Final answer:
A speculum is the instrument used to expand and separate the walls of a cavity for examination purposes, essential in medical diagnostics and procedures.
Step-by-step explanation:
The instrument used to expand and separate the walls of a cavity to make examination possible is a speculum. Speculums are commonly used in medical examinations, such as gynecological exams, to provide visibility to otherwise inaccessible areas of the body. This tool is essential for doctors to view and diagnose internal physical problems effectively. In addition to enabling direct visualization, some speculums may accommodate channels for surgical instruments, allowing for procedures like biopsies or removal of growths.