§12Ὅτι μὲν οὖν εἰ μὴ κάμψις ἦν ἐν τοῖς σκέλεσιν ἢ ἐν ταῖς 12 ὠμοπλάταις καὶ ἰσχίοις, οὐθὲν οἷόν τʼ ἦν ἂν τῶν ἐναίμων καὶ ὑποπόδων προβαίνειν, εἴρηται πρότερον, καὶ ὅτι κάμψις οὐκ ἂν ἦν μηδενὸς ἠρεμοῦντος, ὅτι τε ἐναντίως οἵ τε ἄνθρωποι δίποδες ὄντες καὶ οἱ ὄρνιθες τὴν τῶν σκελῶν ποιοῦνται κάμψιν, ἔτι δὲ τὰ τετράποδα ὑπεναντίως καὶ αὑτοῖς καὶ τοῖς ἀνθρώποις.
Therefore, that if there were no bending in the legs, or in the shoulder-blades and hips, it would be impossible for any of the red-blooded and footed animals to go forward, has been said before, and also that there would be no bending if nothing were at rest, and that humans, being bipedal, and birds perform the bending of their legs in opposite directions, and furthermore, that quadrupeds perform it in a manner contrary both to themselves and to humans.
οἱ μὲν γὰρ ἄνθρωποι τοὺς μὲν βραχίονας κάμπτουσιν ἐπὶ τὰ κοῖλα, τὰ δὲ σκέλη ἐπὶ τὸ κυρτόν, τὰ δὲ τετράποδα τὰ μὲν πρόσθια σκέλη ἐπὶ τὸ κυρτόν, τὰ δʼ ὀπίσθια ἐπὶ τὸ κοῖλον· ὁμοίως δὲ καὶ οἱ ὄρνιθες.
For humans bend their arms towards the concave parts, but their legs towards the convex, while quadrupeds bend their front legs towards the convex, and their hind legs towards the concave; and likewise also birds.
αἴτιον δʼ ὅτι ἡ φύσις οὐδὲν δημιουργεῖ μάτην, ὥσπερ εἴρηται πρότερον, ἀλλὰ πάντα πρὸς τὸ βέλτιστον ἐκ τῶν ἐνδεχομένων.
And the cause of this is that nature creates nothing in vain, as has been said before, but all things towards the best from among the possibilities.
ὥστʼ ἐπεὶ πᾶσιν ὅσοις ὑπάρχει κατὰ φύσιν ἡ κατὰ τόπον μεταβολὴ τοῖν σκελοῖν, ἑστῶτος μὲν ἑκάστου τὸ βάρος ἐν τούτῳ ἐστί, κινουμένοις δʼ εἰς τὸ πρόσθεν δεῖ τὸν πόδα τὸν ἡγούμενον τῇ θέσει κοῦφον εἶναι, συνεχοῦς δὲ τῆς πορείας γινομένης αὖθις ἐν τούτῳ τὸ βάρος ἀπολαμβάνειν, δῆλον ὡς ἀναγκαῖον ἐκ τοῦ κεκάμφθαι τὸ σκέλος αὖθις εὐθὺ γίνεσθαι μένοντος τοῦ τε κατὰ τὸν προωσθέντα πόδα σημείου καὶ τῆς κνήμης.
Consequently, since for all those to whom local movement by means of two legs belongs according to nature, when each is standing, its weight is on this leg, but when they are moving forward, the leading foot must be light in its placement, and when the walking becomes continuous, it must in turn receive the weight upon this foot, it is clear that it is necessary for the leg to become straight again after having been bent, while both the point at the advanced foot and the shin remain stationary.
τοῦτο δὲ συμβαίνειν ἅμα καὶ προϊέναι τὸ ζῷον εἰς τὸ ἔμπροσθεν μὲν ἔχοντος τὴν καμπὴν τοῦ ἡγουμένου σκέλους δυνατόν, εἰς τοὔπισθεν δʼ ἀδύνατον.
And that this should happen and at the same time the animal should go forward is possible when the leading leg has its bend towards the front, but impossible when towards the rear.
οὕτω μὲν γὰρ προενεχθέντος τοῦ σώματος ἡ ἔκτασις τοῦ σκέλους ἔσται, ἐκείνως δʼ ἀνενεχθέντος.
For if the body is carried forward in the former way, the extension of the leg will take place, but in the latter way, it would be drawn back.
ἔτι δʼ εἰς τὸ ὄπισθεν μὲν τῆς καμπῆς οὔσης διὰ δύο κινήσεων ἐγίγνετʼ ἂν ἡ τοῦ ποδὸς θέσις ὑπεναντίων τε αὑταῖς, καὶ τῆς μὲν εἰς τὸ ὄπισθεν τῆς δὲ εἰς τὸ ἔμπροσθεν· ἀναγκαῖον γὰρ ἐν τῇ συγκάμψει τοῦ σκέλους τοῦ μὲν μηροῦ τὸ ἔσχατον εἰς τοὔπισθεν προάγειν, τὴν δὲ κνήμην ἀπὸ τῆς καμπῆς εἰς τὸ ἔμπροσθεν τὸν πόδα κινεῖν·
Furthermore, if the bend were towards the rear, the placement of the foot would occur through two movements contrary to each other, namely, one towards the rear and the other towards the front; for it is necessary in the bending of the leg to advance the extremity of the thigh towards the rear, but for the shin to move the foot from the bend towards the front.
εἰς τὸ ἔμπροσθεν δὲ τῆς καμπῆς οὔσης, οὔθʼ ὑπεναντίαις κινήσεσι μιᾷ τε τῇ εἰς τὸ ἔμπροσθεν ἡ λεχθεῖσα πορεία συμβήσεται.
But if the bend is towards the front, the aforementioned walking will take place not by contrary movements, but by a single movement towards the front.
ὁ μὲν οὖν ἄνθρωπος δίπους ὢν καὶ τὴν κατὰ τόπον μεταβολὴν κατὰ φύσιν τοῖς σκέλεσι ποιούμενος διὰ τὴν εἰρημένην αἰτίαν κάμπτει εἰς τὸ ἔμπροσθεν τὰ σκέλη, τοὺς δὲ βραχίονας ἐπὶ τὸ κοῖλον εὐλόγως· ἄχρηστοι γὰρ ἂν ἦσαν καμπτόμενοι εἰς τοὐναντίον πρός τε τὴν τῶν χειρῶν χρῆσιν καὶ πρὸς τὴν τῆς τροφῆς λῆψιν.
Therefore, man, being bipedal and making his local movement by nature with his legs, bends his legs towards the front for the reason mentioned, but his arms towards the concave reasonably; for they would be useless if bent in the opposite direction, both for the use of the hands and for the taking of food.
τὰ δὲ τετράποδα καὶ τὰ ζῳοτόκα τὰ μὲν ἔμπροσθεν σκέλη, ἐπειδὴ ἡγεῖταί τε τῆς πορείας αὐτῶν καὶ ἔστι ταῦτʼ ἐν τῷ μέρει τῷ ἔμπροσθεν τοῦ σώματος, ἀνάγκη κάμπτειν ἐπὶ τὴν περιφέρειαν διὰ τὴν αὐτὴν αἰτίαν ἥνπερ καὶ οἱ ἄνθρωποι· κατὰ γὰρ τοῦτο ὁμοίως ἔχουσι.
And as for quadrupeds and viviparous animals, since their front legs lead their walking and are in the front part of the body, they must bend them towards the convex for the same reason as humans do; for in this respect they are in a similar state.
διόπερ καὶ τὰ τετράποδα κάμπτουσιν εἰς τὸ πρόσθεν τὸν εἰρημένον τρόπον.
Therefore, quadrupeds also bend their front legs towards the front in the manner mentioned.
καὶ γὰρ οὕτως μὲν αὐτῶν τῆς κάμψεως γινομένης ἐπὶ πολὺ δυνήσονται τοὺς πόδας μετεωρίζειν· ἐναντίως δὲ κάμπτοντες μικρὸν ἀπὸ τῆς γῆς ἂν αὐτοὺς ἐμετεώριζον διὰ τὸ τόν τε μηρὸν ὅλον καὶ τὴν καμπήν, ἀφʼ ἧς ἡ κνήμη πέφυκεν, ὑπὸ τῇ γαστρὶ γίγνεσθαι προϊόντος αὐτοῦ.
For indeed, if their bending occurs in this way, they will be able to raise their feet high; but if they bent them in the opposite direction, they would raise them only a little from the ground, because both the whole thigh and the bend from which the shin grows would come under the belly as the animal goes forward.
τῶν δ᾿ ὄπισθεν σκελῶν εἰ μὲν ἦν εἰς τὸ ἔμπροσθεν ἡ κάμψις, τῶν ποδῶν ὁ μετεωρισμὸς ὁμοίως ἂν αὐτοῖς εἶχε τοῖς προσθίοις (ἐπὶ βραχὺ γὰρ ἂν ἐγίγνετο καὶ τούτοις κατὰ τὴν ἄρσιν τῶν σκελῶν, τοῦ τε μηροῦ καὶ τῆς καμπῆς ἀμφοτέρων ὑπὸ τὸν τῆς γαστρὸς τόπον ὑποπιπτόντων), εἰ δʼ εἰς τὸ ὄπισθεν, καθάπερ καὶ νῦν κάμπτουσιν, οὐθὲν ἐμπόδιον αὐτοῖς γίγνεται πρὸς τὴν πορείαν ἐν τῇ τοιαύτῃ κινήσει τῶν ποδῶν.
And as for the hind legs, if the bend were towards the front, the raising of the feet would be in the same state as for the front legs (for it would occur to a small extent also for these when raising the legs, with both the thigh and the bend falling under the region of the belly), but if it is towards the rear, just as they bend them now, no obstacle arises to them for their walking in such a movement of the feet.
ἔτι τοῖς γε θηλαζομένοις αὐτῶν καὶ πρὸς τὴν τοιαύτην λειτουργίαν ἀναγκαῖον ἢ βέλτιόν γʼ οὕτω κεκάμφθαι τὰ σκέλη· οὐ γὰρ ῥᾴδιον τὴν κάμψιν ποιουμένων ἐντὸς ὑφʼ αὑτὰ ἔχειν τὰ τέκνα καὶ σκεπάζειν.
Furthermore, for those of them that suckle, it is necessary or at least better for their legs to be bent in this way for such a function; for it would not be easy, if they made the bending inwards, to have their offspring under themselves and to shelter them.