8



Democritus of Abdera and certain others who have treated of

respiration, while saying nothing definite about the lungless animals,

nevertheless seem to speak as if all breathed. But Anaxagoras and

Diogenes both maintain that all breathe, and state the manner in which

fishes and oysters respire. Anaxagoras says that when fishes discharge

water through their gills, air is formed in the mouth, for there can

be no vacuum, and that it is by drawing in this that they respire.

Diogenes' statement is that, when they discharge water through their

gills, they suck the air out of the water surrounding the mouth by

means of the vacuum formed in the mouth, for he believes there is

air in the water.

But these theories are untenable. Firstly, they state only what is

the common element in both operations and so leave out the half of the

matter. For what goes by the name of respiration consists, on the

one hand, of inhalation, and, on the other, of the exhalation of

breath; but, about the latter they say nothing, nor do they describe

how such animals emit their breath. Indeed, explanation is for them

impossible for, when the creatures respire, they must discharge

their breath by the same passage as that by which they draw it in, and

this must happen in alternation. Hence, as a result, they must take

the water into their mouth at the same time as they breathe out. But

the air and the water must meet and obstruct each other. Further, when

they discharge the water they must emit their breath by the mouth or

the gills, and the result will be that they will breathe in and

breathe out at the same time, for it is at that moment that

respiration is said to occur. But it is impossible that they should do

both at the same time. Hence, if respiring creatures must both

exhale and inhale the air, and if none of these animals can breathe

out, evidently none can respire at all.