Many instances of what Dr. Carpenter very happily calls “unconscious cerebration” will suggest themselves to the reader. We frequently find suggestions occurring to us suddenly--suggestions which could only have arisen as the result of a train of ideas passing through our minds, but of which we have been unconscious. This function of the brain continues in sleep, but not with so much force as during wakefulness. The movements of the heart, of the inspiratory muscles, and of other organs which perform either dynamic or secretory functions are all rendered less active by sleep; and during this condition the nervous system generally obtains the repose which its ceaseless activity during our periods of wakefulness so imperatively demands. Sleep is thus necessary in order that the body, and especially the brain and nervous system, may be renovated by the formation of new tissue to take the place of that which by use has lost its normal characteristics. From what has been said it will be seen that the brain is no exception to the law which prevails throughout the whole domain of organic nature--that use causes decay. The following extract from another work[1] bears upon this point, and I think tends to its elucidation. “During life the fluids and tissues of the body are constantly undergoing change. New matter is deposited, and the old is removed with ceaseless activity. The body may be regarded as a complex machine, in which the law, that force is only generated by decomposition, is fully carried out. Every motion of the body, every pulsation of the heart, every thought which emanates from the encephalon is accompanied by the destruction of a certain amount of tissue. As long as food is supplied in abundance, and the assimilative functions are not disordered, reparation proceeds as rapidly as decay, and life is the result; but should nutrition be arrested, by any cause, for any considerable period, new matter ceases to be formed, and the organs, worn out, act no longer, and death ensues. “The animal body differs from any inorganic machine in the fact that it possesses the power of self-repair. In the steam-engine, for instance, the fuel which serves for the production of steam, and subsequently for the creation of force, can do nothing toward the repair of the parts which have been worn out by use. Day by day, by constant attrition and other causes, the engine becomes less perfect, and eventually must be put in order by the workman. In the animal body, however, the material which serves for the production of force is the body itself, and the substances which are taken as food are assimilated according to their character by those organs and parts which require them.