The Lungs consist of the bronchial tubes and their terminal dilatations, together with blood-vessels, lymphatics, and nerves, all bound firmly together by elastic tissue. The expansions called " air-cells" * at the end of each final branch of a bronchial tube (Fig. 66) are relatively very large, and their surface is still further increased by the pouches (a, b) which project from them. Their walls are highly elastic, and contain a close network of capillary blood-vessels, supplied by the pulmonary artery and emptying into the pulmonary veins. Through the extremely thin lining of the air-cell, and the thin wall of the capillaries imbedded in it, oxygen is absorbed by the blood from the air in the air-cell and carbon dioxide given up to it. It has been calculated that if the walls of the air-cells were all spread out flat and placed side by side they would cover an area of 2600 square feet. This great surface, therefore, represents the area of the body by which oxygen is received and carbon dioxide given off.

What is bronchitis? How does an attack of bronchitis interfere with breathing?

Of what do the lungs consist? What are the air-cells? How is their surface increased? What do their walls contain ? By what are their capillaries filled, and into what do they empty? What interchange between blood and air occurs in the air-cells of the lungs?

* Cell is here used in its primitive sense of a small chamber or cavity (Latin, cellula), and not in its modern histological signification, as one of the distinct pieces of living matter recognized by the microscope as serving to build up the body by their accumulation and co operation.

The Pleura

The exterior of each lung, except where its bronchus and blood-vessels enter it, is covered by a thin elastic serous membrane, the pleura (Fig. 2). This membrane also lines the inside of the chest. Its surface in health is kept moistened by a small quantity of lymph. In consequence of its smoothness and moisture, during the breathing movements the chest wall and lung glide over each other with hardly any friction.

Pleurisy is inflammation of the pleura. In its early stages it is usually associated with sharp pain on drawing the breath. Later on a large quantity of lymph is often poured out by the inflamed pleura, filling up the cavity which should be occupied by the lung, and pressing the latter up into a small mass, very inefficient for breathing purposes.

The Elasticity Of The Lungs

The lungs are so elastic as to be like a thin india-rubber bag. If we tie a tube tightly into a bronchus and blow in air the lung will dilate, but as soon as we cease blowing and leave the tube open, it will shrink up again. Yet in the chest the lungs always remain so expanded as to completely fill up all the space left for them by the heart and other things contained in the thorax. How is this ?

How large is the surface of the body set apart for oxygen reception and carbon dioxide elimination?

What is the pleura? What does it cover besides the outer surface of each lung? What is its condition in health? What is its use?

What is pleurisy? What symptom usually accompanies its early stages? What happens later?

What do the lungs resemble in their elasticity? How may this elasticity be demonstrated? What is the natural condition of the lungs in the chest?