The science of life : fully illustrated in tone and line and including many diagrams

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

fine channels (the bronchioles) there open little clusters of hemispherical bays, the alveoli, and it is from these alveoli that air is taken up by the blood. In the narrow spaces between the densely-packed alveoli run the capillaries, the arterioles (little arteries), and the venules (little veins) ; it is of the alveoli, the blood-vessels, a little packing tissue, and the orderly labyrinth of ducts which bring air, that the substance of the lung is composed.

The thorax (the cavity of the chest) is a completely enclosed space, with the backbone behind it, the breast-bone in front and the sloping ribs on either side. All these parts are movable upon one another, and by their movements the air is brought to and taken from the alveoli. The movements are of two kinds—movements of the ribs and movements of the diaphragm. The ribs, as has already been said, run round the sides of the thorax from the vertebral column behind to the breast-bone in front ; in between the ribs, connecting each to its neighbours, there is a sheet of vertically disposed muscle-fibres, the intercostal muscles. The action of the ribs depends on two purely structural features. The first is that as the ribs run forward they also run downwards; the attachment of any rib to the breastbone is lower than its attachment to the backbone. The second is that the ribs are curved, lying as they do in the curved walls of the chest, and that when the lungs are empty the convexity of this curve points slightly downwards. During inspiration the actual movement of the chest wall is produced by contraction of these intercostal muscles, which tends to pull the ribs closer together. The uppermost rib, solidly attached to the structures in the neck and shoulder, acts as a fixed point, the other ribs being pulled up towards it. As a result of the anatomical features just noted, this pull has two effects—firstly, because of the downward slope of the ribs, it pushes forwards the lower part of the breast-bone and so increases the front-to-back depth of the chest ; secondly, because of the curvature of the ribs, it pulls their middle parts upwards and outwards, so increasing the breadth of the chest from right to left. These two factors combine in increasing the volume of the thorax, and are assisted in so doing by a simultaneous movement of the diaphragm.

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Fig. 20.

The diaphragm is concave downwards, attached round its edges to the ribs, backbone, and breast-bone, but free in the middle. Its central part is of tendon, but its marginal part consists of radiating muscle-fibres. When these muscles contract the domed centre is pulled downwards ; the diaphragm flattens, pressing on the liver, which lies immediately beneath it, and on the other abdominal organs and at the same time it helps to increase the volume of the thoracic cavity. Let us note in passing that contraction of the diaphragm, since it presses on the organs of the abdomen, must be accompanied by a slight bulging forward of the abdominal wall.

Now the thoracic cavity is itself air-tight, but there lie inside it these two elastic bags,

Windpipe : |

Arteries =I

The heart and lungs, as they lie in the chest.

the lungs, which communicate freely through the trachea with the outer air. Since the volume of the space between chest-wall and lungs cannot change, it follows that any increase of total volume produced by these simultaneous movements of the ribs and diaphragm must involve a sucking of air into the lungs themselves. This is how the lungs are filled. The opposite phase of the breathing rhythm—expiration—is produced chiefly by the elasticity of the lungs and the walls of the chest and abdomen; when the various muscles relax there is a natural recoil which drives air out of the lungs. me During ordinary, unforced breathing inspiration is produced by the muscles moving the ribs and diaphragm, and expiration by the elastic recoil of the parts. During

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