"If... too much blood is supplied to the brain, congestion of the vessels takes place, and irregularity in its action is at once produced; if too little, the brain (and, therefore, the nervous system) becomes first irritable and then lethargic. The quality of the blood supplied is also of great importance. As it courses through the body it has two principal functions to perform — to supply oxygen and to provide nutrition to the different organs of the body; and if it be unable adequately to fulfill either of these functions, a certain disorganization will follow. If the supply of oxygen to the brain be deficient, it becomes overcharged with carbon dioxide, and heaviness and lethargy very shortly supervene. A common example of this is the feeling of dullness and sleepiness which frequently overtakes one in a crowded and ill-ventilated room; owing to the exhaustion of the oxygen in the room by the continued respiration of so large a number of people, the brain does not receive its due modicum, and therefore is unable to do its work properly."