The idea that all mental functions are derived from the brain originated with Hippocrates, but it was largely neglected until the late 18th century, when Franz Gall attempted to link psychology and brain science. Gall took advantage of what was already known about the cerebral cortex. He was aware that it was bilaterally symmetrical and subdivided into four lobes. However, he found that these four lobes were, by themselves, inadequate to account for the forty-odd distinct psychological functions that psychologists had characterized by 1790. As a result he began to analyze the heads of hundreds of musicians, actors, etc., relating certain bony elevations or depressions under the scalp to the predominant talent or defects of their owners. Based on his skull palpation, Gall subdivided the cortex into roughly forty regions, each of which served as an organ for a specific mental function.
While Gall`s theory that all mental processes derive from the brain proved to be correct, his methods for localizing specific functions were deeply flawed because they were not based on what we would now consider valid evidence. Gall did not test his ideas empirically by performing autopsies on the brains of patients and correlating damage to specific regions with defects in mental attributes; he distrusted the diseased brain and did not think it could reveal anything about normal behavior. Instead, he developed the notion that as each mental function is used, the particular area of the brain responsible for that function becomes enlarged. Eventually, a given area may become so bulky that it pushes out against the skull and produces a bump on the head.