In order to understand the rest of Galen’s exposition, the following two points should be kept in mind. First, his terminology was different from ours. He regarded the lateral ventricles of the brain as one paired ventricle and called it the anterior ventricle. He accordingly called the third ventricle the middle ventricle, and the fourth the posterior one. Second, he thought that these ventricles were filled with “psychic pneuma,” a fine, volatile, airy or vaporous substance which he described as “the first instrument of the soul.” (See Rocca 2003 for a detailed description of Galen’s views about the anatomy and physiology of the brain.)