49. Nothing is more common in the whole heaven and in the whole world, than for one thing to be within another, thus an inmost, a middle and an outmost, and for the three to intercommunicate, and for the power of the middle and outmost to be derived from the inmost. That there are these three, one within the other, is evident from each and every thing in the human body. Around the brain there are three tunics, which are called the dura mater, the pia mater, and the arachnoid; and over these is the skull. Around the whole body there are tunics, one within the other, which, together, are called the skin. Around each artery and vein there are three tunics; likewise around each muscle and fibre; in like manner, around all the rest of the things there. So in the vegetable kingdom. How they intercommunicate, and how the inmost enters the middle, and the middle the last, anatomy shows, etc. Thence it follows that it is so with light: that spiritual light, which in its essence is truth, is inwardly in natural light; in like manner, spiritual heat, which in its essence is love, in natural heat. By natural heat is meant natural love, because that love grows warm; and this is covered over by the heat of the blood.