"It is from the blood that life derives the principles which maintain and repair it. The more vigorous, plastic, and rich in nutritive material, so much the more life increases and manifests itself, so much the quicker the reparatory processes restore a lesion to its natural condition. "The blood owes its vivifying properties to the presence of oxygen, which it receives by the respiratory organs; but that nourishing fluid, to complete its physiological _rôle_, needs to receive combustible and organizable material." These Protean principles of the healthy blood form one fifth of its weight. Oxygen unites with the carbon of the food in the blood of animals; carbonic acid is formed and heat evolved. When the atmosphere is vitiated, the oxygenating processes are diminished in ratio to the vitiation. The experiments of Seguin, Crawford, and De la Roche show that in a vitiated and highly heated atmosphere the blood is not thoroughly decarbonized, thereby deranging the nervous system, and affecting the animal functions as well as the mental faculties. The blood is subject to incessant variations. The more feeble the respiration the less rich it is. Man absorbs twenty to thirty quarts of oxygen every hour. The pure air is a real food, and is as necessary for the development and repair of the physical force as the more solid forms of matter. Nine ounces of carbon are consumed every day, and the phenomenon of the expired carbonic acid has its maxima and minima during the day, like the regular variations of the barometer or the tides of the ocean. V. The great nervous prostration and the lack of energy which were observed among the prisoners, were not due entirely to climate. The activity of the nervous mechanism depends greatly upon the supply and purity of the arterial blood. It is the same with the nerve fibres as with the nerve centres, but in less degree. We observe that the exaltation and depression of the nervous power are within the control of man by the administration of certain drugs, or respiration of appropriate gases. The accumulation of bile or urea in the blood diminishes the nerve energy. Many physiologists enumerate moral depressions among the principal causes of epidemics; and this opinion is not strange when we consider how completely the system is under control of the nervous influence, and how much the supply of oxygen and blood to the organs and tissues depend upon the nervous power; and how much, moreover, the integrity of the nervous system depends upon the purity of the blood.