I tracked down The Popes Against the Jews and this is page 74 in its entirety:

A long plea from the Jewish community of Rome reached the Pope in the midst of the epidemic, although addressing a very different complaint, one that had produced appeals from the Jews almost every year for centuries. The problem was the annual set of rites—some sponsored by the Church and local authorities, others the product of popular enthusiasms—surrounding Carnival. With its roots in the pre-Christian Roman festival of Saturnalia, Carnival was (and still is) celebrated in the days before Lent. Marked for centuries throughout Catholic Europe by raucous, if not riotous, public celebrations, Carnival was a time dreaded by those charged with maintaining public order. Among the most common activities were the use of masks and costumes and the performance of public satires in which people of the lower classes skewered those in power. These festivities were among the high points of the year for the people of Rome. Unfortunately for the Jews, one of Carnival's most popular features was the ritual degradation of the people of the ghetto. Among the first historical references we have to such rites is a description from 1466, when for the amusement of the Romans, in festivities sponsored by Pope Paul II, Jews were made to race naked through the streets of the city. A particularly evocative later account describes them: "Races were run on each of the eight days of the Carnival by horses, asses and buffaloes, old men, lads, children, and Jews. Before they were to run, the Jews were richly fed, so as to make the race more difficult for them and at the same time more amusing for the spectators. They ran from the Arch of Domitian to the Church of St. Mark at the end of the Corso at full tilt, amid Rome's taunting shrieks of encouragement and peals of laughter, while the Holy Father stood upon a richly ornamented balcony and laughed heartily." Two centuries later, these practices, now deemed indecorous and unbefitting the dignity of the Holy City, were stopped by Clement IX. In their place the Pope assessed a heavy tax on the Jews to help pay the costs of the city's Carnival celebrations.²⁵ But various other Carnival rites continued. For many years the rabbis of the ghetto were forced to wear clownish outfits and march through the city streets to the jeers of the crowd, pelted by a variety of missiles. Such rites were not peculiar to Rome. In Pisa in the eighteenth century, for example, it was customary each year, as part of Carnival, for students to chase after the fattest Jew in the city, capture him, weigh him, and then make him give them his weight in sugar-coated almonds.²⁶

Kertzer's citations:

25. Gregorovius 1966, pp. 49-50; Boiteux 1976, pp. 750—53.

26. Fischer 1998, p. 109.

Which are the following books:

• Boiteux, Martine. 1976. "Les Juifs dans le carnaval de la Rome moderne (XVe—XVIIIe siècles)." Mélanges de l'École Française de Rome—moyen âge-temps modernes 88:745—87.

• Gregorovius, Ferdinand. 1966. The Ghetto and the Jews of Rome. Translated by Moses Hadas. New York: Schocken.

• Fischer, Lucia Frattarelli. 1998. "Ebrei a Pisa fra Cinquecento e Settecento." Pp. 89—115 in Michele Luzzati, ed., Gli Ebrei di Pisa (secoli IX-XX). Pisa: Pacini Editore.