I have to measure the size of a cavity of a animal body(s). What is more important I need to measure it on several bodies and compare the results. The cavity is small and contains liquid. That liquid can be extracted and that sets the volume of cavity to 0. It's important to note that the tissue surrounding the cavity is elastic. It streches as liquid (e.g. water) is being injected. I need to do one of the following: Inject water into the cavity until the tissue surrounding it stretches to a certain point, kept constant for all the bodies, measure the injected volume and calcualte from there. OR Inject a constant volume of water into cavities and then measure how much the tissue strecthed for each one. Also important thing to note that other structures may be pressing to the elastic tissue with force that is not constant for every specimen. Would that affect the results; should I extract the tissue surrounding the cavity and measure it in specific conditions?