Note, next, that the evolution of thought and emotion varies, other thingsequal, with the supply of blood to the brain. On the one hand, an arrestof the cerebral circulation from stoppage of the heart, immediately entailsunconsciousness. On the other hand, excess of cerebral circulation (unlessit is such as to cause undue pressure) results in unusual excitement. Notthe quantity only but also the condition, of the blood passing through thebrain, influences the mental manifestations. The arterial currents must beduly aerated, to produce the normal amount of cerebration. If the blood isnot allowed to exchange its carbon dioxide for oxygen, there results asphyxia,with its accompanying stoppage of ideas and feelings. That the quantity ofconsciousness is, other things equal, determined by the constituents of theblood, is unmistakably seen in the exaltation which certain vegeto-alkaliescommonly produce when taken into it. The gentle exhilaration which tea andcoffee create, is familiar to all; and though the gorgeous imaginations andintense feelings produced by opium and hashish, have been experienced byfew (in this country at least), the testimony of those who have experiencedthem is sufficiently conclusive. Yet another proof that the genesis of themental energies depends on chemical change, is afforded by the fact thatthe effete products separated from the blood by the kidneys, vary in characterwith the amount of cerebral action. Excessive activity of mind is accompaniedby excretion of an unusual quantity of the alkaline phosphates. §71b. But now after recognizing the classes of facts which uniteto prove that the law of metamorphosis, and in a partial way the law of equivalence,holds between physical energies and nervous energies, let us enter upon theultimate question -- What is the nature of the relation between nervous energiesand mental states? how are we to conceive molecular changes in the brainas producing feelings, or feelings as producing molecular changes which endin motion?
In his lecture on Animal Automatism, Prof. Huxley set forth the proofsthat alike in animals and in Man, the great mass of those complex actionswhich we associate with purpose and intelligence may be performed automatically: and contended that the consciousness which ordinarily accompanies them isoutside the series of changes constituting the nervous coordination -- doesnot form a link in the chain but is simply a "concomitant" or a"collateral product." In so far as it correlates the nervous actionsby which our bodily and mental activities are carried on, with physical forcesin general, Prof. Huxley's conclusion accords with the conclusions aboveset forth; but in so far as it regards the accompanying states of consciousnessas collateral products only, and not as factors in any degree, it differsfrom them. Here I cannot do more than indicate the set of evidences by whichI think my own conclusion is supported if not justified.
One of them we have in the facts of habit, which prove that states ofconsciousness which were at first accompaniments of sensory impressions andresulting motions, gradually cease to be concomitants. The little boy whois being taught to read has definite perceptions and thoughts about the formand sound of each letter, but in maturity all these have lapsed, so thatonly the words are consciously recognized: each letter produces its effectautomatically. So, too, the girl learning to knit is absorbed in thinkingof each movement made under the direction of her eyes, but eventually themovements come to be performed almost like those of a machine while her mindis otherwise occupied. Such cases seem at variance with the belief that consciousnessis outside the lines of nervous communication, and suggest, rather that itexists in any line of communication in course of establishment and disappearswhen the communication becomes perfect. If it is not a link in the line,it is not easy to see how these changes can arise.
Sundry facts appear to imply that consciousness is needful as an initiatorin cases where there are no external stimuli to set up the co-ordinated nervouschanges: the nervous structures, though capable of doing everything requiredif set going, are not set going unless there a rises an idea. Now this impliesthat an idea, or co-ordinated set of feelings, has the power of working changesin the nervous centres and setting up motions: the state of consciousnessis a factor.
Then what we may call passive emotions -- emotions which do not initiateactions -- apparently imply that between feelings and nervous changes thereis not merely a concomitance but a physical nexus. Intense grief or anxietyin one who remains motionless, is shown to be directly dependent on nervouschanges by the fact that there is an unusual excretion of phosphates by thekidneys. Now unless we suppose that in such cases there is great activityof certain nervous plexuses ending in nothing, we must say that the feelingis a product of the molecular changes in them.
