Former 'Tonight Show' Bassist John B. Williams on Hospice, Has Dementia

Legendary bassist John B. Williams -- best known for playing on "The Tonight Show Starring Johnny Carson" -- is on hospice being cared for by 'round-the-clock nurses, TMZ has learned.

His wife, Jessica Williams, tells TMZ ... he recently suffered a fall and subsequently underwent brain surgery as a result of his injuries. Jessica, an accomplished musican herself, says John had dementia prior to the fall, but the fall greatly accelerated its progression.

At this point, John is unable to walk and can't talk, and his health is on the decline. We're told one if his last conversations was with his daughter.

In addition to being the house bassist for Johnny Carson's talk show between New York City and Los Angeles for 7 years, John spent nearly 6 years as a prominent member of The Posse, the house band on "The Arsenio Hall Show." He's also known for his work with some of jazz music's biggest names, including Count Basie and Louis Armstrong.

John grew up around music and the passion never left him even after a stint with the United States Marine Corps in the early 1960s -- he studied music at the experimental "The International College" in Los Angeles beginning in 1974 before forming his own band called Expectations in 1975. Shortly after, he was tapped to join the Nancy Wilson Trio ... where he would play for 25 years.