It is well known that the density of the air diminishes with the atmospheric pressure, that is, in the lower regions of the air, on the sea-coast for example, the air is denser than in elevated regions. Thus in order to absorb the quantity of oxygen necessary for sanguification, it is necessary to respire oftener upon high mountains than when on plains, but this acceleration of respiration is perceptible only when the height is considerable and the distance rapidly passed over. Gay-Lussac, who in his balloon ascension rose to a height of 22,956 feet in six hours, found his respiration disturbed, and greatly accelerated; and having made no movement requiring exertion, he could only attribute this condition to the diminution of the pressure of the atmosphere. But in climbing mountains the movement and efforts of walking are added to the influence of the height; and when the difference in altitude in one day amounts to 6560 feet, a notable acceleration of respiration and quickening of the pulse is observed, which in many instances is accompanied by a peculiar sense of uneasiness, which has been termed mountain-sickness. The most remarkable symptoms are fatigue or rather partial paralysis of the muscular system, and especially of the muscles of locomotion. This paralysis of the legs increases with every step until, having gone a certain distance with increasing difficulty, it is impossible to take another step. A rest of a few seconds is sufficient for the muscles to regain their power, and it seems as if the traveller could go on without fear of a recurrence of the difficulty; but very soon it returns, and a fresh halt is necessary. The higher one goes the shorter the distance that can be passed without resting—from one hundred and fifty steps the distance falls off to one hundred—to fifty—and at last to twenty or thirty. Inclination to sleep, oppression of the heart, and loss of spirit are sometimes added to this periodic exhaustion of strength, and in some persons mountain-sickness is closely analogous to sea-sickness. In others the symptoms are such as are always induced in the respiration, circulation, and in consequence in the muscular system, by violent exercise. Thirty steps in climbing a high mountain cause as much fatigue as a forced march or run on a plain. Respiration, quickened by motion and disturbed by successive efforts, is no longer sufficient for sanguification; the proportion between the venous and the arterial blood is no longer normal; and, above all, sanguineous congestion, which is inseparable from violent exertion, takes place in the lungs, in the brain, and other organs. But as soon as the muscles have relaxed for a few moments, two or three full inspirations rapidly relieve the congestion, while a flood of arterial blood proceeds from the heart to revive the whole organism.
Up to a height of 16,400 feet, man can easily acclimatize himself to the rarefied air. Baron Humboldt saw Peruvians cultivating the land at Antisana, situated 13,454 feet above the level of the sea; and agricultural labour requires the development of an amount of force incompatible with mountain-sickness, even though an energy like that of our European cultivators may not be found. Jacquemont visited villages in Thibet at a height of 16,400 feet. La Paz is situated on the Andes at a height of 12,195 feet, but the inhabitants suffer no inconvenience from the rarity of the atmosphere; though strangers can walk only a short distance without stopping, and they are specially uncomfortable if the young Peruvian ladies mischievously invite them to a few turns in a waltz. It is hardly necessary to remark that these symptoms are not equally urgent in all those who expose themselves to this rarefied air. Some individuals scarcely feel them, and are soon acclimatized, while others suffer greatly for a long period. A host of circumstances and special conditions contribute also to render these symptoms more or less marked, and mountaineers themselves sometimes experience them as well as the inhabitants of less elevated countries.