is the instrument of our bodily activity, shapes itself to the mode in which it is habitually exercised, we seem justined in assuming that the same thing is true of the Cerebrum, which is the instrument of our mental activity. For in no other way does it seem possible to account for the fact of very frequent occurrence, and noticed in a previous paper, that the presence of a Fever-poison in the blood—perverting the normal activity of the Cerebrum so as to produce Delirium—brings within the "sphere of consciousness" the "traces" of experiences long since past, of which, in the ordinary condition, there was no remembrance whatever.
The same occurrence has been noticed as a consequence of accidental blows on the head; though these more commonly occasion the loss than the recovery of a language. The following case of this kind is mentioned by Dr. Abercrombie, as having occurred in St. Thomas's Hospital:
"A man who had been in a state of stupor consequent upon an injury of the head, on his partial recovery spoke a language which nobody in the hospital understood, but which was soon ascertained to be Welsh. It was then discovered that he had been thirty years absent from Wales, and that, before the accident, he had entirely forgotten his native language. On his perfect recovery, he completely forgot his Welsh again, and recovered the English language." (Op. cit., p. 148.)
It seems perfectly clear, then, that, under what we cannot but term purely Material conditions, strictly Mental phenomena present themselves. It is common to the whole series of cases, that the Automatic play of the "Mechanism of Thought" does that which Volition is unable to effect. Whether it be the toxic condition of the Blood, or the simple excitement of the Cerebral Circulation generally, or the special direction of Blood to a particular part of the Brain, it is beyond our present power to tell; but, as all Brain-change is (like the action of any other mechanism) the manifestation of Force, the production of these unusual Mental phenomena, by the instrumentality of an unusual reaction between the Blood and the Brain-substance, is no more difficult of comprehension than that of ordinary forms of Psychical activity, which we have seen reason to regard as the results of the translation (so to speak) of one form of Force into another.
The intimacy of the relation between the Psychical phenomena of Memory and Physical conditions of the Brain is further shown, by the effect of Fatigue and the impaired Nutrition of Old Age in weakening the Memory, and of disease and Injury of the Brain in impairing or destroying it. Every one is conscious of the difference in the activity of the reproductive faculty in which Memory consists, according as his mind is "fresh," or his head feels "tired." The latter state, in which the Automatic activity and the directing power of the Will are alike reduced, is clearly dependent, like the feeling of muscular fatigue, on the deterioration of the Organ, or of the Blood, or of both
