The thyroid gland has a profound influence on the mind, both in its emotional and its intellectual aspects. The emotional features of hyperthyroidism are too well recognized to need stressing. So, too, is the association between gross diminution of thyroid function and the mental incapacity of cretinism. It was suspected, therefore, that even in those types of mental deficiency in which diminution of thyroid function was not recognizable clinically, it might be possible to detect some evidence of abnormality in the thyroid gland by histological study. Further justification for presupposing the possibility of thyroid derangement may be found in the fact that in many conditions an association between the thyroid and thymus has been established, and that in mental deficiency the thymus undergoes premature atrophy.