Cavities and protective boundaries

Locate the major body cavities and the diaphragm.

A helmet protects your head from outside. Your skull also forms a protective enclosure. Several internal spaces are organised in a similar way.

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Cavity

A space or compartment within the body, bounded by surrounding structures.

For example

The cranial cavity is the space inside the skull that contains the brain.

Read all notes for this section

Cavity: A space or compartment within the body, bounded by surrounding structures. The cranial cavity is the space inside the skull that contains the brain.
Diaphragm: A dome-shaped sheet of muscle below the lungs and heart. It also helps breathing. It separates the thoracic cavity above from the abdomen below.
Mediastinum: The central compartment between the lungs. The heart is in the mediastinum, not inside either lung.
Parietal / visceral: Parietal describes a lining against a body wall; visceral describes a covering on an organ. The two pleural layers slide over a thin film of fluid during breathing.

Protect organs in compartments

A body cavity is a space that contains or surrounds internal structures. The cranial cavity inside the skull contains the brain. The vertebral canal inside the spine protects the spinal cord and its coverings. Together these form the main posterior, or dorsal, cavity system.

Use the diaphragm as a boundary

The thoracic cavity is the chest compartment, protected by the rib cage. The diaphragm is a sheet of muscle below the lungs that separates the chest from the abdomen and helps breathing. Below it, the abdominal region continues into the pelvic region without a separating wall.

Distinguish a compartment from a thin space

The mediastinum is the central region between the lungs; it contains the heart and other structures. Each lung has a covering called pleura. Its two layers have a very thin fluid-filled space between them, allowing sliding. This pleural space is not a large empty pocket containing the lung.

Explore the illustration

Look at the whole view first. Then select one label and connect it to the idea you just read.

The main body compartments

Side and front cutaways show the spaces that house the organs. Compare their boundaries and contents, one region at a time.

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01 / 7 Cranial cavity

The cranial cavity is the space inside the skull. It protects the brain. The purple region in the side view continues into the vertebral canal.

The moving outline follows one cavity or boundary at a time. These are simplified teaching cutaways; organ positions are schematic. Dashed boundaries do not represent separating walls. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.

Cranial cavity

The space enclosed by skull bones contains the brain and its coverings.

Go deeper

In-depth reading

Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.

Worked example: following a route down the trunk

Start at the chest. The right and left lungs lie on either side of the mediastinum. The heart lies in this central region, within its own covering, the pericardium.
Move below the lungs to the diaphragm. This muscle forms a real boundary between thorax and abdomen, although vessels and the food pipe pass through openings in it.
Continue down from abdomen to pelvis. There is no second diaphragm separating those regions. The pelvic floor supports the bottom. A drawn line between abdomen and pelvis marks a region, not a wall inside the body.

Explain it yourself

What separates the thoracic and abdominal cavities?

Show the explanation

The diaphragm. In contrast, the abdominal and pelvic regions are continuous with one another.

More detail — when you are ready

These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.

Boundaries help predict contents

The cranial cavity is enclosed by skull bones; its floor includes contributions from frontal, ethmoid, sphenoid, temporal and occipital bones. The vertebral canal continues protection around the spinal cord and nerve roots.

In the thorax, the sternum (breastbone) and costal cartilages (flexible connections from ribs towards the sternum) form the front boundary. Ribs and the muscles between them form the sides; thoracic vertebrae form the back.

The diaphragm forms the floor. The superior thoracic opening communicates with the neck: the thorax is not sealed above by another diaphragm.

Compartments are not all empty spaces

The mediastinum is the central region between the lungs. It contains the heart in its pericardial sac, major vessels, trachea and oesophagus, as well as nerves and lymphatic structures.

The right and left pleural spaces are separate. Each pleura has a parietal layer along the wall and a visceral layer on the lung surface, with a thin film between them.

Similar terminology applies to serous membranes elsewhere. The peritoneal cavity is a potential space between membrane layers; it is not a synonym for the entire abdomen.

Trace the trunk from above to below

The diaphragm separates thoracic from abdominal regions, but structures such as the oesophagus and major vessels pass through it. The abdominal wall is largely muscular, with the lumbar spine and posterior muscles behind.

The abdomen continues into the pelvis without a separating muscular sheet. The pelvic floor supports the lower end.

A kidney can be within the abdominal region while lying behind the peritoneum. Therefore “which body cavity?” and “which membrane compartment?” are related but different questions.