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known to be elements of nerve-cells (their phosphorised constituents); (4) a mass of facts (the outcome partly of pathological observation, partly of experimental destruction of different portions of the nerve-centres) going to show that injury to the brain is attended with some interruption of the psychical activities making up normal consciousness; (5) the important fact that any interruption of the supply of blood to the brain by means of one of the great arteries running to the organ is followed by a profound disturbance if not a suspension of consciousness; (6) the confirmation of this physiological evidence by the results of comparative anatomy, which show that the development of the brain and the degree of intelligence vary in a direct ratio among different species of animals, races of mankind, and individual men.

This relation is clearer in the case of man than of animals as a whole. Though the relation of size and weight of brain to those of the body is in general an index of the intelligence of an animal, there are certain exceptions. As between man and man the size, weight, and specially the degree of structural complexity as shown by richness of convolutions, answer clearly to the degree of mental power manifested.1

§ 11. Special Connexion of Mental Activity and the Cortex. Modern physiology has not only fully established the connexion between the brain and mental activity, but it has gone some way to make it probable that it is the highest centres in the cortex of the cerebral hemispheres which form the immediate physical basis of our mental life, so far at least as this involves clear consciousness. According to this view, it is only when sensory impulses are transmitted to the termination of the afferent fibres in the cortex that a distinct sensation arises. And all volitional initiation of movement takes its start in the cortex.

Recent investigations into the functions of the lower intercranial centres (cerebellum and basal ganglia), while clearly establishing special connexions between these organs and certain classes of sense-impressions and groups of movements, leave us much in the dark as to their exact functions. It is

1 On the relation of size and weight of brain to intelligence, see Bastian, The Brain as Organ of Mind, esp. ch. xviii. and following. The evidence in favour of the brain being the organ of mind is well summarised by Ladd, Physiol. Psychology, pt. ii. ch. i.

probable that so far as consciousness is concerned they are to be viewed as a subordinate mechanism by which more complicated adjustments of efferent to afferent impulses than are possible in the case of the spinal cord and its expansion the medulla are carried out, but adjustments which, owing either to original paths of connexion or to connexions built up by experience and repeated action, do not involve any accompaniment of clear consciousness.

The conclusion is so far only a probability. The idea that the sensorium, in the basal ganglia of the brain, is the seat of conscious sensation is still entertained by some physiologists. It is conceivable that the activities of the lower centres of the brain may contribute elements to the sub-conscious region of our mental life. It may be added that the very texture of the brain, involving a network of open communication between one part and another, appears to exclude the idea of a definite boundary to the circle of nervous action constituting the physical basis of consciousness.1

§ 12. Localisation of Distinct Mental Functions. After thus marking out roughly the boundaries of the "seat" of mental life, there remains the question whether different parts of this region are specially connected with distinct varieties of mental activity.

The attempt of Gall and Spurzheim to connect different faculties-intellectual, emotional, and moral-with definite localities on the surface of the brain has been condemned both by psychologists and physiologists. Their phrenology' involved unscientific ideas both of mind and of the functional activities of the brain.2

More recently the subject has been approached from the physiological side under the heading, the Localisation of cerebral functions. A series of experiments (supplementing the results of anatomical and pathological observation) has been carried out for the purpose of connecting definite regions of the

1 The evidence in favour of making the cortex the seat of conscious activity is summarised by Ladd, Elements, pt. ii. chap. i. The opposite view is urged with much emphasis by G. H. Lewes, Problems of Life and Mind, third series, prob. iii. chap. xiv.

"On the scientific value of phrenology, see Sir W. Hamilton, Lectures on Metaphysics, i. p. 404, Appendix; Lotze, Microcosmus (Engl. trans.), i. p. 339 following; Dr. Bain, On the Study of Character, chaps. ii.-vi.; Volkmann, Lehrbuch der Psychologie, § 30.

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cortex with particular classes of sense-impressions and particular groups of movements.1

Such experiments have undoubtedly established special correlations between certain regions of the cortex and particular groups of psychical elements (sensations and conscious movements) and enable us to speak of particular centres of this and that order of sensations and movements. Thus physiologists are able to mark off, roughly at least, a particular centre for visual sensations, auditory sensations, the movements of the eye-balls, of articulation, and so forth.2

This mapping out of the functions of the cortex, interesting and valuable as it is to the psychologist, has not, so far, sought to assign definite cerebral concomitants for what he distinguishes as distinct psychical activities, e.g., discrimination and assimilation or consciousness of resemblance. Hence the newer cerebral physiology does not as yet furnish the psychologist with a complete substitute for the phrenological scheme, which, bad as it was, boldly aimed at indicating the cerebral counterpart of recognised psychological distinctions.

The close structural connexion of the different portions of the cortex, including the multiform commissural attachment of the two hemispheres, supports the idea that in all modes of conscious activity a considerable area of the cortex is involved. Our conscious life consists, as already pointed out, in complex processes of combination, in comparing and uniting a multitude of elements. And though, as observed too, this process of psychical unification differs from all physical processes, we appear to find the closest physical analogue to it in such a local approximation of a number of sensory and motor impulses as is effected by the complex structural arrangements of the cortex. Thus we may say with some degree of probability that the physical substratum

1 These experiments, carried out on monkeys and other animals, consist on the one hand in stimulating certain well-defined regions by an electrical current and noting the results, and on the other hand in extirpating certain portions and observing what functions are thereby lost. (See Ladd, Elements of Physiol. Psychology, pt. ii. chap. i. § 11 and following; cf. Dr. Ferrier, The Functions of the Brain, 2nd ed. chap. vii. and following; Wundt, Physiol. Psychologie, 5er cap. § 6; and W. James, Principles of Psychology, chap. ii.)

* How far definite demarcation of cortical area answering to particular groups of sensations and movements is possible is a matter of dispute.

of conscious activity is in every case a complex of nerve-processes involving a more or less extended area of the cortical

centres.

§ 13. Correlation of Nervous and Psychical Processes. Having thus conjecturally mapped out the physical substratum of psychical processes, we may inquire into the general correlations between the two sets of operation involved. In what way or ways, it may be asked, does change in the nervous action affect the psychical action? What are the most definite aspects of the concomitance between the two sets of phenomena ?

In seeking to answer this question we must clearly bear in mind that the two classes of phenomena are disparate, and that we can only expect to find a certain amount of correspondence or parallelism between them. It is to be added that, although scientific observation and experiment have shown that a certain correlation obtains between the two, the precise extent of this correlation is as yet unknown. We are thus compelled to eke out fact with conjecture.

§ 13a. Correlations between Elements. In the first place then we may trace out a certain measure of correlation between the elementary psychical and physiological processes involved. That is to say, we may seek to connect differences in the elementary psychical phenomena with certain differences in the underlying physiological actions.

1. Of these correlations the most obvious are quantitative co-variations. Changes of magnitude in the psychical phenomenon appear to be conditioned by similar changes in the nervous process. These quantitative co-variations are resolvable into three, viz., in respect of (a) intensity or strength, (b) extent or range, and (c) of duration. A word or two on each must suffice.

Our psychical states have the attribute of intensity. Thus the sensation answering to a loud sound has greater strength or intensity than that produced by a faint sound. It has been ascertained that all differences in intensity in our sensations are conditioned by changes in the strength of the stimulus at work, and therefore presumably by differences in the intensity or energy of action of the central elements involved.

Again, our psychical states exhibit differences in volume or extensity. Compare the two sensations of heat where a small

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part of the bodily surface is touched by a heated body and where a large area is touched. It is obvious that this difference is connected with the number of nervous fibres engaged, and so with the range of the central excitation.

Once more, our psychical processes occupy time, and it seems probable that their duration is conditioned by that of the connected nervous action. Sensations last in general just as long as the peripheral process of stimulation, and presumably therefore as the central excitation ensuing on this.1

The exact limits of these quantitative correlations will be touched on later. Suffice it to say here, by way of avoiding misapprehension, that the co-variation is not known to be perfectly simple and to obtain in all cases. Thus it is known in the case of sensations that there is a certain intensity of stimulus below which no psychical effect is produced (liminal intensity). And there is evidence that a similar threshold obtains generally in respect of extent and duration. That is to say, a nervous process which involves fewer than the minimal number of central elements, or a shorter than the minimal duration of the process of excitation, is unaccompanied by a psychical concomitant. In what precise manner above these limits changes in the intensity, etc., of the nervous process affect the corresponding aspect of the sensations will have to be spoken of later on.

. 2. In addition to these quantitative correspondences we may point out a certain qualitative correlation. The qualitative aspect of a psychical phenomenon, e.g., of a sensation, is illustrated in the difference between a smell and a taste, or between a bitter and a sweet taste, a difference of kind which cannot be resolved into a merely quantitative difference. It is probable, as we shall see by-and-by, that these differences correspond to differences in the mode or form of the peripheral stimulation, and consequently of the central excitation.

It follows from this general correlation between psychical and physiological action that the degree of complexity of a psychical state is conditioned by that of the underlying physiological process. Highly complex states of mind, such as mixed feelings, would thus involve a number of unlike nervous processes and numerous changes with respect both to the particular central elements engaged and the relative intensities of their action.

1 As we shall see by-and-by, the central process may be prolonged beyond that of the peripheral stimulation, as when we go on having a sensation of light or sound after the external agent has ceased to act.