The cause of the difference of weight between men is involved in much obscu rity.* We may in fact consider the usual weight of a man as his mean weight and the lb. will correspond to the inch, for that is accidental, merely depending upon the units employed, but that the increase of each will be found in an arithmetical progression, and hence, probably, the reason why tall men breathe more than short men.
But the weight can never be the sure guide that the height is, because the former varies at any time in life, even in a few days ; where as the latter varies only at the extremes of life.
4th. Relation of vital capacity to the circum of the notice this here, because the question is so natural, " Has the size of the chest no relation to the vital capacity ?" We do not find that there exists any direct relation between the cir cumference of the chest and the vital capa city. We have found — Height. Circum. Vital Men. ft. in. in. capacity.
11 5 8 35 235 10 5 8 38 226 Therefore, the men with chests 3 inches larger, breathed 9 cubic inches less, or 21 men of the same height, but of different-sized chests, breathed a mean vital capacity of 230 (the due quantity according to TABLE T). We have consolidated the following result upon 994 cases, the height is kept in view, calcu lated at 5 feet 61 inches.
dimensions and the vital capacity ; if, for in stance, one man has a chest 35 inches in cir cumference, and 3 inches mobility, and another man has a chest 40 inches in circumference, and 4 inches mobility, then the latter will surely displace a larger volume of air than the former, but omitting this, we expect as large a vital capacity from a man with a thin and narrow thorax, as from a man with a broad and deep thorax. In fact, aeration need have no re lation to the thoracic dimension ; and, for the same reason, the size of the chest no relation to the vigour of the whole man. Indeed we incline to the contrary, viz. that it is most likely the respiration is most vigorous in the narrow-chested man, when the mobility is greatest. The vigour of the lungs, like every other organ in the body, we believe, has no relation to the dimensions. One person may have a brain 1 lb. lighter, or * less than an other person, and yet their capacity and men tal qualities shall not appear different.
5th. Vital capacity affected by age. — Age affects the breathing movements, but less re markably than the height and weight. Indeed the influence of age was not apparent in the first calculation upon 1012 cases, nor until we took a basis of 1923 cases. Time affects life in two ways, first bringing it to perfection, and then determining that perfection.
There is nothing in this table to confirm that which we had thought would be the main guide to the vital capacity volume ; thus, compare together the first and last 14 men whose chests differ 10 inches, and their vital capacity only 17 inches ; or compare together the first and last columns, the one is perfectly regular and the other most irregular. There is a certain rude relation between the thoracic The column of " difference" exhibits the effect of time upon the breathing volumes.
From 15 to 35 years of age the vital ca pacity is increased, and from 35 to 65 years of age it is decreased in the progression of 19, 11, and 13 cubic inches. We illustrate this by a curve in fig. 709. The curve of the vital capacity will be seen to rise slightly as it passes the perpendicular lines of years until it comes to 35 years of age, after which it keeps declining as it cuts all the succeeding lines of quinquennial periods down to 65 years. We may say, therefore, that the vital capacity increases with the ace up to 30 years, and from 30 to 60 it decreases 43 cubic inches, or 1.43 (nearly l2 cubic inches) per year, or 7 cubic inches in 5 years, or 14i- cubic inches in 10 years. Al though this appears by calculation, yet we do not strictly follow this ratio, as we find by experience that the effect of age may be more diminished, as follows :— From 55 to 65 we have deducted 5 cubic inches per cent, and from 65 to 75 years of age S cubic inches per cent. We have not brought in the effect of time before the age of 55' years. This is supposed to be at the mean weight. The first column is derived from observation, the two second are derived from calculation.
6th. Of the effect of disease upon the vital capacity. — The effect of disease upon re spiration was well known to Boerhaave and Morgagni, they considered that the disturb ance of any organ in the body would dis order the whole function of respiration. Morgagni devotes more than one-sixth of his celebrated work, the " Seats and Causes of Disease," " to diseases which affect respiration." We may safely say all thoracic and abdominal diseases, as tumours, abscesses, and acute in flammations, will affect the respiration. Of the different respiratory volumes, we select that one which requires the most extended mo bility, viz. the vital capacity, which becomes altered to an extent commensurate with that of any disease physically affecting our respiration, it is therefore a test of the pre sence and extent of such diseases. Such movements as command this volume, extend from the neck to the plantar muscles of the feet.