This then is the cause of this involuntary respiration-the impossibility that the fiery element should remain at rest: for as the operation of motion is proper to heat, and we understand that the principle of heat is to be found in the heart, the continual motion going on in this organ produces the incessant inspiration and exhalation of the air through the lungs: wherefore also when the fiery element is unnaturally augmented, the breathing of those fevered subjects becomes more rapid, as though the heart were endeavouring to quench the flame implanted in it by more violent breathing. But since our nature is poor and in need of supplies for its own maintenance from all quarters, it not only lacks air of its own, and the breath which excites heat, which it imports from without for the preservation of the living being, but the nourishment it finds to fill out the proportions of the body is an importation. Accordingly, it supplies the deficiency by food and drink, implanting in the body a certain faculty for appropriating that which it requires, and rejecting that which is superfluous, and for this purpose too the fire of the heart gives nature no small assistance. For since, according to the account we have given, the heart which kindles by its warm breath the individual parts, is the most important of the vital organs, our Maker caused it to be operative with its efficacious power at all points, that no part of it might be left ineffectual or unprofitable for the regulation of the whole organism. Behind, therefore, it enters the lungs, and, by its continuous motion, drawing that organ to itself, it expands the passages to inhale the air, and compressing them again it brings about the exspiration of the imprisoned air; while in front, attached to the space at the upper extremity of the stomach, it warms it and makes it respond by motion to its own activity, rousing it, not to inhale air, but to receive its appropriate food: for the entrances for breath and food are near one another, extending lengthwise one alongside the other, and are terminated in their upper extremity by the same boundary, so that their mouths are contiguous and the passages come to an end together in one mouth, from which the entrance of food is effected through the one, and that of the breath through the other.

Καὶ αὕτη ἐστὶ τῆς ἀπροαιρέτου ταύτης ἀναπνοῆς αἰτία ἡ τοῦ ἀτρεμεῖν τὸ πυρῶδες ἀδυναμία. Ἐπειδὴ γὰρ ἴδιόν ἐστι τοῦ θερμοῦ ἡ κατὰ τὴν κίνησιν ἐνέργεια͵ τούτου δὲ τὰς ἀρχὰς ἐν τῇ καρδίᾳ κατενοήσαμεν͵ τὸ διηνεκὲς τῆς ἐν τῷ μέρει τούτῳ κινήσεως τὴν ἀδιάλειπτον τοῦ ἀέρος ὁρμήν τε καὶ ἐμπνοὴν διὰ τοῦ πνεύματος ἀπεργάζεται. Διὸ καὶ παρὰ φύσιν ἐπιταθέντος τοῦ πυρώδους͵ τὸ ἆσθμα τῶν διακαιομένων τῷ πυρετῷ συνεχέστερον γίνεται͵ ὥσπερ ἐπειγομένης τῆς καρδίας τὸν ἐγκείμενον ἐν αὐτῇ φλογμὸν τῷ νεαρωτέρῳ πνεύματι κατασβεννύειν. Ἀλλ' ἐπειδὴ πενιχρά τίς ἐστιν ἡμῶν ἡ φύσις καὶ τῶν πρὸς τὴν ἰδίαν σύστασιν διὰ πάντων ἐπιδεής͵ οὐκ ἀέρος μόνον ἰδίου πτωχεύει καὶ πνεύματος τοῦ τὸ θερμὸν διεγείροντος͵ ὅπερ ἔξωθεν πρὸς τὴν τοῦ ζώου συντήρησιν διηνεκῶς ἐπεισάγεται, ἀλλὰ καὶ τὴν ὑπερείδουσαν τὸν τοῦ σώματος ὄγκον τροφὴν ἐπίκτητον ἔχει. Διὰ τοῦτο σιτίοις τε καὶ ποτοῖς ἀναπληροῖ τὸ ἐνδέον͵ ἑλκτικήν τινα τοῦ λείποντος δύναμιν καὶ ἀπωστικὴν τοῦ περιττεύοντος ἐνθεῖσα τῷ σώματι͵ οὐδὲ πρὸς τοῦτο τοῦ ἐγκαρδίου πυρὸς μικρὰν παρεχομένου τῇ φύσει συνέργειαν. Ἐπειδὴ γὰρ τὸ κυριώτατον τῶν ζωτικῶν μορίων κατὰ τὸν ἀποδοθέντα λόγον ἡ καρδία ἐστίν͵ ἡ τῷ θερμῷ πνεύματι ζωπυροῦσα τὰ καθέκαστον μέρη͵ πανταχόθεν αὐτὴν ἐνεργὸν εἶναι τῷ δραστικῷ τῆς δυνάμεως͵ ὁ πλάστης ἡμῶν ἐποίησεν͵ ὡς ἂν μηδὲν αὐτῆς μέρος ἄπρακτόν τε καὶ ἀνόνητον πρὸς τὴν τοῦ παντὸς οἰκονομίαν καταλειφθείη. Διὰ τοῦτο κατόπιν μὲν ὑποβᾶσα τὸν πνεύμονα διὰ τῆς διηνεκοῦς κινήσεως καθέλκουσά τε πρὸς ἑαυτὴν τὸ σπλάγχνον͵ ἀνευρύνει πρὸς τὴν ὁλκὴν τοῦ ἀέρος τοὺς πόρους͵ ἀνοχλίζουσά τε πάλιν ἐκπνεῖσθαι τὸ ἐναπειλημμένον παρασκευάζει. Ἐν δὲ τοῖς ἔμπροσθεν ἑαυτῆς προσφυεῖσα τῷ χωρήματι τῆς ἄνω γαστρός͵ ἔνθερμον αὐτὴν καὶ πρὸς τὰς ἰδίας ἐνεργείας κινουμένην ποιεῖ, οὐκ εἰς πνεύματος ὁλκὴν ἀνεγείρουσα͵ ἀλλ' εἰς ὑποδοχὴν τῆς καταλλήλου τροφῆς. Πλησίον μὲν γὰρ πεφύκασιν ἀλλήλων αἱ εἴσοδοι τοῦ πνεύματός τε καὶ τῆς τροφῆς, ἀντιπαρεξιοῦσαι δὲ κατὰ τὸ πρόμηκες ἡ ἑτέρα πρὸς τὴν ἑτέραν͵ καὶ τῷ ἴσῳ μέτρῳ κατὰ τὸ ἄνω συναπαρτίζονται͵ ὥστε καὶ συνεστομῶσθαι πρὸς ἀλλήλας καὶ ἑνὶ στόματι τοὺς πόρους ἐναπολήγειν͵ ὅθεν τῷ μὲν τῆς τροφῆς͵ τῷ δὲ τοῦ πνεύματος ἡ εἴσοδος γίνεται.