Samuel Ball Platner, A Topographical Dictionary of Ancient Rome, London: Oxford University Press, 1929.

Because of Augustus, there were 14 regions in the city of Rome compared to its previous 4. This division into fourteen regions continued in force until the sevent century when an ecclesiastical division into seven regions was introduced and opened the way for the entirely different organization of the Middle Ages.

XII, Piscina Publica. So called from a district within its limits that had formerly contained a public reservoir or swimming bath. This region included the eastern part of the Aventine, and was bounded by the via Appia and Region I, the Aurelian wall, and the vicus portae Raudusculanae and the vicus Piscinae Publicae.

PISCINA PUBLICA: a public bath and swimming pool (Fest. 213), first mentioned in 215 B.C. Liv. xxiii.32.4), situated in the low ground between the via Appia, the Servian wall, the north-east slope of the Aventine, and the area afterwards occupied by the baths of Caracalla (Liv. Fest. locc. citt.; Cic. ad Q. Fr. iii.7.1; Jord. ii.106-107; HJ 183-184). Near it was the headquarters of the lanii piscinenses (CIL vi.167; cf. Plautus, Pseud. 326-328). This pool later gave its name to the vicus piscinae Publicae (CIL vi.975; Amm. Marcell. xvii.4.14), which led from the south end of the circus Maximus across the depression on the Aventine to the porta Raudusculana. The piscina itself was probably fed by local springs, not by the aqua Appia (LA 234-245; cf. Jord. i.1.447, 458), and had ceased to exist in the second century (Fest. 213), but the name clung to the locality (cf. ad piscinam publicam Hippolyt. philos. ix.12, p552; cf. BC 1914, 353), and it was popularly given to Region XII of the city of Augustus. This region was bounded on the north-east by the via Appia, on the south-east by a line extending from the junction of the via Appia and the vicus Sulpicius to the porta Raudusculana, on the south by the line of the Aurelian wall, and on the west and north-west by the vicus portae Raudusculanae and the vicus piscinae Publicae, thus including a very small area inside the line of the Servian wall (BC 1890, 115-137). Piscina Publica was not an official name for Region XII, and we do not know how early it came into use (Pr. Reg. 71-72).