Clinical mercury thermometers. It is printed on them that they are for rectal use.
4.5" long. They are designed so that the full range of the 2-inch-long graduated scale is from 96 to 106 degrees, so these are not the sort of thermometers you would use to tell the temperature of a room, they are designed specifically for measuring the body temperature of higher mammals. They come sterile in a screw-top plastic tube.
They are listed as single-use thermometers, but there is a way around that (using centrifugal force - more below). They retain the highest temperature they have ever recorded. If warmed to a temperature of 101 degrees, it will say 101 degrees. If it cools down to room temperature, it will still say 101 degrees. You can force the mercury back down with centrifugal force however. Hold the thermometer in your hand with the mercury bulb pointing out, and swing your arm like you were pitching a baseball. This has a disadvantage but an advantage as well. The disadvantage is that reusing them is a little annoying (especially if you take a temperature often - or in my case, forcing the mercury down in over 200 thermometers because I stored them in a place that got too hot, I could do it holding 3 or 4 thermometers at a time, but still my arm really hurt at the end). The advantage is that you are guaranteed to get an accurate reading. If you see a temperature of 100.3, you will never have to worry about whether the actual temperature was 100.5 or 100.7 or 100.8 and it just cooled down before you read it, or maybe it shifted around. If you see 100.3, you will know that was the highest temperature it was exposed to and you can take your time reading it. And you can REALLY take your time. I tried putting them in a freezer for months with no effect before I figured out I could centrifuge them to get the mercury down.