cises; we may indulge emotions, and carry out pursuits, and yet not be in a state for storing the memory, or amassing knowledge. Even the incidents that we take part in sometimes fail to be remembered beyond a very short time.
What, then, is there so very remarkable and unique in the physical support of the plastic property of the brain? What are the moments when it is at the plenitude of its efficiency? What are the things that especially nourish and conserve it?
Although there is still wanting a careful study of this whole subject, the patent facts appear to justify us in asserting that the plastic or retentive function is the very highest energy of the brain, the consummation of nervous activity. To drive home a new experience, to make an impression self-sustaining and recoverable, uses up (we are to suppose) more brain-force than any other kind of mental exercise. The moments of susceptibility to the storing up of knowledge, the engraving of habits and acquisitions, are thus the moments of the maximum of unexpended force. The circumstances need to be such as to prepare the way for the highest manifestation of cerebral energy; including the perfect freshness of the system, and the absence of everything that would speedily impair it.
To illustrate this position, I may refer to the kind of mental work that appears to be second in its demand on the energy of the brain. The exercise of mental constructiveness—the solving of new problems, the applying of rules to new cases, the intellectual labor of the more arduous professions, as the law, where a certain amount of novelty attends every case that occurs—demands no little mental strain, and is easy according to the brain-vigor of the moment. Still, these are exercises that can be performed with lower degrees of power; we are capable of such professional work in moments when our memory would not take in new and lasting impressions. In old age, when we cease to be educable in any fresh endowment, we can still perform these constructive exercises; we can grapple with new questions, invent new arguments and illustrations, decide what should be done in original emergencies.
The constructive energy has all degrees, from the highest flights of invention and imagination down to the point where construction shades off into literal repetition of what has formerly been done. The preacher in composing a fresh discourse puts forth more or less of constructiveness; in repeating prayers and formularies, in reading from book, there is only reminiscence. This is the third and least exigent form of mental energy; it is possible in the very lowest states of cerebral vigor. When acquisition is fruitless, construction is possible; when a slight departure from the old routine passes the might of the intelligence, literal reminiscence may operate.
Another mode of mental energy that we are equal to, when the freshness of our susceptibility to new growths has gone off, is search-
