that the part of the system which is at any moment most active owing to the intensity of its neural potential is the part which forms the neural correlate of the psychological abstraction which introspective psychologists and even at times behaviourists call the 'focus of attention1."

Here then in the 'concept' and the 'engram' we seem to have the related thought elements and neural elements which are indispensable for any synthetic or synoptic view of the thought processes on the one side and the neural processes on the other-of thought regarded as a psychological quality of the human mind and the neurological processes which accompany it and furnish its neural correlates. The concept or thought element is the abiding result of a long process of growth or building up from the acquired results of sense-perception, an element which is subject to unceasing change in response to new sense stimuli, and growing with a growth characterized by an increase in mass accompanied by progressive differentiation of structure. On the neural side as the anatomical and physiological correlate of the concept we have the 'engram' of Semon or the neural 'schema' of Drs Head and Holmes consisting of a slowly formed physiological pattern between groups of neurones in different areas of the brain, this pattern also being in continual process of modification and growth like the concept of which we hold it to be the neural correlate. Each psychological entity in thought, each mental symbol or concept, has we think its neural engram, each of these minor engrams forming a small integral part of much larger engrammic systems.

These slowly acquired neural paths may be at one time latent and

1 In addition to what psychologists call the 'focus of attention' there is of course also the accompanying condition for which James used the term 'fringe of consciousness.' We think that the condition denoted by this term has in part as its neural correlate those regions of an active engrammic system where the neural potential is lower than in the track of the main impulse. It is amongst the commonplaces of our everyday experience that any particular part of, for example, an extended visual image in the field of perception may be at one moment in the 'focus of attention' and at another moment in the 'fringe of consciousness,' and that the change from one to the other is under the control of what we usually call 'volition.' Here at once rise two fundamental questions into neither of which we at present propose to enter:

(1) What are the neural correlates of the psychological abstraction which we call

volition?

(2) What are the functional relations in an act of visual perception of the stimuli (a) from the macula, and (b) from other parts of the retina?

The first of these questions seems to us beyond the present bounds of scientific spect lation and its solution must be contingent on our interpretation of the term 'volition.' The answer to the second question is probably within the knowledge of Professor Elliot Smith who has made this subject peculiarly his own.

at another time active, or to use Semon's term 'ecphorized,' owing to the discharge of a neural impulse through them. What is known in the jargon of the day as 'the subconscious mind' has in our view as its neural correlate the multitudinous systems of engrams slowly formed during the entire ontogenetic life process, these engrams being in a latent as distinguished from an active condition. And the 'act of recall' to consciousness, be it total or partial, of the multitudinous mental symbols and their inter-relations has its neural correlate in the activating of those parts of the entire engrammic systems which form the neural correlates of the groups of inter-related concepts which are being for the moment 'recalled' to consciousness.

Just as a concept grows in response to the sensory impulses which form an integral part of the perceptual processes by a growth in bulk or mass accompanied by progressively increasing differentiation of structure, so too with the engram. The sensory impulses from the receptor organs which during the ontogenetic life process are continually changing it from a latent to an active condition lead to a growth by which its neural paths are being continually enlarged by ramification and by an increasing number of neurones being functionally incorporated in it, and in these kindred processes of growth in the concept and the engram we have examples of that power of modification and adjustment which on the biological side is usually called 'adaptation to environment' and on the mental or psychological side is one form of what is usually called 'intelligence.'

This view seems to us to open the way to a physiological explanation of what we call 'memory' to complement the work which has been carried out by psychologists on the psychological aspect of this subject. The 'distributed repetitions' of the psychologists have in our view as their neural correlates the activation at similarly distributed intervals of the same engrammic systems. These engrammic systems may be held to become more permanently established by what the physiologists call 'facilitation' when their periods of activation are suitably 'distributed' than is possible by a single period or more rapidly successive periods of activation, and if such engrammic systems are allowed to remain latent for an indefinite period all power of reviving them may disappear and result in a permanent 'forgetfulness.'

But it is of the essence of what we call 'memory' that the subject matter involved should be capable of being 'recalled' by some agency other than the peripheral receptor organs by means of which it was originally acquired that the engrammic systems which in the acquire

ment of the subject matter were largely formed by successive series of sensory impulses from the receptor organs, should, for the purpose of 'memory,' be capable of being activated by a central agency apart from the receptor organs, and we think that this will be found to be one of the functions of the Optic Thalami.

To the essential organs of the thalami Drs Head and Holmes assigned the function of being in some way the centre of consciousness for the affective side of sensation. If these investigators are right, and we accept their conclusions on this point, then it seems to us that to these organs we may assign a part, by means of some form of conditioned reflexes, in furnishing the affective elements which enter into every act of perception. It is well recognized that every such act involves the commingling and interaction of old knowledge with the new which is being acquired in that act of perception. "For in a perception, as James said, half comes to us from the thing perceived and half out of our own heads." This would seem to involve the activation of engrams from within as well as by sensory impulses from without, of stimulation from a central source as well as stimulation from the peripheral receptor organs. We hold that it is a part of the function of the essential organs of the thalami to provide this central stimulation; that in the processes of perception there is usually an activation of the engrams from the essential thalamic organs supplemented by activation from the peripheral receptive organs, and that these two series of impulses, internal from the essential thalamic organs and external from the peripheral receptor organs, furnish together the 'a priori data' and the 'sense data' involved in every act of perception. In this activation of any group or system of engrams by the essential thalamic organs we find also the neural correlate of that condition which is known to psychologists as 'preperception,' and in the changeful continuity of this neural process by which different engrams become successively activated without stimulation from the peripheral receptor organs we find the neural correlate of what is usually called 'reverie' and also of a large part of the mental processes which fall within the range of what we call 'ratiocination.'

But in 'memory,' in ‘reverie,' and in 'ratiocination,' it is often of

1 "The Artistry of Truth," by Professor S. Alexander, Hibbert Journal, Jan. 1925,

p. 303.

"As it lies in your fancy, then, this object, the reality, is a complex and elusive entity, the sum at once and the residuum of all particular impressions which, underlying the present one, have bequeathed to it their surviving linkage in discourse and consequently endowed it with a large part of its present character," The Life of Reason, by G. Santayana,

I, p. 82.

the essence of the process that we should be able as we say, to roll a subject over and over in what we colloquially call our 'minds' and survey different parts and aspects of it in continually changing and alternating processes of succession; and we hold that the neural correlates of these various mental processes are intimately bound up with the functions of the paths which return from the various parts of the cortex to the optic thalami-the cortico-thalamic paths. It is little material to the primary question of the function of these paths whether they return from all parts of the cortex (Head and Holmes) or only from parts other than the association areas (Bianchi). This is a question which neurology has not perhaps yet been finally able to determine. It is one of the many questions for the future. But a current view of the function of these paths is that they are concerned in transmitting impulses by which the cortex is enabled to exercise in some way a control over the functioning of the thalami.

It is a part of our thesis to offer an alternative view of the function of these paths and we suggest that they are return paths for reflex neural impulses from the cortex which excite relay cells in the thalami, and that these relay cells in turn send stimuli both to the essential thalamic organs and also to the same cortical areas from which the paths conveying the return impulse originated. We think that in this way a multitudinous series of complex conditioned reflexes become established the end of which is to provide a means by which a continual circulation of neural impulse may be maintained without stimulation from the peripheral receptor organs-a circulation which enables any series of engrams to be continuously activated and their psychological correlates or concepts, in their ever-changing relations, kept continuously above the threshold of consciousness and alternated in the various ways which are a condition of reflective thought.

Merely to state this view is to raise at once the whole complicated question of the nature of what we call 'inhibition.' It would carry us beyond the purview of our subject to discuss this question in any detail. We merely note that the term is applied alike (1) to the diminished heart beat resulting from stimulation of the vagus, (2) to the diminished secretion of glands under certain conditions of the sympathetic and para-sympathetic systems, (3) to the diminished activity under certain conditions of some neural organs, and (4) to phases in the processes of the reciprocal innervation of the opposed muscular groups controlling the skeletal framework.

We take it that the term as usually employed connotes an active

or positive neural impulse which diminishes certain kinds of physiological action, and in this sense we think that as applied to a hypothetical restraining action on the thalami by the cortex, and to the reciprocal innervation of opposed muscular groups, it will in the end prove to have been logically inadmissible. We find it stated in works. of physiology that although no inhibitory mechanism has been discovered in skeletal muscles, and although there is no known instance of a peripheral nerve which when stimulated causes relaxation of these muscles, yet the fact that an extensor muscle severed from its distal attachment lengthens when the flexion reflex is stimulated is held to prove the existence of an active inhibitory process. We hold this to be an illogical inference which will in due time be eliminated by the application of Occam's razor. Entia non sunt multiplicanda praeter necessitatem, and we see no necessity for hypothecating the existence of a special inhibitory mechanism to explain this phenomenon: we see only the need of hypothecating a neural mechanism capable of simultaneously diminishing the stimuli to one group of muscles and increasing those to the opposed group, thus securing a co-ordinated and simultaneous intensive adjustment of the neural potential supplying both flexors and extensors. We adopt in short a view something like that expressed by Professor Bianchi in his book on The Mechanism of the Brain and the Function of the Frontal Lobes, viz. that what is usually called 'inhibition' must in many cases be regarded more as a change in neural potential than as the manifestation and result of a special inhibitory mechanism, unless indeed this so-called inhibitory mechanism be regarded as one which acts by re-directing or changing the intensity of the neural potential. Whether future research will confirm Professor Bianchi's opinion that all inhibition is a re-direction of neural potential remains for the future to determine, but we suggest that a similar kind of neural mechanism, whatever it may be, will be found at work effecting the intensive and directional changes of neural impulse in the cerebral engrammic systems and also those involved in reciprocal muscular innervation.

We regard then as one of the factors contributing to the neural sub-strata of reflective thought the formation during the ontogenetic life process of innumerable engrams extending from the thalami to the various cortical lobes connected with their respective receptor organs, visual, auditory, olfactory, tactile, etc.: that these engrams intercommunicate through the association areas; and that, however complex may be their structure and however extended their distribution and range, they each comprise a neural mechanism which, by means of a