Nay, much rather, those members of the body which seem to be more feeble are necessary:
Nay, much more those members of the body, which seem to be more feeble, are necessary:
But instead, there is [absolute] necessity for the parts of the body that are considered the more weak.
Instead, the parts of the body that people think are the weakest are the most necessary.
In fact, so much more necessary are those parts of the body which seem to be weaker.