Much like a professor instructing his students, a military heart surgeon held up a series of chemically preserved human hearts and explained to a Navy court-martial yesterday how doctors try to heal with stitches and valves what has been damaged by disease.

Army Lt. Col. Geoffrey Graeber, director of surgery for the Walter Reed Army Institute of Research, donned a white coat for first-day testimony in the court-martial of Cmdr. Donal M. Billig, the former head of heart surgery at Bethesda Naval Hospital now charged with involuntary manslaughter. Then Graeber testified for hours on the functions of the heart.

At times, Graeber held a wooden model of a heart, three times actual size, in the crook of his arm. Later, he pulled on latex gloves, picked up the yellowish-gray specimens that had been preserved in a formaldehyde solution, and walked back and forth in front of the panel of nine Navy officers to discuss diseased tissue.

Completing his testimony, Graeber, the sole witness of the day for the court proceedings held at the Washington Navy Yard, explained in a clear, authoritative manner a color videotape of a coronary bypass operation.

As Graeber spoke, the panel members, four of whom have medical backgrounds, submitted written questions for clarification.

Later, two of the nine put on latex gloves and felt the heart specimens, which lay on a table in front of the jury box, to judge the texture of the arteries and heart chambers.

Graeber was called by the prosecution to answer questions as a surgical expert. He said during testimony yesterday that he will be returning in the trial as a witness to operations performed by Billig.

Billig, 54, has been charged with involuntary manslaughter in the deaths of five patients at Bethesda Naval Hospital and 24 counts of dereliction of duty in connection with other heart operations he performed there in 1983. At issue in this case, the second in Navy history in which a doctor has been charged with involuntary manslaughter, are Billig's surgical techniques, physical abilities and decisions during the five unsuccessful heart operations.

Graeber's testimony yesterday did not discuss any specific operation in detail and the hearts he used in his testimony were not related to Billig's operations. Information about the five operations that prompted the involuntary manslaughter charges is expected to surface today with testimony from relatives of the five patients who died.