The Medicare and Medicaid statutes are both part of the Social Security Act.
Citations can be confusing, because these titles were added many years after the original Social Security Act of 1935. So if you look at the original Social Security Act, you will not find title XVIII. And yet you will see cases and other documents that refer to the numbering from the Social Security Act.
You will also see cases and other documents that refer to the United States Code, where the laws are codified (in title 42).
The Social Security Act, as amended, is posted on the Social Security Administration's website.
The United States Code is available many places, including on FDSys, from the U.S. Government Publishing Office.
These examples show how § 1801 of the Social Security Act and § 1395 are two versions of the same text and cite each other.
A table from the Office of Law Revision Counsel lists sections of the Social Security Act and their corresponding sections in the United States Code.