The bridge starts with the lines:
Your friend is heavyBut he was readyAnd never stopped for a while
sung to a melody something like this:
The lines "Your friend is heavy / But he was ready" establish a rhythmic pattern, but this is broken by "stopped," which is sung with a value that's shorter than what's expected (a quarter note instead of a half note). In a way, this truncation matches the meaning (even though the line describes how the tortoise "never stopped").
In an opposite manner, "while" is sung with a long note value (two full measures, I think), lending a sense of this duration.
The second half of the bridge is:
While you were sleepingHe went on keepingThe final line in his mind
The line "The final line in his mind" exhibits assonance (the long i). This sound is sustained in the same way that the tortoise maintains his focus on his goal. That the sense is spread across the line break ("He went on keeping / The final line in his mind") also demonstrates this endurance.