When the ductless glands are not in good working condition, there are three principal things which can occasion infection or intoxication. These are deficient nutrition, exposure to cold, and a depressed mental condition. By these the resistance of the cells against the energy of the invading microbes is lowered, and the greater the invasion the easier will be their victory. We will often refer to this in the chapters on personal hygiene, and propose certain remedies for avoiding these predisposing sources of infection and intoxication. ------------------------------------------------------------------------ CHAPTER IV. ON THE AGENCIES THAT GOVERN THE CONDITION OF THE NERVOUS SYSTEM AND MENTALITY. BY treating with thyroid extracts a child that has remained backward in his mental development we can make a curious observation. The child who had previously been a cretinous idiot will not only improve bodily but also mentally, and he will be transformed into an intelligent being with normal mental faculties. The logical deduction is that the thyroid must influence powerfully the condition of our nervous system and mentality. Indeed, the physiological activity of the nervous system and mentality depends entirely on the co-operation of the ductless glands with internal secretion. In fact, we do not think we are going too far in saying that the condition of the nervous system and mentality is mainly governed by these glands. The truth of this assertion is shown by the fact that any alteration of these glands, especially the thyroid and sexual glands, and pituitary body, is always followed by alterations of the nervous system. This is strikingly sustained by the elaborate researches of Sajous who found that the reactions of fluids circulating in all nervous elements corresponded with those of internal secretions and particularly that of the adrenals. Removal of the thyroid also produces far-reaching anatomical changes in the central part of the nervous system which, as we have mentioned, has been described by Albertoni, Tizzoni[87] Blum,[88] Walter Edmund,[89] and others. These consisted of the destruction of nervous cells and nervous processes, chromatolysis, and also the augmentation of the neuroglia, which increases in the same way as the connective tissue in all other organs and tissues. Footnote 87: Arch. per le scienze Mediche, vol. x., p. 45, 1886. Footnote 88: Blum: Virchow’s Archiv, 1899. Footnote 89: Walter Edmunds: Transact. of the Path. Soc. of London, vol. liii., Part 3, p. 343, 1902.