The Respiratory Organs.

THESE organs consist of the windpipe (trachea) ; divisions and mbdivi8ions of the windpipe (bronchia) ; aircells ; and the lung8 or lights.

The Windpipe (trachea) extends from the larynx the seat of the voiceto the third dorsal vertebra, where it divides into two tubes, called bronchia. It runs down the front part of the throat, with the esophagus behind and between it and the spinal column. It is composed mainly of rings of cartilage, one above another.

The Bronchial Tubes are, at the division of the windpipe, two in number, but they divide and subdivide until they become very numerous.

The AirCells or Vesicles are small, bladderlike expansions at the ends of the tubes. They are elastic and swell out when the air passes in. The Lungs fill the greater part of the chest, the heart being the only other organ which occupies much space in the cavity. The size of these organs is large or small, according to the capacity of the chest. Each lung for there are two is a kind of cone, with its base resting upon the diaphragm, and its apex behind the collarbone. They axe concave on the bottom, to fit the diaphragm, which is convex on its upper side. The right and left lungs are separated from each other by a partition called the mediastinum, formed by two portions of the pleura, a smooth serous membrane coming off from the spine and closely enveloping each lung; the heart, covered by the pericardium, lies in the centre, between them. The right lung is divided into three lobes; the left into two. Each lobe of the lungs is divided into a great many lobules, which are connected by cellular tissue. These lobules are again divided into very fine aircells. Besides these, the substance of the lungs is composed likewise of bloodvessels and lymphatics, and is well supplied with nerves. In the fcetal state, before the lungs have been filled with air, they are solid and heavy, something like other flesh, but after all their cells have been filled with air, and breathing has been established, they are exceedingly light and spongy, and float upon water. In cases where infanticide is suspected, and where it is desirable to know whether the child was stillborn., or born alive and killed afterwards, the specific gravity of the lungs, compared with water, will often settle the question.