There were a couple of postings regarding syllables vs. resonance. I'd like to comment. I'm a new user, so I hope I send this to the correct address.
When you play a trumpet a pressure wave leaves the moutpiece/lip interface, reflects somewhere near the bell (depending on the pitch, but that is another issue) and comes back and hits you in the lips. Only certain notes do this in an organized fashion, the ones that resonate. This provides feedback to our lips on resonant notes, hence we feel the slots.
Now, there is a lot of sound in your mouth when you play as well. Also, the oral cavity provides a resonant chamber (a Helmholtz resonator of sorts). It can provide some feed back to the lips and the pitches that it supports can be adjusted by changing the size of the resonator - -- by raising or lowering the tounge.
Finally, when you play, your lips don't produce only one frequency. The input pressure wave function actually resembles a square wave, which is laden with partials. The horn, in addition to sounding a note that you choose with your lips, also supports these partials which are present in such a way as to make the timbre we recognize as a trumpet sound.
OK, I lied. FINALLY, (really), you can also adjust some of those partials slightly by changing the size of the oral cavity providing some extra feedback on some of the upper partials. It is not a big change, but it is audible and can help with higher notes.
I am going to add a commentary on my web site regarding this issue. I will also be adding a detailed "layman's" discussion of the physics of trumpet. There are also many books on this subject listed in my suggested reading section. Regarding this particular issue, I would suggest reading "The Acoustical Foundations of Music" by John Backus. He has an excellent discussion on brasswinds. Also, "The Science of Sound" by Tom Rossing is quite good. They're both in print.