Section 18
SEC. 18. The Collector or other internal revenue officer shall require every return to be verified by the oath of the party rendering it. If the Collector or other officer has reason to believe that the amount of any income returned is understated, he shall give due notice to the person making -the return to show cause why the amount of the return should not be increased, and upon proof of the amount understated may increase the same accordingly. Such person may furnish sworn testimony to prove any relevant facts, and, if dissatisfied with the decision of an internal-revenue officer, may appeal to the Collector of Internal Revenue for his decision under such rules of procedure as may be prescribed by regulations.