Where is the law that requires a Doctor to pay for an income tax?

If the supreme court said that the 16th Amendment did not give congress any new taxing power, and if the supreme court stated that the income tax is a tax on activities (excise tax) and not the income, where in title 26 does it address my activity? I am looking for something like this: "There shall be assessed, levied, collected, and paid annually upon the gains, profits, and income received in the preceding calendar year by every citizen of the United States from any profession, trade, employment, or vocation carried on in the United States or elsewhere, or from any other source whatsoever, a tax"