I think the reading is this:
I' the cave, wherein the BOW, &c.
That is, they are trained up in the cave, where their thoughts in hitting the bow, or arch of their habitation, hit the roofs of palaces. In other words, though their condition is low, their thoughts are high. The sentence is at last, as THEOBALD remarks, abrupt, but perhaps no less suitable to Shakespeare. I know not whether Dr. WARBURTON's conjecture be not better than mine.
III.iii.101 (223,2) I stole these babes] Shakespeare seems to intend Belarius for a good character, yet he makes him forget the injury which he has done to the young princes, whom he has robbed of a kingdom only to rob their father of heirs.—The latter part of this soliloquy is very inartificial, there being no particular reason why Belarius should now tell to himself what he could not know better by telling it.
III.iv.15 (224,2) drug-damn'd Italy] This is another allusion to Italian poisons.
III.iv.39 (225,4) Kings, queens, and states] Persons of highest rank.
III.iv.52 (225,6) Some jay of Italy,/Whose mother was her painting] Some jay of Italy, made by art the creature, not of nature, but of painting. In this sense painting may be not improperly termed her mother. (see 1765, VII, 325, 9)
III.iv.63 (226,7) So thou, Posthumus,/Wilt lay the leaven on all proper men] HANMER reads,
—lay the level—
without any necessity.