Licensed dentists may employ or engage the services of any? person, firm or corporation to construct or repair, extraorally, prosthetic dentures, bridges, or other replacements for a part of a tooth, a tooth, or teeth. A person, firm, or corporation so employed or engaged, when constructing or repairing such dentures, bridges, or? replacements, exclusively, directly, and solely on the written work order of a licensed member of the dental profession as hereafter provided, and not for the public or any part thereof, shall not be deemed or considered to be practicing dentistry as defined in this Article. (1957, c. 592, s. 3; 1961, c. 446, ss. 3, 4; 1979, 2nd Sess., c. 1195, s. 6.)