nd abdomen in
inspiration. 7, 8, The position of the front walls of the abdomen and
chest in expiration.]

_Experiment._ Place the ear upon the chest of a person, and a
murmuring sound will be heard, somewhat like the soft sighings of the
wind through forest trees. This sound is caused by the air rushing in
and out of the lungs, and is peculiarly distinct in the child.

-=-=-=-=-=-=-=-=-=-=-=-=

Explain fig. 97. How may the murmur of respiration be heard?

-=-=-=-=-=-=-=-=-=-=-=-=

483. It is not easy to decide how much air is taken into the lungs at
each inspiration. The quantity, however, must vary in different
individuals, from the difference in the condition and expansion of the
lungs, together with the size of the chest. From numerous experiments,
the quantity, at an ordinary inspiration, of a common-sized man, is
fixed at forty cubic inches. It has been estimated that one hundred
and seventy cubic inches can be thrown out of the lungs by a forcible
expiration, and that there remain in the lungs two hundred and twenty
cubic inches; so that these organs, in their quiescent state, may be
considered as containing about three hundred and ninety cubic inches
of air, or more than a gallon.

484. Respiration is more frequent in females and children than in
adult men. In diseases, particularly those of the lungs, it is more
increased in frequency than the action of the heart. In health, the
smallest number of inspirations in a minute by an adult, is not less
than fourteen, and they rarely exceed twenty-five. Eighteen may be
considered an average number. The quantity of oxygen taken into the
lungs at each inspiration is about eight cubic inches, one half of
which disappears in every act of respiration.

_Observation._ Under different circumstances, however, the consumption
of oxygen varies. It is greater when the temperature is low, than when
it is high; and during digestion the consumption has been found one
half greater than when the stomach was empty.

-=-=-=-=-=-=-=-=-=-=-=-=

483. Can it be ascertained with accuracy how much air is taken into
the lungs at each inspiration? Why not? What is the probable quantity
that an ordinary sized man inspires? How much can be thrown out of the
lungs at a forcible expiration, and how much remains in the lungs?
From these calculations, how much may they contain in their quiescent
state? 484. In whom is respiration most frequent? How in disease? How
in health? How ma