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100 billion cells

‘A baby is born with over 100 billion brain cells. At birth only 25% of the brain is developed. By age three 90% of the brain is developed.’ Catherwood (2000)
anatomy of the brain
Figure 1. Anatomy of the Brain (click for an enlarged image)

There are three major parts to the brain:

  1. cerebrum
  2. cerebellum
  3. brain stem


The cerebrum is divided along the longitudinal fissure into two almost identical halves: the left and the right cerebral hemispheres. The outer layer of each hemisphere is known as the cerebral cortex and it is highly convoluted, with many intricate folds. These serve to increase the surface area of the brain. The left hemisphere controls the movements of the right-hand side of the body and the right hemisphere controls the left side. In most people one of the hemispheres is dominant over the other and, for most people, it is the left hemisphere that is dominant. Therefore, most people are right-handed, as it is the left hemisphere that dominates.

In addition to governing motor movements, each hemisphere appears to have a slightly different function. The dominant hemisphere (predominantly the left) is responsible for logical functions such as reasoning, number skills and scientific thought. The non-dominant right hemisphere is responsible for imagination, insight and an appreciation of shapes and their relationships, such as in 3-D forms.

For many years, people had wondered which hemisphere controlled speech. In 1836, the Frenchman Marc Dax noticed that people who suffered a paralysis of the right-hand side of their body, as a result of a stroke, frequently lost their speech as well. A paralysis of the right side of the body must be due to damage to the left hemisphere, as the left hemisphere controls the right side of the body. As the loss of speech was also due to brain damage and as it was the left hemisphere that was damaged then, Dax deduced, it must be the left hemisphere that controls speech. Speech, then, is said to be localized in one half of the brain, usually the left hemisphere. Specifically, the production of speech is localized in Broca’s area and the ability to comprehend spoken language is localized in Wernicke’s area (see Anatomy of the Language System for further details).

In summary, the cerebrum is responsible for such things as reasoning ability, emotion, memory, motor movements, and speech and language skills.


The cerebellum is positioned towards the back of the skull, underneath the two cerebral hemispheres. Its function is to coordinate voluntary muscle movements. If we did not have a cerebellum then any movements we made would be uncontrolled, hesitant, wavering and inaccurate. The cerebellum acts as a buffer, smoothing out any potential disruptions in our voluntary movements. It functions to help us maintain our posture and balance whilst at the same time we perform complex muscle movements.

Brain stem

The brain stem is the lowest part of the brain and represents the upper extension of the spinal cord. It is concerned with automatic functions, such as heart rate, respiration, blood pressure and maintaining body temperature. It also controls reflex activities such as eye movement and controls many of the drives that are necessary for survival, such as eating, sleeping and sexual activity. In addition, it acts as a sort of relay centre for sensory signals going to the cerebral cortex and motor signals coming from the cortex.


Catherwood, D. (2000) ‘New views on the young brain: offerings from developmental psychology to early childhood education’ Contemporary Issues in Early Childhood 1, 1.

Further information

Want to test your knowledge about brain anatomy take a look at our Brain Facts.