Each outcry of the hunted Hare A fibre from the Brain does tear.
The speaker's still in animal rights activist mode. Like the dog and the horse that came before it (and, really, the robin and the doves—since humans imprisoned them, too), the hunted hare is yet another sterling example of typical human cruelty. And like all those other critters, it is a "world in a grain of sand"—since its plight is the plight of anyone who gets unfairly pursued, tracked down, and killed.
Why the hare's cries tear fibers out of a brain is a tougher question (sounds painful). One way to think of it is that the cruel things people do help make them less human. In Blake's view, it's only genuinely human to do things that involve "Mercy, Pity, Peace, and Love"—but to slaughter people—or hares—is something else.
Hence, being cruel takes something away from you—it tears fibers off your brain, the body part that contains your sense of identity. If you lose too many pieces, you can end up becoming another mindless killer—sort of like Anton Chigurh in No Country for Old Men. You'd be more of a demon or an all-destroying robot than a person.
And here's a question: why does Blake capitalize the words he capitalizes? Think about this when you look at the other lines in the poem, too—then check out "Form and Meter."
A Skylark wounded in the wing, A Cherubim does cease to sing.
Now, it's back to birds. A skylark is, of course, a bird. But a Cherubim (technically, "Cherubim" is supposed to be plural, but Blake doesn't use it that way) is an angel—and not just a cherub of the small, naked, cupid variety. They've traditionally been pictured in ways that are a little more intense, with books and animals and wings popping out all over.
Blake emphasizes that a skylark getting injured isn't just some non-event—it matters, it has real significance. It causes an angelic being to fall silent, probably from sympathizing with its pain. Like Blake wrote in "The Marriage of Heaven and Hell," "All that lives is holy."
The passage also might allude to the Bible (Blake's favorite book). Matthew 10:29 reads: "Are not two sparrows sold for a penny? Yet not one of them will fall to the ground outside your father's care." If an injured or dead sparrow is important to God, the speaker implies, maybe we should take the smallest forms of suffering more seriously. Additionally, take a look at the brief alliteration between "wounded" and "wing." We have more to say about that in "Sound Check."