Concerning the exact function of the thymus, we are a good deal at sea. The latest opinion about the results of extirpation even in young and growing animals is that they are nil. Yet there is a certain justification for proclaiming the thymus the gland of childhood, the gland which keeps children childish and sometimes makes children out of grown-ups. There is a quantity of data for that proposition. In the first place, the curve of rise of growth of the gland seems to coincide with the period of childhood, the curve of its decline with the period of adolescence and the rise of the sex glands. In the past, it was accepted, that with puberty the thymus atrophied and was replaced by some sort of fatty tissue. Nowadays, it is held that secretion cells persist throughout life. When the extent of this persistence is too great, the gland being from five to ten times as large as the normal, a number of other features become prominent to make the extraordinary individual, the status lymphaticus, who amid the hazards of life will react in an extraordinary way. He will be taken up in the consideration of internal secretion personalities. Then there are the varied and remarkable phenomena of thymus enlargement and hyperactivity in childhood itself. When an enlarged thymus is present in an infant, the initiation of breathing in the new-born, the introduction of the newcomer to the oxygen of the air, may be an exceedingly prolonged, difficult, matter. Such a baby is said to be born blue, and the breathing may be stridorous for days, becoming normal for a time, to be followed later by spells of trouble in breathing, breathlessness or breathlessness with blueness, and threatened extinction. Sometimes these spells come out of a clear sky in an apparently healthy child. That some poison, probably an oversecretion of the thymus, is responsible is shown by the relief obtainable by X-ray shrinkage of the gland, or the surgical removal of a part of it.