Three primary mental functions, conscious ness dis tinct from analysis of the facts of Mind has shown that there are three primary mental functions: that is to say : Consciousness, or Feeling: probably It is at least a probable hypothesis, that the three sets corre- of cerebral nerves correspond to the three primary mental sponding thereto. functions. But how are the respective functions of the three sets of nerves to be identified? Here it is important to remember, that although thought and will are usually accompanied by consciousness, they are not always so: there may be unconscious thought, and the facts of somnambulism appear to show that there may be unconscious voluntary determinations. These facts greatly Nerves of strengthen the probability that the nerves of consciousness, at least, are distinct from those of thought and will. Now, how are we to identify the nerves of consciousness? It will be seen that of the three sets of cerebral nerves, as enumerated above, only one is in connexion with the sensory ganglia, leading up from them to the ganglionic substance of the cerebrum. The functions of these nerves must have something to do with the functions of the sensory ganglia. But they are not nerves of sensation. The nerves of sensation are those which are in connexion with the external organs of sense. They may be, however, Nerves of and I believe are, the nerves of consciousness: whether conscious the primary consciousness of sensation, or the secondary ness, how identified. consciousness of memory and thought. those of thought and will. The same kind of reasoning applies to what I believe to be the nerves of thought. The mass of the ganglionic substance of the cerebrum, and of the fibres connecting its different parts, varies as between different species of animals and between different individuals among men, in some kind of approximate proportion to their intellectual powers; and it is doubted by none that the cerebrum is Thought is the organ of thought. Thought, as we have seen, is not necessarily conscious. That is to say, it is in itself unconscious, though it is usually accompanied by consciousness. in itself uncon scious. Being itself unconscious, its seat cannot be in the nerves of consciousness; and I think all analogy is in favour of the hypothesis that the currents of thought are formed in Nerves of thought. the nerve-fibres that connect the different parts of the ganglionic substance of the cerebrum with each other; or, in other words, that thought is due to the mutual action of the ganglionic substance of the cerebral hemispheres, and the nerve-fibres that connect the different parts thereof. It is a very significant fact, that "it is on the very large proportion which the commissural fibres [or those which I regard as the nerves of thought] bear to the rest, that the bulk of the cerebrum of man and of the higher animals appears chiefly to depend; and it is easy to conceive that this condition has an important relation with the operations of the mind, whatever be our view of the relative functions of different parts of the cerebrum."1 will. If my reasoning is assented to thus far, there can be little difficulty in identifying the nerves of will. We find Nerves of a set of nerve-fibres which by their position serve to connect the ganglionic matter of the cerebrum with the corpora striata, which are the motor ganglia for consensual motions. Analogy points to these fibres as the probable channel by which determinations of will, formed in thought, are conveyed to the motor organs. how dis I will here anticipate a possible difficulty. What is the Ideophysiological distinction between ideo-motor actions (to motor and use Dr. Carpenter's expression) and voluntary ones? Sup- actions : pose, for instance, that I decide, on deliberation, to draw tinguished. back from a position of danger, and do so; this is voluntary action. Suppose, on the contrary, that I do not wait to deliberate, but involuntarily shrink back from the thought of danger, perhaps contrary to my better judgment; this is an ideo-motor action, and differs from a consensual one only in the feeling that prompts it being not sensation but consciousness of thought. The stimulus to ideo-motor actions, and the stimulus to voluntary actions, both proceed originally from thought, and consequently from the nerves of thought. But the stimulus to voluntary action comes 1 Carpenter's Human Physiology, p. 577. The theory incomplete. from the ganglionic substance of the cerebrum to the motor ganglia directly, through the nerves of will; the stimulus to ideo-motor action, on the contrary, comes through the nerves of consciousness; and they do not act on the motor ganglia directly, but through the medium of the sensory ganglia, which, as I have shown reason to believe, are the ganglia of consciousness. I admit that the physiological theory of mind, of which I have in this chapter drawn the outline, is but an outline, which may probably never be filled up. Among other deficiencies, it gives no account of one of the most remarkable and yet common of mental facts, namely the act of attention; which may be defined as the direction of consciousness by a voluntary act. In any but the most elementary psychological inquiries, physiology is of little or no use as a guide. NOTE. NERVOUS CURRENTS. ness is menon, Conscious- IF my theory of consciousness is correct, consciousness is always always a a secondary phenomenon. The consciousness of a sensation is secondary due to a current in the nerves of consciousness, produced by a phenocurrent in those of sensation; and, similarly, the consciousness of a thought is due to a current in the nerves of consciousness, produced by a current in those of thought. One current of consciousness may, however, produce another, according to a law to be stated in the next chapter. The current is tinuation of the It is to be observed that the second current is not a mere secondary continuation of the first, but is in every sense distinct. In not a con- order to illustrate my meaning, it will be best to take instances from the working of the electric telegraph. Telegraphic instruments may be so arranged that an electric current sent from Compared London may make a signal by deflecting a magnetic needle at telegraph York, and then go on and make a signal at Edinburgh. But currents. nothing like this takes place among nervous currents. first. to electric The only analogy in telegraphy to the production of secondary fibres are more than con ductors. I may here state, that I think Mr. Lewes is quite right in Nervemaintaining that nerve-fibres are not merely analogous to telegraphic wires. They are, no doubt, conductors of nervous merely energy, but they are also necessary to its production. The mutual action of a nerve-fibre and a ganglion is necessary to the production of a nervous current, just as the mutual chemical action of two substances is necessary to the production of a voltaic current. CHAPTER XXX. CONSCIOUSNESS AND THOUGHT. Ν IN the foregoing chapter, I have attempted an analysis of the different mental functions, only so far as such an analysis illustrates, and is illustrated by, the anatomy and physiology of the organs of mind. That anatomy and physiology are, I believe, most valuable guides for the first few steps in the analysis of mental functions, but beyond the first few steps they fail us altogether. I have no doubt that Mind, in its very highest functions as truly as in mere sensation, is dependent on nervous action; but, beyond broadly discriminating between the nerves of consciousness, of thought, and of will, we have no means of identifying the nervous actions to which particular mental actions are due. All psychology, therefore, except in its most elementary parts, must be studied as a distinct a guide in science, without further help from physiology. This, element- however, is not because bodily and mental functions are really distinct, but only because we are unable to trace the chology, though all connexion. But though it is impossible to trace the demind pendence of mental on nervous action in particular cases, that dependence is proved as a general truth by all analogy, and by much direct evidence: such as the fact that great mental power is never found without a well-developed Proofs of cerebrum1 (though the converse does not appear to be true); that mental exertion, like bodily exertion, exhausts Physiology is useless as any but ary psy depends on nervous action. this. 1 I do not say a large cerebrum. Great intellectual power is sometimes met with in brains not above the average size. But there is certainly no instance of great intellectual power in an abnormally small brain, or in a malformed one. |