Surgical, medical and dental apparatus and instruments and in particular armchairs for surgical operations and seats; armchairs for podology treatment, for otorhinolaryngology treatment and opthalmological treatment, armchairs for ophthalmological treatment and gynaecological treatment, armchairs for dialysis treatment, armchairs for endoscopy treatment; sanitary furnishing items specially for medical studies and in particular for dentistry, for podiatry, for otorhinolaryngology and for ophthalmology practises