attention, in so far as it bears on the physiological basis of popular education.

When the upper half of the human skull is removed, masses of grey nervous matter are exposed, known as the right and left cerebral hemispheres. The outer layers of nerve cells and fibres of these parts of the brain form the cerebral cortex (pallium), or more correctly, the neo-pallium, since the brains of the three lower classes of vertebrates, viz. fishes, amphibians, and reptiles, possess only rudimentary forms of the five layers of nerve cells. and fibres which enter into the formation of the human cerebral cortex. It is to the living substance of these layers of cells that the power of transmuting the specific modes of energy it receives from the sense-organs into psychical or intellectual processes is attributable.

The nerve-fibres which pass from the nervous elements of our eyes, ears, and other sense-organs, terminate in connexion with nerve-cells located in definite areas of the cerebral cortex; these areas are known as sensori-mnemic nervous centres. Thus human beings have visual sensory centres situated in the posterior parts of the brain, auditory nervous centres at the sides of the cerebral cortex, and so on. The nerve-cells of these centres are brought into close relation with one another, and with other parts of the brain by what are called association fibres, because it is along these fibres that the energy discharged by one centre passes to others, and thus an associative system is formed.

The sense-organs are adapted structurally to receive and sift the streams of energy or stimuli which reach them from the outer world and from the movements made by our muscles. By means of the specialised nervous substance which enters into the construction of each of these organs, the energy they receive is transmuted into such a form that on reaching the corresponding cortical nervous centres it produces what we term a sensation.13 The sensation soon passes away, but it leaves an impress on the living mnemic elements of the nerve-cells of cortical centres. This impression takes the form of a latent idea or mental image of the object or movement which has given rise to the impression. Ideas therefore mean the things or the movements, and the contents of an idea, the features of things or of muscular movements. On being re-excited, the living substance on which latent ideas have been established discharges a portion of their working energy, which, in its passage to motor centres-those areas of the brain which control our muscular actions-must pass

13 'What sensations are, we know not, and how it is that anything so remarkable as mental images or ideas comes about as a result of irritating nervous tissue is just as unaccountable as any other ultimate fact of nature.' T. H. Huxley, Elementary Physiology, p. 188.

through what we know as the psychical areas of the cerebral cortex, where it becomes transformed into psychical nerve-force. This, by its action on the elements of the motor centres, leads to the intelligent co-ordinate movements of certain groups of muscles such as those which work the vocal apparatus, or other parts of our bodies or limbs.

The evidence by which the existence of and functions performed by sensory cortical centres is substantiated must here be considered. If, in the lower animals, those parts of the cerebral cortex which are known as the visual centres are destroyed, the animal is rendered completely blind. The same result follows in human beings when the whole of the nervous matter of these centres is destroyed by disease. Under these conditions, although the individual cannot distinguish objects he may be able to think about them, and to hear and have perfect use of his other faculties. On the other hand, if a certain portion only of the visual centres is destroyed, the individual may by aid of the rest of this centre be able to see, but cannot comprehend the meaning of the objects seen; he is mentally blind, because that part of the cerebral cortex has been destroyed on which the latent visual ideas or mental images have been impressed. If another part of the visual cortical centre is destroyed, a person so affected loses the power of distinguishing one colour from another, but may continue to see objects around him and to appreciate their meaning.

Again, the auditory nervous centre is situated in the lower part of the sides of the cerebral cortex. The function performed by one portion of the nervous elements of this centre is to become impressed by the vibrations of sound which reach it through the ears. Latent images of words repeated by another person thus become established in the mnemic elements of this part of the brain. Under ordinary conditions, these charged elements respond to the action of life or of allied stimuli to those which had produced the impression, and the word-sound is reproduced in our memory. If, however, the nervous matter constituting this part of the auditory centres is completely destroyed by disease, a person so affected becomes speechless; the substance in which his latent mental images or ideas of words had been established has been destroyed, and with it the individual's power to make use of the words which formerly existed in his brain, and which he had learnt to employ as symbols to express his thought. And so with the other cortical nervous centres.

In addition to its sensory nervous centres, the human cerebral cortex contains what are called sensori-motor or kinæsthetic centres; that is, the brain possesses aggregations of nerve-cells the functions of whose living matter is to transmute the energy

it receives from sensory centres into nerve-force capable of controlling the movements of groups of muscles. If in a living animal the brain is exposed and a weak electric current applied to definite parts of its motor-cortical area, movements of the animal's limbs, or of special organs, such as the vocal, are brought into action. We may thus map out the motor area of the cerebral cortex into definite spaces, each of which controls the action of a group of muscles, such as those of the fingers, hand, arm, etc.14

Our sense-organs are, therefore, the receivers of energy derived from the outer world and from muscular movements of our bodies and limbs; the cortical sensory centres transform this energy into sensations and latent mental images, or ideas; and discharges of energy take place from the motor-cortical elements which produce definite muscular movements. But in human beings a vast mass of nervous matter intervenes between the sensory and motor centres energy in its passage from the former to the latter has to traverse this mass of intervening cortical matter; and, as we shall endeavour to show, in its passage it becomes psychical nerve-force, and in this form plays on the motor centres, and thus imprints thought and intelligence on our movements or actions.15 The sense-organs have been compared to the receiving station of a telegraphic system, where messages are taken in and despatched to the central office (representing the sensory and psychical nervous centres), where, through the instrumentality of an intelligent agent, the message is despatched to its proper destination, and delivered by a messenger representing a motor centre.

The human brain, unlike that of any other animal, possesses a fully-developed motor speech-centre, which Broca called 'the organ of speech,' because it is through the action of its living matter on the muscles of the vocal apparatus that human beings are able to express their thoughts in spoken words.

It is probable that as a child when we first saw a flower such as a rose,' we asked what it was called, and child-like imitated 14 The size of the sensori-motor cortical areas in the various classes of animals depends on the delicacy and complexity of the movements habitually performed by the muscles under its control, rather than upon the bulk of these muscles. This fact may be demonstrated by comparing the relative size of the sensori-motor cortical centres which control the movements of the muscles of the trunk of a skilled workman with that of the sensori-motor centre which directs the movements of his fingers. Human beings possess a large, welldeveloped sensori-motor centre of speech; in anthropoid apes this area of the cerebral cortex exists only in a rudimentary form.

15 The cerebral cortex, or more correctly the neo-pallium, of human beings covers a superficial area of some 200,000 square m.m.; its cortex is 3 m.m. thick, and contains five layers of nerve-cells. The average bulk of the human brain, the greater part of which consists of its cerebral hemispheres, is 1500 c.c., that of the gorilla is 600 c.c., the bodies of the two animals being of nearly equal weight. See Fig. 16, p. 132, International Scientific Series, vol. xcvii.

the sound and repeated the word 'rose once or more often. In thus articulating this word we brought those muscles of our lips and other parts of the vocal apparatus which are necessary for the production of the word-sound 'rose' into play. Muscular action of this kind is accompanied by the excitation of the senseorgans which form a part of these muscles; their excitation liberates a certain amount of working energy, which passes to corresponding nervous elements located in sensori-motor centres of speech, and leaves on its elements a latent mental impression or idea of the word-sound which has produced the impression. The word rose' thus becomes established in some one or more of the cell-contents of our organ of speech.' If these motor elements are re-excited by similar, or, it may be, by other forms of energy to that which produced the impression, they react in such a way as to excite the muscles of the vocal apparatus to reproduce the sound 'rose.' If from disease that part of the cerebral cortex which forms the motor speech-centre is destroyed, an individual so affected can no longer make use of vocal sounds: he may be able to see, hear, and think, but he cannot express his thoughts in articulate word-sounds, since the specialised nervous matter which regulates the working of the muscles of his vocal apparatus no longer exists.

An object therefore, such as a rose, gives rise at one and the same time to visual, olfactory, and tactual sensations, and to corresponding latent mental images in the cerebro-cortical nervous centres. In addition to these impressions a part of the auditory centre has received and retains in a latent form the word-sound by which we have learnt to distinguish this flower. Lastly, as we have shown, the word 'rose' has become established in a latent form in the nervous elements of cortical-motor centres. As these impressions have been established by energy derived from the same source and at the same time, they become closely connected or associated with one another. 16 Consequently the re-excitation of any one of these centres, as by the sight of a rose, will bring the other centres into action, with the result that a concrete conception of a rose is formed. This conception or thought is the outcome of work performed by the mass of living matter contained in the nerve-cells of the psychical areas of

16 Two principal laws govern the action of associative processes: the first law affirms the principle that each idea reproduces as its successor either an idea that is similar to it in content, or an idea with which it has often appeared simultaneously.' The second law of association is as follows: 'The first idea which is associated with the introductory sensation is determined by its complete likeness or, more frequently, its similarity to the latter.' Dr. A. Bain states that 'the assigning of these laws was the first contribution to a science of human intelligence; while the ultimate shape given to them, whatever that may be, will mark the maturity of at least one portion of that science.' 'Association Controversies.' See Mind, xii. 161.

the brain through which energy, derived from the contents of our ideas, must pass on its way to the motor-cortical centres.

By the excitation or stimulation of the charged elements of the nerve-cells in which latent ideas have been established, a portion of their working energy is released, which passes to those parts of the brain whose function it is to combine (associate) and transform this energy into psychical processes. When we refer to energy released from elements in which latent ideas have been impressed, we mean that this form of energy is derived from the various contents of the idea that is, from the special features of things or movements from which each idea was derived. Streams of this form of energy enter the psychical cortical areas, by preference along those paths that have been most perfectly trained, and there it becomes transformed into psychical nervous force that is, into thoughts or conceptions which, acting on the word-charged motor centres, become manifest in intelligent speech or in the other skilled movements which have enabled man to exist and multiply in the ever-increasing complexity of his environment.

The reproduction of acoustic latent ideas of words sets free energy which, in conjunction with energy derived from other mental images, constitutes the units of thought. The conception of the genesis of thought may therefore be reduced to this formula that our thoughts consist of the association of the contents of ideas, and consequently that our intellectual faculties are derived from energy received from external objects and from the movements of our bodies, acting through the sense-organs on corresponding cortical nervous centres. Sensations, with their correlated latent ideas, form the raw material of thought at rest; the same material, brought into action through the agency of the psychical elements of the brain, acting on motor centres, becomes manifest in the co-ordinated movements of groups of muscles such as those which work the vocal apparatus and other parts of our bodies.

We thus come to appreciate the meaning of Mr. G. J. Romanes' statement, in his admirable lecture on Animal Intelligence' which appeared in the Nineteenth Century for October 1878 (p. 653), that words contain a vast body of ideas in an abbreviated form, which we employ in a manner analogous to that in which mathematical symbols are used. As these contain in a manipulated form the whole meaning of a long calculation, so in all other kinds of reasoning the symbols, which we call words, contain in a concrete form vast bodies of signification (ideas) derived from all parts of the living matter constituting the psychical areas of the cerebrum, but brought as it were to a focus on the elements of the sensori-motor centres of speech.