Given the upsurge of interest in complementary therapies and treatments, medical researchers are gradually being pressured to reassess and expand existing knowledge about the structure and organization of the human body. But in spite of the advances in modern technology, there are vast areas of human physiological activity, which continue to remain undetectable. Such processes, indicates Rudolf Steiner, are connected to spiritual forces. In these revealing lectures Steiner concentrates on the relationship of such forces to the physical organs. In particular, he discusses the organs, which make up our digestive and respiratory systems; the significance of \'warmth\' in the function of the blood and its effects upon the ego; and the evolutionary process implicit in the formation of the spinal column and brain. All are dealt with in a scientific manner that will appeal equally to doctors and therapists, as well as students of Steiner\'s spiritual philosophy. At the same time they are remarkably accessible to the general reader.