Anatomy answers “what is where”
Anatomy is the study of the body’s structure: what parts exist, what they look like and how they are arranged relative to each other. Its companion, physiology, asks how those parts work. The two are closely linked, since structure usually reflects function, but keeping them apart is a helpful way to learn. This article is a map of the territory. It deals with how the human form is described and divided, rather than how each system works.
A shared reference position
If a description depended on whether someone is lying down, sitting or twisting, it would be confusing. To avoid this, anatomists describe everything as though the person were in the standard anatomical position: standing upright, facing forward, feet together and arms at the sides with palms facing forward.
In this position, left and right always refer to the person’s own left and right, not the observer’s.
Direction words
Anatomy has a compact vocabulary for relative location.
- Superior and inferior: toward the head and toward the feet. The nose is superior to the mouth.
- Anterior and posterior: toward the front and the back. The sternum is anterior to the spine.
- Medial and lateral: toward the midline and away from it. The thumb is on the lateral side of the hand in anatomical position, and the little finger is medial.
- Proximal and distal: closer to or farther from the point of attachment, used mainly for limbs. The elbow is proximal to the wrist.
- Superficial and deep: closer to or farther from the surface.
Medical writing often uses terms such as dorsal (back) and ventral (front), which are related ideas.
Planes of section
To describe the inside of the body, anatomists imagine slicing it along flat planes, and imaging methods such as CT and MRI produce pictures along the same planes.
- Sagittal plane: divides the body into left and right parts. A midsagittal cut runs exactly down the middle.
- Frontal (coronal) plane: divides it into front and back parts.
- Transverse (horizontal) plane: divides it into upper and lower parts.
Regions and landmarks
The body is broadly divided into the head, neck, trunk and limbs. The trunk is sometimes mapped into regions to help describe where something is. A common scheme splits the abdomen into four quadrants (upper and lower, left and right) centered on the navel, and a more detailed scheme uses nine regions. These maps help describe where a pain or an organ is located.
Body cavities
Major organs sit inside fluid-filled or membrane-lined spaces called cavities.
- The cranial cavity within the skull holds the brain.
- The vertebral (spinal) canal within the spine holds the spinal cord.
- The thoracic cavity in the chest holds the lungs, heart and other structures. The heart sits in a smaller compartment, and each lung in its own.
- The abdominopelvic cavity below the diaphragm contains the digestive organs, the kidneys, and, lower down in the pelvic portion, the bladder and reproductive organs.
The diaphragm, a dome-shaped muscle, separates the chest from the abdomen. Membranes line these cavities and cover the organs, reducing friction as organs move.
Layers from surface to depth
Moving inward from the skin, the body has layered structure: skin, a layer of connective tissue and fat, muscle, then deeper organs, with bone providing the scaffold. Many organs are wrapped in connective tissue sheaths that anchor them and separate them from neighbors. Knowing the layers helps explain how doctors approach different areas, and why shallow and deep structures can feel different when touched.
Systems at a glance
Organs are grouped into systems, and each is a large topic of its own. A one-line summary of the major ones:
- Skeletal and muscular: support and movement
- Cardiovascular and respiratory: moving blood and exchanging gases
- Digestive: breaking down food and absorbing nutrients
- Nervous and endocrine: fast and slow control and communication
- Urinary: filtering blood and balancing fluids
- Immune and lymphatic: defense and fluid return
- Integumentary: skin, hair and nails
- Reproductive: producing the next generation
How these cooperate is a separate question about function rather than layout.
Normal variation
Textbook diagrams show a typical arrangement, but real bodies vary. The branching pattern of blood vessels, the number of ribs or the shape of muscles can differ between people without causing problems. Bodies also differ in size, proportions and, in some respects, between sexes, and they change with age.
How anatomy is studied
For centuries, knowledge of the body came from dissection, and the history of this effort is part of the story of medical history. Today, scanning technologies let researchers and clinicians look inside living people without surgery. The study of the nervous system’s fine structure, one aspect of the neuroscience field, continues to reveal new detail, as does microscopic anatomy, which examines tissues and cells.
This article is a general introduction and not medical advice.