It consists of a large central mass, the brain and spinal cord, which is formed in the embryo by an infolding of the external surface, much in the same way that a gland is formed; but the connection with the surface is lost in further development and it becomes completely enclosed.

Connected with the central nervous mass, forming really a part of it and developing from it, are the nerves, which appear as white fibrous cords and after dividing and subdividing, are as extremely fine microscopic filaments distributed to all parts of the body.

By means of the nerves all impressions are conveyed to the brain and spinal cord; all impulses from this, whether conscious or unconscious, are conveyed to the muscles and other parts.

The brain is the sole organ of psychical life; by means of its activity the impressions of the external world conveyed to it through the sense organs are converted into consciousness.

Whatever consciousness is, and on this much has been written, it proceeds from or is associated with the activity of the brain cells just as truly as the secretion of gastric juice is due to the activity of the cells of the stomach.