Step 1: The question asks for the first cavity in a forensic post-mortem, and the answer hinges on understanding that autopsy sequence is flexible rather than mandatory.
Step 2: The pathologist tailors the dissection to the clinical and circumstantial picture. If the chest holds the crucial evidence, the thorax is entered first; in an abdominal injury the belly takes priority; a neck-related death may begin at the cervical region.
Step 3: Because the choice shifts with the affected region, none of the three named cavities can be declared the universal starting point. The unifying answer is that any of them may come first.
Step 4: One should also remember that opening the spinal canal is not part of the routine procedure unless the case demands it.
\[\boxed{\text{Any of the above}}\]