The Figure shows the distributions perpendicular to the planes of the galaxies of: (a) the surface density of HI, σHI(R), (b) the surface density of HII regions, σHII(R), and (c) the radio brightness temperature, σTB(R). Details may be found in Berkhuijsen (A.A. 57, 9, 1977; Proc. IAU Symp. 77, 1978); here σHI(R) of the Galaxy was taken from Gordon and Burton (Ap. J. 208, 346, 1976) and σTB(R) of M31 has been revised. Distances to the centre are scaled with the Holmberg radius RH (21, 15 and 7.8 kpc for M31, Galaxy, M33).