Abstract
In a study of the peripheral arteries a period of development lasting until the end of the second decade of life is described. During this time there is evidence of medial hypertrophy and a continued formation of the longitudinal muscular cushions described in the foetus. Bands of longitudinal muscle also appear in the media of certain vessels. It is considered that the findings confirm the functional significance of foetal cushions and further suggest their relationship to the stresses of pulsation. In the later decades the tendency of atherosclerotic plaques to develop at the site of cushions is believed to reflect the importance of mechanical factors in the localization of this disease.