So, how did this 'ideal form' of the sentence become such a priviledged thing? If we speak in contours, or poetic lines, how did the formal period-ended-capital-letter-begun sentence become 'proper', 'correct', 'a complete idea', as everyone learned in every writing class ever?
The notion of a "poetic line" is suspect. Theories about how the lines of a poem should be broken are as varied as the poets who write the poems. My first inclination is to read that quotation with respect to contemporary poetry which has tended to break with inherited form.
Specifically, I think of Charles Olson's pivotal PROJECTIVE VERSE essay. In it he examines--among other things--the role of the typewriter in freeing poets from the constraints of inherited form. He argues that the typewriter, with its "space precisions", gives the poet the same tools as a musical composer. That line breaks can denote breath, blank space too. With the typewriter the poet can "record the listening he has done to his own speech and by that one act indicate how he would want any reader, silently or otherwise, to voice his work."
This sort of idea is what I take the notion of a "poetic line" to be referent to. But here's the thing, it's a snake biting its own tail. Poetry is not created outside of society, and though in many cases it shirks traditional notions of grammar (invented usage), the pressures of standard English usage must still be felt upon it. So what came first? The poetic line or the sentence? Maybe I'm missing the point.
What can be taken away from Olson is this concept of breath, which must absolutely be central to any discussion of the way in which we break our language up. The simple fact of our body prevents us from uninterrupted speech (although, I wonder if those jazz musicians who practice circular breathing can do so). We only hold so much oxygen in our lungs to produce sound. Of course, in the case of writing/reading there is no 'breath'. Although, someone in a recent workshop of mine mentioned that we sound out the things that we read in our own heads (does breath become embedded here?). Additionally, perhaps there is breath in our cognition. How many things can we hold in her heads at one time? Perhaps the sentence rises out of some sort of corporeal necessity. A necessity for things to be digestable.