The large-scale layer structure of the gas spiral arms of Galaxy (figures 1 and 2) was found out for the first time in the galactic longitudes 10 - 40 degrees, of distances of 6 kpc from the centre of Galaxy right up to its external part. 3-4 large-scale layers of thickness 600 - 800 pc is usuall observed in the arms across the plane of Galaxy. The distance between layers in external parts of Galaxy is 1 - 1,5 kpc.

Figure 1. The distribution of the emission of the HI line on wave length 21 cm on radial velosities corresponding to external spiral arms.

Figure 2. The location of layers HI of the external spiral arm.