identical grooves 10 to 80 in the form of an arc spaced apart symmetrically with respect to the transverse axis 2 each groove having its convexity turned towards the transverse axis the width of each arc shaped groove decreases slightly from the longitudinal axis 1 towards the edge 5 of the mattress however a pocket 11 12 21 22 81 82 of an oval shape is disposed in line with the corresponding groove 10 20 80 at each of the ends of the groove and at a short distance from the edge 5 of the mattress this pocket locally doubles the width of the groove there are then in all sixteen pockets in line with the different arc shaped grooves seen in section each arc shaped groove 10 to 80 and each pocket 11 12 to 81 82 has the shape of a v with a rounded bottom the depth of the arc shaped groove 10 is constant over the whole of its length except for the ends where it is substantially greater that is to say more than three times greater in line with the pocket 11 12 to 81 82 as can be seen in fig3 where pocket 11 is visible then between the pocket 11 and edge 5 of the lower face 3 of the mattress groove 10 has in section a ramp shape 13 14 by which its depth decreases progressively to zero in the direction of edge 5 in the absence of such ramps 13 14 once covered with a protecting bag the mattress would have two unaesthetic hollows close to the edge 5 of the mattress it is also clear from fig3 that all the arc.