In order to form for one's self a just notion of the operations which result in the production of thought, it is necessary to conceive of the brain as a peculiar organ, specially designed for the production thereof, just as the stomach is designed to effect digestion, the liver to filter the bile, the parotids and the maxillary and sublingual glands to prepare the salivary juices.
Impressions arriving at the brain make it enter into activity, just as food falling into the stomach excites it to more abundant secretion of gastric juice.
