Brain

 The human brian is made up of more than 100 billion nerve cells called neurons. Each neuron is connected to as many as 25000 other neurons-so the brain has trillions and trillions of different pathways for nerve signals. Girls' brains weight 2.5% of their body weight, on average, while boy's brains weigh 2%. About 0.85 litres of blood shorts through your brain every minute. The brain may be as little as 2% of your body weight, but it demand 25% of your blood supply. An elephant's brain weighs four times as much as the human brain. However, our brains are far bigger in relation to our bodies than those of any other animal. The cerebral cortex is the outside of the brain, and if it was laid out flat, it would cover a bed. The left hemisphere (half) of the brain is usually the word side, where the speech centres are. The right half is the picture side. Conscious thoughts and actions happen in the cerebral cortex. A human brain has a cerebral cortex four times as big as a chimpanzee, about 20 times as big as a monkey's and about 300 times as big as a rat's. Unconscious, automatic activities such as breathing, hunger, sleep and so on are controlled by structures such as the hypothalamus and the hippocampus in the middle of the brain. The cerebellum mainly controls body co ordination and balance.