It ought to be generally known that the source of our pleasure, merriment, laughter, and amusement, as of our grief, pain, anxiety, and tears, is none other than the brain. It is specially the organ that enables us to think, see, and hear, and to distinguish the ugly and the beautiful, the bad and the good, pleasant and unpleasant. Sometimes we judge according to convention; at other times according to the perceptions of expediency. It is the brain, too, that is the seat of madness and delirium, of the fears and frights which assail us, often by night but sometimes even by day; it is there where lies the cause of insomnia and sleepwalking, of thoughts that will not come, forgotten duties and eccentricities. All such tings result from an unhealthy condition of the brain; it may be warmer than it should be, or it may be colder, or moister, or drier, or in any other abnormal state.
For these reasons, I believe the brain to be the most potent organ in the body. So long as it is healthy, it is the interpreter of what is derived from the air. Consciousness is caused by air. The eyes, ears, tongue, hands, and feet perform actions that are planned by the brain, for there is a measure of conscious thought throughout the body proportionate to the amount of air which is receives. The brain is also the organ of comprehension, for when a man draws in a breath, it reaches the brain first, and thence is dispersed into the rest of the body, having left behind in the brain its vigor and whatever pertains to consciousness and intelligence. If the air went first to the body and subsequently to the brain, the power of understanding would be left to the flesh and to the blood vessels; it would only reach the brain hot and when it was no longer pure, owing to admixture with fluid from the flesh and from the blood, and this would blunt its keenness.
I therefore assert that the brain is the interpreter of comprehension. Some say that we think with our hearts, and it is the heart that suffers pain and feels anxiety. There is no truth in this; blood vessels from all parts of the body run to the heart, and these connections ensure that it can feel if any pain or strain occurs in the body. Moreover, the body cannot help giving a shudder and a contraction when subjected to pain, and the same effect is produced by an excess of joy, which heart and diaphragm feel most intensely. Neither of these organs takes any part in mental operations, which are completely undertaken by the brain.