Diaphragm
2 of the 6 encyclopedias on this shelf carry an entry for Diaphragm. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
di'a-fram, a Grecian word meaning a partition wall. In mechanics and anatomy, the term is applied to any thin curtain or partition, as the vibrating diaphragm of a telephone upon which the impulse of the voice falls; the curtain which is used in a photographic camera to cut off the light; and particularly the curtain-like muscle which separates the cavity of the chest from that of the abdomen. The human diaphragm is shaped something like an umbrella, point upward. Its muscular bundles are arranged on the under side somewhat like the rays of an umbrella. When the diaphragm contracts, that is to say, when these muscles swell and shorten, it is stretched more nearly straight across the body, and, as the point sinks, it enlarges the chest cavity, causing the lungs to inhale air. When the diaphragm is relaxed, its elasticity causes it to assume its natural form, and the center of the diaphragm rises, diminishing the upper or chest cavity, thus driving air out of the lungs. Thus we see that taking a breath is largely an action of the diaphragm. Sometimes the diaphragm is affected with periodic twitchings which seem beyond control. One of these, the sneeze, is caused by a sudden relaxation, that is to say, upward bound of the diaphragm. Hiccoughs are caused by spasmodic contractions of the same organ, the action being the exact opposite of a sneeze. In zoology, the diaphragm is a calcareous plate dividing the cavity of certain shells into two parts.
Collier's New Encyclopedia (1921)
an inspiratory muscle, and the sole agent in tranquil respiration. It is the muscular septum between the thorax and abdomen, and is composed of two portions, a greater muscle arising from the ensiform cartilage, and a lesser arising from the bodies of the lumbar vertebræ by two tendons. There are three openings in the diaphragm, one for the passage of the inferior vena cava, one for the passage of the esophagus and pneumo-gastric nerves and the aortic, through which passes the aorta, the right vena azygos, and thoracic duct. It assists the abdominal muscles, which are expiratory, powerfully in expulsion, each act of that kind being accompanied or preceded by a deep inspiration. In optics, an annular disk in a camera or telescope or other optical instrument, to exclude some of the marginal marginal rays of a beam of light.
What Has Changed in the Last Century
The anatomy in both entries still holds: the diaphragm remains the chief muscle of quiet breathing, pierced by the same three openings for the aorta, esophagus and inferior vena cava. Aiton's 1910 account of the sneeze as "a sudden relaxation, that is to say, upward bound of the diaphragm" is the one piece that has not survived — sneezing is now understood as a reflex forced exhalation driven by the abdominal and chest muscles, though the description of hiccups as spasmodic diaphragm contraction is essentially still taught. What has changed is what can be done about it: positive-pressure mechanical ventilators, developed on a large scale during the polio epidemics of the 1950s, can take over the diaphragm's work entirely, and implanted phrenic-nerve pacemakers, pioneered in the 1960s, can make a paralysed diaphragm contract in patients with high spinal-cord injury. In optics the annular "diaphragm" Collier's describes is now usually called the aperture or iris, while the telephone diaphragm Aiton's cites has largely given way to electronic transducers.
Written for this edition, 2026.