Those land animals which walk on all fours approach still closer but their arrangement is much more readily comprehensible than in man. As the animal stands on all fours with head extended, a gigantic cleaver slicing out each vertebra and pair of nerves and slicing straight toward the base of support might be said to divide the body _approximately_ according to the structural and functional arrangement in segments. Yet no segment so separated would exactly correspond to the nerve distribution; there would be enlargement of some organs with extension into the zone previously occupied by their neighbors; enlargement here and atrophy there; invagination of one organ by another and overlapping and intermingling of parts. Even the relation between the spinal cord segments and the vertebrae has departed much from the primitive so that the growth of the vertebrae has exceeded that of the cord and the cord terminates opposite the Lumbar region instead of at the end of the Sacral canal. It may here be remarked that in the human embryo the cord at first occupies the entire length of the neural canal formed within the vertebrae; that in the adult it terminates opposite the lower border of the body of the first Lumbar vertebra and that the nerves, still retaining their original foramina of exit and their relation to the somatic segments, pass downward within the canal to their respective openings and collectively form a brush like mass called “cauda equina.” Causes of Segmental Changes The causes of the change in the shape, form, and relation of the different segments are functional: the body changes to meet the changing needs of its environment and the steady progressive functional development from one species to another. When the animal at last assumes the erect position, doing more intricately and intelligently the bidding of a developing and improving central nervous system, the change of position and the force of gravity bring about a gradual downward, or caudad, tendency of the parts of the somatic segments most remote from the spine and of the nerves which supply them. The nerves, muscles, and bones of the lower extremities change from almost a right angle to an extremely obtuse angle, less obtuse during infancy and more so in the adult. The forelegs become arms and hang at the sides, extending downward from the part of the spine which controls them. The ribs tend more obliquely downward and outward from the spine and the tendency of all the nerves is downward from their attachment to the spinal cord to their emergence from the intervertebral foramina. In the neck and head alone is this rule varied, the tendency of the nerves and some other structures there being to run from the spine either at right angles or upward.