This is the reading of the 4to; the folio reads in the second line, 'Does tyre the ingenieur,' of which it seems almost impossible to make any good sense. 'The essential,' etc., means person, body, form.
"If this poor trash of Venice, whom I trace
For his quick hunting, stand the putting on."
This is the reading of the folio; the 4to 1622 has crush for 'trace'; and Warburton, followed by Steevens, Singer, and Dyce, read brach for 'trash.' These last-named critics read the whole line thus: "If this poor brach of Venice whom I trash." "The jingle," Steevens says, "being in Shakespeare's manner." Now to this I object—first, that this was not Shakespeare's manner, for the apparent instances of it are mostly printers' blunders; and secondly, that Roderigo did not require to be trashed or checked 'for his quick hunting,' for he was always hanging back and ready to give up the chase till urged on by Iago. This last objection also applies to 'trace' in the sense of follow or accompany.
"Can trace me in the tedious ways of art,
Or hold me pace in deep experiments."
1 Hen. IV. iii. 1.
It would also apply, though in a less degree, to train, which would yield a tolerable sense. On the whole, I think that Iago's words may have been praise, which would suit his sneering ironical tone. As to 'brach,' though we frequently find it used of a woman, I believe it was never applied to a man.