Calling Sackville Street the heart of the metropolis is a metaphor, but coming where it does in the text—just before clanking tramcars, creaking mailcars, thudding barrels, thumping printingpresses—the phrase insinuates a near equivalence between human bodies of flesh and blood and civic bodies of wood, steel, and electricity.
If human bodies can be understood as assemblages of interrelated physical systems, then the interrelated physical systems comprising the humming life of a city may perhaps be understood as bodies as well. That analogy is implied by Joyce's use of the word heart as the lead-in to a section describing tramcars departing from the central dispatch station at Nelson's pillar for far-flung termini around the circumference of Dublin, and returning again to the center. In Hades Bloom thinks of the heart as a pump moving blood around the body; in death, "The circulation stops." And when, at the end of Aeolus, an electrical malfunction leaves inbound and outbound cars on eight different tracks "all still, becalmed in short circuit," the circulation of the city stops.
After the initial look at the tramways, the episode glances at other complex systems of transportation and communication: the cars, trains, and boats required to move mail around Ireland and England, and the casks required to move ale. Then the reader's attention is directed to still another kind of circulation: a major newspaper, whose offices are located close to the tram dispatch center and the General Post Office. We learn "HOW A GREAT DAILY ORGAN IS TURNED OUT," and later the hissing paper feeds give the impression of an animal trying to speak: "Sllt. The nethermost deck of the first machine jogged forward its flyboard with sllt the first batch of quirefolded papers. Sllt. Almost human the way it sllt to call attention. Doing its level best to speak. That door too sllt creaking, asking to be shut. Everything speaks in its own way. Sllt."
Joyce's schemas identify the heart as the "organ" of the previous episode. Hades makes frequent references to the heart as the proverbial seat of the emotions, but it also returns repeatedly to the scientific knowledge that the heart maintains life by pumping oxygenated blood through the body. Martin Cunningham says that Paddy Dignam died of a "Breakdown.... Heart." To Bloom, the heart is essentially no different from a mechanical contraption: "A pump after all, pumping thousands of gallons of blood every day. One fine day it gets bunged up: and there you are. Lots of them lying around here: lungs, hearts, livers. Old rusty pumps: damn the thing else." Bloom's thoughts in Aeolus about the owner of the newspaper suggest an analogy: "But will he save the circulation? Thumping. Thumping."
The thumping of the presses returns Bloom to his meditations on organic dissolution. Mechanical and biological processes combine in a vision of senseless pounding activity: "Thumping. Thump. This morning the remains of the late Mr Patrick Dignam. Machines. Smash a man to atoms if they got him caught. Rule the world today. His machineries are pegging away too. Like these, got out of hand: fermenting. Working away, tearing away. And that old grey rat tearing to get in."