The phenomena of memory remain quite incomprehensible if we do not admit their intimate connection with physical changes of the nerve substance. An external impression acting upon receptive nerve-cells is retained by them permanently, as though it were photographed, if it be allowable to explain the unknown by a comparison with the known. It is the blood which carries those substances to the brain which are necessary to the functions of memory. Attention cannot be developed in all its intensity without causing considerable alterations in the circulation. Now when we are absent-minded, images leave no lasting impression on the memory, as no provision is then made by the physical changes in the organism accompanying attention for a more rapid circulation of blood in the cerebral hemispheres. The old notion that the brain was a storehouse in which each idea had its nook where it might stay till needed, is truer than it appears. Modern science has proved that the matter is much more complicated than one thinks. It suffices that the blood should coagulate in the artery which carries it to some convolution, or that a tumour should destroy a part of the brain, for us to lose, as it were, a province of memory. Let us first consider verbal memory. That region of the brain in which it is placed is, generally speaking, the parietal region of the left side; so that anyone who has had a blow on the temple at that side nearly always loses his speech, although he still remembers things and can pronounce their names when they are repeated to him by others, a sufficient proof that the movements of the tongue are not impeded. Sometimes it happens that a person in this condition looks in the dictionary for the missing word, in order to recover the pronunciation of it. In learning a language, we believe that certain cells undertake functions which they did not before possess, that connections with other cells are established, like very intricate nets in which the impressions of nouns and verbs, the graphic representations of ideas and words, are collected. As we exercise ourselves in the language, the blood carries new elements to these cells, and the greater our attention, the stronger become the impressions. Oxidation does not destroy the impression once received, but it weakens it. If we have had no practice for some years in speaking a language, we meet with great difficulties, our communications being made in set, stiff words; but after a few days one regains the former fluency. We might quote cases in which, through illness, a man has completely forgotten a language, recovering it as health returned. Others have forgotten several languages in the order of succession in which they had learnt them, regaining them later in the inverse order to that of acquisition.