The death of a son of a German professor, a few years ago, shortly after an injection of antitoxin to protect him from diphtheria, and the subsequent discovery that the boy was the victim of this condition, caused a widespread interest in and a most careful study of the problems involved. They occur in the thymus more frequently than in any other organ of the body. Normally the thymus begins its involution within a few months after birth, and this process should be completed at puberty. In the status lymphaticus this involution does not occur, but enlargement persists or increases even into adult life, varying in different cases, the weight of the thymus varying from 20 to 135 grams. While the thymus may for some purposes be grouped among the lymphatic tissues of the body, little is known as to its function. Its juice contains leukocytes, which find their way into the general circulation, and it is supposed to have an internal secretion correlated with that of other ductless glands. Injections into dogs of thymus extract produce a fall in the blood pressure, with acceleration of the heart, and, in fatal doses, dyspnea and collapse. While enlargement of the thymus may cause death by pressure on important structures other than the trachea, it produces a type of asthma known as _thymic asthma_, in which death sometimes occurs unexpectedly and rapidly by strangulation. An examination of the thymus rarely shows anything more abnormal than the enlargement of its natural structure, with perhaps acute hyperemia, while occasionally the cut surfaces will exude a milky fluid; there will be found, in addition to these changes in the thymus, a general hyperplasia of the lymphatic system, with enlargement of the superficial and deep nodes, especially the cervical and axillary, the inguinal and those in the abdomen. The spleen enlarges and the Malpighian bodies seem to be packed with lymphoid cells. There may be enlargement of the heart and increase in the thickness of the arterial walls. This is so marked that Virchow suggested the name _lymphatic chlorotic constitution_. It has been suggested that the narrowing of the aortic valve in these cases is due to this lymphoid infiltration. _The relations between rickets and the status lymphaticus_ are so frequent and so conspicuous as to make one suspect a more than casual connection between them. Nearly all cases of lymphatism show the ordinary clinical evidences of rickets. By some such relation may be explained the benefit which accrues in rickets from the administration of the extract of thymus, as well as of the thyroid and the pituitary body.