The largest part of the brain that you see in Fig. is the cerebrum. It consists of two halves or hemispheres joined together by a band of nerve fibres. Interestingly the crossing over of nerve fibres here causes the right half to control the actions.of the left side of the body and vice versa. The outer surface of the cerebrum is the cerebral cortex. It is often referred to as the 'grey matter' because of its colour. It is profusely supplied with blood vessels. The cortex was explored in a very fascinating way, particularly by a Canadian neuro- surgeon, Wilder Penfield: Since 1900's it was known that the brain cpntained no pain :ceptors and.hence it could be operated upon, without making the person unconscious: local anesthesia, the top of the skull could be removed like a cap to expose the cortex. Penfield did exactly this and he stimulated different parts of the cortex, one by one, by matching them with an electric wire or probe. He was amazed to observe the reactions of the dtients. On touching one part of the cortex with the probe, the patients could see, hear or nell or feel! Patients could revive old memories. Some reported hearing the sound of a articular song; one woman felt as if her daughter was in the room talking to her; another. erson could actually recollect the smell of flowers! Stimulation of other regions caused notor responses such as the movement of an arm or leg.