The external surface of the lungs is invested by a 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.

Transverse section of part of the wall of a medium sized bronchial tube.

Fig. 147. 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 unstri-ated muscle, c. Elastic sub-epithelial layer, d. Columnar ciliated epithelium.

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

Section of a portion of Lung Tissue, showing part of a very small bronchus cut across, (f.

Fig. 148. 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.

In the trachea a special muscle exists which can narrow the windpipe 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 vertebrae, 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.

The base of the thorax is connected by a number of strong muscles with the pelvis and spine, whencetions take place in the rate, depth and general type of our respirations without our knowledge. Indeed, if this self-regulating arrangement did not exist, we should have to devote our attention to adapting our respiratory movements to the ever-varying requirements of the gas interchange of the blood.

Drawing of the lateral view of Thorax in the position of gentle inspiration, showing the downward slope ol the ribs.

Fig. 149. Drawing of the lateral view of Thorax in the position of gentle inspiration, showing the downward slope ol the ribs.

Like all other groups of skeletal muscles, those which act on the thorax are regulated by nerves, and work together in harmony. The coordinated movements are so far under the control of the will that any of the groups of muscles may be employed separately, or in conjunction.

But the respiratory differ from the other skeletal muscles, in that they undergo rhythmical coordinated contractions which are not directed by our will, and can be influenced only to a certain extent by it, for they cannot be made to cease altogether.