visible in fig3 the arcuate grooves in the upper wall 15 overlie the arcuate grooves 16 17 in the lower wall the arcuate grooves as described above define rails which support the extendable elements 6 and 7 as can be seen most clearly in fig4 a and 4b each of the extendable elements 5 and 6 comprise an inner metal core 20 the inner metal core defines a vertical arcuate plate 21 the radius of curvature of the arcuate plate is the same as the radius of curvature of the arcuate grooves the upper and lower edges of the arcuate plate 21 are received within the arcuate grooves 16 17 18 so that each metal core 20 may slide along the rail defined by the grooves each metal core also defines an axially extending end piece 23 which extends axially of the seat to which the head rest is fitted and substantially transversely to the arcuate plate 21 the metal core is provided with an outer covering 24 formed of fabric or a layer of plastic which may simulate leather or a layer of real leather the outer covering 24 lies adjacent the rear surface of the arcuate plate 21 and then passes adjacent to the exterior of the end piece 23 the fabric or layer of plastic or layer of leather 24 then extends in front of the metal core 20 and is separated therefrom by means of padding 25 this part of the fabric or layer of plastic or layer of leather thus provides an upholstered front face 7 8 for the extendable elements the extendable elements are moved from the retracted position as shown in fig1 and fig4 a by manually grasping the exposed end parts of the extendable elements and applying.