Section 53. Nothing in sections forty-three to fifty-two, inclusive, shall apply to treatment by a registered physician not practicing dentistry, in case where he deems treatment necessary for the relief of his patients, or prevent a registered dentist of another state or his assistant from operating at a public clinic under the auspices of a duly organized and reputable dental college or association, or prevent a student of a reputable dental college, incorporated under the laws of this commonwealth and granting degrees in dentistry, from performing operations as part of the regular college course, or prevent the surviving spouse, executor or administrator of a registered dentist who has died, or the spouse of one who is incapacitated, from continuing for not more than three years his business under a registered dentist, or prevent a dental intern registered as provided in section forty-five A from engaging in the practice of dentistry as authorized by said section. A registered dentist shall have the same right to prescribe or buy drugs or medicines for use in the conduct of his practice, as a registered physician of the commonwealth.