For some time now, doctors have used the inability to feel sensation at particular locations on the surface of the skin to diagnose where nerves that exit the spine may be compressed (or damaged). A lack of sensation on the skin along the outer edge of the foot, for example, can mean that the nerves that exit between the last vertebrae of the spine and the first vertebrae of the sacrum are pinched between the vertebrae. Similarly, a lack of sensation in the little finger can signal that the nerves that exit between the last vertebrae of the neck and the first vertebrae of the chest are somehow impinged on by the bones on either side.
Conversely, if there is feeling throughout the body to the surface of the skin, the spine is aligned so that the bones do not impinge on the nerves. In my experience, an effort to have feeling through the entire body to the surface of the skin can generate support for the spine and even bring the spine into alignment, as the effort to feel generates an involuntary response in the muscles of posture.
Shunryu Suzuki once said in a lecture:
“You should feel each part of your body doing zazen separately. Each part of your body should participate completely in zazen.”
(Tassajara, June 28 1970; from Bill Redican’s edit on)
Another way to say this would be, when we are conscious of a particular part, it is the contribution of our feeling of that particular part that serves to complete the whole.