From Medical Instrumentation for Healthcare, by Cromwell L, Arditti M, Weibell FJ, Pfeiffer EA, Steele B, Labok J.  Published by Prentice-Hall, 1976.  Page 272.

Found in “Diseases of the Chest”, published by American College of Physicians and Laennec Society of Philadelphia. Council on Post-Graduate Medical Education, World Bank Publications, 1968, page xxix.