A casing defining an internal chamber having a skinfold formed therein when it is applied to a patient's skin, said fold coming into contact with the lower edges of the chamber. The chamber is defined by two lateral walls and by two transverse walls, at least one of said transverse walls being formed of a flap capable of having a pivoting motion, to induce the coming closer and the drawing away from each other of the lower edges of said transverse walls in contact with the skinfold. The flap is hinged in the vicinity of its upper end on the lateral walls. The pivoting of the flap is obtained by means of a geared motor fixedly assembled within said flap, and having its output shaft rotating a cam, received in a cam race fastened to one of the lateral walls of the internal chamber.