Hi,

We necropsy lots of mice and the procedure we use is to place a looped
piece of thin string around the trachea. Using forceps, the trachea is
pinched above the loose ligature, and the needle of a small syringe filled
with formalin is inserted into the trachea below the ligature and the
formalin is then syringed gently into the lungs. The ends of the ligature
is then tightened to prevent formalin escaping. If folk have difficulty in
pinching the tracea with forceps and tightening the ligature at the same
time, small artery forceps are used to pinch the trachea.

Hope this helps.

Steve