It wasn’t monkey testicles that Yeats would have had transplanted but goat ones. Although your recollection of the donor species was incorrect, it was fairly common to have testicular xenotransplants in the 20’s.
I am speaking of one “Doctor” John R. Brinkley, a/k/a “the goat gland doctor”. Brinkley was a quack, through and through, but a horrifyingly successful one. He performed over 16,000 xenotransplants of goat testicles or testicular tissue into men. Supposedly, the goat testicles would “reinvigorate” older men, prevent or treat a whole host of ailments, and restore libidos.
These “treatments” weren’t cheap, either: $750 during the 1920’s, or over $9,000 in today’s dollars. For $5,000 (equivalent to almost $61,000), he would implant human testicles obtained from death-row inmates.
When I say “transplant”, you shouldn’t think of the type of careful, meticulous, difficult microsurgery that I was discussing in the Staff Report. Brinkley apparently just kind of tossed the new ones in and crudely sutured them to the existing equipment.
I didn’t find out about Brinkley until after I had written the Report, first hearing about his antics from bibliophage.
Yeats was not one of Brinkely’s patients, but he was a bit obsessed with “stimulating his muse” (as it were) in later life. According to Brenda Maddox in Yeats’s Ghosts, Yeats did have an operation for “rejuvenation by experimental revitalization of the aging puberty gland.” Today, we call this procedure a vasectomy. The production and role of testosterone was not known at this time, so a vasectomy was thought to boost sex drive by retaining sperm in the testicles.
Yeats became associated with Brinkley through jokes, mainly. Although Yeats had his operation in 1934 after Brinkley was out of business, the goat gland doctor was so famous that any operation to boost libido was bound to be linked to him. Brinkley had not only a major and internationally-known practice, but also a nationwide radio call-in show, owned multiple radio stations, and almost became governor of Kansas in a write-in campaign! As soon as Yeats disclosed to his friends that he had his operation, jokes about goat glands were circulated. They still are in some places.