That on every occasion when this movement is made, the abdomen must be fully expanded-not held in or drawn upward. That great emphasis must be given to the importance of bringing the chin slowly but vigorously downward against the chest before the inward and backward movement is begun. This insures a proper stimulation of the thyroid gland. CHAPTER V: Stimulating, Straightening and Strengthening the Spine The human spine bears the same relation to the body as a whole as the trunk of a tree does to the rest of the tree. If the trunk is strong the entire tree is sturdy and vigorous. If the spine is strong the body as a whole possesses a similar degree of strength. Therefore, the necessity for a strong spine is readily apparent. This strength is necessary not only because the spine is what may be termed the foundation for our entire physical structure but also because therein are located the nerves that radiate to each organ and every minute part of the body. These spinal nerves control the functional processes of all our bodily tissues and structures. If the spine possesses a proper degree of strength, if the bony structure is properly proportioned, and if the alignment of all the vertebrae is everything that can be desired, you are then practically assured of the pulsating vitality which is a part of superb health. It is an interesting fact that the spine is the central and fundamental structure of all the higher organisms on this earth. In the course of the evolution of life on this planet there developed from the very simplest forms of animal organisms two different higher forms of life--on the one hand the vertebrate animals, possessing an internal skeleton, and on the other hand the insects, clams, crustaceans and other creatures that have their skeletons on the outside, as one may say, in the form of shells. The legs of an insect, for instance, are small tubes with the muscles inside. The limbs of vertebrate animals, on the other hand, have the muscle outside the bone. Invertebrates commonly have the main nerve trunk in front, or underneath, instead of at the back, and likewise often have their brains in their abdomens. Some of them, such as the grasshopper, even hear with their abdomens. But all vertebrata have the great nerve trunk at the back, contained in the spine and with a bulb on the front or upper end constituting the brain. In fact, a vertebrate animal is primarily a living spine, and all other parts of the body are in the nature of appendages. The limbs, for instance, and in the higher animals the ribs and other parts of the skeleton, are simply attached to the spine, or are offshoots from it. In the fishes these limbs take the shape of fins. In the higher developments of life they assume the form of legs.