'deceived by his monkeys'

I was rather entranced by a quote on Radio 4's In Our Time this morning (or, 'The-Melvyn-Bragg-Clever-Programme' as I like to refer to it, because it always makes me feel at least 33% cleverer by proxy after listening!) referring to Galen*, the Roman physician and medical theorist as being 'deceived by his monkeys' 

*nope, no Roddy McDowell references you ignoramus!