Cushing and Branch's work was a key step in the early development of surgery of the mitral valves, later realized by Cushing's students Elliot Carr Cutler and Claude Beck in 1924. "Experiments on canine heart valves were performed repeatedly during the last quarter of the 19th century.... Most of the experimenters had attempted to study the physiologic and anatomic effects of artificial lesions....With the contribution by Cushing and Branch there is evidence of a new phase. The possibility of a surgical attack on valvular disease in man is now envisioned clearly and discussed overtly in a purposeful manner. The valvular lesion that figures most prominently in the discussion is metral stenosis.... Cushing and Branch used direct transthoracic exposure of the heart, the instrument (McCallum's valvulotome) being passed through the myocardium... The intention was to procure long-term survival of the animals and to observe long-term effects" (Jarcho, "Experiments on heart valves (1908) by Harvey Cushing and J.R.B. Branch," Am. J. Cardiol., 36 (1975) 506-508).
"The contribution of Cushing and Branch is of particular importance because it was the first real proof that operative creation of valvular defects could be carried out with a high degree of certainty, that each attempt would be successful and with a sufficiently low mortality (they reported 11 recoveries in 25 attempts) to hope that, with improvements in technique, the risk could be almost negligible." (Cutler, Levine and Beck, "The surgical treatment of mitral stenosis," 1924).
Digital facsimile from PubMedCentral at this link.