In birds wide bronchial tubes pass through the lung tissue to reach large air cavities. The walls of the tubes are studded with the openings of innumerable air cells lined with capillary blood vessels. The terminal air cavities are not vascular as in the mammalian lung.

STRUCTURE OF THE LUNG AND AIR PASSAGES.

The respiratory apparatus of mammals consists of (1) vascular sacs filled with air, known as the lung alveoli; (2) channels

by which these sacs are ventilated—the air passages; (3) motor arrangements, which carry on the ventilation of the lungs-the thorax.

1. The lungs are made up of innumerable minute cavities (alveoli), with thin septa springing from the inner surface so as to divide the space into several compartments or air cells. Each of these cavities forms a dilatation on the terminal twig of a branching bronchus, and may be regarded as an elementary lung. The aggregate of these cavities, and the branches of the

air passages and vessels distributed to them make up the structure of the lung.

The walls of the cavities are formed chiefly of fine elastic fibres, and the surface is lined with exceptionally delicate and thin-celled epithelium. Supported in the delicate framework of elastic and connective tissue is the remarkably close-set network of capillaries, in which the blood is exposed to the air. The delicate wall of the vessel and the thin body of the epithelial lining cell are the only structures interposed between the blood and the air. Pleura.-The external surface of the lungs is invested by a

Transverse section of part of the wall of a medium-sized bronchial tube. X 30. (F. E. Schultze.)

a. Fibrous layer containing plates of cartilage, glands, etc. b. Coat composed of unstriated muscle. c. Elastic sub-epithelial layer. d. Columnar ciliated epithelium.

serous membrane, the pleura, which is reflected to the wall of the thorax from the roots of the lungs, and completely lines the cavity in which they lie. Thus the lungs are only attached to the thorax where the air passages and great vessels enter, the rest of their surface being able to move over the inner surface of the thorax, and to retract from the chest wall if air be admitted into the pleural sac.

2. The air passages are kept permanently open during ordinary breathing by the elasticity of their tissues. The trachea and bronchi have special cartilaginous springs for the purpose. These

are closely attached to the fibro-elastic tissues which complete the general foundation of the walls of the tubes. The air passages are throughout lined with ciliated columnar epithelium, which, at the entrance to the infundibula, loses its cilia, and is converted into a single layer of flattened cells.

The air passages are supplied with muscle tissue of different kinds. Besides the ordinary striated muscles that control the opening of the anterior and posterior nares and pharynx, a special set surrounds the upper part of the larynx, and is capable

Section of a portion of Lung Tissue, showing part of a very small bronchus
cut across. (F. E. Schultze.)

a. Fibrous layer containing blood vessels. b. Layer of unstriated muscle. c. Layer of
elastic fibres. d. Ciliated epithelium.

of completely closing the glottis, and thus shutting off the lung cavities, and proper air passages from the outer air. (Fig. 146.)

In the trachea a special muscle exists which can narrow the wind pipe by approximating the extremities of the C-shaped springs that normally preserve its patency.

In the bronchial tubes a large quantity of smooth muscle cells exist, for the most part arranged as a circular coat, which is best developed in the small tubes (Fig. 148, b). As we pass from the large to the smaller bronchi the walls become thinner and less rigid, and the cartilaginous plates and fibrous tissue gradually

diminish, while on the other hand the muscular and elastic elements become relatively more abundant.

3. The thorax, in which the lungs are placed, is a bony framework, the dimensions of which can be altered by the muscles which close in and complete the cavity.

The framework is a rounded blunt cone, composed of a set of bony hoops, the ribs, attached by joints to a bent pliable pillar, the vertebral column, and held together in front by the sternum, to which they are attached by resilient cartilaginous springs. The ribs slope downward and forward, and are more or less twisted on themselves about the middle of the shaft.

The first pair of ribs, which encircles the apex of the thoracic cone, forms part of a short flattened hoop. It slopes downward in front to reach the sternum. Each succeeding rib from above downward increases in the amount of its slope downward and forward, and in the obliquity of its shaft.

The floor of the thorax is formed by a dome-shaped muscle, the diaphragm, which bulges with its convex side into the cavity, and separates the thoracic from the abdominal viscera. The upper outlet is closed around the trachea by several muscles, which pass obliquely upward from the upper part of the thorax to the cervical vertebræ, and hold that part of the chest in position. These muscles can elevate as well as fix the first rib, as will be seen when speaking of the muscles in detail. The intervals between the ribs are filled up by two sets of muscular fibres, which cross one another at right angles, and are attached to the margins of the neighboring ribs.

FIG. 149.

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The base of the thorax is connected by a number of strong muscles with the pelvis and the spine, whence

they pass upward to the lower ribs. The anterior muscles pull down the sternum and anterior part of the ribs. The posterior fix and extend the last rib.

From a mechanical point of view the thorax may be regarded as a specially arranged bellows, the dimensions of which can be increased in all directions.

Within this bellows are the lungs, which may be regarded as an elastic bag, the interior of which communicates with the outer air by an air-pipe, the only way by which the atmosphere can reach the interior of the bellows. When the framework enlarges, the pressure of the atmosphere forces a stream of air into the elastic sac, so as to distend it, and thus fill the space caused by the expansion of the framework.

By the motions of the framework a stream of air passes in or out of the sac; a small quantity of the air in the bronchi is thus changed at each breath, and a certain standard of purity kept

up.

In order to fully understand the motions by which the thorax is enlarged, a more detailed knowledge of the anatomy of the bony case and its muscles than can be given here must be acquired.

RESPIRATORY MOVEMENTS.

Physiologically the motions are divided into two sets—(1) those which enlarge the thoracic cavity, and cause the air to rush into the lungs, called inspiration; and (2), those which diminish the size of the thorax and force out the air, called expiration.

No action of life is more familiar than the rhythmical movements of respiration. The slow, quiet rise and fall of the chest and abdomen are the signs most commonly sought as indicative of life; for every one knows that constant ventilation must go on in order that the blood may readily obtain the necessary amount of oxygen, and get rid of carbonic acid gas, the ordinary diffusion that takes place in the motionless chest being quite insufficient to remove the heavy carbonic acid gas from the lungs.

The rhythm of the respiratory movements may be represented graphically by recording the changes in the diameter or circum