I inferred this to be the paragraph where the expression was taken from;
"No, the romance and the beauty were all gone from the river. All the value any feature of it had for me now was the amount of usefulness it could furnish toward compassing the safe piloting of a steamboat. Since those days, I have pitied doctors from my heart. <em>What does the lovely flush in a beauty's cheek mean to a doctor but a "break" that ripples above some deadly disease? Are not all her visible charms sown thick with what are to him the signs and symbols of hidden decay? Does he ever see her beauty at all, or doesn't he simply view her professionally, and comment upon her unwholesome condition all to himself? And doesn't he sometimes wonder whether he has gained most or lost most by learning his trade?."</em>
<u><em>Explanation</em></u>:
There is a reasonable possibility that the expression "<em>break</em>" is referring to a break or shift in perception a doctor may have for a patient who has a lovely smile but is suffering from a deadly disease.
In other words, what he sees may trigger a mental shift in his perception, which is highlighted on the statement made further in the paragraph which says; "Are not all her visible charms sown thick with what are to him the signs and symbols of hidden decay?." Meaning the Doctor has gotten a "break" and thus no longer sees the beauty in the patient.
