The item which will engage our attention principally here relates to the primary function of the nerve cell--to store up vital forces in the form of highly unstable chemical compounds,[17] which may upon slight disturbance be broken down, the static energy represented in their union thus becoming dynamic. Those who have given special attention to the matter seem to agree that all activity, physical as well as mental, involves the expenditure of a portion of this energy.[18] It may perhaps be mentioned in passing that when this conception was first being presented some persons hastily constructed the theory that what people had been calling mind was nothing more nor less than a certain mode of manifestation of this mysterious but yet physical force. While abundant evidence, gained from various sciences by recent research, leads one to the conviction that in some unknown manner psychical and neural processes are closely co-ordinated, yet not a single investigator of standing claims that they are identical. There is doubtless among some in our day too great a tendency, unconscious though it may be for the most part, to declare that a description of the physical correlates or antecedents of mental phenomena fully accounts for the latter in respect alike of their nature and their modes of manifestation; but those who find themselves coming to such conclusions might be both interested in and benefited by examining the opinions of great naturalists and psychologists who have reflected long and profoundly upon the world-old problem of the connections between body and mind--such men as Lotze,[19] Darwin,[20] Romanes,[21] Wallace,[22] Fiske,[23] Drummond,[24] Wundt,[25] and many others of equal scientific attainments. [Footnote 17: For chemical formulæ of some of the compounds, see Ladd, Outlines of Physiological Psychology, p. 13.] [Footnote 18: For the opinions of investigators, as Mosso, Lombard, Maggiora, Kraeplin, and others, see Pedagogical Seminary, vol. ii, No. 1, pp. 13-17; Scripture, The New Psychology, chapter xvi; and Educational Review, vol. xv, pp. 246 _et seq._] [Footnote 19: Microcosmus, p. 162.] [Footnote 20: Descent of Man, p. 66.] [Footnote 21: Mental Evolution in Man, pp. 218 _et seq._] [Footnote 22: Darwinism, p. 469.] [Footnote 23: Destiny of Man in the Light of his Origin.] [Footnote 24: Ascent of Man.] [Footnote 25: Human and Animal Psychology, pp. 5-7 and 440-445.]