Mars is my Destination by Frank Belknap Long

I had no way of knowing how long I remained on the outer fringes of what was probably just a weakness-produced blackout before the outlines of the hospital room wavered back, becoming so clear again that I could see the foot of the bed, and a glass-topped table covered with small bottles and a roll of gauze bandage that looked about as big as a liquid fuel cylinder.

Someone who couldn't have been the doctor was sitting in a chair by the bed, leaning a little forward, his eyes level with mine. I was more than startled. An ice-cold measuring worm came out at the base of my spine and started inching its way upward, bunching itself up and lengthening out again, the way measuring worms do when they're trying to decide if you're just the right fit for a human-style coffin.