All the passages in the body are connected with the centers, which are most important in spiritual development, and it is upon the cleanliness and purity of these passages that spiritual development depends. Besides these nine passages it is necessary to keep the skin in a proper condition for spiritual purposes. It is from the mystical conception that humanity first learned the idea of clothes. There have been times when certain races painted their skin, and by certain yogis the body was covered with ashes; in ancient times the body was covered with the bark of trees.