duty calls, stops dialogue

Same manner of leave taking with the same function in both instances of ending the dialogue.  Duty before pleasure!  The professional man, soldier or surgeon, has real work to do. 

"Do you remember the Malay chief and his red horse?"

I.F. "Remember them! It is a splendid picture of glorious bravery—of heroic action!''

"And now, sir, your eloquence must not detain me from 'drill.' There are a half-dozen fine young fellows here who have not had even so good an opportunity as this to put in practice their theoretical knowledge."

("Scenes Beyond the Western Border," September 1851)
and Scenes and Adventures in the Army

"Euthanasia, Mr. Purser, is something like your will-power: I doubt its authenticity as a scientific term—begging your pardon again. It is at once imaginative and metaphysical,—in short, Greek. But," abruptly changing his tone, "there is a case in the sick-bay that I do not care to leave to my assistants. Beg your pardon, but excuse me." And rising from the mess he formally withdrew. 

(Billy Budd)

Resonances here run deep. This practical-minded surgeon in Billy Budd, as William B. Dillingham points out, justly may be regarded as "one of Melville's narrow men of science." (think Cadwallader Cuticle and Margoth).  In the example from "Scenes Beyond the Western Border," the just-concluded prairie dialogue with "I. F." featured talk of Trelawny with a special nod to Trelawny's ridiculously cruel surgeon Van Scolpvelt as "the eccentric and inhuman martyr of science."

And remember, "I. F." stands for "Imaginary Friend."