To determine the volume of air in the spi rometer. — The graduated scale 13 is attached to the receiver, and made to extend down wards on the outside of the reservoir, so as always to be in relation with the index 2. On this scale 0 corresponds with the top of the receiver, or rather with the highest point to which the water can be made to rise within it. The number of cubic inches is shown by She degree upon the scale pointed to by the index 2, which corresponds with the level of the water in the receiver. But the water in the reservoir seen behind the slip of glass may not be level with the water within the receiver, just as the level of the water in a pneumatic trough may be higher or lower than the level of the water contained in a glass receiver standing upon the shelf. To know when these are level, depress the receiver until the coloured fluid in one leg of the bent tube, or inverted syphon, 5, stands level with that in the other leg, as at 6 ; then the water contained in the receiver, and that external to it, are level to each other ; and the air within the receiver is of the same density as that without.
Immediately the plug 15 is replaced, and the hands withdrawn from the receiver, the latter will be seen to ascend some half-inch, the water behind the slip of glass to fall, and the coloured fluid in the bent tube to be. unequal. This is caused by the excess of weight in the counterbalance (11, 11,), which is what necessitates the observation of the coloured fluid in the bent tubes and the cor rection above directed. The scale is gra duated to degrees, each of which measures two cubic inches.
To correct the respired volume for tem perature. — The table of the vital capacity volume is calculated at 60° Fahr. The tempe rature of a volume of air displaced out of the lungs into the spirometer is reduced at once to the temperature of the water in the spiro meter. This, according to the season of the year, may he 50° or 80°. Now 338 cubic inches at 50° would occupy 337 cubic inches at 60°, and 330 at 80°, would be 317 at 60°. For eight months out of the year there needs no correction. But a correction is necessary, when a thermometer in the room stands much above or below 60°. We may estimate the change in the bulk of air as for every degree (Fahr.) of variation of temperature ; thus if a man breathe, in winter, 295 cubic inches of air into the spirometer, when the thermometer in the room stands at 55°, being 5 degrees below 60°, then 2.93, must be added to the 295 cubic inches, making 297.95, or, in round numbers, 298 cubic inches. On the other hand, if the vital ca pacity be determined at 215 cubic inches, when the thermometer stands at 72°, which is 12° above 60°, = 5 must be deducted ; making the corrected observation 210, instead' of 215 cubic inches.
In the absence of the spirometer, the mea sure of the mobility of the ribs, by means of a common tape measure is of much value.* To measure the mobility of the thorax with a tape measure, pass the tape measure round the chest under the waistcoat, over the region of the nipples, request the person to in spire deeply, and note that circumference, then to expire deeply, and again note the circumference, the difference is what we term the mobility. This is a rough measure ment, but of no little value in doubtful cases of chest disease. This difference, or mobility, in men of all statures should be about 3 inches, if it is found only 2k inches the examination should be carried further ; sometimes the mobility extends to inches, but this is excessively rare. As a general rule, when we find the mobility three inches, we find the vital capacity volume correspond with our table. Sometimes the mobility may be good, and the vital capacity bad, because, as we have already noticed, we may move the walls of the chest without breathing.
The vital capacity is a constant quantity ; habit will not increase it. But this volume is disturbed directly, and modified by five circum stances : 1st, by height ; 2nd, by position ; 3rd, by weight ; 4th, by age ; 5th, by disease.
1st. Of the effect of height.
The vital capacity volume bears a striking relation to the height of the individual exa mined ; so that, if we take a man's height, we can tell the volume of his vital capacity. We show this by a curve in fig. 710., as before ; let the perpendicular lines represent the heights increasing inch by inch from the left towards the right ; the single continuous line is the curve of the vital capacity, which gradually ascends as it passes over the perpendicular lines. The heights extend from 5 feet to 6 feet; above six feet the observations are few. Whe ther the vital capacity volume maintains the same regular progression beyond this point remains to be determined. If we draw a line in a perfect arithmetical ascent of eight units for every increasing height, the line of vital capacity will be observed to run nearly pa rallel with it; therefore this volume increases with the increase of stature. The figures at the bottom represent the vital capacity in cubic inches, being the mean of the observa tions under each height. The following table (TABLE T.) places the subject more in detail : A cursory inspection of the table shows that the vital capacity increases with the height ; this is without any consideration as to age, weight, or circumference of the chest. For clearness, we arrange it in a more re duced form, as in TABLE U.