The Vital Capacity indicates the mobility of the walls of the chest, but by no means varies according to the dimensions of the cavity; for differences in the dimensions occur, irrespective of height, age, or weight, whereas the vital capacity increases according to the weight in persons of less than 11 1/2 stones, and in persons above that weight is said to diminish at the rate of a cubic inch per pound; it increases also with age up to the period from the thirtieth to the thirty-fifth year, then diminishes 1 1/2 inches every year; and it increases regularly with the height of the individual. This increase of vital capacity according to height is all the more interesting, as it was contrary to the expectation of the observer who first noted it (Hutchinson); because, as he justly observed, height depends principally on length of limb, and he could not see how that could affect respiration. The student, however, will observe that length of limb gives increase of surface exposed to contact with the atmosphere, and liable to be cooled thereby, and that the person who has the greatest amount of surface requires the greatest amount of combustion in his tissues to keep up the temperature of the body, and consequently requires more oxygen, and gives off more carbonic acid than others.

It has been recently shown that a distinct influence is exerted by climate on the vital capacity; it being found that in the course of a voyage, the capacity, being measured in the north and south temperate zones, and in crossing the equator, rises in the tropics and falls again on reaching temperate latitudes. This curious phenomenon apparently depends on the lungs containing less blood and a greater volume of air in hot climates, so that they are more compressible in expiration. It is an accompaniment not of increased but of diminished respiration (Rattray).

The advantage of a large chest may be easily understood; for the activity of respiration corresponds with the vital capacity, and not with the thoracic dimensions; it is regulated by conditions throughout the body, and not by the size of the organ; therefore the smaller the lungs the greater the work thrown on each portion of them, and the greatest work of all is thrown on each portion when small lungs are combined with great height. Drill masters are right in teaching that an erect figure is good for the lungs; for the ribs are so attached to the vertebral column, that when the column is bent forwards their elevation is prevented; and that the full expansion of the chest requires a straight back may be easily demonstrated by taking in as full a breath as possible in the stooping posture, and then rising and throwing the shoulders back, when it will be found that an additional quantity of air can be inhaled. The stooping posture renders part of the lung useless, throws the work on the rest, and leaves a smaller amount of residual air with which to dilute that which is inhaled.