I can't explain the grammar of this. Can you?
I have never written a sentence this long!

In the heyday of youthful greediness and ambition, when the mind, dazzled by the vastness and variety of the universe, must needs know everything, or rather know about everything, at once and on the spot, too many are apt, as I have been in past years, to complain of Cambridge studies as too dry and narrow: but as time teaches the student, year by year, what is really required for an understanding of the objects with which he meets, he begins to find that his University, in as far as he has really received her teaching into himself, has given him, in her criticism, her mathematics, above all, in Plato, something which all the popular knowledge, the lectures and institutions of the day, and even good books themselves, cannot give, a boon more precious than learning; namely, the art of learning.

Good old Google. I found this, which helps, in Quinion.
I still couldn't explain it to somebody else.

NEEDS MUST WHEN THE DEVIL DRIVES
[Q] From Jonathan Harpaz, Israel: “Does the following expression/idiom exist: needs must when the devil drives? If so, is it British or American and when did it originate?”
[A] The expression does exist, and as it happens is one of the older proverbs in the language, somewhat predating the USA. Shakespeare uses it in All’s Well that Ends Well: “My poor body, madam, requires it: I am driven on by the flesh; and he must needs go that the devil drives”. However, it is actually older—the earliest I can find is in John Lydgate’s Assembly of Gods, written about 1420: “He must nedys go that the deuell dryves”.
The form you quote is the usual modern one, but it isn’t so easy to understand, as it is abbreviated and includes needs must, which is an semi-archaic fixed phrase meaning “necessity compels”. The Shakespearean wording makes the meaning clearer: if the devil drives you, you have no choice but to go, or in other words, sometimes events compel you to do something you would much rather not.

Don't have quite the breath control to dive into that sentence and come up alive on the other end, but I've always taken the phrase must needs to mean must, of necessity. Is that what you're looking for Dr Bill?

Dear Faldage: my problem was that I didn't recognize a quote from Shakespeare, and was unable to construe it as
modern English.