While anal-eroticism is unmistakeably present in all of Joyce's works, it was the function of micturition which held the most pleasurable associations for him. The Letter usually ends with an act of micturition, a 'pee ess' (111.18). (In one version it bears the subscribed address: 'Dubblenn, WC', 66.18). The 'P.S.' to the full statement of the Letter (619.17) was originally written 'Ps!' for the post-script is a flow of urine: 'amber too'. It forms a subsidiary stream proceeding from the 'main body' of water, and hence, if the six main paragraphs of the Letter are the verbal embodiment of Anna Livia, the P.S. is to be identified with her small daughter, Issy. This identification is of major importance, for it is Issy who tempts her brothers with the sound of her micturition to which, Siren-like, she bids them 'Lissom! Lissom!' (21.02, 571.24); hearing the same command - 'Ps!' - which lures him on to sexual perversion, Earwicker falls from grace in the Phoenix Park. This is the primal temptation for Joyce; all of history springs from Man's first obedience. 

Hart, Clive / Structure and motif in Finnegans wake