This process, as we affirm, the name-giver named inspiration and expiration. And all this movement, active as well as passive, takes place in order that the body, being watered and cooled, may receive nourishment and life; for when the respiration is going in and out, and the fire, which is fast bound within, follows it, and ever and anon moving to and fro, enters through the belly and reaches the meat and drink, it dissolves them, and dividing them into small portions and guiding them through the passages where it goes, pumps them as from a fountain into the channels of the veins, and makes the stream of the veins flow through the body as through a conduit.

Let us once more consider the phenomena of respiration, and enquire into the causes which have made it what it is. They are as follows:-Seeing that there is no such thing as a vacuum into which any of those things which are moved can enter, and the breath is carried from us into the external air, the next point is, as will be clear to every one, that it does not go into a vacant space, but pushes its neighbour out of its place, and that which is thrust out in turn drives out its neighbour; and in this everything of necessity at last comes round to that place from whence the breath came forth, and enters in there, and following the breath, fills up the vacant space; and this goes on like the rotation of a wheel, because there can be no such thing as a vacuum. Wherefore also the breast and the lungs, when they emit the breath, are replenished by the air which surrounds the body and which enters in through the pores of the flesh and is driven round in a circle; and again, the air which is sent away and passes out through the body forces the breath inwards through the passage of the mouth and the nostrils. Now the origin of this movement may be supposed to be as follows. In the interior of every animal the hottest part is that which is around the blood and veins; it is in a manner on internal fountain of fire, which we compare to the network of a creel, being woven all of fire and extended through the centre of the body, while the-outer parts are composed of air.

τούτῳ δὲ δὴ τῷ γένει τὸν τὰς ἐπωνυμίας θέμενον ἀναπνοὴν καὶ ἐκπνοὴν λέγομεν θέσθαι τοὔνομα. πᾶν δὲ δὴ τό τ΄ ἔργον καὶ τὸ πάθος τοῦθ΄ ἡμῶν τῷ σώματι γέγονεν ἀρδομένῳ καὶ ἀναψυχομένῳ τρέφεσθαι καὶ ζῆν· ὁπόταν γὰρ εἴσω καὶ ἔξω τῆς ἀναπνοῆς ἰούσης τὸ πῦρ ἐντὸς συνημμένον ἕπηται͵ διαιωρούμενον δὲ ἀεὶ διὰ τῆς κοιλίας εἰσελθὸν τὰ σιτία καὶ ποτὰ λάβῃ͵ [79a] τήκει δή͵ καὶ κατὰ σμικρὰ διαιροῦν͵ διὰ τῶν ἐξόδων ᾗπερ πορεύεται διάγον͵ οἷον ἐκ κρήνης ἐπ΄ ὀχετοὺς ἐπὶ τὰς φλέβας ἀντλοῦν αὐτά͵ ῥεῖν ὥσπερ αὐλῶνος διὰ τοῦ σώματος τὰ τῶν φλεβῶν ποιεῖ ῥεύματα.

Πάλιν δὲ τὸ τῆς ἀναπνοῆς ἴδωμεν πάθος͵ αἷς χρώμενον αἰτίαις τοιοῦτον γέγονεν οἷόνπερ τὰ νῦν ἐστιν. ὧδ΄ οὖν. [79b] ἐπειδὴ κενὸν οὐδέν ἐστιν εἰς ὃ τῶν φερομένων δύναιτ΄ ἂν εἰσελθεῖν τι͵ τὸ δὲ πνεῦμα φέρεται παρ΄ ἡμῶν ἔξω͵ τὸ μετὰ τοῦτο ἤδη παντὶ δῆλον ὡς οὐκ εἰς κενόν͵ ἀλλὰ τὸ πλησίον ἐκ τῆς ἕδρας ὠθεῖ· τὸ δ΄ ὠθούμενον ἐξελαύνει τὸ πλησίον ἀεί͵ καὶ κατὰ ταύτην τὴν ἀνάγκην πᾶν περιελαυνόμενον εἰς τὴν ἕδραν ὅθεν ἐξῆλθεν τὸ πνεῦμα͵ εἰσιὸν ἐκεῖσε καὶ ἀναπληροῦν αὐτὴν συνέπεται τῷ πνεύματι͵ καὶ τοῦτο ἅμα πᾶν 79c οἷον τροχοῦ περιαγομένου γίγνεται διὰ τὸ κενὸν μηδὲν εἶναι. διὸ δὴ τὸ τῶν στηθῶν καὶ τὸ τοῦ πλεύμονος ἔξω μεθιὲν τὸ πνεῦμα πάλιν ὑπὸ τοῦ περὶ τὸ σῶμα ἀέρος͵ εἴσω διὰ μανῶν τῶν σαρκῶν δυομένου καὶ περιελαυνομένου͵ γίγνεται πλῆρες· αὖθις δὲ ἀποτρεπόμενος ὁ ἀὴρ καὶ διὰ τοῦ σώματος ἔξω ἰὼν εἴσω τὴν ἀναπνοὴν περιωθεῖ κατὰ τὴν τοῦ στόματος καὶ τὴν τῶν μυκτήρων δίοδον. τὴν δ΄ αἰτίαν τῆς ἀρχῆς αὐτῶν θετέον τήνδε. [79d] πᾶν ζῷον αὑτοῦ τἀντὸς περὶ τὸ αἷμα καὶ τὰς φλέβας θερμότατα ἔχει͵ οἷον ἐν ἑαυτῷ πηγήν τινα ἐνοῦσαν πυρός· ὃ δὴ καὶ προσῃκάζομεν τῷ τοῦ κύρτου πλέγματι͵ κατὰ μέσον διατεταμένον ἐκ πυρὸς πεπλέχθαι πᾶν͵ τὰ δὲ ἄλλα ὅσα ἔξωθεν͵ ἀέρος.