49. Nothing is more common in the whole heaven and in the whole world, than for one thing to be within another; thus there is an inmost, a middle, and an outmost; and these three intercommunicate, and the power of the middle and outmost are derived from the inmost. That there are three things, one within the other, appears from each and all things 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 taken all together are called the skin. Around each artery and vein there are three tunics; likewise around each muscle and fiber; in like manner around all the rest which are there. In the vegetable kingdom the case is the same. How these parts intercommunicate, and how the inmost enters the middle, and the middle the ultimate, is shown by anatomy, etc. Thence it follows that the same is the case with light; that spiritual light which in its essence is truth, is interiorly in natural light; likewise that spiritual heat which in its essence is love, is in natural heat. By natural heat is meant natural love, because that love becomes warm; and this is clothed with the heat of the blood.