There is disclosed a portable instrument including an optical device (4; 5; 6) for measuring a physiological quantity, particularly the heart rhythm, and data emission and/or reception means, the optical device including at least one light source (41; 51; 61) for subjecting a portion of an organic tissue (10) to a light emission and at least one photoreceptor (42; 52, 53, 54; 62, 64, 66) for detecting the intensity of the light emission after propagation in the organic tissue. The optical device also forms the data emission and/or reception means, said at least one light source and/or said at least one photoreceptor being arranged for respectively emitting data to an external unit or receiving data from said external unit.