How do I know when a sentence is complete?

I'm having trouble figuring out why certain sentences seem to continue on when it seems as though they should have stopped, and a new sentence should come afterward, not continue on. There are sentences with the subject and the verb and the complete thought, but they continue on when I thought they should have a period right after them. Here's an example: Sontag begins the premise that every historical era "reinvents the project of spirituality," wherein spirituality means the totality of an eras ideas and the resolution of painful structural contradictions inherent in the human situation." I figured that after spirituality, the sentence should have ended. I know that its a continuation, but I'm so used to the fact that a complete sentence shouldn't have a comma right after it, connecting another full sentence without the conjunction. The first sentence doesnt seem to be a subordinate clause, and Im wondering why it has a comma right afterward. Can someone tell me the mistake I'm making?