284 RESPIRATION IN MAN. 
do by entering the lungs through the windpipe, and inflating 
them (or blowing them out), so as to increase their size in 
proportion to the increase of the space they have to fill. In this 
manner the lungs are made constantly to fill the cavity of the 
chest, however great may be the increase in the latter. But 
if we were to make an aperture through the walls of the chest, 
the air would rush directly into its cavity, when the move- 
ments of inspiration are performed, and the lung of that side? 
would not be dilated. The same thing would happen if there 
were an aperture in the lung itself, allowing free communica- 
tion between one of the larger bronchial tubes and the cavity 
of the chest ; for the air, although still drawn-in by the wind- 
pipe, would pass directly into the cavity of the chest, rather 
than dilate the lung, which would thus become entirely useless. 
Such an aperture is sometimes formed as the result of disease ; 
and if the action of both lungs were thus prevented, death 
must immediately take place from suffocation. 
334. The extent of the respiratory movements varies con- 
siderably ; but in general it is only such as to change about 
the seventh part of the air contained in the lungs. (It may 
be generally noticed, that every fifth or sixth inspiration in 
Man is longer and fuller than the rest.) Their rate varies 
according to age, and to the state of the nervous system ; being 
faster in infants and in young persons than in adults; and 
more rapid in states of mental excitement, or irritation of the 
bodily system, than in a tranquil condition. In astate of rest, 
from 14 to 18 inspirations take place every minute in an 
adult, and at each about 20 cubic inches of air are drawn-in ; 
but both the depth and frequency of the inspirations are con- 
siderably increased by exercise. Taking an average alterna- 
tion of activity and repose, it appears that about 360 cubic 
feet of air pass through the lungs every twenty-four hours, 
or 15 cubic feet every hour; and as the air which has once 
passed through the lungs contains about 1-24th part of 
carbonic acid, about 15 cubic feet of that gas, containing 
nearly 8 ounces of solid carbon, are thrown-off in the course 
of twenty-four hours. 
335. Now carbonic acid, when diffused through the atmo- 
sphere to any considerable amount, is extremely injurious to 
1 Each lung has its own cavity ; the two being completely separated 
from each other by the pericardium (§ 43). 
