Fine dark Ha filaments 'fibrils' form at the limb, apparently in most of the middle chromosphere corresponding to an altitude between 1500-2000kin and 4000km. The space in between filaments is corona and the transition layer. The 'cool' gas in fibrils is protected by the magnetic field against the conductive flux out of the 'hot' corona. Therefore the fibrils stretch up to 4000 km where their temperature is about 18000 K and the density about 5 x 10 ~ cm -z. The gas in the fibrils is ionized by electronic collisions and by the external ultraviolet radiation. The second level of the hydrogen atoms in fibrils is populated by recombinations, electronic collisions and by Ly-a quanta. The calculated optical thickness of the fibrils in Ha is about 1, it explains the absorption features on the spectroheliograms. The gas pressure in fibrils is lower than the coronal pressure, and the pressure equilibrium is achieved by a magnetic field of about 1.5-2 G. In the active regions the photospheric fields are stronger, therefore the fibrils in active regions are wider and show more contrast. The emission of the fibrils at the limb is explained by the scattering of the solar radiation. The temperature in arches reaching as high as 5000-6000 km, is stabilized near the top by the HexI emission. Thus the middle chromosphere is essentially a collection of magnetic arches.