I know Nagini is a magical snake but...

I have just re-read the first chapter of Goblet of Fire for about the 15th time and it has just occurred to me that, well, mammals have mammary glands and other things (snakes for example)don't. I realise that brighter people than me have probably been asking this for the last 7 years, maybe even have asked JKR, but how exactly (or even roughly) do you milk a snake? I'm not surprised that Wormtail is reluctant and given the way 'milked' is used in countless slash fics I've read I keep thinking of it as an unpleasant euphemism. Though I'm sure that JKR just isn't very good at zoology.