Our knowledge of this subject is derived in part from observations made on workmen engaged in underground avocations requiring them to labor under greatly increased barometric pressure; in part from observations made on patients undergoing the compressed-air treatment; and in part from experimental researches on animals.

Not to enter too far into details out of place here, it suffices to state that the inhalation of compressed air lessens the respiratory movements and the number of heart-beats, and increases the arterial tension. The effect of this change in the conditions of the circulation is to diminish the amount of blood in the veins and auricles, and to increase it relatively in the ventricles (Sanderson). On ophthalmoscopic examination of the fundus oculi it has been ascertained (Von Vivenot) that the blood-vessels of the retina diminished in size with the increase in the pressure. Compressed air does not appear to influence materially the production of carbonic acid, but it does the absorption of oxygen (Von Liebig). The changes induced in the phenomena of life under high barometric pressure have been carefully studied by Paul Bert, who seems to have established that the evil effects of such pressure are largely due to the formation of bubbles of gas in the blood. He finds that nitrogen is dissolved in the blood during the time of high pressure, and returns to the gaseous state when the pressure ceases. He further ascertained that the bubbles of gas obstructed the circulation at various points, especially in the lumbar portion of the spinal cord, causing paraplegia. When the accumulation of gas was sufficient in amount, the pulmonary circulation was obstructed and the heart paralyzed by the sudden distention. MM. Blanchard et Regnard have studied the changes taking place in the spinal cord under the action of nitrogen. Numerous haemor-rhagic foci, and evidences of parenchymatous myelitis, were discovered in the entire cord. The myelitis presented the usual characteristics: hypertrophy and varicosity of the axis cylinder, and granular change at various points, but least pronounced in the lumbar portion of the cord, and most decided in the dorsal region.