IV.

"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 _role_, 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.