<Dd> "Physiologists divide nerve - fibers, which form the nerves, into two classes, afferent and efferent . Impressions are made on the peripheral afferent fiber - endings; these create sensations which are transmitted to the center of the nervous system . Efferent nerve - fibers carry impulses out from the center to their endings . Most of these go to muscles and are therefore called motor impulses; some are secretory and enter glands; a portion are inhibitory their function being to restrain secretion . Thus, nerves carry impulses outward and sensations inward . The activity of these nerves, or rather their fibers, may become excited or allayed by impingement, the result being a modification of functionating--too much or not enough action--which is dis - ease ." </Dd> <P> Chiropractors use and have used various terms to express this concept: subluxation, vertebral subluxation (VS), vertebral subluxation complex (VSC), "killer subluxations," and the "silent killer ." </P> <P> They, along with some physical therapists and osteopathic physicians, have also used another term, BOOP, meaning "bone out of place ." </P> <P> Some chiropractors have described the disagreements within the profession about the concept, and have written skeptically about BOOP as an antiquated idea . In 1992 one wrote: </P>

When was x-ray first used to study vertebral subluxations by the chiropractic profession