But since the Divinity alone is free from needs, while human poverty requires external aid for its own subsistence, nature therefore, in addition to those three powers by which we said that the whole body is regulated, brings in imported matter from without, introducing by different entrances that which is suitable to those powers. For to the fount of the blood, which is the liver, she furnishes its supply by food: for that which from time to time is imported in this way prepares the springs of blood to issue from the liver, as the snow on the mountain by its own moisture increases the springs in the low ground, forcing its own fluid deep down to the veins below. The breath in the heart is supplied by means of the neighbouring organ, which is called the lungs, and is a receptacle for air, drawing the breath from without through the windpipe inserted in it, which extends to the mouth. The heart being placed in the midst of this organ (and itself also moving incessantly in imitation of the action of the ever-moving fire), draws to itself, somewhat as the bellows do in the forges, a supply from the adjacent air, filling its recesses by dilatation, and while it fans its own fiery element, breathes upon the adjoining tubes; and this it does not cease to do, drawing the external air into its own recesses by dilatation, and by compression infusing the air from itself into the tubes. And this seems to me to be the cause of this spontaneous respiration of ours; for often the mind is occupied in discourse with others, or is entirely quiescent when the body is relaxed in sleep, but the respiration of air does not cease, though the will gives no co-operation to this end. Now I suppose, since the heart is surrounded by the lungs, and in the back part of its own structure is attached to them, moving that organ by its own dilatations and compressions, that the inhaling and exhaling of the air is brought about by the lungs: for as they are a lightly built and porous body, and have all their recesses opening at the base of the windpipe, when they contract and are compressed they necessarily force out by pressure the air that is left in their cavities; and, when they expand and open, draw the air, by their distention, into the void by suction.

Ἀλλ' ἐπειδὴ μόνον ἀπροσδεές ἐστι τὸ Θεῖον͵ ἡ δὲ ἀνθρωπίνη πτωχεία τῶν ἔξωθεν πρὸς τὴν ἰδίαν σύστασιν ἐπιδέεται͵ διὰ τοῦτο ταῖς τρισὶ ταύταις δυνάμεσι͵ δι' ὧν ἔφαμεν ἅπαν οἰκονομεῖσθαι τὸ σῶμα͵ ἐπίῤῥυτον ἔξωθεν ἐπεισάγει τὴν ὕλην͵ διαφόροις εἰσόδοις τὸ κατάλληλον αὐταῖς εἰσοικίζουσα. Τῇ μὲν γὰρ πηγῇ τοῦ αἵματος͵ ἥ τις τὸ ἧπάρ ἐστι͵ τὴν διὰ τῆς τροφῆς χορηγίαν ὑπέθηκε. Τὸ γὰρ ἐπεισαγόμενον ἀεὶ διὰ ταύτης τὰς τοῦ αἵματος πηγὰς βρύειν διὰ τοῦ ἥπατος παρασκευάζει͵ καθάπερ ἡ ἐπὶ τοῦ ὅρους χιὼν διὰ τῆς οἰκείας ἰκμάδος τὰς κατὰ τὴν ὑπώρειαν αὔξει πηγάς͵ διὰ τοῦ βάθους τὸ οἰκεῖον ὑγρὸν ἐπὶ τὰς κάτω φλέβας συνθλίβουσα. Τὸ δὲ ἐγκάρδιον πνεῦμα διὰ τοῦ γείτονος ἐπεισάγεται σπλάγχνου͵ ὃ καλεῖται μὲν πνεύμων͵ ἐστὶ δὲ τοῦ ἀέρος δοχεῖον͵ διὰ τῆς ἐγκειμένης ἀρτηρίας τῆς ἐπὶ τὸ στόμα διηκούσης͵ τὸ ἔξωθεν πνεῦμα ταῖς ἀναπνοαῖς ἐφελκόμενον. Ὧ κατὰ τὸ μέσον ἡ καρδία ἐνειλημμένη͵ κατὰ μίμησιν τῆς τοῦ ἀεικινήτου πυρὸς ἐνεργείας͵ ἀδιαλείπτως καὶ αὐτὴ κινουμένη͵ οἷόν τι ποιοῦσιν ἐν τοῖς χαλκείοις αἱ φύσαι͵ ἕλκει τε πρὸς ἑαυτὴν ἐκ τοῦ παρακειμένου πνεύμονος πληροῦσα τῇ διαστολῇ τὰς κοιλότητας καὶ τὸ πυρῶδες ἑαυτῆς ἐκριπίζουσα͵ ταῖς ἐχομέναις ἀρτηρίαις ἐμπνεῖ. Καὶ τοῦτο ποιοῦσα οὐ διαλείπει͵ τὸ μὲν ἔξωθεν διὰ τῆς διαστολῆς εἰς τὰς ἰδίας κοιλότητας ἕλκουσα͵ τὸ δὲ παρ' ἑαυτῆς διὰ τῆς συμπτώσεως ταῖς ἀρτηρίαις εἰσκρίνουσα. Ὅ μοι δοκεῖ καὶ τῆς αὐτομάτου ταύτης ἀναπνοῆς αἴτιον ἡμῖν γίνεσθαι. Πολλάκις γὰρ ὁ μὲν νοῦς ἄσχολός ἐστι πρὸς ἑτέρους ἢ καὶ παντάπασιν ἠρεμεῖ λυθέντος ἐν τῷ ὕπνῳ τοῦ σώματος͵ ἡ δὲ ἀναπνοὴ τοῦ ἀέρος οὐ διαλείπει͵ μηδ' ὁτιοῦν συνεργούσης εἰς τοῦτο τῆς προαιρέσεως. Οἶμαι γάρ͵ ἐπειδὴ περιείληπται τῷ πνεύμονι ἡ καρδία καὶ προσπέφυκεν αὐτῷ κατὰ τὸ ὀπίσθιον ἑαυτῆς μέρος͵ ταῖς ἰδίαις διαστολαῖς καὶ συμπτώσεσι συγκινοῦσα τὸ σπλάγχνον͵ τὴν τοῦ ἀέρος ὁλκήν τε καὶ ἐμπνοὴν ἐκμηχανᾶσθαι τῷ πνεύμονι. Ἀραιὸς γάρ τις ὢν καὶ πολύπορος καὶ πάσας τὰς ἐν αὐτῷ κοιλότητας πρὸς τὸν πυθμένα τῆς ἀρτηρίας ἀνεστομωμένας [248] ἔχων͵ συστελλόμενος μὲν καὶ συμπίπτων τὸ ἐναπολειφθὲν τοῖς κοίλοις πνεῦμα κατ' ἀνάγκην ἐκπιέζων προΐεται. Ὑποχωρῶν δὲ καὶ ἀνοιγόμενος ἐπισπᾶται τῇ διαστάσει πρὸς τὸ κενούμενον διὰ τῆς ὁλκῆς τὸν ἀέρα.