Reforming Last-Resort Lending: The Flexible 
Open-Market Alternative
The most fundamental of the Federal Reserve System’s many responsibilities is that of serving as the U.S. financial markets’ ultimate source of liquidity. Federal Reserve notes, along with account balances held by private depository institutions at the various Fed banks, are the U.S. economy’s final means of payment, and hence its most liquid assets, the scarcity of which is a crucial determinant of the scarcity of other liquid assets.
Source: heritage.org