Introduction-this is the way all things breathe in and out.
'In all [animals] there are tubes of flesh, empty of blood, stretched all over the
surface of the body, and over their openings the outermost surface of the skin
is pierced through with close-packed holes, so that the blood is hidden but a
free passage is cut through for the air by these holes.'
When the blood rushes
from them, the air rushes in
with a mad gush. .
and when the blood runs back the air breathes out.
It is like what happens when a girl plays with a clepsydra.
When she closes the vent at the top and dips the clepsydra into the water, no
water enters; it is prevented by 'the weight of air falling on the many holes'
of the strainer at the bottom..
until she unblocks the compressed [air-]stream; then, as the air leaves,
the due quantity of water enters.
In the same way, when there is water in the clepsydra and the vent at the top
is closed by the hand, air pressure from the outside,
exerted upwards on the strainer at the bottom, holds in the water ...
until she lets go with her hand; then in turn, the opposite happens-as air
enters [through the vent at the top] the due amount of water flows out.
In the same way, when the blood in the body 'rushes back again to the inmost
part' a stream of air enters .
and when it runs back again an equal stream [of air] breathes