In Canto XIII Dante and Virgil come to another ledge, and climb higher up, into the cornice of the Envious. It might be worth noting that while we have these terms in English, for Dante and his readers there are other connotations for the words he uses to enumerate the 7 deadly sins. Pride is not what we might ordinarliry think of as pride in common usage: it is more of a perverse self-love, a placing of oneself as the center of all things, and hence playing at God. So too when we come to Envy, it is not merely "being jealous" of what someone else has or is. It is a bitter, resentful attitude at those things. Envy is not simply wishing you had your neighbour's "ox or ass", as the last commandment declares of covetousness. It is a resentfulness that your neighbour even has those things in the first place. So Envy involves a deep-seated, almost angry, attitude towards one's fellow man. If Pride was a sin which involved looking too much at one's own self, then envy involves looking too much at one's neighbour.
As they come up, there are no carvings of the scenes as there was in the last Canto. The reason is this: at the beginning stage of repentance, the soul learns by example. So at the first cornice we had examples of both Pride and Humility: we begin by simply avoiding the one and imitating the other. It is of no use for the beginner to try to understand a "theology of virtue" if he or she does not actually begin to practice it. And it might even be said that one will never understand a theology of virtue unless one practices it.
There is no guide here to tell the pilgrims where to go, and so Dante plots his course by "looking up, staring at the sun" (XIII.13). As a poetic image, the sun has long been a representation of spiritual and intellectual illumination, and as the image was baptized by Christian writers, has now come to represent the light of Christ, or the grace of God. Remember it was Lucia (light) who carried Dante up the slope earlier. So without the help from other guides, Dante now takes his direction from "the sun".
Those who are repenting of the sin of Envy are found huddled against the cliff side. Dante and Virgil come to see that this group has their eyes sewn shut with iron thread. I think the imagery serves two purposes. First, it is obvious from that this sin is particularly associated with sight, with looking at one's neighbour more than one should. And so the image of repentance addresses the physical "source" of that sin. In addition, have one's eyes sewn shut with iron thread indicates that the removal of the temptation, the removal of the habit of sin, is something which must be "forced" upon us; we have lost the ability to "not sin". This is detox for the soul: they cannot look upon others, even if they wanted to. This is both an inward and outward image. Yes, the image is physical, but Dante knows, and will tell us later, that the true sources of sin are internal, they are the false or distorted ways in which we love ourselves, our neighbours, or the things in this world. Sin has an inward source, but it is manifested outwardly in the physical world.
Since the souls in this cornice have no sight, there are no carven examples of Envy and Generosity. Instead they use their voices to shout out examples. This is also worth considering: the examples ( the the knowledge of the difference between) Envy and Generosity are not written in stone (as with Pride and Humility), but come by voice. The examples of Generosity are spoken by the penitents, but the examples of Envy are not; they are rather spoken by other voices (eg, the voice of Cain, who, envious of his brother Abel, slew him - XIV.132 ff). This example of Envy, at the end of Canto XIV, helps set up the transition to the next cornice - the place of the Wrathful. It shows in the person of Cain how one sin naturally leads into another related sin.