Part 30 (2/2)

What is necessary before they can be adapted to the wants of the body?

By what organs are these changes effected? 464-474. _Give the anatomy of the respiratory organs._ 464. Name the respiratory organs. What organs also aid in the respiratory process? 465. Describe the lungs.

[Ill.u.s.tration: Fig. 89. A back view of the heart and lungs. The posterior walls of the chest are removed. 1, 2, 3, The upper, middle, and lower lobes of the right lung. 8, 9, 10, The two lobes of the left lung. 6, 13, The diaphragm. 7, 7, 14, 14, The pleura that lines the ribs. 4, 11, The pleura that lines the mediastine. 5, 12, 12, The portion of the pleura that covers the diaphragm. 15, The trachea, 16, The larynx. 19, 19, The right and left bronchia. 20, The heart. 29, The lower part of the spinal column.]

Explain fig. 89.

466. Each lung is enclosed, and its structure maintained by a serous membrane, called the _pleura_, which invests it as far as the root, and is thence reflected upon the walls of the chest. The lungs, however, are on the outside of the pleura, in the same way as the head is on the outside of a cap doubled upon itself. The reflected pleur in the middle of the thorax form a part.i.tion, which divides the chest into two cavities. This part.i.tion is called the _me-di-as-tinum_.

[Ill.u.s.tration: Fig. 90. The heart and lungs removed from the chest, and the lungs freed from all other attachments. 1, The right auricle of the heart. 2, The superior vena cava. 3, The inferior vena cava. 4, The right ventricle. 5, The pulmonary artery issuing from it. _a_, _a_, The pulmonary artery, (right and left,) entering the lungs. _b_, _b_, Bronchia, or air-tubes, entering the lungs. _v_, _v_, Pulmonary veins, issuing from the lungs. 6, The left auricle. 7, The left ventricle. 8, The aorta. 9, The upper lobe of the left lung. 10, Its lower lobe. 11, The upper lobe of the right lung. 12, The middle lobe. 13, The lower lobe.]

_Observation._ When this membrane that covers the lungs, and also lines the chest, is inflamed, the disease is called ”pleurisy.”

466. By what are the lungs enclosed? What is the relative position of the lungs and pleura? What is said of the reflected pleur? Explain fig. 90. What part of the lungs is affected in pleurisy?

467. The lungs are composed of the ramifications of the bronchial tubes, which terminate in the bronchial cells, (_air-cells_,) lymphatics, and the divisions of the pulmonary artery and veins. All of these are connected by cellular tissue, which const.i.tutes the _pa-renchy-ma_. Each lung is retained in its place by its _root_, which is formed by the pulmonary arteries, pulmonary veins, and bronchial tubes, together with the bronchial vessels and pulmonary nerves.

468. The TRACHEA extends from the larynx, of which it is a continuation, to the third dorsal vertebra, where it divides into two parts, called bronchia. It lies anterior to the spinal column, from which it is separated by the oesophagus.

469. The BRONCHIA proceed from the bifurcation, or division of the trachea, to their corresponding lungs. Upon entering the lungs, they divide into two branches, and each branch divides and subdivides, and ultimately terminates in small sacs, or cells, of various sizes, from the twentieth to the hundredth of an inch in diameter. So numerous are these bronchial or air-cells, that the aggregate extent of their lining membrane in man has been computed to exceed a surface of 20,000 square inches, and Munro states that it is thirty times the surface of the human body.

_Ill.u.s.tration._ The trachea may be compared to the trunk of a tree; the bronchia, to two large branches; the subdivisions of the bronchia, to the branchlets and twigs; the air-cells, to the buds seen on the twigs in the spring.

470. The AIR-VESICLES and small bronchial tubes compose the largest portions of the lungs. These, when once inflated, contain air, under all circ.u.mstances, which renders their specific gravity much less than water; hence the vulgar term, _lights_, for these organs. The trachea and bronchial tubes are lined by mucous membrane. The structure of this membrane is such, that it will bear the presence of pure air without detriment, but not of other substances.

467. Of what are the lungs composed? How retained in place? 468. Where is the trachea situated? 469. Describe the bronchia. What is the aggregate extent of the lining membrane of the air-cells? To what may the trachea and its branches be compared? 470. What is said of the air-cells and bronchial tubes?

[Ill.u.s.tration: Fig. 91. A representation of the larynx, trachea, bronchia, and air-cells. 1, 1, 1, An outline of the right lung. 2, 2, 2, An outline of the left lung. 3, The larynx 4, The trachea. 5, The right bronchial tube. 6, The left bronchial tube. 7, 7, 7, 8, 8, 8, The subdivisions of the right and left bronchial tubes. 9, 9, 9, 9, 9, 9, Air-cells.]

What membrane lines the trachea and its branches? What is peculiar in its structure? What does fig. 91 represent?

_Observation._ The structure of the trachea and lungs may be ill.u.s.trated, by taking these parts of a calf or sheep and inflating the air-vesicles by forcing air into the windpipe with a pipe or quill. The internal structure may then be seen by opening the different parts.

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