the bloodvessels are dilated or contracted as required,[75] and the consequent supply of blood becomes more or less copious.[76] The cells which receive no blood must suspend their action; those which receive a larger supply can, on the contrary, operate more powerfully. The will which regulates the distribution of blood, when incited by a group of presentations temporarily predominating, thus resembles a servant who is constantly occupied in a room in carrying out the behests of his master: to light the gas in one place, in another to turn it up higher, in another to turn it off partly or wholly, so that at one moment this, and at another that, corner of the room becomes bright, dim, or dark. The preponderance of a group of presentations allows them during their period of power to bring into their service, not only the brain-cells, but the whole organism besides; and not only to fortify themselves through the representations which they arouse by way of association, but also to seek certain new sense-impressions, and repress others, in order, on the one hand, to obtain new excitations favourable to their persistence--new original perceptions--and on the other hand, through the exclusion of the rest, to ward off such excitations as are adverse to their persistence. For instance, I see in the street a passer-by who for some reason arouses my attention. The attention immediately suppresses all other presentations which, an instant before, were in my consciousness, and permits those only to remain which refer to the passer-by. In order to intensify these presentations I look after him, _i.e._, the ciliary and ocular muscles, then the muscles of the neck, perhaps also the muscles of the body and of the legs, receive motor impulses, which serve the purpose only of keeping up continually new sense-impressions of the object of my attention, by means of which the presentations of him are continuously strengthened and multiplied. I do not notice other persons who for the time come into my field of vision, I disregard the sounds which meet my ears, if my attention is strong enough I do not perhaps even hear them; but I should at once hear them if they proceeded from the particular passer-by, or if they had any reference to him. This is the ‘adaptation of the whole organism to a predominant idea’ of which Ribot speaks. This it is which gives us exact knowledge of the external world. Without it that knowledge would be much more difficult of attainment, and would remain much more incomplete. This adaptation will continue until the cells, which are the bearers of the predominating presentations, become fatigued. They will then be compelled to surrender their supremacy to other groups of cells, whereupon the latter will obtain the power to adapt the organism to their purposes.