For a rational being, to act in accordance with nature is also to act in accordance with reason.
Standing straight - or held straight.
Rational beings collectively have the same relation as the various limbs of an organic unity - they were created for a single cooperative purpose. The notion of this will strike you more forcefully if you keep on saying to yourself: 'I am a limb of the composite body of rational beings.' If, though, by the change of one letter from I to r [melos to meros], you call yourself simply a part rather than a limb, you do not yet love your fellow men from your heart: doing good does not yet delight you as an end in itself; you are still doing it as a mere duty, not yet as a kindness to yourself.
Rational beings are social beings. Whenever we act, we should do so according to our nature. We accept the results of the universe and world of which we are a part. We work (well) with other social beings. We control our own attitude with regard to the universe and the world as well as all people and things in the universe and world. This is how a rational being acts in accordance with nature.
Do you hold yourself the the standards of virtue or are you compelled to?
In the last passage, Marcus continues with the theme of rational beings. We need to recognize our relationship with the universe and world. Are you a co-operating limb or a stubborn limb? As a limb, you have to not only perform your duty, but you must love and accept what the whole body desires. Work well and serve others and love it. You must recognize this.
(see also Citadel p. 201, 230, 240, 258, 289)