Now with regard to the body. Everything except the breath and the ether within the body is material, is mortal, is solid, is definite. The essence of that which is material, which is mortal, which is solid, which is definite is the Eye, for it is the essence of sat (the definite).

In plain words: Now the same teaching is applied to the human body. Everything in the body except the breath and the space inside it is solid, mortal and fixed. The eye is the essence of these solid parts.