All the higher animals, as we know, have evolved from the fishes and reptiles, and all in common possess a spine which in its fundamental characteristics is very much the same now as when it was first evolved. In other words, the spine is a bodily structure as old as the rock-ribbed hills. It has stood the test of time, and therefore must be regarded as the most highly perfected mechanical structure in the body. Its strength combined with its flexibility and its perfect adjustment as a container for the central nervous system, makes it perhaps the most wonderful structure in the body outside of the brain and the spinal cord itself. While other organs and features of the body have been changed and modified to such an extent in the various species which have been evolved that they can hardly be recognized as having a common origin, yet the spine has remained substantially the same. It is true that the spine has been shortened in many species as the result of the loss of the tail, but this means only the dropping off of a part of it and does not greatly alter its fundamental character. The human spine, however, differs from that of other animals in respect to its suitability for the erect posture. Man is the only animal in the world who can straighten his body and stand perfectly erect. Even the anthropoid apes when standing on their feet assume a somewhat oblique position. The vertebral column in animal life was first developed on the horizontal plane, and so, naturally, when man was evolved and adopted the erect position, certain modifications of the spine were necessary. A new strain developed on the vertebral column which was due to the new position, and so there came about certain changes in its structure. For one thing the spine became less flexible and gained in stability, especially in the lower sections. The sacrum, for instance, is created by the fusing together of several vertebrae into one bone for the sake of greater strength and stability. The sacrum in man is much broader than in animals, for it must supply solidity and strength to the lower part of the spine, thus adapting it to the vertical position, and in the same way the lower vertebrae generally are comparatively broader and heavier, gradually decreasing in size and tapering toward the top of the spine like the trunk of a tree. This particular feature of the human backbone is worthy of special consideration because it is the upper section of the spine, in which the vertebrae are smaller and tapering, that weakness is most likely to exist. It is in this upper section of the spine that strength is most needed in order to preserve it in perfect alignment, and keep the body properly erect. And it is for this reason, as the reader will see, that exercises affecting the upper parts of the spine are most important. Therefore I have given them special attention.