The architecture and chemical constitution of the neural elements indicate unmistakably, it seems, that they were so constructed that in their functioning they would be amenable to the law of the conservation of energy, and recent investigations have produced some experimental evidence in support of this view. Hodge,[26] who succeeded in making microscopical examinations of living nerve cells while under stimulation, demonstrated that the cell by this treatment was depleted of its contents, as revealed in the gradual reduction of its size. In corroboration of these results it was found that the cerebral cells in animals were larger in the morning than after a day's activities, indicating that depletion must have taken place during waking life, followed by recuperation in sleep. Some interesting data relating in a way to this matter are easily gained in the laboratories by the use of the plethysmograph, which is designed to record the degree of blood pressure in different parts of the body. This instrument may be put upon the wrists and head, for instance, and it may be observed, when a person is subjected to certain influences whether, there is any alteration in blood supply in either region. It may be noticed, as a matter of fact, that when one is required to think diligently upon any problem, or being asleep is awakened or even disturbed by a noise in his environment, the volume of blood decreases in the wrist and increases in the head.[27] This same phenomenon is shown by experiments with the scientific cradle.[28] The inference from these data seems reasonable, that mental activity causes an expenditure of nerve force, which Nature seeks to replenish by inciting an unusual flow of nutritive-bearing fluid to the cerebral cortex. It has been shown, in further illustration of this law, that thought increases the temperature of the head, indicating that heat is generated through molecular activity; and also that psychical action increases waste products in the system, which may be derived only from the degradation of substances in nerve cells.[29] So information obtained from various other sources points toward the conclusion that in all activity energy stored in nerve cells is dissipated. [Footnote 26: For a complete statement of methods and results, see Hodge, American Journal of Psychology, vol. ii, pp. 3 _et seq._; and vol. iii, pp. 530 _et seq._] [Footnote 27: See Pedagogical Seminary, vol. ii, pp. 12 _et seq._] [Footnote 28: Ibid., _op. cit._] [Footnote 29: Cowles, Neurasthenia and its Mental Symptoms, pp. 17 _et seq._]