism (compare Chapter II., § 2). In the 
general division of labor among its organs, certain cells situated 
at the surface of the body become especially sensitive to external 
stimuli. The special function of these cells accordingly becomes 
that of receiving the action of such stimuli, of modifying this action 
in accordance with their own peculiar structure, and thus of set- 
ting up in the conducting nerves the neural process which is prop- 
agated to the central organs. It is obvious, then, that the struct- 
ure and grouping of the superficial cells must bear some definite 
relation both to the external stimulus and also to the nerve-fibres 
Avhich convey inward the nervous impulse occasioned by it. The 
end-organs of sense may then all be described as special adapta- 
tions of the superficial cells to the different kinds of stimuli. With 
such special adaptations the peripheral terminations of the nerve- 
fibres must be connected. For the end-organs, as it were, look both 
outward and inward. They act as mediators between those different 
modes of external molecular motion which can occasion sensations 
in us, and the nerves which convey the results of this motion, when 
it has been changed into a nerve-commotion, onward to the central 

§ 2. In the end-organs of the special senses the fibrils of the 
sensory nerves, as a rule, terminate in cellular structures which have 
the morphological significance of metamorphosed epithelial cells. The 
end-organs of smell and taste show this characteristic development 
most clearly. These end-organs are, in genei-al, made up of cells 
which, posteriorly, pass into nerve-threads that are gathered to- 
gether into the sensory nerve of the special sense ; and which, an- 
teriorly, pass into conical or fusiform processes. The simplest type 
of an end-organ may then be described as follows : A hair-like pro- 
cess extending outward, and connected by a sensitive cell with a 
nervous filament extending inward. Such processes are probably 


extreinelj' sensitive to external stimuli ; and perhaps peculiarly so 
to tlie chemical changes which, at least in the case of three of the 
special senses (smell, taste, and sight), are their immediate excit- 

All the end-organs of sense may be regarded as modifications of 
the type described above. Only a small part, however, of what are 
ordinarily called " the organs of the special senses " {e.g., the nose, 
the mouth, the ear, the eye, the skin) belongs, strictly speaking, 
to the nervous system. By far the greater part consists of me- 
chanical contrivances, designed to prepare the external stimuli and 
conduct to the true nervous apparatus the impulses they occasion. 
These non-nervous mechanical contrivances, however, modify the 
nature of the stimulus in so important a manner as to merit some 
brief description in our consideration of the nervous mechanism, 

§ 3. Besides the end-organs of sense, histology points out another 
kind of terminal apparatus. The efferent nerves, in order that they 
may stimulate the muscles, must have some special form of attach- 
ment to them. Special contrivances for connecting the motor 
nerves and the muscles are actually discoverable. We distinguish, 
then, two classes of end-organs : first, End-organs of Sense, and, 
second. End-organs of Motion. 

§ 4. Among the end-organs of sense, those of Smell have been 
least successfully investigated. That portion of the mucous mem- 
brane of the nose which clothes the upper region of the nasal cavity 
and is marked by a brown-yellow color — the region of the expansion 
of the olfactory nerve — is called " regio olfactoria ;" it contains the 
end-organs of smell. Here Ecker and Eckhardt (in 1855) discovered 
two different kinds of cells ; but we are indebted to Max Schultze for 
the first detailed description of them. The epithelial portion of the 
olfactory organ is supposed to be constructed upon the same type in 
all the vertebrate animals. Of the two kinds of cells which the last- 
mentioned investigator described, one is called " epithelial," the other 
"olfactory." The epithelial cells are the larger, have an oval nucleus 
of considerable size, and extend through the whole epithelial layer. 
Their external half appears more or less cylindrical or columnar (at 
least in the Triton and Proteus), and is described by some observers ' 
as striated longitudinally. The form of the inner half of these cells 
is varied. The olfactory cells are spindle-shaped, with a large, 
round nucleus, and very long, fine processes. The external process 
is elongated into a stiff hair, at least in many cases, although 
Schultze considers that in man the olfactory cells have no cilia. 

' See Professor Babucliin in Strieker's Human and Comparative Histology, 
iii., p. 207 f. 



These cells are surrounded by the epithelial cells. Most physiol- 
ogists follow Schultzein holding that the two kinds of cells are dis- 
tinct both in form and in function, and that only the " olfactory " 
cells are connected with the end-fibrils of the nerve of smell ; Ex- 
ner ' and others, however, believe that the distinction is not a fixed 
one. In his opinion the structure of one is merged into that of the 
other, and both are connected, though in a different manner, with 
the subepithelial net-work in which the fibres of the olfactory nerve 
are lost. The exact histological relation of the fibrils of the ol- 
factory nerve to the epithelium of the regio 
olfactoria is not yet made out. It is probable, 
however, that the finest of these fibrils, after pen- 
etrating the epithelial layer, closely embrace the 
large epithelial cells and enter into connection 
with the inner extremities of the olfactory cells. 
According to Exner, the fibres of the nerve do 
not pass over directly into the processes of the 
end-organ cells, but are lost in a net-work whose 
interstices are filled up with granules of nervous 
matter. The first pair of cranial nerves, the ol- 
factorius, which, as we have already seen on p. 84, 
is really a lobe of the brain itself, is the specific 
nerve of smell. 

§ 5. The contrivance for applying the stimulus 
to the end-organs of smell is very simple ; in gen- 
eral it is only necessary that a current of air, in 
which the stimulating particles float, shall be pjg 40 
drawn through the nasal passages over the mu- 
cous membrane of the regio olfactoria. Even am- 
monia and camphor, when placed under the nos- 
trils, have no smell so long as the breath is held or drawn through 
the mouth. In quiet inspiration much the greater part of the cur- 
rent of air is conducted to the pharynx du-ectly, and comparatively 
little reaches the ridge situated above the nasal dam at the back of 
the nose, where the end-organs of smell are placed. In full inspir- 
ation, and still more when short and deep draughts are drawn 
through the nasal passages, a considerable amount of the air 
is forced over the sensory parts. By snuffing we increase the 
amoimt of air drawn into the region by first creating a partial vac- 
uum in its cavity. In expiration the breathing passage is so located 
as to carry nearly all the air past the sensory parts without striking 
them. For this reason smelling is almost exclusively confined to 
' Sitzgsber. d. Wiener. Acad., Ixiii., p. 44 f. and Ixv., p. 7 f. 

—Olfactory Cells 
and Epithelial Cells 
from the Mucous Mem- 
brane of the Nose, ^""/l 
(After Schultze.) 


inspiration ; it has been disputed whether the current of expiration 
can be smelled at all. But Debrou showed that the odor of orange 
blossoms, when water tinctured with them has been drunk, can be 
detected in the expired air. The cui'rent which passes through 
the anterior part of the nasal passages seems to be the more impor- 
tant. This is probably the reason why the loss of the nose is so fre- 
quently attended with loss of the sense of smell. 

§ 6. The end-organs of Taste are situated in certain papiike, 
found on the upper surface of the root of the tongue, on the bor- 
ders and apex of the tongue, and in some cases on the anterior por- 
tion of the soft palate. These papillae of the tongue, ave' the jMpil- 
Ice circumvallakn and the jMjnUccfungiformes. The lateral portions 
of the former are pre-eminently the regions of the mucous mem- 
brane of the tongue where the end-organs of taste are found. The 
same organs are also found more sparsely distributed in the fungi- 
form papillae. The cii'cumvallate papillae are composed of connective 
tissue, which is invested by a jDavement epithelium arranged in 
laminae. The epithelial layer is thinner than elsewhere at the sides of 
the papillae, in which the end-organs of taste (gustatory flasks or 
bulbs) form a zone that extends upward to about the level at which 
the papillae are no longer protected by their lateral wall. In the 
fungiform papillae the end-organs api^ear in the epithelium which 
covers their upper surface, and in the side surfaces. A. Hoffmann 
also found them in the papillae of the region of the soft palate. It 
is more doubtful whether they exist, as has been alleged, on the epi- 
glottis. The papillce filiformes, which are sometimes classed with 
the two others, probably have nothing to do with sensations of 

Methods of experimenting to discover what surfaces are sensitive 
to taste arc not easily made exact, because the stimulus must be in 
solution to excite the end-oi'gans, and because the nature of the ex- 
citatory changes is chemical. There is scarcely a spot from the lips 
to the stomach which some physiologist has not described as be- 
longing to the organ of taste. But the regions where the above- 
described papillae, with their gustatory flasks, are found, are doubt- 
less the principal — and probably they are the only — sensitive 
surfaces. Considerable differences exist, however, among different 
species of animals, and even among different individual men — es- 
pecially as to the sensitiveness of the tip and edges of the tongue, 
and of the anterior surface of the palate. All the evidence tends to 
show that the gustatory flasks are the sole end-organs of taste. 

' Comp. Briicke, Vorlesungen iiber Pliysiologie, ii., p. 257; and von 
Vintscligau in Heimauu s Handb. d. Plijsiol. , III., ii., p. 147. 



Fig 41. — Giistatoi-y Bulbs from the Lateral Gustatory 
Organ of the Kabbit. ^=%. (Engelmann.) 

§ 7. The microscopic structure of tlie end-organs of taste is de- 
scribed in substantially the same way by all investigators, al- 
though these structui-es vary 
considerably, according to 
their position, and accord- 
ing to the different species 
of animals. In general they 
are like a glass knob with a 
short neck, and with its 
length somewhat greater 
than its greatest width. 
Hence they are called" gus- 
tatorij knobs" or "bulbs" 
(so Henle), or, better, " gus- 
tatory flasks " (so M. Schultze). They occupy flask-shaped cavities 
of the epithelium, which they completely fill. Their lower or inner 

part rests on the connective 
tissue of the mucous mem- 
brane ; theu' upper and more 
slender part is sun'ounded by 
epithelial cells and has an 
opening, or pore, of from ^jqVb" 
to ysVo '^f ^^ inch in diameter, 
at the surface of the epithe- 
lium. The margin of this pore 
is usually formed by placing 
several cells together, but 
sometimes by a single cell which appears perforated with a round 
hole. Each of the gustatory flasks consists of from fifteen to thirty 
long, thin cells, arranged like the leaves of a bud in closely com- 
pressed rows around the axis. 

All the gustatory flasks are comjjosed of two kinds of cells : 
some are, essentially, epithelial cells, and have probably no direct 
connection with the nerves ; the others are highly differentiated 
structures, are probably directly continuous with the nerve-fibrils 
and are thought to be true gustatory cells. The epithelial or iu- 
vesting-cells are long, narrow, spindle-shaped, bent, with a nucleus 
well marked ; the outward end is pointed, the central end branch- 
ing. The gustatory cells are thin, long, and highly refractive of 
light, with nearly the whole body of the structure occupied by an 
elliptical nucleus. The body of the cell is elongated into two pro- 
cesses, of which the upper or peripheral is tolerably broad and 
bears a short and fine point like a hair or pencil-point. This point 

Fig. 49. — Transverse Section through a Papilla 
Circumvallata of a Calf. Showing the arrange- 
ment and distribution of the gustatory buib. 
2^/j. (Engelmann.) 



lies in a canal, in the epithelial layer, and rarely projects from the 
pore of the flask. The lower or central process of the cell is much 
attenuated, and usually divides into two branches. A direct con- 

Fis. 43. — Isolatecl Gustatory- 
Bulb, from the Lateral 
Gustatory Organ of the 
Kabbit. ^""/j. (Engelmann.) 

Fig. 44.— a. Isolated Gustatory Cells, from the Lateral 
Organ of the R.abbit ; 6, an Investing and Two Gusta- 
tory Cells, isolated but still in connection, ^""/i. (En- 

nection of these processes with the fibrils of the gustatory nerve is 
assumed by all investigators. The manner in which the nerve-fibres 
terminate within the papillae is different in different animals. 

The gloaso-pharyngeal nerve is the principal nerve of taste. It is 
distributed to the back of the tongue, enters the circumvallate pa- 
pillae, where it forms a minute plexus, interspersed with nerve-cells, 
from which both meduUated and non-medullated fibres pass to the 
base of the gustatory flasks. The lingual branch of the trigeminus 
has also some claims to be, in a minor degree, a nerve .of taste. 
Sehiff ' considers it as designed for sour taste, with a slight sensi- 
tiveness to bitter also. 

§ 8. In considering the end-organs of Touch, attention should 
be directed to the great variety of sensations which are grouped 
together under the word "touch," in the broadest meaning appli- 
cable to it. The question is thus raised whether any histological 
difference is to be detected in the nervous apparatus which may 
serve as a physical basis for the difference in the sensations. We 
may set aside for the present all consideration of the feelings of 
pain, of exertion and fatigue, and the so-called " common feeling " 
and "muscular sense." The question is thus reduced to this nar- 
row form : Can histology point out two specifically distinct kinds of 
end-organs in the skin, one of which serves for sensations of tem- 
perature, and the other for sensations of pressure ? 

§ 9. Histological examination shows that the sensory nerves dis- 
tributed to the skin — the general organ of touch — terminate in two 

' Molesch. Unters.. X., p. 406 f., as referred toby von Vintschgau in Her* 
mann's Haudb. d. Physiol., III., ii., p. 171 f. 



ways, either in free end-fibrils or in special constructions called 
"tactile corpuscles" or "end-bulbs." The different varieties/ all, 
however, essentially alike, of these special end-organs of touch have 
been named after as many different investigators. Their general 
office is that of modifying and multiplying the effect of the stimu- 
lus upon the nerve-fibres which terminate in them. The so-called 
" corpuscles of Pacini " were the fii'st end-apparatus to be discovered 
in connection with the peripheral termination of the sensory nerves ; 
they were seen more than one hundred and fifty years ago by Vater. 
In man they are constantly present in the subcutaneous connective 
tissue of the palms of the hand and of 
the soles of the feet ; but are most 
numerous in the palmar surfaces of 
the fingers and toes, especially the 
third phalanges, although they occur 
in the neck, arms, etc. In some places 
they are visible to the naked eye as a 
minute gTain of from -^^ to i of an 
inch in diameter. They may be said 
to be nothing more than the ends of 
medullated nerve-fibres remarkably 
thickened.^ Each corpuscle consists 
of layers of connective tissue, arranged 
concentrically and more closely packed 
near the centre ; these surround a cav- 
ity containing a soft nucleated mate- 
rial, into the interior of which the 
nerve penetrates. Here the nerve- 
fibre, having become a naked axis-cyl- 
inder, appears to terminate in a little 
bulb. Examination with the highest 
powers of the microscope shows that the axis-cylinder of the fibre 
is fibrillated, and that the terminal bulb consists of finely granular 

Closely allied to the foregoing structures are the so-called " end- 
bulbs of Krause." These are small capsules of connective tissue in 
which nuclei can be detected. In them the nerve-fibrils of touch 
terminate either in a coiled mass or in a bulbous extremity. They 
are from ^^-^ to yoVo ^^ ^^ inch in diameter, and exist in the con- 

' On the different kinds of terminal corpuscles, a principal monograph is by 
Fr. Merkel, Ueber die Endigungen der sensibleu Nerven in der Haut der 
Wirbelthiere, Rostok, 1880. 

'^ So Biesiadecki in Strieker's Human and Comparative Anatomy, ii., p. 232. 

Fig. 45. — Corj^iuscle of Pacini (or Vater) 
from the Mesentery of the Cat, (After 
Frey. ) a, nerve with its sheaths ; 6, 
system of tunics constituting the cap- 
sule of the corpuscle ; f, axial canal, 
in which the nerve-tibre ends. 



Fig. 46. — End-bulbs from the 
Conjunctivfi of the Human 
Eye. (After KoUiker.) 1, 
has two nerve-fibres which 
form a coil within the end- 
bulb ; 2, has a fatty core. 
The nerve-fibre of 3 ends 
within in the form of a knot. 

junctiva of the eye, in the tongue, the Hps, the floor of the buccal 
cavity, etc. The " corpuscles of Wagner " (or Meissner, -who has 
furnished most of the details) may be described as oval-shaped 
bodies, made up of superimposed laminae and 
bearing some resemblance to a miniature fir- 
cone. The medullated nerve-fibres, like 
" creeping roots," wind beneath the cutane- 
ous papillse, and here and there penetrating 
them, terminate in the corpuscles. Within 
the corpuscles, 
according to 
Kolliker, the 
fibrils form two 
or three coils, 
and finally join 
together in 
loops. These 
tactile end-organs are most constant 
and numerous in the terminal pha- 
langes of the fingers; they occur in 
smaller numbers on the palm and 
back of the hand, on the sole and 
back of the foot, and sometimes on 
the nipple, lips, etc. They are seated 
in the papillse of the skin. Meissner 
counted four hundred papillse in -^V 
of an inch square on the third pha- 
lanx of the index-finger, and found 
these corpuscles in one hundred and eight of them. Their long 
diameter lies in the direction of the papillse and extends from -g^-g^ 
to Yj-Q ^^ ^^ ^^^^ » ^^®y ^^'® about -g-J-jj- of an inch in thickness. 

§ 10. Since the surface of the skin is in general sensitive to press- 
ure and to temperature, it follows that the special structures de- 
scribed above as occurring in parts of this surface, cannot be the 
sole end-organs of touch. Modern histology has demonstrated 
the presence of an intricate plexus of non-meduUated nerve-fibres 
which end in free extremities between the cells of the mucous 
layer. This terminal plexus of nerve-fibres is also the end-organ 
of so-called general sensibility and of touch. 

None of the attempts hitherto made to establish specific relations 
between the varieties in the structure of the tactile end-organs and 
the varieties of the sensations which they administer can be joro- 
nounced successful. Ivrause has tried to deduce from the coustruc- 

tG. 47. —Corpuscles of Touch. (After 
Frey.) a, from the soft skin of the 
duck's bill ; b and c, from the papillse 
of the tongue of the same animal. 


tion of the corpuscles of Pacini their fitness to act as the end- 
organs of pressure ; but these corpuscles are wanting in many 
parts of the body that are sensitive to pressure. Wagner con- 
sidered the corpuscles which bear his name to be special organs of 
touch. But it has been shown by Merkel that these corpuscles are 
nothing but aggregates of more elementary forms, the so-called 
" tactile cells." Some have argued that the end-bulbs of Krause and 
the corpuscles of Pacini are the organs of general feeling {sensus 
communis) ; but others, with more probabihty, assign this function 
to the free nerve-endings ; while Merkel is of opinion that the latter 
are specifically concerned in sensations of temperature. Nothing 
is known on this point beyond the fact that the skin, within which 
the sensory nerve-fibres terminate, either in free ends or in special 
tactile corpuscles, is the organ for all the varieties of sensation 
brought under the most general meaning of the word " touch." 

The more precise manner in which the terminal fibres of the 
nerves of touch stand related to the individual tactile cells is also 
still in doubt. Some investigators consider that the fibres enter 
into the very protoplasm of the cells (Merkel, Frey) ; others that 
they spread themselves on end-plates superimposed on the cells 
(Retzius, Ranvier). 

§ 11. With the exception perhaps of the ear, the Eye is by far 
the most elaborate and comphcated of the end-organs of sense. 
This is true of those portions of it which are designed merely to 
bring the external stimulus to bear upon the nervous structure, as 
well as of this structure itself. Considering it as a whole, we 
may say that the peripheral organ of sensations of light and color 
is an optical instrument constructed on the plan of a water camera 
obscura, with a self-adjusting lens, and a concave, sensitive, nervous 
membrane as a screen on which the image is formed. 

§ 12. The eyeball consists of three coats or tunics inclosing 
three translucent refracting media. Since, however, the front part 
of the outer one of these coats is itself translucent and refracting, 
the number of refracting media in the eye is really four. (1) The 
first or external coat consists of two parts : (a) the Sclerotic or 
posterior five-sixths part ("white of the eye"), which is a firm, 
fibrous membrane formed of connective tissue intermingled with 
elastic fibres ; and (b) the Cornea, or translucent anterior one-sixth 
part, which is circular and convex in form, and covered with con- 
junctival epithelium. The cornea rises and bulges in the middle 
like a watch-glass. (2) The second coat, or tunic of the eye, also 
consists of two parts : these are (a) the Choroid coat, which com- 
prises much its larger portion, is of a dark brown color, due to ita 



pigment cells (except in the case of albinos), and is abundantl;f 
provided with nerves and blood-vessels ; and (b) the Iris, a circular, 
flattened, disk-shaped diaphragm in front of the lens (the colored 
part of the visible eyeball), bathed with aqueous humor, and hav- 
ing in its centre a circular aperture called the ' ' pupil "' of the eye. 
The anterior border [corpus ciliare) around the iris consists of the 

JPi: ci% 


PiQ. 48. 



-Horizontal Section through the Left Bye. ^/j. (Schematic, from Gegenbaur.) 

ciliary muscle and the ciliary processes. (3) The Retina is the 
third or inner coat of the eye. It is a delicate membrane of ex- 
quisite ti'ansparency and almost perfect optical homogeneity ; it 
has a highly comj)lex structure, consisting of nine or ten layers, th^ 
truly nervous portions of which contain nerve-fibres, nerve-cells, 
and special end-organs, together with connective tissue and blood- 
vessels. The inner surface of the retina is moulded on the vitreous 


body, and it extends from the entrance of the optic nerve nearly 
as far forward as the ciHary processes. 

§ 13. The eyeball has four translucent refracting media. The 
first of these — enumerating inward from the outside front — is (1) 
the Cornea, already spoken of as the anterior one-sixth of the 
outer coat of the eye. (2) The Aqueous Humor fills the space 
between the cornea and the lens, and is divided by the iris into 
two chambers, of which the front one is much the larger. It is 
limpid and watery ; it holds in solution the salts of the blood- 
serum, with traces of organic substances. (3) The Crystalline Lens 
is situated between the iris and the vitreous body. It is a transpar- 
ent biconvex lens, with its antero-posterior diameter about one-third 
less than the transverse diameter. It consists of a capsule and in- 
closed body. It is of " buttery consistency," composed, like an 
onion, of a number of easily separable layers. Each layer consists 
of fibres which, within the layer are, as a rule, radial. Between 
the entire ciliary part of the retina and the corresponding part of 
the vitreous humor is interposed a structureless membranous body, 
to which the edge of the lens is attached, and which radiates out- 
ward and maintains the lens in tension. It is called the suiipen- 
sory ligament, (or Zonula of Zinn) and its office is very important 
in the accommodating of the eye to different distances. (1) The 
Vitreous Humor consists of a number of firm sheets or layers 
(lamellae), between which fluid is contained, built iuto a body that 
is, optically considered, transparent and homogeneous. It occupies 
most of the space inclosed by the tunics of the eye. It is thought 
to be a gelatinous form of connective tissue, and is composed most- 
ly of water with salts in solution, of proteids and mucin, fats and 
extractive matters — especially urea. Its peculiar structure is of 
little significance for the physiology of the eye. 

§ 14. Of the appendages or accessory parts of the eye — such as 
the eyebrows, the eyelids, lachrymal apparatus, muscles of the eye-' 
ball — only the mechanism by which the eye is moved in its or- 
bit has any special significance for physiological psychology. 
The building-up of a world of visible objects, and even the forma- 
tion of a so-called " field of vision," is dependent upon the great 
mobility of the eye. The eyeball is moved in its bony socket, 
where it is embedded in a mass of fat as in a socket-joint, by sis 
muscles, which are attached to it somewhat like the bridle to the 
horse's head. Four of these muscles spring from the bony wall 
near the point where the optic nerve enters, extend through the 
length of the socket and pass directly to the eyeball, where they 
are attached to it, one above, one below, one on the outer, and one 



on the inner side, (the recti; intemiis and externus, superior andinfe* 
rior). In moving both eyes up or down, the same muscles in both 
contract simultaneously ; in moving the eyes to the right, the outer 

Fig. 49. — Muscles of the Left Human Eye, 
seen from above, rs, rectus superior ; 
re, i-ectus externns ; and rit, rectus 
internus ; os, superior oblique, with 
its tendon, t, which runs through the 
fnembranous pulley, ?«, at the inner 
wall of the cavity of the eyeball. 

Fig. 50. — Muscles of the Left Human Eye, seen 
from the outside. Ir, levator of the upper eye- 
lid, which covers the rectus superior, rs, re, os, 
as in the preceding figure ; rif, rectus inferior ; 
oi, inferior oblique. 

muscle of the right eye and the inner of the left, contract simul- 
taneously (and mce versa) ; in turning both eyes inv^ard to converge 
them upon a near object, the two inner muscles contract together. 
We cannot move the eyes so that the optical axes do not either 
meet or remain parallel ; we cannot look with one eye upward and 
the other downward, nor with one eye to the left and the other to 
the right ; nor can we voluntarily turn the eyes farther apart than 
when their axes are parallel. 

The other two of the six muscles of the eye are called oblique. Of 
these one is superior and internal ; it does not pass directly forward 
from its place of origin, at the posterior aperture through which 
the optic nerve enters to the eye, but first runs through a ring, then 
turns around, and is attached obliquely to the upper surface of the 
eyeball. The other oblique muscle begins at the inner wall in the 
socket, passes under the eye-ball, and is attached to it opposite to 
the superior oblique muscle. The two oblique muscles combine 
with the four recti to move the eyes in various directions which 
would be impossible for the latter alone. 

§ 15. The problem which is to be solved by the end-organ of 
vision may be stated in a general form as follows : A mosaic of 
localized sensations must be so constructed that changes in the 
quantity, quality, local relation, and sequence of these sensations 


shall be quickly interpreted as indicative of the size, shape, lo- 
cality, and motion of external visible objects. The most important 
part of the solution of this problem falls tipon the nervous struct- 
ure of the retina. It is itself a mosaic of nervous elements, the 
excitation of which may vary in quality, quantity, local coloring, 
and sequence of the different elements excited. But in order that 
the retina may exercise its function with the precision and delicacy 
of detail for which its structure fits it, the rays of light reflected 
from a single point of the surface of the visible object must excite a 
single one, or at most a small and definite group, of the retinal 
nervous elements. The sensations thus occasioned can then un- 
dergo a systematic arrangement by the mind. It is the work of 
the translucent refracting media of the eye to apply the stimulus to 
retinal elements exactly discriminated, and in an order correspond- 
ing to the object ; that is to say, the cornea, the humors of the eye, 
and the lens must form an image on the retina. To show the pos- 
sibility of this by calculating how the general laws of optics apply 
to the special structure of the eye, as anatomy describes it, and to 
make the calculations accord approximately with the facts, has been 
the labor of a number of investigators, especially of Helmholtz and 
his pupils. To the results of this labor only a brief allusion must 

§ 16. The four media of the eye constitute a system of refracting 
surfaces, each of which is separated from the one adjoining by a 
circular cut, as it were, in the whole refraction -substance. Espe- 
cially is this true of the lens with its concentric layers. The " image " 
formed upon the first member of this system of surfaces, by its re- 
fraction of such bundles of rays, from the object, as all lie in a plane 
at right angles to the axis of the system, thus becomes an " object '' 
for the second refracting surface of the system ; and the image- 
formed by the second an object for the third ; and so on. The re- 
sult of any number of such refractions will accordingly always be 
an image whose points lie in a plane at right angles to the axis of 
the system of refracting surfaces, and which, as a whole, is in true 
perspective to the original object. The last image and the object 
are geometrically similar. 

In tracing the course of the rays of light through the refracting 
media of the eye, two things must be taken into the account : (1) 
the indices of refraction of these media, and (2) the geometrical 
form and position of all the limiting surfaces. (1) The means for 
attaining a knoAvledge of the former is by taking the average result 
of an examination of a number of eyes supposed to be normal 
Fortunately for science, death has, for the first twenty-foiir hours, 



little or no effect in changing the indices of refraction of the eys. 
Krause ' found the mean index of refraction of the cornea to be =: 
1.3507, of the aqueous humor = 1.3420, of the vitreous body = 

Fig. 51. — Median Section through the Axis of the Lens of the Bye. (Schematic, after Babuchin.J 

1.3485. But Helmholtz (subsequent observers have agreed better 
with his result than with Krause's) found the two latter indices of 
refraction to be = 1.3365 and i= 1.3382, 
respectively. The lens of the eye, espec- 
ially, is not homogeneous throughout as 
to its index of refraction. Each layer has 
its own index, and the amount of the 
index of each layer increases regularly 
toward the kernel of the lens. The work 
of refraction done by the lens is, there- 
fore, greater even than that which could 
be done by a homogeneous lens with an 
index of refraction equal to that of the 
kernel, or most highly refracting part of 
the lens. 

(2) The position and form of the separating surfaces of the re- 
fracting media can be only approximately determined in the living 
eye. Three of these surfaces are of chief importance — the anterior 
surface of the cornea, and the anterior and posterior surfaces of the 
lens. The convexity of the first of these three is found to depart 
perceptibly from a sphere ; it is greater toward its edge than at its 
vertex, where it resembles rather a section of an ellipsoid. The 
advantage of such a shape is seen in the fact that the images 

' Krause's experiments refer to rays of the wave-length to which the bright- 
est place in the solar spectrum corresponds ; that is, to the place at the end of 
the first third, or quarter between D and E. The refraction-index of water for 
these rays he assumed at =1.33424. 

Fig. 52. — View of the Lens 
Profile. Vi- (After Arnold.) 


formed when the pupil is expanded are thus made sharper than 
they could otherwise be. No observable refraction takes place on 
the posterior surface of the cornea, because the difference between 
the indices of refraction of the cornea and of the aqueous humor is 
so slight that the faint images from this surface vanish by proxim- 
ity to the stronger ones refracted from the front part of the cornea. 

§ 17. The power of altering the refracting conditions of the eye, 
so as to enable the media to form a single perfect image on the 
retina, for varying distances of the object, is called its power of 
" accommodation " or adjustment. Plainly such adjustment of the 
eye cannot take place, like that of a camera obscura, by changiuo- 
to any appreciable extent the distance of the lens from the screen 
on which the image is formed. It must therefore take place, either 
by increasing the indices of refraction of the media of the eye, or 
by increasing the curvature of one or more of the refracting surfaces. 
It is now known to be due to changes in the convexity of the lens, 
principally, if not wholly, of its anterioi- surface. The posterior 
apex of the lens remains unmoved. There are several methods of 
experiment which demonstrate that in accommodation for near dis- 
tances the front of the lens becomes more strongly arched. When 
accommodation is taking j)lace, the pupil may be seen not only to 
contract, but also to draw its edge forward. Helmholtz calculated 
the amount of this forward movenient for two cases at about J^y and 
-gJg of an inch, respectively. Moreover, by an ingenious contrivance 
the image reflected from the anterior surface of the lens may be 
watched as it becomes smaller and more distinct on adjustment 
for near distances, thus showing that the surface from which it is 
reflected has increased its curvature. 

It is obvious that the mechanism for adjusting the eye must be 
under the brain's control, since adjustment is voluntary ; and that 
it must consist of muscles which lie within the eyeball. The ac- 
cepted hj-pothesis concerning the nature and action of this mechan- 
ism was first proposed by Helmholtz. This investigator assumes 
that the lens, when the eye is at rest, does not have the form which 
corresponds to a condition of equilibrium in its own elastic power. 
If it were not held in by its surroundings, it would be more arched 
than it is both before and behind. But it is kept flattened by the 
radial tension of the susjjenso7-i/ ligament ; when this tension is with- 
drawn the lens becomes curved by the action of its own elasticity. 
The withdrawal of the tension is accomplished by the action of the 
ciliary muscle, the fibres of which have their point of fixation at the 
edge of the cornea, and run from here in the direction of a merid- 
ian toward the equator o fthe eye. When the ciliary muscle cou- 



tracts, the free ends of its fibres are drawn toward its fixed ends 
on the edge of the cornea ; the radial tension of the suspensory hg- 
ament is thus relaxed, and the lens is allowed to assume its natural 
form under the equipoise of its own elastic forces. 


fun. Sciuemmii 
Jiiy. Jiect. iridis 


Fig. 53. 

Proc. ciliarix 

railiarer circulurev 


-Sectional View of the Connections of the Cornea, Ciliary Muscle, Ciliary Processes, etc. 
'"/j. (Gegenbaur.) 

The occulo-motor nerve furnishes the fibres that serve the ciliary 
muscle ; these fibres run in the posterior strands of its roots. Their 
central place of origin is in the posterior part of the floor of the 
third ventricle ; stimulating the front division of this part produces 
accommodation of the lens ; stimulating the back division of the 
same part produces contraction of the pupils. Stimulation still 
further back, where the third ventricle passes into the aqueduct of 
Sylvius, produces contraction of the internal rectus muscle of the 
eye ; and the innervation of this muscle is, of course, regularly con- 
nected with adjustment for near distances. Thus all the mechan- 
ism of accommodation, both that of the central organs and that of 
the end-organs, is made to work together for the production of an 
image upon the retina. 

§ 18. Given the formation of the image upon the retina, it is fur- 
ther required in order to vision that this physical process should 
be changed into a physiological process. We now examine briefly 
the mechanism by which such a change is accomplished. [The 
reader is referred to the larger specific treatises for the detailed 
theory of the schematic, the emmetropic, the myopic, and the hy- 
permetropic eye.] The retina, or inner tunic of the eye, contains 
the nervous elements by whose action the system of refracted rays 



is changed into a mosaic of nerve-commotions. But light does not 
act as a stimulus to the nei'vous substance, either fibres or cells, 
unless it have an intensity which is neai'ly deadly to that sub- 
stance. Since we are able to see the feeblest rays of the moon as 
reflected from white paper, the nervous excitation which is the con- 
dition of vision cannot be produced by the direct action of light on 
the nerve-fibres or nerve-cells of the eye. A photo-chemical sub- 
stance and process, as well as a special end-apparatus, seems there- 
fore to be necessarily involved in the problem which is given to 
the retina to solve. 

§ 19. The nervous and other elements of the retina are arranged 

Outer surface. 
10 ^^^^m^WS^msmm!mPm«^fr?^y^'^' 10 Layer of pigment cells. 

9 Layer of rods and cones. 

S Membrana limitans externa. 

7 Outer nuclear layer. 

f .... Outer molecular layer. 

y Inner nuclear layer. 

[5reJ'?|tdg*|!'„'K| 4 . . . .Inner molecular layer. 

. Layer of nerve-cells. 

. . . Layer of nerve-fibres. 

=^^-:^:|^^^: J . . . .Membrana limitans interna. 
Inner surface. 
Fig. 54. — Diagrammatic Section of the Human Retina. (Schultze.) 

in tlie following ten layers, counting from within outward and 
backward : (1) the membrana limitana interna, which is the retinal 


f- .i„<; A/ ' ' ■ ' — 

Fig. 55.— Diagrammatic represen- 
tation of the Connections of the 
Nerv(' - fibres in the Retina. 
(Schultze.) Tlie numbers have 
the same reference aa in Fig. 54. 


border toward the vitreous body ; (2) the 
layer of optic nerve-fibres distributed from 
the papilla where this nerve breaks in 
through the tunics of the eye ; (3) the 
ganglion-cell layer ; (4) the imier molecular 
layer ; (5) the inner nuclear layer ; (6) the 
outer molecular layer ; (7) the outer nu- 
clear layer ; (8) the membrana limitans 
externa ; (9) the bacillary laj'er, or layer 
of rods and cones ; (10) the pigmenl-epiihe- 
lium layer. The membranes (Nos. (1) 
and (8)) are not really uninterrupted 
layers, but an extremely fine network. 

By no means all the retinal substance 
is nervous. Indeed, the numerous radial 
fibres {fibres of Midler) which seem to 
penetrate its entire thickness are now held 
to be in great part elements of the suj)- 
porting tissue ; moreover, the whole con- 
nective substance is a kind of sponge-like 
tissue, in the gaps of which the true ner- 
vous elements lie embedded. The gaps 
thus filled are especially large in the 
second, third, fifth, and seventh layers. 

A description of the undoubtedly ner- 
vous elements of the retina includes the 
following particulars : (a) The retinal 
fibres of the optic nerve lie parallel to the 
surface, are non-medullated, and extreme- 
ly fine ; in general, they are arranged in 
ray-like bundles, radiating on all sides 
from the place of the entrance of the 
nerve. The arrangement is special at the 
yellow spot, so as to surround, and not 
cover it. This nerve-fibre layer is thickest 
at the papilla of the retina, and diminishes 
continuously from this sjjot toward the 
ora serrata ; at about one-third of the 
distance it becomes single. (6) The gan- 
glion-cells, which form the principal part 
of layer No. 3, like the multipolar cells of 
the rest of the cerebro-spinal sj-stem, 
have one large process of more trans- 



lucent appearance. This process subdivides into fibrils of vanishing 
fineness, that enter and are lost in the next layer. At the yellow 
spot these cells are eight or ten deep ; from this centre they dimin- 
ish toward the ora serrata, where spaces are found between the 
cells, (c) The nervous elements of the inner molecular layer (No. 4) 
are not clearly made out. They probably consist of extremely fine 
filaments, which are connected with the external processes of the 
ganglion-cells, (d) Most of the nucleus-like bodies of the inner 
nuclear layer (No. 5) are probably nervous. Each such body has 
two processes — one directed inward, the other outward. The 
former is thought to be connected with the filaments of the 
inner (No. 4), and the latter with those of the outer (No. 6) molec- 
ular layer. («) In the outer molec- 
ular layer (No. 6) are nervous fila- 
ments, like those in No. 4, which are 
probably connected with the external 
processes of the inner nuclear layer. 
Here are also found numerous star- 
shaped cells probably not nervous. 
(/) In the outer nuclear layer (No. 
7) the undoubtedly nervous elements 
preponderate. Each nucleus-like 
body in this layer is connected by a 
radial fibre with one of the nervous 
elements of the rod- and- cone layer 
(No. 9). These nuclear bodies are 
called rod-granules and cone-granules 
respectively, and are to be distin- 
guished, not only by their connection 
with these elements, but also by 
their size and position ; the latter are 
larger, and lie on the more external 
side of the layer, {g) The layer of 

rods and cones (No. 9) consists of a multitude of elongated bodies 
arranged side by side, like rows of palisades, with their largest ex- 
tension in the radial direction. These bodies are of two kinds — 
one cylindrical, and called "rods of the retina," the other rather 
flask-shaped, and called "cones of the retina." 

The rods extend the entire thickness of the layer, and are about 
3^„ inch in length, but the cones are shorter ; the rods are 
about TXoTTo ^^^^^ ^^ diameter, the smallest cones of the central 
depression ^ o|) qq inch. The inner ends of both are continuous 
with the rod-fibres and cone-fibres of the outer nuclear layer. 

Fig. 56. — Diagrammatic Section of tne 
Posterior Part of the Retina of a Pig. 
80»/i. (Schultze.) 7, part of outer nu- 
clear laj"er ; 8, membrana limitana ex- 
terna ; 9, rods n.nd cones. Each of the 
cones, which are in very close apposition, 
contains in its inner segment a highly re- 
tractile bod3', the func ion of which is 



Each rod or cone is composed of an inner and outer segment 
or limb ; the latter is highly retractile, the former only feebly so. 
The inner limbs appear under the microscope like a mass of pro- 
toplasm. The appearance of a most delicate longitudinal line in 
the inner and outer segments has led to the belief that a nerve- 
fibril is, as it were, drawn through their axis. The description 

Fig. 57. — Rods and Cones of the Human Retina. (Schultze.) 
A, showing inner segments of the rods, .s' s s, and of the 
cones, zz'; the latter in connection with the cone-nuclei and 
fibres as far as the outer molecular layer. """Z]. A inner 
segment of a cone with a cone-nucleus. ^200^^, C, isolated 
interior portion of a cone. 

Fig. 58. — Rod and Cone from 
the Human Retina, preserved 
in perosmic acid, showing 
the fine fibres of the surface 
and the different lengths of 
the internal segment, '"""/i. 
(Schultze.) The outer seg- 
ment of the cone is broken 
into disks which are still ad- 

of the two shows that there is no essential anatomical difference 
between the rods and cones ; nor are we able to distinguish any 
difference in their physiological significance. The distribution of 
the two elements is different for different parts of the retina. In 
the yellow spot only cones appear, but these are of more slender 
form, and of increased length, so that not less than one million are 
supposed to be set in a square y^ inch ; ' while not far from this 
' See Le Conte, Sight, p. 58. New York, 1881. 




Figs. 59 and 60.— Superficial Aspectof the Arrangement 
of the Rods and Cones in the Retina. *"%. (Schultze.) 
The former is from the region of the macula lutea ; 
the latter from the peripheral region. 

spot each cone is surrounded by a crown-shaped border of rods. 
Toward the ora serrata the cones become continually rarer. In 
close connection with the 
rods and cones stand the 
cells of the pigment-epithe- 
lium. These cells form a 
regular mosaic of flat, six- 
sided cells, which send out 
pigmented processes 
tween the outer limbs of the 
rods and cones. 

The fibres of the optic 
nerve are supjDOsed to be 
connected with the rods and cones by means of the ganglion -cells, 
and of the radial fibres in which the granules of the outer and inner 
nuclear layers are embedded. 

§ 20. Two minute portions of the inner surface of the retina re- 
quire to be distinguished from the rest of its area ; the yellow spot 
(macula lutea) and the " blind spot " [papilla optica). The yellow spot 
S Ch is of oval shape, about -^-^ of an 
inch in its long diameter, and has 
in the centre a depression called 
ihQ fovea centralin. It is the place 
of clearest vision, and the physi- 
ological centre of the eye. About 
\ of an inch inside the eye from 
the middle of the yellow spot is 
the middle of the papilla, or place 
where the optic nerve breaks into 
the retina. The blind spiot, or 
portion of the retina which can 
be experimentally shown to be 
inoperative in vision, has been 
proved by Helmholtz to corre- 
sj)ond in both size and shape to 
that covered by this papilla. Its diameter is about J^ or J-j of an 
inch, varying considerably for different eyes. It is wanting in all 
the nervous elements. 

§ 21. In answer to the question, What elements of the retina are 
directly affected by the light ? both anatomy and physiology refer 
to the layer of rods and cones. This layer alone possesses that 
mosaic nervous structure which appears to correspond to the de- 
mands made upon the end-apparatus of vision. It can be demon.- 

PlG. 61.— Equatorial Section of the Right Eye, 
showing the Papilla of the optic nerve, the 
Blood-vessels radiating from it, and the 
Macula lutea. 7i- (Henle.) S, sclerotic; 
Ch, choroid ; and R, retina. 


strated that the waves of light pass through the structure of the 
retina, and that the nervous process must begin in the back part of 
this structure. Indeed, it is possible, by an experiment (devised by 
Purkinje), to perceive with one's own retina the aborescent figure 
formed by the shadow of the blood-vessels expanded upon its front 

§ 22. We have already seen (Chapter I., §§ 14, 15) that a chemi- 
cal process may reasonably be conjectured to accompany the action 
of the nerves in general. Undoubtedly a photo-chemical process 
is concerned in vision. But after all the careful researches of many 
observers, especially of Ktihne ' and his pupils — it is difficult to 
point to any results of chemical investigation which serve better to 
define the exact nature of the physiological action of the end-oi'- 
gaus of the eye. The relation of the light to any chemical pro- 
cesses which may take place in the gray substance of the retina can 
be only indirect. The opto-chemical hypothesis must, therefore, 
regard the epithelial cells, with which the end-fibrils of the optic 
nerve are in physiological connection, as the bearers (Trager) of cer- 
tain photo-chemically decomposable materials or visual substances 
(Seh^toff^e) ; these substances, however, cannot excite chemically 
the irritable part of the visual cells — the protoplasm of the inner 
limbs of the rods and cones — without being themselves decomposed. 
Visual substance is necessarily some kind of matter easily decom- 
posable by light, or chemically sensitive to light. The first process, 
then, in the excitation of the optic nerve, is the decomposition by 
the light of some substance found in certain epithelial elements of 
the retina. The second process is the action, as visual excitants 
(Sehreger), of the decomposition-products of the epithelial cells 
upon the protoplasm of the end-organs. But in order that such 
decomposition-products may act as excitants of the end-organs of 
vision, the visual substance must be rightly placed — that is, it must 
be in local connection with the protoplasm of the outer limbs of 
the rods and cones. The relation of the two last layers of the 
retina is such as to secure this necessary connection. We are as 
yet unable, however, to say what are the visual substances which 
the successful working of the opto-chemical hypothesis demands. 
The location of the pigmentum nigrum, and the changes produced 
m it by light, favor the conjecture that this substance is of the 
most fundamental and general importance for visual sensations. 
Visual purple may also be supposed to be a visual substance. The 
fact that light of different wave-lengths effects changes in this pig- 

' The few statements here given are taken for the most part from the article 
of this investigator in Hermann's Handb. d. Physiol., III., i. , pp. 335 fE. 


ment with different degrees of speed, suggests the view that it is 
related to the susceptibility of the eye for different colors. But 
since invertebrates do not have the visual purple ; since the cones 
(a thing which no one doubts) see without this purple, and since 
the rods of some animals, such as hens and doves, and the rods of 
the ora serrata, perform their functions without it, this pigment 
can scarcely be said to be the only visual substance. The opto-chem- 
ical hypothesis, then, seems to require several colored visual sub- 
stances. Moreover, since animals can see with bleached retinas, 
and albinos have the power of vision, we are compelled to assume 
also a colorless visual pigment. As to the nature of the chemical 
changes necessary to be produced in the protoplasm of the outer 
limbs of the rods and cones by the action of the decomposition- 
products of the visual substances, we are quite ignorant. 

§ 23. The end-organ of hearing is the Ear. But in this case, as 
in that of the eye, a very large part of the apparatus of sense is sig- 
nificant simjDly as a contrivance for applying the stimulus to the 
true end-organ, to the differentiations of epithelial cells and nervous 
cells connected with the terminal fibrils of the sensory nerve. The 
entire human ear consists of three parts, or ears ; namel}^ the ex- 
ternal ear, the middle ear, or tympanum, and the inner ear, which 
is also called the " labyrinth," from its complex construction. 

I. The External Ear — exclusive of the cartilaginous plate which is 
extended from the side of the head — consists of (a) the concha, a 
deep hollow, and {b) the external meatus, or passage leading from the 
bottom of this hollow to the drum of the ear. The concha is prob- 
ably of little or no use in sharpening our perceptions of sound ; 
for if a tube be inserted so as to secure a canal for the air to the 
drum of the ear, the entire concha may be filled with wax, and the 
result is to increase rather than diminish the sharpness of the 
sound. It is possible, however, that vibrations of more than one 
thousand in a second are concentrated by reflection ' from the con- 
cha. The external ear appears to be of some service in perceiv- 
ing the direction of sound. Kinne's experiments seem to show 
that — as Harless ' thought— the cartilage of the ear can be thrown 
into sympathetic vibration with certain acoustic waves, and so re- 
inforce the sound. At best, however such work done by the con- 
cha is small. 

The most patent office of the external meatus is the protection 
of the ear-drum ; the passage is so curved that the drum cannot be 

' See Hensen, Physiologie d. Gehors, in Hermann's Haadb. d. Physiol., 
III., ii., p. 23. 

-Article Horen, in Wagner's Handworterbuch d. Physiol., FV , 1853. 



reached from the outside in a straight hne. Helmholtz called at- 
tention to the fact that certain tones of a high pitch resound 
strongly in the ear when the meatus is of normal length, but cease 
so to resound when its length is increased artificially. The meatus 
probably, therefore, modifies certain tones by its own resonant 
action — strengthening the high ones, and deadening the low, in 
some degree. 

Various simple experiments — such as placing a resounding body 
in contact with the teeth — prove that the surrounding cranial bones 
conduct sound to the ear. It is probable, howevei", that the path 
of such conduction is not, for the most part, as was formerly sup- 
posed, directly to the inner ear by way of the cranial and petrous 
bones, but indirectly, through the ear-drum and bones of the middle 
ear to the fenestra ovalis. The amount of direct conduction pos- 
sible, has not as yet been determined precisely. 

§ 24. n. The Middle Ear, or Tympanum, is a chamber irregu- 
larly cuboidal in form, and situated in the temporal bone, between' 
the bottom of the meatus and the inner ear. Its outer wall is (a) 

Fio. 62.— Dnim of the Right Ear with the Ham- 
mer, seen from the inside. 2/,. (Henle.) 1, 
chorda tympani ; 2, Eustachian tube ; *, ten- 
don of the tensor tympani muscle cut off close 
to its insertion ; m a, anterior ligament of the 
malleus ; M c p, its head ; and M 1, its long 
process. S t p, Spina tympanica posterior. 

Cliorda tijDiEani 

Fig. 63.— Side Wall of the Cavity of the Tym- 
panum, with the Hammer (M) and the Anvil 
(.J). The former shows the connection of its 
handle with the drum. T, Eustachian tube. 
^/,. (Gegenbaur.) 

the membrana tympani, which consists of three layers — an external 
tegumentary, an internal mucous, and the intermediate membrana 
propria, composed of unyielding fibres arranged both radially and 
circularly. In the inner wall, which separates the tympanum from 
the labyrinth, are two openings or windows — the fenestra ovalis, 
which corresponds to the vestibule of the labyrinth, and the fenestra 
rotunda, which corresponds to the tympanic passage in the cochlea. 
Near its anterior part the tympanum opens into (6) the Eustachian 


tube, a canal which communicates with the nasal compartment of 
the pharynx. 

(c) The auditory bones are three in number, called Malleus, 
Incus, and Stapes, and arranged so as to form an irregular chain 
stretched across the cavity from the outer to the inner wall of the 
tympanum. The malleus has a head, separated by a constricted 
necli from an elongated handle ; its handle is connected with the 
centre of the membrana tympani ; its head articulates with the in- 
cus. The incus has a body and two processes. On the front sur- 
face of the body is a saddle-shaped hollow, in which the head of 
the malleus fits ; the short process is bound by a ligament to the 
posterior wall of the tympanum ; the 
long process ends in a rounded pro- 
jection {os orbicular e) through which 
it articulates with the stapes. The 
stapes, or stirrup-shaped bone, has " "' 

a head and neck, a base and two 
crura. The head articulates with 
the incus ; from the constricted neck 
the two crura curve inward to the 
base, which is attached to the fenes- 
tra ovalis. These bones are moved 
on each other at their joints by {d) 
two or three small muscles — the ten- „ ^, „ ^.^ „ . ^. 

Fig. 64. — Bones of the Ear, as seen in their 
SOr tympani, the stapedius, and, more connection from in front. Vi- (Henle.) 

I, Incus (anvil _), of which lb is the short, 
doubtfully, the taxator tympani. Ihe and Ilthe Icng, i>rocess; c. its body, and 
n 1 e L^ • • j. i • j. xi P'> ^^^ process for articulation with the 

first 01 these is inserted into the st&^'esiprocessusorbiciaaris.) M, Malleus 
TT J.1 J. T L (hammer), of which Mc is the neck, Mcp 

malleus, near the root, and serves to the head, mi the long process, and Mm the 

tighten the tympanic membrane by erpUuhm^cp.^' '"'^'' ^''^™''^' ''"^ '*' 
drawing the handle of the malleus 

inward ; the stapedius is inserted into the neck of the stapes, 
but its function is doubtful — apparently it draws the stapes from 
the fenestra ovalis, and so diminishes the pressure of the chain 
of bones in that direction. The laxator tympani is inserted into 
the neck of the same bone, and its action has been supposed by 
some to be antagonistic to that of the tensor tympani ; but its 
muscular character is now denied by most observers. 

§ 25. The general of&ce of the tympanum may be described as 
that of transmitting the acoustic waves to the inner ear, while at the 
same time modifying their character. Some modification is neces- 
sary in order that these waves may occasion such \T.brations in the 
elements of the inner ear as shall be adapted for the excitation of 
its end-organs. The acoustic motion of the molecules of air, in the 


form in wliicli it reaches the ear-drum, has a large amphtude, but 
a small degree of intensity. This motion must be changed into one 
of smaller amplitude and greater intensity ; and it must be trans- 
mitted, with as little loss as possible, to the fluids of the labyrinth. 
The transmitting vibrating media must also have the power of an- 
swering to the different tones of any pitch perceptible by the ear. 
The description of the manner in which this apparatus of membrane 
and bones solves so complicated a mechanical problem belongs to 
the physics of anatomy ; it has been worked out with great detail 
by Helmholtz and others, although certain points still remain un- 
solved. "VVe can here only indicate one or two particulars. 

A flat membrane, evenly stretched, whose mass is small in pro- 
portion to the size of its superficies, is easily thrown into vibration 
by the impact of acoustic waves upon one of its sides. Such a 
membrane resjjonds readily to tones which approach its own funda- 
mental tone ; but if divergent tones are sounded the membrane is 
unaffected. A motion which consists of a series of harmonious 
partial tones cannot then be repeated by such a membrane in the 
form in which the air brings it. If, then, the membrane of the 
tympanum were not so arranged and connected as to have no pre- 
pondei'ating tone of its own, it could not be the medium of our 
hearing a great variety of tones. The projaerty of taking up with 
the vibrations, as it were, of a large scale of tones is secured for the 
tympanum by its funnel-shaped form and by its being loaded. It 
is contracted inward into a dej)ression of the right shape by means 
of the handle of the hammer ; it is therefore unequally and only 
slightly stretched, and has no fundamental tone. It is also load- 
ed with the auditory bones, Avhich deprive it of every trace of such 
a tone and act as dampers to prevent long-continued vibrating. 
Moreover, since the apex of its funnel bulges inward, the force of 
the vibrations from all sides is concentrated in vibrations of greater 
intensity in the centre, where it is spent in setting the chain of ear- 
bones in motion. 

The acoustic vibrations of the auditory bones, which are occa- 
sioned by the movements of the ear-drum, are not longitudinal, 
but transverse ; they do not, however, resemble the vibrations of a 
stretched cord or a fixed pin. They do not vibrate by reason of 
their elasticity, but like very light small levers — vibrating as a sys- 
tem, with a simultaneous motion around a common axis. Direct 
observation of these bones in motion shows that their sympathetic 
vibrations vary greatly for tones of different pitch and similar in- 
tensity, from a scarcely observable motion to a surprisingly great 



The effect of the muscles of the tympanum upon the transmis- 
sion of tones of different pitch is not as yet clearly demonstrated. 
In general, the stretching of the tensor muscle, within the hmits 
which have thus far been investigated, seems to weaken the higher 
much less than the lower tones. But the tension of the drum un- 
der the influence of this muscle does not indicate the slightest 
change on passing from low to high tones. The stretching of the 
tendon of the stapedius muscle has no observable influence on the 
acoustic vibrations of the tympanum. 

§ 26. The Eustachian Tube, when in its normal position, is neither 
closely shut nor wide open. Its office is to effect a renewal of the 
air in the tympanum, to maintain the equilibrium of atmospheric 
pressure on both sides of the tympanic membrane, and to convey 
away the fluids which collect in the tympanic cavity. If it re- 
mained open, so as to permit the acoustic waves of the air fi'om the 
mouth to enter, our own voices would be heard as a roaring sound, 
and the passage of air inward and outward during respiration 
would affect the position and tension of the tympanic membrane. 
That it is opened, however, on swallowing, Valsalva proved two 
centuries ago. For if we keep the nose and mouth closed and then 
swallow, with the cheeks blown violently out, a feeling of press- 
ure is felt in the ears and the hearing is weakened. These effects 
are due to the forcing of the air through the Eustachian tube into 
the tympanic cavity. The tube is thus of indirect service in re- 
spect to the physiological functions of the middle ear. 

§ 27. III. The Internal Ear, or Labyrinth, is the complex organ 
in which the terminal fibrils of the auditory nerve are distributed 
and the end-organs of hearing situated. It lies in a series of cav- 
ities channelled out of the petrous bone. It consists of three parts 
—the Vestibule, the Semicircular Canals, and the Cochlea. In each 
osseous part a membranous part is suspended, corresponding to it 
in shape, but filHng only a small portion of the bony cavity which 
contains it. It is in the labyrinth that the acoustic waves trans- 
mitted by the tympanum are analysed and changed from a physi- 
cal molecular process to a nerve-commotion, by the special end- 
apparatus of hearing. 

(A) The Vestibule is the central cavity of the internal ear ; it is 
the part of the labyrinth which appears first in animals and is most 
constant. The membranous vestibule is composed of two sac-like 
dilatations-the upper and larger of which is named utriculus, the 
lower mcculas. In its outer wall is the fenestra ovalis ; its anterior 
wall communicates with the scala vestibuli of the cochlea, and at its 
posterior wall the fine orifices of (B) the Semicircular Canals open 



into the utriculus. These canals are three in number, are bent so 
as to form nearly two-thirds of a circle, and are about, an inch in 
length and ^^ of an inch in diameter. They are called the supe- 

No. 1. 

No. 3. 

No. 3. 

Ke vaa 
Ks \ \ ^^ 


Fig. 65. No. 1, Osseous Labyrinth of the Left Ear, from below ; No. 2, of the Right Ear, from 
the inside ; No. 3, of the Left Ear, from above. (Henle.) Av, aqueduct of vestibule ; Fc. fossa 
of the cuchlea ; Fee, its fenestra {rotunda) ; Pv, fenestra of the vestibule {ovalU) ; ha, external 
ampulla ; h, external semicircular canal ; Tsf, traetua spirnH-i foraminosus ; vaa, ampulla of 
the superior semicircular canal ; vc, posterior semicircular canal ; and vpa, its ampuUa. 

rior, the posterior or vertical, and the external or horizontal canals. 
The contiguous ends of the superior and posterior canals blend to- 
gether and have a common orifice into the vestibule. They all 


Fio. 66.— Osseous Cochlea of the Right Ear, ex- 
posed from in front. ■•/, . (Henle.) t, section 
of the division-wall of the cochlea ; +t, upper 
end of the same. Fee, Fenestra ; H, hamulus ; 
Md, modiolus ; Ls, lamina spiralis. 

Fig. 67. — Cross-section through the Acoustic 
Nerve and the Cochlea, ^/j. (Henle.) Nc, 
nerve of the cochlea ; Nv, nerve of the vesti- 
bule ; St, .scala tympani ; Sv, scala vestibuli ; 
and between them the ductus cochlearis, Dc. 
Ls and Md, as in preceding figure. 

have a regular relative position, their planes being at right angles 
to each other. Near the vestibule they dilate to about twice their 
average diameter and form the so-called ampullce. Both the osseous 




vestibule and the osseous canals contain a fluid (the penlymi^h), in 
which the membranous vestibule and canals are suspended ; the 
membranous labyrinth is also distended with a similar fluid (the 

(C) The Cochlea is by far the most complex part of the laby- 
rinth ; it is about ^ of an inch long, and is shaped like the shell of 
a common snail. It, too, consists of a membranous sac embedded in 
the osseous cavity. The whole passage of the cochlea is imperfectly 
divided into two canals by a partition-wall of bone, which is wound 
2|- times around an axis (the modiolus), from the base to the apex, 
somewhat like a spiral stair-case. It is called the osseous lamina 
spiralis. Of the two canals or passages thus formed, the one which 
faces the base of the cochlea is called the scala tympani ; since it 
has its origin in the cir- 
cular aperture (fenestra 
rotunda) which leads to 
the tympanic cavity. The 
other, which faces to- 
ward the apex, opens 
into the vestibule, and 
is called the scala vesti- 
buli. At the apex of the 
cochlea these two scalae 
communicate with each 
other through a small 
hole [helicotrema). The 
division of the mem- 
branous cochlea is com- 
pleted by a membrane 
(the basilar membrane, or membranous spiral lamina), which bridges 
the interval between the free edge of the osseous spiral lamina and 
the outer wall of the passage ; it is attached to this wall by the spiral 
ligament. Another membrane (the membrane of Reissner) arises 
from a spiral crest (limbus, or crista spiralis) attached to the free 
edge of the osseous lamina, and extends to the spiral ligament, so 
as to form a small aqueduct between it and the basilar membrane 
(the scala intermedia, or ductus cochlearis, or canal of the cochlea). 
It is in the vestibule, in the ampullae of the canals, and in the scala 
intermedia that the nervous end-organs of hearing are to be found. 

§ 28. The auditory nerve, on approaching the labyrinth, divides 
into a vestibular and a cochlear division. The former enters the 
vestibule and subdivides into five branches — one for the utriculus, 
one for the sacculus, and one for each of the three ampullae. In 



3. — Section through one of the Coils of the Cochlea. 
(Schematic, from Gegenbaur.) 



each of these dilatations the membranous wall forms a projecting 
ridge, called the crista acoustica. The endothelial investment of 
the crista is elongated into columnar cells, intercalated between 
which are fusiform cells. Each of the latter, according to Max 
Schultze, and others, has the peripheral and the central process 
with which we are already familiar in the nerve-cells of other end- 
organs of sense. The peripheral process projects into the eudo- 

lymph as an auditory 
hair ; while the central 
extends into the subendo- 
thehal tissue where the 
nerve-plexus of the audi- 
tory nerve ramifies, with 
the terminal branches of 
which it is probably con- 
tinuous. According to 
more recent observers 
(Eetzius and others) the 
auditory hairs are con- 
nected with the columnar 
cells, and do not project 
into the endolymph, but 
into a soft material of in- 
distinctly fibrillar struct- 
ure. The inner surface 
of the epithelium of the 
crista is thus clothed with 
a thick-set " wood " of 
these hairs. Max Schultze 
found their length to be 
about ^1^ inch — their ul- 
timate ends, however, be- 
ing too fine to discriminate. Calcareous particles, called " ear- 
stones " (otoliths) apj)ear in both saccule and utricle, embedded in 
a soft matrix and lying in contact with the nerve-epithelium. In 
the vestibule the hair-like prolongations of the epithelial cells are 
more scanty than in the ampullae. 

§ 29. The terminal nerve-apparatus of the cochlea is even far 
more complicated and remarkable. The cochlear branch of the 
auditory nerve pierces the axis of the cochlea (modiolus) and gives 
off lateral branches which pass into the canals of the osseous spiral 
membrane. Here they radiate to the membranous spiral lamina, 
and are connected with a ganglion of nerve-cells ; beyond the gan- 

Fig. 69. — Scheme of the Nerve-endings in the Ampullse. 
(After Kiidiiiger.) 1, membranous wall of the anipiillag, 
with a structureless border. 2 ; through which the nerve- 
fibre, 3, sends its axis-cylinder, 4 ; 5, plexiform connection 
of the nerve-fibres ; 6, auditory cells ; 7, supporting cells ; 
8, auditory hairs. 



glion they form a plexiform expansion, from wliicli the delicate 
fibrils — losing their medullary sheath and becoming extremely 
fine axis-cylinders — pass through a gap in the edge of the lamina 
into the organ of Corti. The connection of their ultimate fibrils 
with the cone-cells of this oi'gan may be assumed, but is difiicult to 

The organ of Corti is situated on that surface of the basilar 
membrane which is directed toward the ductus cochlearis. Its 
structure is a wonderful arrangement of cells. Some of these cells 
are curved, elongated, and placed in two groups — an inner and an 
outer. They are called the "rods," or "pillars," or ''fibres of 

Fig. to. — Organ of Corti in the Dog. ^00/^. (Waldeyer.) h — c, homogeneous layer of the basilar 
membrane ; u, its vestibular layer ; v. its tympanal layer ; d, blood-vessel ; f, nerves in spiral 
lamina ; g, epithelium of spiral groove ; A, nerve-fibres passina; toward inner hair-cells, /, k ; I, 
auditory hairlets on inner hair-cells ; / — l^, lamina reticularis ; m, heads of the rods of Corti 
jointed together ; the inner rod seen in its whole length ; the outer one broken off : n, cell at 
base of inner rod ; p, q. r, outer hair-cells ; s. a cuticnlar process probably belonging to a cell 
of Deitcrs : t, lower ends of hair-cells, two being attached by cuticular processes to the basilar 
membrane ; to, a nerve-fibril passing into an outer hair-cell ; «, a sustenticular cell of Deiters. 

Corti." The cells of the inner grouj) rest by a broad foot on the 
inner part of the basilar membrane, project obliquely forward and 
outward, and expand into a dilated head ; the cells of the outer 
gi'oup rest in the same way, incline forward and inward, and fit 
into a depression in the head of the cells of the inner group. The 
two thus make a boio, which arches over an exceedingly minute 
canal (the canal of Corti) formed between them and the basilar 
membrane. These rods of Corti increase in length from the base 
to the apex of the cochlea. The basilar membrane is composed 
of fibres arranged in a transverse direction, so that each rod rests 
upon one, or upon a pair of these fibres. Internal and almost 
parallel to the inner group of rods is a row of compressed conical 


cells with short and stiff hair-like processes [inner hair-cells). 
External and almost parallel to the outer group are four or 
five rows of hair-cells {outer hair-cells) which are attached to the 
basilar membrane, while their other extremity projects as a brush 
of hairs through the reticular membrane (membrane of Kulliker). 
This latter membrane is a very delicate framework, perforated 
with holes, through which the hairs of the outer hair-cells project, 
and which extends from the inner rods to the external row of hair- 
cells. It acts as a support for the ends of these cells. The inter- 
val between the outer hair-cells and the spiral ligament is occupied 
by cells of a columnar form (the sujpporting cells of Hensen). The 
organ of Corti is covered over and separated from the endolymph 
of the ductus cochlearis by the so-called membrana, tectoria. 

§ 30. The problem before the labyrinth of the ear is in part the 
same as that solved by the tympanum, namely, the problem of con- 
veying the acoustic waves to the true end-apparatus of hearing. 
The repeated shocks of the stirrup at the fenestra ovalis — and per- 
haps, in far less degree, the pulsations of air at the fenestra ro- 
tunda — produce waves in the fluid of the labyrinth. Any mole- 
cular oscillations of this fluid, thus occasioned, cannot, however, 
act directly as the appropriate stimulus of the sensations of sound. 
Since the dimensions of the whole mass thrown into vibration are 
so small in comparison with the length of the acoustic Avaves that 
the extension of the shock from the stirrup would be practically 
instantaneous tln-oughout, and since the surrounding walls may be 
regarded as absolutely immovable by any such impact, the laby- 
rinth-water would act as an incompressible fluid. It would, there- 
fore, be unsuitable for the transmission of various kinds of acoustic 
waves. But different parts of the labyrinth are capable of yielding 
to the waves in the fluid caused by the repeated shocks of the 
stirrup. Four such places, into which, as they yield, the fluid of the 
labyrinth can retreat (as it were) are designated by Hensen ; ' these 
are the two openings of the aqueduct of the vestibule, the mem- 
branes of the aqueduct of the cochlea, the pores of the blood-vessels 
in the bone, the membrane of the fenestra rotunda by bulging out 
into the tympanic cavity. Impulses started in the fluid of the 
labyrinth would thus result in its movement back and forth, so as 
to produce a friction of the end-apparatus. This friction would 
be increased by the action of the otoliths, or minute calcareous 
particles, found in the fluid. Thus the waves started at the fenes- 
tra ovalis would be diffused over the vestibule and into the scala 
vestibuli of the cochlea, where they would flow to its head, being 
' In Hermann's Handb. d. Physiol., III., ii., p. 106. 


prevented by the separating membrane from entering the scala 
tj'inpaui. To what extent these waves flow through the helico- 
trema, or small hole at the apex of the cochlea, into the scala tjm- 
pani, and wliat are the exact relations between the waves in this 
latter scala and those in the scala vestibuli — cannot be stated con- 
fidently. Nor can the exact part of the basilar membrane at which 
the excitation of the end-organs by the oscillations of the structure 
begins, be indicated with certainty. This membrane is, however, 
undoubtedly thrown into vibration through the unequal pressure 
of the moving fluid ; and by its vibration it excites the nervous 
structures Avith which it is intimately connected. 

§ 31. A still more difficult problem for the labyrinth to solve 
may be described in one word as a problem of "analysis." The 
inner ear is not, indeed, contrived so as to reproduce changes in 
the form of the acoustic oscillations, as such, after the manner in 
which these changes can be made apparent to the eye or to touch. 
But all our analogies for the analysis of composite tones — the 
" clangs ■' or musical notes of ordinary experience — are derived from 
the process of sympathetic vibrations. We are led, then, to inquire 
whether any part of the structure of the ear is capable of enough 
such sympathetic vibi'ations to account for the experience which 
we have in recognizing all the possible degrees of pitch in the scale 
of musical sounds. The structure must also be such as to receive 
the impressions produced by a number of simultaneous tones, com- 
posing a harmony. Moreover, it must be such as to represent 
tones that follow each other in rapid succession, as do the notes of 
a melody. The sympathetic vibratory apparatus of the labyrinth 
must therefore cease its vibrations immediately upon the cessation 
of the sounds in sympathy with which it vibrates. In other words, 
it must either have a damper, or be so constructed as to return 
at once to a state of rest without such a damper. It must be capa- 
ble of being thus excited, and of returning to a state of rest, no 
fewer than five hundred times in a second, since the crackling 
of electric sparks, between which the interval is no more than .002 
of a second, can be heard as distinct noises. Still further, the end- 
apparatus of hearing must suffice for all kinds of noise, as distin- 
guished from musical (ones ; and it is extremely difficult to see how 
the same apparatus which serves for the analysis of the clang can 
also suffice for all the various sensations of noise. 

The manner is not known in which the auditory hairs and stones 
and cells of the vestibule and ampuUse, and the rods of Corti, the 
fibres of the basilar membrane, and the conical hair-cells of Dei- 
ters, in the cochlea, actually discharge the required functions. The 


structure of the end-apparatus in the vestibule and semicireulai: 
canals is plainly not adapted to the analysis of musical tones. The 
otoliths found in the vestibule, and the hairs of the ampullse, are 
not capable of regular sympathetic vibrations ; moreover, they form 
no scale of structures corresponding to the scale of sensations of toue. 
This fact has led to the assumption that these organs are designed to 
act as the end-organs of noise instead of musical sound. The more 
complicated structures of the ductus cochlearis do seem, on the 
contraiy, to be adapted for the required analytic functions. It was 
first argued by Helmholtz that the bov^s formed by the rods or 
fibres of Corti are enough in number to constitute such a scale of 
structures that this work of analysis can be assigned to them. 
Some three thousand of these fibres, arranged in rows upon the 
basilar membrane hke the keys of a piano-forte, if distributed over 
seven octaves would give about thirty-three for a semitone. They 
might then be supposed to be elastic ; and since they differ in 
size, to be tuned for particular sounds, so that the sympathetic 
vibration of each one of them cori-esponds to the sensation of a 
given toue. But the rods of Corti are stiff and not easily vibratory ; 
and their office is probably simply to constitute a support for the 
hair-cells. Moreover, birds, which are undoubtedly capable of ap- 
preciating musical notes, have no rods of Corti. 

Hen sen has shown ' that the basilar membrane is itself in a good 
degree gi'aded to pitch ; its continuous structure and expansion in 
size from the beginning to the end of the ductus cochlearis en- 
courage the assumption that its individual radii act like stretched 
strings to respond to the different tones, from the lowest to the 
highest. The calculations of Helmholtz have tended to confirm the 
view of Hensen.. It is assumed, then, that the parts resting upon 
this membrane would be moved up and down, and that the excita- 
tion of the conical hair-cells — with which the terminal fibrils of the 
auditory nerve are supposed to be connected — is thus brought 
about. The number of the acoustic cells is claimed to be about 
great enough to correspond to the demands made upon the organ 
which shall be instrumental in the physical analysis required as a 
basis for the sensations of musical tones. The claim is at best 
doubtful. As Hensen himself remarks,^ the possibility is by no 
means excluded that the working of this complicated and delicate ap- 
paratus may be altogether different from that conjectured by all such 
theory. In other words, the physiology of the peripheral mechanism 
of hearing is as yet in a very incomplete and unsatisfactory state. 

1 Zeitschrift f. wiss. Zool., XIII., p. 481 f. 

2 In Hermaun's Ilaudb. d. Physiol., III., ii., p. 104 f. 


8 32. A brief description of the End-Organs of Motion, or motor 
end-plates, will suffice for our purposes. In general, the termina- 
tions of the efferent nerves are connected either with electrical 
organs (as, for example, in the torpedo), or with secretory glands, 
or with the muscular fibre. We consider only the last of these 
three cases. 

After an efferent nerve has entered the substance of the so-called 
voluntary or striated muscle, it subdivides among the individual 
muscular fibres, separating these fibres from each other. Such 
nerve-twigs usually lose their medullary sheath, and their axis- 
cylinder splits up into fibrils, whose exact mode of termination has 
been much debated. It appears now to be demonstrated (by 
Kiihne, Margo, Eouget, and others) that the axis-cylinder itself 
pierces the sarcolemma or sheath of the muscular fibre ; that the 
neurilemma becomes continuous with the sarcolemma ; ' and that 
the fibrils, into which the axis-cylinder divides, form a flat, branch- 
ing mass within certain peculiar, disk-shaped bodies situated inside 
the sarcolemma, and called "motor end-'plates.'" In the non-striated 
(or non-voluntary) muscles, the nerves divide and subdivide to form 
more and more minute plexuses of nerve-fibres, which are distrib- 
uted in the connective tissue that separates the muscular fibres from 
each other. The exact relation between this extremely minute in- 
tramuscular network of fibrils and the nuclei of the cells of mus- 
cular "fibre " is not yet made out. 

The shape and structure of the motor end-plates are different for 
different animals, and even for different muscles of the same ani- 
mal. Indeed, the mode of the termination of the motor nerves in 
the muscle appears to be somewhat distinctive of the different 
parts of the muscular structure. Sometimes the axis-cylinders are 
somewhat enlarged, with strongly granular corpuscles attached or 
adjacent. Sometimes a granular mass with its nuclei forms a kind 
of base or floor for the terminal nerve-fibres ; and this eminence 
may be elongated, elliptical, or circular. But the character and 
variety of these forms are of no particular interest to psychology, 
even as approached from the physiological point of view. 

' The question of histology is debated, whether the neurilemma actually 
becomes continuous with the sarcolemma. Strictly speaking, according to 
Kiihne, it does not ; but then, strictly speaking, it is not continuous with it- 
self. It is, as we have seen (p. 36 f), divided by the annular constrictions into 
members which are separate structures. It is to be considered as fringed 
out on its edge and cemented to the sarcolemma [See on this subject the 
monograph. Die Verbindung d. Nervenscheiden mit dem Sarkolemm, Sepa- 
ratabdruck aus der Zeitschrift fiir Biologie, by Kiihne.] 


§ 1. The life of the individual man, so far as it can be made an 
object of immediate observation and scientific description begins 
as an undifferentiated germ, "without apparent distinction of bodily 
organs or of physical and psychical activities. This living germ 
undergoes a development. Before it can be subjected to ordinary 
inspection it has unfolded itself into an elaborate organism ; and, 
in its normal relation to the other sj'stems of this organism (mus- 
cular, respiratory, metabolic, reproductive, etc.), the nervous system 
has acquired all its complex mechanism, consisting of an indefinite 
number of parts. Wliat are the different stages of the development 
of this nervous system, and what are the laws according to which 
its different factors and organs become differentiated, it belongs to 
the science of Embryology to describe. But it belongs to psychology 
to make such doubtful inferences as suggest themselves concerning 
the psychical activities that are to be ascribed to the unfolding 
mind of the embryo. Psychology, indeed, attempts in such a case 
to form a picture of those earliest and most obscure mental states, 
the elements of which can no longer be rejDroduced or recombined 
in the developed consciousness of the adult. To this fact is due, 
in part, the doubt which clings to all such inferences. But this 
doubt is also due to the fact that embryology itself is so incomplete, 
even in respect to its possession of single facts, and yet more in- 
complete in respect to its power to set forth any system of general 
truths and laws. 

Our knowledge of all the earlier states and changes of con- 
sciousness is wholly a matter of the interpretation of states and 
movements of the bodily organism, in terms of our own conscious 
mental experience. If, then, it were found that certain physical states 
and motions of the human embryo need for their interpretation 
the assumption of preceding or accompanying mental states, we 
should have the right to carry our psychological principles back to 
the life of this embryo — even back to its beginning in the undif- 
ferentiated germ from which the whole development proceeds. 


But as the case now stands, the proper physical science cannot 
claim to have furnished us with the requisite description of these 
antenatal-physical movements and states. Little use for the main 
purposes of Physiological Psychology, therefore, can be made of 
facts accessible as to the embryonic development of man. "We 
might even seem warranted in passing by the whole subject with 
two or three general observations like the following : The two-fold 
life of man, both nervous mechanism and mind, begins in what is 
apparent only as a physical unity, in that system of moving mole- 
cules which constitutes the living germ. Out of this unity, and in 
indissoluble connection with it, the two-fold human life then pro- 
gressively develops. The mechanism unfolds itself, increases the 
complexity of its molecular activities, runs its course of changes, and 
is broken up again into its material elements. The mind manifests 
itself in primitive activities, unfolds itself, increases the complexity 
of its psychical life, and then ceases to make itself known through 
the physical mechanism, when the mechanism itself is dissolved. 
And all the while the molecular mechanism and the mind are most 
closely and mysteriously correlated in their development as a to- 
talit}', and in their particular activities. 

But in spite of the fact that embryology furnishes psychology 
with scanty material for any extended and trustworthy conclusions 
with regard to the earliest activities and development of the mind, 
at least a sketch of its principal outlines, so far as the nervous sys- 
tem is concerned, seems desirable. Of knowledge fi*om direct ob- 
servation concerning the early development of the human embr^^o 
there is exceedingly little. Yet the comparatively few facts which 
are indisputably known, throw considerable light upon the nature 
and functions of the human nervous mechanism. Moreover, in cer- 
tain most important particulars there is good reason to believe that 
the earliest history of the development of the embryos of other 
animals is substantially like that of the human embryo. The very 
first things in the life of the chick — or better, one of the mammals 
—for example, may be described as probably holding good in all 
important respects for the life of man. And when those diifei'ences 
wlaich are most strikingly human begin plainlj'^ to appear, they 
show what parts of the nervous system are most worthy of em- 
phasis as distinctively connected with man's mental life.' 

§ 2. The immature ovarian ovum of the common fowl — lil^e that 

' The following description is taken to a large extent, and in some places 
almost verbatim, from Foster and Balfour's Elements of Embryology, London, 
ly83, and F. M. Balfour, Comparative Embryology, vol. ii. , pp. 177 ff., Lon- 
don, 1881. 


of every other animal — presents the characters of a simple cell. It 
is seen to consist of a naked protoplasmic body which contains in 
its interior a nucleus (the germinal vesicle) and within this a nucle- 
olus (the germinal sjjot). It is enclosed in a capsule of epithelium, 
called the "follicle," or "follicular membrane." As the ovum ma- 
tures, the body of it grows in size and a number of granules make 
their appearance in the interior ; while the outermost layer of the 
protoplasm remains free from them. But as the granules grow 
larger in the centre, other granules appear also in the periphery 
of the ovum. The germinal vesicle, during the growth of the ovum, 
travels toward the periphery where the protoplasm surrounding- 
it i-emains comparatively free from granules. Accessory germinal 
spots make their appearance. The cells of the follicular membrane, 
which were at first arranged in a single row, now become two or 
more rows deep ; and, whereas the immature ovum is naked, its 
superficial layer is now converted into a radiately striated mem- 
brane. Still later, a second membrane appears between this striated 
membrane and the cells of the follicle ; and the former disappear- 
ing as the ovum approaches maturity, the second membrane (called 
the "vitelline") remains alone. Other changes which take place 
after the ovum has ripened and has been discharged into the ovi- 
duct, it is not necessary to describe. They result in the formation 
of the accessoi-y parts of the egg. The only essential constituent 
of the body of the ovum is an active living protoplasm. 

§ 3. Impregnation takes place in the upper portion of the oviduct, 
and consists in the entrance of a single spermatozoon into the 
ovum, followed by the fusion of the two. The spermatozoon itself 
may be considered as a cell, the nucleus of which is its head. On 
entering the ovum, the substance of its tail becomes mingled with 
the protoplasm of the ovum ; while the head enlarges, moves to- 
ward and fuses with a part of the substance of the ovum, thus 
constituting the nucleus of the impregnated e^Q. In this manner 
the jDhysical and mental peculiarities of both parents are trans- 
mitted or carried over to the offspring by means of the actual fu- 
sion of substance derived from the bodies of both. 

§ 4. A process known as segmentation or "yolk-cleavage" follows 
the fecundation of the ovum. This process consists in a succes- 
sive division of the ovum into a number of cells, from which all the 
cells of the full-grown animal are, as it were, the lineal descendants. 
This process has many variations among the different animals. 
The chief peculiarity among the mammals is that the whole mass of 
the yolk is subject to this change. 

By segmentation the germinal disk of the ovum is broken up 



into a large number of rounded segments of protoplasm, called the 
blastoderm. Of these segments those that lie uppermost are smaller 
than those beneath. The beginning of the two layers into which 
the blastoderm divides is thus made. The behavior of the nucleus 
formed by the union of substance from the male and the female, 
during the process of segmentation, has not been so satisfactorily 
traced ; it appears probable, however, that a pi'ocess of division goes 
on in it also. Other nuclei, thought to be derived from the primi- 
tive nucleus, make their appearance immediately below the blasto- 
derm. The distinction between the upper and lower layers of the 
blastoderm now becomes more obvious, for the segments of the 
former arrange themselves side by side, with their long axes vertical, 
as a membrane of columnar nucleated cells ; while those of the 
latter continue granular and round, and form a close, irregular 
net-work of cells, whose nuclei are not easily seen. 

§ 5. The principal difference between the ovum of a mammal and 
that of a bird depends upon the amount and distribution of the 
food-yolk. The ovum of the mammal is small — the human ovarian 
ovum being only from 
t\s ^^ -ih of an inch 
in diameter — because 
it contains so little 
food-yolk ; but this 
small supply is dis- 
tributed uniformly 
throughout. In con- 
sequence of the above- 
mentioned difference, 
the ovum is able to 
break up into seg- 
ments through the 

whole of its protoplasmic mass. As the process of segmentation 
goes on, the differences among the ova of different species of ani- 
mals become more clearly marked. For example, in the rabbit, 
although the details are differently described by different observers, 
at the close of the process of segmentation the ovum appears to be 
comj)osed of " an outer layer of cubical hyaline cells, almost en- 
tirely surrounding an inner mass of highly granular, rounded, or 
polygonal cells." In a small circular area, however, the inner mass 
remains exposed. The outer cells soon close over the exposed spot 
(called by van Beneden, hlaatopore), and thus form a superficial 
layer. A narrow cavity then appears between the two layers, 
which extends so as to separate them completely, except in the 

Fig. Tl. 

Fig. 72. 

[GS. 71 and 73.— Fructified Human Egg of 12-1.3 days, seen 
from the surface and the side. In the centre o£ the former is 
what Keichert considers the embryonic area. 


region near to the spot originally exposed. The enlargement of 
the ovum and of the cavity together, soon give the whole structure 
the appearance of a vesicle with a thin wall and a large central 
cavity. This vesicle is called the blastodermic vesicle. The greater 
part of its walls is composed of a single row of outer flattened 
cells ; while an inner lens-shaped mass of cells appears attached to 

Fig. 73. — Vascular Area and Embryonic Area of the Embryo of a Babbit, seven days old. ^^j^, 
(Kolliker.) o o, the vascular or opaque area ; ag, embryonic area; pr, primitive streak and 
groove ; rf, medullary groove. 

a portion of the inner side of the outer layer. The " blastodermic 
vesicle " enlarges rapidly ; its inner mass of cells loses its lens-like 
shape, becomes flattened, and spreads out on the inner side of the 
outer layer. Its central part remains thicker and forms an opaque 
circular spot on the blastoderm, which is the beginning of the area 
where the embryo is to fol-m (the embryonic area). 

§ 6. The immediately subsequent history of the development of 
the mammalian ovum, until the appearance of the so-called "primi- 
tive streak," is less perfectly understood : Foster and Balfour ' speak 
of the following description as "tentative." In the embryonic 
area the cells of the inner mass become divided into two distinct 
strata, an upper one of i-ounded cells which lies close to the 
flattened outer layer, and a lower one of flattened cells (the "hypo- 

' Elements of Embryology, p. 316 f. 



blast"). The former becomes fused with the outer layer, and 
thus gives rise to a layer of columnar cells (the " epiblast "). In 
this way the embryonic area consists of two layers of cells ; the 
upper one of which is the epiblast, and the under one the hypoblast. 

The blastoderm at first, then, consists of only two layers, which 
constitute a double-walled sac (the gastrula) ; but a third layer 
soon makes its appearance between the other two. These three 
l^jevB— epiblast, mesoblast, and hypoblast— are called "germinal 
layers " and are found in the embryos of all forms of vertebrate, 
and most forms of invertebrate animals. The middle one, or meso- 
blast, arises from certain parts of the other two primitive layers, in 
a manner which need not be described. From these three germi- 
nal layers, all the different parts of the organism of the animal are 
developed. The history of the development of every animal in its 
earlier stages is, therefore, a narrative of the changes which take 
place in the three layers of the blastoderm. The hypoblast^ is 
the secretory layer ; and from it almost all the epithelial hning 
of the alimentary tract and its glands is derived. The mesoblast 
is the source of "the skeletal, muscular, and vascular systems, and 
of the connective tissue of all the parts of the body. But it is the 
epiblast which produces the central and peripheral nervous system, 
the epidermis, and all the most important parts of the organs of 
sense. It is to the development of the epiblast exclusively, then, 
that we now direct our attention. 

§ 7. The process of differentiating the layers of the embryo is 
intimately connected with another, which results in forming a 

Fig ''4 -Primitive Stroak of the Embryo of a Rabbit, eight days and nine hoiars old. 22"/,. 
(Kmliker) No medullary groove has yet been formed, ax, primitive streak: pr primitive 
groove f;;/, primitive fold ; ect, ectoderm (or epiUast) ; mes, mesoderm (or mesoblast) ; ent, en- 
toderm {hypoblast). 

structure known as the primitive groove. This process is substan- 
tially alike in mammals and in birds. A short sickle-like thickening 
of the blastoderm, which afterward becomes a " narrow strap-like 
opacity"— due to a forward propagation (linear proliferation) of 


epiblast cells in a straight line — arises near the junction between 
the pellucid and the opaque areas of the blastoderm, and stretches 
inwai'd upon the embryonic area ; it is called the primitive streak. 
The median line of the primitive streak then shows a shallow fur- 
row, running along its axis. This furrow is called the primitive 
groove. (Compare Fig. 73.) 

§ 8. Now occurs the formation of the medullary groove. In that 
portion of the embryonic area which is in front of the primitive 
streak, the axial pai't of the epiblast thickens ; two folds arise along 
the boundaries of a shallow median groove ; the folds meet in front, 
diverge behind, and then enclose between them the front part of the 
primitive streak. These are the medullary folds, and they constitute 
the first definite features of the embryo. The part bounded between 
these folds is called the " medullary plate ; " its supreme impor- 
tance in the embryo ajopears in the fact that it is the portion of the 
epiblast which gives rise to the central nervous system. At about 
the time of the development of the medullary groove (a little earlier) 
an important change is taking place in the constitution of the 
hypoblast in front of the primitive streak. An opaque line ap- 
pears, as seen from the surface, and is continued forward from the 
front end of the streak, but stops short at a semicircular fold near 
the front part of the pellucid area. This fold is the future head- 
fold of the embryo. The opaque line is due to a concentration of 
cells in the form of a cord ; it is the beginning of what is known as 
the nolochord. It is to subsequent changes in connection with the 
notochord that we are to look for the development of the distinct- 
ively vertebral structure of the animal. 

§ 9. From this point onward the shaping of recognizable parts 
of the embryo proceeds rapidl}'. The pellucid area, which was at 
first quite flat or slightly curved, has, in the process of its growth, 
suffered a " tucking in " — as it were — ^of a portion of the blasto- 
derm, in the form of a crescent. It is this tuck which, when viewed 
from above, appears as a curved line marking the margin of the 
medullary groove. Thus the blastoderm is at this spot folded in 
the form of the i-eversed letter 8 ; the fold is the one already re- 
ferred to as the "head-fold." Of the two limbs of this 8-fold, 
the upper is continually growing forward and the lower is contin- 
ually growing backward. As the head-fold enlarges rapidly, the 
crescentic groove becomes deeper ; and at the same time, the over- 
hanging margin of the groove rises up above the level of the blasto- 
derm. The medullary folds meantime increase in height and lean 
over from either side toward the middle line. They soon come in 
contact in the region which will afterward become the brain, and 



thus form a tubular canal (the medullary or neural canal), although 
they do not for some time coalesce. As the upper limb or head of 
the embryo becomes more prominent, the medullary folds close 
rapidly, and, in the region of the head quite coalesce. The open 
medullary groove is thus converted into a canal or tube, which is 
closed in front but remains open behind. The fi-out end of this 

Fig. 75. — Pore-part of an Embryo-chirk at the 
end of the second day, viewed from the Dorsal 
Side. 'I/,. (Kolliker.) V h. fore-brain ; A b I. 
occular vesicles ; JIh. mid-brain : /?/*, hind- 
brain : /T, part of the heart seen bulginsr to 
the rig'ht side; Vom. vitelline veins; J/"*', 
medullary canal, spinal part ; Mr', raednllary 
wall of the mid-brain ; U w, proto-vertetral 

Fig. 76 — Embryo of a Rabbit, eight days and 
fourteen hours old. ^-'"/j (Kolliker.) a p. 
pellucid area ; v. anterior edge of the circuit 
of the head; A', fnre-brain ; //', region of 
later mid-brain ; h'", position of the hinder 
brain ; hz, po.sition of the heart ; rf, medullary 
groove ; I'lr, medullary ridge ; uw. meso- 
blastic somite; pz, lateral zone ; si?, vertebral 

neural canal — having a more rapid grovrth than the rest — dilates 
into a small bulb or vesicle, the ca\ity of which remains continuous 
with that of the rest of the canal, while its walls are similarly formed 
of epiblast. This bulb is the so-called first cerebral vesicle ; and the 
lateral processes which soon grow out from its sides are called optic 
vesicles. Behind the first vesicle, a second, and afterward behind the 
second vesicle, a third is soon formed. Thus these three brain-buds, 




& medulla 

or germinal Drains, are made. At the level of the hind end of the 
head, two shallow pits appear (the auditory piti^) which are the rudi- 
ments of the organ of hearing. Thus the closing-up of the medul- 
lary canal has converted the original medullary groove into a 
neural tube ; and three cerebral vesicles have been grown which 
are to develop into the fore-brain, the mid-brain, and the hind- 

§ 10. The most important changes which now take place in 
the development of the nervous mechanism, are connected with 
the growth of the three cerebral vesicles and with the flexure of 
the medullary canal. The front portion of this canal— that is, the 
fore-brain with its vesicles — in consequence of inequalities of 

growth in the different 
parts of the brain, be- 
comes bent downward ; 
this is the commence- 
ment of the cranial flex- 
ure. As the flexure pro- 
gresses, the front portion 
becomes more and more 
folded down, so that the 
second vesicle, or mid- 
brain, comes to project in 
front of it. From the 
front part of the fore- 
brain the vesicles of the 
cerebral hemispheres 
grow out and swell lat- 
erally, so as to make two 
buds corresponding to 
the two hemispheres of 
the brain. Each of these side-buds has a cavity which is continu- 
ous behind with the cavity of the fore-brain ; each cavity becomes a 
lateral ventricle of the brain. The original vesicle of the fore-brain, 
having ceased to occupy its front position, is developed into the 
parts surrounding the third ventricle. In the hind-brain, or third 
cerebral vesicle, the part nearest to the mid-brain becomes marked 
off by a constriction ; the hind-brain is thus separated into two 
parts — the rudimentary cerebellum with the pons in front, the 
rudimentary medulla oblongata behind. 

§ 11. Various differentiations of the lining of the epiblast, which 
is involuted along the cerebro-spinal cavity, take jDlace. Through 
the length of the neural canal this lining is thickened at each 



N. trig. 

Fig. 77. — 4, Brain of an Embryo of the Rabbit. B, Brain 
of an Embryo of the Ox. In both cases the side-wall of 
the left hemisphere is removed. (After Mihalkovics.) 


side, so that tlie cavity is no longer circular, but resembles a narrow 
vertical slit. In the region of the cerebral hemispheres the sides 
and floor of the canal are much thickened, but in the region of 
the third and fourth ventricles, its roof becomes excessively thin, 
^ so as to foi-m a membrane consisting of scarcely more than a single 
layer of cells. 

§ 12. Another important event, at about this stage in the 
development of the embryo, is the formation of the cranial and 
spinal nerves. The cranial nerves sprout out of a continuous 
band (the neural band), composed of two plates, which connects 
the dorsal edges of the neural canal with the external epiblast. 
This band separates from the epiblast and becomes a crest on the 
roof of the brain, with its two plates fused together. The crest 
extends forward as far as the roof of the mid-brain. As the roots 
of the cranial nerves grow centrifugally and become established, 
the crest connecting them is partially obliterated. The posterior 
roots of the spinal nerves are outgrowths of a series of median 
processes of cells that appear on the dorsal part of the cord. These 
outgrowths are symmetrically arranged, and attached to the walls 
of the cord ; but their original attachment is not permanent. Such 
rudimentary posterior spinal nerves divide subsequently into three 
portions— a rounded portion nearest to the cord, an enlarged 
middle portion forming the rudiment of a ganglion, and a periphe- 
ral portion forming the commencement of the nerve. The origin 
of the anterior roots of the spinal nerves is less satisfactorily made 

§ 13. In the further development of the hind-brain the medulla 
oblongata undergoes changes of a somewhat complicated character. 
Its roof becomes extended and thinner ; where the two lateral halves 
of the brain were at first united (at the raphe) a separation takes 
place, so that the sole union of the two sides is by a single row of 
cells. The thin roof of the fourth ventricle is thus formed. The 
floor of the whole hind-brain becomes thickened, and on its outer 
surface a layer of longitudioal non-medullated nerve-fibres appears. 
The roof of the anterior part of the hind-brain, which has become 
thickened instead of thinned out — thus forming the rudimentary 
cerebellum — is developed, first, by the formation of the median lobe 
(or vermiform process) and, afterward, by the swelling of its sides so 
as to constitute the cerebellar hemispheres. 

§ 14. The changes in the development of the mid-brain (or 
mesencephalon) are comparatively simple. When the cranial flex- 
ure has taken place, the mid-brain is left at the front end of the 
axis of the body, as a single vesicle with a vaulted roof and a curved 



floor, whose cavity is known as the aqueduct of Sylvius. The cor- 
uora quadrigemina of the two sides are marked off from each other 

by the appearance of a vertical 
furrow about the sixth month ; 
and about a month later a 
transverse depression sepa- 
rates the anterior (nates) and 
posterior [testes) pairs. The 
thickening of the floor of the 
mid-brain gives rise to the 
crura cerebri. 

§ 15. Of the two divisions 
into which the fore-brain has 
already become divided, the 
posterior constitutes the so- 
called " thalamen-eephalon." 

Pig. 78.— Head of the Embryo of a Sheep, cut Tllis bodv is at first a simple 
through the middle. 3/]. (Kiilliker.) u, under ■ -, \, n j. • m 

jaw ; z, tongue ; s, septum ?iarmm ; occipitale vesicle, formed OI Spmule- 
basilare ; th. thalamus opticus; vt, roof of the , i n •,! m e 

third ventricle: cp. posterior commissure ; m//, sliapcd CellS, Wltll WailS OI 
mid-brain divided by a fold into two parts ;/, falx - .„ j.i • i Ti„ 

cerebri ; /', terminal plate of the fore-brain. At nearly Unilorm thlCkUCSS. ItS 
the prolongation of the line of fm is the foramen 
of Monro, t, tentorium cerebelli ; cl, cerebellum ; 
pi, plexus of the fourth ventricle. 

floor gives rise to the optic chiasm 
and the origin of the optic nerves, 
and to the rudiment of the infundi- 
bulum ; and its sides become thick- 
ened to form the optic thalami, while 
the interval between them enlarges 
toward the base and constitutes the 
cavity of the third ventricle. The 
more complicated changes which its 
roof undergoes give rise to the pineal 
gland and other small surrounding 
structures. It is the anterior and 
larger portion of the fore-brain which 
constitutes the rudiment of the cere- 
bral hemisiDheres. In this cerebral 
rudiment, also, a floor and a roof may 
be distinguished. The former is de- 
veloped into the principal basal gan- 
glia, the striate bodies ; the latter into the structures of the cerebral 
hemispheres proper. The formation of the striate bodies (corpora 
striata) is in fact due to thickenings of the walls of the floor of this 

Fig. 79 —Brain of Human Embryo of five 
months, with Basal Ganglia laid bare. 
Natural size. (Kolliker.) st, corpus 
striatum ; o, optic thalamus ; la, ante- 
rior lobe (lunatus) of the cerebellum, 
and Ip, posterior lobe of the same ; ss, 
semiltinariR superior, and si, inferior ; 
p, pyramid. 



rudiment. The laying of the commissures is the characteristic 
feature of the development of the mammalian hemispheres. These 
Tire the anterior commissure, the fornix, 
and the corpus callosum. But into the 
details of this process we do not need 
to enter. One characteristic of the em- 
bryonic development of mammals is the 
early enlargement of the cerebral hemi- 
spheres ; in the human embryo they are 
even by the tenth week much larger 
than all the other parts of the brain. 
At this time they are hollow bodies 
with comparatively thin upper walls, the 
lateral ventricles being dilated and com- 
municating with each other through a 
wide opening, and with the thii'd ven- 

by the foramen of Monro. They 

from befoie backward, and thus 
cover up, one after the other, the optic 
thalami, corpora quadrigemina, and cere- 
bellum. Their floor keeps on thicken- 
ing, and. thus the striate bodies become 
greatly enlarged, and project upward into 
the lateral ventricles, giving these cav- 
ities their arched form. 

The following table, exhibits the rela- 
tions, with respect to their development, in which the different 
parts of the brain stand to its fundamental rudiments : 



— Brain and Spinal Cord 
Fcetiis, four months old. 
(KOlliker.) h, hemispheres of 
the cerebrum; m, corpora quad- 
rigemina (or mesencephalon) ; 
c, cerebellum; too, medulla ob- 
longata ; S.9, spinal cord with 
its brachial and crural enlarge- 

f 1. Prosencephalon, 
I Fore-brain. 

I. Anterior prima- j 

xy vesicle. 

1 2. Tlialamencephalon, 
1^ Inter-brain. 

II. Middle primary J 3. Mesencephalon, 
vesicle. | Mid-brain. 

4. Epe ncephalon, 
J Hind-brain. 

I 5. Metencephalon, 
1^ After-brain. 

III. Posterior prima- 
ry vesicle. 

f Cerebral Hemispheres. Cor- 
! pora Striata, Corpus Callo- 
j sum. Fornix, Lateral Ven- 
[ tricles, Olfactory bulbs, 
f Thalami Optici, Pineal gland, 
J Pituitary body, Third Ven- 
[ tricle, Optic nerve (prima- 
I rily). 

f Corpora quadrigemina. Crura 
) Cerebri, Aqueduct of Syl- 
) vius. Optic nerve (secon- 
l darily). 
' Cerebellum, Pons Varolii; 

anterior part of the Fourth 

j Medulla Oblongata, Fourth 
( Ventricle, Auditory nerve. 

^ Taken from Quain's Anatomy (Ninth Edition), II. 

p. 828. 



Fissura parieto- 

Fig. 81. — Brain of a Six-months Human Embryo. Natural size, 
(Kolliker.) ol, olfactory bulb ; /«, fissure of Sylvius ; c, cere- 
bellum ; p, pons Varolii ; /, flocculus ; o, olive. 

The more important convolutions and sulci of the cerebral hemi- 
spheres (those called "primitive ") result from the folding of the 

whole substance of 
the wall of the hemi- 
sphere ; the less im- 
portant (the so-called 
"secondary") consist 
merely of depressions 
and elevations of its 
more superficial por- 
tion. The former ap- 
pear earlier — the first 
of the primitive sulci 
being the fissure of 
Sylvius, which is visi- 
ble before the end of 
the third month. By 
the end of the seventh 
month almost all the principal features of the cerebral hemispheres, 
both convolutions and sulci, are already fixed. 

§ 16. The nervous parts of the eye are differentiations of certain 
lateral growths of the germinal brain-buds, called the "optic 
vesicles." The optic vesicles are outgrowths from the sides of the 
first cerebral vesicle, and are originally connected with it by short 
and wide stalks ; at first they stand out at nearly right angles to 
the axis of the embryo. The stalks soon become narrower and thus 
form the rudiments of the 
optic nerves ; ' at the same 
time the rudiments of the 
retina are formed from the 
vesicles themselves. The 
bulb of the optic vesicle is 
made into a cup with two 
walls by doubling it upon 
itself ; thus a second optic 
vesicle or "optic cup" is 
produced, as distinguished 
from the original one. The 
lens of the eye is made by 
thickening some of the superficial epiblast and involuting it in- 
ward over the front of the optic cup, or secondary optic vesicle. 

' But His and Kclliker suppose these nerves to be formed by secondary em- 
anation from tlie chiasm or nervous centre. 

Fig. 82. — LouKitudinal Sections of the Eye of an Embryo, 
in three stages. (From Remak.) 1, commencement 
of the formation of the lens, I, by rlepression of a part 
of h, the corneous layer ; u. r, the primitive ocular vesi- 
cle is doubled back on itself by the depression of the 
commencing lens. 2. the depression for the lens is now 
encloscil, with the lens beginnine' to be formed on tho 
inner side: the optic vesicle is more folded back. 3, 
a third stage, in which the secondary ojjtic ve.sicle, g I, 
begins to be formed. 


This involution has at first the form of a pit, then of a closed 
sac with, thick walls, then of a solid mass. The cavity between 
the two walls of the optic cup is closed up by bringing the walls 
into contact. The subsequent development of the different parts 
of the eye is conditioned upon the fact that the walls of the 
optic cup grow more rapidly than does the lens, and that their 
growth, does not take place equally in all portions of the cup. It 
is by changes in the surrounding mesoblast, which takes on the 
character of an investment, that the outline of the eyeball is defi- 
nitely formed (the choroid and sclerotic). The vitreous humor 
also is a mesoblastic product which is supposed to originate as a 
kind of transudation through the so-called choroid slit. Of the 
two walls, the inner or anterior is originally somewhat thicker ; 
and since, in most parts of the cup it grows more rapidly, it con- 
stantly increases in relative thickness. But just in front of a line 
which afterward becomes the ora serrata, both layers soon cease 
to thicken and then completely coalesce ; thus the hind portion or 
true retina becomes marked off from the ciliary ridges and the iris, 
while the wide opening of the optic cup is narrowed into a smaller 
orifice that constitutes the pupil. By differentiations of the inner 
or anterior wall of the hind portion of the optic cup — its cells mul- 
tiplying rapidly and undergoing morphological changes while the 
wall is thickening — the different layers of the retina are formed. It 
is a significant fact that in its early stage this wall resembles the 
brain in its structure, and may be considered as a part of that organ. 
It is not necessary to enter into a more detailed description of the 
development of the different parts of the eye. 

§ 17. The ear originally appears on either side of the hind-brain 
as an involution of the external epiblast, sunk in a mass of the 
mesoblast. It is then simply a shallow pit with a wide-open 
mouth. The mouth closes up and the pit then becomes a closed 
vesicle (the otic vesicle) which is lined with epiblast and sur- 
rounded by mesoblast. As the walls of this vesicle thicken, its 
cavity enlarges. The shape of the vesicle is at first nearly spheri- 
cal, but it soon becomes triangulai', with the apes of the triangle 
directed inward and downward. It is by elongating this aj)ex that 
the rudiment of the cochlear canal is formed. Part of the vesicle 
becomes stretched into a long, narrow, hollow process (the recess us 
vestibidi), and from the outer wall of the main body two pi-otube- 
rances grow, which are the rudiments of the vertical semicircular 
canals. These parts of the auditory labyrinth are soon more clearly 
defined. The cochlear canal is further elongated and curved ; the 
recess us is also stretched out more ; and from a new protuberance 


tlie horizontal canal is developed. Another protuberance, which 
becomes apparent at the inner commencement of the cochlear 
canal, is converted into the sacculus by being constricted on either 
side. The rest of the cavity, into which all the other parts open, 
may now be called the utricidus. Dilatations of the semicircular 
canals form the ampullae. When the cochlear canal has reached two 
and a half coils, the thickened epithelium of its lower surface 
forms a double ridge, from which the organ of Corti is developed. 
For the details of the structure of the labyrinth we refer to the 
previous description of this end-organ of sense. 

§ 18. All the coarser differentiations of structure to which refer- 
ence has thus far been made are only the expression — as it were — • 
of certain histogenetic changes which have been secretly taking 
place. The laj'ing down of delicate threads of nervous tissue, the 
proliferation of nerve-cells along definite lines of movement, have 
resulted in combining these elements by a living process into the 
organs of the neiwous mechanism. The white matter of the cord 
is supposed to result from a difierentiation of the outer parts of its 
superficial cells into longitudinal nerve-fibres ; the latter remain, 
however, for a considerable time without their medullary sheath. 
The white matter first appears in four patches at the front and 
back of either side, in which the individual fibres seem like smaU 
dots. The gray matter of the cord is formed by a differentiation 
of the principal mass of the Avails of the medullary canal. The 
outer cells first lose their epithehal-like arrangement, and then 
become converted into true nerve-cells, with prolongations that 
constitute nerve-fibres. The early histological character of the 
parts of the brain which lie back of the cerebral hemispheres is 
very similar to that of the spinal cord. In the floor of the hind- 
brain and mid-brain a superficial layer of delicate nerve-fibres is 
early formed. The cells internal to the nerve-fibres give rise to 
the epithelial layer which lines the cavities of the ventricles and to 
an outer layer of gray matter. In the fore-brain the walls of the 
hemispheres become divided into two la3'ers, between which the 
fibres of the crura cerebri interpose themselves. The inner layer 
unites with these fibres to give rise to most of the white matter of 
the hemispheres ; the outer layer of rounded cells becomes further 
differentiated into the outer part of the gray matter, which has 
comparatively few cells, and a deeper layer with numerous cells, 
the latter forming the principal mass of the gray matter of the 

§ 19. The preceding description of the outlines of the develop- 
ment of the human nervous mechanism is derived for the mosi 


part, from 'the study of other embryos than those of the human 
species. It is probably, however, substantially true for the latter 
also. It is valuable for the purposes of Physiological Psychology, 
chiefly as emphasizing vphat has already been said concerning the 
structure and functions of this mechanism in its developed form. 
The nature of the process by which the nervous system is devel- 
oped, as well as the nature of the developed structure and its func- 
tions, as far as physical science can go at all, leads us in the direc- 
tion of a mechanical theory. But in respect to both, such a theory 
is at present in an exceedingly fragmentary and uncertain condition. 
Further investigations may largely remove the present limitations. 
But the most complete theory possible can hardly be more than 
a statement of the order and extent of physical changes, the real 
causes and meaning of which it lies beyond the power of a mechani- 
cal theory to give. 

The impregnated ovum does, indeed, become converted into the 
developed organism by an evolution that, at every step in its course, 
appears as an alteration in the arrangement of material molecules, 
under conditions derived from the original nature of the molecules 
themselves, from their necessary relations to each other, and from 
the action of their total environment. By division of that which 
was single into several parts, by bending of that which was straight, 
by stretching in one direction and compressing elsewhere, by swell- 
ing and dilating in the various outlines under the influence of press- 
ure, by folding and tucking in so as to close up an opening here 
and form another there, by laying down cells of the same kind in 
right lines or grouping them in masses, etc. — in brief, by motion 
of pai'ticles of matter in such way that the motion of each is con- 
ditioned upon that of the others, the nervous mechanism is built 
up. What it can accomplish in the way of further molecular mo- 
tion, after it is thus built up, depends of course in large measure 
"Upon what it is made to be by the very process of building. Bow 
far it is possible even to propound a mechanical theory of the build- 
ing process belongs to the speculations of embryologists to con- 
sidei*. It is our next problem to consider as a whole the few data 
upon which it has been thought possible to base a mechanical 
theory of the behavior of the nervous system after it has once beea 
constructed as a result of the embryonic process. 


§ 1. The macliine-like nature of much of tlie structure and move- 
ment of the human body does not escape the most ordinary obser- 
vation. When this body, either as a whole or with respect to some 
of its parts, changes its position in space, its various masses sup- 
port and act upon each other in essentially the same manner as 
the masses of matter which compose the parts of any machine con- 
structed by human skill. Such movement is possible for it, because 
its framework of boues has a rigidity sufficient to sustain the other 
less rigid organs ; and because the bones are so divided, and yet 
articulated, that they can assume different relations toward one an- 
other in accordance with the simplest principles of mechanics. 
The laws of the lever, of the pulley, the ball-and-socket joint, etc., 
need no modification when applied to this particular machine of the 
human body. 

The action of certain other of its parts, which do not belong 
to the bony framework but which are of muscular or epithelial 
stnicture, is also plainly of the same machine-like character. The 
movement of the heart, for example, is in part to be explained as 
that of a pump with chambers and valves ; and the flow of the 
blood thi'oug-h the arteries as that of a fluid pumped through con- 
duits, of unlike and changeable sizes. So, too, the lungs may be, 
with considerable propriety, compared to bellows which alternately 
suck in and expel the surrounding atmosphere. The optics of the 
eye and the acoustics of the ear are special only so far as the stnict- 
ure of the organs makes necessary a special application of the gen- 
eral laws of those sciences. Moreover, the distribution of the 
fluids among the tissues of the body takes place under the laws 
which govern the distribution of fluids generally when separated by 
membranes which they can permeate. Nor is the chemistry of the 
same tissues and fluids by any means wholly unlike that with which 
the experiments of the laboratory make us familiar. When, how- 
ever, we begin to speak of those changes of relative position which 
take place at extremely minute distances among the molecvdar ele- 


ments of which the larger masses of the body are composed, we 
seem compelled to drop the conception of a machine and to seek 
both another conception and another title. 

The very attempt, then, to explain the motion of the more purely 
machine-like parts of the human body, leads us to consider certain 
activities of other parts for which the word "mechanism " is more 
appropriate. The movement of none of the more or less rigid or- 
gans of the body originates within these organs themselves. The 
changes of relative position in the parts, with which the ordinary 
laws of mechanics deal, imply antecedent molecular changes in 
other parts with which these laws cannot deal. The motion which 
finds its final expression in the swing of the arm, or of the leg, in 
the lifting of a weight, and even in the contraction of the heart, or 
in the rising and falling of the chest, does not begin in arm, or leg, 
or ribs, or diaphragm, or cardiac muscles. The change of position 
of so considerable masses of matter is but the summing-up of in- 
numerable minute molecular changes which began within the body, 
but outside of the masses themselves. If, for example, we inquire 
as to what causes the bones to move — however strictly their mo- 
tion, once begun, may follow the laws of mechanics — the answer is 
to be found in the pull of the tendons, or cord-like structures, 
which are attached to them. And if we then inquire, What causes 
the tendons to pull upon the bones by means of their attachment ? 
the answer must be, That it is the contraction of the muscles which 
pulls upon the tendons. 

The next step in following this chain of causes, however, intro- 
duces us to a different class of considerations from any of the fore- 
going. For we cannot say that the contraction of the muscles is 
caused by the pull of the nerves upon them. The movement of 
muscular fibre in contraction is an altogether dififerent affair from 
the movement of the bones as they are pulled by the muscles ; nor 
do the nerves act upon the muscles as the muscles act upon the 
tendons. The elasticity of the muscles is, indeed, a mechanical 
quality, like that of which we avail ourselves in the construction of 
machines. But the quality of elasticity does not fully explain the 
behavior of the so-called muscle-nerve machine when its muscular 
tissue is contracting or relaxing. Yet the Hving muscle, in itself 
considered, may certainly be looked upon as a molecular mechan- 
ism. It is a system of minute particles of matter which act upon 
each other at indefinitely small distances ; and which, when any 
motion is set up at one part of it, propagates such motion accord- 
ing to laws that are given in the very constitution and arrangement 
of the particles themselves. This is precisely what we understand 


by a pbj-sical molecular mechanism. The office of the nerve with 
respect to the muscle is simply, as we know, to start that molecular 
activity which it is the function of the irritated muscle itself to ex- 
ercise. The nerve, however, cannot perform its office of irritating 
the muscle without being in a state of molecular commotion called 
the " excitement " of the nerve. And, further, this excited condition 
of that jDart of the nerve which is in immediate contact with the 
muscle is itself a state of the nerve which has been propagated from 
a distant point of the nervous matter. All the machine-like move- 
ments of the masses of the body require us, therefore, to look for 
their origin in minute molecular changes that originate in its ner- 
vous elements. And for the further account of these neural molec- 
ular changes we are to look to a mechanical theory of the nervous 

§ 2. The basis for a general view of the nervous system as a 
mechanism has been laid in all the preceding examination ; and 
it cannot be denied that the results of this examination are such 
as to dispose us favorably toward the attempt to develop such a 
view into a complete mechanical theory. Physical science, as a 
matter of course, strives to establish such a theory. It knows no 
other way of considering any group of phenomena when brought 
before it for examination. To deny totally the application of the 
conception of a mechanism to the action of the nervous system 
would be to refuse to apply to its phenomena the same scientific 
treatment which we apply to all other physical phenomena. To 
limit, a priori, such application would be to restrict improjperly, on 
merely theoretical grounds, the area of the phenomena with which 
such science is entitled to deal. The fact that molecular changes 
here are correlated with another class of phenomena which we call 
" mental," in no wise destroys the propriety of pushing our physical 
science of the nervous system to its furthest possible limits. The 
movements of all material bodies, whether in the elemental shape 
of the molecules, or in the shape of the same molecules when aggre- 
gated into masses, as well as the laws under which such bodies in 
movement act and react upon each other, constitute the legitimate 
sphere of physical science. But it is to a system of interacting 
molecules that the conception of mechanism especially ajDplies. 
The aim of physical research with regard to any given system of 
this kind is, therefore, not accomplished until all the movements 
of its different parts are explained in the light of a consistent 
mechanical theory. This general principle of all physical science 
neither needs nor permits a special exception in the case of the 
human nerves, organs of sense, and brain. 


On the other hand, the very unsatisfactoiy condition of the data 
for a mechanical theory of the human nervous system has been im- 
plied in each of the preceding chapters. It -will appear all the 
more plainly now as we present briefly a statement of two or three 
such theories in the form in which it has been found possible for 
different investigators to state and to defend them. Nor can we 
express much confidence that physics and physiology combined 
will ever be able to point to a complete theory of so intricate and 
delicate a mechanism as this nervous system. Moreover, we do not 
by any means affirm that a purely mechanical treatment, however 
complete, would of itself suffice to furnish a satisfactory understand- 
ing of all the phenomena ; or even that the phenomena in general 
could by any possibility be brought solely under the terms of such 
treatment. We only affirm the unrestricted right of jDhysical sci- 
ence to attempt, in the light of the conception of mechanism, an ex- 
planation of the nervous system as well as of all other physical 
subjects ; and also its right to its ^Dersistent faith that — So far as 
■physical science can explain any such subject, all the s^Decial difficul- 
ties of the nervous system can be fitly considered only in this way. 

§ 3. The chemical constitution and structural form of the ele- 
ments of nervous matter require that the system which they com- 
pose should be regarded in the light of the concej)tiou of mechan- 
ism. It is true that physical science cannot give an accurate descrip- 
tion of the chemical processes which take place in the formation of 
the nerve-fibres and nerve-cells, or during their functional activity ; 
it cannot do so much as this for the living tissues generally. But 
it finds here the same chemical elements which exist elsewhere 
in nature, esj)ecially the four elements, oxygen, hydrogen, nitrogen, 
and carbon. It nowhere finds these elements behaving differently 
in the nervous system from the way in which it is their nature to 
behave elsewhere, under similar circumstances. And the fact that 
precisely similar circumstances do not occur to induce the same 
combination and interaction of these elements outside of the ner- 
vous sj'stem, is traced back to its causes in a succession of occur- 
rences that all have the character belonging to the chemistry of 
living tissues. We know of no sap which is suitable for forming- 
organisms in general, but which is itself a perfectly homogeneous 
fluid. Nucleated granules in the very chemical constituents which 
give conditions to all the subsequent activity of the molecules, are 
revealed by microscopic examination of those cells from which the 
whole body springs. This fact, together with the character of the 
subsequent process, may lead some to insist that a certain special 
form of energy (called "vital force," or by some less obnoxious 


title), is marslialling the minute particles under its superior control 
But such way of considering the phenomena — whether admissible 
or inadmissible — does not at all help us to dispense with the purely 
mechanical point of view. In the original living germ with which 
the organism began, and in all its subsequent development, every 
chemical change in nervous matter is nothing more than a move- 
ment of physical molecules, strictly under the conditions furnished 
by their constitution and previous arrangement. 

The general significance of the chemical constitution of ner- 
vous matter, with reference to a mechanical theory of the nervous 
system, is by no means wholly obscure. It is obvious that all the 
energy expended in the movement of the body as a whole, or of its 
larger masses, originates in minute molecular changes. The latter 
changes have, of course, a direct relation to the chemical constitu- 
tion of the tissues in which they occur. The muscular fibre can 
contract because its molecules admit of that rearrangement in 
which the contraction essentially consists ; for doing the amount of 
work implied in such rearrangement, this fibre is, of course, depen- 
dent upon its own chemical constitution. But the source of the 
excitation of the muscle is to be found in antecedent molecular 
changes within the nervous system ; indeed, all the changes that 
are to be summed up in the work done by and within the rigid 
masses of the body have their origin here. It accords, then, with 
the mechanical conception of the nervous system that its chemistry 
should be just such as we have seen that it actually is. Nervous 
matter holds in store a large amount of energy that is easily dis- 
posable ; of energy that will be yielded freely and rapidly if any- 
thing occurs to start the process within the system of molecules 
of which such matter is composed. For the molecules are of such 
kind as readily break up and recombine their elements in simpler 
forms ; in doing this they render kinetic a large amount of energy 
which they have previously held latent. 

No mechanical theory of the nervous system can explain the 
meaning of all the various structural forms which the elements of 
this system assume. It cannot be told, for example, what pecuUar 
place in the mechanism belongs to the different shapes of nerve- 
cells, bipolar, multipolar, stellate, etc. Nor can a complete picture 
be drawn of the dift'erences in character which the nerve-commotion 
takes as it passes from the nerve-fibres to the nerve-cells, or from 
one nerve-cell to another. We can only insist upon the undoubted 
general fact that all these structural forms have whatever signifi- 
cance belongs to them, because they are themselves molecular 
structures, capable of undergoing, in relation to each other, those 


very changes in which the functional activity of the nervous system 

§ 4. There can be no doubt that the arrangement of the nervous 
elements into a system corresponds to the conception of mechanism. 
A certain work of "concatenating" the different physical systems of 
the body, and of adjusting its relations to the changes in its 
environment, requires to be accomplished. This problem demands 
a three-fold exercise of function ; it is a problem in the construction 
of a mechanism. The nervous system actually is of threefold con- 
struction ; its threefold construction is the answer which it prac- 
tically makes to the above-mentioned problem. One part of the 
complex problem consists in the conversion of certain of those 
molecular motions which take place in nature outside of the living- 
organism into molecular motion within the tissues of such organism. 
The solution of this part of the problem is furnished by the end- 
organs of the nervous system. The end-organs are those special 
mechanisms which are adaj)ted to convert the molecular motions 
called stimuli into the molecular motions called neural excitation. 
That by far the larger portion of the eye and ear, for example, acts 
in a purely mechanical way, there is no doubt. It is the office of 
the great mass of the eye to transmit and refract the rays of light ; 
of the ear to transmit and condense the acoustic waves. But when 
the nervous elements of the retina and of the organ of Corti re- 
ceive the physical processes transmitted to them, they transmute 
these physical processes into physiological neural processes ; in 
doing this they act as special molecular mechanisms. 

The second j)art of the complex problem before the nervous sys- 
tem consists iu the conduction in all necessary directions of these 
neural processes ; only on this condition can distant parts of the 
nervous system act, as it were, in view of each other, and thus the 
whole body be bound into a living unity under the influence of 
changes in its environment, and iu the ideas and impulses of the 
mind. The nerve-fibres solve this part of the problem. This they 
do by acting as mechanisms, which have such a molecular constitu- 
tion and function that a commotion, started at any point in tbe 
physical elements of the system, spreads from molecule to molecule, 
in accordance with the laws of the system. 

The third part of the same complex problem requires for its solu- 
tion structures and functions still more intricate and inexplicable. 
Incoming molecular disturbances must be modified and redistributed 
so as to give rise to outgoing molecular disturbances along definite 
tracts, in order that definite groups of muscles may be made to con- 
tract. Only in this way can the whole physical organism, by a so- 


called reflex activity, adjast its condition, in view of the presence of 
given kinds and degrees of stimuli. Moreover, the vital functions — 
the movements that control respiration, digestion, circulation of the 
blood and of other fluids, etc. — must be united so as to work to a 
common end, and with the modified forms and degrees of their re- 
spective energies, which the changing circumstances require. Still 
further, not only must the neural processes set up by the end-or- 
gans and conducted inward by the afferent nerves have a place of 
meeting in j)i'oximity with the centres of origin for the correspond- 
ing efferent impulses ; but all the neural processes in this place of 
meeting must also be so modified and made mutually dependent 
that they can be correlated, under psycho-physical laws, with the 
processes of mind. It is the central organs which alone possess 
the molecular construction and functions necessary for such won- 
derful reflex and automatic activities. In their highest form — the 
hemispheres of the human brain — they solve the problem of pro- 
viding a system of molecules, whose constitution and changes may 
be immediately related with the phenomena of mind. These central 
organs are extremely intricate physical structures. It cannot be 
pretended that even a beginning has been made toward a satisfac- 
tory theory of their functional activity considered as a special case 
in molecular physics. But this fact does not affect the confidence 
which is based upon what is known of physical structures in gen- 
eral, that in these organs, the changes wliich take place are essen- 
tially of the same order as are those with which the science of mo- 
lecular physics has elsewhere to deal. They are modes of motion in 
which the behavior of each molecule, regarded as a constituent 
element of the system, is conditioned upon the constitution and 
behavior of the other members of the same system. That is to 
say, the central organs must be regarded in the light of the con- 
ception of mechanism. 

§ 5. The general office of the nervous system may, then, be de- 
scribed in somewhat the following manner. The development of a 
rich and varied life, both animal and intellectual, requires a great 
store of sensations and of motions. Tlie sensations are jDrimarily 
designed to serve as signs of changes in the environment of the 
animal to \^hich his condition must be adapted by movement of 
his l)odily parts ; but they are also to serve as a basis for intel- 
lectual attainment and development. The forces of external nature 
continually storm the peripheral parts of the animal's body. In 
order that any of these forces may act as the stimuli of sensations, 
they must be converted into molecular motions within the tissues 
of this body. In order, further, that the masses of the body may 


constantly be readjusted to tlie external changes of which the sen- 
sations are signs, the molecular motions must, in turn, be converted 
into movements of these masses. In other words, a process of con- 
stant interchange must take j)lace between the animal organism 
and external nature. 

Disturbances in one part of the body, by the play upon it of nat- 
ure's energ}', instead of becoming injurious or destructive, are 
thus made serviceable through inducing the needed disturbances 
of other j^arts of the same body. The equilibrium on which life 
depends is maintained. Moreover, the material necessary for self- 
conscious development, for a growing knowledge of the so-called 
outside world, is furnished through the conduction of these dis- 
turbances to their common meeting-places iii the central organs. 

To accomplish the general work of equilibrating the interaction 
of the different jjarts of the body, of readjusting its condition to 
the changing condition of its surroundings, some special construc- 
tion and arrangement of material molecules is necessary. If the 
work is to be done in a highly elaborate way, a very intricate ar- 
rangement of an indefinitely great number of chemically complex 
molecules is necessary. Such an an-angement is the human ner- 
vous system. But just because its arrangement and function are 
of this kind, it is a " mechanism." As a highly complex molecular 
mechanism it utilizes the disturbances which arise from the en- 
vironment. It binds together all the other systems of the body in 
living reciprocity of energies and. functions. Its superficial parts 
are so constructed that they can be set in motion by various forms 
of physical energy — by light, heat, sound, chemical change, etc. ; 
they are also adapted, fitly to modify the impressions thus received. 
The molecules of its conducting nerves are so constituted and ar- 
ranged that they can indicate the path along which the disturbance 
thus occasioned must pass ; they can dictate the conditions and 
laws under which its course must be completed. The molecules of 
its central organs are capable of assuming inconceivably varied re- 
lations to each other, of thus transmuting and redistributing the 
nerve-commotions which reach them along the incoming tracts, and 
even (it would seem) of starting automatically outgoing disturb- 
ances in response to self-conscious sensations and ideas. 

But all the foregoing offices of the nervous system are nothing 
but the jnovements of physical elements, in constant reciprocal de- 
pendence upon each other, though in response to excitations lying 
outside of the system itself. To move thus is the function of a 
molecular mechanism. So far as science can control the different 
parts of the nervous system for experimental purposes, it finds 


them behaving in such a manner as to make a plain demand for a 
physical and mechanical theory in explanation of their behavior. 

§ 6. The foregoing description of the nervous system as a mech- 
anism, like all similar descriptions, undoubtedly lacks scientific 
quality. It is neither exact nor in such form as to admit of ex- 
l^erimental verification. It is largely based upon conjectures, full 
of gaps and assumptions ; and were it pressed at every point for 
proof, it would be obliged to rely much upon general principles in 
mechanics (the special applications of which to the case in hand are 
by no means certain or obvious), and even to indulge in hopes and 
promises with reference to the future, rather than present demon- 
stration. May we not know more precisely the nature of the mo- 
lecular changes which constitute the functions of nerve-fibres and 
nerve-cells ? Cannot physical science help us to complete these be- 
ginnings of a theory ? 

In answer to the question just raised we have already seen how 
little satisfaction is afforded on applying to the science of chemistry. 
On general principles of physical science there can be little doubt 
that the excitation and conduction of nerve-commotion is dependent 
upon a chemical change in the nervous tissue itself. Moreover, we 
know that the j)rocess of conduction in the nerve requires each of 
its molecules to act upon the neighboring elements as the condition 
of the process continuing. Nor can this process itself be a mere 
impartation of motion, from molecule to molecule ; on the contrary, 
the phenomena of electrotonus seem to show that it must also con- 
sist in the setting free of energy which exists latent within the 
molecules of the nerve-substance. These molecules contain, then, 
by virtue of their constitution, stored or potential energy which is 
converted into kinetic energy in the propagation of the process of 
excitation, and which is expended, in part, in either inhibiting or 
increasing the energy of that process. Such potential energy can 
scarcely be other than chemical. 

Accordingly, we should be tempted to describe the process of 
progressive excitation of the nerve somewhat as follows : Every 
element of the nerve, by reason of its highly complex and unstable 
chemical constitution, contains a large store of energy ; the excite- 
ment of the nerve consists in the explosive decomposition succes- 
sively of these elements of the nerve ; and the result of the decom- 
position is the setting free of the stored energy to be expended in 
part in the excitation of the next adjoining elements. The process, 
then, is not altogether unlike the burning of a line of powder 
grains. Such an hypothesis, however, would at once have to answer 
several difficult questions. "Why does not the whole of the exjilosive 


substance burn up, instead of only an amount of it approximately 
proportional to the strength of the stimulus which sets the process 
ao-oing ? Analogies may indeed be found in the union of chlorine 
and hydrogen under the action of light. What checks the process 
in the nerve as a whole, and what limits it quantitatively in a differ- 
ent way at different points in its course, so as to give the phenomena 
of anelectrotonus and catelectrotonus ? (comp., Chap. III., § 19 f.). 
Moreover, direct observation has as yet discovered no indisputable 
evidence of functional chemical changes in the nerve-fibres. If 
such changes exist at all they are exceedingly small. 

§ 7. Allusion has been made (p. 119 f.) to the fact that the effect 
of several excitations of a nerve-stretch is compounded, as it were, 
in the action of the attached muscle. That is to say, excitations 
which are simultaneous, or which follow each other with sufficient 
and not too great rapidity, are summed up in the nerve, like mo- 
lecular waves of nerve-commotion piled upon each other. Besides 
such phenomena of " summation," there exist analagous phe- 
nomena of so-called " interference ; " and, further, of the facilitat- 
ing effect which one excitation has upon others following it along 
the same paths of conduction, especially in the central organs. 
These and similar phenomena tempt us to consider the activity of 
the nervous substance in terms of an exceedingly complex sum in 
the addition and subtraction of molecular disturbances of a wave- 
like character. Elaborate experiments have been made to deter- 
mine the laws under which such summation or interference of 
electrical excitations takes place. Thus G. Valentin' assumes that 
the case of the nerves comes under the general theory of molecular 
waves that may either be piled upon each other, or may interfere 
with each other. The interferences he calls " positive " when the 
currents are moving in the same direction, " negative " when they 
are moving in opposite directions ; and such currents may, of 
course, be either ascending or descending. The character of the 
interferences is to be defined by the way in which the nerve-muscle 
machine responds to these four kinds of interference. The inter- 
ference has a heightening effect (is erhohende) when the result 
indicated by the behavior of the muscle is greater than the sum of 
two single effects from the partial excitations that are compounded ; 
a depressing effect when the result is less than this sum. If the 
effect of the interference is such as to reduce the result to zero, 
it is called inhibitory. Valentin concludes that, in case of inter- 
ferences of excitations from one and the same current (with respect 

' In PfliJger s Archiv, vii. (1873), pp. 458-496, article on the Interfer- 
ences of Electrical Excitations. 


to degree, direction, etc.), the character of the effects produced 
depends upon the original molecular constitution of the nerve. 
Just as its constitution is decisive with regard to the nature of the 
muscular contractions that follow a single excitation of the nerve, 
so is it also decisive with regard to the results of interference. 
These results, moreover, conform to the same laws after decapita- 
tion or poisoning as before. And further, the same rules govern 
in the case of interferences of two currents, if both the currents 
are of about the same degree of strength. Finally, according to 
Valentin, the same rules belong to the interferences that occur in 
cases of reflex action, or of the stimulation of motor nerves through 
the sensory, as those which apply to the direct stimulation of the 
motor nerves. It is apparent that the only net gain from the fore- 
going experiments consists in the information that the molecular 
constitution of the nerves themselves determines all the variable 
elements in the results of exciting them. But this would be an 
assumption fairly made by every attempt at a physical science of the 
nervous functions. And inasmuch as we can make no such veri- 
fiable statements concerning the nature of this molecular constitu- 
tion as will serve the purposes of a precise mechanical theory, it is 
hard to see what advance has been gained toward the construction 
of such a theory. 

The jDhenomena caused by the reciprocal action of different ex- 
citations within the central nervous system are, of course, much 
more complex and difficult to bring under a theory of molecular 
wave-like impulses, than are the phenomena of the comparatively 
simple nerve-muscle machine. A fortiori, molecular physics is 
unable to propose a satisfactory theory for the central organs. 
According to, Exner,' many of the phenomena are covered by the 
general principle that one excitation acts io facilitate or, as it were, 
smooth the path for others passing, after only a brief interval, along 
the same course. This principle he distinguishes from that of 
''summation," when applied to reflex action. The latter term Exner 
applies to the accumulation in the central organ of excitations 
which, taken singly, are too weak to produce any reflex motion, but 
which by their combined strength do produce such motion. The 
principle of "facilitation," however, refers to the condition of the 
central parts after the passage through them of a stimulus which 
has already called forth some reflex action. Exner's experiments 
led him to conclude that the motor excitation of some one ex- 
tremity from the brain (that is, by stimulating, in the brain, the 
so-called motor area of the extremity) facilitates the subsequent pas- 
' Article in PflLiger's ArcLiv., xxviii., pp. 487 ff. 


sage of reflex stimulus affecting the same extremity ; and, con- 
versely, stimulating an extremity reflexly facilitates the passage of 
a subsequent motor excitation from the area of the bi'ain to the 
same extremity. Thus, for example, the reflex motions of the fore- 
leg of a rabbit, produced by stimulating the toes of that leg, were 
found to be increased in intensity if the so-called cerebral motor 
centre of the fore-leg was also stimulated. Different reflex excita- 
tions also may facilitate each other's effect in the same way. For 
example, the sensory stimulation of the left foot has the effect of 
facilitating the reflex act which, as it might appear, would relate 
only to the right foot and its motor area in the central organ ; and 
such reflex action of the right foot facilitates the contraction set 
fz'ee in the same foot by stimulating the left-foot section of the spi- 
nal cord. Exner was unable, however, to obtain any inhibitory 
effect upon the motion of the extremities by stimulating various 
other places of the cortex of the cerebrum, or by stimulating the 
cerebellum. He also found that when one side of the cortex of the 
cerebrum is stimulated by electricity so as to produce a condition 
of tetanus in one extremity of the animal, the results of two excita- 
tions — one as a reflex from the foot and one directly from the same 
side of the brain — are compounded in a way which seems incom- 
patible with any known form of the summation and interference 
of molecular wave-like disturbances. 

Indeed (to return to the simpler case), Griitzner ' seems justified 
in saying that, strictly speaking, we cannot without qualification 
even represent what takes place when two currents of electricity 
act in combination upon a nerve, as though it were a matter of the 
addition or subti'action of their separate effects. For it is possible 
that an electrical current of an intensity equal to the d,mount of the 
natural nerve-current (current of rest = a) and the current used as 
stimulus (current of action = b), taken together (a + b), will not ex- 
cite a nerve that shows no current at all, although the latter (b) 
alone will excite the nerve if just previously the former {a) was 
present in the nerve. The currents already existing in the nerve, 
when the exciting current is applied, are, therefore, not simply 
added to or subtracted from the latter ; but they produce molecu- 
lar changes of an unknown kind which tend to induce the origina- 
tion of so-called " cathodic " and "anodic" places in the nerve — 
that is, places of exalted and places of depressed excitability. 
Thus a weaker current will excite the nerve when it is in a condi- 
tion of exalted excitability , a stronger current may fail to excite 
the nerve when in a condition of depressed excitability. 

' See Pfliiger's Arcliiv, xsviii., p. 144 f. 


How obscure and complicated are the molecular conditions con- 
nected with the excitation of the nerve is further shown by the 
effect of giving different treatments to the cross-section of the nerve. 
If the nerve is simply cut, its behavior under stimulation is differ- 
ent from that which occurs when it has been bound before the 
cross-section is made. Binding the nerve produces, for some min- 
utes after cross-section, a large increase of its excitability in the 
immediate neighborhood of the injui-ed place ; this is true for all 
kinds of stimuli, including the electric current in both directions. 
From five to ten minutes subsequently, however, the making of the 
current in the opposite direction to the current induced by cross- 
section has frequently a diminished rather than an increased effect. 

§ 8. On the whole, it would appear, then, that the ability to lay 
even a basis for a strictly scientific molecular theory of the nervous 
mechanism depends upon the ability satisfactorily to explain the 
electrical process in the nerves and their consequent behavior under 
electrical stimulation. It would by no means follow that a com- 
plete theory for the comparatively simple phenomena of the nerve- 
muscle machine would furnish the sure clew, not to say the full 
explanation, of all the activities of the nervous system. On the 
contrary, the evidence is overwhelming that the working of the 
complete nervous mechanism involves other principles than those 
which may be deemed sufficient for the case of the single nerve 
and muscle when under electrical stimulation. But, plainly, the 
more complex case cannot be solved without fii'st solving the far 
less complex one. And yet the simplest possible case of nervous 
molecular mechanism — the case that can be brought under the 
most favorable experimental conditions — has thus far proved to 
lie beyond oui" power to find a satisfactory scientific solution. 

The two most important principles which must enter into any 
mechanical theory for explaining the behavior of nerves in relation 
to electricity are, according to Hermann : ' (1) the law of electrical 
excitation, and (2) the law of the so-called current of action. The 
phenomena upon which these laws are themselves based are chiefly 
the phenomena of electrotonus and the phenomena of negative 

It is a fact (see p. 114 f.) that the passage of the electrical current 
through a nerve-stretch produces in the nerve a changed condi- 
tion of excitabilit}^ called electrotonic. This condition is, however, 
different for different parts of the nerve-stretch. It is dependent 
upon the nearness of each part to the electrodes, it being greatest 
in their vicinity. It is dependent on the strength of the polarizing 
' Handb. d. Physiol., II., i., p. 193. 


current and on the length of the stretch through which it flows. 
Its intensity is greater on the side of the anode than on the side 
of the cathode. The condition may be said to be one of increased 
excitabihty in the region of the cathode, of diminished excitabihty 
in the region of the anode. Helmholtz found that the time of the 
development of the electrotonic condition is not perceptibly later 
than that of the electrical current which excites it ; the condition 
originates at the moment of making, and ceases at the moment of 
breaking, the polarizing current. Du Bois-Reymond concludes, 
thereupon, that the electrotonic condition is spread over the nerve 
with a speed equal to that of the process of excitation. 

It is also a fact (see p. 117 f.) that, in the case of the nerve-stretch 
as well as in that of the muscle, the galvanometer shows the pas- 
sage of a current when one of the electrodes is placed at its cut 
end and the other at its equator. It is a fact that this so-called 
natural current, or current of rest, is diminished by the stimula- 
tion of the nerve with an interrupted current, or by other means 
of exciting it — the diminution being shown by the return of the 
needle of the galvanometer toward the zero-point (the so-called 
" vegative variation "). 

§ 9. The two principal theories which have hitherto attempted 
to account for the above-mentioned facts are the theoi'ies of du 
Bois-Reymond and of Hermann. Du Bois-Reymond ' assumes 
that in the substance of the nerve there exists an arrangement of 
electro-motive molecules embedded in an imperfectly conducting 
medium. Each molecule is like a minute battery with positive and 
negative poles ; and the molecules present their positive surfaces to 
the longitudinal surface of the nerve, their negative surfaces to the 
cut ends or transverse sections of the nerve. The presence of these 
molecules gives rise to currents in the medium which surrounds 
them. Owing to the imperfect conductivity of the medium, such 
currents flow in more or less concentric lines at some distance 
from each molecule. The current which exists in the nerves 
(exists, according to du Bois-Reymond, as natural to the nerve and 
previous to its injury by cross-section), and which is made obvious 
by the deflection of the needle of the attached galvanometer, may 
therefore be regarded as the resultant of the numerous unobserv- 
able currents belonging to the several molecules. In this way the 
so-called " cui'rent of rest " is to be explained. Du Bois-Reymond 
is forced to account for the fact that such natural currents are 

' The views of du Bois-Reymond are to be fouud in his Untersuchungen 
liber thierische Electricitat, 1848-49, and Gesammelte Abhaudluugen, etc., 



either exceedingly small or wholly wanting in an uninjured mus- 
cle by a very artificial hypothesis as to a so-called parelectronomic 
region at the place where the ends of the muscle come into contact 
with the tendons. His theory of electrotonus and of the negative 
variation of the nerve-current is too complicated and doubtful to 
be even stated here ; it is enough to say that his assumptions as to 
"peripolar" and "bipolar" molecules, and the efiect of the elec- 
trical current in reversing the molecules, etc., have little to com- 
mend them to the practical workers in modern physics. 

§ 10. The theory which Hermann,' and those who agree with 
him, would substitute for the theory of du Bois-Reymond takes 
its point of starting from a discovery made by Matteucci some 
years ago. In 1863 this truly great investigator noticed phenom- 
ena similar to those of the electrotonic condition of the nerves in 
over-spun wires moistened with a conducting fluid. If an electri- 
cal current is conducted to the moist covering of such a wire, the 
needle of the galvanometer shows along every part of the wire the 
presence of a current in the same direction with the primary cur- 
rent, but with the strength of the former diminishing as the dis- 
tance increases fi'om the points where the latter is applied to the 
wire. No such cun-ent arises, however, if the wire is made of amal- 
gamated zinc and its covering is moistened with a solution of sul- 
phate of zinc. It appears, then, that the electrical condition of the 
wire, when a current is conducted to it, depends upon the limiting 
surfaces of its metal centre and of its moistened covering being po- 
larizable. Very recently ^ Hermann has, as he thinks, still further 
shown the possibility of explaining all the electrotonic properties 
of the nerves after the analogy of Matteucci's discovery. A con- 
ductor consisting of a central and a covering substance, with polar- 
izable hmiting surfaces, as soon as a momentary electiic current is 
conducted through any portion of it begins successively to exhibit 
a current of the same kind at every other place in it ; the more 
• distant the place from the one to which the cui-rent is applied the 
later its appearance there, so that at the most distant places such 
current may begin after it has for some time ceased at the primary 
place. Now, in an analogous manner, every nerve-fibre may be as- 
sumed to consist of a centre and covering substance, with polarizable 
limiting surfaces. In the nerve-fibre the limiting surfaces needed 
for the theory are j)erhaps actually to be found between the axis- 

' The views of Hermann may be found in liis Untersucliungen znr Physiol- 
ogie d. Muskeln u. Nerven, 1867-68, and in numerous later papers in PfliigerV 

^ Pfliiger's Arcliiv, 1885, xxxv., p. 23 f. 


cylinder and the medullary sheatli. Griinliagen,' however, affirms 
that the polarization of the limiting surfaces of the nerve-fibre is a 
consequence rather than a cause of the current in electrotonus. 
The first and fundamental cause of this current he considers to be 
the characteristic difference in the resistance, as conductors, of the 
kernel and the covering of the nerve-stretch. 

The so-called " natural current," or " current of rest," Hermann 
does not consider it necessary to explain. What appears to be a 
natural current Hermann holds to be in all cases the result of in- 
jury. It is to be considered, then, as due to the peculiar molecu- 
lar condition into w^hich certain parts of a nerve-stretch are thrown 
by their mechanical or chemical destruction. In fact, whenever a 
nerve is cut across, or any of its fibres are injured, the molecules 
thus disturbed begin at once to die ; they then become negative to- 
ward the other uninjured parts of the nerve. It is because of this 
change in the dying molecules that the electrical current is devel- 
oped. But all the parts of a wholly untouched and unexcited nerve 
are, according to Hermann, " isoelectric." It is not necessary to 
give the experimental evidence by which this investigator strives 
to prove his opinion ; it is enough to say that this evidence is 
strong, and nearly, if not quite, conclusive. 

Accordingly', Hermann regards the negative variation as not due 
to the diminution of any current previously existing, but rather as 
a manifestation of the electro-motive forces which come into opera- 
tion at the moment, and at the seat, of excitation. This current is, 
therefore, the only true " current of action." Its rise and flow are 
explained by the fact that every excited part of a nerve-stretch be- 
comes negative toward all the other parts. As this wave passes 
along the nerve-fibre, each minute portion becomes first negative 
and then positive toward the adjoining minute portions ; and hence 
the so-called "ad-terminal" and "ab-terminal currents" appear 
along the nerve-stretch as fast as successive parts of its substance 
reach their maximum of negativity. The excess of the ab-terminal 
over the ad-terminal current manifests itself as the so-called '^'■neg- 
ative variation." 

The phenomena of electrotonus Hermann explains, as has already 
been said, upon the basis of Matteucci's experiments. An inner 
polarization, such as takes place between the wire and its moist- 
ened covering, takes place between the substance which constitutes 
the core of the nerve and one of its sheaths. The electrotonic cur- 
rent is, therefore, simply due to an escape of the polarizing cur- 
rent. It is wanting in the dead nerve, because the inner polariza- 
' Pfliiger's Archiv, xxxv., p. 534 f. 


tion belongs only to the nerve in its living state ; it is stopped bj 
ligature or by crusbing, because the nervous substance is thus 
made into dead, indifferent substance, and the functional continu- 
ity of the nervous core is destroyed. His detailed explanation of 
" tetanic action-currents " and " phasic action-currents," and of the 
jDhysiological phenomena of electrotonus and catelectrotonus, need 
not be repeated. The one principle to which Hermann -would re- 
duce all the electrical j)henomena derived from the cut nerve- 
stretch is this : All excitable j^^otoplasm, when dying or irritated, 
becomes negative toward its own univjured and unirritated parts. 
Such is the nature of its electro-motive reaction. 

§ 11. Objections have been made to the theory of Hermann, but 
they can scarcely be said to be so formidable as those which he 
brings against the theory of du Bois-Keymond. The most forcible of 
them is, perhaps, the following : If the so-called currents of rest were 
due solely to the negativity of the dying jjortion of the substance, 
we should not expect that the current from the equator to the cross- 
section would be greater than the current from a point nearer the 
cross- section, seeing that the resistance is greater in the former case. 

Hermann is himself ready to admit,' however, that no simple 
scheme of polarization will fully satisfy the conditions of the prob- 
lem offered by the behavior of the nerve-muscle machine under 
electrical stimulation. " The platinum wire, with its moist sheaths, 
is no model of the irritable nerve ; it is only a model of its elec- 
trotonic properties." We must, therefore, after the discussion of 
all analogies resort again to the unknown molecular constitution 
and properties of the substance of the nerve, as being sui generis, 
for oui' explanation of its peculiar physiological properties. Its 
functions are a species of molecular change, connected, to be sure, 
both with chemical changes and with other mechanical changes of 
a wave-like character, and yet unlike them all ; and these molecular 
changes, when the nerve is excited, ai^e propagated from point to 
point along its course with a speed and according to laws which 
have already been stated (see Chapter IH.). But further than this 
we cannot as yet go with confidence in af&rming a mechanical the- 
ory of even that simplest element of the nervous mechanism for ex- 
perimental purposes — namely, the nerve attached to the muscle 
and constituting the nerve-muscle machine.^ 

' See Handb. d. Physiol., II., i., p. 195 f. 

- Further information upon the two theories of Hermann and du Bois-Rey- 
mond may be found in Foster's Text-book of Physiology, pp. 101 ff. See, also, 
a brief statement of Hermann's theory in the Journal of Physiology, I., pp, 


§ 12. A confession of ignorance might fitly close the entire dis- 
cussion as to the possibility at present of a precise mechanical 
theory of the nervous system. For on resuming the larger and 
more Complicated inquiry, as to how the physiological functions of 
all the nervous organs in their mutual relations may be explained 
according to any known laws of molecular science, we are obliged 
to approach this inquiry with an acknowledged inability to deal 
satisfactorily even with the much simpler case of one of the ele- 
ments of this system. The peculiar forms and laws of the molec- 
ular activity of the entire nervous mechanism certainly cannot be 
understood until we are able to describe and explain the molecu- 
lar activity of a single nerve-muscle machine. A statement of an 
elaborate theory, framed with a view to meet the whole case, by a 
distinguished authority, cannot fail, however, to possess a certain 
interest and value. Accordingly, we shall refer briefly to the 
theory of Wundt.^ 

Wundt begins his discussion of the mechanics (or molecular 
physics) of nervous substance by stating two possible ways of ap- 
proaching the subject. It is conceivable that we might directly 
investigate the chemical and physical constitution of the nervous 
elements, and the changes they undergo in the exercise of their 
physiological functions, with a view to construct a theory of so- 
called nerve-force by induction from such investigation. But the 
preferable — because the much more promising — way of procedure 
is to assume that the processes which take place in the nervous 
system are modes of molecular motion connected with each other, 
and with the forces of external nature, under the general principles 
of molecular physics ; and then, arguing deductively^ to make such 
a combination and application of these principles as will serve to 
meet all the demands of the case. It scarcely need be said that 
Wundt adopts the latter method. 

Assuming, then, the general principles of molecular physics, and 
especially the lav/ of the conservation of energy, it is possible to 
show how living beings may be brought under the control of these 
principles. Such beings, through the regularity with which the 
making and breaking of chemical combinations goes on within 
them, take a noteworthy part in the continuous process of inter- 
changing potential and kinetic (inner and external) energy. It is 
the nervous system, in all the animals that have one, from which 

'To be found, in part, in his Untersuchungen zur Mechanik der Nerven, 
and in later and more complete form in the chapter (vi., Part I.) " Physio- 
logische Mechanik der Nervensubstanz," in his Grundziige der physioiogi- 
schen Psychologie. Leipzig, 1880. 


this process is controlled. The process itself is a species of com- 
bustion ; the nervous system keeps going those functions which 
effect the process, regulates the setting free and distributing of 
the heat, and determines the muscles to movement. The source 
of the special activities of the nervous system itself lies in the nat- 
ure of the chemical combinations which compose it. 

The nervous system regarded as unaffected by stimuli — that is, 
as unexcited — may be theoretically compared to a fluid in a condi- 
tion of equilibrium. But, in fact, the nervous system is never in a 
condition of perfect equilibrium. For, not only is there a ceaseless 
play of energy internal to this system, in which the atoms separate 
from the old combinations as nervous substance to form new com- 
binations as the same substance ; but a continuous process also 
goes on by which the molecules of the nervous substance are broken 
up to form less complex but more stable compounds. Moreover, 
the building of the nervous substance itself out of the nourishment 
brought tc it is a process the reverse of that last mentioned ; it 
is a process, that is to say, in which the more stable chemical com- 
pounds of other substance are broken up and their atoms used to 
form the more complex but more unstable molecules of the nervous 
substance. The process of change from the less stable to the more 
stable combinations represents the setting-free of stored or poten- 
tial energy ; the reverse j)rocess represents the storing of energy 
and the vanishing of kinetic or actual energy. That energy which 
is made apparent by the former process Wundt calls "positive;" 
that which is stored xip, when the more stable combination disap- 
pears, he calls "negative." Positive molecular energy of the ner- 
vous system is recognized as heat set free, as contraction of the 
muscles, etc. ; its negative molecular energy exists in the form of 
heat becoming latent, or of inhibitory action upon the course of 
excitation in the nerves, etc. 

In accordance with the foregoing theory of positive and negative 
molecular energy, as due to the chemical activity of the nervous 
substance, Wundt would explain the process of excitation and con- 
duction in the nerve-fibres. No simple conduction of motion, of 
course, takes place in the nerve ; but certain molecular processes, 
peculiar to the constitution of the nerve, are set up at one point by 
the stimulus, and are then conducted successively to other points 
along its stretch. In all cases when a nerve is irritated two classes 
of opjDosed effects are set up in its substance ; the one is directed 
toward the production of external energy (seci'etion, stimulation of 
the gaijglion-cells, movement of the muscles, etc.), the other toward 
the control of the energy thus set free. The former is positive, 


the latter negative or inhibitory. The general law for all excita- 
tion of the nerves is, that by the application of stimulus the posi- 
tive as well as the negative molecular energy of the nervous sub- 
stance is increased. Stimulating the nei've accelerates both the 
recombination of the atoms of its highly complex molecules in 
less complex but more stable forms, and also the escape of the 
atoms from these forms and their return to the more complex and 
less stable combinations. The i*enewal of the nerve depends upon 
the i-estitution of the more complex molecules ; but the work 
which the nerve does external to itself depends upon that process 
of combustion in which the complex molecules break up and pass 
into more stable but less complex forms. The latter process 
involves, of course, the exhaustion of the nerve. External energy 
(work done outside of the nerve) can then only take place in case 
the positive molecular energy is more accelerated than the negative, 
by the application of the stimulus. 

The entire sum of positive molecular energy which is set free 
when a nerve is ii'ritated may be reckoned as distributed in three 
directions : a part is spent in the continuous excitation of the 
nerve ; another part becomes heat ; still another part is converted 
into negative molecular energy. In this way the peculiar molec- 
ular condition which the nerve-fibre leading from the peripheral 
region assumes, when it is irritated, is imparted to the central re- 
gion of the nerve-cell. 

§ 13. The application of the foregoing theory to the central 
organs of the nervous mechanism requires us to take account of 
the fact that a greater intensity of the stimulus is needed to move 
a muscle through a collection of ganglion-cells than directly by 
stimulating the nerve-fibre connected with the muscle. We are to 
conclude, then, that the nervous substance of the central parts 
offers a far greater resistance to the conduction of the jDi'ocess of 
excitation than is offered by the nerves themselves. On the other 
hand, the central organs are in a condition to develop within them- 
selves a far greater amount of work ; that is, to convert into 
kinetic form a vast sum of energy stored in their chemical con- 
stitution. The proofs which "Wundt brings forward for this view 
are derived from the phenomena of summation of inhibition, and 
of so-called "reflex-poisons," etc. A detailed discussion of such 
phenomena leads to the conclusion that, when " summation " (com- 
pare pp. 223 ff.) takes place, the several excitations along the cen- 
tripetal tracts have been conducted to different sensory central 
regions, and have then passed from them, as a result of their being 
simultaneously excited, over into the same motor elements of the 


central organ; but when "inhibition" takes place, such excitations 
have been conducted so as to come together and counteract each 
other in the same sensory central region. The external conditions 
of those relations which obtain among the different senses and 
sensations are to be found, partly in the constitution of the organs 
of sense, and partly in the nature of their respective stimuli. 

When speculating as to the molecular changes, with respect both 
to positive and also to negative energy, which take place in the 
central organs, our point of starting must be taken from a condition 
of equilibrium assumed to exist in their ganglion-cells. Excitation 
of the central organs, like irritation of the nerves, increases both 
kinds of nervous energy. But the positive molecular energy of the 
central organs is relatively little increased by a momentary excita- 
tion. The result of repeated excitation, however, is to make the 
positive condition largely predominate in the whole central region. 
An excited ganglion-cell is in a condition analogous to that of the 
nerve-stretch at the anode when a constant current is passing 
through it. In the nerve, as a rule, the nervous substance is used 
up, and the process of storing energy goes on in only a very par- 
tial manner. In the cells the production of the complex molecules 
in which energy is stored predominates, as a rule. 

The fundamental properties of nervous matter — mechanically 
considered — are (1) to receive external impressions in order by 
them to be determined in its own molecular condition ; and (2) to 
transform potential energy into kinetic, partly under the immedi- 
ate, and partly under the progressive, influence of these impres- 

Wundt also proposes an elaborate and highly speculative view 
of the molecular constitution and functions of the ganglion-cells. 
Every such cell possesses, he thinks, two regions (although the 
word " regions " is not to be interpreted locally). These regions 
are called "peripheral " and "central," because the former is as- 
sumed to stand in more intimate x-elations to the peripheral ner- 
vous substance, with respect to its own reactions under stimula- 
tion. Excitations which reach the central region of a ganglion-cell 
induce a propagation of the processes set up in this region to the 
other or peripheral region. In the same way do excitations which 
first touch the peripheral region necessitate the spreading of the 
form of molecular energy set free here over into the central region. 
When a process of excitation is frequently conducted in a definite 
direction through some ganglion-cell, such direction is favorably 
disposed toward the conduction of future excitations which may 
reach the same cell. Whether the excitation of any particular 


nerve-fibre connected with a ganglion-cell results in an excitatory or 
an inhibitory effect depends upon the nature of its connection with 
the cell. 

But we refrain from further statement of a theory so largely con- 
jectural. Nothing i-emains but to repeat a confession of igno- 
rance and of inability even to suggest a satisfactory solution for so 
complex a problem in molecular physics as is offered by the human 
nervous system. 

§ 14 A review of various molecular theories proj)osed to account 
for the nervous mechanism, either as a whole or in any of its parts, 
makes plain the important fact that such theories are all obliged 
to assume the origin and continuance of a peculiar molecular 
structure for this mechanism. In other words, no attempt to 
explain how the nervous system acts can avoid the conclusion that 
the determining factor in the explanation must be found in what 
the nervous system is. The physiological functions of the nerve 
depart when the nerve dies. The nerve dies when it is severed 
from the ganglion-cell. Both cell and nerve must, therefore, con- 
stitute a living molecular unity, in order that their normal physio- 
logical functions may be performed. The explanation of these 
functions assumes the molecular constitution of the organs them- 
selves. But how shall we explain, in accordance with the known 
laws of molecular physics, the origin and preservation of such a mo- 
lecular constitution ? It is the business of biology rather than of 
physiology to attempt an answer to this question. But the question 
itself asks from science the performance of a task no smaller than 
that of framing a mechanical theory of life. Biological science can, 
as yet, do little toward framing such a theory. Throughout our en- 
tire discussion of the nervous mechanism we have carefully avoided 
raising an inquiry as to the nature of life, as to the source and con- 
ditions of that very molecular constitution which determines the 
nature and working of this mechanism. We have simply assumed 
and argued that, taking the nervous system for what it really is 
and really does, its structure and functions admit of scientific ex- 
planation, so far as such explanation is possible at all, only when 
they are regarded as belonging to a molecular mechanism. The 
question of a mechanical theory for the origin and constitution of 
living organisms in general lies outside of the inquiries of Physio- 
logical Psychology. 

§ 15. One other important question has also thus far been 
avoided. What is the relation of the mind to the working of the 
nervous mechanism ? Can the mind set this molecular mechanism 
at work, or can it in any way determine the character of its func- 


tions ? As far as our consideration of the nervous system has gone 
hitherto, all might very well have been the same without the exist- 
ence of a single act of conscious thought or feeling occurring in 
any relation whatever to this system. Given the molecular mechan- 
ism as it is constituted and conserved by the forces which control 
as long as life continues ; and given the necessary impact of out- 
side forces ujDon the end-organs, and the projDer changes of blood 
within the central organs ; and it has been assumed that this mech- 
anism would exercise its functions in ways thus far described. 
But the consideration of another class of phenomena is now to be 
introduced ; these are the phenomena of human consciousness, the 
phenomena of 3Iind. The question whether such phenomena can 
be true causes of any of the changes in the molecular mechanism 
is a part of the general question as to the correlations that exist 
between two classes of facts. The answer to such general question 
belongs to the following divisions of our work. 




§ 1. Ordinaey observation recognizes the fact that the phenomena 
of consciousness are more or less definitely correlated with the 
condition of the body. Certain alterations in our mental states, on 
account of the injury of any of its masses, as well as a constant de- 
pendence of those states upon the way some of the masses stand 
related to each other and to the outside world, impress the fact upon 
our daily experience. It is by no means so obvious that the ner- 
vous substance has any peculiar relation to the thoughts and feelings 
of the mind. For the functions of the nervous system are not ex- 
ercised in giving information as to itself, its own condition and 
changes. By aid of these functions we have presented in con- 
sciousness a more or less clear picture of the condition and changes 
of the superficial parts of the body. In the same way a knowledge 
is gained of the successive states of tension belonging to the 
muscles in movement, and even — though rather obscurely — of the 
place and condition of the internal organs. But as long as they 
are healthy and excited with only a moderate intensity of their 
stimuli, the nerves do not even reveal their own existence ; and 
when they are injured or unduly excited, the notice they furnish 
of the fact comes in the form of painful feeling which we have 
learned to localize, not in the nervous substance itself but in the 
adjacent parts of muscle and skin. Attention may be called, how- 
ever, to the peripheral nerves by the accident or the dissecting- 
knife which exposes them to sight. In the case of the central 
nervous organs, and especially in the case of the brain, there is little 
in ordinary experience which leads to a suspicion of their signifi- 
cance or even of their existence. 

It is not very strange, then, that no general recognition of the 
supreme importance of the brain, in relation to the phenomena of 
consciousness, is to be found in early history. It is true that 
Plutarch ' and Theophrastus " inform us of the opinion of the 

' DePlacitis Philosophorum, IV., 17, 1. 
' De Sensu, § 35 f. 


pliysieian Alcmaeon, who is said to have been a younger contem- 
porary of Pythagoras, and who regarded the brain as the common 
meeting-place of the senses. The same view is also ascribed to the 
celebrated Hippocrates. Later on Plato accepted it. But Aris- 
totle,' the greatest of all thinkers in antiquit}', the son of a phy- 
sician, especially educated in physical science, and well acquainted 
for the time in the dissection of animals, regarded the brain as a 
lump of cold substance, quite unfit to be the seat and organ of the 
sensus communis. This important office he ascribed rather to the 
heart. The brain he considered to be chiefly useful as the source 
of fluid for lubricating the eyes, etc. 

§ 2. The opinion of Exner," however, who supposes that feeling 
in no way immediately informs us that we think with the head, 
still less with the brain or the cortex of the cerebrum, seems some- 
what extreme. Concerning the contents oi the cranial cavity, indeed, 
we get no direct information from the feelings connected with 
the exercise of its functions. But we certainly localize in the head 
certain phenomena of consciousness that are inextricably inter- 
woven with the processes of thought. The act of attention results 
in feelings which indicate that the muscles of the eye are being in- 
nervated ; or in the more indefinite and diffused sense of strain 
produced by contracting the skin of the forehead and adjacent 
parts of the face. The special sensations of hearing, smelling, and 
tasting, which impress so strongl}^ our conscious life, are frequently 
referred to the head. The same thing is true of many of the sen- 
sations of sight — particularly of such as appear when the eyes are 
closed, in the form of after-images, or spectra, or indefinite and 
changing color-spots, seated in the upper front part of the face. 
Moreover, that inchoate and sometimes half-articulated language, 
with which we support our ti'ains of thought, even when we are not 
conscious of resortiog to the expedient of " talking to ourselves," is 
felt to be going on within the head. When one has been engaged 
for some time in intense thought, or in eager and concentrated 
observation, one is suddenly made aware of more or less painful 
feelings which are somewhat indefinitely ascribed to the same 
cerebral region. Men commonly lean the head upon the hand 
in supporting meditation ; or rub it vigorously to awaken 
the powers of memory and reasoning ; or stroke it to relieve the 
disagreeable sensations which follow severe mental excitement. 
Headache, of more or less intensity, thus becomes associated with 

1 See De Partibiis Animalium, 052. b. 5; (II., 7); 656, b. 22 (II., 10); De 
Juvent. , 4(!7, b. 28 ; and De Anima. III., 1 and 2. 
' See Hermann's Haudb. d. Physiol., II., ii., p. 193. 


"active exercise of the intellect. The head is wearied with thought ; 
and not only so, but also with intense physical exercise. The dis- 
comfort which bodily strain produces in the hinder regions of the 
head are an indication, although of only a very general kind, that 
processes have gone on in that locality which are of great impor- 
tance to the succeeding states of consciousness. All this apparent 
testimony of immediate feeling is, doubtless, somewhat exaggerated 
in an age so distinctively " nervous" as our own ; and this fact may, 
perhaps, account in part for the inclination of the ancients to em- 
phasize the more obvious connection of mental jDhenomena with the 
heart, and other lower visceral organs, to the neglect of all connection 
of these phenomena with the brain. But it cannot well be doubted 
that a certain amount of testimony from immediate feeling as to the 
important relation which exists between the state of mind and the 
contents of the cranial cavity, belongs to all human experience. 

However uncertain the witness of immediate feeling upon the 
point in question may be, very little observation of others is needed 
to amplify and confirm its witness. We are not infrequently led to 
notice how quickly and profoundly the states of consciousness are 
changed by injuries to the brain. The effect of a blow upon the 
head in suspending consciousness is decisive of this question. The 
intimate local connection between the organs of sense and the 
brain leads naturally to the conclusion that the avenues of sensa- 
tion and of perception have in the latter a kind of gathering-j)lace, 
as it were. It is but a step from this conclusion to a recognition 
of the truth that the physiological significance of the contents of 
the cranial cavity consists in their afibrding a field upon which all 
the impressions of sense can meet together, and so furnish the basis 
and material of comparative thought. Indeed, it was this line of in- 
quiry which probably led certain ancient anatomists, like Herophilus 
and Galen, to locate the soul, or psychical principle, in the brain, 

§ 3. A great multitude of physical considerations, advanced by 
modern science, place beyond doubt the supreme importance of 
the brain in its influence upon the phenomena of consciousness. 
It has already been stated (Part I., Chapter HI., § 7) that the free 
circulation of arterial blood, with its supjDly of oxygen, is a 
necessary condition for the fulfilment of the functions of all the 
central organs ; this necessity is especially marked in the case of 
the brain. The stoppage of one of the great arteries leading to 
this organ, either by compression in the neck, or by embolism at 
some point along its course, at once produces profound dis- 
turbances and even complete cessation of consciousness. It has 
been calculated that, while the weight of the entire encephalon is 


only about one-forty-fifth of that of the body, the supply of blood 
used uj) there is not less than about one-eighth of the whole supply. 
This expenditure is indicative of the large amount of work done 
by the intercranial organs. 

More delicate measurements seem to show that the temperature 
rises and falls in the whole cerebral area, or at particular cir- 
cumscribed regions of the cortex, in close connection with the 
psychical activities. Thus Dr. Lombard found, by measurements 
with exact thermo-electric apparatus, that the temperature of the 
head during waking hours varies rapidly, though slightly (less 
than y^^° C.) ; and that these variations " appear to be connected 
with different degrees of cerebral activity. . . . Every cause 
that attracts the attention — -a noise, or the sight of some person or 
other object — produces elevation of temperature. An elevation of 
temperature also occurs under the influence of an emotion, or 
during an interesting reading aloud." Similar examinations have 
been carried still further by Schiff,' who has appUed extremely del- 
icate thermoscopic instruments directly to the cerebral substance 
of certain animals (comp. Part I., Chapter III, § 21). He finds 
that the arrival of sensorial impressions is followed by a rise of 
temperature, in certain special areas of the cortical substance, where 
— as he supposes— these impressions are diffused; he also con- 
cludes that any resulting psychical activity is itself connected with 
a still further rise of temperature than that which the sensorial 
impressions alone engender. Schiff 's conclusions, therefore, point 
not only to the localization in the entire brain of functions connected 
with the phenomena of conscious psychical life, but also to some 
distribution of such functions among its various areas. In the 
same general direction are the conclusions of Byasson" and others, 
as to an increase of waste in the tissues of this organ, which 
corresponds, to some extent, at least, with the amount of thought 
accomplished. This investigator found that the quantity of sul- 
phates and phosphates excreted, in comparison with the quantity 
estimated as entering into his diet, was notably increased in pro- 
portion to the amount of his mental work. That is to say, in con- 
nection with an increase in the number and intensity of the 
psychical operations stands an increase in the functional activity of 
the cerebral cells, as shown by the expenditure of their phos- 
phorized constituents.^ 

' Archives de Physiologie, 1870, p. 451. 
'' In the Jour, d, Anat. de Robin, 1869, p. 557 f. 

'■' See the chapter of Luys on tlie Physico-chemical Phenomena of Cere- 
bral Activity ; The Brain and its Functions. 



§ 4. Comparative anatomy also indicates the importance of the re- 
lation between the size, structure, and functions of the intercranial 
nervous mass and the jDhenomena of mind. It shows, first of all, a 
general but indefinite correspondence betvs^een the size and weight 
of the brain of any species of animal, as compared with the weight 
of its entire mass, and the place of the same species in the scale of 
intelligence. This fact is roughly exhibited by the following com- 
parative table : ^ 


Tunny-fish . . . 
Land tortoise. 



Elephant . . . . 
Salamander . . 















Finch 1 

Eagle 1 

Pigeon 1 

Rat 1 

Gibbon 1 

Young cat 1 

Sai — ape 1 


Doubtless other tables might be compiled which would lead to 
less satisfactory conclusions than the one given above. Even in 
this table we note that the elephant stands lower than the sala- 
mander or the sheep, both of which animals are, in fact, far in- 
ferior to the elephant in intelligence. Large allowance must also 
be made in certain cases for peculiarities of physical structure ; for 
example, the tortoise is rated lower than he would be were it not 
for his heavy shell. The law itself is confessedly subject to re- 
markable and unexplained exceptions ; at best it holds good only 
in a very general way. For example, the relative weight of the 
brain is not greatly different in the dolphin, in the baboon, and in 
man. It is much greater in the infancy and youth of the human 
species than in middle life or old age. In the male child at birth 
it is about as one to six or seven (according to Tiedemann, 1 to 
5.85 in the male, and 1 to 6.5 in the female). The brain grows with 
great rapidity for the first few years — the increase during the first 
year being estimated at about one cubic centimeter daily. But the 
rest of the body increases so much more rapidly that by the end of 
the second year it is about 1:14 ; by the end of the third year, 1:18. 
It increases in absolute weight until well on into middle life, and 
then after middle life diminishes at about the average rate of one 

' Taken from Hermann's Handb. d. Physiol., XL, ii., p. 193. as compiled by 
Exner on the basis o£ the works, in part of Carus, and in part of J. Miiller. 
The figures of comparative weight between the brain and the body are some, 
what differently given by other authorities. 


ounce in a decade. The average relative weight of the adult brain 
is one-fortieth or one-fiftieth. Tiedemann found that the relative 
weight of the brain is dependent upon the absolute weight of the 
body, and is relatively greatest with light persons. The human 
brain is, however, absolutely heavier than that of any of the ani- 
mals except the elephant (8-10 lbs.) and the whale (5-6 lbs.). 

Much pains has been taken, by actually weighing diiferent 
human brains, or by calculating their weight on the basis of careful 
cranial measurements, to establish a law connecting the amount of 
the iutercranial nervous mass with the comparative intelligence of 
races and of individuals.' The average weight of the brain of the 
adult European is, for the male, fi'om 46 to 52 ounces ; for the fe- 
male, from 42 to 46 ounces. Boyd gives the average weight of the 
brain of the male, at the period of life when it is most developed 
(twenty-five to forty years of age), as 46.8 ounces (1,321 grams, 91 
centigrams). This difference between the sexes is not wholl}' de- 
pendent on difference in bulk of body, but is an important sexual 
distinction. The brain of man is on the average ten per cent, above 
that of woman ; the difference in average stature is about eight per 
cent. Many human brains rise above the upper average ranges ; 
others fall below the lower average ranges ; and yet no marked 
peculiarities of mental development are necessarily connected with 
these variations. Considerable quantities of the substance of the 
brain may be lost (at any rate from some areas of the cortical sur- 
face) without perceptibly changing the mental life. In 278 cases 
of males, the maximum weight of brain was found to be 65 ounces, 
the minimum 34 ounces ; in 191 cases of females, the maximum 
was 56 ounces, the minimum 31 ounces.' Numerous instances of 
large excess in the average weight of brain-mass by individuals 
eminent for intelligence are on record : for example, Byron scarcely 
under 79 ounces ; Cromwell, only 77 grains less, or 78.8 ounces 
(although Vulpian thinks that the national spirit has exaggerated 
both these instances) ; Cuvier, 64.5 ounces ; Abercrombie, 63 
ounces ; Spurzheim, 55 ounces ; Sir J. Y. Simpson, 54 ounces ; Web- 
ster, 53.5 ounces ; Agassiz, 53.4 ounces ; Chalmers, 53 ounces. 
Other persons? of eminence, however, have had brains of only aver- 
age, or of under average weight ; thus C. F. Hermann, 46.5 ounces, 
and J. F. L. Hausmann, 43.3 ounces. Moreover, brains of high 
weight not infrequently occur without evidence of unusual mental 
capacity, or even in the case of those mentally inferior. Kecord is 

' On the relations of the Brain witli respect to weight and mass, see Schwalbe, 
Lehrb. d. Neurologie. ii., pp HSO ff. Erlangen, 1881. 

-Results obtained by Sims, Clendinning, Tiedemann, and J. Reid. 


made of four male brains, beloBging to persons of no repute for in- 
tellectual ability, which ranged from 62.75 ounces to 61 ounces ; of 
another such, which weighed 60.75 ounces ; of the brain of a boy 
of fourteen which weighed 60 ounces. In the West Riding Asy- 
lum ' for the Insane, out of 375 males examined, the weight of the 
bi*ain in 30 cases was 55 ounces or upward ; out of 300 females 
examined, in 26 cases it was 50 ounces or upward. Several persons 
afflicted with dementia were found to have brains weighing more 
than 60 ounces. On the contrary, idiots, almost without exception^ 
have brains far below the average in weight ; as a rule, the brain 
of such an unfortunate does not weigh so much as 30 ounces. 
Cases of microcephalous idiots are on record whose brains weighed 
only 10.5, or even 8.05 ounces. Here, again, however, singular ex- 
ceptions must be admitted ; for in a few cases the brains of idiots 
have reached the average weight, and have even, in rare cases, con- 
siderably surpassed it. 

Although the data adduced to show that the average weight of 
brain in the more highly civilized races is greater than in the savage 
races, are by no means abundant or conclusive, yet they are suffi- 
cient to create a reasonably strong presumption in favor of this 
view. Calculating from the size of the cranial cavity, as ascertained 
by measurement of a large number of skulls, it is inferred that the 
average weight of brain in the African, Australian, and Oceanic 
races generally, falls from 1 to 4 ounces below that of the more 
highly civilized European. It is further noted that there is almost 
a complete absence of cases rising above the higher ranges — above 
54 ounces, for example ; and that there is not the same difference 
between the two sexes in the uncultivated as in the cultivated, 
peoples. Davis calculated the average weight of brain among the 
Chinese to be about equal to that of the Caucasian race in Europe ; 
among the Sandwich Islanders to be some thirty grams less. The 
surpi'isingly low weight of the brain of the Hindus is in part a 
function of their smaller weight and bulk of body. It ma}' fairly 
be urged in objection, that by the method of measuring skulls 
taken somewhat at random we should be likely to find a note- 
worthy absence of such exceptional cases in certain quarters 
among the European races ; and that the relative increase in size 
of the female brain among uneducated peoples is probably, in part 
at least, the result of the response of the nervous system to the 
demand made upon it for the hard labor performed by the women 
among such peoples. 

Any law which refers the intensity and range of the mental 
^ For these facts see the Encyclopsedia Britaunica, uiuth ed., I., p. 879 f. 


activities directly to the size and weight of the nervous mass of the 
brain must, therefore, be held only very loosely. It is to be ex- 
pected that many unexplained exceptions will meet us, whether we 
compare men with the other animals, or certain races of men 
with others, or individual men with one another. No intelligent 
physiologist would now think of making mere mass the test of 
mental capacity. 

§ 5. A more intimate relation of dependence exists between the 
amount of intelligence and the complex structure of the brain as 
arising to a large extent from the development of the cerebral 
hemispheres — that is, from their relative size and esjDause, and 
from the number and depth of their convolutions. In other 
words, wealth of expanded and convoluted cerebral hemispheres 
is, in some general way, a measure of the richness and intensity of 
mental life. This conviction becomes stronger the more carefully 
the comparative anatomy of the cerebi^um, and the development of 
the cerebral hemispheres in the human embryo, are examined. 
The forms of brain found permanently in fishes, amphibians, 
reptiles, birds, and the lower mammals, are extremely similiar to 
those shown in succession by the developing brain of the higher 
mammals, and especially of man. The most distinctive feature of 
man's superior brain is the marked development in the size, num- 
ber, and depth of the convolutions of the hemispheres. In fishes 
generally, both cerebrum and cerebellum are very small ; but the 
ganglia connected with the organs of sense, especially of vision, are 
relatively large. In amphibia the cerebral hemispheres are rel- 
ativel}' enlarged ; are advanced backward still farther in reptiles ; 
while in birds the vesicles of the mid-brain are partially hidden by 
the development of the hemispheres. In the lower mammals the 
enlargement of these same organs by growth backward continues, 
and their two parts become connected by a commissure ; but they 
still remain comparatively meagre in size and simple in structure, 
without much distinction of lobes or division into convolutions. It 
is only in the most elaborately developed brains of the higher 
mammals that the occipital lobe enlarges backward so as to cover 
mid-brain, cerebellum, and medulla oblongata ; and that the frontal 
lobe spreads forward over the nasal cavities so as to constitute a 
development of forehead. Meantime the convolutions apparent on 
the cerebral surface increase in number and depth. 

The theory suggested by comparative anatoniy is confirmed by 
the probable view of Meynert, that the whole of this cortical I'egion 
of the cerebrum is a great "projection-field" on which the sensory 
impulses are marshalled and systematically ordered (to serve, as it 


Tvere, for the physical bans of mental phenomena), as they arrive 
from the peripheral regions and are distributed over the outgoing 
motor tracts. Certain striking exceptions to the principle of this 
theory must, however, be acknowledged. Within each great group 
of animals considerable variations occur in the degree of cerebral 
convolution, such that it cannot be said accui-ately to measure the 
degree of intelligence. For example, among mammals the in- 
sectivora have brains "poorest" in convolutions, the herbivora 
are "richest," and the carnivora stand between; the ruminants, 
although rather dull and incapable of being taught, have numer- 
ous and deep convolutions enough to rank them much higher than 
their real intelligence deserves. The marmoset, on the other hand, 
the relative weight of whose brain is as 1 to 18, shows a compara- 
tively smooth and non-convoluted surface, in striking contrast with 
that of other monkeys. 

Trustworthy data are as yet wanting to place beyond doubt the 
probable opinion that the brains of less highly civilized races and 
less highly intellectual individuals are relatively poor in develop- 
ment of the cerebral hemispheres. The human embryo is, indeed, 
an illustration in miniature of the truth of this opinion ; the older 
it becomes the more distinctly marked are the lobes of the cere- 
bral hemispheres, and the more numerous and deep are their con- 
volutions. The brains of idiots are said, as a rule, to be jjoor in 
convolutions ; this fact is doubtless connected with the embryonic 
condition in which many of them have remained through arrested 
development. Hermann Wagner,' on the basis of measurements 
made by his father, undertook to estimate the comparative total 
siu'face of the cei'ebral hemispheres of four brains, viz.: of two 
males of noteworthy intelligence (Gauss, the mathematician, and 
Fuchs, the physician), of a male laborer (Krebs), and of a female 
in middle life. By weighing carefully the amount of gold-foil laid 
on uniformly, which was required completely and closely to en- 
velop all the convolutions of these brains, Wagner concluded that 
the area of concealed surface was, in each case, approximately 
equal to that exposed. The total surfaces of the four brains were 
thus found to measure — of Gauss, 2,196 square centimeters; of 
Fuchs, 2,210 ; of the woman, 2,041 ; of Krebs, 1,877. It is a tempt- 
ing but rather insecure generalization which concludes from so 
sew cases that the richness of the cerebral convolutions (the total 
surface, both that exposed and that concealed by the sulci), is a 
general direct measure of the intelligence. 

§ 6. Other interesting attempts have been made to measure the 
1 Maassbestimmuiigen d. Oberflaclie d. grossen Geliiriis. Cassel, 1864. 


intelligence of the animal by the relative size and structure of 
the iutercranial nervous mass, and so, definitely, to establish a dii^ect 
relation between the two ; we notice especially those of J. Miiller,' 
and of Meynert.^ The great physiologist, Miiller, held that the 
position of an animal in the scale of intelligence may be estimated 
by comparing the hemispheres of his bi'ain with the corpus quad- 
rigeminum. According as the latter organ is relatively large, and 
lies behind the hemisi^heres, uncovered by them, the animal is low 
in the scale of intelligence ; according as the hemispheres increase 
in size, and so envelop and bury beneath them the relatively small 
corpora quadrigemina, the animal stands high in that scale. This 
statement, however, scarcely covers anything more explicit than 
the general fact that relative increase of the cerebral hemispheres 
is indicative of progressive mental life. Meynert has pointed out 
other important relations between parts of the brain, by which he 
proposes to measure the intelligence. In the entire mass of the 
crura cerebri we may recognize two parts, an upper (tegmentum), 
which stands in direct connection with the optic thalami and the 
corpora quadrigemina, and a lower (crusla), which is connected 
through the lenticular nuclei of the striate bodies with the cere- 
brum. Now the greater the hemispheres are in comparison with the 
corpora quadrigemina, the greater must the mass of the crusta be iu 
comparison with that of the tegmentum. The development of the 
pons Varolii is also essentially dependent on that of the crusta, for 
the fibres of the latter enter into the former ; the arching of the 
pons is therefore connected with the development of the hemi- 
spheres. In general, then, the relative development of the entire 
tract represented by the crusta, or lower jDart of the crura cerebri, 
and the nucleus lenticularis, the fibres of which expand in the cere- 
brum, is — according to Meynert — a measure of an animal's intelli- 
gence. In man the mass of the crusta on the level of the corpora 
quadrigemina exceeds that of the tegmentum ; in the other mam- 
mals the reverse is true.^ 

§ 7. The above-mentioned facts of comparative anatomy, with 
many others similar, show plainly the unique significance which the 
masses of the brain, and especially the cerebral hemispheres, have, 
as related to the phenomena of self-conscious mind. They may be 
supplemented and confirmed through other facts furnished by 
physiology, esjDecially of the experimental kind. Upon this point, 

' Handb. d. Physiol, d. Meiisclieii, 1844, I., p. 702 f. 
^Sitzgsber. d. Wiener Acad., LX., iii. (1869), pp. 447-462. 
^For a brief but judicious discussion of this subject, see Briicke, Vorles' 
ungen iiber Physiologie, 1884, II., pp. 52 ff. 


for the present, reference is simply made to the results of inves- 
tigation as already set forth in Part I. (see especially Chapter 
IV.). Physiology demonstrates that the nervous impulses, so far as 
they I'esult in sensation, pass along centripetal tracts which con- 
verge from every portion of the periphery toward the brain ; and 
that, so far as they x'esult in motion following upon idea and voli- 
tion, they pass along centrifugal tracts diverging from the same 
central masses. It thus confirms the same theory which studies of 
the anatomical structure of the nervous system suggest, namely, 
that in these masses, and especially upon the cortex of the cere- 
brum, is the common meeting-place of both kinds of impulses. The 
section or injury of any nerve-tract, even in the spinal cord, apart 
from indirect and secondary influences, does not affect the psychical 
functions. In such an event, the parts of the body lying j)eriphe- 
rally from the point of interruption are simjDly withdi'awn from all 
direct connection with sensations or volitions. Sensory impulses, 
then, no longer occasion sensations ; ideas of motion and volitions 
to motion, of the parts thus disconnected, become of no effect in 
producing the customary result. It has also been made obvious 
that, in proportion as the masses of an animal's bi-ain are removed 
or incajDacitated from performing their functions, the evidences of a 
varied and complex mental experience are diminished. The simple 
spinal cord of a frog, acting as a nervous mechanism, will perform 
a few wonderful feats ; joined with the medulla oblongata, optic 
lobes, and other lower parts of the brain, it will give largely in- 
creased signs of psychical phenomena ; it would not be claimed, 
however, that the cerebral hemispheres of this animal — relatively 
insignificant as they are when compared with those of the higher 
animals — are of no special importance for its highest psychical life. 
Essentially the same thing, though in more emphatic form, is true 
of all animals of a higher grade of intelligence. 

§ 8. In the case of man, the cerebral hemispheres are, aj^par- 
ently, the only portions of the nervous system, between the size, 
condition, and molecular activity of which and the phenomena of 
consciousness there is a direct correlation. If, then, we are to speak 
of mental activities as " localized " at all, the locality must be in 
the cortex of the cerebrum. The position that, in the case of man, 
the spinal cord and all the intercranial organs below the cerebral 
hemispheres, are incapable of acting as the immediate physical 
basis of mental states, is confirmed even by those experiments upon 
other animals, which seem at first sight to discredit it. The hypoth- 
esis that consciousness has a seat in the spinal cord of the frog ; 
that, in fact, we may properly speak of the decapitated animal as 


having a soul — has been urged by eminent j)hysiologists (Pflnger, 
for example). That the cord alone is capable of various purpose- 
ful activities, such as serve, under certain circumstances, as signs 
of a psychical experience, may be demonstrated by experiment 
(comp. Part I., Chapter IV., §§ 4 ff.). But unless one is prepared 
to maintain that all purposeful activity, as resulting from excited ner- 
vous substance, must be correlated with phenomena of conscious 
sensation and volition, one can scarcely assume with confidence 
that such phenomena accompany the movements of the decapitated 

"What the nervous mechanism will do, when set agoing by the 
appropriate stimuli, depends not only on its oi'iginal structure, but 
also on its acquired habits of action. That this law holds good 
even for the mechanism of the hemispheres of the brain is obvious 
from various facts. Stimulating those regions of the cerebral cor- 
tex which are connected with definite groups of muscles, in the case 
of the adult animal (for example, a dog), does not call out the same 
responses in the animal newly born (that is, under nine or ten days 
old). The case of the bird which has lost its cerebral hemispheres, 
and which executes motions by means of the lower basal ganglia, 
that seem to indicate a complex psychical life (comp. Part I., 
Chapter IV., § 20) is less easy of solution. Are we to consider 
such an animal still capable of "sensation," " perception," and "vo- 
lition ? " If this question means whether any phenomena continue 
to occur such as correspond to those conscious experiences of our 
own to which we apply the above-mentioned words, then we must 
confess our inability to answer it. 

In general, we know extremely little of the conscious mental life 
of the lower animals. What we conjecture is wholly dependent on 
the interpretation, given in terms of our human consciousness, to 
motions of their bodies resembling those which express definite 
conscious states in ourselves. But a large part of our own 
bodily activity is ordinarily not definitely correlated with any con- 
scious mental activity ; for example, breathing, winking, swallow- 
ing, changing the posture of the body in sleep and in states of 
jDrofound meditation, and especially the very complex operations 
involved in walking, singing, playing on musical instruments, or 
handling a tool, etc. In all these and similar cases, we find that 
the intricate and purposeful play of the mechanism is by no means 
necessarily connected with a corresponding series of conscious sen- 
sations and volitions. But in proportion as the hemispheres of an 
animal's brain become relatively developed, not only their abso- 
lute but also their relative significance is increased. The influence 


of the brain proper upon the voluntary movements of an animal 
is greatei', the higher the animal stands in the scale of cerebral 
development and of intelligence. A frog, or a fowl, deprived of 
its hemispheres, can do what is quite impossible for a dog or an 
ape in the same condition. If, theu, man's nervous mechanism, 
especially in case it has been trained to elaborate co-ordinated func- 
tions, can, without any corresponding accompaniment of mental 
phenomena, accomplish so much which ajppears significant of the 
most elaborate psychical activities ; a fortiori, it is likely that we 
may make this mechanism, working without consciousness, account 
for most of what is done by the hen or pigeon without its cerebral 
hemispheres. Moreover, experimental physiology undoubtedly 
tends toward accounting more and more fully for the most com- 
plex bodily motions under the tei-ms of physical mechanism. 

The most marked result of an animal's loss of the cerebral 
hemispheres is the sudden and great, or total departure of its 
intelligence. This fact is, of course, confirmatory of the impres- 
sion that the functions of these hemispheres, and of them alone, 
constitute the physical basis of its intelligence. We confess, 
however, our inability to affirm that the " psychical life " of every 
animal is inseparably bound to its continued possession of these 
organs. There may possibly be a varied psychical life of animals 
that have no brain. Yet in the case of the higher mammals, and 
especially in the case of man, we need not hesitate to affirm the 
probability of such an inseparable connection. The physical basis 
of the phenomena of human consciousness is pre-eminently, if not 
exclusively, the convoluted cortex of the cerebrum. 

§ 9. It is impossible, accordingly, to avoid raising the inquiry 
whether some more definite scheme of the localization of cerebral 
functions may not be discovered. The cerebral cortex is itself a 
very complex organ, or system of organs. Its different regions 
are marked by comparatively slight, and yet not insignificant, dif- 
ferences of structure ; they stand in different local relations and 
nervous connections with one another and with the ganglia lying 
below. This outlying rind of gray nervous matter is, of course, 
not a homogeneous mass. It is made up of innumerable nervous 
elements combined in various ways and multiform connections. 
It may be regarded, then, as a complex of organs. The question 
therefore arises : Have the different members of this complex of 
organs different relations to definite motor activities in the pe- 
ripheral regions, and to the various phenomena of conscious men- 
tal life ? or, in other words : Have different parts of the cere- 
bral hemispheres all the same office and value in relation to the life 


of sensation and voluntary motion ? This is the question generally 
understood under the term — "the localization of cerebral func- 
tion. " 

§ 10. Most of our definite knowledge concerning the functions 
of the other parts of the nervous mechanism creates a presumption 
in favor of some localization of cerebral functions. All the different 
parts of this mechanism are, indeed, constructed by combining 
variously a few elements of essentially the same structure ; all of 
them likewise are capable of exercising essentially the same neural 
functions. But each part of this mechanism has also its special 
functions. Thus we found that the different nerves become classi- 
fied functionally ; some are motor, voluntary or involuntary, some 
inhibitory, some secretory, some sensory, etc. Hints of a certain 
kind of classification may be discovered for the smaller ganglia or col- 
lections of nerve-cells. In making transverse sections of the cord, 
different regions with different functions appear. Considered lon- 
gitudinally, the cord is capable of being more or less definitely 
divided into several so-called centres, with specifically different 
functions. Localized centres, where specific kinds of reflex-motor 
activity have their particular seats, are fairly crowded together in 
the medulla oblongata. All the lower parts of the encephalon 
appear subject, in a measure, to the principle of localization. Shall 
we, then, stop short in our attempts at differencing the functions of 
the locally separate parts of the nervous system just at the point 
where we reach the most complex and extended organ, or I'ather 
collection of organs, which this system contains ? 

§ 11. Notwitlistanding the strong presumption in favor of the 
localization of cerebral function, the beginnings of a successful 
attempt to establish this theory are only about fifteen years old. 
The doctrines of Gall, Spurzheim, and others in the older school 
of phrenologists, proved so inconclusive as to bring contempt 
ujDon subsequent attempts to divide the hemispheres of the brain 
into different functional areas. Moreover, certain indisputable facts 
seemed to render impossible the assured beginnings of a theory 
of cerebral localization. Considerable portions of the human brain, 
it was found, might be lost without destroying any one sensory or 
motor function. Moreover, the gray matter of the cerebral hemi- 
spheres, it was then thought, could not be directly excited by elec- 
tricity or by other forms of stimuli. The greatest experimenters 
in i)hysiology, such as Longet, Magendie, Flourens, Matteucci, van 
Deen, Budge, and Schiff, declared against the localizing of cerebral 
function. In 1842 Longet ' afiS,rmed that he had experimented upon 

' Anatomic et physiologie du sjsteme nerveux, etc., Paris, 1842, i., p. 644 f. 


the cortical substance of dogs, rabbits, and kids, had irritated it me- 
chanically, cauterized it with potash, nitric acid, etc., and had passed 
galvanic currents through it in different directions, without obtaining 
any sign whatever of resulting muscular contraction. In the same 
year Flourens ' asserted, on the basis of numerous experiments in 
extirpation, that the lobes of the cerebrum perform their functions 
with their whole mass ; that there is no special seat for any of the 
cerebral activities ; and that even a small remnant of the hemi- 
spheres can serve all the uses of their collective functions. 

So great was the authority of the distinguished names just men- 
tioned, that their confident opinions gained general credence. The 
evidence brought forward by Broca and others seemed, however, 
to show some special connection between a single convolution of 
the frontal lobe and the complex activities of articulate speech ; 
and the anatomist, Meynert, held the opinion that the structure 
and connections of the cerebrum show its anterior poi'tion to be 
in general used for motor, its posterior for sensory functions. In 
1867 Eckhard repeated the significant observation which had been 
made by Haller and Zinn more than a century before : namely, 
that, on removing parts of the cortical substance of an animal's 
brain, convulsive movements occur in its extremities. 

§ 12. It was not until 1870 that the "epoch-making" experi- 
ments of Fritsch and Hitzig " began the modern era of investiga- 
tion into this subject. These observers announced the fact that the 
cerebral cortex of dogs is, at least in certain minute areas of it, ex- 
citable by electricity. They pointed out the further fact that, while 
some parts of the convexity of the cerebrum ai'e capable of motor 
excitation and others not, the motor parts lie in general to the 
front, the non-motor to the rear of this convexity'. By stimulating 
with an electrical current the so-called motor parts, co-ordinated 
contractions of the muscles in the opposite half of the body are 
obtained. Of such so-called "motor centres" they indicated, in 
their first announcement, the following five : One for the muscles 
of the neck, another for the extension and adduction of the fore- 
limb, another for the bending and rotation of the same limb, 
another for the hind-limb, and lastly one for the face. From such 
facts they drew the conclusion that the principle announced by 

' Recherclies experimentales sur les proprietes et les fonctions du systeme 
nerveux, etc., p. 99 f. 

^See the article by G. Fritsch and E. Hitzig in the Archiv f. Anat., Phy- 
siol., etc., 1870, pp. 300-332 ; and subsequent articles by Hitzig in the same 
Archiv, 1871, 1873, 1874, 1875, 1876 ; also his work, Untersuchungeu iiber 
das Gehirn, Berlin, 1874. 

2j4 evidence for localization. 

Flourens is demonstrably false. We must rather admit, say they 
that " certainly several psychical functions, and probably all, are 
shown to have their point of entrance into matter or of origin from 
it at circumscribed centres of the cerebral cortex." ' The same 
principle was subsequently defended at length by Hitzig, and the 
number of so-called cerebral centres increased. The most note- 
worthy facts which these experimenters first made clear and de- 
monstrable have since been verified by many investigators. Many 
of these facts may, with care and skill, be verified by any observer. 
Dr. Terrier in particular has used the method of Fritsch and Hitzig 
to map out the hemispheres of the brain of the monkey into no 
fewer than fifteen kinds of centres. The testimony of human pa- 
thology, and the evidence of comparative anatomy and of histology, 
have also been largely drawn upon either to confirm or to confute 
the conclusions originally based on experiments with animals. Be- 
fore considering the conclusions themselves, it is necessary to 
understand the true nature and extent of the various kinds of evi- 

§ 13. Exner has well said that " a physiology of the cerebral cortex, 
in the sense in which there is a physiology of the muscle, etc., scarcely 
exists at the present time." The reasons for such a deficiency lie 
partly in the very nature of this organ and the place it holds with- 
in the animal economy ; as well as partly, perhaps, in certain prej- 
udices which have hindered the physical theory of a material struct- 
ure so intimately related to the action of the mind. The cerebral 
cortex of the animals is exjDerim en tally approached only by over- 
coming immense difficulties. Moreover, those physical and chemi- 
cal processes of the cerebral substances, to which we must look for 
any strictly scientific understanding of its physiology, are placed 
almost utterly beyond reach of investigation. Seasoning must fill 
up with conjecture the great gaps that lie between a very complex 
series of physical occurrences, only a part of Avhich are observable, 
on the one side, and on the other, an equally complex group of 
psychical occurrences. The latter belong to a different order of 
phenomena from the former ; and, moreover, in the case of the 
lower animals — which must be selected almost exclusively for ex- 
periment — we know nothing of these psychical occurrences except 
through physical signs that are peculiarly liable to misinterpreta- 
tion. The result is that our conclusions on the localization of cere- 
bral function must be reached by considering a great multitude of 
complicated facts, many of which appear to take sides with contend- 

' Archiv. f. Anat, Physiol., etc., 1870, p 332. 
*See Hermanns Handb d. Physiol., II., ii., p. 189. 


ing cliampions of dijBferent theories who ahke appeal to them. It 
is only by observing the du'ections in which the different Hnes of 
evidence seem to point in common, that we can reach even a prob- 
able opinion upon a few points. 

§ 14. Three great lines of evidence, leading from three great 
groups of facts, must be considered. These are the evidence from 
exjDerimentation, the evidence from pathology, and the evidence 
from histology and comparative anatomy. Each of the three has 
its peculiar advantages and value ; each also its peculiar difficulties 
and dangers. It is only by regarding the combined testimony of 
the three that the highest probability at present possible can be at- 

Experimentation with a view to discover the localized functions 
of the cerebral cortex is of two kinds, stimulation and extirpation. 
Here, too, what has already been said (Parti., chap. IV., § 14) con- 
cerning the difficulties of the same mode of investigation in the 
sub-cerebral regions of the encephalon must be recalled and made 
more emphatic. All experiment by stimulation of certain areas of 
the hemispheres of the brain relies, of course, upon the argument 
that those areas whose stimulation is followed by tlie movement of 
definite groups of muscles are especially connected with such groups 
of muscles. The further assumption is likely to be made that these 
areas constitute the special organs which Lave control, as it were, 
of the same muscles. Since it seems to be a general princij)le that 
the sensory and motor nerve-tracts distributed to any region of the 
periphery come into tolerably close local relations to each other 
somewhere within the entire field of the cerebrum, it would seem to 
follow that some special connection exists between certain classes 
of sensations and volitions and the circumscribed areas of cortical 
substance pointed out by experiment. It should not be forgotten, 
however, that the excitation of any group of muscles, by applying 
stimulus to some area of the cerebral cortex, proves only that this 
area is somehow connected with such group of muscles. It still 
remains to be shown that sensory impulses, on arriving from such a 
peripheral portion of the body, serve as the physical basis for the 
psychical phenomena of sensation solely within this circumscribed 
central area ; or that conscious volitions, in order to be followed 
by motion in this peripheral portion, must give rise to the mole- 
cular commotion of the same area. 

§ 15. By far the most efficient and manageable stimulus for ex- 
perimenting upon the localization of cerebral function is the electrical 
current. Mechanical or chemical irritation may, however, be em- 
ployed in certain cases. The use of the electrical current incurs, 


of course, tlie danger of its diffusion. Important objections, based 
upon this fact and upon other grounds connected with the use of 
electricity, have been raised to the conclusions of Hitzig.' To 
Hitzig's claim that the electrical currents which excite the so-called 
motor areas are "very weak," and therefore unable, at a very slight 
distance from the place of the application of the electrodes, to affect 
the nervous substance, Hermann replies that, on the contrary, con- 
sidering the effect antecedently to be expected, these currents are 
" surprisingly strong," and that the brain, in diffusing the currents, 
must act like any other substance {e.g., a mass of copper) of similar 
form — that is to say, the distribution of such a cuiTent in the sub- 
stance of the brain is a purely geometrical function of the form of 
this substance and of the position of the electrodes. Moreover, it 
is found that increasing the strength of the current ajDplied to a 
so-called "motor area" invariably increases the size of the cortical 
region thrown into activity. That extra-polar conduction actually 
takes place in the substance of the brain has been shown by Dupuy, 
and by Carville and Duret ; contraction of the muscle of the rheo- 
scopic frog and deflection of the needle of the galvanometer, at re- 
mote distances from the electrodes, prove that the current passes 
along the whole extent of the cerebral hemisphere. The excitability 
of the cortical substance continues for hours after its exposure to the 
air, or after acids have completely destroyed its external third por- 
tion. If the cortical area be separated by a circular cut from all 
connection with the nervous substance below, it is still excitable with 
only a slight increase in the strength of the stimulus applied. Or 
if the gray substance of the surface be wholly removed, and the 
electrodes plunged in the blood of the cavity of one of these so- 
called motor areas, the customary results follow. Still further, the 
size of the circle within which the minimum amount of stimulus, 
when applied to certain gyri, will serve to excite the hind-limb of 
the animal, remains about the same whether the amount of cortical 
surface contained in the circle be largely increased by a sulcus 
crossing it, or not. 

From facts like the foregoing it is argued that, while beyond 
question the application of a given amount of stimulus to certain 
gyri of the cortical surface will produce definite motor results, we 
cannot ajBSrm those gyri to be the true cortical centres of such 
motion. Such gyri have accordingly been regarded by some as 
merely connected with the excitation of motion in a mechanical 

1 See especially the article of Hermann describing investigations under- 
taken by him in company with von Borosnyai, Luchsinger, and others, 
Pfluger's Archiv (1875), x., pp. 77 fE. 


way, through their service in conducting the electrical stimulus to 
other regions of the brain, especially to the basal ganglia. The 
ai'gument for the theory of localization would need to show, how- 
ever, that the electrical current stimulates these areas immediately 
to the exercise of their central nervous functions, and does not 
simply pass through them to excite other nervous matter lying 

To the foregoing objections the advocates of the theory of locali- 
zation make the following among other replies : " The effect of irri- 
tation of the basal ganglia is capable of exact estimation ; " ' and 
definite localized contraction of single groups of muscles, such as 
comes from stimulating certain areas of the cortical surface, does 
not follow from irritating the basal ganglia. Stimulation of other 
areas of the cortical surface which lie nearer to the basal ganglia — 
for example, of the island of Reil, which immediately overlies the 
corpus striatum — causes no movements. On the contrary, it was 
found by Carville and Duret that the phenomena evoked by stimu- 
lating the motor areas persist, even after the destruction of the 
corpus striatum. Moreover, when the animal is deeply etherized, 
the excitability of the cortical regions is partially or wholly lost.^ 
Since the physical conductivity of the gray nervous substance is 
not impaired by the anaesthesia, the loss of function must be due 
to the functional condition of this substance. More conclusive do 
the facts appear to be, which show that the nature of the motor 
reaction following upon the application of stimulus to the cortical 
substance is peculiar. Many observers have found that a stronger 
stimulation is necessary to bring about the same motor results 
after the cortical surface is removed ; this is what we should expect 
on the theory of localization, but the reverse of what would be 
true if the effect of the current was transmitted unchanofed through 
this surface. Then, too, Franck and Pitres' have shown that the 
effect of the electrical current is retarded in the gray matter ; the 
difference of time, as dependent upon whether the stimulus is 
applied to the gray matter or to the white lying beneath, being 
about 0.015 second. This interval must be spent in evolving, 
under the influence of the stimulus, the distinct neural function 
which belongs to the gray matter. Finally, the excitation is appar- 
ently reinforced in strength by the functional activity of the cor- 
tical substance, since — as we have just seen — a stronger stimulation 
is needed to produce the same result after this substance is re- 

1 Ferrier, The Functions of the Brain, London, 1876, p. 133 f. 

2 See Hitzig in Archiv f. Anat., PliysioL, etc., 1873, p. 403. 
2 Archives de physiologie, 1875. 


moved ; such reinforcement is the peculiar property of the central 

It seems obvious, therefore, that experiments with electrical 
stimulation of the cortical surface demonstrate a special connection 
between certain more or less definitely circumscribed areas of 
that surface and definite groups of muscles ; they also create a 
strong jDresumption that this connection is not merely anatomical 
or structural, but also functional. 

§ 16. The second kind of direct experimental evidence is de- 
rived from observing the effects of extirjoation. It is natural to 
arsrue that those areas of the brain, the loss of which is followed 
by the loss or disturbance of motion in definite groups of muscles, 
or by the loss or disturbance of any class of sensory impressions, 
are functionally related in a peculiar way to such muscles or organs 
of sense. But the application of this argument is encompassed 
with many difficulties. In the first place, it is impossible at 
each stage of the experiment — which often includes several days 
or months of observation — to know precisely what the condition 
of the brain is. Post-mortem examination of the brain reveals only 
what was the final effect of the experiment in destroying its 
tissues. The rise and fall of local or extensive inflammations, the 
progress of degeneration in the nerve-tracts and of abscesses result- 
ing from the primary lesions, etc., cannot be followed by the ex- 
perimenter in detail. Nor can he directly observe the formation 
and education of the tissue as it is called upon for an increase in 
the amount of its former functions, or perhaps for the discharge of 
functions partially new. As a rule, then, it is found that the 
effects of extirpation change from time to time ; some of them are 
of first importance and cannot well be overlooked, and others are 
so delicate and minute as almost wholly to escape observation ; 
some speedily pass away, others more slowly, still others perhaps 
not at all. The difficulties are, of course, especially great when 
we try to deal with effects upon the animal's sensory apparatus and 
his psychical world of sensations and perceptions. To tell whether 
an animal sees, hears, feels, smells, and tastes, or not ; and to tell 
precisely in what sense it exercises these functions — whether, for 
example, its deficiency is " soul-blindness " in any of its various 
degrees — are not tasks which it is easy to perform, or about the 
coiTCct pei'formance of which one can indulge in a boundless 

The demonstrative value of both kinds of experimental evidence 
— electrization and extirpation — is much lessened by the fact that 
it is almost wholly derived fx'om the lower animals. Ethical con- 


siderations, which few investigators dare even occasionally to dis- 
regard, forbid that the living human brain should be made the 
subject of similar experiment. In ordei', then, to draw any safe 
conclusions from this evidence, it is necessary not only that the 
application of the principle of localization in general should be as- 
sumed, but also that some right should be gained to transfer to 
the human brain from the map of the cortical surface of the ani- 
mal's brain, the so-called motor and sensory areas which have been 
determined by experiment. But it is not even in all cases clear, 
jDrecisely what convolutions or parts of convolutions of the human 
cerebrum correspond to those previously marked out on the brain 
of the animal. Moreover, in the effort to make any such transfer- 
ence of the argument from the animals to man, we meet again with 
the insuperable difficulty of forming a correct mental picture of 
the psychical life of the animals. 

§ 17. The evidence from human pathology for the localization of 
cerebral function has a peculiar value ; but it has also its peculiar 
puzzles and dangers. Such evidence is free, indeed, from the ob- 
jections which arise against all attempts to carry the argument over 
from the cerebral hemispheres of the lower animals directly to 
those of man. Nature and human intercourse are less kind to this 
wonderful mass of nerve-cells and nerve-fibres than the electrodes 
and knife of the physiologist are compelled to be. Accident and 
disease destroy, either suddenly or progressively, the different 
areas of the cortical substance of the human brain. They have, in 
various cases, made such a variety of attacks upon it as to cover all 
the areas of both hemispheres. If, then, we had a large collection 
of cases in which the lesions were definitely circumscribed, or the 
progress made by the destruction of tissue Avas accurately recorded 
for every stage ; and if we had also a correspondingly definite and 
accurate description of the motor and sensory disturbances occa- 
sioned by these lesions, we might perhaps be able to make a toler- 
ably conclusive induction. But losses of brain-tissue, when caused 
by accident and disease, have not the same circumscribed limits 
which can be observed by the knife or corroding acid of the physi- 
ologist. Lesions of the cortical areas entirely free from complica- 
tion with lesions in the sensory and motor tracts below are compar- 
atively infrequent. Cases of total destruction of any so-called "area " 
on both hemispheres, and of such area alone, rarely or never occur. 

Furthermore, it is only by careful post-mortem examination that 
the precise extent of the pathological changes can be known ; this 
examination, at best, reveals simply the last state of the case. 
The reports oi post-mortem examinations are also, as a rule, lacking 


, in precision. On the other hand, the symptoms of motor or sensory 
disturbance are rarely described, from beginning to end, with suffi- 
cient accuracy of detail to be of great service. Many large losses 
of cerebral substance are followed by no sensory or motor disturb- 
ances which can be distinctly traced. In large numbers of cases 
where such disturbances arise, they in time pass almost or quite 
wholly away. For these and other reasons the best evidence at- 
tainable from pathological cases, when collected and sifted, appears 
surprisingly confusing and self-contradictory. Pathology has, there- 
fore, furnished the common fund of cases from which the most di- 
verse and even contradictory theories have drawn at sight their stock 
of so-called proof. It has been used as the careless and false witness 
upon which either party, and all parties to the suit, could call for 
precisely the testimony desired. An increase of information and 
care on the part of those who have opportunity for ante- and post- 
mortem observation of such cases will doubtless, in time, cause 
pathology to yield much more assured results. 

§ 18. The third kind of evidence to which the principle of the 
localization of cerebral function may appeal comes from compara- 
tive anatomy and histology. Comparative anatomy, however, gives 
us evidence of only the most genei'al kind. Combined with exper- 
iment by electrical excitation, it shows that, on the whole, the 
higher the structure and intelligence of the animal, the more nu- 
merous and moi'e definitely marked are the " excito-motor areas " 
which may be discovered on the hemispheres of its brain. Only 
traces, as it were, of such areas can be found upon the cerebral 
hemispheres of the frog or the pigeon ; only a few areas can be 
doubtfully pointed out for the i-at or guinea-pig. The indications 
are clearer and more numerous of localized cerebral function in 
definite centres of the brains of the rabbit and the sheep. But it 
is in dealing with the cerebral convolutions of the more highly 
specialized brains of the dog, and particularly of the monkey or the 
man-like ape, that the proofs of the theory become most abundant. 
While, then, the argument from all the other animals to man is 
uncertain and should be used only with great caution, the general 
drift of comparative anatomy encourages us to place the greater 
confidence in it, the more nearly the bi-ain of the particular animal 
from whose case we wish to draw the inference resembles the 
brain of man. At the same time, the rash confidence with which 
the brain of the monkey has been mapped out in detail, and human 
pathology thereupon ransacked with the purpose of finding some 
warrant for copying this map upon the brain of the human species, 
cannot be too carefully avoided. 


Histology supplements and confirms the other evidence by show- 
ing that the structure and connections of different parts of the 
cerebrum are such as we should expect them to be, in case the 
functions of the parts were such as experimentation and pathology 
seem to have discovered. The modern arts of microscopy and 
photography have made possible an increasingly accurate knowledge 
of the intimate structure of the brain. Many great difficulties, 
however, still remain in the way of such perfection of this knowl- 
edge as will make it available as a secure foundation for a theory 
of the localization of cerebral function. At present the histology 
of the human cerebral hemispheres is not in a condition to take 
the place of a leader of physiological experiment and pathological 
observations. Its office is still rather that of rendering supple- 
mentary evidence in correction or confirmation of the evidence 
from the other two sources. Thus, for example, if Gliky's belief 
that he traced the nerve-tracts from the so-called motor centres of 
the cerebral hemispheres as they bend around the striate bodies 
and run into the crusta of the crura cerebri should be demon- 
strated, this fact would constitute an item of confirmatory evidence 
furnished by histology to experimental physiology and pathology, 
in favor of their general theory. 

§ 19. According to the foregoing view of the nature of the three 
kinds of evidence available, it would seem that, in collating and 
estimating the combined proofs from them all, the following course 
of inquiry should be pursued. The indications of experiment upon 
the cerebral hemispheres of the animals— especially of those most 
closely allied to man in their cerebral structure — by the two 
methods of stimulation and extirpation, must first be gathered and 
carefully weighed. Only those conclusions upon which the two 
methods are found to yield substantially the same results should be 
selected for further testing. The instances of localization of cere- 
bral function thus detected in the other higher mammals must 
then be allowed to suggest to pathology the questions it should 
undertake to answer with reference to man. In other words, ex- 
perimentation with the other animals suggests and strengthens the 
hypothesis which human pathology must try to satisfy. But in 
undertaking to test such hypothesis, pathology must be both fair 
and comprehensive in its observations. All the accessible patho- 
logical cases must be sifted and those only selected to bring for- 
ward as evidence which have the definite nature, and have received 
the careful examination recorded in detail, that are necessary to 
make them of real value. The corrective or confirmatoiy evidence 
of histology must then, so far as possible, be summoned to aid in 


forming our final conclusions. It is not until all the kinds of proof 
unite with a large and substantial agreement, if not with an abso- 
lute uniformity, that we can feel the utmost confidence attainable 
in our results. If it be found that certain regions of the cerebral 
hemispheres of the higher animals are the only ones to respond 
when stimulated with movements in definite peripheral parts of the 
body, and that the injury of those same central regions alone, or 
chiefly, causes motor and sensory disturbances in the same periph- 
eral parts ; if it also be found that lesions of the corresponding- 
regions of the human brain are alone, or chiefly, followed by similar 
motor and sensory disturbances, and that lesions of other regions 
alone are rarely or never followed by these same disturbances ; and, 
finally, if it be found that these same cortical regions have in the 
human body a special anatomical connection with these same pe- 
ripheral parts ; then we have reached the most conclusive evidence 
attainable for a theory that the cerebral functions are localized in 
the case of man. But precisely what is meant by such " localiza- 
tion " may still remain more or less a matter of dispute. We con- 
sider now a summary of the evidence according to the foregoing 



§ 1. On attempting to make an induction from all the three kinds 
of evidence which may be adduced in answer to the question, 
whether the different functions of the cerebral cortex have special 
relations to its different localities, no other difficulties are on the 
whole so great as those which come from so-called "negative cases." 
These negative cases force the inquirer to undertake a detailed ex- 
perimental and pathological examination. " That the cortex of the 
cerebrum, the undoubted material substratum of our mental opera- 
tions," says Ecker,' "is not a single organ, which is brought into 
play as a whole in the exercise of each and every psychical function, 
but consists rather of a multitude of mental organs, each of which 
is subservient to certain intellectual processes, is a conviction which 
forces itself upon us almost with the necessity of a claim of reason." 
But even the proposition that the brain is the "material substratum 
of our mental operations," is very far indeed from having the char- 
acter of a rational necessity. The further proposition that the cor- 
tex of the cerebrum " consists of a multitude of mental organs," is 
an inadequate statement of a conclusion which, at the very best, 
we can adopt only as the result of a long series of complex and con- 
flicting researches. Li fact, considerable areas of the cortical sur- 
face appear, at first, not to have any immediate relation to any psy- 
chical function whatever. 

The first general principle to be admitted in all attempts at a 
theory of the localization of cerebral function is, then, of a nega- 
tive character. This principle is based upon the negative results 
of physiological inquiry. Considerable areas of the cortical sur- 
face do not respond with motor activities when stimulated. Con- 
siderable portions of the cortical substance may be extirpated or 
lost by disease without the destruction or appreciable disturbance 
of any motor, sensory, or more purely intellectual functions. To 
such an astonishing extent is this true as to throw temporary doubt 
not only over the whole theory of the localization of cerebral func- 
■ The Convolutions of the Human Brain, p. 1. London, 1873. 


tion, but even over tlie statement that the cerebral cortex, as a whole, 
is the only " material substratum " of mental operations. 

§ 2. Attention has already been called (Chap. L, § 11) to the fact 
that Longet, Flourens, and other great physiologists, considered 
the cerebral hemispheres to be active as a whole in all their func- 
tions, and this, partly, because they found them not irritable by the 
electi'ical current. The discovery of Fritsch and Hitzig in 1870 de- 
monstrated that a part of the hemispheres of the dog, and a part 
only, gives signs of being excited by the application of stimulus. 
This part they called " motor," and located, in general, in the fore 
part of the hemispheres ; behind lay the region called " non-mo- 
tor," because it gave no response on being stimulated.' Even with- 
in this so-called "motor" region the early reseai'ches of these in- 
vestigators pointed out only five spots of a small fraction of an inch 
in diameter (the electrodes were, as a rule, separated not more than 
2-3 mm.) that could be more definitely related to the movement of 
certain groups of muscles ; between and around these spots lay the 
much larger areas of negative resrdt. Subsequent experiments 
added a few more such irritable areas to the map of the cerebral 
hemispheres of the dog. A large number of so-called centres, cover- 
ing an increased amount of the cortical surface, have been pointed 
out by Ferrier and others on the cerebral hemispheres of the mon- 
key. Fully half of this number, however, cannot be regarded as 
having anything like a demonstrable character ; and much fault has 
justly been found ^ with many operators upon the brains both of 
monkeys and of dogs, for their lack of precision in experiment, and 
haste in drawing conclusions. 

Experiments in extirpation also show that considerable areas 
of the cortical substance may be removed without perceptibly im- 
paiiing any of the motor or sensory functions of the animal. In- 
deed, even when the loss of the cortical substance, thus artificially 
produced, extends over almost an entire hemisphere, or over a large 
portion of both hemispheres, the operation may not result (in the 
case of the dog, ordinarily does not result), in the pervmnent and 
complete loss of any siDecific function, motor or sensory. So true is 
this that one eminent observer, Goltz, has maintained, on the basis 

' Archiv f. Anat., Physiol., etc., 1870, p. 311. 

^ See, for example, Munk s strictures of Ferrier, Ueber d. Functionen d. 
Grosshirnrinde, Berlin, 1881. pp. 14 fE. (also p. G f. ; 36 f. — " roh w<ir operirt, 
roh beohachtet, roh geschlossen ''''). On the other hand, the charge of careless- 
ness in experiment, and of illogical conclusions is freely made against Munk 
himself, both by advocates of rival theories of localization, like Dr. Yeo and 
others, and also by opponents of all theories of localization, like Goltz, Lob, 
and others. 


of many experiments in extirpation, that it is chiefly the quantity of 
the cerebral substance destroyed, in large measure irrespective of 
the locality, which determines the nature and extent of the result- 
ing psychical disturbances. The arguments of Goltz (as he him- 
self admits) do not answer those urged for a certain kind and de- 
gTee of the localization of cerebral function. But his experiments 
furnish a large number of facts which emphasize' the negative char- 
acter of many of the results of experiment. This fact is in itself 
undeniably unfavorable to any theory which would map out the en- 
tire cortical surface into so-called centres or areas, to be considered 
as separate organs of particular psychical processes. 

§ 3. The negative evidence from certain cases in human pa- 
thology is yet more astonishing and perplexing. At first sight it 
seems to suggest the conclusion that the mind can dispense, with- 
out impairment, with a considerable mass of brain-substance, no 
matter from what region it be subtracted. Many cases of large le- 
sions of the cerebral hemispheres in man, with no resulting disturb- 
ance of the psychical functions, are recorded.' 

Berenger de Carpi tells of a young man who had a foreign body 
of four fingers' breadth square driven into the substance of bis 
brain until it was buried. Much of this substance was lost when 
the foreign body was removed, and more yet some thirteen days 
later. Nevertheless, the patient lived for a long time in the enjoy- 
ment of all his faculties. 

Longet was acquainted with an army ofiicer who had lost, by a 
wound in the parietal region, a large quantity of brain-substance ; 
yet he remained mentally vivacious and showed no other result of 
the lesion than a tendency to grow tired easily. The same authority 
communicates ^ the case of an Italian whose skull was crushed in 
the right parietal region by a stone. So much of the substance of 
the brain was lost on the wound being dressed, and subsequently 
through a fall from his bed (on the eighteenth day) and through 
intoxication (on the thirty-fifth day), that the attendant physician 
calculated the lesion must have reached down nearly to the corpus 
callosum. The man, however, lived without any apparent impair- 
ment of psychical functions ; but we note in this case a permanent 
laming of the limbs of the left side. 

' See the list of such cases in Ferrier, the Localization of Cerebral Disease, 
London, 1878, pp. 25 ff.; Hermann, Handb. d. Physiol., II., ii., pp. 333 ff.; 
Briicke, Vorlesungen liber Physiol., II., p. 57 f. Wien, 1884; and the works 
cited by the two former, especially Pitres, Lesions du centre ovale, Paxis, 

^ Recorded, however, by Quesnay. 


A remarkable case is narrated by Brticke, " on the authority ol 
a certain Dr. Kratter. By a blow from a stone on the parietal 
region of the skull, one Ivan Mussuhn was thrown to the ground ; 
but within two hours he recovered so that he himself went to 
the " praetor " and entered complaint against his assailant. For 
twenty days he hved in apparently full possession of his powers 
of motion, sensation, and intelligence ; on the twenty-first day he 
suddenly died. The entire left cerebral hemisphere was found on 
examination to be a disorganized mass. It is to be noticed, how- 
ever, that the autopsy did not take place until some eighteen hours 
after death, and that we have no good means of judging what the 
condition of the injured hemisphere was during the twenty days 
preceding his sudden death. 

Remarkable instances of defective brains are also on record ; for 
example, the case which Lallemand nari'ates of a person of normal 
psychical constitution in whose cerebrum the entire place of the 
right hemisphere was, after death, unexpectedly found to have 
been filled with a serous fluid. Here again, however, there had 
been lameness of the left side of the body from birth. 

Extensive lesions without marked motor or sensory disturbances 
occur by far most frequently in the frontal lobes of the cerebral 
hemispheres. Yet similar negative cases are by no means infre- 
quent also in the occipital and temporo-sphenoidal lobes. Trous- 
seau narrates the case of an officer who was shot through the head 
in the middle of the frontal lobes, and who showed until death, 
which occurred from inflammation, no signs of. any kind of paral- 
ysis. The work of M. Pitres ^ contains a large collection of cases, 
in which the frontal lobes have been the seat of extensive disease, 
of softening, or of abscess, without any symptoms of laming what- 
ever ; in most of which, also, no disturbance of psychical con- 
dition was observed. That sudden extensive lesions may occur in 
this region without inducing sensory or motor paralysis, is shown 
in a marked way by the celebrated "American crowbar case." ^ 
By prematui'e discharge of blasting powder an iron bar, three feet 
seven inches in length and one and one-fourth inch in diameter, 
was driven through the brain of a young man. The missile en- 
tered at the left angle of the jaw, and passed through the top of the 
head near the sagittal suture in the frontal region ; it was picked 
up at some distance off, covered with blood and brains. The pa- 

' Vorlesungen iiber Physiol., 11., p. 57. 
'^ Lesions du centre ovale. 

" See the paper in the Am Journal for Med Sciences, by Dr. Bigelow, July, 
1850, and the one read before the Masa. Med. So. by Dr. Harlow, June, 1868, 


tient, although, for the moment stunned, recovered in a few min- 
utes so as to ascend a flight of stairs and give to the surgeon an 
intelhgible account of his injury. He Hved twelve and a half years 
afterward, with no noticeable impairment of his sensory-motor 
powers. Examination of the skull showed that the substance de- 
stroyed by the bar must have been confined to the frontal region, 
with the possible exception of the tip of the temporo-sphenoidal 

Boyer narrates the case of an epileptic child, that showed, how- 
ever, no other abnormal nervous phenomena, whose entire temporal 
lobe on the left side was found to have been destroyed. Instances 
of extensive lesions in the occipital lobes, without any resulting 
sensory or motor disturbances, might also be given. 

§ 4. It must be confessed, in the words of Exner,' that the 
understanding of cases of this sort "is made more difficult rather 
than easier by recent researches." Nevertheless, a large amount 
of concurrent testimony from all three main sources of evidence 
proves that some theory may be framed in acknowledgment of a 
more definite localization of cerebral function. Such theory can 
be most clearly established Avith respect to the cerebral region 
esj)ecially concerned in the motor functions. This region is the 
one Ijing about the great central fissure, or fissure of Rolando ; 
more precisely still, it embraces the gyrus centralis anterior, the 
gyrus centralis posterior, and the prolongation of the two on the 
median surface of the brain in the lohulus paracentralis. (Comp. 
Figs. 87 and 88). More definite localizations still, of smaller re- 
gions within the larger one — e. g., for the upper limbs, for the lower 
limbs, for the separate fingers, etc. — are more doubtful ; they can 
by no means appeal to the same amount of evidence as that at com- 
mand of the more genei-al induction. 

§ 5. The evidence from experiments in stimulation indicates that 
we are to look for the so-called ^^ motor areas'" in the above-mentioned 
convolutions about the fissure of Rolando. The original experi- 
ments of Fritsch and Hitzig ^ located the five motor areas as fol- 
lows : The centre for the muscles of the neck (marked A in the 
figure) in the middle of the pree-frontal gyrus at the spot where its 
surface falls off steep ; the centre for the extensor and adductor of 
the fore-limb, at the outermost end of the post-frontal gj^rus in the 
region near the end of the frontal fissure (-1^ in the figure) ; the 

^ 111 Hermann's Handb. d. Physiol., II., ii., p. 834. 

» See Archiv f. Anat, Physiol., etc., 1870, p. 312 f . ; comp. Taf. IX B. by 
Hitzig in the same Archiv for 1873, from which the accompanying figure is 



centre for the bending and rotation of the same limb, a little 
farther back (+ in the figure); the centre for the hind-limb, in 

the post- frontal gyrus but to- 
ward the median line of the 
^ hemisphere and back of the 

* preceding two centres (4^ iu 
f the figure) ; the facial centre, 

# in the middle part of the 
gyrus lying above the fissure 

of Sylvius (4~0 in the figure). 

** These experimenters found 

■^ also that the muscles of the 

o back, tail, and abdomen, were 

excited to contraction by 

stimulating points lying be- 

'*' tween those marked as above ; 

but they could not definitely 

circumscribe the cortical areas 

Fig. 83.— Hitzig's Motor Areas on the Cortex of the -^Jiich WCre to be aSSiffUed tO 
Dog. The left hemisphere belongs to one animal, ° 

the right to another; o, the mlmis cruciatUH, tlieSC mUSclcS. 

around which the gyrus sigmoideus bends ; oooo, i. i.i i, i. 

area for the face. The other symbols are explained J3y retercnce tO tUe CUart 

*'' *''^ ^''^- of the numerous " centres of 

electrical irritation " Avhich Terrier ' claims to have discovered on the 
cerebral hemispheres of the monkey, it will be seen that they are set 
close together in the two 
central convolutions 
{gyri centrales, called 
by Ferrier the " ascend- 
ing frontal" and "as- 
cending parietal ") and 
in the immediately ad- 
joining parts of the 
frontal and temporo- 
sphenoidal convolu- 
tions. Thus, the cen- 
tres (2, in part), (3), (4, 
in part), (5, in part), 
(6), (7), (8), (9), and 
(10), are located in the 
anterior central (" ascending frontal ") convolution ; (2, in part), 
(4, in part), (11, in part), and (a), (b), (c), (d), are placed on the poste- 
' See The Functions of the Brain, pp. 141 fE. , 149 f. , and 305 f . London, 1876, 
In the second edition (1886), pp. 240 ff. 

Pig. 84.— Areas on the Left Hemisphere of the Monkey, by 
stimulating which Ferrier obtains motion in definite groups 
of muscles. 


rior central (" ascending parietal ") convolution ; the centres (12) and 
(5, in part) are situated on parts of the superior and middle frontal 
convolutions adjacent to the anterior central ; and (14) on the su- 
perior temporo-sphenoidal convolution. 

Further and more recent information seems to render the experi- 
ments in the electrization of the cerebral areas of animals more 
available for use in confirming the general argument in behalf of 
some kind of localization of cerebral function in the case of man. 
Luciani and Tamburini, as well as other experimenters, have agreed 
with Hitzig and Terrier in finding small, circumscribed motor cen- 
tres on the cortex of the dog, monkey, rabbit, and other animals. 
Some experimenters (Bochefontaine and Vulpian, e.g.) claim to 
have discovered that the minute areas, at first excitable, after a 
time cease to be so ; and that other areas, at first not excitable, 
afterward become excitable — that is, a displacement of the excit- 
able points takes place. 

More recently still, it has apparently been discovered ' that Ex- 
ner's view (to be explained subsequently) of the existence of " ab- 
solute " and " relative " motor fields in the case of man is probably 
applicable to the animals also. Paneth found that a number of 
minute areas (or spots) for each one of the several groups of muscles 
could be detected as lying in the larger " excitable zone " of the 
cortex. These areas, vmlike the immediately surrounding ones, 
could be excited when cut around, but not when cut heneMh ; the 
fibres whose function it is to bring a definite group of muscles to 
contraction seem then to proceed directly from these cortical spots 
to the lower parts of the brain. A number of such belong to each 
muscle excitable ; but only two general " fields " are distinguish- 
able, within which all the isolated motor spots are located ; one 
field is for the extremities, the other for the orbicularis palpebrarum. 
The former is situated in the posterior division of the gijrus sig- 
moideus. The minute areas for the different muscles of the ex- 
tremities are sharply limited ; they do not wholly cover each other ; 
and those for any special muscle (the extensor digitorum of the fore- 
foot, etc.) are of small extent in comparison with the field or zone 
which may be looked on as common to all the extremities. The 
excitability of the different muscles is not all alike ; this Paneth ex- 
plains by assuming that the number of nerve-elements assigned to 
each is not alike. 

§ 6. Experiments in extirpation confirm, at least in a general way, 

' See the art. of J. Panetli, "Ueber Lage, Ausdehnung, \ind Bedeutung der 
absoluten motorisclieu Felder," etc., in Pfliiger's Archiv, xxxvii. (1«85) pp. 
530 fE. 


the above-mentioned results of experiments in stimulation. The 
destruction of the substance of those cortical areas which respond 
to the current of electricity with the co-ordinated movement of def- 
inite groups of muscles, causes a temporary or permanent impair- 
ment of the functions connected with the same groups of muscles. 
In their first report Fritsch and Hitzig called attention to certain 
experiments of their own in removing from the cerebral hemispheres 
of two dogs the nervous substance of the centre which had already 
been fixed upon by them as that for the " right fore extremity " 
of the animal. These experiments they found confirmatory of the 
views derived from stimulation. The animals operated upon, 
when sitting or standing or running, used the right fore-leg un- 
skilfully ; this part of the body, however, showed no marked dimi- 
nution of sensibility under hard pressure. Other observers have 
since performed many similar experiments ; — especially Ferrier on 
monkeys, and Goltz and Munli on dogs. Among them all no oth- 
ers are so carefully refined as are those of Munk." But the very 
refinement of these experiments subjects them to more of distrust, 
in certain particulars. 

§ 7. The earlier experiments of Munk were confined to the convex 
surfaces of the parietal, occipital, and temporal lobes of dogs ; they 
consisted in removing clean-cut circular bits of the cerebral sub- 
stance about three-fifths of an inch in diameter and one-twelfth of an 
inch thick — sometimes simultaneously, from the symmetrical areas 
of the two hemispheres, and sometimes with an interval between the 
two operations. Munk's general conclusion is stated as follows : 
If a line be drawn from the terminal point of the fissure of 
Sylvius vertically toward the falx cerebri, it will mai'k, approxi- 
imately, the limits of two spheres that are sharply distinguished 
experimentally — namely, an anterior motor and a posterior sensory 
sphere." Extirpations in front of this line always occasion dis- 
turbances of motion, those back of it never so. More precisely, 
the cerebral convolutions of the dog may, according to Munk, be 
mapped out into the several spheres and regions indicated in the ac- 
companying figure (Fig. 85). It will be noticed that three of these 
regions (namely, C for the hind leg, D for the fore leg, and E for 
the head) correspond pretty accurately to the centres of stimulation 
fixed upon by Fritsch and Hitzig. Extirpations of the cortical sub- 
stance in these i-egions, of only a few millimetres broad and not 
more than two deep, are regularly followed by definitely localized 

' See his Gesammelte Mittheilungen aus d. Jaliren 1877-80, in the book, 
Ueber d. Functionen d. Grosshiruriude. Berhu, 1881. 
2 Munk, ibid., p. 11. 



distiu'bance of motion. For example, let the region (Z)) be removed 
from the left cerebral hemisphere of a dog. At the end of from three 
to five days and after the fever from the operation has subsided, 
abnormal phenomena connected with the fore-leg of the opposite 
side will be observed. If any other limb of the animal than the 
right fore-leg be touched lightly, the dog will look quickly around ; 

Fig. 85.— Areas on the Brain of the Dop. (According to Munt.) A, centre of the Bye ; B, of 
the Ear ; C, of the sensations of the hind Leg- ; D, of the fore Log ; E, of the Head ; F, of the 
Apparatus for protecting the Eye ; G, of the Region of the Ear ; H, of the Neck : J, of the 

and if of a bad temper will try to bite the offending hand. He 
will also quickly withdraw any other limb when it is subjected to 
even very slight pressure. But hard pressure and pinching or 
sticking of the right fore-leg is either followed by no result, or else 
by mere withdrawal of the limb, as though in reflex motion, without 
any attention being paid to the attack. Moreover, this particular 
limb, unlike all the others, can be put into unnatural and uncom- 
fortable positions — can be bent, stretched, set on the ground with 


the back of the foot down, etc. — without any resistance on the 
animal's part or any apparent disposition to remove it to the 
normal and comfortable position. According to Munk, the animal 
has apparently lost all mental picture of this one limb, and there- 
fore all power to move it intelligently and voluntarily. If he has 
been accustomed, on call, to put the right leg into his master's hand, 
he will now respond with the left instead of the right foot to the 
same call. The dog no longer handles his food with the right 
foot. In running he slips on that foot. If he is drawn to the 
edge of a table and the right leg forcibly stretched out over it, he 
will allow the leg to hang down thus, although evidently aware of 
the dangerous position in which this places him. Such an animal 
can, however, still walk and run, using all four limbs. The " gross 
mechanism " of motion — to borrow Munk's phrase ' — still acts as it 
did before; but the so-called "cerebral" or intelligent quality in 
the management of this particular limb has been lost. 

Gradually the phenomena which indicate impairment of cerebral 
function as related to the movement of the foi-e-leg diminish in 
magnitude. Less pressure is then necessary to secure the with- 
drawal of the injured limb ; the dog is less surprisingly unskilful 
in its use. At the end of four or five weeks the moi-e marked 
symptoms of his loss of function have probably disappeared ; at the 
end of eight or ten weeks it may be difficult or impossible to dis- 
tinguish his movements from those of a perfectly sound animal. 
If, however, the size of the pieces of cerebral substance taken from 
any of the so-called " motor regions " be somewhat larger than that 
indicated above, recovery is slower and more imperfect. In the 
opinion of Munk, if the extirpations are considerably enlarged the 
restitution of function is never complete. 

§ 8. That explanation of the phenomena which regards the va- 
rious cerebral regions, that seem somehow specially connected with 
motor activities, as true " motor centres," — that is, as areas of the 
cerebral cortex that have for their peculiar function the initiating 
of definite motor impulse on occasion of the idea and volition to 
move definite portions of the peripheiy of the body, — is rejected 
by Munk. All the regions marked G — J, belong rather to what he 
calls the "feeling-sphere" of the cerebral hemispheres. It is an 
undoubted fact that the definite co-ordination of the limbs, from the 
higher cerebral centres, depends upon feelings of contact and press- 
ui'e of the skin, and upon muscular feelings or so-called feelings of 
innervation. The effects of extirpating centres like (C), (D), and 
(E), is due, therefore, first to the sudden loss, and subsequently to 
' Ueber d, Functionen d. Grosshinirinde, p. 47. 



the gradual restitution of these feelings, and of their correspond- 
ing mental representations, with respect to given groups of mus- 

Schiff ' agrees with Munk in the view that the real loss of function 
due to the extirpation of the above-mentioned cerebral regions is 
sensory rather than motor ; he considers that the imj)airment of the 
power of moving these parts is only an expression of the loss of the 
sense of touch in the same parts ; in other words, it is tactile an- 
aesthesia. He calls attention to the significant fact that an animal 
thus operated upon will freely allow parasites and insects to gather 
on that surface of the skin whose corresponding cortical area has 
been removed. Schiff also finds that the use w^hich the higher apes 

Fig. i 

-Areas on the Brain of the Monkey. (According to Munk.) The letters have the same 
reference as in tho preceding figure. 

make of their limbs for grasping the rounds of a trellis or ladder 
is not permanently impaired by removing to considerable depth 
the convolutions about the central sulcus, unless the trellis or ladder 
be turned at an angle of 60'' to 70°, so as to convert the animal's 
walking into climbing. Apparently the animal cannot climb be- 
cause he is unable to form a mental picture of the next round so 
as to i-each out and grasp it. Schiff therefore concludes, that " all 
motions are suppressed (by extirpating the cerebral substance) 
which, on being excited by the higher senses, receive a special 
supervision on the side of cerebral sense, in relation to direction, 
extent, and succession." He also asserts, in opposition to the 
conclusions of Goltz, that the injured animal never recovers the 

1 See Pfliiger's Archiv, xxx. (1883), pp. 313 flf. 

274 STiMULATioisr AjStd extirpation. 

powers it has once really lost ; in other words, it is not possible to 
extir^Date any of the centres, excitation of which produces a given 
motion, without effecting some permanent result. 

§ 9. The conclusions of Munh and Schiff undoubtedly have cer- 
tain facts of experiment in theu" favor, but they can scarcely be said 
either to cover all the facts or to be wholly consistent with certain 
pai'ticular ones. Goltz ' agTces substantially with Munk in finding 
that destruction of the cerebral substance of the frontal lobe causes 
the animal to execute movements of the limbs of the opposite side 
in a coarse and unskilful manner. He also finds that the tactile 
sense is temporarily impaired, although by giving increased atten- 
tion the animal is able to feel the slightest touch on any area of 
the skin. Indeed, deep and extensive lesions in this region may 
be followed by hj^DcrEesthesia. The muscular sense, on the other 
hand, seems permanently to suffer. Goltz's conclusions are squarely 
contradictory of all those which find any permanent laming of any 
muscle as the result of even the most extensive destruction of the 
cortical substance in the so-called "motor field." His theory lays 
more emphasis on the general impairment of intelligence which re- 
sults fi'om removing any considerable amount of the substance of 
the brain, from whatever region it may be taken. 

A still more recent investigator^ calls attention anew to the facts 
that an animal deprived of the " motor sphere " cannot use the ex- 
tremities as hands ; cannot hold the foot out on call, or push away 
the hand by which its chin is grasped, or stretch out the limb so 
as to grasp the dish containing its food. These phenomena imply, 
he thinks, some severance between the organ of will and the nerves 
which execute the will. The motor centres are to be limited, it is 
claimed, almost exclusively to the gyrus sigmoideus, and those for 
feelings of the skin and muscles to the region lying above the fis- 
sure of Sylvius. 

On attempting to reconcile all the results of experiment upon 
the animals with one another, and with the facts of human pathol- 
ogy, it must be admitted that great difficulty is experienced ; and 
even more difficult}'^ when the effort is made to frame a consistent 
theory which shall cover them all. On the whole, however, it 
seems obvious that a certain region or sphere of the cortex of the 
brains of the higher animals is entitled to be called " motor" in a 
special sense ; and that this region corresponds in a general way to 

' See his article in Pfli'ger's Archiv, xxxiv. (1884). pp. 450 fP. 

" Bechterew, art. " Wie siiid die Ersclieiuiuigen zu verstelien die nach 
Zerstorimg des motorischeu Riudeufeldes an Thiereu auftreteu ; " Pfliiger, 
XXXV. (1885), pp. 137 tf. 


that which (as we shall soon see) pathology indicates as specially 
motor in the case of man. Stimulation of various minute areas in 
this region is followed by the movement of definite muscles of the 
body ; extirpation of this region in its entirety, or in part, is fol- 
lowed by special disturbances of the motor functions of the animal. 
These disturbances are not of the kind which indicates so much 
a laming of any particular muscle, as a loss of cerebral, and so in- 
telligent, quality in respect to the handling of the extremities. 
They probably imply more or less of all those various kinds of psychi- 
cal disturbances and impairments of function, by some one of which 
exclusively the different investigators are wrongly inclined to ac- 
count for the phenomena which they observe. Extensive lofeses of 
cerebral substance in the motor region result in the loss of those 
tactile sensations and muscular sensations, by means of which the 
animal localizes and interprets the meaning of objects, and adapts 
the finer movements of its limbs accordingly. They also impair 
the power to express the volition of the animal by motor impulses 
started, in accordance with the sensations and images of motion, in 
the appropriate area of the brain. Moreover, such loss of the 
powers of sensation, sense-perception, and skilful motion, neces- 
sarily implies more or less of loss of intelligence. 

§ 10. It will always be difficult to designate precisely what fac- 
tors in the animal's complex sensory-motor activities drop out as the 
result of the removal of a certain area of cortical substance from the 
brain of a dog or monkey ; and whether these factors are exclusively 
sensory or exclusively motor, rather than both sensory and motor. 
It is doubtful if enough can ever be known, concerning the mental 
life of the dog or the monkey, to determine confidently in this way 
the question of the localization of psycho-physical functions. The 
phenomena of human pathological cases indicate, however, that in 
man the corresponding general area of the cerebrum — that is, the 
convolutions on both sides of the central fissure and the lobulus 
paracentralis — is especially concerned in both sensory and motor fac- 
tors for co-ordinated action of the limbs. Without adducing further 
confirmatory evidence from experiment upon other animals, we 
pass to the consideration of the evidence from human pathology. 
The results of experiments in stimulation and extirpation upon the 
lower animals are not to be transferred in toto, as a matter of 
course, to the human cerebrum ; they are rather to be consulted 
as indicating the precise nature of the questions to be proposed to 
pathology, and of the answers to these questions which are antece- 
dently probable. 

From this point onward our chief reliance must be placed upon 



Exner's ' careful and scientifically classified investigations. The 
method pursued by this investigator is described at length by him- 
self. ° Exner began, with true German thoroughness, by reading 
several thousand cases of cerebral disease which had been followed 
by post-mortem examination ; the catalogue of works thus consulted 
by him occupies more than twenty pages. From all these cases he 

Fig. 87. — Lateral View of the Human Brain. (Schematic, Ecker.) F, frontal, P, parietal, O, 
occipital, and T, temporo-sphenoidal lobes. S, fiBsure of Sylvius, with S'. the horizontal, and 
8", the ascending ramus ; C, sulcus centralis ; A, anterior, and B, posterior, central convolu- 
tions ; Fl, ra, F8, superior, middle, and inferior frontal convolutions ; fl, superior, f2, infe- 
rior frontal sulci; f3, sulcus praeceiiiralis; PI, superior, and P2, inferior parietal lobule; the 
latter, the gyrus supra marginalis, and P9', the gyrus angularis ; ip, sulcus interpiirietalis : cm, 
end of calloso-margii'al fissure ; Ol. 02, 08, occipital convolutions ; po, parietooccipital fissure ; 
o, transverse, and o2, inferior longitudinal sulcus; Tl, T2, T3, tempore sphenoidal convolu- 
tions; and tl, t2, tempero sptienoldal fissures. 

then made a collection of such only as could safely form the basis 
of a scientific induction. The conditions of admittance into this 
collection were as follows : Both the history of the disease and the 
description of the post-moi'tem condition must be trustworthy, full, 

' Untersuchnngen iiher d. Localisation d. Functionen in d. Grossliirnriude 
d. Menscheii. Wien, 1881. 
•' Ibid, p. G f. 



and unambiguous ; and there must have been no other lesion than 
the one in the cerebral cortex, either elsewhere in the brain or in 
the spinal cord, to complicate the legitimate inferences. Only two 
exceptions to the latter rule were, for reasons peculiar to themselves, 
admitted. Nearly all cases in which symptoms indicative of diffuse 
meningitis occurred were also excluded. In this cautious way one 

Fig. 88. — View of the Human Brain from Above. (Schematic, Ecker.) The letters have the 
same reference as in the preceding figure. 

hundred and sixty-nine test-cases were secured from the thousands 
recorded. These test-cases were then tabulated on three sets of 
maps, according to the following methods of induction : (1) The 
method of negative cases, (2) the method of "reckoning per cent.," 
(3) the method of positive cases. 

The method of negative cases (if the number of such cases were 
large enough) would result in showing what regions of the cerebral 
hemispheres, if any, are not necessarily connected with motor or 



sensory functions — both, or either one respectively. The charts 
constracted by this method would, accordingly, have only those 
convolutions and parts of convolutions left blank, or unmarked, 
in which no lesion had occurred that was not followed by some 
given kind of motor or sensory disturbances. The method of per- 
centage was designed to show the amount of probability that a 
given small area of the cerebral cortex will be hit by disease, as 
it were, in case the lesion has been followed by a given kind of 
motor or sensory disturbance. For this purpose the entire sur- 
face of one hemisphere was mapped out into three hundred and 

A 9 

Fig. 89. — Median Aspect of the Right Hemisphere. (Schematic, Ecker.) CO, corpus callosum. 
G-yri : G-f, fornicatns ; H, hippocampi (with its sulcus, h), and TJ, uncinatus : PI', praecuneus; 
Oz, cuneus ; oc, calcarine fissure, with its two rami, oc' and oc'' ; D, gyrus descendens ; T4, the 
lateral, and T5, the medial, gyrus occipito-iemporal.R. 

sixty-seven quadrilateral areas, all small and yet of somewhat dif- 
fei'ent sizes. As the different selected cases were recorded by 
painting the area of the lesions on this set of maps, the intensity of 
the color used would, of course, deepen in proportion to the cer- 
tainty of a connection between that particular area and some par- 
ticular sensory or motor function. Thus a joerfect black would 
indicate one hundred per cent, of cases in which a given quadri- 
lateral was hit when a given disturbance of function had followed ; 
pure white, nought per cent, of such cases. The third method (that of 
positive cases) is the one usually relied upon to prove (?) the theory 
of localization of cerebral function from pathology ; it is justly re- 
garded by Exner as the least conclusive, as never of itself forming 


a basis for anything beyond conjecture. Its principle is the as- 
sumption that the region where the lesions connected with certain 
disturbances are most thickly crowded together, is the required 
cortical area with its specific function. 

§ 11. The result of Exner's comprehensive induction from patho- 
logical cases, as based on all three of the methods just described, 
fixes almost beyond doubt the so-called "motor areas" of the hu- 
man cerebrum. [For understanding Exner's induction, consulta- 
tion of Ecker's charts, found on p. 276f., figs. 87, 88, and 89, will be 
found helpful.] The field of wholly ^'latent lesions" — that is, of le- 
sions which are not necessarily followed by any disturbances of 
either sense or motion — covers a large part of the surfaces of both 
hemispheres ; it is not, however, precisely the same for them both. 
While Exner's collection of cases comprised 67 lesions of the right 
hemisphere, and 101 of the left, the absolute number of latent 
lesions was the same (namely, 20) for both hemispheres. The 
chances that a lesion of the right hemisphere will not be followed 
by any disturbance of function are, therefore, about fifty per cent, 
greater than the chances that the same thing will occur in the left 
hemisphere. On the right hemisphere the entire surface, with the 
exception of the two gyri centrales, the lobidus paracentralis, and 
certain small portions on the convex and inferior surfaces of the 
occipital lobe is latent. On the left hemisphere the latent region 
is of less extent. This result may be regarded as a restatement, 
on a basis of scientific induction, of the well-known fact that ex- 
tensive lesions can occur in the frontal, temporal, and occipital 
lobes, without being followed by any sensory or motor disturb- 
ances. But it also confirms the impression that the portions of 
the cerebral cortex lying about the fissure of Rolando are entitled 
to be called " the exquisitely motor parts of the cortex." 

Yet more precisely, the motor region on either hemisphere may 
be, according to Exner, marked out by the method of negative 
eases, and by the method of percentage of cases. The former meth- 
od shows that, for the upper extremities, the corresponding cortical 
region on the right hemisphere is the lobulus paracentralis, the gyrus 
centralis anterior (with the exception of a small part of its lower 
end) and the upper half of the gyrus centralis posterior. The latter 
method further confirms the foregoing conclusion. It shows that 
the "absolute field" for the upper extremities — the field, that is, 
within which lesions are always followed by impaired motion of 
these extremities — covers quite completely the same parts of cere- 
bral convolutions ; while the " I'elative field," or portion in which 
more than fifty per cent, of cases of lesions are followed by similar 


disturbances, extends over the remaining half of the gyrus centralis 
posterior, the posterior third or half of the three frontal convolu- 
tions, the anterior half of the parietal lobe, and more of the neighbor- 
ing median surface. Corresponding to the better motor education 
of the right arm is the fact that its motor region on the left hemi- 
sphere is more extended. Here the absolute field comprises the 
lobulus paracentralis, the three upper quarters of both gyri centrales, 
and the greater part of the upper parietal lobe. Portions of the 
median surface of the occipital lobe may also belong to this field. 
The relative field for the upper extremity on the left hemisphere 
includes the posterior half of the gyrus frontalis superior, almost 
the entire convex surface of the other frontal gyri, the parietal lobe 
at large, and the upper part of the occipital lobe. 

More specific localization of cerebral areas, corresponding to the 
different parts of the upper extremities, can as yet be accomphshed 
only with much less confidence and in a conjectural way. The 
method of positive cases seems to designate the gyrus centralis an- 
terior as the special cortical area for the hand ; with a probability 
that the area for the extensors of the hand lies in its middle part, 
and the area of the thumb somewhat below in the same gyrus. 

§ 12. Exner's collection contained 75 cases of disturbances of 
motion in the loiver extremities ; 26 lesions being on the right, 49 
on the left hemisphere. The methods both of negative cases and 
of percentage agree in indicating that the " absolute " cortical field of 
the left leg comprises the lobulus paracentralis, the uppermost third 
(as far as the lower end of the sulcus frontalis superior) of the 
gyrus centralis anterior, portions of the corresponding third of the 
gyrus centralis posterior, and some small areas behind and below on 
the lobulus quadratus, — all, of course, in the right hemisphere. The 
" relative field " of the same limb on the same cerebral hemisphere 
includes both lower thirds of the central convolutions, the back 
parts of the frontal convolutions, the parietal lobules, and the up- 
per portion of the occipital lobe. On the median surface of the 
brain, the posterior part of the gyrus frontalis superior and the 
anterior half of the lobulus quadratus belong to this field. On the 
left hemisphere the absolute cortical field for the right leg includes 
the lobulus paracentralis, the upper half of the gyrus centralis pos- 
terior, and most of the upper portion of the parietal lobe. A small 
lateral part of this lobe, and on the median surface the lobulus 
quadratus, and perhaps the cuneus, must be added to complete the 
relative field of this lower extremity. Exner does not consider it 
possible, as yet, to be more precise in designating the cerebral fields 
for the lower limbs of man. 


§ 13. On comparing with each other the foregoing conclusions, 
it is apparent that the absolute field for the upper extremities en- 
tirely covers the corresponding field for the lower extremities ; but 
the gyrus centralis anterior and lower half of the gyrus centralis 
posterior belong only to this field for the upper extremities. The 
relative fields, too, for both arms and legs, have a similar relation in 
extent and intensity. There is considerably greater probability, 
therefore, that a lesion of a given size in the motor region will af- 
fect the arms than that it will affect the legs ; indeed, the collection 
of Exner shows but one case in which the motions of the legs were 
disturbed and not those of the arms. This greater " sensitiveness " 
— if we may so speak — of the cortical region of the upper extrem- 
ities, corresponds to the fact that their motion is more distinc- 
tively cerebral and intelligent than that of the lower extremities. 

§ 14. In this same " exquisitely motor " region of the cerebral 
cortex, and in the most nearly adjacent regions of the frontal and 
parietal lobes, certain other cerebral fields corresponding to definite 
muscles or groups of muscles may be localized, conjecturally. By 
the method of percentage the cerebral area for those muscles to 
which the facial nerve is distributed may be rather indefinitely in- 
dicated as lying in the lower half of the gyrus centralis anterior, and 
lower third of the gyrus centralis posterior, on the right hemisphere ; 
while, on the left hemisphere, it appears to be moi'e definitely fixed 
at a small strip that belongs to the gyrus centralis anterior, and lies 
between the places where the inferior and superior frontal gyri 
spring from this central gyrus, but nearer the first of the two. 
Both methods of induction apparently unite in indicating the 
cortical region for the tongue as lying where the middle and lower 
frontal gyri meet with the anterior central gyrus. In the nine cases 
of the collection in which the muscles of head and neck were af- 
fected, the lesions were all situated in one of the central convolu- 
tions ; but a more definite localization within the limits of these 
convolutions does not apjjear to be possible. As to the localization 
also of the cerebral field for the muscles of the eyeball, including 
that for raising the upper lid, pathology is able only to say in a 
general way that this field appears to fall within the general motor 
area as thus far pointed out. Exner thinks it certain that the 
rectus internus muscle of one side, and the rectus externus of the 
other side, are innervated from the same hemisphere of the brain. 
This we should also argue from their ordinary physiological func- 

§ 15. Positive cases of a nature to strengthen the foregoing in- 
duction as to the cerebral areas especially connected with the upper 


and lower extremities might be indefinitely multiplied. Especially 
interesting are those where disuse, through accident or disease, of 
one of these extremities has been found, post-mortem, to have re- 
sulted in atrophy of the corresponding cortical fields. That is to 
say, the cortical region, being unused on account of the loss of 
function in the peripheral member, has itself paid the penalty of 
all failure to exercise the normal functions ; it has lost in size and 
strength. For example, atrophy of the upper end of the gyrus 
centralis anterior of the right hemisphere, and of its prolongation 
in the lohulus paracenlralis, was in one case found to have resulted 
from the amputation of the left leg twenty years before death. 

§ 16. The conclusions of other authorities as to the motor re- 
gions of the cerebral cortex in man — especially of Lepine,' and of 
Charcot and Pitres'^ — as based on pathology, confirm those of Exner 
in the main, as well as also in some interesting particulars ; any di- 
vergences arise almost wholly from the effort to make distinctions 
more nicely than the present condition of the facts will warrant. 
The most general conclusions of these investigators may be summed 
up as follows : ^ " The cortex of the cerebral hemispheres in man 
may be divided, functionally, into two parts ; motor and non-motor, 
according as destructive lesions do or do not cause permanent par- 
alysis of the opposite side of the body." . . " The motor zone in- 
cludes only the ascending frontal and ascending parietal convolutions 
and the paracentral lobule." It may be concluded then, as a well-es- 
tablished induction, that the convolutions on either side of the fissure 
of Rolando (the gyri centrales anterior and posterior) and the con- 
nected lobule on the median surface of the brain (lohulus paracen- 
tralis) are in the highest degree especially connected with the mo- 
tion of the extremities of the body ; that adjacent parts of the 
frontal and parietal lobes are thus connected in a less degree ; that 
the cortical region for the arms lies, on the whole, anterior to that 
of the legs ; and that, probably, the region for the hand is near the 
middle part of the front central convolution, and that for the tongue 
where the middle and lower frontal convolutions meet the front 
central. More precise localization of the motor functions of man 
must as yet be made with a lower degree of confidence. Beyond 
these general statements lies the undefined field of conjecture. 

§ 17. It cannot be said that histology and comparative anatomy 

' Localisation dans les maladies cerebrales. Paris, 1875. 

^ Localisation dans les maladies du cerveau, Paris, 1876; Revne mensuelle 
de Med. et de Chir.. 1877-1879; Etude critique et clinique de la doctrine 
dans I'ecorce des hemispheres cerebraux de I'homme, Paris, 1883. 

'' See Brain, July, 1884, p. 270 f. 


much affect the strength of the argument for the localization of the 
cerebral motor regions, as derived from experimentation and pa- 
thology ; whatever evidence they do furnish, however, is confirma- 
tory of the conclusions reached above. In this connection refer- 
ence maybe made to the conclusion of Meynert,' that the paths 
of the sensory nerves run more toward the occipital, and those of 
the motor nerves toward the frontal region of the cerebrum. The 
existence of nerve-cells of gigantic size, resembling those found in 
the motor region of the spinal cord, which Betz discovered in the 
motor regions of the cerebrum of the dog, the monkey, and of man, 
is an indication in the same direction. It should also be mentioned 
that the pathological researches of Pitres ^ into the results of lesions 
in the medullary substance lying between the cerebral cortex and 
the basal ganglia, seem to show that such as occur in the fronto- 
parietal portion of this substance cause paralysis of motion and de- 
generation of the motor tracts. Finally, the general structure of the 
cerebrum and the courses of its nerve-tracts, as already considered 
(Part I., Chapters 11. and IH.), are, in the main, accordant with 
the facts of experimentation and pathology. 

§ 18. The remarkable degree of coincidence in locality which 
obtains among those circles whose extirjDation is followed by dis- 
turbances of motion (disturbances due, in the opinion of Hitzig, 
to destruction of the physical basis of the animal's control over its 
limbs, but, in the opinion of Schiff, rather due to tactile anaesthe- 
sia) suggests the following question : Is not the cortical field of 
tactile sensation in the extremities of man coincident, in the main, 
or even in particular, with the field for the motion of the same ex- 
tremities? An affirmative answer to this question woiild seem 
reasonable, even prior to experimental and pathological evidence. 
The sensory and motor mechanisms are, of necessity, most inti- 
mately connected, locally, in all the central organs. This state- 
ment is certainly true of the spinal cord and of the inferior parts 
of the braiu. Moreover, in consciousness the sensations which 
guide the volitions in all the finer uses of the peripheral parts of the 
body are very promptly, and even almost inextricabl}', interwoven 
with the volitions. In walking, talking, handling a tool, or playing 
a musical instrument, to be unable to experience certain delicate 
sensations is to be unable to will the execution of corresponding 
nicely adjusted motions ; whereas the appearance of the associated 
sensations may instantaneously call forth the requisite volitions. 
It would seem, then, that the cerebral mechanism for both the 

* Sitzgsbr. d. Wiener Acad., LX., heft iii., p. 455 f. 
^ Lesions du centre ovale. Paris, 1877. 


sensory and the voluntary motor factors of these complex functions 
must be composed of elements having the closest local connection. 
Certain indisputable facts of pathology form, however, a strong 
objection to such an identification of the cerebral fields of motor 
function and tactile sensory function. Many cases of motor dis- 
turbance occur without the disturbance of sensation in the same ex- 
tremity ; and cases of sensory disturbance withoiit corresponding 
motor disturbance, although much less frequent, are by no means 
very rare. How, then, can these facts of pathology be reconciled 
with any hypothesis which locates in the same cerebral region the 
so-called " fields " for both classes of function ? 

The answer which Exner gives to the foregoing question is per- 
tinent, but not wholly conclusive. No absolute cortical field for 
disturbances of tactile sensation in the extremities of the body can, 
indeed, be pointed out ; that is to say, there is no portion of the 
cerebral cortex, lesions of which are invariably and necessarily fol- 
lowed by tactile anaesthesia, hyperaesthesia, etc., in definite parts of 
the periphery. But that the entire relative field of sensations of touch 
in the extremities corresponds with that of the motor activities, is 
made highly probable by the method of positive cases. After ex- 
cluding doubtful cases — in which the patient comjplained rather in- 
definitely of a feeling of " heaviness" or " numbness," etc., in some 
area of muscle or skin — Exner's collection was found to contain 
22 cases where marked disturbances of tactile sensations seemed 
clearly made out. Of these 22 cases no fewer than 16 were located 
wholly in the two central convolutions ; and 3 of the remaining 6 
extended for several millimetres into the same convolutions. Of 
those still remaining, the one farthest removed from the " exquisitely 
motor " region was in the gyrus angularis, and therefore in a portion 
of the relative motor field which has a considerable per cent, of in- 

On the basis of so complete an agreement of the positive 
cases, Exner feels warranted in affirming that "the tactile cortical 
fields for the different divisions of the body coincide in general 
with their motor cortical fields." It is to be noted, moreover, that 
the percentage of the cases of disturbance of tactile sensations 
occurring on the right hemisphere is more than twice as large as 
that of the left. Sensibility seems, then, to be the predominating 
function of the right hemisphere, as motion is of the left. This 
fact, when taken in connection with the greater liability of the left 
hemisphere to be the seat of cerebral disease, accounts in part for 
the less frequent occurrence of sensory disturbances following le- 
sions in this general area. Moreover, we are warranted in assum- 


ing that the cortical fields, in which the nervous impulses occasion- 
ing tactile sensations are projected, are connected with each other, 
and with the ascending sensory tracts, in a very complicated way. 
The manner of this connection is doubtless different for the different 
areas of muscle and skin. Nor does it appear that the sensory areas 
are so well differentiated as the corresponding motor areas ; although 
one case, at least, can be pointed out in which loss of sensation in 
the thumb and index-finger was the definite result of a lesion of 
the very limited cortical region already conjecturally assigned to 
these members. Finally, it must be remembered that those descrip- 
tions of pathological cases on which all our inductions have hitherto 
been based, are very liable to be faulty with respect to slight dis- 
turbances of sensibility. 

It is a general conclusion, then, which is entitled to a large de- 
gree of confidence, that both the gyri centrales, the lobulus para- 
centralis, and the most nearly adjacent parts of the frontal and 
parietal convolutions, constitute a cortical region especially related 
to both the motor and the sensory functions of the extremities of 
the body. 

The view of Exner concerning the nature of the motor area in 
man is, on the whole, greatly strengthened by the most recent con- 
clusions of Luciani. This experimenter finds ' that total or partial 
extirpation of the " motor zone " in the dog and the monkey is 
uniformly followed, not only by motor paralysis, but also by cuta- 
neous and muscular anaesthesia. The " motor " sphere and the 
" tactile " sphere are largely coincident in these animals ; and " in 
all experiments upon the tactile sphere there was a manifest and 
constant crossing of the relations between the peripheral sensory 
fibres and their respective cortical centres." "What one calls 
'motor zone ' is the central focus of the large portion of the senso- 
rial sphere visible on the external aspect of the hemisphere." 

§ 19. The testimony of the facts upon which reliance must be 
placed in the effort to localize the cerebral field for sensations of sight 
and hearing in man is by no means so satisfactory as the foregoing. 
Experiment upon animals by stimulation is of no direct value ; it 
could at most only discover the cortical regions especially related 
to some of the motions of the eye or ear and their surrounding 
parts. Our conclusions from the method of extirpation also must 
always be somewhat uncertain, since we infer the sensations of the 
animal only by interpreting his motions into terms of our own seK- 
consciousness. It is not strange, then, that the leading experi- 
menters differ irreconcilably in certain of their conclusions. There 
' See au abstract of liis results, in Brain, July, 1884, pp. 145 ff. 


is pretty general agreement at present, however, as to the localiza* 
tion of sight somewhere in the occipital lobe. Hitzig ' found that 
the removal of certain gyri in the posterior lobes of the dog pro- 
duced blindness of the opposite eye, combined with a paralytic 
dilatation of its pupil ; stimulation of the same gyri produced con- 
traction of the pu j)il. Ferrier ^ claims that destruction (by cauter- 
ization chiefly) of the gyrus angularis of apes produces blindness of 
the opj)osite eye, and this loss of function alone ; stimulation of the 
same region causes movements of the eye. He therefore considers 
this convolution as pre-eminently the cortical centre of sight. But 
Munk, after numerous experiments upon dogs, and some upon 
monkeys, locates the centre of sight above and behind the place 
assigned it bj' Ferrier — namely, in the upper and hinder part of the 
occipital lobe ; the gyrus angularis, on the contrary, he makes the 
cortical region for the tactile sensations of the eye. Munk's ex- 
pei'iments are so minute in carefulness, and his conclusions so 
based upon detailed analysis of the phenomena, that they perhaps 
deserve to suggest to pathology the exact form in which to put its 
inquiry. They are, undoubtedly, excessive, however, in the refine- 
ment to which they would carry the principle of localization. 

§ 20. Munk details the following among other phenomena which 
result from extirpating the region marked A^ (see Fig. 85) from the 
brain of a dog. The animal thus operated upon is in a condition 
to Avhich the name of " psychical blindness " (Seelenhlindheit) is 
given ; but it has suffered no other obvious impairment of its sen- 
sor}^ or motor functions. By " jDsychical blindness " is meant the 
inability of the dog to form those visual mental images or ideas 
which give it the meaning or interpretation, as it were, of its visual 
imjDressions. This includes the loss of the use of that portion of 
the retina which is necessary for distinct vision, and of the immedi- 
ately surrounding retinal parts. If the region Aj be removed from 
both hemispheres of the brain, when the animal has recovered from 
the inflammatory reaction, it will still move about freely, guiding 
itself by sight even under difficult circumstances. But it does not 
recognize by sight the dish from which it has been accustomed to 
take food or water, the companions with which it has formerly 
played, the man who has been its keeper, the threatening hand or 

1 Centralb. f. d. med. Wissenschaft, 1874, p. 548. 

2 The Functions of the Brain, p. 164 f. In the second edition (p. 271 f.) 
Ferrier acknowledges that he was in error in localizing the visual centres in 
this gyrus to the exclusion of the occipital lobes. For a very telling criticism 
of this position of Ferrier, see Munk, Ueber d. Functionen d. Grosshirurinde, 
p. 14 1 


whip, the burning coal held before its face. It still retains its gen- 
eral intelligence and makes constant and diligent investigation into 
the objects by which it is surrounded. As time passes, it gradually 
learns to recognize again all these visual objects. The more com- 
plex and infrequent of the objects are the last in the process of re- 
covery to receive interpretation. At the end of three to five weeks 
after the operation, the injured animal may be said to have recov- 
ered ; its restlessness and curiosity have subsided in proportion to 
the progress made in the knowledge of visual impressions ; it ^.s 
itself at last, its " soul-blindness " having departed. It may be 
shown, moreover, that this recovery consists in learning anew the 
meaning of visual impressions ; or, in other words, in acquiring 
anew the stock of visual ideas that has been blotted out of the ani- 
mal's mind by extirpating the cortical centre of sight. For if the 
dog be carefully kept, for a long time, fi'om any given kind of ex- 
perience — for example, from being struck with a Avhip or biu-ned 
with a coal — it will give no sign of " psychical sight " in relation to 
these particular objects. More remarkable still is the fact that, 
according to Munk,' in certain cases, after the extii'pation of Aj, a 
single visual image or two — for example, the motion of the hand 
commanding the dog to hold out the foot — may be retained. Ex- 
tirpations of the cortical surface on the occipital lobe in the regions 
marked A — that is, before, beneath, or in front and above, the sight- 
centre A, — cause disturbances of sight in a less degree. Such phe- 
nomena Munk considers explicable by the hypothesis that, while a 
large part of the area of the occipital lobe is the seat of the percep- 
tions (?) of sight, the visual images of memory are especially con- 
nected with the so-called sight-centre A^. "When, then, all, or nearly 
all, of the field of sight, in the widest sense, is extirpated from both 
hemispheres, complete and permanent " soul-blindness " results. 
The cortical projection-field corresponding to the entire retinas of 
both eyes, its accumulations of old visual ideas, and capacity for 
receiving new ones, has been wiped out. 

Munk endeavors to establish a still more minute differentiation 
of function in the cortical field of sight as corresponding to the ret- 
inal field of sight. ^ Each retina, he holds, stands for the most part 
in connection with the visual sphere of the cortex of the opposite side 
of the brain ; only a small part — namely, the extreme lateral por- 
tion of the retina — is in connection with the cortical sphere of the 
same side. This lateral poi-tion of the retina seems to be of differ- 
ent dimensions in different races of dogs. Further, the retina ia 

' Die Functionen d. Grosshirnrinde, pp. 23, 34, 119 f. 

* See the " Fuufte Mittheiluug " of his Work, as cited before. 


projected, as it were, on the cortical field of vision in and about Aj, 
in such manner that its lateral area corresponds to the lateral area 
of the cortical sphere on the same side ; its inner area to the median 
area of the cortical sphere on the opposite side. ; its upper area to 
the front area of the cortical sphere on the opposite side ; its lower 
area to the hinder area of the cortical sphere on the opjDosite side. 

In monkeys, as well as dogs, Munk finds that the sight-centre is 
not, as Ferrier at first supposed, the gyrus angularis, but rather the 
convex surface of the i^osterior lobes. Small circular extirpations, 
of not more than two-fifths or three-fifths of an inch in diameter, 
from this region are followed by disturbances of vision, and by these 
alone. If the whole convex surface of one lobe is extirpated, the 
animal has cortical blindness for those halves of both retinas that 
are on the same side as the lesion. If the convex surfaces of both 
posterior lobes are destroyed, the animal becomes entirely blind ; 
no restoration of cerebral function subsequently takes place, unless 
some considerable parts of the edges on the upper surface of at 
least one lobe have escaped destruction. The cortical projection- 
field for the visual impressions of the monkey differs from that of 
the dog simply in having the lateral part of the retina, which cor- 
resjDonds to the cortical area of the same side, much more extended. 
Accordingly, extirpation of the lateral half of the left cortical sight- 
centre, and of the median half of the right cortical sight-centre, 
produces in the monkey total cortical blindness of the left eye. 

§ 21. The searching examination which the views of Munk have 
received has resulted in throwing doubt over some of his alleged 
facts, and in discrediting several most important points in his hy- 
pothesis. This is true especially of the work of Lob and Luciani, 
both of whom have gone thoroughly over the ground covered by 
Munk and come to conclusions dissenting from him. The former ' 
has minutely investigated the effects of destroying Munk's visual 
centre A^, and even his entire visual sphere in the case of dogs. 
He finds, contrary to Munk, that no blindness of the clear spot of 
vision in the opposite eye is produced even by the most extensive 
lesion of this area ; that losses of the cortical substance in the 
area bordering on the lateral part of the visual sphere {i.e., in 
Munk's auditory sphere) also produce disturbances of vision ; that 
other disturbances of motion and intelligence also follow destruc- 
tion of this area ; and that disturbances of sight may follow lesions 
in other than the occipital lobes, especially in the frontal lobes. 
This last conclusion agrees with the results obtained by other ob- 
sei"vers (Ki'iwotorow, Luciani and Tamburini, and especially Goltz), 
' See articles in Pfliiger's Arcliiv, xxxiv. , pp. 67 ff. 


and must be accepted as correct. The more permanent disturb- 
ances which undoubtedly do follow injury of the occipital lobes 
are thought by Lob to be due to what is called a " homonymous 
lateral hemiamhlyopia " (or weakness of the corresponding lateral 
half of the eye) on the opposite side. Munk's whole theory of 
" psychical blindness " as due to the extirpation of visual percep- 
tions and images, and of recovery from such blindness as due to 
special education of the animal in forming new mental images, is 
rejected by Lob. 

The admirable observations of Luciani also tend to disprove 
many of the particular conclusions of Munk, while at the same 
time showing how relatively important are the occijntal lobes in re- 
spect to the cerebral and psj'chical elements of vision. These lobes, 
together with the angular gyrus, are in a peculiar degree the re- 
gion on which the animals are dependent for " psychical " vision 
— that is, for "discernment of things, and a right judgment con- 
cerning their properties and their nature," by sight. 

The foregoing general conclusions from experiment with the ani- 
mals as to the especial importance of the occipital lobes for intelli- 
gent (or " psychical ") vision are, on the whole, in accordance with 
the indications from human pathology. Even Lob testifies that 
after extirpating part of the occipital lobes he has never observed 
a ?7iere motor disturbance without one of vision also ; whereas after 
extirpating part of the parietal lobes he has never observed a dis- 
turbance of vision loithout a motor disturbance. 

§ 22. The answer of pathology to the question, whether the cere- 
bral field especially connected with visual sensations and ideas is 
the same in man as in the dog and the monkey, is not unambigu- 
ous. The method of negative cases, according to Exner,' yields no 
certain results; no "absolute field" for vision can as yet be indi- 
cated on the cerebral cortex. The methods of percentage and of 
positive cases, however, point clearly to the occipital lobe as the 
visual field, and to the upper end of the first gyrus occipitalis (01, in 
Ecker's charts ; see p. 276 f.) as its most intensive portion. Li six out 
of seven cases of disturbances of vision due to cortical lesion the seat 
of the lesion was here. The region of less intensity extends over 
both the first and second occipital convolutions, the cuneus, and 
the adjacent part of the lobulus quadratus. Confirmatory evidence 
may be found in the cases of several persons for a long time blind, 
whose brains have been found on post-mortem to be atrophied 
above the place where the parieto-occipital fissure emerges from 
the median surface upon the convex surface of the occipital lobe. 
' Untersucliungen iiber d. Localisation, etc., p. 60. 


It should be said, on the other hand, that lesions of the occipital 
lobes are very frequently latent, and that extensive injuries of this 
cortical field in man are recorded which v^^ere followed by no 
marked disturbance of sight. 

§ 23. Histology also has some evidence to contribute regarding 
the nervous connections of the retinas of the eyes with the cerebral 
cortex. The amount of crossing which the fibres of the ojDtic nerve 
undergo in the optic chiasm has been the subject of much debate. 
It undoubtedly differs in different animals, and depends uj^on the 
structures of both retina and brain, and upon the relations of the 
two. The researches of von Gudden ' and others have tended to 
show that each optic nerve contains both a bundle of nerve-fibres 
that is crossed and one that is uncrossed, in the optic chiasm or 
beyond it, towai'd the cerebral connections of the nerve ; and that 
the former bundle increases and the latter diminishes in size, on 
the whole, in the higher orders of animals as compared with the 
lower. Biesiadeclii and others claim, on the contrary', that there 
is total decussation of the otitic nerves in the monkey and in man, 
as well as the lower animals. Charcot ° has propounded a yet more 
elaborate scheme of decussation. In the case of man there is still 
doubt, therefore, how far — if at all — the retina of each eye is repre- 
sented on the cortical surface of both hemispheres of the brain. 
That the cortical region especially concerned in the sensations, 
perceptions, and images of sight is in the occipital lobe, and es- 
pecially on its upper convex surface, is a highly probable conject- 
ure. But for the settlement of further details we must await the 
development of the evidence. In the work of this develoj^ment, 
experiment with aiaimals can only suggest the question which a 
more careful collation of a growing number of cases in human 
pathology will perhaps finally answer ; meanwhile the evidence of 
histology may be used to confirm or modify the conclusions estab- 
lished, more or less conjecturally, on the basis of pathology.^ 

§ 24. The localization of other sensory functions in so-called 
" fields " or " centres " on the hemispheres of man's brain — of hear- 
ing, taste, and smell — is even more doubtful. Little confidence can 
be placed in any conjectures thus far put forwai'd. The tempta- 
tion is naturally strong to suspect that those regions of the cortex 
unoccupied by such motor and sensory functions as we are able to 

' Grafe's Archiv f , Ophthalmologie, 1874, Abth. ii. ; 1875, Abth. iii. ; 1879, 
Abth. i. 

- Le Progri's Medical, August, 1875. 

■'For a further description of plienomena and cases, and for a defence of his 
own views, see Ferrier, The Localization of Cerebral Disease, pp. 110 fiE. 


locate should have the other mental phenomena assigned to them. 
In this way the entire brain appears to be made of some definite 
value and use. Convolutions which are located where they are 
unapproachable for purposes of experiment, and in which compar- 
atively few cases of lesion occur, are peculiarly provocative of con- 
jecture. In such Jields of the cerebral cortex, theories of localiza- 
tion may roam at will. The auditor^' centre is assigned by Terrier ' 
to the superior temporo-sphenoidal convolution ; but the evidence 
adduced in proof — such as the pricking-ujD of the animal's ears, 
etc, — is highly unsatisfactory. The same centre is located by 
Munk '^ at the region Bl, for its greatest intensity, and with less 
intensity in the adjacent regions marked B ; but since the entire 
region on both hemispheres must be extirpated (an almost certain- 
ly deadly operation) in order that the animal may become wholly 
" soul-deaf," and since we have no sure means for ascertaininsr 
precisely to what deficiency we should ascribe the failure of the 
animal to respond intelligently to sounds, Munk's experimental 
proof is likewise unconvincing. Luciani, with much more proba- 
bility, considers the " auditory sphere " to extend over the whole 
cortical area of the temporo-sphenoidal lobe, and probably also the 
cormi ammonis. 

The centres of smell and taste are located by Ferrier close to- 
gether in the subicidum and neighboring parts of the lower temporo- 
sphenoidal convolutions ; the centre of touch in the gyrus hijjpo- 
campi and hippocampus major. Munk,^ however, regards these 
centres of Fei'rier as " phantasms." He is strongly inclined, on the 
basis chiefly of one well-differentiated case, to localize smell in the 
gyrus JdiopocampL It is difficult to see how anything sufficiently 
definite for scientific purposes can be known as to distui-bances of 
taste in a dog or a monkey. No adequate evidence is j)rocurable as 
yet for an induction from human pathological cases in regard to 
the cortical fields of any of these so-called lower senses. 

§ 25. To the foregoing remark a possible exception must be 
allowed for the sense of hearing. In. this connection belongs the 
noteworthy localization of the cerebral functions concerned in the 
utterance and interpretation of articulate speech. The various de- 
ficiencies in the power of producing and interpreting articulate 
sounds, whether as spoken or written, which are due to lesions of 
the cerebral cortex, may be grouped together under the general 

' The Functions of the Brain, p. 171 f. ; comp. The Localization of Cerebral 
Disease p. 132 f. 

- Ueber d. Functionen d. Grosshirnrinde, p. 23 f . ; 40 f. 
3 Ibid., p. 129. 


term "aphasia." For about a decade previous to the discoveries 
of Fritsch and Hitzig, in 1870, the facts Avhich seemed definitely to 
connect the loss of speech with a certain region of the left cere- 
bral hemisphere were nearly all to which any advocate of the local- 
ization of cerebral function could confidently appeal in behalf of 
his theory. As long ago as 1825, Boillaud located the articulation 
of words in the frontal lobes. Subsequently (1836) M. Dax main- 
tained the proposition that " lesions of the left half of the enceph- 
alon are coincident with forgetfulness of ^the symbols of thought." 

In treatises of the years 1861-1865, Broca first announced the 
substantially true discovery that the gyrus frontalis inferior on the 
left side of the cerebrum is especially concerned in using the pow- 
er of siDcech. This circumstance he connected with the fact that 
men generally use the left hemisphere more than the right for the 
expression of thought with the I'ight hand and arm, whether in 
writing or in the mechanical arts. The literal meaning of the 
statements made hj Broca — such as that this part of the brain is 
"the seat of the faculty of articulate language " ' — is, however, not 
simply inappropriate to the facts ; it is even absurd. There is no 
one " faculty " of language which can, in any possible meaning of 
the word, be regarded as having its "seat " or locality confined to 
some particular region of the brain. Speech involves, in a very 
complicated and large way, all the faculties ; strictly speaking, then, 
it cannot be located, with all its attendant operations of self-con- 
scious, rational mind, in any one cerebral area. But that the 
phenomena of aphasia show some special connection of certain 
cerebral centres with the complex process of apprehending and ex- 
pressing articulate language, seems entitled to credit as an induc- 
tion based upon a wide range of facts. Of course, in this particu- 
lar attempt at localization of function, no real help can be derived 
from experiments upon the lower animals. 

§ 26. The jDhenomena of various classes, among which the truly 
aphasic cases must be discriminated, vary all the way from those 
resembling the results of momentary inattention — such as that of 
the German professor who certified in writing, "A. B. has attended 
my remarkable lectures in chemistry with inorganic assiduity "■ — to 
the impairment and utter loss of speech in progressive paralysis 
with dementia." A few of the more curious and instructive in- 

' Sur le siege de la faculte du langage articnle, etc., Bull, de la Soc. anat.. 
August, 1861 ; Du siege de la faculte du langage articule dans I'hemisphere 
gauche du cerveau. Bull, de la Soc. d'anthropol. , June, 18(55. 

'For the whole subject, see the great monograph of Kussraaul, in Ziemssen'a 
Cyclopaedia, xiv., pp. 581-875. 


stances furnish facts like the following : The aphasic patient may 
be entirely speechless, and yet understand what is said to him, and 
be able to write his wishes down on paper. Some thus afflicted re- 
tain the power to pronounce words of one syllable, but are obliged 
to resort to writing in order to communicate anything fm-ther. Oth- 
ers possess a small stock of words, which they make more serviceable 
with expressive gestures. Others, still, are simply able to speak " a 
few senseless, and often very extraordinary, syllables and words." 

Among the surprising phenomena of the disease of aphasia, 
none are perhaps more so than those occasioned by the ability 
to utter certain syllables or words, when accompanied by an utter 
inability to put the same letters into slightly different combination. 
One patient, who could say "Bonjour, monsieur," tolerably well, 
could not pronounce the word "bonbon" at all. Another, whose 
vocabulary was almost entirely limited to the meaningless syllables, 
" cousisi," was quite unable to utter either " coucon " or "sisi." 
The celebrated case of the aphasic Le Long, reported by Broca, 
was that of a man confined to five words for his entire vocabulary. 
These words were, " oui, non, tois instead of trois, toujours, and 
Le Lo instead of Le Long." The first two and the last were used 
with their appropriate meaning; "tois" indicated all ideas of 
number whatever ; and " toujours" was the word used when the 
patient could not express his meaning by gestures and the other 
four words. It appears, then, that Le Long could pronounce the 
r in " toujours," but not in " trois," and the nasal sound in " non," 
but not in his own name. In another class of cases, the aphasic 
person can utter only a few or no words spontaneously and cor- 
rectly, but can repeat and write without difficulty words that are 
spoken before him. Such inability is sometimes called "simple 
aphasia of recollection." Different classes of words, as a rule, slip 
fi-om the memory in succession, as it were. Proper names are 
most frequently forgotten ; then substantives generally, and some- 
times verbs, adjectives, pronouns, and all other parts of speech. 
"The more concrete the idea," says Kussmaul,' "the more readily 
the word to designate it is forgotten, when the memory fails." 
Many cases of disease occur where the patient has lost the power 
mentally to find the appropriate words, although his power of ar- 
ticulation is unimpaired. Such disturbances of speech may, or may 
not, be accompanied by a corresponding impairment of general in- 
telligence. This complication increases the difficulty of studying 
the phases of this disease. 

Aphasia may also be accompanied by so-called "word-deafness" 
' Ziemsseu's Cyclopaedia, xiv. , p. 759. 


and "word-blindness." Persons thus afllicted hear words as con- 
fused nmrmurings, or see them as bluri'ed images. The individ- 
ual letters may be intelligently heard or read, but their combina- 
tion has become unintelligible. The same thing sometimes happens 
with figures ; as in the case of the accountant who could read the 
sum 7G6, figure for figure, but did not know what the figure 7 
meant as placed before the two 6's. At other times the disturb- 
ance of speech takes the form of grammatical ataxy, as it were, or 
of verbal delirium — a medley of words, partly in themselves signifi- 
cant and partly unmeaning. 

The agraphia, or inability to express thought in written language, 
which not infrequently accompanies aphasia, may be incomplete, 
or absolute and literal. Some patients, who have formerly been 
highly cultivated, become unable to produce a single letter with 
the pen. Others can write long rows of letters, but arrange them 
for the most pai't in meaningless fashion, with an intelligible word 
occurring here and there. In brief, the phenomena more or less 
closely connected with the disturbance called aphasia are exceed- 
ingly complex and various. 

§ 27. In the effort to classify so many complicated facts, and to 
distinguish among them such as are of truly cerebral origin, Ex- 
ner' makes the following distinctions: First, there are cases 
where the understanding of the words is affected ; and such loss 
may constitute the chief or the entire part of the aphasia. The 
patient can then hear and articulate, but the "acoustic image" of 
the word as the symbol of an idea has jDcrished. In a second form 
of aphasia, the inability concerns the clothing of the result of 
thought (the idea) in words — whether for purposes of spoken or of 
written expression. In most such cases it is simjDly the appro- 
priate word which is forgotten. In the third class of cases, the 
aphasic person can foi'm the idea and select the word approj)i'iate 
to express it, but cannot bring about those processes of central in- 
nervation which are necessary to initiate the expression. All these 
three forms may combine variously, and all may be connected with 
disturbances of speech which are not to be localized in the cerebral 
cortex, but which have their origin at some point in the extra-cor- 
tical nerve-tracts concerned in speech. The very elaborate analysis 
of Kussmaul leads him to make the following statement: "All 
disturbances of speech can be brought under two great classes^ 
according as the connection between the conception and the word 
is impeded in the direction from the former to the latter or, vice 
versa, from the latter to the former. When the first happens, the 
' In Hermaun's Handb. d. Physiol. , II., ii., p. 343 f. 


expression suffers; when the second, the understanding." By the 
1 ist word, however, we must naean the " understanding " as apphed 
especially to articulate speech. For aphasic persons are often very 
intelligent in carrying on the trains of thought necessary to suc- 
cess in games of skill, or in the expression of feeling in music ; and 
if we accept, even with considerable allowances, the intelligent 
testimony of Lordat concerning his own mental condition when 
aphasic, they sometimes exercise the mind in abstract reasoning 
of a high order, even when unable to recall a single word appro- 
priate for the expression of their thoughts. 

In all true aphasia, then, the connection between ideas and ar- 
ticulate language is interrupted within the cerebral cortex. Is it 
possible to indicate any region of this cortex, lesions in which are 
regularly accompanied by aphasic symptoms ? or, in other woi'ds, 
Can the function of articulate speech, so far as this consists in the 
ability to apprehend and successfully to will its acoustic and visual 
symbols, be localized in the cerebral hemispheres ? In answer to 
this question it must be admitted that no absolute field for aphasia 
can be pointed out ; that is, besides the region where lesions are 
connected in by far the greater number of cases with aphasic dis- 
turbances, other regions of the cerebral hemispheres only some- 
limes thus connected may be pointed out. 

§ 28. In a large percentage of cases of disturbance of speech due 
to cerebral lesions, the posterior third of the third frontal convolu- 
tion and the other regions bordering on the fissure of Sylvius 
(island of Eeil, and immediately adjacent parts of the parietal and 
temporal lobes) are the seat of the lesions. Aphasia is far more 
frequently due to changes in the left than in the right hemisphere 
of the brain. Dr. Seguin, out of 260 cases, calculated the propor- 
tion of aphasias due to lesions on the left side, as compared with 
those due to lesions on the right, to be as 243 : 17 or 14.3 : 1. Such 
disparity is far too great to be attributed to the comparative fre- 
quency with which the left hemisphere in general is the seat of 
lesions. In Exner's ' collection of cases, out of 81 lesions resulting 
in aphasia, all but one were on the left hemisphere (in three cases, 
however, the right was also involved), and in that one the trouble 
was only temporary. Such facts have led to the theory that, in all 
but left-handed men, speech, like other motor functions, is chiefly 
left-brained ; remarkable cases of left-handed people who have be- 
come aphasic through lesions on the right hemisphere are actually 

' Functionen in d. Grossliirnrinde, p 51 f. 
^ See Kussmaul and his citations, p. 739 f. 


Of the left hemisphere, the gyruii centralis anterior and the 
adjacent convokitions of the frontal lobe, but especially the pos- 
terior part of the third (lower) convolution, have much the highest 
intensity as seats of aphasia lesions. In 53 carefully collected 
cases by Lohmeyer,' 60 were on the left hemisphere, 24 in the 
lower frontal convolution, 34 in this convolution and neighboi-ing 
parts, 13 in the island and adjacent parts, 6 in the island alone. 
Exner's collection, however, did not show that the "intensity" of 
the lower is any greater than that of the middle frontal convolution, 
or of the two upper temporal convolutions. This collection con- 
tained, moreover, five cases in which lesions were seated in the 
lower left frontal convolution loithout any resulting aphasia. Exner 
therefore justly concludes that the " cortical field " of speech, like 
the corresponding fields of all the motor functions, is really much 
more extended than has generally been supposed. He is himself, 
nevertheless, inclined to localize, yet more definitely, so-called 
"ataxic aphasia" in the third frontal convolution, "word-deaf- 
ness" in the middle gyrus temporalis, and agraphia in the lower and 
front part of the parietal lobe ; that is, in the neighborhood of 
the motor region for the upper extremities. So specific localiza- 
tion can hardly, however, be safely based on the restricted number 
of cases which Exner considered. 

Lohmeyer gives 2 cases of aphasia following lesions in the an- 
terior portion of the frontal, 3 in the parietal, and 4 in the occipital 
lobe, Exner gives 3 cases in which the central convolutions were 
alone the seat of disease ; 2 in which the temporal and parietal 
lobes were alone affected ; 1 in which the only lesion was in the oc- 
cipital lobe. In the only sense in which the brain can be spoken 
of as the " seat of the faculty of articulate language," we must ad- 
mit that the evidence confirms the following assumption of Kuss- 
maul : " It is, a liriori, probable that an enormous association-tract 
in the cortex has been assigned to speech, even though the key- 
board of sound may be confined to the anterior cortical regions." 

§ 29. The moi'e ardent and positive advocates of the theory 
of locally specific cerebral functions find it exceedingly difiicult to 
refrain from seating general intelligence, or the powers of percep- 
tion, memor}^, comparison, etc., as applied to all the objects of 
cognition, in some particular so-called "field "or "area "of the 
brain. At present the frontal lobes offer themselves as the most 
convenient region for such pre-empting of the cerebral domain. 
The general propriety of considering the connection which un- 
doubtedly exists between the central nervous mechanism and men- 
' Arcliiv f. Klin. Cliirurgie, XIII., p. 309, as cited in Kussmaul. 


tal phenomena, under any spatial terms whatever, will occupy our 
attention later on. It is enough at present to say that the experi- 
mental and pathological evidence do not warrant us in assigning 
such pre-eminence to the frontal lobes. Extensive lesions may oc- 
cur in these lobes with little or no diminution of so-called general 
intelligence. On the other hand, small lesions in other regions of 
the brain are not infrequently productive of comparatively profound 
mental derangement or loss of function. Moreover, lesions localized 
in those areas of the cerebral cortex which have thus far been con- 
sidered—namely, the parietal, occipital, and temporo-sphenoidal 
lobes— are, of necessity, connected with more or less impairment 
of intelligence. 

There can be no doubt that the mental processes which we describe 
by the word "intelligence " are all closely related to the basic sensory 
and motor activities that are chiefly localized elsewhere than in the 
frontal lobes. An animal that is " soul-deaf " or " soul-blind " has, 
so far forth, an impaired intelhgence. The same thing is eminently 
true of the man afflicted with aphasia in any of its severer forms. 
The loss of intelligence is not necessarily (or even probably) due to 
the partial destruction of that functioning of the hemispheres in 
general which results in intelligence ; it is rather due to the fact 
that the support which all ideation receives from the audible and 
visible symbols of the idea— whether chiefly as respects its forma- 
tion or as respects its expression — has become impossible. The 
impairment of any considerable area of tactile sensations, especially 
as localized in those parts of the body which are most used in per- 
ception through such sensations {e.g., the hand), also occasions a 
certain loss of intelligence. The restrictions which cerebral disease 
introduces into the number and nicety of the sensory and motor 
functions are, of course, much less important when they come upon 
minds already /»rni67ierf, as we say, " with a stock of ideas." Still, 
even in such cases a basis of sensations and volitions constantly 
underlies, as it were, all the higher and pre-eminently intellectual 
mental processes. 

§ 30. In spite of the evidence adduced, a few experimenters still 
either wholly reject the principle of the localization of cerebral 
function, or else urge arguments against carrying it out even with 
the limitations which the foregoing conclusions have observed. 
Among such experimenters the most prominent is perhajDS Goltz. ' 
The method of extirpation practised by Goltz was that of wash- 
ing away the substance of the cerebrum by streams of water sent 

' See especially liis treatises as collected in the book, Ueber d. Verriclitungen 
d. Grosshiriis, Boun, 1881 ; and Pfliiger's Arcbiv for 1876, 1877, and 1879. 


through orifices broken at selected places in the skulls of dogs. 
This metbocl has the advantage of saving bleeding ; it has the dis- 
advantage of not definitely localizing the injuiy. Its author has 
applied it with great care and skill to a large number of animals, 
many of which he has succeeded in keeping alive for months, even 
after the removal of considerable areas from both hemispheres 
(in one instance tbe brain-substance, calculated to have weighed 
originall}' 93 grammes, had been reduced to 13). The principal 
conclusions drawn from his expei'iments by Goltz are adverse 
to the theories of localization held by Ferrier, Munk, Luciani, and 

Goltz's conclusions may be summarized as follows. No impair- 
ment of intelligence follows the loss of a large amount of cortical 
substance from one side of the brain ; but loss of any considerable 
amount of substance from both sides — whether in the frontal, 
posterior, or temporal lobes — produces a permanent impairment of 
all the functions, which corresponds in a general way to the amount 
of the loss. Every sense, and the inteUigence of every sense, is 
thus weakened ; for tlie cerebral elements of sense are impaired or 
destroyed [Hirnsehschwache, etc.). For example, a dog which has 
been trained to give his paw on command loses the power to do so 
in consequence of such loss of brain-substance, and never regains it. 
It is not possible, by extirpating any amount of the substance of 
the cortex on either side or on both sides, to produce a permanent 
laming of any muscle of the body, or a total loss of sensibility in 
any of its parts. It is, however, possible, according to Goltz, by 
repeated removal of the cerebral substance on both sides, gi-adually 
to reduce an animal to a condition of almost complete idiocy — to 
an elaborate eating, drinking, and walking "reflex-machine." The 
removal of as much as 4 grammes from each hemisphere produces, 
as he calculates, a considerable degree of idiocy. No part of the 
cortex of the brain can, then, be called the exclusive organ or centre 
of intelligence or feeling ; but the psychical functions of sensation, 
volition, ideation, and thought are connected with all of its parts. 
The quantity of the cerebral substance i-emoved determines the 
amount of the general impairment of mental powers, instead of the 
locality from which the removal is made fixing the quality of mental 

It must be admitted that the facts discovered by Goltz, and the 

conclusions which he reaches, seem at first strongly opposed to all 

localization of cerebral function. But they are not really so ; nor 

is it quite coi-rect even to say, as Foster ' does, that Goltz's results 

' A Text-book of Physiology, p. 649. New York, 1880. 


are "absolutely opposed" to tliose of Munk. In fact, Goltz' him- 
self asserts that destruction of the parietal lobes produces a greater 
permanent disturbance of feeling, and destruction of the occipital 
lobes a greater permanent disturbance of sight. In general, an 
animal operated upon in the two hind-quarters of the cerebrum is 
more stupid than one which has suffered loss of the fore-quarters ; 
the former is duller of sight, hearing, smell, and taste ; the latter is 
duller in respect to skin-sensations. The effect of injury to the 
posterior parts of the brain is therefore much more marked in de- 
pressing the intelligence of the animal (as shown in sense-percep- 
tion). Moreover, Goltz ^ claims that he has never rejected the 
possibility of a localization of the functions of the brain. He con- 
firms ^ the conclusions of Fritsch and Hitzig, by saying that he has 
often seen mechanical excitation of the parietal lobes j^roduce mo- 
tions in the limbs of the opposite side. His facts and arguments 
are rather directed against tliat form of the hypothesis of localiza- 
tion which seats the different functions in small circumscribed 
areas ^ and then, w^hen forced by facts, conceives of them as also ca- 
pable of hopping about from one of these areas to another, like a 
bird from twig to twig in the branches of a tree. Furthermore, a 
detailed comparison of the experiments of Goltz with those even 
of Munk shows that the results of the two are in the main recon- 
cilable ; if only it be remembered that the former has not always 
precisely defined the areas of brain-substance removed, nor suffi- 
ciently taken into account the undoubted results obtained by others 
from definitely circumscribed lesions ; and that the latter has, cer- 
tainly, in many cases been more precise and confident than a fair 
view of all the facts will warrant. 

§ 31. Three principles may be laid down as summing up the 
results reached by inference upon the basis of experiment with 
respect to the localization of function in the cerebral cortex. '" The 
first principle is to be accepted in the form of a general postulate 
derived from a study of the other parts of the nervous system, and 
confirmed on attempting to apply it to the cerebral hemispheres. 
It may be stated as follows : the different elementally parts of the 
nervous system are all capable of performing its different sj^ecific 
functions when, and only when, they have been brought into the 
proper connections and have been exercised in the performance of 

' Verrichtungen d. Grossliirns, pp. 114 f. and loO f. ; and art. in Pfliigers 
Archiv. xxxiv., pp 450 ff. 

^ Verrichtungen d. Grossliirns, p. 163. ^Ibid., p. 165. *Ibid.. p. 169. 

' Compare the five general laws o£ central functions given by Wundt, 
Grundzuge d. pliysiolog. Psycliologie, i., p. 224 f. 

300 sujVimakt of results. 

tliose functions. This principle includes two important laws which, 
we know, hold good throughout the whole nervous mechanism, and 
which he at the physical basis of important psychical facts and laws ; 
they are the law of Specific Energy and the law of Habit. Different 
combinations of the elementary parts of the nervous system, form- 
ing composite parts or organs, have different values and functions 
in the general economy of the system. Every nerve-fibre, every ele- 
ment of an end-organ, or of a central organ, may be said to have a 
specific function, and to discharge that function in the exercise of 
a specific energy. As to how far the capacity for this specific energy 
is dependent upon the specific molecular structure of the element- 
ary parts, we are only able to conjecture ; but about its depend- 
ence upon the connections in which the elementary parts stand with 
each other, there can be no doubt. 

Moreover, the elementary parts of the nervous system, inasmuch 
as they have the general adaptation necessary to the performance 
of nervous functions, can " learn " (so to speak) to perform the 
more specific of these functions — but only in case they stand in 
appropriate connections. The repeated action of the nervous ele- 
ments in specific functions fits them the better to act in the same 
functions. The effect of the exercise of any function in the past 
may be " stored uj) " so as to increase the facility of the nervous 
structure to exercise again every similar function. Thus, different 
elementary parts of the nervous system, if at first forced by circum- 
stances to become active in a given way, may by repeating the 
activity gain a position of facility and value like that belonging to 
other parts whose so-called normal action lies in this particular 
way. This law of habit in the nervous system explains much of 
the behavior of the nerve-muscle machine, or of the decapitated 
frog, etc., under artificial stimuli ; it also underlies the theory of the 
sensory-motor effects attributed to centres in the spinal cord and 
basal ganglia ; it throws light upon the physical basis of our ex- 
perience in learning to walk, talk, play upon musical instruments, 
or handle tools, as well as upon the transmission from generation 
to generation of minute bodily characteristics. Both the law of 
specific energy and the law of habit undoubtedly apj^ly to the func- 
tions of the cerebral cortex. 

The remaining two of the three principles alluded to above may 
be said to follow from the first ; they are the princij)le of localized 
function and the principle of siibstitulion. The former asserts 
that, in the normal condition of the nervous system, all jjarts have 
not the same definite functions. Inasmuch as the functions of the 
different elementary parts necessarily depend upon the manner 


in whicli tliey are combined and connected, the composite parts 
or organs thus formed must also have certain normal functions. 
But such composite parts or organs have, of course, a definite local- 
ity ; hence the functions of the nervous mechanism must be more 
or less definitely localized. Nor can the principle be suspected 
of a disposition to stop short off and abdicate its authority, when 
we reach the region of the cerebral cortex. There is nothing in 
the structure of the cortex to show why the general law of differen- 
tiation of function should be inapplicable there. On the contrary, 
everything in both its anatomy and physiology indicates that the 
principle of localized function does apply, in some sort, to the cere- 
bral hemispheres. 

So-called "centres," or "areas," or "fields," of the cerebrum are 
in no case, however, to be regarded as portions of its nervous sub- 
stance that can be marked oft' by fixed lines for the confinement 
of definite functions within rigid limits. These areas are some- 
what different for different brains of the same species ; the}' widen 
when a heightened energy is demanded of them ; their centres 
are neither mathematical points nor very minute collections of 
cells. They are not composed of elements which have, each one, 
a fixed and unchangeable value, and a definite function, as though 
the number of mental operations assigned to a locality needed to 
be jDrecisely matched by the separate nerve-fibi'es and nerve-cells of 
the locality. Nor are these areas perfectly isolated localities ; on 
the contrary, they obviously overlap each other in certain cases. 
According to the true statement of Luciani, "the single centres 
in the sensory-motor zone are so completely bound up with, and, 
so to speak, let into one another, that it is not possible to divide 
them with a clear and definite line, such as is the case when the 
cortex is incised and removed ; so that in destroying a centre one 
necessarily eliminates a portion of the neighboring centres." Nev- 
ertheless, there is no doubt that the cerebral functions connected 
with the different sensations and motions of the peripheral parts of 
the body are not all alike exercised by all parts of the cerebrum. 
They are assigned specifically to those regions which alone have the 
proper structure and stand in the proper relations. 

Furthermore, the functions of the cerebrum are not absolutely 
confined to those centres with which, under ordinary circumstances, 
they are chiefly or wholly connected ; in which, that is to say, they 
are localized. If such centres, for any reason, become incapacitated 
or relatively unfitted to perform their normal functions, the same 
functions may be performed by other areas of the cerebral cortex, 
provided these areas also stand in the proper connections. This 


is the principle of substitution. It is clue to its working that 
animals subjected to exj)eriments in extirpation, as a rule, so 
largely recover the powers of sensation or motion which they have 
temporarily lost. It is on this class of phenomena that Goltz rests 
much of his argument. In the cerebral hemispheres, however, the 
principle of substitution does not overstep all limits, nor does it 
operate arbitrarily. The portions of the same hemisphere that 
are just adjacent to the so-called centres — the larger areas sur- 
rounding or contiguous to the smallei' — and, on account of its bi- 
lateral structure, the corresponding portions of the other hemi- 
sphere, are in general those best capable of exercising such substi- 
tutive functions. It may be doubted whether these portions do 
not, in all ordinary cases, cover the entire limits within which the 
principle of substitution can act. Such substitutive functions im- 
prove under the law of habit to which the organs of the cerebral 
cortex are subjected. 

The connections between the different cerebral areas and their 
functions are so complex and subtile that physiological science wiU 
need a long time to disentangle them ; it may be doubted whether 
it will ever succeed in doing this completely. The connections 
among the phenomena of conscious sensation, volition, ideation, 
and thought are at least equally subtile and complex. Will psy- 
chology ever disentangle these connections ? 

The bearing of the subject on our conclusions concerning the 
nature of the mind and its connection with the body will be con- 
sidered elsewhere. 


§ 1. The world of ordinary experience consists of a great number 
of so-called '•' things " that are known to us by their distinguishing 
qualities. Although each one of these things is believed to be a 
separate existence, they are all perceived as having certain common 
characteristics, and as standing in certain relations to each other, 
of space, time, and action. It is with the things, their common 
qualities and mutual relations, that unreflecting practical life is 
chiefly concerned. But even without special reflection, everyone 
learns that his knowledge of such external objects dejDends upon 
the kind and degree of the effect they exercise upon his conscious- 
ness through the senses. Attention is thus turned from the things 
themselves to the sensations produced in us by their action. The 
variety of such sensations, at first bewilderingly great, is soon re- 
duced to some order by a classification referring them to the dif- 
ferent organs through which they come. Thus, certain sensations 
are received through the nose, others through the mouth, the ear, 
the eye, or the skin — especially as covering that part of the body 
(the hand) which is most active in touch. Smell, taste, hearing, 
sight, and touch are the five classes of sensation, as the grouping 
is made by the unprejudiced judgment of all. 

A further rough and scientifically inadequate classification takes 
place among the sensations of the same sense. Those of smell, in- 
deed, defy classification, whether popular or scientific. Among 
tastes, the most familiar are easily distinguished ; such are the 
sweet, the sour, and the bitter. The two principal classes of sensa- 
tions of sound are easily discriminated, as either noises or musical 
tones ; the former are further classified as respects the character of 
the feeling which accompanies them, and the latter as high or low 
in pitch. The different more prominent colors — including black 
and Avhite— are recognized by all persons of normal vision as 
modes of the sensations of sight ; hence the colors commonly 
named, and the various so-called "shades" of these colors. That 
more than one class of sensations arise through the skin is shown 


by the popular use of the word to "feel." Things /eeZ hard and 
soft, smooth and rough, as weU as warm and cold. But things are 
also said to feel heavy or light. The feeling by which their weight 
is estimated, however, is only ascribed in a very indefinite way to 
the parts of the body that are chiefly concerned in passively sup- 
porting, or actively lifting, or pushing against their weight. The 
particular use of tactual feeling, as well as the general use of the 
muscular sense, in gaining this class of sensations is little noticed 
by ordinary reflection. 

§ 2. All the sensations are also regarded as having some place 
in a scale of degrees of sensation ; they are either strong or faint, 
or else He somewhere between the two extremes. They are also 
habitually thought of as related in time, and as being connected 
with the motion in space of the objects that occasion them. Of 
the molecular action of their stimuli upon the end-organs of special 
sense ; of the hidden chemical, electrical, or other processes con- 
nected with the activity of the peripheral and central nervous sys- 
tem ; of the physiological, psycho-physical, and psychological laws 
under which the mind reacts in the form of simple sensations, and 
combines these sensations into the composite objects of sense ; of 
all these and other similar matters, the unreflecting conception of 
sensation takes no account. 

§ 3. It is obvious that the analysis of sense-percepts which suf- 
fices for working-day life will in no respect answer the demands of 
science. Its " common-sense " character is a distinct mark of its 
inadequacy. An adequate scientific treatment of this branch of 
Physiological Psychology requires at least four things : (1) to dis- 
tinguish the simple sensations from those complex objects of experi- 
ence with which alone our adult consciousness is familiar ; (2) to 
point out the varieties of quality and degrees of quantity which be- 
long to these sensations, and to discover the laws which relate them 
to changes in the form and intensity of their stimuli ; (3) to show 
how the simple sensations are constructed by the mind into the so- 
called "presentations of sense" under mental laws of time-form 
and space-form ; and (4) to indicate how far, if at all, the higher 
mental activities of association, memory, will, and judgment, may 
be brought under laws similar to those upon which the formation 
of these presentations of sense depends. It is upon these four 
heads of inquiry that modern psychology, as studied from the 
psycho-physical point of view, has expended most of its painstak- 
ing researches. Its success has been by no means complete. All 
these fields of inquiry still include many unanswered questions ; all 
of them present the results of researches that seem in various re- 


spects conflicting. Yet it is precisely in these fields that modern 
psychology has achieved its most brilliant successes. It has thrown 
a flood of new light upon the essential nature and growth of hu- 
man experience. It has profoundly influenced the current views 
on metaphysics. It has contributed important factors toward tlie 
solution of certain questions of interest to ethics and religion. It 
has given us a new jDoint of view for I'enewiug the ancient debate 
between Materialism and Spiritualism. 

§ 4. The distinctions with which scientific analysis begins are to 
a large extent received from ordinary experience. Some of the 
most essential of the distinctions are confirmed by the results of 
this analysis. They all, however, require to be carried farther and 
to be fixed with much more of accuracy than belongs to the im- 
pressions of common life. New distinctions also have to be intro- 
duced. For example, scientific investigation maintains the differ- 
ence between sensations of smell and sensations of taste ; but it 
points out what is not ordinarily apparent — namely, that certain 
results commonly referred to the latter sense really belong to the 
former. It also adds the sensations of the muscular sense to the 
classes popularly described ; and it discriminates more clearly be- 
tween two distinct kinds of sensation that have the skin for their 
organ — namely, temperature and pressure. 

Psycho-physical science, moreover, accepts the common distinc- 
tion between the quality and the quantity of the different sensa- 
tions. But it describes with all possible accuracy the limits Avithin 
which alone this distinction can be carried out. It shows that the 
quality and quantity of sensation are inseparably connected ; that, 
as Lotze held (a view confirmed by von Kries and others), changes 
in quality can be distinguished from changes in intensity, with 
perfect confidence, only in the case of sensations of hearing. It is 
possible that even here the distinction is largely made on the basis 
of complex experience. Very intense sensations of heat and cold so 
far change their specific character as to tend to pass into each other, 
or, perhaps, to become submerged in a common tone of painful 
feeling. Minimum sensations of heat and pressure are difficult to 
distinguish from each other ; maximum sensations of pressure are 
likely to lose the characteristic quality of touch and be displaced 
by sensations of pain. To treat scientifically of the quality of 
sensations requires, then, a large amount of the most careful 

§ 5. It is essential, in the first place, to distinguish " simple 
sensations" from " presentations of sense," or those complex ob- 
jects of consciousness which result from an act of mental synthesis 


on the basis of several simultaneous affections of sense. As respects 
developed experience, the simple sensation is a necessary fiction of 
psycho-physical science. Consciousness is scarcely more able di- 
rectly to analyze a presentation of sense into those factors out of 
which it originated than it is to analyze a drop of water into its 
component oxygen and hydrogen gases. Simple sensations, there- 
fore, are not objects which can be examined in the direct light of 
introspection. Yet they are factors which, as scientific analysis 
shows, actually enter into all such objects as can properly be sjDoken 
of under the term "presentations of sense." Any sensation which 
is absolutely unanalyzable with respect to distinctions of quality', 
and which, therefore, cannot be considered as consisting of com- 
ponent parts, is called simple. It is distinguished as a sensation 
from all other elementary forms of feeling or knowledge, by the 
relation which it sustains to the jDresentations of sense. A sensa- 
tion, unlike the feeling of grief, of desire, or of weariness, etc., is a 
potential factor of a material object. Through the senses we know 
"things;" not, indeed, as though they apj^eared before the mind 
by immediate apprehension in the form of exact copies of extra- 
mental realities. But every sensation is an affection of the mind 
recognized as connected with an extra-mental reality, through the 
activity of the senses. Simple sensations are those elementary 
factors, themselves indecomposable, out of which the presentations 
of sense are composed. The objects of sense, however, do not have 
the character of mere compounds of simjDle sensations. Sensations 
must not only be associated and compounded, but also localized 
and projected without (that is, set in systematic relations of sj)ace- 
form), in order to constitute the objects of sense. 

§ 6. The foregoing remai'ks suffice to indicate, in a preliminary 
way, what is the nature and value of the psycho-phj'sical investi- 
gation of sensation. We inquire, in the next two chapters, as to 
the Quality of Sensations. The inquiry, when conducted from the 
psycho-physical point of view, involves an answer to three questions : 
(1) What is the precise locality in the organism where the specific 
excitation which occasions each kind of sensation originates ; and 
what is the natui-e of the action of the stimulus in producing such 
excitation? (2) What are the kinds of sensations which appear in 
consciousness as the result of the various excitations? (3) What are 
the laws by which the quality of the sensations is related to the 
kinds of excitation? Neither of these three questions can be 
answered completel}'. The investigation of the first is much re- 
stricted by our almost complete ignorance of those processes in 
the central organs that are in all cases the proximate internal 


stimuli or immediate antecedents of the sensations. Moreover, our 
knowledge of the intimate structure of the end-organs of sense, 
and of the nature of the physical processes which excite them, is 
still very incomplete. The detection of obscure but important dif- 
ferences in the qualities of conscious states of sensation is by no 
means easy ; it requires great skill, strict and trained attention, 
and unwearied rej)etition of experiment. But these conditions of 
success have a great effect in altering the quality of the sensations 
themselves. Besides all this, remarkable idiosyncrasies not infre- 
quently appear ; and language can only imperfectly describe even 
the most common factors of the varied and living experiences wdth 
which science tries to deal. 

In investigating the laws that define the relations between our 
subjective experience, called sensation, and objective phenomena 
in the shape of physical energy acting uj^on the nervous mechanism, 
there is often the greatest doubt as to what manner of laws are be- 
ing investigated. They may be considered as purely physiological, 
or as psycho-physical, or as purely psychological. It is not strange, 
therefore, that different theories exist for accounting for all the 
more important groups of facts, deijending uj^on the emphasis laid 
by different investigators upon the value of each of the three possi- 
ble modes of explanation. The truth is, that each sensation is sepa- 
rated by a series of intricate j)hysiological and psychical processes 
from the application of the stimulus in the gross, as it were, to the 
end-organ of sense. 

§ 7. The authority of one great law is involved, as a silent 
assumption, in all discussion of the quality of sensations. This law 
is known as the law of the Specific Energy of the Nerves. It has 
already been shown (Part I., chap. I., § 35) that any such dis- 
tinction of the kinds of nerve-fibres as denies their possession of 
common functions cannot be maintained. But the phenomena 
of sensation cannot be explained without a much more extended 
application of this law than has thus far been found necessary. 
Distinctions of quality in sensation depend upon the excitation of 
specific corresponding elements of the nervous system. That only 
the optic nerve is capable, when excited, of exercising the physio- 
logical function upon which sensations of light and color depend, 
does not admit of doubt ; the same specific quality cannot be 
denied to the functional activity of the nerves of smell, taste, hear- 
ing, and touch. Moreover, in the end-organs of each of these 
senses, provision must be made for a further differentiation of 
function. What is the nature of the evidence, and what conclu- 
sions must be drawn from it, will be best appreciated at a later pe- 


riod in the discussion. Meantime we find ourselves obliged to as- 
sume the existence of some law of the specific euex-gy of the nervea 
of special sense. 

§ 8. Little of a scientific character is known concerning Sensa- 
tiona of Smell, considered as respects their qualit}'. Anatomy, 
chemistry, and physics fail to furnish definite information on this 
point ; experimental physiology as aj)plied to the lower animals is, 
of course, unsatisfactory ; and the appeal to human consciousness 
asks for an anatysis of which it is incapable. It has already been 
shown (Part I., Chap. V.) that the part of the mucous membrane 
of the nasal passages known as regio olfactoria contains the end- 
organs of smell ; the specific stimulus of the organs in this region 
is applied as borne thither by the current of air, and almost, if not 
quite, exclusively in the act of inspiration. In order that any sub- 
stance may act through the end- organs on the nerve (olfaclorius) 
which is spread out in this region, it must either exist in gaseous 
form or else be vaporizable under given conditions of temperature. 
The degree of temperature at which different substances become 
odorous therefore varies according to their i^hysical characteristics. 
For example, arsenic, which at ordinary temperatures is inodorous, 
when raised to a dark-red heat is vaporized and the vapor excites 
an intense sensation of smell. Fluid bodies which give off an 
odorous i-eek, when brought in fluid form into contact with the 
mucous membrane of the regio olfactoria have no smell ; if this 
membrane is soaked in fluid of any kind whatever, it loses for a 
time the capacity to be excited witli olfactory impressions. E. H. 
Weber ' discovered that if the head be placed with the nostrils 
pointing upward, and the nasal jDassages be then filled with pure 
water, sweetened water, or a mixture of water and eau de Cologne, 
after these passages are emptied the sense of smell is, in all cases, 
temporarily lost ; even when Cologne is used, with the exception 
of the instant at which the fluid is poured in, no odor can be per- 
ceived. Subsequent observers have confirmed the experiments of 
Weber. One investigator ^ lost all sense of smell, even for acetic 
acid and ammonia, for a period of half a minute ; another for five 
minutes, and the sense in its full acuteness did not return for nearly 
half an hour. Whether this effect of the fluid is due to impair- 
ment of the end-apparatus of smell by soaking it (so Valentin), or 
to the mechanical barrier which the layer of foreign substances 
interposes between the odorous particles and this apparatus (so 
FrohHch), we cannot say ; it may be due to both causes. Contrary 

1 See Archiv f. Anat , Physiol., etc.. 1847, p. 257. 
sprolilicli, in Sitzgsber. d. Wiener Acad., 1851, VI., p. 322. 


to the assertion of Wundt,' that probably no gases or vapors, except 
atmospheric air and its constituents, are absolutely inodorous, so 
far as we have present information a number of gaseous and va- 
porizable substances are so ; and no reason is known for such 
apparent exceptions to the rule. 

§ 9. The stimulus of smell is supposed to consist in certain ex- 
ceedingly minute particles contained in the odorous gas or vapor 
which is drawn in with the current of air over the mucous mem- 
brane of the regio olfactoria. The question is as yet scarcely de- 
cided, whether other forms of stimulus, besides these odorous 
particles — mechanical, electrical, thermic, or so-called subjective — 
can excite the sensation of smell. Tlie older experimenters (Volta, 
Pfaff, Fowler, and Humboldt) failed to obtain any certain proof 
that the electrical current is an excitant of this sense. In one place, 
however, Pfaff speaks of a sensation resembling the smell of sul- 
phur as caused by the apj)lication of electricity to the sensory pas- 
sages of the nose. Ritter (in 1798) experimented by using bits of 
graphite and zinc thrust into these passages, and also by holding 
one pole of a battery in the hand and placing the other in the nos- 
tril. In the latter way he thought that he excited a genuine sj)e- 
cific sensation of this sense. He describes the positive pole in the 
nostril as producing an inclination to sneeze and a trace of a smell 
like that of "ammonia ; " the negative pole placed there does away 
with this inclination and produces a kind of " sour " smell. Such 
phenomena are probably, however, all to be assigned to the nerves 
of taste, touch, and common feeling. More recent investigations 
have done little to remove the reasons for doubt." The smell of 
phosphorus which is developed by the action of the electrical ma- 
chine is probably due to the ozone set free ; it is not a case, then, 
of the direct excitation by electricity of the sensation of smell. 
Some physiologists (notably Valentin) have observed that this sen- 
sation may be awakened by mechanical stimulation, such as strong 
vibi'ation of the nostrils, violent sneezing, etc. ; others have failed 
to produce this specific sensory effect in such ways. It does not 
appear that thermic stimulation will excite the sensation of smeU. 

Experiments to prove that subjective sensations of smell may be 
produced b}' injecting odorous substances into the veins of animals 
are very uncertain. Human pathological cases, in spite of the cus- 
tomary indefiniteness of the patient's testimony as to the nature of 
his sensory affection, show that compression of the olfactory nerve 

' Gruiidziige d. physiolog. Psycliologie, i. , 384; comp von Vintschgau, in 
Hermann's Kandb d. Physiol., HI., ii., p 261 f. 

^See Roaeutiial, iu Archiv f. Auat., Physiol., etc., 1860, pp. 217 ff. 


by tumors, etc., may produce sensations of smell. Disturbances of 
the central organs, such as occur in certain cases of insanity, may 
doubtless have the same result. The powerful effect which some 
odors have upon the brains of certain persons, so that nausea, gid- 
diness, and other disturbances of feeling result, scarcely needs 
mention ; it cannot all be resolved into mental associations con- 
nected with the sense-impressions. 

§ 10. No approach can be made toward a scientific classification 
of the kinds of smells.' This specific sensation must, however, be 
carefully distinguished from the other forms of feeling with which 
it is most closely allied. Many supposed sensations of taste are 
really sensations of smell. Substances like ammonia and acetic 
acid powerfully excite the sensations of touch and common feeling 
through their action on the trigeminus as well as the olfactory 
nerve. Other sensations of touch and of the muscular sense are 
reflesly occasioned in such cases, and blend with the specific sensa- 
tions of smell in the total mental result. But of all the attempts to 
classify the qualitatively jDure sensations of this sense, none can be 
said to have any scientific value. The division into pleasant and 
unpleasant smells depends upon the idiosyncrasies of individuals ; 
to some the smell of burning feathers, of assafoetida, of valerian, or 
of rank cheese, is pleasant. Frohlich's " classification into those 
which excite merely the olfactory nerve, and those which call out 
other sensations reflexfy through their action on the trigeminus, is 
purely physiological and not psycho-physical ; moreover, it does 
not apply to sensations of smell, as such. When we classify the sen- 
sations according to the objects which produce them — as j)ractieally 
we are obliged to do — we are not distinguishing the qualities of 
our feeling ; the smell of a rose does not belong to a class of sen- 
sations as does a sour taste or the color red. No known principle 
will bring order out of the bewildering complexity of this sense. 

Sensations of smell cannot, like those of pressure, hearing, and 
sight, be schematized or represented as standing in any definite 
local or mathematical relations to each other. Smells cannot be con- 
ceived of as having a scale of pitch, or triangle of color-tones. As 
Wundt ' declares, the sensations of smell form " a discrete mani- 
foldness Avhich has an unknown arrangement." 

§ 11. The properties which any substance must j)ossess in order 
to be odorous, and the nature of the action of the odorous particles 

' For the entire subject, see von Vintschgau, in Hermann's Handb. 4 
Physiol., IIL, il, p. 2G6 f. 

•^Sitzgsber. d. Wiener Acad., 1851, VI., p. 322 f. 
^Physiolog. Psychologie, i , p. 386. 


upon the end-organ of smell, are wholly unknown — as much so 
now as when, more than a half-century since, Cloquet confessed the 
complete ignorance of the scientific world on these matters. A 
great variety'' of phenomena appear, but no known laAV has control 
of them. Some plants are odorous by day alone, others by night 
alone ; still others only in the morning. Some plants have a smell 
when dried ; others give off only a weak odor when dry, but a 
stronger one when moistened. Of course, the effect of any odorous 
substance depends upon the ease wdth which it may be vaporized, 
and the speed and extent of its diffusion through the atmosphere. 
Camphor, musk, and other similar substances are distinguished for 
their long-continued and far-reaching effects. 

The discovery of Eomieu, in 1756, that small bits of camphor on 
the surface of water have a rotary motion, has called out various 
investigations in the line suggested by this fact. Provost subse- 
quently (1799) observed that other odorous bodies have a similar 
motion on the surface of water, and that a very thin layer of water 
on a perfectly clear plate or glass withdraws itself as soon as jduI- 
verized camphor is laid upon it. More recently still, Liegeois has 
noticed the same phenomena, wholly or in part, exhibited by some 
two hundred odorous substances of either vegetable or animal struct- 
ure. Minerals, according to this observer, do not behave in the 
same way. Some of these odorous substances seem to inhibit or 
check the rotar}^ motion in others. He concludes that we are jus- 
tified in believing odorous substances to have the power, especially 
when in contact with water, of setting up a motion of these outside 
particles which distributes them through the atmosphere so that 
they reach the mucous membrane of the nasal passages. Just how 
they act upon the end-apparatus there it is impossible to say. The 
researches of Tyndall ' and others as to the influence which odorous 
particles of different substances have upon the capacity of the air 
to absorb heat may possibly be combined with the foregoing re- 
searches in a way to suggest some tenable hj-pothesis touching the 
nature and action of the stimuli of this sense ; but thus far, as has 
been said, we cannot go beyond a confession of ignorance. 

§ 12. The condition of scientific attainment as to sensations of 
taste and their stimuli is only little better than that as to the allied 
sense of smell. The adeqnate specific stimulus for the nerves of 
this sense consists in certain tastable substances ; such substances, 
however, do not excite the end-apparatus unless they act upon it 
under definite conditions. Only fluid bodies, or such as are at 
least to some small degree soluble in a fluid or menstruum, excite 
' Heat as a Mode of Motion, pp. 341 ff. New York, 1868. 


sensations of taste ; absolutely insoluble bodies are, without excep- 
tion, tasteless. This fact may be due to the concealed position of the 
inner cells of the gustatory flasks, which is such that they cannot be 
reached by substances undissolved. By no means all soluble sub- 
stances have a taste. No known law regulates the relation between 
the solubility of bodies and their power to excite sensations of this 
class. It is disputed whether any of the gases are direct excitants 
of the end-organs of taste. The monograph of A. Stick ' maintains 
the tastable character of certain gases, on the ground that a stream 
of them, let fall upon the tongue when dry (so that they cannot well 
be absorbed by the saliva), produces the peculiar sensations of 
taste which these gases are known to possess. A stream of car- 
bonic-acid gas, for example, when acting on the dry edge of the 
tongue, has a taste which is described as sweetish sour. It is diffi- 
cult, however, to secure such a degree of dryness of the tongue as 
will not leave a moist capillary layer ; diflficult, also, to exclude all 
the connected sensations of smell and common feeling.'' 

It is doubtful whether the sensation of taste can be excited by 
mechanical means ; and there is no j)roof that heat can irritate the 
gustatory nerves. Certain authorities of the first rank have indeed 
described specific sensations of taste as mingled with the feelings 
which follow rubbing, pricking, and pressing the tongue. Henle 
observed a saltish taste to be excited by passing a current of air 
over the tongue ; Wagner a bitter taste by pressing down the base 
of the tongue with the dry finger ; Dr. Baly an acid or a saltish 
taste by repeatedly and lightly tapj)ing the end of the tongue. 

The long-debated question as to the electrical stimulation of this 
sense seems now to be decided affiinnatively.^ It was discovered in 
1752 that the application of two different metals to the tongue is 
followed by a peculiar sensation of taste. Volta recognized the 
fact that the effect of the metals is due to the electrical current 
called out between them. If the cathode is laid upon the upper 
surface of the tip of the tongue, a sensation is produced by the cur- 
rent passing out which is variously described as metallic, acid and 
metallic, or bitter and metallic, etc. ; but if the anode is applied to 
the same spot, the sensation produced by the entering current is 
described as acid, or acid and metallic, or bitter and metallic. In 
the former case, not infrequently, a strong cuxTent is needed to pro- 

'Ueber d. Schmeckbarkeit d. Gase, Berlin, 1857; article in Annalen des 

■•* See von Vintschgau, in Hermann's Handb. d. Physiol., III., ii., p. 19G f. 

^ The whole question is discussed by von Vintschgau, ibid., p. 181 f.; and 
Pfluger's Archiv, xx., pp. 81 ff. 


duce any sensation at all. Since the discovery of electrolysis, it 
has been objected that these effects are due to the decomposition 
of the fluids of the mouth and the consequent accumulation of free 
acid at the positive and free alkali at the negative pole ; the}' are 
therefore not to be ascribed to the direct action of the electrical 
cui-rent on the end-apparatus of sense. Experiments by du Bois- 
Eeymond, Rosenthal, ' and others have been directed toward an- 
swering this objection. The former showed that when a chain of 
four persons is arranged in such manner as to send a current of 
electricity through the tongue of one, the eyeball of another, and 
the muscles of a frog-preparation held by two of the four, the same 
current will cause simultaneously an acid taste, a flash of light, and 
a movement of the animal's muscles, Rosenthal discovered that, 
if two persons touch the tips of each other's tongues while one 
holds in a moist hand the positive and the other the negative pole, 
an electric current Avill cause the first person to have an alkaline and 
the second an acid taste. Still other experiments confirm the 
opinion that sensations of this sense may be directlj' due to elec- 
trical stimulation. Attempts have been made to prove the possi- 
bility of exciting subjective sensations of taste by injecting tasta- 
ble substances into the veins of animals ; but the psychology of 
the subject has reaped no results from these attempts. Most of 
the alleged cases of such subjective origin are probably due to 
substances really brought to the tongue in the saliva. It is worth 
remarking here that sensations of taste rarely or never mingle in 
our dreams. 

§ 13. The question whether atastable substance excites precisely 
the same sensation when applied to all portions of the organs of taste 
is a difficult one to answer satisfactorily (see Part I, Chap. V., § 
6). The tabulated results of different experimenters upon this 
question disagree considerably. Such disagreement is suggestive 
of idiosyncrasies of taste, and of doubt whether the different shades 
of the same class of sensations are either nicely discriminated or 
uniformly described by most persons. Descriptions which speak 
of the sensations aspi'ickly, piquant, cooling, etc., show, of course, a 
combination of sensations of common feeling with those of special 
sense. The minor varieties of taste may be occasioned in a manner 
similar to that of the less important shades of color-sensations. It 
seems tolerably well established that sweet and sour are tasted 
chiefly with the tip of the tongue ; bitter and alkaline with its roots. 
The experiments of two of the principal observers, Horn and Picht, 
agree in the conclusion that nearly all substances (even sugar) call 
' Ueber d. elektrischen Geschmack ; Archiv f. Anat., Physiol., 1860, p. 217 f. 

314 SENSATioisrs or smell and taste. 

out a bitterish taste when applied solely to the papilloe circum- 

§ 14. Most of the different kinds of tastes admit of being con- 
sidered as compounds of a few simple sensations of this sense with 
each other and with sensations of smell, touch, common feeling, and 
muscular sense. Many so-called tastes are really chiefly smells. 
Physiologists generally distinguish four principal classes of tastes — 
sweet, bitter, salt, and sour. Wundt ' adds to these four the alka- 
line and the metallic. But possibly the alkahne may be considered as 
a modification of the salt ; and the metallic is probably a compound 
taste, although its analysis is by no means easy. The attempt has 
been made by Valentin and others to reduce this number to two — 
the sweet and the bitter. The sour is thus considered as not a pure 
sensation of taste, but as predominatingly a sensation of touch. 
Acids in concentrated form certainly bring into action the nerves of 
feehng ; but in very dilute form they seem to excite purely the sensa- 
tion of taste. The same thing is true of saltish substances. The 
bitter and the sweet are agreed by all to have the character of pure 
sensations of this sjDecific sense. Powerful reflex sensations of the 
muscular sense are occasioned by strong stimulation of the nerves 
of the tongue, and these sensations blend with the specific sensa- 
tions of taste. There is no satisfactory reason to be given for 
classing the sensation of nausea under the sense of taste. 

The primary forms of taste are combined, in the greatest variety, 
with an indefinite number of shades under each of them. The 
hypothesis of four or more specifically different forms of the end- 
apparatus corresponding to the primary forms of sensation — for 
example, "bitter-tasting" nerve-fibres, "sweet-tasting" nerve- 
fibres, etc. — offers, under the law of the specific energy of the 
nerves, an opportunity for explaining all the phenomena of this 
sense somewhat similar to that embraced by the so-called Young- 
Helmholtz theory of color-sensations. 

§ 15. Concei'ning that in tastable substances which fits them to 
excite the end-apparatus of the gustatory nerves, or concerning the 
molecular action of such substances, we have no information what- 
ever. No scale of stimuli, considered as differing in the rapidity of 
their vibration and corresponding to a scale of resulting sensations 
differing in pitch or tone, can be made out for sensations of taste. 
The great difficulties which accompany experiments uj)on this 
sense, and the fact that the most fundamental questions concerning 
its activities are still unanswered, place it in an unsatisfactory posi- 
tion only less hopeless than that occupied by the kindred sense of 
' Playsiolog. Psycliologie, i., p. 382. 


smell. We have in the case of taste, however, the very great ad- 
vantage of being able, at least loosely, to classify the sensations 
Avhose quality we are considering. 

§ 16. On passing to the consideration of sensatiojis of sound much 
more help is received from the science of physics. But modern in- 
vestigations, in the form in which they concern us, do not go back 
of the great work of Helmholtz,' who made the entire field peculi- 
arly his own. Since the first appearance of this work, the subject 
has also been greatly enriched by the original researches of Oetting- 
en,^ Mach,' Preyer,* Hensen,' Stumpf," and others. In speaking 
of the stimuli of these sensations, we are still comj)elled to refer 
chiefly to the vibrations of air, which are only remote excitants of 
the end-organs of this sense. Neither physics nor physiology has 
yet been able to fix the precise locality in the organism (the ner- 
vous structure of the cochlea) where the immediate stimulation of 
the end-apparatus takes place ; or to tell what is the exact nature 
of its action. We are obliged, then, to confine ourselves in the 
main to considering a relation between the vibratory energy of the 
air and certain states of consciousness, without attempting to ex- 
plain the many intermediate links. 

All sensations which arise in the mind by means of the irritation 
of the auditory nerve are called sensations of sound. The word 
" sound " is thus used by psychology for a wholly subjective affair, 
which has no more resemblance to those vibrations which physics 
designates by the same word than has the taste sweet to the un- 
known physical properties that produce it. The trained mind, or 
"trained ear," as we say, has indeed the power du-ectly to analyze 
a compound musical sound into its constituent elements. But each 
of these elements is purely a sensation, a subjective affair. It car- 
ries in itself no token that it has been produced by vibrations of 
any kind ; or that it sustains any numerical relation whatever to 
the vibrations of which some other sensation of sound is composed. 
We know nothing directly, through sensations, either of the struct- 

1 Die Lelire von d. Tonempfindungen als pliysiolog. Grundlage f. d. Tlieorie 
d. Musik, Braunschweig, 1st edition, 1863; 2d edition, 1865; 3d edition, 
1870 ; 4th edition, 1878. 

'^ Harmoniesystem in dualer Entwicklung, 1866. 

^ Various contributions in the Archiv f. Ohrenheilkunde and elsewhere 
(especially the Sitzgsber. d. Wiener Acad.). 

■* Ueber d. Grenzen d. Tonwahrnehmung, 1876 ; Sitzgsber. d. Jen. Ge- 
sellsch. f. Med., 1878 ; Akustische Untersuchungen, 1879. 

5 In Hermann's Handb. d. Physiol. , III. , il. , pp. 3-142, and works by the 
same autlior there referred to. 

" Toupsychologie, Leipzig, 1883 (Vol I. only). 


ure of the ear or of vibrating strings and particles of air, or of the 
mathematics and physics of music. 

Sounds are of two classes — tones, or musical sounds, and noises. 
The former are due to periodic motions of sonorous bodies ; the 
latter to non-periodic. Noises are those sounds which, objectively 
considered, are wanting in tlie periodic regularity of stimulation 
which characterizes all musical sounds, and, subjectively considered, 
in the peculiar, pleasant modification of consciousness which the 
latter produce. But noises accompany almost all tones ; and, con- 
versely, tones may be detected by the trained ear as mingled with 
the noises of every-day life. No plaj'er of the violin avoids all noise 
of scraping from the bow ; no stroke of a workman's hammer, or 
slamming of a door, that does not start and catch up into itself 
some trace of musical tone. The interest of science has hitherto 
been almost wholly concentrated upon musical sounds, and little 
has been done by either phj'sics or physiology toward tlie analysis 
of noises. It is characteristic of a noise, according to Helmholtz,* 
that there is a quick and irregular alternation of different kinds of 
sensation of sound. This distinctive character can generally be 
detected " by attentive aural observation without artificial assist- 
ance." "We can compound noises out of musical tones ; as, for ex- 
ample, by striking together all the keys of an octave on the piano. 
Hensen'"' distinguishes three " categories of unmixed noises " — the 
" beats " or pulsations which disturb the purity of musical tones ; 
the crackle, crack, or crash ; and hissing sounds. These three shade 
into each other, and, when mixed with different kinds and quantities 
of musical sounds, make up the noises which we hear on every hand. 

§ 17. Musical sounds differ, not only in quality, but also in 
quantity or intensity of sensation as dependent upon the ampli- 
tude of the vibrations which produce them. With respect to their 
qualitij they are distinguished as either simple or complex, accord- 
ing as they result from one set of regularly recurrent (periodic) vi- 
brations of a given number in a given unit of time, or result from 
a combination of two or more sets of such vibrations. The musi- 
cal sounds of ordinary experience are complex. The blending of 
the simple tones into the complex tone is not so complete, however, 
that it cannot be at least partially analyzed directly by a trained 
ear. The complex sound, which results from this compounding 
of the contrasts or coincidences of several simple musical sounds, 
may be called by the term " clang " — in this meaning borrowed 
from the usage of the German. The quality of tones considered as 

1 The Sensations of Tone, etc., p. 11 f. London, 1875. 

2 Hermann's Handb. d. Pliysiol., Ill, ii., p. 17. 


simple sensations is their pitch, which varies according to a scale of 
states of consciousness that are immediately apprehended and com- 
pared with each other, and that are discovered by objective meth- 
ods to correspond to a scale of changes in the number of the vi- 
brations of the waves which occasion them. The pitch of tones is 
therefore spoken of as "high" or "low," according to the place 
which we assign to the resulting sensations in this scale. Such 
place in the scale may be considered either with respect to the re- 
lation of any particular tone to the upper or lower limits of the 
scale, or with respect to the relation of the different tones to one 
another. " Clangs," or complex tones — the musical sounds with 
which we are made acquainted by all ordinary experience — have 
also a variable quality called timbre, or " color-tone ; " the timbre 
of the clang is dependent upon the pitch, number, and relative in- 
tensity of the simple tones which compose it. Thus a note having 
the same place in the musical scale (for example, a of the once- 
marked octave — 440 vibrations) sounds differently, as we say, on 
the piano, violin, cornet, or when sung hy the human voice. The 
pitch of the tone as produced by all these different methods is the 
same ; but its color-tone is determined by the character of the 
over-tones which are blended with the fundamental tone. 

§ 18. Tlh.Q pilch of tones depends npon the rapidity of the peri- 
odic vibrations (the number in a given unit of time — usually one 
second) wliich occasion them, or — what is the same thing — upon 
the length of the sound-waves. This class of sensations, however, 
has both an upper and a lower limit ; that is to say, vibrations ei- 
ther below or above a certain number per second, or — what is the 
same thing — -wave-lengths that are either shorter or longer than a 
given limit, produce no sensations of musical sound. The difficulty 
of determining these limits is great, because the intensity of ex- 
tremely low or high tones has to be enormously increased in order 
that the}^ may be heard at all ; because the perceptions of the 
acoustic sense are so very blunt near the limits that the different 
sensations are almost certain to be confused ; because distracting 
sensations of common feeling mingle in these ranges of tone with 
the sensations of sound, and because near the lower limits the 
over-tones — especially the octave above — become so strong as to be 
mistaken for the fundamental tones. On account of these diffi- 
culties the older investigators made numerous mistakes. Indi- 
vidual j)eculiarities are also very important in determining sensa- 
tions of pitch. Some persons can hear tones below or above those 
audible to most others. Helmholtz ' thought that sensations of 
' The Sensations of Tone, p. 268. London, 1875. 



tone begin to cease when the vibrations fall below 34 per second ; 
some tuning-forks of great size, which vibrated only 28 times per 
second, seemed to him, however, to have a trace of tone in the form 
of a "weak drone." Preyer ' found that while 14 vibrations pro- 
duced no tone that he could hear, at 16 vibrations he was able to 
hear a tone ; others could distinguish a musical sound only at 19 
or 23 vibrations. The same observer experienced as a sensation of 
musical sound more than 40,000 vibrations per second ; Turnbull 
found that the majority of those with whom he experimented 
could not hear more than about 20,000 to 22,500 vibrations per sec- 
ond, and only one — a musician — heai'd 30,000 ; Despretz succeeded 
in producing with tuning-forks audible tones that had 32,000 vi- 
brations. Blake thinks that persons with defective ear-drums are 
able to hear tones of higher pitch, reaching even 50,000 vibrations. 
Vibrations slower than 28 to 30 per second produce in most ears 
only a buzzing or groaning sound ; the more acute tones are unpleas- 
ant, or even painful, and finally inaudible to all ears. These results 
cannot be considered as very concordant or precise. They show, 
however, that the range of the average human ear is rather more 
than nine octaves, reaching from about A„_ of the sub'contra octave 
(27-j vibrations per second) to above c' of the seven-times-marked 
octave (16,896 vibrations per second). 

The following table " gives the pitch of all the musical tones audi- 
ble to the human ear, in the key of C major, on a scale in which 
a' is fixed at 440 vibrations. Only about seven of the rather more 
than eleven octaves of the table are, however, usable in music ; 
these seven reach upward from C ^ of the contra, or fz-om A„ of the 
subcontra octave, to 6^ — namely, the seven or seven and a half 
octaves of the modern piano. 





Subcontra octave 

Contra octave 

Great octave 

Small octave 

Once-marked octave 

Twice-marked octave 

Thrice-marked octave 

Four-times marked octave 
Five-times-marked octave . 
Six-timt-s-marked octave .. 
Sevcii-timcs-marked octave 
Eight-times-iiiarked octave 












18% 6 


74 J:^ 











41 H 



















110 , 






440 ! 



880 i 



1,760 ' 



3,520 ; 










.3015/,, Cj 

fil^ C, 

123?^ ; c. 

2473<? : c, 

495 c', 

990 c2 

1,980 cs 

3,960 o^. 

7,920 c6, 

15,840 c6, 

31.680 c'i 

, T>2, etc. 
, Dj, etc. 

D, etc. 
(i, etc. 
, d', etc. 
, d2, etc. 
, d'i, etc. 

d", etc. 
, d^, etc. 

d^, etc. 
. d^, etc. 

' Grenzen d Tonwahrnehmiiiig, p. 23 f. 

- Taken from Stumpf, Tonpsychologie, I., p. xiv. , and giving the German 
scale ; tlie Frencli fixes a' at 435 vibrations ; the theoretical pitch in England 
gives 512 for c'K 


§ 19. The sensitiveness of the ear to differences of pitch varies 
greatly with different individuals, and for the different octaves of 
the musical scale. Preyer found that unpractised persons, within 
the octaves from c to c^ (132-1,056 ^dbrations by the table, but 
128-1,024 by the scale adopted for his experiments), distinguish 
a difference of from 8 to 16 vibrations as producing a distinct dif- 
ference in the sensation of pitch. Extreme cases of deafness to 
differences in pitch are recorded ; as, for example, that of the man' 
who, in the middle part of the scale, could not distinguish an in- 
terval of less than a third, and, in the higher and lower parts, of 
less than a seventh.^ Persons insensitive to differences of a tone or 
half-tone, who are sometimes said "not to know one note from 
another," are by no means infrequently met with. Differences of 
the two ears of the same person, in the fineness of this kind of per- 
ception, are common enough ; in certain cases the difference may 
amount to a half-tone or more. Sensitiveness to pitch is generally 
capable of rapid cultivation, and may reach a high degree of per- 
fection in persons Avho have what is called " a good natural ear" for 
musical tones, if the ear be also highly trained. Such persons may 
become able to discriminate differences in the sensations caused by 
changing the number of vibrations not more than a third of a single 
vibration per second, in the region of the scale between a' and c'. 
In the octave from 6' to h' more than 200 tones are distinguish- 
able. But above and below this region the distinctions jDos-sible 
are less fine ; above c^ even well-trained ears commit errors in iden- 
tifying two notes that differ by 100 or even by 1,000 vibrations. It 
appears, then, that not only the musical quality of tones, but also 
the power of distinguishing differences in them, diminishes rapidly 
as we approach the upper and lower limits of the scale. 

The fineness of the possible distinctions of purity of interval also 
differs for different individuals and for different intervals. The 
following table is compiled by Hensen ^ from data drawn from 
Preyer 's investigations. The bracketed numbers of the first column 
indicate the proportion in which the vibrations of the different 
intervals stand to those of the fundamental tone ; the quotient 
n : n = i, the variation from the pure interval which was fovind 
detectable in each case ; V = the number of vibrations off from the 
pure interval which is the least distinguishable ; and aS" is the de- 

1 Reported by Grant Allen, in Mind, 1878, p. 157 f. 

^ Comp the lengthy and interesting discussion on "Individualitat des Sin- 
nes und Gedachtnisses fiir Tonqualitaten," in Stumpf, Tonpsychologie, I., pp. 
263 ff. 

''See Hermann's Handb. d. Physiol., III., ii., p. 114. 



nominator of the fraction which indicates the sensitiveness of the 
ear to the purity of each interval. 


Fourth (1.333) 

Fifth (1.5) 

Minor Sixth (1.6) 

Major Third (1.25) j 

Minor Third (1.20) , 

Octave (2.0) 

V/holeTone (1.125) 


n' . 




































Immediate judgment of abf;ohUe tone (as the «' carried in mind 
by musicians) is possible ; judgment between two tones as to 
which is higher or lower in pitch is also immediate, and may be 
exercised independently of everything except the two sensations 
themselves. The latter judgment is the common power of mind 
belonging to this sense ; the former is, as a rule, exercised only by 
skilled persons, and by them only very imperfectly. Experiments 
of Stumpf,' upon himself and three other musicians, showed that 
the mistakes in judgment of absolute tone amounted, in the lower 
region of the scale (from G^ to B^), to 15^-100^ of the trials ; in 
the middle region (from a-cj\ or from g-e'), to Qfo-lQ'fo ; in the 
ujjper region (from g'^-f'^, or from/^-a^), to 7^-80^^. Only-one of 
the four persons experimented upon seemed to approach the point 
of infallibility. Judgment of absolute tone is, therefore, a different 
matter from that which makes distinctions in intervals or in the 
least observable differences of pitch, and is much more precarious. 

§ 20. Those psychologists appear to be in the right who claim 
that some power of the mind immediately to judge differences of 
quality in pitch, purely as such, must be assumed in order to ac- 
count for the foregoing phenomena." Such judgment, however, 
may be, and ordinarily is, much assisted by auxiliary discrimina- 
tions of other sensations which blend with those of musical tone. 
Among such secondary helps the most important are the muscular 
sensations which accompany the innervation of the larynx and other 
organs used in producing musical tones. For we ordinarily inner- 

' Ton p.sychol ogle, I., pp. 305 ff. 

s On tin's Rubiect. comp. Lotze, Medicin. Psychologie, pp. 265 ff., 480 f. ; 
Strieker, Studien iiber d, Association d. Vorstellungen, 1883. p. 2 1; G. E. 
Miiller, Zur Grundlftcung d. Psycho-physik, Berlin, 1878, pp. 276 ff.; and 
Stumpf, Toupsychologie, I., pp. 134 ff. 


vate these organs (at least in an inchoate and partial way) — that is, 
"we sound the note to ourselves — when trying carefully to judge of 
its pitch. But the niceness of these muscular sensations is not 
great enough, even when most highly trained, to account for the 
discriminations of the " good ear." The trained musician can de- 
tect by ear a difference in quality between two tones of 400 and 
400^ vibrations per second ; but the most skilful singer — Jenny 
Lind, for example — scarcely succeeds in singing in quarter-tones. 
Moreover, the relative powers of larynx and ear by no means keep 
pace with each other in the same person. It should also be re- 
membered that all our ordinary discriminations of musical sound 
apply to composite tones, or " clangs ; " in discriminating these we 
are aided by the color-tone, or tone-feeling, which belongs to each 
note as sounded by some sonorous body with whose peculiarities 
we are previously more or less acquainted. 

It follows, then, that the judgment is supplied, by the. varying 
qualities of musical tones, with the means for arranging them in a 
continuous series which may be symbolized by different positions 
assigned along an uninterrupted straight line. Of any three un- 
like tones, one must be, and only one can be, arranged as respects 
pitch between the other two. And whenever any two tones, as m 
and n, are given, another sliding tone, which begins with m and 
ends with «, is possible. Moreover, within the bounds of our ex- 
perience of tones, as we advance along the scale toward either the 
upper or the lower limit, we see no tendency in the qualities of 
the sensations to approach each other. In this respect the scale 
of sound-tones is wholly different from that of color-tones. There 
are not two ways, for example, of getting from a' to c^ (one 
through b\ c^ etc., and the other through g\ f, etc., around to 
e\ d,^ and then c"), as there are two ways of going from yellow to 
blue (i.e., through green and blue-green, or through violet, red, 
and orange). We speak, then, of the series of tones as a constant 
and infinite series ; although, of course, no series of states of con- 
sciousness is really infinite, and although the upper and lower 
limits of the musical scale, as well as the limits of the least ob- 
servable differences between two tones, are not constant but vari- 
able for different individuals. 

The symbolism taken from relations of space, which we employ 
when we speak of certain acoustic sensations as " high " and of 
others as "low" in pitch, or when we distinguish so-called "in- 
tervals " between the tones as large and small, is strictly applicable 
only to the complex tactual, visual, and muscular sensations that 

accompany the acoustic. In sounding the lower tones with the 


voice the organs are depressed ; in sounding the higher, they are 
elevated. Low notes have a certain breadth and gravity which 
corresponds to the foundations of a spatial structure ; as sensations 
they require more time to come into and depart from conscious- 
ness, as it were. A great intensity and slower tempo belong to the 
bass-viol than to the violin. We read iqj for the notes of highest 
pitch, and doivn for those of lowest pitch, in the written musical scale. 
§ 21. We have seen that tones, like rays of light, come to us as 
compounded into "clangs;" these really composite tones being 
esteemed as single notes in ordinary experience. The nature of 
such composition determines the so-called "timbre," or "color- 
tone," of the notes. Each sensation of a clang is a summing-up in 
consciousness of several absolute qualities of musical sound ; the 
stimulus which occasions this complex subjective state is a complex 
sound-wave made up of the contrasts and coincidences of several 
single waves that have the character of simple pendulum vibrations. 
The quality of each clang depends upon the form of this complex 
sound-wave. We need not consider in detail the physics and 
mathematics of such complex waves. It is enough to observe that 
those single tones whose vibrations stand in simple mathematical 
relations to each other, when combined into a clang, cause a pe- 
culiarly pleasant sensation ; those whose vibrations stand in com- 
plex mathematical relations make, when combined, an unpleasant 
sensation. In an octave of the musical scale the eight different 
notes stand in the following ratios to each other.' 

D : E 

9 . 5. 

¥ • 4 

9 : 10 




: A 

: B 

: C 



. .5 
• 3 







That is to say, while the tone G makes one vibration, D makes 
nine-eighths, and E makes five-fourths, etc. ; or while G makes 8 
vibrations D makes 9, E makes 10, etc. Of these relations in the 
number of vibrations the simplest is, of course, that of the octave, 
1 : 2. The acoustic waves which constitute the stimuli of each 
complex sensation called a " clang," accordingly, also permit of 
being regarded as the summing-up of the waves of a fundamental 
tone and of certain partial tones belonging to the fundamental 
tone. These partial tones, or "over-tones," are called "the har- 
monics" of the " clang," or single compound tone. 

§ 22. When two or more "clangs "are sounded together, the re- 
sult is what is called either a "chord " or a " discord." The former 

1 For the mathematics and physics of tones, see Hensen, in Hermann's 
Handb. d. Physiol., III., ii , pp. 4fE. 


is a pleasant, tlie latter an unpleasant, complex of sensations ; con- 
sonance and dissonance are thus spoken of as qualities of sensations 
of musical sound. Thus, if c and c' are struck together upon a 
weil-tuued piano, the combination of clangs is a chord, or harmo- 
nious musical sound ; but if c and d, or c and c sharp, or c and 
its seventh, h above, are simultaneously sounded, then the com- 
bination of tones is unpleasant. Cases of consonance and disso- 
nance differ from those just considered under the term "clang" 
only with respect to the relative strength of the partial tones as 
compared with the fundamental tones : in the clang the over-tones 
are weak as compared with the one fundamental tone ; but in the 
chord or discord the fundamental tones of the other clangs are, of 
course, strong, and stand in powerful relations of consonance or 
dissonance both toward the fundamental tone of the lowest clang 
and toward its partial tones. All the partial tones of the different 
combined clangs enter into the formation of the total result pro- 
duced. According to the table already given (p. 322), the Octave 
is the most perfect possible consonance (1 : 2); then the Twelfth 
(1 : 3), the Fifth (2 : 3), the Fourth (3 : 4), the Sixth (3 : 5), the ma- 
jor Third {4 : 5), the minor Third (5 : G). With the relation of the 
Third we come upon the borders of dissonance ; indeed, the ancient 
Greeks and Komans considered the Third a dissonance, and avoided 
it in singing, because, as Helmholtz supposes, their ears were more 
sensitive to "beats " than ours. The consonance of the Sixth and that 
of the Fourth have also been much disputed. The major Sixth 
and major Third are called by Helmholtz " medial consonances ; " 
the minor Third and minor Sixth, "imperfect consonances." 

An analysis of the harmonics of these consonances yields the fol- 
lowing results,' which show the amount of coincidence belonging 
to the acoustic waves of the different tones when combined in a 
chord with a fundamental tone. 

Octave j"'g'"'"'g'^^°"' 
( c' I c'^ g' c'^ 

Twelfth ]«l-^:il^l^V^^!^li!. 

Fifth j 


Major Third | ° 
( e 

c' g' c- e- 

gi d'^ 
ci g' c' e^ 

c^ g' c'^ 

b' e'^ 

The major Sixth is similar in the form of its harmonics to- the 
major Third. 

' Conap. Helmholtz, The Sensations of Tone, p. 281 f. 


Two psycho-physical causes for the characteristic feelings which 
belong to sensations of consonance and dissonance, respectively, 
may be assigned with more or less of probability. The first is that 
proposed by Helmholtz.' The feeling of dissonance which is pro- 
duced by sounding together two notes that differ only by a semi- 
tone is found to be increased when the difference in the pitch of 
the notes is still further diminished. Successive shocks called 
"beats " occur, less frequently but more decidedly and unpleasantly, 
as the pitch of the notes becomes more nearly the same. The feel- 
ing of dissonance is found to reach its height when the number of 
beats is about 30 per second. For example, if 6' (495 vibrations) 
and c^ (528 vibrations) are struck together, the number of beats 
is 33 (528 — 495=33), and the dissonance is very strongly marked. 
In all marked dissonances such beats occur at the rate of from 20 
to 40 in a second. The unpleasant effect in consciousness is an- 
alogous to that produced by all sudden and rapid intermission of 
stimulation ; as, for example, the flickering of light or the scraping 
of uneven surfaces over the skin. The feeling of consonance is due 
to the absence of beats. In addition to Helmholtz's negative reason, 
Oettingen has jDroposed the positive one, that the pleasantness of 
harmony is due to what he calls the " tonicity " and " phonicity " 
of certain intervals and combined notes. " Tonicity " is the prop- 
erty of being recognized as a constituent of a single fundamental 
tone which is designated by the name "tonic." "Phonicity" is 
that property of a chord or interval which consists in the possession 
of certain partial tones that are common to all tones. The first of 
these qualities of harmony seems to ally the pleasure it yields to 
that which follows even the obscure and only half-conscious per- 
ception, as it were, of all relations, as such, between our sensa- 

§ 23. In order that the physical apparatus of hearing may act 
as the organ of those wonderfully fine discriminations which belong 
to the most analytic of all the senses, it would seem that it must 
possess an outfit of end-organs with structure sufficiently minute 
to serve as a basis for a satisfactory development of "local signs." 
The number of the cells of Corti, and of their separate terminal 
auditory nerves, has been calculated by Hensen '^ at about 16,400 ; 
by Waldeyer ' at 20,000. It is doubtful, howevei^ whether even 
this large number will suffice to account for that niceness of audi- 
tory discriminatioDS which we have seen to be possible. 

' The Sensations of Tone, p. 255 f. 

■^ In Hermann's Handb d. Physiol, III., ii., p. 115. 

^ Strieker s Gev,ebelehre, II., p. 954. 


§ 1. The analysis of the qualities of different Sensations of Sight 
is much more intricate than that of any of the other senses. They 
may all be described as sensations of color and light ; but an in- 
definite number of colors is known to experience, and as many 
grades of the sensation of light. Moreover, the quantity of the 
white light which acts as stimulus upon the eye has an important 
effect upon the quality of the resulting color-sensation ; in other 
words, the tone of the color is dependent upon the amount of white 
light which is mixed with the " saturated " spectral color. The size of 
the colored object and the resulting breadth of the sensation, as 
well as the intensity of the stimulus and the time during which it 
acts, also affect the quality of the sensation. Still further, the same 
stimulus produces different sensations as it falls upon different por- 
tions of a normal retina ; while a considerable class of persons are 
color-blind, or incapable of certain kinds of color-sensations. The 
previous condition of the retina, and the relations between the con- 
tiguous portions when any considerable area of it is under stimu- 
lation, must also be taken into account. The fundamental laws 
governing sensations of sight can, therefore, be discovei'ed only by 
excluding for the time many of those variable elements which, in 
fact, always enter into the determination of the exact quality of 
such sensations. Thus defining the first problem before us, we 
find that it may be stated in the following terms. What sensations 
result from the stimulation of a sufficiently small, but not too small, 
area of the most central part of a normal retina, for a given time, 
when it is not fatigued and the eye is at rest, and with neither too 
great nor too small intensity of a given kind of light? Such sen- 
sations may be called (though somewhat ineptly) normal sensations 
of color. When the foregoing question is answered we may go 
on to consider the most important variations possible on account 
of various forms of departure from the so-called normal conditions 
of sensation. 

§ 2. The ordinary stimulus, the application of which to the eye 
gives rise to the sensations of sight, is light — or certain exceedingly 


rapid oscillations of luminiferous ether. Some forms of mechani- 
cal and electrical stimuli also produce the same sensations. Any 
violent shock to the eye, such as a blow upon the back of the head, 
maj^ fill the whole field of vision with an intense light. The action 
of mechanical pi*essure of moderate intensity upon a limited j^art 
of the retinal elements may be studied by rolling the eyeball in- 
ward and using the fingernail, or a small, blunted stick, upon the 
outer surface of the closed lids. By such stimulation disks of 
light (called 2^hosphenes), with darkly colored edges, are produced 
in the field of vision of the closed eye. Some observers have 
claimed that very strenuous exertion of the apparatus for accom- 
modation occasioned in their eyes similar phenomena ("phos- 
phenes of accommodation "). On making or breaking a weak elec- 
trical current sent through the eye, the entire field of vision is 
lighted up ; the constant current also seems to excite the optic 
nerve. The quality of the sensations thus excited is found to de- 
pend upon the direction of the current through the nerve. When 
the current is ascending, the place where the nerve enters the ret- 
ina appears as a dark disk upon a field of vision that is bright- 
er than it, and of pale violet-color ; when it is descending, as a 
bright bluish disk on a field of dark or reddish-yellow color. The 
retina has also a " light of its own " {Eigenlicht) ; for its nervous 
elements are rarely or never inactive, but have a continuous tonic 
excitation. Hence the most gorgeous and varied coloring is often 
seen when the eyes are closed in a darkened room. This normal 
light of the retina is not constant either in degree or in quality ; 
both the form and the color of the different minute parts of the 
field of vision, as lighted by it, are very changeable. It may be 
said to have the rhythmic movement of all tonic excitation. Such 
excitation is supposed to be due to chemical effects, wrought by 
the changing supply of blood, upon the nervous elements of the 
retina and (perhaps, also) of the central organs of the brain. The 
peculiar action of the ascending and descending electrical current 
has been thought by some ' to be due to its catelectrotonic or 
anelectrotonic effect upon the central organs by way of the optic 
nerve. Aubert has estimated the retina's own light to be about 
equal (in his case) to half the brightness of a sheet of white pajier 
when seen in the full light of the planet Venus. 

§ 3. The place where the light acts (and here, as is supposed, only 
indirectly through photo-chemical— and perhaps electro-motive — 
changes in the pigments of the eye) upon the end-organs of vision 

' See Fick, Physiolog. Optik, iu Hermann's Handb. d. Physiol., III., i. , p 



must be located at the back of the retina in the rods and cones 
(see Part I., Chap. V., §§ 18-22). The argument b}' which we have 
connected the analytic power of vision with the structure of this 
nervous layer ma}' be carried yet further into details. It appears 
likely that each element of the structure — at least in some parts of 
the retina — should be regarded as an isolated sensitive spot, which 
corresponds on the one side to definite excitations from the appro- 
priate stimuli, and on the other side to the smallest localized sen- 
sations of color and light. In order that two visual sensations 
may be seen as separate, yet side by side, in an object, two neigh- 
boring retinal elements must be excited by the stimulus. This 
implies that the breadth of retinal surface stimulated must be, at 
least, about that of the distance between two such elements. With 
this hypothesis the facts of histology and exi^erimental physiology 
agree fairly well. 

The degree of accuracy which sight can attain is dependent 
upon the size of the retinal elements directly affected by the 
light. ' Hooke observed that no one can distinguish two stars as 
two, unless they are apart at least 30" ; few, indeed, can distin- 
guish them when less distant from each other than 60'. E. H. 
Weber could not perceive as separate two lines whose distance 
did not cover at least 73" of the angle of vision ; Helmholtz puts 
the limit of his sharpness of vision at 64". The numbers 60", 
64", and 73", in the angle of vision, correspond to a size of the 
retinal elements varying from 0.00438 mm. to 0.00526 mm.; and 
this agrees very closely with the 
calculated breadth (by Kolliker) 
of the thickness of the cones in 
the yellow-spot — namely, 0.0045 
mm. to 0.0055 mm. (0.000177 in. 
to 0.0002165 in.}. If white lines 
be drawn on a dark ground so 
closely together as to approximate 
this limit of vision, they will ap- 
pear, not straight, but knotted and 
nicked. This fact is due to the 
action of the stimulus on the mo- 
saic of rods and cones, as seen by the accompanying figure (No. 90). 
The diminishing sharpness of vision as we move away on the sur- 
face of the retina from its most central area corresponds to the 

' See Helmholtz, Handb. d. Physiolog. Optik, Leipzig. 1867, p. 215 ft.; 
Fick, in Hermann's Handb. d. PhjsioL, HI., i., p. 152 f.; von Kries, ArcMv 
f. Anat. u. Physiol., Physiolog. Abth., 1883 (Appendix), p. 24 f. 

Fig. 90. — A shows the appearance of lines 
drawn veiy clopely together, which is sup- 
posed to be due to their falling upon the 
nervous elements of the retina in the man- 
ner shown by B. 


comparative paucity of the nervous elements which enter into the 
structure of the peripheral parts. 

§ 4. Excluding consideration of those changes in the quantity, 
as such, of visual sensations which are produced by changes in in- 
tensity of the light, and confining our attention to what has already 
been defined as the normal action of the eye (comp. p. 325), we treat 
scientifically all the different sensations of sight when we describe 
(1) the wave-lengths of the different kinds of colored light, or pure 
color-tones, and (2) the relations in which the different colors 
stand with respect to the amounts of white (or colorless light) and 
saturated light (or light of pure color-tone) which enter into them. 
The foregoing distinctions in the quality of our color-sensations 
may be confirmed by an appeal to experience. Eed is unlike yellow 
in "color-tone," and both are unlike blue ; but orange is more like 
either red or yellow than it is like blue, while violet is more like 
blue than it is like either yellow or red. Yet we distinguish colors 
of the same class (red, green, or violet) as being like or unlike 
with respect to their " brightness ; " and in resj)ect of brightness, a 
certain shade of red may differ more from another shade of red than 
it differs from some shade of yellow, green, or blue. The bright- 
ness of a color is, scientifically speaking, dependent both upon 
the degree of saturation which the color possesses and upon the 
total intensity of the light. 

§ 5. A color-tone is said to be " pure " or " saturated " when it is 
free from all admixture of other color-tones. Pure or saturated 
color-tones can be obtained only by use of the spectrum, which, 
on account of the different refrangibility of the different colored 
rays that compose it, analyzes the compound ray of white light into 
its constituent color-tones. By stimulating with different simple 
rays those nervous elements which have the same local situation 
at, or very near, the pole of the eye, we test the question whether 
each special color-sensation corresj)onds to a special physical con- 
struction of the stimulus. It is thus discovered that the compound 
ray of sunlight, so far as it stimulates the human eye, is made up 
of components formed by oscillations varying all the way between 
about three hundred and seventy billions and about nine hundred 
billions per second ; and that the color-tone of the sensation changes 
as the number of these oscillations changes. The following table ' 
exhibits these facts on the scale of Fraunhofer's lines, which mark 
those portions of the spectrum where its principal colors appear 
most obvious to the normal eye. 

' Taken from Fick, Physiolog. Optik, iu Hermann's Handb. d. Physiolog., 
Ill, i., p. 173. 



Name of the line. 

Number of vibra- 
tions per second. 

Wave-length in the air. 

















Rays of light which have a number of oscillations less than four 
hundred and seventy billions |Der second, so far as they affect the 
retina at all, occasion the sensation of Red ; and this sensation does 
not vary essentially in quality when the oscillations are four hundred 
and forty to four hundred and sixty billions. But when their number 
increases beyond four hundred and seventy billions (C) the quahty 
of the sensation changes rapidly, takes on a yellow tone (Orange- 
yellow), and finally, at about five hundred and twenty-six billions 
(D), corresponds to what we definitely call Yellow. This yellow 
becomes greenish as the oscillations increase in number, until they 
reach about five hundred and eighty -nine billions (E), when Green 
appears. (Changes from yellow to green occupy only a small zone 
in the spectrum.) The green in turn becomes bluish ; at six hun- 
dred and forty billions (F) Blue begins to appear. From this point 
to seven hundi-ed and twenty-two billions (F-G) the color-tones 
that lie between blue and violet are run through ; beyond the latter 
number Violet comes to view. 

The color-tones of the spectrum are, therefore, not sharply sepa- 
rated, but pass gTadually into each other. The nearer together two 
colors are situated in the spectrum, the more nearly do they corre- 
spond in the quality of their sensations. Nor has the spectrum any 
sharply defined limit at either end, but passes gradually into black 
— more gradually at the violet than at the red end. The energy 
of the ultra-red rays, as measured by their physical and chemical 
action, is greater than that of the more highly refrangible rays. 
The fact that these rays do not excite visual sensations must, then, 
be due to the structure of the retina. The ul(ra-\iolei end of the 
spectrum has been made visible for a certain extent by experiment ; ' 
it produces the sensation of a glimmer of lavender-gray color. Our 
inability to perceive these ultra-red and wZ^ra-violet rays is not to 
be considered an imperfection of the eye, as Tyndall thought. It 
is rather purposeful, and of the greatest importance for vision ; 
since, if these ultra rays were visible, the clearness of objects would 
'- See Helmholtz, Physiolog. Optik, p. 232 f. 


be much disturbed by tbe chromatic aberration of the refracting 
appai'atus of the eye.' 

§ 6. Besides the foregoing distinctions of color-tones, the im- 
pression made by the green-yellow of the spectrum (D-E, and im- 
mediately about D) is by far the strongest ; or, as we should say, 
this color is naturally the "brightest " of the spectral colors. From 
the region immediately around D, the brightness of the color-tones 
diminishes toward both the red and the violet ends of the spectrum 
— at first quickly, then more slowlj', and then more quickly again. 
Such a relation cannot be due to the spectrum as an objective 
affair ; for if we measure by other physical means the amount of 
energy belonging to its different regions, we find that of the red 
rays (which are by no means brightest) to be strongest. "We must. 

Fig. 91. — (From Fick.) The letters on the horizontal line stand for Fraunhofer's lines. The 
ordinates of the interrupted curved line show the brightness of rays as seen ; the ordinates of 
the dark curved line, the intensity of the rays as measured by calorific effect. 

then, seek an explanation in the structure of the retina, and conclude 
that it is peculiarly sensitive to stimulations by oscillations of about 
five hundred and fifty billions per second. The sensitiveness of the 
retina to slight variations in color-tone, as dependent upon differ- 
ences in the wavelengths of the stimulus, is also different at different 
portions of the spectrum. It is greatest in the green and blue- 
green regions (D and F). 

The following table represents both the foregoing laws. The 
numbers of the second and third columns show the relative bright- 
ness with which the different colors of the spectrum appear to the 
eye, as calculated by different methods and by two observers. It 
will be seen that the results agree substantially, though by no 
means perfectly. In the last two columns the letter sstand for 
Fraunhofer's lines, and the figures give the fractional variation in 
the wave-lengths which produces an observable variation in the 
color-tone for different regions of the spectrum.'' 

' See Fick, Compendium d. Physiologie, 2d edition, p. 181 f • ; and Her- 
mann's Handb. d. Physiol., III., i., p. 181 f. 

■■* See Helmholtz's Physiolog. Optik, p. 317 f.; von Kries, in Archiv f. Anat. 
u. Physiol., Physiolog. Abth., 1882 (Appendix), pp. 56-76 ; Fick, in Hermann's 
Handb. d. Physiol, III., i., p. 174 f. ; MandelstammandDobrowolsky, in Aichiv 
f. Ophthalmologie, XIII., ii., p. 899, and XVIII., i , p. 66. 





















Mandelstamm and Dobrowolsky. 

Red, B 

Orange, C 

Reddish -yellow, D 

Yellow, D-E 

Green. E 

Blue-green, F 

Blue. G 

Violet, H 

B .. 

E .. 

1 1 5 

id I 


— L-^ 

- § 7. The colors of every-day experience, like its musical tones, are 
not simple and pure color-tones, such as are obtained by spectral 
analysis ; they are composite. Inquiry must therefore be raised as 
to the effect produced in sensation from the co-working of two 
homogeneous ra^'s of light upon the same elements of the retina 
under all the normal conditions to which reference was previously 
made. In pursuing this inquiry no direct assistance can be ob- 
tained from the discriminations of consciousness ; for sensations of 
color, unlike those of musical clang, cannot be mentally analyzed 
into their constituent elements. The science of optics makes us 
acquainted, however, with the following facts : When the wave- 
lengths of the two colors mixed vary but slightly (a few billions of 
oscillations in a second) from each other, the color resulting from 
the mixture lies between, and may be recognized as a " shade " of, 
the colors mixed. By selecting for mixture color-tones that lie 
apart at all possible distances along the spectrum, an indefinite 
number of impressions of color may be obtained, which all differ 
from those obtained by the homogeneous colors. These mixed 
color-impreHHions, however, are not all different from each other ; 
so that the number of the qualities of resulting sensations is far less 
than that of the compound physical processes which stimulate the 
retina. Their character depends both upon the place of the spec- 
trum from which the simple color-tones are selected for mixture, 
and also upon the relative intensity of the ones selected. For ex- 
ample, if a ray of four hundred and fifty billions of oscillations per 
second (red) be mixed with one of seven hundred and ninety billions 
(violet), anew series of impressions of color (the purples) is attained 
by varying the intensities of the two. These impressions are more 
or less like red or like violet, according to the relative amounts of 
the rays of four hundred and fifty billions and of seven hundi-ed and 
ninety billions which enter into the mixture. Moreover, there are 
found to be two ways of advancing by this process of mixing color* 



tones toward any one of the composite colors. Thus, we may pass 
from yellow to blue either through green-yellow, green, and blue- 
green, or through orange, red, purple, and violet. The following 
table ' is of interest in this connection. Where two colors are given 
as resulting from the mixture, the variation is to be understood as 
dependent upon the prevailing intensity of one of the two compo- 


Red and Yellow 

Orange and Yellow-green 

Yellow and Green 

Yellow-green and Blue-green . . 

Green and Cyanic Blue 

Blue-green and Indigo 

Cyanic Blue and Violet 

Red and Yellow-green 

Red and Green 

Violet and Blue-green 

Violet and Green 

Violet and Orange 

Red and Cyanic Blue 

Red and Indigo 

Tone of the color obtained by mixture. 






Cyanic Blue 


Orange or Yellow 

Orange or Yellow or Yellow-green. . . . 

Indigo or Cyanic Bine 

Indigo or Cyanic Blue or Blue-green. 


Indigo or Violet 


Degree of 




Very whitish. 










Slightly whitish. 

§ 8. The number of colors distinguishable by the human eye is 
not easily stated with accuracy ; like the number of musical tones, 
it varies with different individuals. The usual number of seven 
fundamental colors, as fixed by Newton, with the intent of forming 
an octave in the scale of color-tones, has no sufficient claim to 
acceptance. Six of the seven — namely, red, orange, yellow, green, 
blue, violet — are indeed names in common use. But indigo, as an 
intermediate tone, or kind of semitone, between blue and violet, 
has perhaps no more real right to recognition than various other 
intermediate color-tones. Bonders " puts the number of color-tones 
distinguishable in oil-colors at one hundred ; von Kries ' the rec- 
oo-nizable number of spectral tints at about two hundred and 
thirt}'. But each of these yields different sensations of color ac- 
cording to the degree of its saturation or purity, due to freedom 
from admixture of white light. Another series of variations of sen- 
sation must be allowed for, which are due to differences in " bright- 
ness " or intensity. Introducing these two variable elements, von 
Kries calculates the number of distinctions of color-sensations, 
possible for all degrees of purity of tone and intensity of light, at 

' Made according to investigations by J. J. Miiller, and taken from Tick, in 
Hermann's Handb. d. Physiol., III., i., p. 190. 
'^ Archiv f. Ophtbalmologie, XXVII. 
2 Arcbiv f. Auat. u. Physiol., Physiolog. Abth., 1882 (Appendix), p. 58 f. 



about five hundred thousand to six hundred thousand. This num- 
ber stands midway between the " many milHons " of which Au- 
bert speaks and the five thousand allowed by Douders. Herschel 
thought that the workers on the mosaics of the Vatican must have 
distinguished at least thirty thousand different colors. 

§ 9. Experiment also shows that if certain color-tones with a 
given intensity are united on the retina, the result is a sensation 
unlike that of any other of the colors, whether pure or mixed. 
This sensation we call "white," and the two colors which by their 
admixture produce it are called " complementary." Complementary 
colors may be mixed upon the retina in various ways ; either by al- 
lowing two spectral rays properly selected to be superimposed at 
the same spot, or by blending the reflected images of two colored 
wafers, or by blending the du'ect visual impressions of colored 
surfaces on a swiftly revolving top or wheel, etc. But however 
mixed, the resultant sensation is that of a so-called " white " color 
in which all trace of the constituent elements is lost. Following is 
a table of complementary colors : ' 





Relation of 



Green-blue . . . 

1,600 {^i 




Gold-yellow .. . 
Gold-yellow. . . 



Green-yellow. . 




Indigo blue. . . 
Indigo-blue. . . 


§ 10. If the foregoing facts and laws are held to be true of the 
" normal " connection between light and visual sensations, then 
various classes of circumstances must be taken account of as "ab- 
normal," which, nevertheless, enter into all our daily experience 
with this sense. Indeed, the connection between stimulus and 
sensation is not the same for different individuals who possess sub- 
stantially the same color-sensations ; frequently the complementary 
colors for two different indi%T.duals are not precisely the same. 
Even the two eyes of the same individual often differ percejDtibly 
in this regard. Important changes in the quality of the sensations, 
other than those directly ascribable to changes in the wave-lengths 
of Hght, take place when the intensity of the light approaches ei- 

' Taken from Helmlioltz, Pliysiolog. Optik, p. 277. The numbers are given 
in hundred milliontlis of a Parisian inch, and may be reduced to millimetres 
by multiplying by 27.07. 


tliei- a masimum or a minimum. At the maximum intensities of 
the stimulus all sensations of color-tone cease, and even homoge- 
neous rays appear white. Previous to reaching this maximum, 
red and green pass over into yellow. At the mininium intensities 
of light every color-tone except the pure red of spectral saturation 
appears colorless when seen alone on a perfectly black ground. 
The different colors appear and disajjpear, as such, at different 
degrees of intensity of the stimulus — green, among them all, re- 
maining visible in the weakest light. They all also change their 
tone as the light which falls on them diminishes ; but it is scarcely 
possible to describe the law of this change, on account of the great 
difficulty of distinguishing color- tones in very weak light. 

§ 11. Changes of color also take place when the time of the 
action of the light is reduced to a minimum. Sensations of satu- 
rated color can be produced by instantaneous illumination of the 
spectrum with the electrical spark. More time is needed, however, 
to produce these sensations with smaller intensities of the light. 
The different colors, even when of the same brightness, appear to re- 
quire different amounts of time in order to reach the maximum of 
their effect— red, 0.0573; blue, 0.0913 ; green, 0.133 of a second.' 
The tone of the color varies with the duration of the impression as 
well as with the intensity of the Hght. Very minute objects, too, 
appear of a different color on account of their size. In general, the 
larger the surface, the less the intensity of the light necessary to 
produce the sensation of any particular color-tone ; the greater the 
intensity of the light, the smaller the surface which will suffice for . 
such sensation. Fick '' has shown that the color-sensations derived 
from small dislinct points support each other, as it were, in the 
same way as the contiguous points of a colored surface. For if we 
make with a fine needle a single hole (of about 0.6 mm. in diameter) 
in a sheet of paper and look through it at colored paper distant 
some six and a half metres, the color of the paper cannot be dis- 
ting-uished. But if the number of holes be as many as sixteen, 
the color can be distinguished at the same distance, even when the 
holes through which we look are smaller. Subsequent experiment^ 
has shown that the smaller the distance between the single perfo- 
rations, the greater the distance at which the eye can recognize 
colors through them. In general, then, two weak sensations, each 
of which belongs to one eye, may fuse together into one strong 

' According to Kunkel, in Pfluger's Archiv, ix. , p. 207. 

* Pfluger's Archiv, xvii., p. 152. 

* See Dobrowolsky, in Pfluger's Archiv, xxxv., p. 536 f. 


§ 12. Very important changes in the visual sensations occur as 
dependent on the place of the retina which is stimulated. In this 
respect a great difference exists between the central and the pe- 
ripheral parts. The entire field of this organ may be somewhat 
indefinitely divided into three zones — a central or polar, a middle, 
and an outer or peripheral. It is probably true that the periph- 
eral parts of the retina produce no sensations which cannot be 
produced by stimulating the central zone.' But it is equally true 
that, under the same circumstances, the same stimulus produces 
a markedly different effect upon sensation when applied to differ- 
ent localities of the retina. Rays which, falling on the polar zone, 
produce the impression of red, yellow, or green, all make an im- 
pression of yellow when they fall on the surrounding zone (a few 
millimetres from the fovea centralis) ; and this yellow is so much 
the paler, the greener the impression on the polar zone. Rays 
which make on the polar zone the impression of blue or violet make 
on the outer zone the impression of blue ; and this blue is so 
much the paler, the nearer the imi:)ression on the polar zone is to 
green. It follows, then, that whereas thei'e is at the central zone 
an indefinite number of color-tones possible, this number is re- 
duced to comparatively few impressions at the middle zone ; while 
all color-tones gradually become indistinguishable and are lost on 
passing through the outer zone. These great changes in sensi- 
tiveness to color are not accompanied by similar changes iii sen- 
sitiveness to colorless light ; it even appears that regions of the 
retina distant about 30° from its centre are more sensitive to light 
than is the polar zone. 

A certain proportion of persons (perhaps one-twentieth or more) 
appear to have a defective structure of the retina, which may be 
described as corresponding in the polar zone to that of the normal 
retina in the middle or even the outer zone. Such persons are said 
to be "color-blind." The farther outward this imperfect condition 
of the retina extends, the nearer does the defect approach to total 
color-blindness.'' In most cases of this defect there is a partial or 
complete insensitiveness to the red rays ; these rays are especially 
liable to be confused with the dark-green or the yellow. The spec- 
trum is thus shortened at the red end. Cases of so-called violet- 
blindness, as reported by Donders and Stilling, are much more rare 
and doubtful. In total color-blindness only shades of gray from 

' See von Kries, Archiv f. Anat. ix. Physiol., Physiolog. Abth., 1883 (Ap- 
pendix), p. 90. 

'^ See Fick, Zur Tlieorie d. Farbenblindbeit, p. 213 f. ; and in Hermann's 
Handb. d. Phyfaiol., III., i., p. 206 f. 

336 SENSATioisrs of sight. 

white to black are visible. In general, the attempts to make out a 
spectrum for the color-blind are unsatisfactory, since we can only 
be sure as to what color-tones appear like or unlike to them ; we 
cannot, on the contrary, be sure that their abnormal sensations are 
like any of our normal sensations — in other words, that what they see 
when red light falls on the retina corresponds to any of our color- 
tones. The three or four cases reported where one eye of a person 
has been normal and the other color-blind are, of course, especially 
valuable ; since they offer an opportunity to compare immediately 
the sensations of the normal with those of the pathological eye. 
These cases, according to von Kries ' show that the two funda- 
mental colors to which the color-blind are reduced may be con- 
sidered as either red and blue-green or greenish-yellow and blue- 

§ 13. Important modifications of the normal action of the eye are 
also caused by the previous coj^cZiiion of the retina, or by the contem- 
poraneous condition of parts of it contiguous to those on which the 
light falls. The former fact explains the phenomena of " inertia " 
and " exhaustion ; " the latter, the phenomena of " contrast." The 
reaction of the sense of sight is relatively very sluggish ; or — in 
other words — the inertia of the eye is relatively great. This fact 
is undoubtedly due to the chemical nature of the stimulus which 
acts directly upon its end-organs. The light requires time in order 
to effect those photo-chemical changes on whose action upon the 
nervous elements of the retina our sensations of light and color 
depend. On the other hand, if we close the eyes after looking 
intently upon any bright object, the image of this object remains 
for some time, and only slowly fades out of sight. Such an image 
is called a "positive after-image," because its bright and dark lines 
and surfaces correspond to those of the original object. The delay 
which the sensations undergo, both in forming and in fading away, 
is said to be due to the inertia of the retinal structure. It is, of 
course, a law of all nervous excitation and action that it requires a 
certain amount of time for beginning and for changing its char- 

White positive after-images (as Fechner, Helmholtz, and oth- 
ers have shown) pass quickly through greenish-blue to indigo- 
blue and then to violet or rose-color. But "negative after-images " 
are due to the exhaustion of the retina. If the eye be intently 
fixed for some time on a small square of black lying upon a sheet 
of white paper, and then suddenly turned upon the white surface, 
a bright square appears, moves about with the eye, and slowly 
' ArcLiv f. Anat. ij.. Physiol., Physiolog. Abth., 1883 (Appendix), p. 153 £ 


fades away. If wg look for a long time at a green surface and then 
direct the eye upon'a white one, the latter appears for a moment to 
be of a red color. In general, the color of the negative after-image 
is such that, when combined with the color of the object, the two will 
produce white. In other words, the color of such an image is " com- 
plementary " of the color of the object. Such facts as the foregoing 
must in some manner be brought under the law which applies to all 
the elements of the nervous system, but especially to the end-organs 
and the central organs ; these organs become wearied by continuous 
use, and require time for recovery of their suspended or diminished 
functions. Precisely how the application is to be made lo the case 
of the retina is, however, a matter of the general physiological the- 
ory of vision which cannot as yet be stated with perfect certainty. 
The phenomena of exhaustion are among the most important for 
the formation of such a theory. Investigations in this direction 
have led to the discovery that none even of the spectral colors are 
perfectly saturated, since each of them can be made to appear more 
so by looking at it with an eye wearied by the complementary 
color. ' Red is most nearly saturated, blue and yellow next, and 
green least of all. 

§ 14. The different ]3arts of the retina are interdependent in the 
production of sensation ; or — to employ the statement of Wundt ^ 
— " The sensation which arises through the stimulation of any given 
point of the retina is also a function of the state of other immedi- 
ately contiguous points." Hence arise, in part at least, the phe- 
nomena of contrast, which are of two kiiads — contrast of bright- 
ness and contrast of color-tone. The fundamental fact in the first 
class of contrasts is this : every bright object appears brighter with 
surroundings darker than itself, and darker with surroundings 
brighter than itself. These phenomena are explained by Helm- 
holtz ^ as deceptions of judgment, such as we are accustomed to in 
our estimates of distances. To this explanation, however, Fick,* 
Hering,^ and others oppose strong and apparently conclusive ob- 
jections. They would explain the same phenomena by the modify- 
ing influence of the excitation of one part of the retina upon the 
excitation of contiguous parts. Such influence does not always 

^ Comp. Helmlioltz, Physiolog. Optik, p. 279 f. ; Exner, in Pfliiger's Archiv, 
i., p. 389; and see, especially, von Kries, Archiv f. Auat. u. Physiol., Phy- 
siolog. Abth., 1882 (Appendix), p. 115. 

- Physiolog. Psychologie, i., p. 439. 

3 Physiolog. Optik, pp. 388 ff. 

* In Hermann's Handb. d Physiol., III., i., p. 231 f. 

^ Sitzgsber. d Wiener Acad, June, 1872, and December, 1873. 


take the form of depressing the excitabihty of the contiguous 
parts ; on the contrary, stimulating certain elements for some time 
may finally involve contiguous ones in a secondary way. This fact 
they consider to be the true explanation of the spreading of a 
bright object on a dark background, whose after-image becomes a 
clear band of light around the dark image of the bright object. 
When colored instead of white light is used in experimenting under 
the law of conti'ast, phenomena similar to those of complementary 
colors are obtained.' A small square of white on a surface of green, 
when covered with a transparent sheet of tissue-paper, appears as 
red on a surrounding surface of a whitish hue ; on a red ground it 
appears as green, on a blue ground as yellow, and vice ver'sa. There 
is the same dispute over these as over the other phenomena of 
contrast. Shall they be considered as cases of deception of judg- 
ment, or do they admit of a physiological explanation ? Mere 
cases of deception they cannot well be. The theory which ascribes 
to each part of the retina an influence upon other contiguous parts 
is the most satisfactory form of a physiological explanation. But 
such physiological explanation seems to need supplementing by 
reference to induced conditions of the central organs, concei'ning 
the nature of which we ai'e thus far almost entirely ignoi'ant. 

§ 15. It will readily be seen that a theory which shall satisfac- 
torily account for the complicated phenomena of visual sensations 
is difficult to establish. Physiological optics will probably never 
be able to explain in detail the individual sensations of light and 
color. But each claimant to present such theory must, as Wundt ' 
maintains, account for the following- four main classes of facts : (1) 
The subjective i-elations of the color-tones, and the fact that they 
may all be graded downward, as it were, into colorless light ; (2) 
the law of the mixing of all the colors from three (or more) funda- 
mental color-tones ; (3) the phenomena of after-images ; and (4) 
the phenomena of contrast. Among all the hypotheses hitherto 
proposed to account for the quality of visual sensations, that brought 
forward by Young, and elaborated and applied b}' Helmholtz, is by 
far the most prominent. This hypothesis takes its point of start- 
in<T from the undoubted fact that, by admixture of a few so-called 
fundamental color-tones, we can produce all the other colors, as 
well as the sensation called "white." There are said to be /Aree 
such color-tones, because this is the smallest number which will 
account for the facts. Of these three, green must be one, since, in 
the spectrum of colors, this tone has no complementary color. Green 

' See Helmlioltz, Physiolog, Optik, pp. 388 E. 
^ Physiolog. Psycliologie, i , p. 450. 



being fixed, the other two color-tones must be chosen from near the 
ends of the spectrum, and in such a way that, when combined with 
spectral green, they will produce white. Ked (carmine-red, ac- 
cording to Fick) and either violet (so Young and Helmholtz) or blue 
(indigo-blue, Fick) best fulfil the required conditions. It is, then, 
assumed, by the Young-Helmholtz theory, that in every portion 
of the retina which is susceptible to color there exist three kinds 
of nervous elements, the excitation of which separately would pro- 
duce three distinct kinds of sensations ; and that each kind of ele- 
ment is capable of producing only that kind of sensation which is 
peculiar to itself. It apparently follows that each of these three 
kinds of nervous elements has its special form of end-apparatus, the 
excitability of which differs from that of the others ; that is to say, 

Fig. 92. — Diagram from Fick, illustrating the Young-Helmholtz Theory. (For explanation, see 

the text. ) 

there are fibres of red color-sensation, whose end-apparatus responds 
specifically to rays of small refrangibility ; fibres of green color- 
sensation, whose end-appai*atus responds to rays of medium re- 
frangibility : and fibres of violet or blue color-sensations, whose 
end-apparatus responds to rays of great refrangibility. We must 
suppose, however, since we cannot directly analyze into their com- 
ponents the sensations which appear in consciousness, that no one 
of the three kinds of elements is ordinarily excited alone. Every 
actual sensation of color is therefore a complex affair, whose char- 
acter is determined by the relations in which each one of the three 
intensities of excitation stands to both the others. In explanation 
of this assumption the following diagram is proposed.' (See Fig. 
92.) The curved lines E, G, and B represent the three kinds of 

' Taken from Fick's Physiolog. Optik, in Hermann s Handb. d. Physiol., 
III., i., p. 198; comp. Helmholtz, Physiolog. Optik^ p. 291. 


nerves sensitive to the three fundamental color-tones — K to red, G to 
green, B to blue (indigo). The curves described by them show the 
strength of the excitation exercised by the stimulus, corresponding 
to the colors of the spectrum, upon each kind of nerves. The per- 
pendicular lines indicate the colors of the spectrum ; and the waj 
these lines cut the curves shows the relative strength of the excita- 
tion of each kind of nerves w^hich is combined to produce these 

It should be gratefully acknowledged that the Young-Helmholtz 
theory affords a brilliant explanation of a great many of the phe- 
nomena of sensations of light and color. It is most successful with 
those that relate to the mixijig of colors and to complementary 
color. The hypothesis cannot be said, however, to be wholly ade- 
quate and satisfactory. One of its most intelligent advocates (Fick) 
admits that it cannot explain the following cardinal fact : Every 
ray of light which, so long as it is confined to a moderate extent of 
the polar zone, makes the impression of a saturated color produces 
a whitish impression, almost devoid of color-tone, as soon as it is 
limited to an extremely minute portion of the retina. This is the 
very opposite of what the hj'pothesis would lead us to expect ; for, 
according to it, extremely minute impressions on the retina ought 
to isolate the particular kind of fibres, and so yield tlie purest 
possible color-tone. The facts of histology seem rather adverse 
than favorable to the theory, although not much stress can be laid 
upon them alone. Moreover, it does not satisfactorily explain the 
facts of contrast of colors and of color-blindness. The most re- 
cent investigations seem to indicate that cases of color-blindness 
cannot be accounted for by dropping out one fundamental kind of 
nerve-fibres, as the Young-Helmholtz theory supposes.' Various 
other important objections are raised by its opponents (especially 
by Hering, Wundt, and others). 

§ 16. In order to supply the alleged defects of the Young-Helm- 
holtz theory of color- sensations, several other theories have been 
devised — notably those of Hering and of Wundt. The former* 
differs from most other investigators in his view of the nature of 
the changes of sensation which take place as we, in experience, 
run through all the different shades of gray from white to black. 
All such changes Hering considers analogous to those alterations 
in the quality of our sensations that would be produced by jpassing 

' See von Kries, Archiv f. Anat. u. Physiol., Physiolog. Abth., 1882 (Ap- 
pendix), pp. 1 84-153. 

^ B. Hering, Zur Lehre vom Lichtsinne, Sitzgsber. d. Wiener Acad., 6 papers, 

wutstdt's theoey of coloes. 341 

the eye over a surface on which the different color-tones almost 
insensibly shaded into each other. Hering, therefore, proposes six 
(or three pairs instead of three single ones) fundamental color- 
tones — namely, black and white, green and red, blue and yellow. 
The changes which give rise to sensations of black, green, and blue 
are ascribed to the process of "construction " of a so-called visual 
substance ; those which give rise to white, red, and yellow are as- 
scribed to the "destruction" of such visual substance. The three 
pairs of color-tones are thus made antagonistic rather than com- 
plementary. But the hypothesis of Hering appears to involve more 
uncertain assumptions, and to explain fewer facts, than the one it 
would displace. Moreover, the assumption that white, and its shades 
down to black, may be considered as color-tones, instead of altera- 
tions in the brightness of the true color-tones, is generally denied. 

The theory of Wundt ' emphasizes the difference in processes 
rather than in the kinds of retinal elements. It involves the fol- 
lowing principles : (1) In every excitation of the retina two dif- 
ferent processes are set up, the variations of which follow differ- 
ent laws ; one of these is a " chromatic " process (which gives us 
color-tones), and is a function of the length of the waves of light ; 
the other is "achromatic," and is also dependent upon the wave- 
lengths, but varies only in intensity and remains in character the 
same. (2) The achromatic excitation consists in a "uniform pho- 
to-chemical process," which reaches its maximum at yellow and 
falls off toward both ends of the spectrum. (3) The chromatic 
excitation is a "polyforra photo-chemical pi'ocess," which changes 
continuously with the wave-lengths of light. The extreme differ- 
ences of this length are such as to produce effects that approximate 
each other ; while the effects of certain different intervening wave- 
lengths are related in such a way that opposed phases of one and 
the same movement equalize each other perfectly. (4) Evei'y pro- 
cess of excitation of the retina outlasts the stimulation for a certain 
time, and exhausts the sensibility of the nerve-substance for that 
particular form of stimulation. The positive after-images are to be 
explained by the persistence of the retinal excitation, the negative 
by exhaustion. (5) The difficult phenomena of contrast are to be 
explained by the general principle that all impressions of light and 
color are experienced in relation to each other. In other words, 
they fall under the general law of relativity. 

§ 17. Von Kries'' has subjected all the principal theories of color- 

' See Physiolog. Psychologie, i. , pp. 450 if. 

- See Arcliiv f. Anat. u. Physiol., Physiolog. Abth., 1883, Appendix, pp. 



sensations to a most searching criticism as considered in the light 
of all the facts. He naturally finds serious defects in them all, but 
arrives at the following highly important conclusions. The photo- 
chemical facts concerned in vision compel us to adopt a theory of 
component elements rather than one of changes qualitatively alike 
and arranged in a continuous series. This would seem decisive 
against the theory of Wundt, Only by the aid of assuming the 
varied combination of such elements can we explain the phenomena 






Fig. 93. — Color-Triangle, from Fick. (For explanation see text.) 

of exhaustion. Three series of components are apparently requisite : 
one for the bright and dark, but colorless, sensations, and two 
color-tone series — a red-green series, and a yellow-blue series. 
White is, nevertheless, not to be considered as belonging to the 
three, since it corresponds to all the color-tones whenever they 
reach a minimum of saturation. The processes corresponding to 
these three series of components ma}' be located at dilferent places 
in the nervous apparatus of vision — either more centrally or more 


peripherally. The articulation and adjustment, as it were, of the 
three processes von Kries would assign to the central organs. And 
here we reach the extreme limits, not only of our assured knowl- 
edge, but also of our power to frame a plausible theory ; for it ap- 
pears that all theories must either leave certain important facts un- 
explained, or else make further assumptions concerning nervous 
processes — especially in the central organs of vision — of the exist- 
ence and influence of which upon the sensations there can be no 
doubt, but of the precise natui-e of which we are completely ig- 

§ 18. Much ingenuity and painstaking have been expended in de- 
vising some form of si/inbolism which should represent to the ej'ein 
geometrical relations the laws of the sensations of light and color. 
Obviousl}' the sensations of this sense cannot, like those of hearing, 
be symbolized by the relations of points along a straight line. 
Color-tones, unlike musical tones, form a series of qualitatively differ- 
ent sensations that, at certain places in the scale, separate from each 
other with varying degrees of rapidity, and then toward the broken 
ends, as it were, of this scale, tend to approach each other again. 
Such relations are most successfully set forth by a triangle, which 
maybe constructed as in the foregoing figure ' (93). In this triangle 
the different color-tones may be regarded as tying together along 
the cuiwed line, from red to violet, and the difference in any two 
color- tones as measiu'ed by the angle which two lines make when 
drawn from the point W through the 
points occupied on the curve by the 
two color-tones. For example, the 
difference between red and violet is 
less than that between red and green, 
as is indicated by the fact tbat the 
angle B TF//is smaller than the an- 
gle EWG. 

By Fig. 94 " the relations of the 
color-tones as contrasting with, and 
complementary of, each other are rep- 

l-esented. Of the two concentric Fig. 94.— scheme for showing the Rela- 

circles, each color in one corresponds *^°''' °^ Coior-tone (see text). 

to the complementary color of the other. If the color inducing 
the contrast is represented by a segment of the inner circle, the 
coincident segments of the two circles represent the direction in 
which the induced change is moA-ing, as it were. For example, 

' Taken from Fick, in Hermann's Ilandb. d. Plij'siol., III., i., p. 184. 
■•^ Taken from Wundt, Physiolog. Psycliologie, i., p. 442. 


since the segment green coincides with purple, and red coincides 
with blue-green, green on a red ground is modified as it would be 
if blue-green were mixed with it ; and red, as it would be if purple 
were mixed with it. 

§ 19. At least two specifically different forms of sensation — namely, 
Pressure and Temperature — have generally been admitted to have 
their organ in the skin. ' The claims of various other kindred forms 
of feeling to be considered as primitive factors of our sense-percep- 
tions, arising from the activity of the skin as an end-organ of sense, 
are more doubtful. Sensations of motion, of innervation and weari- 
ness of the muscles, the so-called " common sensations " (or sensa- 
tions of the sensus communis), the sensations of pain or pleasure, and 
those delicate shadings of sensation, as it were, which constitute 
the " local coloring " of all the feelings to which we assign a definite 
place in the fields of sight and touch, are all closely allied to sensa- 
tions of pressure and temperature. But some of these forms of 
feeling — as, for example, the so-called sensations of motion and of the 
sensus communis— oxe undoubtedly complex modifications of certain 
simpler states of consciousness ; others of them, as the sensations, 
of muscular weariness, of pain, of innervation, and " local coloring," 
may possibly have, in jDart, a central origin. As a rule, they lack 
the characteristic quality of being components of the " presenta- 
tions of sense," as this quality belongs to all genuine sensations. 
Sensations of "local coloring" have, indeed, a most important part 
to take in the formation of the " presentations of sense ; " but they 
are, in the realm of touch and of muscular feeling, as infinitely and 
delicately varied (and even more difficult of description) as are the 
finest shadings of musical tones or color-tones. 

§ 20. A sixth sense, however, and a sixth organ of sensations 
must doubtless be recognized as constituted by the muscles and 
the various kinds of feeling which their action occasions. These 
muscular sensations, when combined with those of the skin, give 
certain complex feelings of motion on which the adjustment of the 
body to its environment is so dependent. The long-continued dis- 
pute concerning the presence of sensory nerve-fibrils in the muscles 
may be said to be settled affirmatively. '^ Certain subjective phe- 
nomena cannot be accounted for by ascribing the so-called muscular 
sensations to feelings of central innervation, or by identifying them 

' On the physiology of the skin, see Goldscheider, art. Neue Thatsachen 
liber die Hautsinnesnerven, Archiv f. Anat. u. Physiol., Physiolog. Abth., 
Supplement-Band, pp. 1-104. 

^ See, especially, Sachs, iu Archiv f. Anat. u. Physiol., 1874, pp. 175 f., 491 
f . , and 645 f . 


with the sensations of pressure through the skin. ' Bernhardt " found 
that the degree of sensitiveness to different weights, when lifted by 
the foot or the finger, was little or not at all diminished by exclud- 
ing all central innervation of the muscles through an act of will. 
The discrimination of differences of weight was not greatly impaired 
when the limb was bent by an induction -shock sent through the 
muscles instead of by motor impulses arising in the brain. The 
muscular sensations cannot, therefore, be due to such central activ- 
ity. Investigation also shows that the muscular sensations sup- 
plement those of pressure in the skin in all our estimates of the 
position and motion of the limbs ; these two are, therefore, not 
identical. Moreover, without assuming the existence and aid of 
such sensations we cannot account for that nice control of the mus- 
cles which, especially in the case of the eye, is so indispensable a 
prerequisite, not only for adjusting their action to the ends desired, 
but also for gaining an exact knowledge of the position and motion 
of objects in the outside world. The precise manner, however, in 
which the muscular sensations originate, through that stimulation 
of the sensory nerves which the contraction of the muscular fibre 
occasions, is as yet unknown. Nor can they easily be separated 
and classified into kinds, apart from the sensations of pressure with 
which they are in actual experience constantly allied. Their chief 
interest to psychology centres in the help which they furnish to the 
mind in forming itfe " presentations of sense." 

§ 21, Sensations of Pressure are dependent upon the excitation 
of the sensory nerves of the skin through their appropriate end- 
orgfins. The excitation of the trunk of any of these nerves at some 
point along its course may produce the feeling of pain, but does not 
produce those definite sensations of pressure which we are able to 
localize so accurately and discriminate so nicely as to their degree. 
Precisely which of these end-organs are specifically related to sen- 
sations of pressure neither histology nor experimental physiology 
has thus far been able to determine (see Part I., chap. V., § 10), 
The ordinary stimulus of the end-organs of the skin active in these 
sensations consists in their compression or expansion by contact 
with some external object which either rests upon them or upoa 
which they rest, or which is moved over or against them, or over or 
against which they are moved. Such stimulus may, of course, vary 
both in form and in degree. The quantity and succession of the 
sensations of pressure, as well as the manner in which they com- 
bine with one another and with sensations of the muscular sense, 

' Comp. Funke, in Hermann's Handb. d. Physiol., III., ii, , p. 359 f. 
2 Archiv f . Psychiatrie, III. , p. 627. 

346 SENSATiOTsrs or the skin. 

have a marked effect in determining their characteristic "tone" 
of feeling. In respect to quality pure and simple, sensations of 
pressure scarcely admit of a scientific classification. We localize 
them in the field of touch ; we make an important use of them in 
connection with sensations of muscular origin, for constructing the 
field of vision and for giving to different objects their respective 
places in this field ; but in ordinary experience we do not directly 
recognize kinds of the simple sensations of pressure as we do of 
tastes, smells, tones, and colors. A distinction is sometimes made 
between " light touch," or touch proper, and sensations of press- 
ure or weight. But the distinction, so far as it leaves out of ac- 
count the muscular sensations, has hitherto been one only of de- 
gree and not of kind. 

The more recent and thorough investigations of Goldscheider * 
have led him to distinguish two si^ecifically different sensations 
which enter into what is ordinarily called the feeling of pressure. 
This distinction is based upon facts experimentally ascertained. If 
a very fine point of metal, wood, or cork, be touched lightly to the 
skin, it will be found to awaken a definite sensation, such as is en- 
titled to be called a" sensation of pressure," only at certain minute 
spots in any given area of the skin. This sensation, when the 
pressure is very light, is described as lively and delicate, often 
accompanied by the feeling of being tickled. On increasing the 
pressure upon these same spots the sensations change their char- 
acter somewhat, and become as though some small, hard kernel 
were pressed uj)on the skin (" Kdrniges Gefnhl "). Between these 
distinctively "pressure-spots" it is not possible to excite by i^ress- 
ure the same characteristic sensation. Stimulation of the inter- 
mediate spots, on the contrary, produces a dull, indefinable, "con- 
tent-less " sensation ; and when the pressure is increased, a sense 
of being pricked or stuck. Both of these kinds of sensation, when 
the pressure is still further increased, pass over into painful feeling ; 
but the character of the pain in the two is different. 

The arrangement of the " pressure-spots " is analogous to that of 
the temperature-spots (to be described subsequently). They occur 
much more frequently in certain areas of the body than in others. 
They are placed in chains, as it were, sometimes more and some- 
times less thickly set. These chains ordinarily radiate from a kind 
of central point, and run in such directions as to form either circu- 
lar, longitudinal, or pyramidal figures. Their direction is seldom 
identical with that of the temperature-spots. In tlie opinion of 

' Archiv f. Anat, u, Physiol., 1885, Pliysiolog. Abth., Supplement-Band, pp. 
76 fE. 



Goldscheider the spots of both kinds correspond to the terminal 
points of the nerve-fibres of two specifically different kinds of nerves 
distributed over the skin. But whereas aU the area of the skin is 
well covered with such nerves as give us the general dull and in- 
definite feeling of contact, the nerves of the sensation of pressure 
are much more unevenly distributed. It need scarcely be said 
that, other things being equal, they are most numerous in the areas 
of the skin most sensitive to touch. The different pressure-spots 
themselves differ in sensitiveness ; some are much more easily ex- 
cited than others. The sensations themselves come under the 


Fig. 95.— Arrangement of Pressure- spots (Goldscheicler). A, dorsal and radial surface of the 
first phalanx of the Index fioger ; B, membrane between thumb and index finger; 0, dorsal 
surface of forearm ; D, back ; B, inner surface of forearm ; F, back of hand. 

general laws of exhaustion, practice, etc., as these laws apply to 
the whole mechanism of sense. 

The attempt has been made, on the other hand, to identify, in 
kind, sensations of pressure (especially those of light touch) and 
sensations of temperature.' E. H. Weber observed that cold bodies 
resting on the skin appear heavier, and warm lighter, than they 
really are. A single silver dollar of the temperature of 25°-19.5° 
Fahr. appeared to be of the same weight as two dollars of the tem- 
perature of 98.5°-100.5° Fahr. Wunderli also argues the identity 
of these two classes of sensations on the ground that, if certain parts 
of the skin are lightly touched with cotton or slightly warmed by 
approaching a heated surface to them, through a square opening in 

' For a discussion of this qiiestion, see Funke, in Hermann's Handb. d. 
Physiol., III., ii., p. B20 f. 


a piece of paper laid upon the skin, the two sensations thus occa- 
sioned are frequently mistaken for each other. But Szabadfoldi 
has much Aveakened the force of Weber's experiment by showing 
that small wooden disks when heated to 122° Fahr. often feel 
heavier than those which are really larger but are not thus warmed. 
And Wunderli's observation at best holds good only for compara- 
tively obtuse parts of the skin, especially the back. Moreover, if 
the same stimuli should serve to excite both the pressure-spots 
and the temperature-spots, this would not prove the identity of the 
two sensations. 

Finally, the physiology of the sense of temperature re-enforces 
the indubitable testimony of consciousness, and leads us to the con- 
clusion that from beginning to end — in the character of their stimuli, 
of their nervous processes, and of the resulting modifications of 
feeling — the sensations of pressure and the sensations of tempera- 
ture are qualitatively distinct. They have in common only the 
organ in which their apparatus is located, and the fact that both 
kinds of sensations are constantly associated most intimately in 
time and space. 

§ 22. Sensations of Temperature, therefore, form a second dis- 
tinct species which have their origin in the excitation of the nervous 
end-apparatus of the skin. Whether their end-apparatus is locally 
the same as that upon the excitation of which the sensations of 
pressure are dependent, it has seemed rmtil very lately impossible 
to say. But recent investigations (especially of Blix,' Goldscheider,'' 
and Donaldson') point unequivocally to the conclusion that certain 
definite spots of the skin, and these only, are susceptible to irrita- 
tions of a kind to result in sensations of temperature. Such spots 
are insensible to pain (even the pain of temperature), and a needle 
can be run into them without being felt ; they are probably also in- 
sensible to pressure. What is more remarkable still, the existence 
of " heat-spots " and " cold-spots " — or minute localities of the skin 
sensitive to heat but not to cold, and conversely — seems demon- 
strable. By using a machine which locates the stimulus micro- 
metrically, the topography of the skin may be mapped out, and 
extremely minute spots indicated which respond to irritation with 
sensations of pain, of pressure, of cold, and of heat — respectively. 
These different kinds of sensation-spots appear never to be super- 

' Zeitschrift f. Biol., 1884, XX., pp. 141 ff. 

^ Moiiatshefte f. prakt. Dermatol., 1884, III., Nos. 7, 9, 10 ; 1885, IV., No. 1 ; 
and art. in Archiv f. Anat. u. Pliysiol., 1885, Physiolog. Abtli., Supplement- 

^ Reseai'ch on the Temperature-sense, reprinted from Mind, No. XXXIX. 


imposed. They are not located alike on the symmetrical parts oi 
the same individual, or on the corresponding parts of different in- 
dividuals. An accurate mapping out of the different areas of the 
skin, with respect to their temperatui'e-spots, is difficult ; since ex- 
periment soon blunts the sense, and even the approach of a heated 
or cooled point raises or lowers the temperature over a considerable 
area. But, in general, such spots occur in lines that radiate from 
centres generally coincident with the roots of the hairs, in those 

Fig. 9C.— Arrangement of Temperature-spots. A, heat-spots ; and B, cold- spots— from the palm 
of the left hand (GoMsoheider). 

regions of the skin where such appendages are found. These lines 
often run so as to cross each other, forming figures of various shapes, 
— triangles with rounded corners, etc. Heat-spots are, on the 
whole, less abundant than cold-spots ; but inpartsof the body where 
the skin is most sensitive to either heat or cold the corresponding 
class of sjDots is relatively frequent. Temperature-spots may be 
divided into first-class and second-class (so Goldscheider) according 
to the strength with which they react on moderate stimulation. 
Some spots are roused only by excessive temperatures. The same 
object feels cool to one spot, ice-cold to another. 

The electrical current when applied to these spots is thought to 
call out the corresponding specific sensations. Goldscheider con- 
siders that he has succeeded in exciting definite temperature-sensa- 
tions by applying electricity to the trunks of the nerv'es distributed 
to certain areas of the skin. This would appear to be almost a 
demonstration that the nerves of this sense are specific, and of 
two kinds — nerves of heat-sensation and nerves of cold-sensation. 
Puncturing a temperature-spot also gives rise to temperature-sen- 
sations. The discriminative sensibility of the temperature-spots 
is found to be much finer than that of the tactile sensations. 
Everything which produces a change in the temperature of the 
skin acts, of course, as a stimulus for the sensations of heat and 

§ 23. The above-mentioned discoveries as to the specific energy 
of the nerves and end-apparatus of the skin, interesting as they are. 


have not yet been completely brougiit into rational connection with 
our experience of temperature-sensations and our knowledge of 
the general laws of nervous action. It is obvious, however, "^iat 
the principles of contrast, of relativity, and of exhaustion, must 
bear a large part in the explanation of all these sensations. Sen- 
sations of temperature have apparently a certain dependence on 
the temperature of the thermic apparatus itself. This law has been 
elaborated and defended in detail by Hering,' in the following 
form : " As often as the thermic apparatus at any spot in the 
skin has a temperature which lies above its own zero-point we 
have a sensation of heat ; in the contrary case, a sensation of cold. 
Either sensation is so much the more marked, or stronger, the 
more the temperature of the thermic appai-atus at the time varies 
from the temperature of its own zero-point." By the "zero-point" 
of any part of the skin is meant the exact objective temperature 
which at that part will produce no sensation of either heat or cold. 
Such zero-point is, of course, different for different parts of the 
body, according as they are or are not exposed, and are or are not 
well supplied with arterial blood, etc. It also changes in connection 
with changes in the temperature of the surrounding air or of the 
bodies with which the skin is in contact. By this principle a great 
number of our ordinary sensations of temperature are explained 
by Hering. The finger and the nose are colder than the inside 
of the mouth, because they are exposed to radiation of their heat. 
On passing from a room of a given temperature into one of either 
higher or lower temperature we experience at first certain sensa- 
tions of temperature while the zero-point of the thermic apparatus 
is becoming adjusted to its new surroundings. After such adjust- 
ment has taken place these sensations may cease — to be renewed 
in the revei'se direction, however, on a return to the former sur- 
roundings. This adjustment has its limits ; it is dependent chiefly 
upon the evaporation of the skin and upon the circulation of the 

If the surroundings are more than so hot or so cold, they may 
excite constant sensations of temperature. Among the induce- 
ments to sensations of heat at any locality of the skin, Hering men- 
tions the following as prominent in our ordinary experience : All 
checking of the radiation of heat, while the blood-supply remains 
unaltered ; all contact with a medium or object of higher tem- 
perature — and this according to the ease with which such medium 
or object parts with its heat ; and all increase of heat in the skin 

' In Hermann's Haiidb. d. Physiol., III., ii., p. 419 f . ; and Sitzgsber. d. Wie- 
ner Acad., LXXV., Abth. 3, p. 101 f. 


coming from the interior of the body, as in the sudden hj'pergemia 
which takes place in blushing. Inducements to sensations of cold 
are as foUows : Increased convection of the heat of the skin by the 
suiTounding medium, while the blood-supply remains unchanged 
(as when the wind blows over the hand or face, esj)ecially if the 
skin be moist) ; contact with objects which have the same (or even 
slightly higher) objective temperature as the surrounding air, but 
convey the heat from the skin more rapidly than it ; contact with 
or j)roximity to objects colder than the skin ; lessening of the 
interior warmth of the body — for example, by contraction of the 
blood-vessels which supply a given portion of the skin. Ordinary 
experience makes us famihar with many of the phenomena which 
come under all these cases. 

The determination of the e'K^act zero-point of different parts of the 
body is a matter of great difficulty. The rise and fall of the tem- 
perature of the thermic apparatus, in connection with that principle 
of exhaustion which aj^plies to all the nervous mechanism, and es- 
pecially to certain of the end-organs of sense, doubtless account 
(at least partially) in some way for the well-known phenomena of 
contrast in temperature-sensations. "Weber showed that if the 
hand be held for a minute in water of the temperature 54.5° 
Fahr., and then in water of 64.4" Fahi*., a sensation of heat will be 
felt for a few seconds, although the latter would have felt cold to 
the hand if placed in it at first. Moreover, if we hold one hand in 
moderately cold water, and dip the other repeatedly in the same 
water, the sensation of cold is stronger in the latter, although the 
temperature of the hand held in the water is really lower. But, 
according to an experiment of Goldscheider's, if one hand be left 
for ten seconds in water of the temperature of 104° Fahr., and then 
both hands immersed in cold water, the warmed hand will feel the 
cold less distinctly than the other. This latter investigator, how- 
ever, is inclined to dissent from Hering's theory, and retui-n to the 
theory of E. H. Weber. "Weber held that the rising of the tem- 
perature of the skin is felt as heat, and its sinking as cold. 

After-images of temperature-sensations seem also to exist. But 
when a surface of the skin has been warmed or cooled, and the 
after-image has faded quite away, it is said that it can be called 
back b}' light mechanical irritation ; this is especially true of sen- 
sations of cold. The phenomena of exhaustion are noticed in sen- 
sations of tempei'ature. Our perception of the absolute degree of 
temperature, and of minute variations in its degree, is most acute 
for places in the scale lying close to the normal temperature of the 
skin. It would seem, on the whole, as though the phenomena 


of contrast of sensations of temperature, as well as of color, require 
for their satisfactory explanation a knowledge (possibly of the 
action of the central organs of the nervous system) which we do 
not yet possess. 

E. H. Weber also showed that the amount of the skin which is 
stimulated has a marked influence on the quality of the resulting 
sensation. The temperature of the same fluid does not feel pre- 
cisely the same to a single finger and to the entire hand. This 
experience is similar to that which has already been described in 
the case of sensations of color. It appears explicable in the case 
of the skin from what is now known about the existence of a cer- 
tain variable number of heat-sjDots and cold-spots. In the same 
way, in part, may we explain the fact that smooth objects, which 
therefore come into contact with a larger portion of the skin — like 
leather, paper, wood, glass, and porcelain — appear colder to the 
whole hand even when they have the same objective temperature 
with it. 

§ 24. Nothing whatever is known as to the exact manner in which 
changes of temperature act upon the thermic apparatus to excite 
it ; the recent discoveries ajDpear to make such action all the more 
difficult of conception and description. Since the terms " hot " 
and " cold " are in physics only relative, it is hard to see why ab- 
solutely different apparatus, with a distinct local position, should 
be used (as Goldscheider's discoveries indicate) for the sensations 
corresponding to each. Moreover, on Hering's hypothesis, how 
are we to account for the fact that heat-spots and cold-spots are in 
turn stimulated by the same objective temperature according to 
the rise and fall of the zero-point of tbe entire region of the skin ? 
Possibly it may be found that certain chemical or electrical changes, 
dependent upon the increase or decrease of that mode of molecular 
motion which jDhysics calls " heat," are the proximate stimuli of 
the two classes of end-organs of the temperature-sense. Gold- 
scheider supposes that the difference in sensitiveness of different 
areas of the skin to temperature must be ascribed to the anatomi- 
cal distribution of the heat- sensitive and cold-sensitive fibres, re- 
spectively. But he does not show us what kind of nervous con- 
trivance would satisfy all the conditions which are imposed by the 
complicated facts of experience. 

A- Herzen ' considers himself to have demonstrated, by patho- 
logical cases and experiment upon animals, that sensory impulses 
of cold, like those of touch, pass along the posterior strands of the 
spinal cord ; and that the sanje region of the brain {Gyrus sigmot- 
' See Pfliiger's Archiv, 1885, pp. 93 fE. 


deus) is the " centre " for both. Sensitiveness to heat can be re- 
tained, it would seem, after sensitiveness to cold has been lost. 

§25. In closing the subject treated in the last two chapters, 
attention is again called to the large amount and cumulative char- 
acter of the evidence afforded by the special sensations, considered 
as respects their quality, for the law of the Specific Energy of the 
Nerves. It is impossible to account for the above-mentioned phe- 
nomena without carrying this law to a great length in its applica- 
tion to the special senses. We may not be able to affirm — as does 
Fick,' for example — that two sensations are distinguishable as re- 
spects quality only in case they are occasioned by two individually 
different elements of the nervous system. For we have seen that 
the quality of sensations depends upon their quantity, upon their 
relation to preceding and contemporaneous sensations, and upon 
considerations other than merely the one of what particular nerve- 
fibre or element of the end-apparatus was acted upon by the stim- 
ulus. Moreover, there is no warrant for saying that identically the 
same nervous apparatus cannot be excited variously according to 
the nature of the stimulus which acts upon it, or according to the 
combination with other parts of the system into which it enters for 
the time. It is obvious, however, that the differentiation of func- 
tion, and the assignment to specifically distinct apparatus of par- 
ticular nervous impressions corresponding to particular mental 
states, is carried to a great length in the special senses. In this 
differentiation of function it is not wholly or chiefly the nerve- 
fibres, as such, which should be taken into account ; it is also the 
minute subdivisions of the eud-organsof sense, and the connections 
set up within the corresponding regions of the central organs. In 
accounting for those complex sensations which appear in ordinary 
consciousness, the law of permutations and combinations has, of 
course, to be considered. A vast variety of such sensations maybe 
made up by changing the relations to each other of comparatively 
few simple elements. But in each of the senses our analysis, when 
carried to its utmost limit, leaves a number — in some of the senses 
very large — of simple sensations, which apparently must have their 
physical basis in the excitation of specifically distinct elements of 
the nervous mechanism. 

The sense of smell apparently requires that the law of the specific 
energy of the nerves should be carried to such a length as almost 
to reduce it to an absurdity. Histology has discovered only one 
essential kind of olfactory end-organ, and that of comparatively 
simple structure ; and yet experience gives, as the result of its ex- 
1 In Hermanns Handb. d. Physiol., III., ii., p. 166. 


citation, a bewildering variety of sensations so specifically different 
as to baffle all our attempts to classify them. From the case of 
this sense an argument may then be derived which leads in ei- 
ther direction. It may be objected to the law that it is absurd to 
sup23ose a complexity of the end-organs of smell such as to corre- 
spond to each specific kind of olfactory stimulus with a specific sen- 
sation — for example, the smell of musk, or of sulphuretted hydro- 
gen. It may be replied to the objection that, in the case of the 
ear, there are at least 16,000 or 20,000 distinct forms of auditory 
end-apparatus corresponding to the different musical tones ; and 
it is therefore by no means impossible that the entire regio ol- 
factoria may contain enough siDecifically different forms of its own 
peculiar end-apparatus to suffice for all the simple sensations of 

The sense of taste does not occasion so many difficulties in rela- 
tion to the law of the specific energy of the nerves. It is thought 
possible by most physiologists to reduce all the sensations of taste 
to four, or at most six, different species. It is easy to suppose 
as many specifically different forms of the nervous apparatus cor- 
responding to the different classes of sensations — sweet and sour, 
salt and bitter, alkaline and metallic. In spite of the fact that 
such a classification appears satisfactory to most authorities, experi- 
ence is reluctant to confirm it. Many of the complex tastes, even 
when separated from their accompanying sensations of smell, are 
scarcely resolvable into combinations of the above-mentioned 
simple tastes. Into which of the six, for example, would experi- 
ment resolve the gustatory sensations which come from chewing a 
bit of chocolate, or of a nut from a black-walnut tree ? 

The strongest defence of the most extreme form of the theory of 
the specific energy of the nerves has hitherto been found in sensa- 
tions of musical sound. Here we undoubtedly have a wide range 
of qualitatively distinct states of consciousness which are ap- 
parently dependent ujDon the excitation of a correspondingly large 
number of distinct nervous elements. From sensations of sight, 
although many points of the prevalent theory are still obscure and 
unsatisfactory, a considerable force of evidence bearing in the same 
direction may be obtained. It seems almost certain that the 
numerous states of consciousness which result from stimulating 
the different nervous elements of the retina are due to combina- 
tions of a comparatively few kinds of such elements, each of which 
responds in a specific way to a special order of stimulus. Yet this 
is not precisely what the theory of specific energy seems to de- 
mand. For the different color-sensations all appear as simple and 


unanalyzable states of consciousness. None of them are twofold, as 
sensations. We are at a loss to say why, according to the theory 
of specific energy, each sensation should not result from the ex- 
citation of one, and only one, kind of nervous elements. 

The recent discoveries as to the existence of pressure-spots, 
heat-spots, and cold-spots in the skin add important evidence to 
that ah-eady existing in favor of the law under discussion. It will 
further appear, when we consider the process of locaKzation in the 
so-called " geometrical senses " of the eye and the skin, that the 
very possibility of such a process demands a strict and far-reaching 
application of the law of the specific energy of the nerves. Pre- 
cisely how we are to state and limit this law, neither its opponents 
nor its advocates have as yet been able satisfactorily to show. The 
exact expression of the theory waits for further evidence from 
experiment, although there can be little doubt that in its main 
features it is already secure. 



§ 1. By an act of mental analysis, which all men readily perform, 
changes in the amount of sensation are distinguished from changes 
in its quality. This distinction obviously requires for its perform- 
ance nothing beyond what is immediately given in consciousness. 
All sensations appear there as differing among themselves, not only 
with respect to the nature of the impression which serves to classify 
them into groups (as sensations of sight, sound, etc.), but also with 
respect to the degree in which each particular impression possesses 
the sphere of conscious attention and feeling. The best illustra- 
tion of an alteration in the intensity of sensation, while its charac- 
teristic quality remains unaltered, may be derived from musical 
tones. The dying-out of a single note when the bow is drawn 
with decreasing force across the string of a violin, or a single 
key of the piano is struck and the pedal held, may be considered 
as a change in the quantity of sensation, while its quality is un- 
changed. A more complex case is the experience we have when 
approaching to, or receding from, a bell that is sounding or a 
steam-whistle that is blowing. Noises of a certain complex quality 
— such as slamming, hissing, grating, etc. — are continually de- 
scribed as very loud, moderately loud, or of weak intensity. So, 
too, when approaching a white or colored light, with our attention 
fixed upon it, we generally disregard almost wholly the changes 
in its color-tone which take place, and consider chiefly the changes 
in its intensity and apparent size. The pressure of diffei'ent 
weights upon different parts of our skin is ordinarily regarded as 
the same in quality and as varying only in amount and locality. 
The same thing is true, in almost precisely the same way, with 
sensations of temperature. The thing we touch is called slightly 
cold or very cold, somewhat warm or hot, our attention being 
directed chiefly to the quantum of sensation which it calls forth. 
In other words, it is generally the same kind of pressure and tem- 
perature, with a varying degree of intensity, of which we are 


It Is more difficult, however, even in the most indefinite way, to 
separate the quantities of our sensations of smell and taste from 
the changes in quality of the same sensations. A concentrated 
sweet or acid so strongly excites a variet}^ of forms of feeling which 
mingle indistinguishably with the specific sensations of taste that 
we are compelled to attend to the very decided qualitative changes 
which are taking place. The increased intensity of the sweet or 
sour we may indeed speak of as "very" much of the same sensa- 
tion which was excited in less degree by the diluted form of the 
stimulus ; but we are more likely to regard it as constituting a 
complete change in the kind of taste. In the same manner, atten- 
tion is forcibly directed toward the kind of sensation which results 
from increasing the quantity of any specific sensation of smell. 

It is further obvious that the distinction which we make between 
changes in the quantity and changes in the quality of our sensa- 
tions is to some extent applicable for comparing the sensations of 
different senses. And here the distinction, when applied to sub- 
species under certain specific forms of sensation, affords us a 
means of transition for such comparisons. Some yellows are 
bright and others dull ; and the same thing is true of the reds and 
the blues. The sours, the sweets, the bitters, may be compared 
with each other as respects the degree of intensity which they pos- 
sess. We may next, in a very indefinite way, compare the quantities 
of the sensations of the different senses as they appear side by side, 
or successively, in consciousness. We are ordinarily satisfied, 
however, with simply describing the varying degrees of intensity 
possessed by our different sensations as " weak " or " strong " (with 
or without the emphatic " very "), or as only " moderate." Thus 
we may judge that both the light which we see and the tone which 
we hear (either simultaneously or one immediately after the other) 
are, or are not, to be classed together under the same one of these 
three grades of intensity. 

§ 2. That changes in the intensity of our sensations are not, in 
fact, independent of changes in their specific nature has already 
been proved (Chap. TV"., § 4). Only in the case of musical tones 
are we able at the same time to attend carefully to both the quan- 
tity and quality of our' sensations, and so discover with perfect 
confidence that the former is changing while the latter remains un- 
changed. Even in this case, since the tones which we ordinarily 
hear are composite, any considerable alteration of their intensity 
changes also their tone-coloring, through the alteration which it 
produces in the comparative intensities of the overtones. Any in- 
crease iu the brightness of a particular color invariably changes its 


characteristic color-tone. A white of less intensity is not merely 
less white, but becomes a gray ; and by constantly diminishing its 
intensity white can be shaded through the different grays toward 
black, which is certainly not a feebler degree of the sensation of 
white. The same dependence of quality on quantity is true in all 
sensations of smell, taste, pressure, and temperature. It would be 
a mistake, however, on this account to consider " quantity " of sen- 
sations as only another name for shades of quality, or to deny that 
we can apply terms of measurement to these reactions of the mind 
upon the excitation of the nervous apparatus of sense.' Scientific 
analysis confirms the distinction made by ordinary experience be- 
tween "the way" we feel and "how much" we feel in any particular 

§ 3. All descriptions of the changing intensities of sensations, 
when made on the basis of ordinary experience solely, leave the 
subject in a very indefinite and unscientific form. That a certain 
noise is louder or weaker than another of precisely the same kind, 
one may be quite ready to afiirm ; one may even be ready to say 
that one judges this noise to be about twice or three times as loud 
as the other. But when more precise estimates are demanded, one 
is obliged to hesitate before giving them. Is this musical tone ten 
(or a hundred) times as loud as the other ; or is it only nine and 
nine-tenths (or ninety-nine and nine-tenths) as loud ? Few would 
venture so nice an estimate with any confidence. Yet the case of 
sound is much more favorable than that of most of the senses for 
forming an exact judgment as to its intensity. It would be difficult 
under the most favorable circumstances to affirm that the sensa- 
tion of the light a is twice or three times as bright as that of the 
light h ; or that of the shadow x one-half or one-third as bright as 
y. The comparative intensities of different color-tones are yet more 
difficult to fix subjectively — even in the most indefinite way. This 
particular yellow may seem about as bright a color, of its kind, as 
does the red near it, of its kind. But the precise moment could 
not readily be told when the blue of the sky appears exactly 
twice as intense as the green of the grass. Still further, all esti- 
mates of the quantity of sensation approach the point at which 
they lose their meaning and tend to become absurd, when we com- 
pare, for example, sensations of smell or taste with those of press- 
ure, temperature, or sight. We never say : The rose smells as 
sweet as it looks red ; or the lemon is twice as sour as the sky is 
blue. And yet each qualitatively different sensation is assumed to 
have its place somewhere in that scale of intensities through which 
' Comp. Ktumi)i', Ton psychologic, I., p. 347 f. 


the different qualities may run ; each may, therefore, be compared 
with every other, with respect to the general position which it oc- 
cupies in its characteristic scale. 

§ 4. All things to which terms of quantity apply admit of some 
kind of measurement and comparison with respect to their quantity. 
Sensations, to be sure, are not "things," but rather modes of the ac- 
tivity of mind, excited through the nervous mechanism of sense. 
Nevertheless, since, like material things, they admit of some appli- 
cation to themselves of the terms of quantity ; and since they vary 
in their absolute and relative degrees of quantity, it is not strange 
that experimental science has endeavored to measure sensations, 
and to state laws for their comparison and mutual relations. The 
general question of the quantity of sensation involves an answer to 
two subordinate inquiries. Of these two the first concerns the 
limits within which the different sensations may vaiy in quantity, 
and yet remain sensations of the same sense ; the second concerns 
the law of the relation which is maintained within the limits among 
the various sensations compared. But neither of these questions 
can be answered directly. Sensations cannot be kept constant in 
quantit}-, and measured by the direct application of physical stand- 
ards, whether with a view to fix their absolute or their relative 
magnitude. They are all, however, under ordinary circumstances, 
connected with the action of different forms of physical energy 
upon the nervous system ; that is to say, they are caused by the 
application of stimuli to the nerves, and the changes in the amount 
of the sensations are dependent upon changes in the intensity of 
the stimuli which occasion them. These stimuli admit of changes 
in quantity, which, theoretically at least, are measurable objectively, 
with more or less exactness. Resulting changes in consciousness 
can only be measured by attentive judgment, which directly dis- 
criminates the sensations as varying in intensity, and as being 
greater or less, one than the other, in the scale of impressions 
which experience has framed. 

The problems of the measurement of sensation may then be 
stated as follows : (1) To determine how little and how much of each 
kind of stimulus will produce respectively the least and the 
greatest quantity of each kind of sensation of which the mind is 
capable, or to find the quantitative limits within which sensations of 
each sense are possible ; and (2) to determine the law of the relation 
under which changes in the intensity of sensations, as estimated 
in consciousness, are dependent upon changes in the intensity of 
the stimuli. 

§ 5. Unexpected and insuperable difficulties, however, stand in 


the -way of a direct solution of either of the two above-mentioned 
problems, even in the modified form in which they were last stated. 
For, in the first place, it is only with respect to sensations of press- 
ure and of the muscular sense that we can measure objectively the 
physical energies which act on the nervous end-organs, with much 
approach to perfect exactness.' The amplitude of the acoustic waves 
in the air which originate from a given source would indeed admit 
of exact measurement ; but the modifications which these waves 
undergo before they reach the nerve-cells and nerve-fibres of the 
inner ear are so complicated as to make it impossible to calculate 
accurately the amount of the physical stimulus which is directly 
applied to the end-organs of healing. The photo-chemical and 
thermic effects of light may be measured objectively. But this 
light is not the direct physical stimulus for the fibres of the oj)tic 
nerve, or even for the end-organs of the retina ; and we have no 
sufficient means for estimating the amount of those chemical changes 
in the visual substances, or pigments of the eye, which are supposed 
to be the immediate excitants of the terminal apparatus of vision. 
The case is yet more hopeless with respect to the senses of taste 
and smell ; inasmuch as we do not even know what pi'operties smell- 
able and tastable substances must possess in order to influence the 
nerves of those senses. The objective measurement of the stimulus 
for sensations of temperature also is made difficult by the fact that 
its amount is dependent upon the zero-point of the skin itself, 
since this point is different at different times and for different 
areas of the entire surface, and is always difficult of precise deter- 

Moreover, could we measure with perfect exactness the intensity 
of the stimulus as it is applied directly to the appropriate end- 
organs of sense, our knowledge of the intensity of the necessary 
physical antecedents of the resulting sensations would be far 
enough from complete. How do the end-organs modify the quan- 
tities of the stimuli before they transmit their effect to the conduct- 
ing nerve-fibres ? Precisely how much further modffication do 
these quantities receive in transmission to the central organs, at 
the hands of the conducting nerve-tracts ? What are the laws which 
control the reception, diffusion, and modification of the different 
intensities of the transmitted nerve-commotions, within those parts 
of the nervous mechanism (the central organs), where they become 
the immediate occasions of the rise and change of sensations in the 
mind ? These are questions to which we are absolutely unable to 
give any satisfactory answer. 

' Comp. Wuudt, Philosopliische Studien, 1883, II., hefti., pp. 10 ff. 


§ 6. But if an exact objective measurement of the physical stim- 
uli is intrinsically difficult, an exact subjective measurement of the 
sensations themselves is inherently impossible. Such subjective 
measurement can exist at all only in the form of a judgment which 
compares two or more sensations with a view to pronounce whether 
they are equal in intensity ; or, if unequal, which is the greater and 
which the less of the two. But we have seen that the ordinary 
estimate of the absolute strength of a sensation is able simply to 
assign to it an indefinite position in the scale of its kind. With 
certain exceptions, scientific analysis can do little to exclude the 
uncertainties of the ordinary estimate. These exceptions are all of 
the following kind : Where two sensations of the same quality are 
produced, either simultaneously on difierent corresponding areas of 
the same organ or successively (with the most favorable interval be- 
tween) upon the same area, by amounts of stimulation that are very 
nearly or precisely equal, the attentive mind can discriminate the 
minute differences, or exact equality, of the intensities of these two 
sensations, with a great degree of nicety. The problem of meas- 
uring the quantity of sensations depends, therefore, upon obtaining 
the least observable differences in intensity for each kind of sensa- 
tions, and for every point along the scale of degrees of intensity. 

But in this connection another occasion for doubt and debate 
arises. Is "the least observable difference" of two sensations it- 
self a constant quantity ? The affirmative answer to this question 
is assumed by Fechner ' and all strenuous advocates of the law 
which he defends. It has even been argued that to hold another 
than the affirmative view involves a contradiction in terms." What 
can be meant, it is asked, by a "least obsei'vable difference " in in- 
tensity between two sensations, unless it be that this difference is 
a constant unit for the measurement of those sensations of the 
same kind which lie near the same point in the scale ? If the dif- 
ference is more than just observable, then of course it is not the 
least observable ; if it is less, then it is not observable at all — that 
is to say, there is no change in sensation. But to this argument 
the following reply is pertinent : The "least observable difference " 
is not itself a mental entity or a mental state, that can be measured 
and used as a unit for measuring the quantity of other mental 
states. For example, if the addition of n to the stimulus *S^ is the 
smallest amount that will produce such a change in the mental 
state X as to cause it to pass over into x\ which the mind recog- 

' Elemente d. Psychophysik (1860\ i., p. 54 f. ; In Sachen d. Psychophysik 
(1877\ p. 45 f. ; Revision d. Hanptpunkte d. Psychophysik (1882), p. 18 f. 
■ Comp. the first edition of Wundt's Physiolog. Psychologie, p. 294. 


nizes as having a greater quantity of sensation than x, such fact is 
to be stated and accepted as a mere fact ; it does not follow, how- 
ever, that we may conclude that x' — x = A, and that this A is en- 
titled to a name (" least observable difference ") and a rank among 
the mind's experiences by way of sensation. There are no seiisa- 
tl07^s (whatever physical occasions of sensations may exist) except 
those that appear in consciousness ; ex hypothesi, there appear in 
consciousness only x and x', and no sensation whatever Ijing be- 
tween the two in intensity. We judge, indeed, that the intensity 
of x', now present in experience, is greater than was the intensity of 
X, now remembered as an image of past experience ; but A (or x' — x) 
is a mere abstraction, a figment of the experimenter's brain, and not 
a real experience of the person with whom he is experimenting. 

Moi'eover, if A were capable in any case of being regarded as a 
unit of subjective measurement, it would by no means follow that 
its mental value is a constant. That n, or the amount of stimulus 
which must be added to S in order to produce an observable change 
in the quantity of sensation, is not constant we know beyond doubt. 
For the different senses, for different individuals, for different de- 
grees of the absolute stimulus {i.e., value of S), for different con- 
ditions of the organs of sense, this amount n is constantly varying. 
The amount of A may also be held to vary, according to psycho- 
logical changes in the means and power of mental discrimination, 
such as we have no way of measuring objectively. For we must 
again insist upon the fact that the real quantity of a sensation is 
not the same thing as the estimated quantity of the same sensation. 
The " least observable difference " would not, therefore, necessarily 
be the same as the least real difference, between two sensations.' 
It is not the mind's custom to attend accurately to the changes in 
quantity of its sensations as such. Properly speaking, many con- 
siderable changes in our sensations, as we may judge by the guid- 
ance they give to the bodily motions and the mental train, do not 
appear in consciousness with a label of exact quantitative measiire- 
ment, as it were, attached to them. 

It is therefore obvious, from the great difficulties which belong 
inseparably both to the objective measurement of the stimuli of 
sensation, and to the subjective measurement of the resulting sen- 
sations, that any law of their relation can have only an indefinite 
statement and a secondary value. 

§ 7. Two methods of determining the lower limit, or minimum 
of stimulus producing a sensation, are possible. In the use of one 
method, a weak stimulus, but somewhat above the amount needed 
' Comp. Stumpf, Tonpsychologie, I., p. 51 f. 


to produce a sensation is applied ; its intensity is then diminished 
by minute gradations until the exact point is readied and noted at 
■which it ceases to produce any sensation at all. In the use of the 
other method a stimulus too weak to produce any sensation is first 
applied ; its intensity is then very gradually increased until it 
begins to produce the smallest observable sensation. Both ways 
may be combined, and thus the " sensitiveness " of each organ of 
sense, and of each part of each organ, may be determined. Such 
sensitiveness increases, of course, in inverse ratio to the amount of 
stimulus necessary for producing any sensation at all, or for pro- 
ducing a sensation estimated as having a definite degree of energy. 
The effort to determine the lower limit of sensations of sight and of 
sound is embarrassed by the facts that the retina is always under 
excitation from the chemical changes going on in its pigments, and 
therefore has a certain quantum of so-called " light of its own," 
and that such a thing as " absolute stillness " cannot probably be 
secured for the ear. Total absence of sensation in the ear, could 
it be secured, would not be comparable to the black which we see 
with the eyes closed.' 

The upper limit, or maximum amount of stimulus which the ner- 
vous organism can receive, cannot be determined experimentally. 
The use of excessive quantities of stimulus is not only too fatigu- 
ing but also too dangerous to the structure of this organism (for 
example, of blinding light upon the eye, stunning noise in the ear, 
etc.) to admit of successful experiment in this dii-ection. Moreover, 
the application calls out so much of those varied forms of feeling 
which are allied with all the specific sensations as to overwhelm 
the latter with the former. Very concentrated, sour, or bitter solu- 
tions, or very intense odors, are not simply tasted and smelled ; they 
are oho felt with aU the adjoining parts of the body. Very strong- 
light and very loud noise do not simply heighten the specific sensa- 
tions of sight and hearing, they rather destroy them in a flood of pain- 
ful feeUng. We may affirm in general, however, that the " capacity " 
of each sense varies directly as the amount of stimulus which it can 
receive. The " circuit " or range of the sensations of each sense 

C 1 

may then be said to be — where _ stands for the measure of the 

sensitiveness, and C for the measure of the capacity, of each sense." 

^ On the question whether absolute stillness is possible, and whether the ear 
has any sensation comparable to the black of the eye, see Lotze, Medicinische 
Psychologie, p. 218 ; Volkmauu von Volkmar, Lehrbuch d. Psychologie, 
1884, I., p. 273 ; and Stumpf, Tonpsychologie, I., p. 380 f. 

2 See Wundt, Physiol og. Psychologie, i. , p. 324. 


§ 8. There are tliree methods of determining experimentally tke 
least observable differences in sensations. These are called, (1) the 
method of least observable difference ; (2) the method of average 
errors ; (3) the method of correct and mistaken cases. Of the 
three methods, the first bears the name which suggests the real 
subject of investigation in them all. This method is divided by 
Wundt ' and others into two — namely, the method of mean grada- 
tions of sensation, and the method of minimum changes of sensation. 
Bat these are really only two modes of applying one method. In the 
one case an attempt is made to form a scale of stimuli whose intervals 
correspond to equally large intervals in our estimate of the resulting 
sensations, by judging what amount of the stimulus produces a sensa- 
tion (M) that lies exactly midway between two other sensations (A and 
O) separated by a clearly perceptible interval (hence A : M : : M : 0). 
Between A and M another middle terra, the sensation of magnitude K, 
may then be sought and found ; and so on until the limit of observ- 
able differences is reached. This mode is, however, less comprehen- 
sive and fruitful than the second mode of applying the same principle. 
The " method of minimum changes in sensation " seeks directly to 
establish, all along the scale of intensities of the stimuli, that change 
in their strength which is just enough (and no more than enough) 
to produce a minimum change in sensation. Such minimum 
change may be conceived of as standing just on the ''threshold" 
of our power to make distinctions in the degrees of strength with 
which our sensations are apprehended in consciousness.^ 

The " method of average errors" (2) begins by fixing upon some 
given sensation which is known to bo caused by a given intensity 
of stimulus ; the attempt is then made also to fix upon another 
stimulus, by means of the sensation it produces, as being exactly 
equal to the former. The trial results in a number of guesses that 
are more or less out of the way. By averaging all the cases of trial, 
the degree of sensitiveness to distinctions is discovered. In other 
words, the method attempts to determine, at each point along the 
scale and for each kind of stimulus, the differences in the strength 
of stimuli that are just below the amount necessary to make an ob- 
servable difference in the resulting sensations. 

In the " method of correct and mistaken cases " (3) minute ad- 
ditions or subtractions of the amount of stimulus are made, with 
the intent of seeing how many cases of right and how many of 

' See Wundt Pliysiolog. rsycliologie. i. , p. 325 f., and comp. his Philosoph- 
ische Studien, 1881, p. 8 f. 

-Called "Unterschiedsschwelle" by Fechuer, Elemented. Psychophysik, 
i.,p. 242. 


wrong guessing, respectively, will result for each of the different 
positions in the scale of the stimuli, and for each kind of stimulus. 
If, then, the proportion of the number of correct to mistaken guesses 
is kept the same for all points of the scale, the amount of change 
in the stimulus necessary for this may be held to measure the sen- 
sitiveness to differences which belongs to each of these points. 
Thus, let n = the whole number of guesses, and r — the number 

of right guesses ; then — = the sensitiveness to differences. But 

the positive value of this quotient being kept unchanged, the 
amount of stimulus added to or subtracted from the original 
amount will measure the sensitiveness to differences for all points of 
the scale. Tiiis method has been largely used and warmly defended 
by Fechner ' in experimenting with sensations of pressure. Much 
doubt has, however, been thrown upon the use made of it by this 
observer ; and especiall}'' upon the propriety of reckoning the doubt- 
ful cases one-half to the right and one-half to the wrong guesses.^ 

A comparison of the above-mentioned methods shows that they 
ai'e all simply different ways of measuring the sensitiveness of the 
mind to minute differences in the quantity of its sensations as de- 
pendent upon changes in the intensity of the stimuli. They should 
never be employed, therefore, without taking into account the 
fact that various other causes, besides such objective changes in 
the stimuli, always co-operate to determine the degree of this 
mental sensitiveness. To eliminate these other factors from the 
calculation is by no means easy. 

§ 9. The one law which claims to be a scientific expression of the 
relations between changes in the intensity of stimuli and changes 
in the quantity of the resulting sensations is that known by the 
name of E. H. Weber. This observer originally used the method 
of least observable differences as applied to sensations of pressure 
and to the measurement of lines by the eye.' " Weber's law " has 
been elaborated, confirmed by a vast amount of espei"iment, and 
defended as a psycho-physical pi'inciple of the widest application, 
by Fechner (in the works referred to, note, p. 361). The significant 
addition which' Fechner has made to Weber's law consists in the 
assumption that all just observable differences are equally great. ^ 

' Elemente d. Psychophysik, i., pp. 98-120. 

" On this point see, especially, G. E. Miiller, Grundlegung d. Psychophy- 
sik, p. 36 f. ; and Wundt, Physiolog. Psychologie, i. , p. 330 f. 

^ Especially in articles on the sense of toiich, in R. Wagner's Handworterb. 
d. Physiologie, III., ii. ; and Archiv. f. Anat, Physiol., etc., 1835, pp. 153 ff. 

* On this point comp. Funke, in Hermann's Handb. d. Physiol. , III. , ii. , p. 
349 f. ; and Wundt, Philosophische Studien, II., Ileft 1, p. G f. 


It is therefore also called " Fechner's law." As an empirical law 
it attemjDts to put into scientific form, on the basis of experimen- 
tal investigation, the truth of ordinary experience — namely, our 
estimate of the difference in amount between two sensations is 
not directly proportioned to the real difference in their stimuli, 
but the latter must increase faster than does the former. For ex- 
ample, the difference in the intensity of the shadows cast by one 
and by two wax tapers is very perceptible in a dimly lighted room, 
but is altogether unobservable in open sunlight ; or the strength 
with which two clocks tick can be discriminated with much nicety, 
but not the amount of noise made by two successive discharges of a 

In other words, if we assume that the least observable difference 
in sensations may be regarded as a constant quantity, then, in 
order to produce this increase or decrease in the amount of sen- 
sation, the addition or subtraction of a much greater amount of 
stimulus is needed for the higher than for the lower portions of the 
scale. Weber's law undertakes to tell us how much greater such 
required amount of stimiilus must be. It admits of statement in 
the several following ways : The difference between any two stimuli 
is experienced as of equal magnitude in case the mathematical re- 
lation of those stimuli remains unaltered ; or, If the intensity of 
the sensations is to increase by equal absolute magnitudes, then 
the relative increase of the stimulus must remain constant ; or. The 
strength of the stimulus must ascend in a geometrical proportion 
in case the strength of the sensation is to increase in an arith- 
metical proportion.* 

1 See Wundt, Physiolog. Psychologie, i., p. 335. For the detailed mathe- 
matical discussion and expression of Weber's law the reader is referred to 
the technical works, especially of Fechner and G. E. Miiller. A simple state- 
ment of Weber's principle may be given as follows : Let H= the intensity of 
the light of one-half of a white field ; j-^ = the smallest fraction of stimulus 
added to H that will produce an observable increase in this intensity ; and 
JET H — ~ = the intensity of the other half of the same field. Then let 8 = 
the sensation produced by II; S + s = the sensation produced by 11+ 77777. 
and s will, of course, represent the so-called least observable difference at this 
point in the scale. We have, then, H produces S; H + 77577, or |{ji ZT, pro- 
duces S + s; lolf £[+ LP. " ^ , or igi-ISr) H, produces 8 + s +s; and so on. 
That is to say, if s is to be kept of the same magnitude, then i/must be mul- 
tiplied by the same magnitude {{f,}-,)- 

The three fundamental formulas which Fechner has employed to state and 
demonstrate the law are the following : Let 8 be the magnitude of the sensa- 
tion caused by the stimulus 2, and AS a just observable increase in this sen- 


The empirical data upon which the advocates of Weber's law rely 
are very numerous, but their value and trustworthiness are often 
much diminished by the fact that most experimenters have failed 
to isolate sufficiently the exact problem which it was desired to 
solve. Nevertheless, the data show that the law summarizes many 
facts reasonably well within a certain range of sensations lying near 
the middle of the scale of quantity. Near both the upper and the 
lower limits the law fails to prove applicable ; even in the regions 
and under the circumstances which are most favorable it is only 
approximately true. Many fluctuations of unknown significance 
and origin occur in all the senses. 

§ 10. In determining the least observable sensations of touch, 
the result is largely dependent upon the presence of muscular sen- 
sations also. It further depends upon the method in which the 
comparison is made ; for, as Weber discovered, an actually present 
sensation can be compared with the remembered image of one just 
past better than two present sensations can be compared. The in- 
terval of time and the locality of the organ have also a great influ- 
ence. Most persons observe a stronger sensation of pressure when 
the weight is laid on the left than when it is laid on the symmetri- 
cal place of the right side. The same amount of surface must be 
covered, and the objects compared must have the same temperature, 
in order to secure trustworthy results of experiment. Weber 
found that, when the interval was fifteen to thirty seconds, under 
the most favorable circumstances, 14|- could be distinguished from 
15 grammes, or 14^ from 15 ounces. That is, some persons can 
distinguish weights which differ as 29 : 30, by the sensations of 
pressure they occasion, when laid on the volar side of the last 
phalanges. By raising the weights the nicety of discrimination can 
be increased so as to be represented by the proportion 39 : 40. 

sation wliicli is caused by an increase of the stimulus = A2. Let (7 be a con- 
slant dependent on tbe values chosen for ;!? and 2. Then AiS = -^. Let it 
be further assumed that A/S remains constant whatever valiies for 8 and A2 
are assumed; then dS = -^, and by integration S=C log. 2, which is 
Fechner's "fundamental formula." But if the stimulus is just belotcthe least 
observable amount, and be = 2% then substituting in the above formula we 
have = C log. 2' ; from which Fechner derives formula No. 2 (the formula 

of measurement), namely, 8 =^ C log. — , which means that the magnitude of 

the sensation is " negative,^'' in case the stimulus sinks below the least observ- 
able = 2'. If two sensations {8 and 8) are observably different, then 6' — 8' 
= C (log. 2— log. 2') ; this is called the "formula of difference," and means 
that the difference in the intensity of two sensations is proportional to the 
logarithm of the quotient of the magnitudes of their stimuli. 


By an extended series of experiments with weights ranging from 
300 to 3,000 grammes Fechner ' emploj^ed the method of correct 
and mistaken cases to confirm Weber's law as applied to combined 
sensations of pressure and of the muscular sense. Some experi- 
ments were made with both hands ; others with the right or left 
separately. The weight used to add or subtract was either 0.04 or 
0.08 of the absolute weight. The results showed that the law held 
only approximately for all the series of experiments, and not abso- 
lutely for any one series. As calculated by G. E. Miiller ° they 
give, instead of a constant quotient to express the degree of sensi- 
tiveness (as Weber's law requires), a quotient varying from ^j-^vg- for 
•weights of 300 grammes to ^^ for weights of 3,000 grammes. Nor 
can Fechner's effort to correct the variation, by introducing after- 
ward a conjectural allowance for the weight of the arm itself, be 
considered successful.^ Biedermann and Lowit, by the method of 
just observable differences, obtained results departing widely from 
Weber's law.* By experimenting with weights varying from 10 to 
500 grammes they found that the sensitiveness to pressure rose 
with the increase of the weights from 10 to 400 grammes, and then 
fell off rapidly, as the following table will show : 

Quotient of sensitiveness. 



The trustworthiness of these results is impaired, however, by the 
fact that no method, except the doubtful one of directing "atten- 
tion " exclusively to the sensations of pressure, was employed to 
exclude the disturbing effect of the muscular sensations. The 
same observers concluded, also, that the fineness of the muscular 
sense, when isolated, does not vary according to Weber's law. They 
fixed it at ^\ for weights of 250 grammes, j{j for weights of 2,500 
grammes, -^^ for weights of 2,750 grammes. 

That Weber's law does not hold good, near the lower limits, for 

1 Elemente d. Psjcliopliysik, i,, p. 183 f. 

^ Zixr Grundlegung d. Psychopliysik, p. 197. 

^ In Sauhen d. Psychopliysik, p. 198. 

* See Heriiig, Sitzgsber. d. Wiener Acad., LXXIL, Abth. iii., p. 343 f. 

solute weight. 

Least observable diflference. 




















sensations of pressure, and of muscular innervation and movement, 
is admitted by all. The absolute sensitiveness of these sensations 
differs greatly, as has been shown (p. 346 f.), for different localities 
on the surface of the body. Aubert and Kammler found the light- 
est weight which produced a sensation of touch to be 0.002 gramme 
on the forehead, temples, and dorsal side of the forearm and hands ; 
0.003 gramme for the volar side of the forearm ; 0.005 gramme 
for the nose, lips, chin, eyelids, and skin of abdomen ; 0.005-0.015 
gramme for the volar side of the fingers ; and 1 gramme for the 
fingernails and skin of the heel. This kind of sensitiveness has 
been thought to be chiefly dependent upon the number of the 
nervous elements present in the skin, its thickness, the character 
of its tension over the underlying parts, etc. ; but its variations are 
by no means parallel with those of the sharpness of the sense of 
locality. The foregoing and similar conclusions all need to be re- 
vised in the light of Goldscheider's determinations of the pressure- 

§ 11. Extraordinary difficulties accompany the attempt to apply 
Weber's law to sensations of temperature. As has alread}^ been seen 
(p. 350 f.), we do not know exactly what to measure — whether the 
rising and falling of the thermic apparatus, or its actual tempera- 
ture in relation to its own zero-point ' — as constituting the quanti- 
tative changes in the stimuli. Even Fechner admits that Weber's 
law does not apply to the sensitiveness of the hand to changes in 
temperature when it is itself cooling off; but he thinks the law 
holds good approximately for degrees of warmth varying between 
25° and 37.5° C. (77°-99.5° Fahr.), if 18.71° C. (65.66° Fahr.) be 
taken as the zero-point. The assumption of this zero-point is, 
however, arbitrary. No general rule for the quantity of sensations 
of temperature can well be given except this : the akin is most 
sensitive to changes which lie near its oion zero-point. In compar- 
ing two temperatures it is most favorable to nice discrimination 
that one should lie slightly above, the other slightly below, this 
point. The degrees of the thermometer between which the maxi- 
mum of sensitiveness is attainable are given differently by differ- 
ent observers: By Nothnagel, 27^-33° C. (80.6°-91.4° Fahr.) ; by 
Lindemann, 26°-39° C; by Alsberg, 35°-39° C; by Fechner, 12°- 
25° C. — where it is so great as not to be easily measurable by a 
good quicksilver thermometer (about \° Fahr.). Cold and heat alike, 
when applied for some time, reduce greatly the sensitiveness of 
the skin to minute changes of temperature ; by heat it can be so 
dulled as not to distinguish alterations of less than ^° or f° Fahr.; 
' So Heriug, see Hermann's Handb. d. Physiol., III., ii., p. 430. 


bj cold it can be rendered insensible to changes measuring from 
2^ to 5i-=. 

We have already seen (comp. p. 348 f.) that the sense of tempera- 
ture depends for its fineness upon the extent and locality of the sur- 
face excited. Weber found that water at 29^-° E., in which the 
Avhole hand was immersed, seemed warmer than that at 32^ R., to a 
single finger. Nothnagel placed the following values upon the fine- 
ness of discrimination, for minute variations in temperature, of dif- 
ferent parts of the body : Middle breast, 0.6° C; sides of the same, 
0.4°; middle of the back, 1.2°; sides of the same, 0.9°; hollow of the 
hand, 0.5°-0.4°; back of the same, 0.3°; partsof upper and lower arm, 
0.2°; cheeks, 0.4°-0.2°; temples, 0.4°-0.3°. More recent investiga- 
tions have shown that the table of sensitiveness for the different parts 
of the body must take account of the division of the tempei'ature- 
sense into two species, and of the locality of the heat-spots and 
cold-spots in all such different parts. On the basis of experiment 
with areas of the skin whose topography with respect to the tem- 
perature-sense had previously been investigated, Goldscheider has 
given a lengthy statement ' of the sensitiveness of different parts 
of the body. 

Thus he finds that the skin of the head is, in general, little 
developed for the sense of cold, and only in a few places for the 
sense of heat. The sensitiveness of the forehead to cold is intense, 
but to heat only moderate ; that of the breast to cold moderate 
along the sternum, and elsewhere very intense, while to heat it is 
only moderate except near the uiiDj^les ; that of the back everywhere 
very intense to cold, and only moderate to heat ; while in all parts 
of the hand the intensity of sensitiveness to both cold and heat 
is alike. 

In general, the skin in the median line of the body seems much 
less sensitive to changes in temperature than at its sides ; and the 
number of thermic elements (according to Goldscheider, the dis- 
tributory fibrils of the temperature-nerves), the thickness of the skin, 
etc., are determining factors. 

§ 12. The possibility of executing or appreciating a musical 
passage in which the intensity of the successive notes is brought 
to a certain standard of memory, or in which these notes are nicely 
shaded so as to constitute a crescendo or a diminuendo, ajDpears to 
depend upon appljdng to sensations of sound some law resembling 
that of Weber. It is partly by comparing such sensations with 
their images in memory that the singer or player reproduces certain 

' See the Arcliiv f. Auat. u. Physiol., Physiolog. Abth., 1885, Supplement- 
Band, pp. GO If. 


notes previously executed, with about the same stress of tone.* 
Moreover, in order to shade the relative intensities of successive 
tones, our appreciation of their differences needs to be much greater 
for those that have a low degree of intensity. Many obstacles, 
however, stand in the way of determining either the lower limit 
or the least observable difference for sensations of sound. The 
general dif3S.culty which belongs to investigating the intensity of 
sensations, even under the most favorable circumstances, is here 
enhanced by the facts, that the pitch and timbre of each clang have 
much to do with our judgment of its strength ; that different ears 
differ so widely in their organic susceptibility, while the mind is 
peculiarly sensitive to changes of feeling and judgment connect- 
ed with sensations of sound, and thus very weak sensations are 
vacillating and unsteady in consciousness, and sounds appear and 
disappear in the ear while the degree of stimulus and of attention 
are unchanged ; that the reflection and interference of the acoustic 
nerves, their distance and direction, and the absence or presence of 
" entotic " sounds, are so influential ; and, finally, that it is impossi- 
ble to discover a sounding apparatus of definitely ascertainable and 
uniform intensity of action. 

§ 13. None of the means employed for determining the amount 
of stimulus necessary to produce the weakest sensations of sound, 
or the least observable differences in such sensations, are entirely 
satisfactory. The method of listening to noises made by falling 
weights is rendered uncertain by the fact that the character, both of 
the weight an'd of the surface on which it strikes, has so much in- 
fluence. Moreover, it is a matter of dispute whether the intensity 
of the stimulus is to be measured by the product of the mass into 
the height from which the body falls (m x h) or into the square- 
root of that height (in x Vh). It is possible that neither of these 
measurements is exact. ^ Assuming the former to be correct (noise 
= m X h), by using a sound-pendulum A. W. Volkmann found that 
differences in intensity are observable when they stand in the pro- 
portion 3 : 4. Vierordt, on the other hand, concluded that the latter 
measurement (noise =. m-^h) is more nearly correct; and by assum- 
ing Vierordt's view, and using iron balls that fell vertically on a vi- 
bx'ating plate, Norr ^ attempted to fix a unit of measurement. This 
unit h& made = 1,500 milligramme-millimetres vsdth the ear distant 
50 ctm. from the soiu'ce of the sound. Experimenting with sounds 

' Comp. Stumpf, Tonpsychologie, I., p. 345 f. 

''■ Comp. Wundt, Physiolog. Psycliologie, i., p. 341 and note ; and E. Tiscber, 
in Philosophische Studien, I., heft 4, p. 543 f. 
- Zeitschrift f. Biologic, 1879, XV., p. 297 f. 


ranging in intensity from those a little above the least observable to 
those of unpleasant strength (1.71-524167.8 times the unit), and 
dividing the entire scale into 7 groups, w^ithin each of which about 
1,000 experiments were conducted, he found that the proportion of 

right guesses to the entire number made ( — ) remained approxi- 
mately constant — that is, that Weber's law holds for sounds of vary- 
ing iutensit}'. A large allowance, however, had to be made for 
relations of time ; the percentage of correct guesses being about 8.7 
larger when the sound of greater intensity followed that of less 

More recently, E. Tischer ' has apjDarently added some evidence 
to the validity of Weber's law by experimenting with an improved 
form of the method of Vierordt and Norr. Keeping one of the two 
sounds to be compared at a constant intensity, he increased or 
diminished the other until from 4 to 6 successive correct guesses 
as to their relative value were obtained. But the fact that, when 
the second stimulus was diminished until certainty of judgment 
was obtained, very considerable unexplained variations from the 
results expected by Weber's law occurred leaves much doubt still 
hanging over the matter. In order to harmonize the conflicting- 
results, the proposal has been made to introduce another function 
into the formula, noise = m h or my/h. All the investigations, 
therefore, still leave the question of the applicability of Weber's 
law to sensations of sound in a rather uncertain condition. 

Little or nothing has been accomplished by expetiment to de- 
termine whether the same law applies to the intensity of musical 
tones. Among the various factors which enter into our judgment 
of the intensity of tones, the " color-clang " is especially influential." 
Absolute pitch and intervals of pitch are also very important. In 
general, tones and noises of a higher pitch, with an equal objective 
intensity of stimulus, are judged to be louder than those of a lower 
pitch. Konig showed that a tuning-fork of the pitch c must have 
about four times the amplitude of vibration required by one of the 
pitch C, in order to produce upon the ear the same effect from the 
same distance. 

§ 14. The various attempts to determine the lower limit of sound 
for the human ear have not resulted in any precise statement. 
Schafhiiutl, after experiments in as near perfect stillness as possi- 
ble, at midnight, fixed the limit at the noise made by a cork ball of 
1 milUgrarame weight (about 0.0154 gr.) falling from a height of 1 

' Wundt's Philosophisclie Studien, 1883, I., heft 4, pp. 495 fE. 
'■'Comp. Stumpf, Toiipsychologie, I., p. 364 f. 


millimetre (0.03937 inch). Boltzmann and Topler have reached 
results which Hensen' considers to be as accurate as possible. By 
measuring the compression of the air at the end of an organ-pipe of 
181 vibrations per second, they calculated that, even under circum- 
stances not as favorable as possible, the ear responds with sensation 
to an amphtude in the vibration of the molecules of the au' not more 
than 0.00004 mm. at the ear, or about j\ the wave-length of green 
light. The mechanical work done upon the ear-drum in a single 
vibration of such small intensity is reckoned at not more than j^-g 
billionth kilogrammetre ; or only about J^ of that done upon the 
same surface of the pupils of the eye by a single candle at the 
same distance. These calculations indicate that the motions in 
the cochlea which excite the end-organs of sense are astonishingl}' 
minute — far too minute to be observed even by the microscoioe. 
Yet the sharpness of hearing maybe enormously increased by dis- 

§ 15. Judgments of the intensity of sounds are dependent also 
upon practice, and upon other psycho-physical conditions such as 
determine the nicety of all judgments of quality. Small impres- 
sions of noise are apt to have their intensity underestimated ; the 
inclination to do this has been attributed to the influence of our 
custom of withdrawing attention from them altogether under ordi- 
nary circumstances.^ The fact that sounds to which we become 
accustomed lose most of their intensity in consciousness must be 
explained chiefly under the same law of mental habit ; it cannot, on 
the other hand, be largely due to the physiological law of exhaus- 

§ 16. Attention was early called to the law of judgment in esti- 
mating the quantitative relations of sensations of sight, on account 
of its connection with astronomical observation. In the preceding 
century French physicists had already begun to investigate the 
sensitiveness of the eye to varying intensities of light. Bouguer, in 
answer to the question, What force must a light have in order to 
make a more feeble one disappear? placed the fraction of least 
observable difference in the intensities of two shadows at gJj. That 
the magnitudes of the stars are not to be classified according to 
their absolute brightness as determined by photometric observa- 
tions was, of course, assumed by Sir John Herschel when he made 
the latter vary in the series 1 : ^ : |- : J^, while the former vary in 
the series 1:2:3:4. That the least observable difference in the 
intensity of two sensations of sight is absolutely much smaller for 

' See Hermann's Handb. d. Physiol.. III., ii , p. 117 f. 
^ So Stumpf, Tonpsychologie, I., p. 388. 


those of the lowest grade of intensity is a truth needed to explain 
many every-day experiences. For example, the finer gradations of 
shade in a lithograph or photograph are not lost when we take it 
from the open sunlight into a rather dimly lighted room ; we can 
also observe them through smoked glass, if it be not too black. 
Through the same media we can measure rather delicate shades of 
brightness on the clouds. We observe, however, that in all such 
cases either too great or too weak intensity of the light destroys 
our power to distinguish the finest gradations of its intensity. 

§ 17. It has already been shown (p. 326) that the retina is never 
free from light of its own which has a varying intensity ; this fact 
greatlj' increases the difficulty of fixing accurately either the lower 
limit or the least observable difference of visual sensations. lu the 
effort to apply Weber's law to sensations of color, the laws of change 
in the quality operate to obscure the laws of change in the quantity 
of the sensations. Experiments with shadows for the sake of testing 
Weber's law were first conducted by A. W. Volkmann and others, 
under the direction of Fechuer.' By measuring the distance to 
which a candle must be removed from an object in order that the 
shadow produced by its light might disappear in that of another 
candle of like intensity situated at a fixed near distance from the ob- 
ject, the quotient for the least observable difference was found to be 
y-J-y. This quotient was also found to remain nearly constant for 
absolute intensities varying from 1 to 38.79. If, however, the light 
of the background diminished to 0.36 in intensity, marked varia- 
tions in the law occxuTed ; the difference in the brightness of the two 
shadows had then to be greater than yj-o to be observable. Later 
experiments of the same observer yielded results less favorable to 
Weber's law.^ The quotient was found to vary from j^ for weak 
intensities of light to j^^j for stronger intensities. 

By using rotating disks and comparing the grayish circles made 
upon them when revolving rapidly, through the admixture of small 
black stripes with the white of their surfaces ("Masson's Disks"), 
Helmholtz ^ found the medium value of the quotient of least observ- 
able difference to be y^^ ; this quotient is not constant, however, and 
increases, especially for sensations near the upper or the lower limit. 
By changing the method somewhat, Aubert obtained a variation 
of yJ-^ to yI-o i^ the degree of sensitiveness to differences in the 
brightness of lights, even when not going above the middle of the 
scale of intensity. Experiments with such intensities as lie nearest 

' See Elemente d. Psychophysik, p. 148 f. 

* A. W. Volkmann, Physiolog. Untersuchungen, I., p. 56 f. 

^ Physiologisclie Optik, p. 315 f. 


the limits showed much greater dej^artures from Weber's law. Just 
above the lower limit, au addition of even :^ to ^ to the stimulus 
might be necessary in order to produce an observable difference 
in the resulting sensation. Similar results have been obtained by 
Delboeuf, but, on the whole, more favorable to Weber's law than 
the results of Aubert. 

A more accurate and carefully guarded series of experiments 
than any of the foregoing is recently reported by Dr. Emil Kj-ae- 
pelin.' This experimenter used the method of just observable dif- 
ferences as applied to Masson's disks when looked at through gxay 
glasses of varying intensity. The utmost care seems to have been 
taken to exclude disturbances from changes iu the adjustment of 
the eye, retinal exhaustion, reflection of light from surrounding- 
objects, etc. Three groups of experiments were conducted — one 
by daylight, one by candlelight, one by lamplight. Both eyes 
were experimented upon ; and both directions of alteration in the 
intensity of the stimulus (stronger following weaker, and vice versa) 
were employed. Kraepelin concludes that for the unexhausted eye, 
with a good power of accommodation, the fraction which gives the 
least obseiwable difference remains constant, while the intensity of 
the light varies between values of 1,000 and 9.61 of absolute inten- 
sity as fixed for his experiments. That is, within these limits the 
law of Weber holds good as expressing with closely approximating 
accuracy the results of experiment. 

The experiments of Dobrowolsky ^ and Lamansky ^ with light of 
the different spectral color-tones shows that, with these sensations 
also, Weber's law holds approximately good for moderate intensi- 
ties, but is subject to considerable variations a3 we approach the 
upper and the lower limits. The former used the method of com- 
paring a white surface with one in which colored light had been 
mixed with the white. On changing the absolute intensity of the 
light between values of 1 and 0.0302, only a slight variation in the 
quotient indicating the least observable difference of intensity ap- 
peared for the color red. This quotient was found, by the same 
observer,^ however, to be very different for different color-tones : thus 
for red, -^^j ; yellow, J^- ; green, -^^ ; blue, y^^ ; violet, j^. Laman- 
sky and others have made the sensitiveness to changes in the inten- 
sity of color-tones greatest for green instead of violet ; and have ob- 
tained other results different from those obtained by Dobrowolsky. 

§ 18. The minimum of the intensity of light ajDpreciable by the 

1 In Wundt's Philosopliisclie Studien, 1884, II., lieft 2, pp. 306 ff. 

^ Pfluger"s Arcliiv. xii . p. 441 f. 

3 Arcliiv f. Ophthalmologie, XVII., i., p. 123 f. ■* Ibid., XVIII., i., p. 74 f. 


eve under the most favorable circumstances was fixed by Aubert at 
^^ of that reflected from white paper in the light of the full-moon. 
This result can only be considered as approximate. The individ- 
ual factor in all such calculations must be held to be very large and 
variable ; especially, perhaps, if we admit that there is a class of 
so-called " sensitives " to whom the ends of an electro-magnet 
when excited appear luminous, as Eeichenbach's experiments seem 
to show. Weber applied his own law to so-called extensive sensa- 
tions of sight. He showed that in judging of the comparative 
length of lines the least observable difference is, for each person, a 
tolerably constant fraction of the absolute length of the line with 
which the comparison is made. This fraction is different for differ- 
ent persons ; and has a range from J-^ to j^. Fechner ' defends 
the validity of the law for lines of lengths varying between 10 and 
240 mm. (| to 9^ in.), with the eye removed from 1 ft. to 800 mm. 
(12-32 in.). The lower limit for such cases has been fixed by A. W. 
Volkmann at lines of length from 0.2 to 3.6 mm. It is obvious, 
however, that we are here not dealing with pure quantity of visual 
sensations, but with judgments of local relation which, in case the 
eyes are moved, have their basis, at least partly, in our power to dis- 
criminate minute differences in the sensations of the muscular* sense 
connected with such movements. 

§ 19. The law of Weber can, of course, derive little or no sup- 
port from sensations of taste and smell. In the case of these two 
senses our knowledge of both series of quantities — of the intensity 
of the stimulus and of the amount of specific sensation which re- 
sults from its application — is altogether too inadequate to admit 
of trustworthy comparison. We cannot measure forms of energy 
like those by which smellable particles and tastable solutions act 
on the end-organs of sense, until we have a unit of measurement and 
some information as to what the object is to which the standard 
should be applied. Nor can we compare amounts of sensations 
that are so largely matters of indiridual origin and capricious 
change, and that are so overlaid with other forms of feeling, as are 
the sensations of these senses. Moreover, the element of time — both 
as respects the interval elapsing between the two sensations com- 
pared and also the order in which the sensations follow each other 
— is here a very important influence. 

The irdensl.ty of taste depends upon a variety of circumstances 
besides the objective quantity of the stimulus. Among these cir- 
cumstances is the extent of surface excited. Camerer ^ found by 

' Elemente d. Psychophysik, i , p 211 f. 

- See Zeitschr. f. Biologie, 1870, VI., p. 440 f. 


experimenting with common salt in solutions of different degrees 
of concentration that the number of correct guesses inci-eased almost 
in exact proportion to the number of gustatory papillse upon which 
the solutions were placed. Certain mechanical and thermic con- 
ditions also have a great influence. Substances even in fluid form, 
when quickly swallowed, have little taste ; pressing and rubbing 
against the gustatory organs, movement of the tastable matter in 
the mouth, increase the excitatory effect of the stimuli. It is 
doubtful whether this effect is due solely to the mechanical result 
of spreading the stimulus over the surface and urging it into the 
pores against the end-orgaus of the sense, or in part also to some 
direct physiological cause. The influence of temperature on the 
intensity of sensations of taste is well known. Weber showed that 
if the tongue is held for |- to 1 minute in very cold water, or in wa- 
ter of about 125° Fahr., the sweet taste of sugar can no longer 
be perceived. Cold also destroys for a time the susceptibility to 
bitter tastes. Keppler '• endeavored to test Weber's law by deter- 
mining the sensitiveness to minute changes in the four principal 
kinds of taste ; and arrived at a negative result. Fechner, how- 
ever, considers that Keppler's experiments with common salt confirm 
Weber's law, and that his other experiments were not adapted to 
yield any assured result. We can only repeat the statement that 
other causes than mere increase in the quantity of the stimulus so 
largely determine the intensity of the resulting sensations as to 
discredit any arguments from the experiments either for or against 
applying Weber's law to sensations of taste. 

§ 20. The exj)eriments of Valentin' and others, to determine how 
weak solutions of various substances will excite the end-organs of 
taste, are chiefly valuable as gratifying our curiosity. The figures 
are not to be accepted as exact, but as showing in general the ex- 
treme fineness of this sense, and the great difference of different 
substances in their power to excite it. Valentin found, for exam- 
ple, that 0.24 gramme of a solution containing 1.2 per cent, of cane- 
sugar excited the sensation of sweet ; a solution containing j|-g- 
part of common salt was scarcely detectable ; of sulphuric acid 
To oVo'o' could be discerned, yro ottotf ^°^ '■> extract of aloes contain- 
i^^o g'o oVTro could be distinguished from distilled water ; -33^0 
of sulphate of quinine was plainly observable, and the observer 
thought he could detect a slight trace of bitter when the solution 
was diluted to foTr^oin) o^ ^^i^ substance. In general, a smaller 
absolute quantity of stimulus, when in a relatively concentrated 

' Pfluger's Archiv, 1869, ii., p. 449 f. 

"^ In his Lehrb. d. Physiol, d. Menscheu, 2 ed., Abth. 2. 


solution, will suffice to excite tlie end-organs of taste.' It will 
readily be seen that the minimum of some of these substances 
which will give rise to a sensation under the most favorable cir- 
cumstances is exceedingly small. 

§ 21. The intensity of sensations of smell is also largely depend- 
ent on other causes than changes in the quantity of the stimuli. 
The amount of sensation appears to be largely governed by the 
extent of surface excited ; since it is greater when we smell with 
both nostrils, and with the current of inspiration which carries the 
exciting particles over more of the sensitive membrane. No as- 
siu'ed results on this point, however, have yet been reached. Val- 
entin supposes that a smaller number of odorous particles will 
excite sensation if presented in a concentrated rather than a dilute 
form. When the intensity of the stimulus increases beyond a cer- 
tain point, the character of the resulting sensation changes — often- 
times from a pleasant to an unpleasant tone of feeling. All are 
familiar with the fact that a large increase of some smells — for 
example, musk — does not give the same kind of sensation. This 
sense has a great degree of "sharpness," or power to be excited 
by small quantities of stimulus, as distinguished from "fineness," 
or power to distinguish minute variations in the sensations. It is 
undoubtedly different in different species of animals, as dependent 
upon unknown differences in their psycho-physical constitution ; 
but it is tolerably uniform among men where there is the same 
cultivation of it, and the same concentration of attention. It is 
well known that certain animals have an astonishing fineness of 
smell, and are able by it even to detect the individual variations 
that are quite imperceptible to man. Little value can be attached 
to the results reached by experiments to fix the least number of 
smellable substances which can excite the human end-organs of 
this sense. In general, we can say that incredibly small quanti- 
ties of some substances will suffice. Valentin found that a curx'ent 
of air containing -g-oiiVoTr of vapor of bromine excited a strong un- 
pleasant sensation. Atmosphere polluted with even jYiyl-u-o-Tr ^^ 
sulphuretted hydrogen could be detected. It was calculated by 
this observer that -jo"o J-oo u "^^ ^ milligramme of alcoholic extract of 
musk is about as little as can be perceived. The effect of constant 
over-excitement of the organs of this sense, in deadening their sen- 
sibility, is too well known to require illustration. No argument 
for or against Weber's law can safely be drawn from sensations of 

§ 22. A review of the preceding facts confirms what was previ- 
' See Camerer's table in Pf