Reasons for not using the word mesial:
1. Only dentists will understand it.
2. It sounds like a slightly up-market version measly, which will mislead anyone gamely trying to guess what on earth you're going on about.
3. It is in fact a synonym for medial, so you might as well use that, as it is much more guessable and you might even pass as a scientist or mathematician.
4. Even better, you could use the phrase in the middle, which means the same thing in most circumstances (except dentistry, where it means towards the middle).
5. Well, you don't want to look like a dork, do you.
Word Not To Use Today: mesial. This word comes from the Greek mesos, which means middle.