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.
It seems almost symbolic and indicative of the purpose of creation that the body, which is less strong and vigorous in Man than in the lower animals, should tend more and more obliquely downward from its central axis, while the cranium, containing a highly specialized mass of cells and fibres, the organ of Mind, which marks Man’s supremacy in the animal kingdom and is his crowning glory, is reared above the body it dominates.
In all the form changes which mark the growth of the body the organs are arranged to afford the greatest possible economy of space and convenience for use. This perfect and matchless mechanism adapts itself to the changing habits and environments and to the quality and needs of the Mind which inhabits it.