Is it acceptable to have the joining words meanwhile and however in the middle of a sentence? I have always thought that they should open a sentence, unlike other conjunctions such as because or although which can used at the beginning of a sentence or in the middle.
These words can be used within parathetical commas (as pointed out by Roaring Fish +1) or as a conjunctive adverb following a semicolon. When used with a semicolon they begin an independent phrase which is linked using the word. For example:
It can occur in both melodic and harmonic lines; however, it is subject to certain restraints. *
While the second independent phrase could stand on its own as a sentence, there may be reasons why one might choose to conjoin the two phrases using a semicolon and the conjunctive adverb.