481 a 20φύσιν ἰόντων. εἴ τε1 περίττωμα πάσης τροφῆς ἐστί, ποίᾳ διαπέμπεται τοῦτο; κατὰ μὲν γὰρ τὴν ἐκπνοὴν οὐκ εὔλογον· ἀντιλαμβάνει γὰρ εὐθύς. λοιπὸν δὲ δῆλον ὅτι διὰ τῶν τῆς ἀρτηρίας πόρων. τὸ δ᾿ ἐκκρινόμενον ἤτοι λεπτότερον ἢ παχύτερον. ἀμφοτέρως δ᾿ ἄτοπον· εἰ <γὰρ>2 τοῦτο πάντων 25ἔσται καθαρώτατον, <πῶς λεπτότερον;>3 εἰ δὲ παχύτερον, ἔσονταί τινες πόροι μείζους. εἰ δ᾿ ἄρα κατὰ τοὺς αὐτοὺς λαμβάνει καὶ ἐκπέμπει, τοῦτ᾿ αὐτὸ παράλογον καὶ ἄτοπον. ἡ μὲν οὖν ἐκ τῆς τροφῆς αὔξησις καὶ διαμονὴ σχεδὸν ταῦτα.
II. Ἡ δ᾿ ἐκ τῆς ἀναπνοῆς, ὥσπερ Ἀριστογένης οἴεται (τροφὴν γὰρ οἴεται καὶ τὸ πνεῦμα πεττομένου4 30τοῦ ἀέρος ἐν τῷ πνεύμονι5· τοῦτο δ᾿ εἰς τὰ 481 bἀγγεῖα διαδίδοσθαι, <καὶ>6 τὸ περίττωμα πάλιν ἐκπέμπεσθαι) πλείους ἔχει τὰς ἀπορίας. ἥ τε γὰρ πέψις ὑπὸ τίνος; εἰκὸς μὲν γὰρ ὑπ᾿ αὐτοῦ, καθάπερ καὶ τῶν ἄλλων. αὐτὸ δὲ τοῦτ᾿ ἄτοπον, εἰ μὴ διαφέρει τοῦ ἔξω ἀέρος· οὕτω δ᾿ ἡ θερμότης ἂν 5πέττοι. καὶ μὴν καὶ παχύτερον αὐτὸν εὔλογον εἶναι μεθ᾿ ὑγρότητος τῆς ἀπὸ τῶν ἀγγείων ὄντα καὶ τῶν ὅλων ὄγκων, ὥσθ᾿ ἡ πέψις ἂν εἰς τὸ σωματῶδες εἴη. τὸ δὲ περίττωμα, εἴπερ γίνεται λεπτότερον, οὐ πιθανόν. ἄλογος δὲ καὶ ἡ ταχυτὴς τῆς πέψεως. εὐθὺς γὰρ μετὰ τὴν εἰσπνοὴν ἡ ἐκπνοή. 10τί οὖν τὸ οὕτω ταχὺ μεταβάλλον καὶ ἀλλοιοῦν; ὑπολάβοι γὰρ ἄν τις μάλιστα τὸ θερμόν, καὶ μαρτυρεῖ οὕτως ἡ αἴσθησις· ὁ γὰρ ἐκπνεόμενος θερμός.
their true nature. Again, if all food has a residue, how is it expelled in the case of breath? It is not reasonable to suppose that it is by exhalation; for this directly follows inhalation. Clearly the alternative is that it takes place by the channels of the windpipe. Now that which is excreted is either finer or coarser; but either alternative is absurd. How can it be finer, if breath is to be the purest substance? and if it is coarser, some of the channels must be proportionately wider. And if it receives and discharges by the same channels, this in itself is improbable and strange. So much for the theory that the growth and maintenance of breath are due to food.
II. The theory that they are due to respiration,The physiology of breath in the body. as Aristogenes supposes (Tor he thinks that breath too is a form of food, the air being digested in the lung; this is absorbed into the several receptacles, but the residue is expelled again), involves even more difficulties. What is the agent in this digestion? Presumably the breath, just as it is in other cases. But this is in itself improbable, unless it differs from the outside air; in which case its heat might be digestive. Again, it would seem probable that the air should be coarser when combined with the moisture from the vessels and from the solid parts in general; so that digestion would tend towards corporeality. But that the residue becomes finer cannot be believed. Again, the rapidity of the digestion is unreasonable; for exhalation occurs directly after inhalation. What then is it which causes such rapid change and alteration? One might suppose that it is heat, and the senses give evidence of this, for the air breathed out