Aschoff's studies of the aorta show that, "in infancy the elastic laminæ of the media stand out sharply defined, well separated from each other by the muscle layers, which are well developed.... From childhood there is to be observed a slowly progressive increase in the elastic elements of the media. Not only do the individual lamellæ seen in cross-sections become thicker, but also they afford an increasing number of fine secondary filaments feathering off from these and crossing the muscle layer, so that now they are no longer sharply defined, but more ragged upon cross-section. This progressive increase attains its maximum at or about the age of thirty-five, and from now on for the next fifteen years the condition is relatively stationary. After fifty there is to be observed a slowly progressive atrophy of the elastica. The media becomes obviously thinner and presumably weaker." (Adami.) It has also been found (Klotz) that after the age of thirty-five, the muscle of the media begins to exhibit fatty degeneration which after fifty years is well marked. The fatty degeneration may then give place to a calcareous infiltration or the fibers may undergo complete absorption. It would appear that the thinning of the aortic media is due not so much to the atrophy of the elastic tissue as to that of the muscle tissue. The elastic tissue does lose its specific property and the artery thus becomes practically a connective tissue tube. Scheel has made very careful measurements of the ascending, the thoracic, and the abdominal aorta, and the pulmonary artery. He found that from birth to sixty years, the aorta became progressively wider and lost its elasticity. The pulmonary changed little, if at all, after thirty to forty years, and where before it was wider than the aorta, it now was found to be smaller. In chronic nephritis both were widened. The continuous increase of width and length of the aorta stands in reverse relationship to the elasticity of its walls. Although the division of the lesions into nodular, diffuse, and senile has been the usual one, it is better to separate three groups into (1) nodular, (2) diffuse or senile, and (3) syphilitic. There is more known about the histology of the syphilitic form and the lesions which consist of puckerings and scars seen on opening an aorta just above the valves, and on the ascending portion of the arch are characteristic. A macroscopic examination suffices in most cases for a definite diagnosis.