The central ring of the Wheel is the hub upon which the whole thing turns. We see three animals chasing each other by the tail; a pig, a cock and a snake. These three symbolize the three great roots of samsaric existence; ignorance, desire and aversion.
Ignorance is literally "not knowing" (i-gnosis) an etymology that works in Pali as well (avijja = a + vijja.) It is here especially not knowing the reality of our existential dilemma as beings trapped in the samsaric round. This comes from not seeing the three characteristics of unsatisfactoriness, impermanence and insubstantiality that are inherent in all conditioned phenomena. Instead, the worldling sees the things of the world as satisfying, lasting and real.
The pig is an excellent symbol of this. He roots around in his trough full of delicious potato peels and mash, growing fatter day by day. The farmer is a beloved figure who altruistically fills the trough for him. He never knows until too late the reality of his situation. (I am here being somewhat anachronistic, the modern factory method of pig rearing being a hell realm from the very beginning.)
The pig is the being in ignorance, the trough the world and the mash are sensual pleasures. The farmer is Mara in his dual aspects as the Tempter and the Lord of Death.
The cock in this drawing is the symbol of desire. By "desire" is here meant the craving for pleasures of the five bodily senses; beautiful sights, harmonious sounds, delicious tastes, fragrant aromas and sensual touches.
The cock in the barnyard lives for his pleasure, indeed the reason the farmer keeps him is to service his hens. The cock never understands the transitory nature of the pleasures which fuel his insatiable desires. And what is the summit of his experience? Some mad clucking and a little thrill and soon he is back to his unsatisfied state.
This is the perennial nature of all sense experience. It is transitory and incapable of giving lasting happiness or satisfaction. The worldling in the grip of sense desire is as senseless as the barnyard cock, even though his desires may be more refined. Even the sophisticated sense pleasures of music and art are intrinsically the same; unsatisfying, fleeting and unreal.
The third animal in the trinity is the snake as symbol of aversion or ill-will. This is the negative form of the same energy which manifests positively as desire. It is driven by the same thing, the experience of the senses. In this form it is the negative desire to destroy ugly sights, harsh sounds, bitter tastes, foul odours and painful sensations of the body. It is based on the same fundamental misunderstanding of the true nature of these phenomena as just fleeting insubstantial experiences of the senses.
The snake is a symbol of aversion because he works by poison. Aversion is very much a poison in all in its manifestations as hatred, anger and the urge to hurt and kill. But in the worldling affected by these emotions, it is he himself who is primarily a sufferer from the poison, and not the object of his hatred.
The three animals are seen as chasing one another in a circle, because this is the eternal nature of the three great roots. Each feeds upon the others and gives birth to more of the same in a vicious circle. Because of fundamental ignorance about their true natures, worldlings desire some experiences and detest others. Because of their desires they hate those people and things which thwart them. Because of their hatred, they desire power to destroy. And the blindness and intoxication of both lust and hatred create even greater delusion and ignorance, even to the point of madness.
This cycle is primordial and beings have been trapped in it since the beginingless beginning. What is more, it is unbreakable within the terms of its own self-contained world, which is the world of samsara or conditioned reality. There is simply no way that any variation within the play of desire, aversion and delusion can ever transcend itself. The way out points to that which is beyond all this.