The human body
The brain and the nervous system
The brain, spinal cord, and peripheral nerves carry and process signals. They support sensation, movement, coordination, and automatic regulation of body functions.
These structures contribute to special senses, including sight, hearing, and balance. Their specialized tissues detect stimuli and work with the nervous system to convey information.
The brain
The central organ of the nervous system. Its interconnected regions support perception, movement, memory, language, and the regulation of bodily functions.
- MeasuredAbout 86 billion neurons and about 85 billion other cells
- MeasuredThe cortex, its wrinkly outer layer, is about 82% of the brain’s mass but holds only about 16 billion neurons (about 19%)
- MeasuredMass: 1,336 g in men, 1,198 g in women
- EstimateAbout 2% of body weight, but about 20% of the energy used at rest
The spinal cord
The cable of nerve tissue running inside the spine, about 42–45 cm long. It carries signals between the brain and the body and handles some reflexes on its own.
The eye
The eye focuses light through the cornea and lens onto the retina, where about 90 million rods and 4–5 million cones turn it into nerve signals.
- Measured92 million rods (78–107 million) and 4.6 million cones (4.1–5.3 million) in each retina
In Endo-View
In the human atlas you can peel the body layer by layer down to these organs, turn it, take it apart and click any part for its card.
Every number is labelled measured, an estimate or drawn: how we know.
More of the body
The heart and blood vessels The lungs and airways The digestive system The kidneys and the urinary system
The journey to life
What Happens to Sperm After Ejaculation How Sperm Get Through the Cervix: Cervical Mucus Sperm in the Uterus: The Race and the White Blood Cells Sperm in the Fallopian Tube: Waiting for the Egg The Human Egg Cell: Its Size and Its Layers How Fertilisation Works: Sperm Meets Egg The Zygote and the First Cell Divisions What Is a Blastocyst? Days 3 to 6 How Implantation Works: Days 6 to 12 The Fetus from Week 10 to Week 20 Pregnancy Week by Week: The Fetus to Week 40 How Birth Works: Labour and the First Breath The Embryo Week by Week: Weeks 4 to 10