Within the central nervous system a sensory-motor system proceeds from roof to roots. It constitutes a core from which life is managed. From it motor impulses flow to the body, sensory impulses return to it. This system responds to intention and regulates motion. All speaks to shared functions in the rider and the horse of this sensory-motor systems in equitation.
Not all impulses of the sensory-motor system reach the brain or, for that matter, need its involvement. Many short-circuit in various spots of the central nervous system. An example: if you want to get up from a chair and walk to the window, you find yourself getting up and walking. You need not employ your mind to do so. These short-circuits, or reflexes, make for riding. A horse, in other words, reads the rider’s intentions and, in a work-sharing process along definitely defined lines fulfills the rider's will. The rider sits and uses his sensory system. The horse moves employing its motor system. Curiously in this process it needs not abandon its senses. It agrees with the rider and continues to navigate obstacles and grounds.
As concerns the correlations of human/equine nerves, which permit this work-sharing, three details are worth knowing. They are the harmony of different skeletal structures, the shortening of the human central nervous system to the first lumbar vertebra and a similar, however much less pronounced shortening in the horse and the before mentioned peculiar innervation of the skin in so called dermatomes.
Human and equine nervous systems in principle are alike. Human and equine bone structures, however, differ principally. The horse moves on four legs. The greater part of its vertebra, that is the cantilevered bridge, which at its center bears the rider, is in horizontal position. The S-shaped neck vertebra carries the head, which can rise to the skies or descend to the ground, and supports the horse's facial features. The human walks on two legs. His spinal column is in upright position. Supported by the sacrum in it he can bend and/or rotate. His head balances on the spinal column's upper end.
Man's upright spinal column and the horse’s multiply-shaped vertebra make for different motions. And yet, despite these differences the postures of two-legged man and the four-legged horse do correspond, as does the distribution of their dermatomes and peripheral nerve roots, respectively.
The extreme shortening of the human central nervous system to the uppermost lumbar vertebra results in an awkward distribution of peripheral nerves. One would expect these to exit directly, each pair proceeding from the central nervous system to corresponding exit holes right there, as they do in most of the horse. However in the human this system gets lost. Beginning with neck vertebra three, peripheral nerve roots do not correlate with exit holes. Instead peripheral nerves travel in the vertebral canal yet outside of the central nervous system for progressively longer distances. There is an order in the exit progression and this order is maintained. However, from lumbar vertebra one only peripheral nerves travel down. This phenomenon is called the horse’s tail. Resultingly nerve sections, which correspond to the human's lower body are stacked up in thorax vertebrae ten to twelve above, which it so happens also provide for the human spinal column's greatest flexibility.
How peculiar, the reader may think. And yet, purpose and reason are at work. In a world subject to a steady pull of gravity such an arrangement, barring the risk of pressure on the peripheral nerves, is possible only in the upright vertebra. The human is born with a full-length spinal cord. Its shortening is the result of growth. In other words, vertebrae grow. The central nervous system within does not. This particular process appears intimately linked to the extended learning phases in man's early life. With the development of intellectual facilities such as self-perception, consciousness and abstract thought. A foal gets up and immediately may run. Man takes time. He learns to sit, crawl, stand and walk before finally he also may run.
The shortened human central nervous system supports the correlation of human/equine nerves. Lumbar and sacral nerve sections are positioned mid-back in the area of the spinal column's greatest flexibility thus constituting the rider's central steering.
