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of the body. For since what is hard and resistant does not receive sensory activities, as can be seen both in the bones within us and in the plants in the earth, in which we perceive a certain form of life in their growing and being nourished, yet the resistance of the underlying matter does not, on the other hand, admit sensation; for this reason it was necessary for a sort of wax-like structure to be laid down for the activities of sensation, able to be imprinted with the perceptual impressions, neither becoming confused through excessive moisture—for 244 that which is being imprinted would not remain in the moist substance—nor resisting the impression through an immoderate degree of hardness—for that which is unyielding is unimpressible by the forms—but having a mean between softness and firmness, so that the living creature might not be deprived of the most beautiful of natural activities, I mean sensory motion. Since, then, that which is soft and pliant, having no activity from what is firm, would be completely immobile and without joints, like sea-lungs; for this reason nature mixes into the body the firmness of bones, and having united these to one another through fitting articulation, and having tightened their joints with the ligaments of sinews, thus it grew around them the flesh which is receptive of sensations, arranged on the surface to be more resistant to suffering and more vigorous.

Upon this firm nature of the bones, therefore, as upon certain load-bearing pillars, having placed the whole weight of the body, it did not implant the bone as an undivided whole in the entire structure. For man would have remained immobile and inactive, if he had been so constructed, like some tree remaining in one place, with neither the alternating movement of the legs advancing his motion forward, nor the service of the hands being useful for life. But as it is, by this design it has contrived the instrument to be capable of movement and action, by implanting in the body the impulse and power for motion through the voluntary spirit that extends through the sinews. From this comes the service of the hands, so varied and versatile, and suited for every design. From this come the rotations of the neck, and the inclinations and nods of the head, and the activity of the jaw, and the opening of the eyelids occurring with a signal, and the movements of the other joints are activated by certain sinews being tightened or loosened, as if by some machine. But the power passing through these has a certain self-commanding impulse, being activated in each part by a voluntary spirit according to some economy of nature. And the root and origin of all the movements through the sinews has been shown to be the sinewy membrane that surrounds the brain. Therefore we no longer think we need to inquire curiously about the vital parts, that is, about what this particular part is, since the motive faculty has been shown to reside in it. But that the brain contributes something very great to life, the contrary event clearly shows. For if the membrane around it should suffer any wound or rupture, death immediately follows the injury, with nature not holding out against the wound even for an instant, just as when some foundation is undermined, the whole building collapses along with the part. Therefore, since when this suffers, the destruction of the whole living creature is manifest, this would properly be acknowledged to have the cause of life. And since, when those who have ceased to live, the heat inherent in their nature is extinguished, the dead body becomes 245 cold, for this reason we have perceived the vital cause also in heat. For since death followed upon its departure, it must necessarily be acknowledged that the living creature subsists by its presence. And of such a power we have understood the heart to be, as it were, a certain fount and origin, from which pipe-like channels, branching off from one another in many ways, diffuse the fiery and hot spirit to the whole body. But since it was entirely necessary for some nourishment to be present for the heat from nature; for it is not possible for fire to remain on its own, unless it is nourished by suitable fuel; for

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σώματος. Ἐπειδὴ γὰρ τὸ σκληρόν τε καὶ ἀντίτυπον τὰς αἰσθητικὰς ἐνεργείας οὐ κατα δέχεται, ὡς ἔστιν ἰδεῖν ἐπί τε τῶν ἐν ἡμῖν ὀστέων, καὶ τῶν ἐν τῇ γῇ φυτῶν, ἐν οἷς ζωῆς μέν τι κατανοοῦμεν εἶδος ἐν τῷ αὔξειν καὶ τρέφεσθαι, οὐ μὴν παρεδέξατο ἐναντίως ἡ ἀντιτυπία τοῦ ὑποκειμένου τὴν αἴσθησιν· τούτου χάριν ἔδει καθάπερ κηροειδῆ τινα κατασκευὴν ὑποτεθῆναι ταῖς κατ' αἴσθησιν ἐνεργείαις, δυναμένην τοῖς ἀντιληπτικοῖς τύποις ἐνσφραγισθῆναι, μήτε συγχεομένην δι' ὑπερβαλλούσης ὑγρότητος· οὐ γὰρ 244 ἂν διαμείνοι ἐν τῷ ὑγρῷ τὸ τυπούμενον· μήτε ἀντι τυποῦσαν ἐν τῇ ἀμετρίᾳ τῆς πήξεως· ἀσήμαντον γὰρ πρὸς τοὺς τύπους ἐστὶ τὸ ἀνύπεικτον· ἀλλὰ μέσως ἔχουσαν μαλακότητός τε καὶ στεῤῥότητος, ὡς ἂν μὴ τοῦ καλλίστου τῶν κατὰ τὴν φύσιν ἐνεργημάτων, τῆς αἰσθητικῆς λέγω κινήσεως, ἀμοιροίη τὸ ζῶον. Ἐπειδὴ τοίνυν τὸ μαλακόν τε καὶ εὔεικτον μηδεμίαν τὴν ἐκ τῶν στεῤῥῶν ἔχον ἐνέργειαν, ἀκίνητον ἂν ἦν πάντως, καὶ ἀδιάρθρωτον, κατὰ τοὺς θαλασσίους πνεύμονας· διὰ τοῦτο καταμίγνυσιν ἡ φύσις τῷ σώματι τὴν τῶν ὀστέων στεῤῥότητα, καὶ ταῦτα πρὸς ἄλληλα διὰ τῆς προσφυοῦς ἁρμονίας ἑνώσασα, τοῖς τε διὰ τῶν νεύρων συνδέσμοις τὰς συμβολὰς αὐτῶν ἐπισφίγξασα, οὕτως αὐτοῖς τὴν δεκτικὴν τῶν αἰσθήσεων περιέφυσε σάρκα, δυσπαθεστέραν τε καὶ εὐτονωτέραν τῇ ἐπιφανείᾳ διειλημμένην.

Ταύτῃ τοίνυν τῇ στεῤῥᾷ τῶν ὀστέων φύσει, οἷον στύλοις τισὶν ἀχθοφόροις, ὅλον τοῦ σώματος ἐπιθεῖσα τὸ βάρος, οὐκ ἀδιαίρετον ἐνέφυσε τῷ παντὶ τὸ ὀστέον. Ἦ γὰρ ἂν ἀκίνητός τε καὶ ἀνενέργητος ἔμεινεν, εἰ οὕτω κατασκευῆς εἶχεν ὁ ἄνθρωπος, καθάπερ τι δένδρον ἐφ' ἑνὸς τόπου μένον, μήτε τῆς τῶν σκελῶν διαδοχῆς ἐπὶ τὸ πρόσω προαγούσης τὴν κίνησιν, μήτε τῆς τῶν χειρῶν ὑπουργίας χρησιμευούσης τῷ βίῳ. Νυνὶ δὲ μεταβατικὸν εἶναι καὶ πρακτικὸν τὸ ὄργανον διὰ τῆς ἐπινοίας ταύτης ἐμηχανήσατο, τῷ προαιρετικῷ πνεύματι τῷ διὰ τῶν νεύρων διήκοντι, τὴν πρὸς τὰς κινήσεις ὁρμήν τε καὶ δύναμιν ἐνθεῖσα τῷ σώματι. Ἐντεῦθεν ἡ τῶν χειρῶν ὑπουργία, ἡ ποικίλη τε καὶ πολύστροφος, καὶ πρὸς πᾶσαν ἐπίνοιαν ἐπιτηδεία. Ἐντεῦθεν αἱ τοῦ αὐχένος περιστροφαὶ, καὶ τῆς κε φαλῆς ἐπικλίσεις τε καὶ ἀνανεύσεις, καὶ ἡ κατὰ τὴν γένην ἐνέργεια, καὶ ἡ τῶν βλεφάρων διαστολὴ ἅμα νεύματι γινομένη, καὶ τῶν λοιπῶν ἄρθρων αἱ κινή σεις ἐν νεύροις μέν τισιν ἀνασπωμένοις ἢ χαλωμέ νοις, καθάπερ ἐκ μηχανῆς τινος, ἐνεργοῦνται. Ἡ δὲ διὰ τούτων διεξιοῦσα δύναμις αὐτοκέλευστον ἔχει τινὰ τὴν ὁρμὴν, προαιρετικῷ πνεύματι κατά τινα φύσεως οἰκονομίαν ἐν τοῖς καθέκαστον ἐνεργουμένη. Ῥίζα δὲ πάντων ἀπεδείχθη καὶ ἀρχὴ τῶν κατὰ τὰ νεῦρα κινήσεων, ὁ τὸν ἐγκέφαλον περιέχων νευρώδης ὑμήν. Οὐκέτι οὖν ἡγούμεθα δεῖν πολυπραγμονεῖν περί τε τῶν ζωτικῶν μορίων, τὸ τοιοῦτόν ἐστιν, ἐν τούτῳ δειχθείσης τῆς κινητικῆς ἐνεργείας. Ὅτι δὲ μέγιστόν τι συντελεῖ πρὸς τὴν ζωὴν ὁ ἐγ κέφαλος, ἐναργῶς τὸ ἐξ ἐναντίου συμβαῖνον δηλοῖ. Εἰ γάρ τινα τρῶσιν ἢ ῥῆξιν ὁ περὶ αὐτὸν ὑμὴν πάθοι, εὐθὺς ἐπηκολούθησε τῷ πάθει ὁ θάνατος, οὐδὲ πρὸς τὸ ἀκαρὲς τῆς φύσεως ἀντισχούσης τῇ τρώσει, ὥσπερ θεμελίου τινὸς ὑποσπασθέντος ὅλον τὸ οἰκοδόμημα συγκατεσείσθη τῷ μέρει· οὗ τοίνυν παθόντος πρό δηλός ἐστιν ἡ τοῦ παντὸς ζώου διαφθορὰ, τοῦτο κυρίως ἂν τῆς ζωῆς τὴν αἰτίαν ἔχειν ὁμολογοῖτο. Ἐπειδὴ δὲ καὶ τῶν παυσαμένων τοῦ ζῇν κατασβεσθείσης τῆς ἐγκειμένης τῇ φύσει θερμότητος, τὸ νεκρωθὲν κατα 245 ψύχεται, διὰ τοῦτο καὶ ἐν τῷ θερμῷ τὴν ζωτικὴν αἰτίαν κατενοήσαμεν. Οὖ γὰρ ἐπιλελοιπότος ἡ νεκρό της ἐπηκολούθησεν, ἀνάγκη πᾶσα τῇ παρουσίᾳ τού του συνεστάναι τὸ ζῶον ὁμολογεῖσθαι. Τῆς δὲ τοιαύτης δυνάμεως οἶον πηγήν τινα καὶ ἀρχὴν τὴν καρδίαν κατενοήσαμεν, ἀφ' ἧς αὐλοειδεῖς πόροι πολυσχιδῶς ἄλλος ἐξ ἄλλου διαφυόμενοι, παντὶ τῷ σώματι τὸ πυρῶδες καὶ θερμὸν διαχέουσι πνεῦμα. Ἐπεὶ δὲ πάντως καὶ τροφὴν ἔδει τινὰ τῷ θερμῷ συμπαρεῖναι παρὰ τῆς φύσεως· οὐ γὰρ ἐνδέχεται τὸ πῦρ ἐφ' ἑαυ τοῦ μένειν, μὴ διὰ τοῦ καταλλήλου τρεφόμενον· διὰ