Sir. Alexis St. Martin never needed a cannula because the skin and tissue surrounding his wound would never heal despite many attempts by Dr. William Beaumont. Cannulas. however, are essential in tracheostomies (not to be confused with tracheotomies) in human patients (children and adults) and in cattle, sheep, mice, and other animals’ digestive studies. Many times cannulas are used to prevent tissue from closing around the fistula. After time, one can take out the cannula and the fistula will remain open due to complete tissue healing. We go back to our previous examples to demonstrate this, the fistula caused by tracheostomies will remain open to help the human patient breathe, and after a couple of months of the cannula being present, and the ruminal fistula in the UC Davis’ infamous fistulated cows will remain open even after the cannula is removed. In another example, earrings which keep the hole in your ears open can also be called cannulas.