Which structure in mammals separates the thorax from the abdomen?

The structure that separates the thorax from the abdomen in mammals is the diaphragm. This dome-shaped muscle plays a crucial role in the respiratory system. When the diaphragm contracts, it creates a vacuum that draws air into the lungs, facilitating inhalation, and when it relaxes, it helps push air out during exhalation. This partition not only aids in respiration but also contributes to the proper functioning of the thoracic and abdominal cavities by maintaining pressure differences that are essential for various physiological processes.

The other options do not pertain to this anatomical separation. A dimer typically refers to a chemical entity formed from two identical small molecules, while a dichotomous key is a tool used in biological classification to aid in the identification of organisms. Diastole refers to the phase of the cardiac cycle when the heart muscle relaxes, allowing the chambers to fill with blood, and is unrelated to the separation of body cavities.