The period of saturation begins for the cerebral cell. The power of retention of external excitations is already on the brink of decay. New acquisitions of heteroge neous elements which do not form a portion of the circle of youthful knowledge become very difficult, if not impossible. We know how painful the labour of learning a foreign language, so easy for the young child, becomes for the adult ; how rebellious the memory is as regards the registering of new words ; and with what an expenditure of intellectual force we retain the vocabulary of languages with which we were not familiar in childhood. We also know how blunt, even in the domain of common things, the retentive power of our memory, and consequently our powers of application in general, become, if we have to learn things that are quite new to us ; and how, for instance, we with reason look upon it as impracticable to acquire a special technical education, and commence a new career after forty years of age.
At this period of life first impressions still faithfully persist in the memory, but nevertheless they have a tendency to diminish in intensity, and it us necessary to vivify them by incessant labour, to stimulate them anew by placing the cerebral regions where they are stored up in identical conditions, by similar impressions of equal intensity, so as to prevent their becoming extinct ; just as we keep up a fire by continually supplying it with fresh material.
As the entire human frame begins to suffer from the effects of senescence, which occurs in different individuals at very different periods, the cerebral cells, like the other elements of the organism, suffer a premature decay.
They grow old histologically ; they become more or less infiltrated with fatty granular matter ; they cease to be transparent, shrivel up, and from a dynamic point of view insensibly lose a portion of their sensibility and their special retentive power ; so that, as foci of organic phosphorescence, it may be said that they are extin guished within certain circumscribed localities of the cere bral cortex, and consequently cease to preserve a record of their first impressions. Thus it is that the general
phenomena of mental activity undergo a perceptible decay proportional to the sum of the cerebral elements superannuated. In the a:',ed, memories sometimes dis appear in an isolated manner ; sometimes those which are not maintained by regular exercise become extinct; sometimes the general faculty of memory fails altogether, and in its decay involves the progressive blunting of the most lively sentiments.
A strange phenomenon now occurs—we perceive, contrary to what a priori would seem most probable, that in old persons, as in patients with dementia, old memories remain the freshest and most vivid, while recent facts, impressions which occur at the very moment, are unperceived and treated as if they did not exist. It is probable that at this period of life, the cells of the sensorium, altered in their essential constitution, have become lazy, and incapable of erecting themselves in the presence of recent external impressions and that this state of torpidity of the elements of the sensorium for new excitations, leaves the field free to the older ones which, not being obscured by more lively impres sions, continue to vibrate without opposition, and thus perpetuate the last phosphorescent gleams of a far-off past which is dying.*