Out of desperation, you start trying to call for help, convince the scientists that they can't do this, you're a human being, a living, sentient creature with rights! But your lack of teeth and a tongue prevent any meaningful communication, all you can do is make incoherent blubbering noise. One of the scientists glances over at you with irritation, almost like you'd look at an annoying, yappy little dog - and hits a button on a console.
Just out of your notice, a tube-like tentacle starts snaking its way upwards through one of the holes in the grate below you. It's smooth, shiny black, and almost looks more like liquid than solid. It weaves its way upwards, but you're too busy trying to shout for help to notice it until it's practically in front of your face. You've barely realized its presence when the tendril starts snaking towards your mouth.
You clap your jaws shut and hopelessly try to twist and turn your face away, but the tentacle has much more freedom of motion than you do - and your empty mouth doesn't have much to offer in the way of resistance. A thin tendril slips through the edge of your lips, oozes past your gums as though it were fluid, and once it's found its way into your mouth, starts expanding rapidly.
It forces your jaws open and fills every available space in your mouth, and you gag as it finally forces its way down your throat and into your esophagus. It's cold and slimy, and although everywhere it touches seems to start tingling and eventually going numb, you can still feel it as it works its way downwards, settling into your stomach. Once there, it start rhythmically pulsating - pumping your stomach full of... who knows what.
Unbeknownst to you, while this is happening, a second tentacle is emerging from the grate below you, but this one has a much different target. Your eyelids shoot wide open as you suddenly feel something cold and slimy pressing against your anus. The tentacle in your mouth holds you in place, making your struggles even more useless than before, your writhing movements pressing and squeezing the walls of your stomach against the slimy intruder. You try to clench your sphincter muscles to stop the second tentacle, but it's no use - like the first one, as soon as a tiny thread of it worms its way in, then the rest follows easily. It burrows its way into your intestines and then, like the first, starts steadily, rhythmically throbbing, leaving you pinned helplessly, a tentacle stuck in either end.
Finally, a mechanical arm extends from the ceiling, equipped with a more ordinary-looking hose with a nozzle at the end. It takes aim at you and starts spraying you with some sort of thick, goopy liquid. It circles you, moving mechanically up and down, until you're completely coated in the stuff. For a brief moment you panic about breathing, as the goo seeps into your ears and nose, but it seems as though the tentacle in your mouth has branched off into your trachea as well, and is doing your breathing for you.
And then, it seems, you're just left like this. Completely helpless, with your face and entire body coated in the mysterious sludge, unable to see, hear or feel anything but the two tentacles pulsating in both ends. But then, out of some combination of the goo absorbing into your skin and whatever is being pumped inside of you, your whole body starts to feel strange.