The theory found in the Timaeus, of the passing round of thebreath by pushing, by no means determines how, in the case of theanimals other than land-animals, their heat is preserved, andwhether it is due to the same or a different cause. For if respirationoccurs only in land-animals we should be told what is the reason ofthat. Likewise, if it is found in others also, but in a differentform, this form of respiration, if they all can breathe, must alsobe described.Further, the method of explaining involves a fiction. It is saidthat when the hot air issues from the mouth it pushes thesurrounding air, which being carried on enters the very place whencethe internal warmth issued, through the interstices of the porousflesh; and this reciprocal replacement is due to the fact that avacuum cannot exist. But when it has become hot the air passes outagain by the same route, and pushes back inwards through the mouth theair that had been discharged in a warm condition. It is said that itis this action which goes on continuously when the breath is takenin and let out.But according to this way of thinking it will follow that we breatheout before we breathe in. But the opposite is the case, as evidenceshows, for though these two functions go on in alternation, yet thelast act when life comes to a close is the letting out of thebreath, and hence its admission must have been the beginning of theprocess.Once more, those who give this kind of explanation by no means statethe final cause of the presence in animals of this function (to witthe admission and emission of the breath), but treat it as though itwere a contingent accompaniment of life. Yet it evidently hascontrol over life and death, for it results synchronously that whenrespiring animals are unable to breathe they perish. Again, it isabsurd that the passage of the hot air out through the mouth andback again should be quite perceptible, while we were not able todetect the thoracic influx and the return outwards once more of theheated breath. It is also nonsense that respiration should consistin the entrance of heat, for the evidence is to the contrary effect;what is breathed out is hot, and what is breathed in is cold. Whenit is hot we pant in breathing, for, because what enters does notadequately perform its cooling function, we have as a consequence todraw the breath frequently.
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It is certain, however, that we must not entertain the notion thatit is for purposes of nutrition that respiration is designed, andbelieve that the internal fire is fed by the breath; respiration, asit were, adding fuel to the fire, while the feeding of the flameresults in the outward passage of the breath. To combat thisdoctrine I shall repeat what I said in opposition to the previoustheories. This, or something analogous to it, should occur in theother animals also (on this theory), for all possess vital heat.Further, how are we to describe this fictitious process of thegeneration of heat from the breath? Observation shows rather that itis a product of the food. A consequence also of this theory is thatthe nutriment would enter and the refuse be discharged by the samechannel, but this does not appear to occur in the other instances.
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