Bone them young
The cruelties depicted in Ruby: the Pride of the Ring turn Bloom's thoughts to acts that he has personally witnessed in circuses: "Cruelty behind it all. Doped animals. Trapeze at Hengler's. Had to look the other way. Mob gaping. Break your neck and we'll break our sides. Families of them. Bone them young so they metemspychosis." The training of trapeze acrobats from a very young age, so that they could carry on a family tradition, strangely brings him back to the word whose appearance in the novel Molly has asked him about.
"Hengler's Royal Circus" is mentioned in Eumaeus as well as Calypso. Like most of the larger Victorian circuses, it staged its acts in large permanent structures (often round, hence the name) rather than in tents. Frederick "Charles" Hengler (1820-87), one of the great circus managers of the 19th century, directed the construction of such buildings—sometimes called "hippodromes," to reflect his emphasis on equestrian acts—in various Scottish and English cities. When his circus came to Dublin, they apparently performed in the Rotunda, a large round performance space on Rutland Square. By the end of the century circuses were a staple of middle-class entertainment.
Along with bareback riding tricks, Victorian circuses featured human acrobatic brilliance in the form of "Trapeze" acts, stunts performed in the air on swinging horizontal bars suspended from ropes, including the "flying" tricks initiated in Paris by Jean Leotard in 1859. Many trapeze artists grew up in families—a tradition which the Wallendas and many others have continued to the present day. When Bloom thinks "Bone them young," he is apparently using the verb in the sense of intensively studying in preparation for something, as in "Bone up on your Italian before the trip."
How such boning might cause young people to "metempsychosis" (another noun that Bloom uses as a verb) is anybody's guess. It is a very strange sentence. One possible interpretation is that Bloom is thinking of trapeze artistry as a kind of exotic life form: early training helps children metamorphose into the spectacular butterflies that eventually flit from bar to bar. (When he tries to explain metempsychosis to Molly "in plain words," he briefly confuses it with metamorphosis.)
Gifford infers that "Bloom has apparently witnessed a trapeze accident at the circus," but it is possible that he has merely been disturbed by the business of exploiting danger to sell tickets. Mobs will gape at actual falls, but they also will gape at feats that might result in a fall, their pleasure at the athleticism heightened by their awareness of the potential for bodily injury. It seems somehow fitting of Bloom's timidity and pacifism to suppose that he "Had to look the other way" merely because some death-defying trick was whipping the crowd into a frenzy.