PRACTICAL PHASES OF MENTAL FATIGUE. 513 



Drummond,* ~VVundt,f and many others of equal scientific attain- 

 ments. 



The architecture and chemical constitution of the neural ele- 

 ments 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,:}: 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 re- 

 duction 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 de- 

 signed 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 vol- 

 ume of blood decreases in the wrist and increases in the head.* 

 This same phenomenon is shown by experiments with the scientific 

 cradle, j | 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 illustra- 

 tion 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. A So information obtained from various other sources 

 points toward the conclusion that in all activity energy stored in 

 nerve cells is dissipated. 



Recent experimental studies have given us reasons for believ- 

 ing that nerve cells in different individuals yield up their energy 

 in response to stimulation with varying degrees of readiness. Q Ex- 



* Ascent of Man. f Human and Animal Psychology, pp. 5-7 and 440-445. 



\ For a complete statement of methods and results, see Hodge, American Journal of 

 Psychology, vol. ii, pp. 3 et seq. ; and vol. iii, pp. 630 et seq. 



* See Pedagogical Seminary, vol. ii, pp. 12 et seq. \ Ibid., op. cif. 

 A Cowles, Neurasthenia and its Mental Symptoms, pp. 17 et seq. 



() Educational Review, op. cit. 

 VOL. LV. 37 



