From the Clark Panel Report:
The possibility that the path of the bullet through the neck might have been more satisfactorily explored by the insertion of a finger or probe was considered. Obviously the cutaneous wound in the back was too small to permit the insertion of a finger. The insertion of a metal probe would have carried the risk of creating a false passage in part, because of the changed relationship of muscles at the time of autopsy and in part because of the existence of postmortem rigidity. Although the precise path of the bullet could undoubtedly have been demonstrated by complete dissection of the soft tissue between the two cutaneous wounds, there is no reason to believe that the information disclosed thereby would alter significantly the conclusions expressed in this report.