Internal cavities are spaces within the body that can hold organs, fluids, or other structures. Common examples include the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which contains the stomach, intestines, and liver. These cavities are lined with membranes that help protect the organs and allow for movement. Cavities can also refer to hollow spaces in bones, such as the sinuses in the skull. These air-filled spaces help lighten the weight of the skull and contribute to voice resonance. Understanding internal cavities is essential for studying human anatomy and physiology.