Structure

Structure

Further information: Brain size

Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter^[7]

The adult human brain weighs on average about 3 lb (1.5 kg)^[8] with a size (volume) of around 1130 cubic centimetres (cm³) in women and 1260 cm³ in men, although there is substantial individual variation.^[9] Men with the same body height and body surface area as women have on average 100g heavier brains,^[10] although these differences do not correlate in any simple way with gray matter neuron counts or with overall measures of cognitive performance.^[11] Neanderthals, an extinct subspecies of modern humans, had larger brains at adulthood than present-day humans.^[12] The brain is very soft, having a consistency similar to soft gelatin or soft tofu.^[13] Despite being referred to as "grey matter", the live cortex is pinkish-beige in color and slightly off-white in the interior. At the age of 20, a man has around 176,000 km and a woman about 149,000 km of myelinated axons in their brains.^[14]

General features

Drawing of the human brain, showing several important structures

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer with a convoluted topography.^[15] Underneath the cerebrum lies the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look different from any other brain area. The same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.