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

BOOK 1

violent or forced breathing, however, other factors are brought into play—additional muscles of the chest and back assist inspiration, and forceful contractions of the abdominal wall assist expiration. In men, the principal factor in quiet breathing is the movements of the diaphragm. In women, on the other hand, the principal part is played by the upper ribs; this is because at certain times the female abdomen has a precious and delicate charge, which might be injured by rhythmical pressure from the diaphragm.

In this way the air in the alveoli of the lungs is constantly renewed. The alveoli are thin-walled structures ; each is bounded by a single layer of flattened cells, extremely thin. Immediately outside this layer and closely pressed against it is the tangled mass of capillary blood-vessels, also very thin-walled, so that in the lungs blood comes into as close a relation with air as it does with hungry tissue-cells in other organs. As in the latter case, the interchange of dissolved substances takes place by a simple physical process of diffusion. The inner surfaces of the alveoli are damp ; oxygen from the air inside them dissolves in this layer of moisture, and from this it can readily diffuse through the thin alveolar and capillary walls into the blood.

Ordinary room air is for various reasons unfit for so great an intimacy as this: it is only an improved and chastened air that is admitted to the alveoli. A very considerable process of purification and preparation has had to occur en route from the nostrils. Air carries, for example, floating bacterial spores—minute organisms in a state of suspended animation waiting for a chance to get into our warm and nourishing blood. ‘The outer surfaces of our bodies are fortified against invaders by a strong, resistant layer of skin, and it is only when this skin is torn that parasites can enter from that quarter. In the lungs things are different, for the delicate alveolar lining could put up no effective resistance against them; they have to be prevented from getting there. It is therefore necessary to filter the inspired air to remove this danger. Moreover, air contains dust, and some kinds of dust (such as flinty particles) can cause serious trouble if they reach the delicate lung membrane. The chief filtering organ, both for bacteria and dust, is the nose.

The cavity of the nose is divided into a maze of passages by shelves of bone, projecting from its walls and twisted into complicated and bizarre shapes. The thin

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THE SCIENCE OF LIFE

GHAPTER 2

layer of living tissue covering these partitions and the lining of the trachea and bronchi throughout are provided with thousands of glands, each producing a continual trickle of sticky mucus, and also with other cells whose duties are to distribute the mucus evenly over the surface and to keep it moving slowly towards the throat. The latter cells illustrate a kind of motion, very different from muscular movement, of which we have said nothing hitherto. Each cell has, rising from its free surface, a number of fine, whiplike projections—cilia, by whose continual lashing it keeps in motion the fluid layer covering it. Air as it enters the nose leaves its coarser particles of dust entangled by the hairs in our nostrils. As it passes among the labyrinthine curlings of the nasal bones, most of the finer particles stick to the layer of mucus as flies do to a fly-paper; finally, in the trachea and bronchi, any particles that have got through the nose are caught in the same way. It will be seen that the nose is the most important part of our filtering system—hence the importance of breathing through the nose.

In fresh country air the amount of dust is small, and practically none of it gets through this protective labyrinth into the lungs. In towns, on the other hand, the air carries soot and various other consequences of human concentration, and the alveoli of towndwellers are usually definitely grubby. In some of the less popular occupationscoal-mining, for example—the air inhaled is so loaded with particles that the normal protections are inadequate, and the alveoli and finer bronchioles become choked. The lungs of an agricultural labourer are pink ; those of a coal-miner are black.

Besides the filtering of dust and bacteria, there are other duties that the nose performs. In the first place, air is cold—imagine an incessant cold draught blowing into a man’s chest and against the walls of his heart ! But as it filters through the warm channels of the nose the chill of cold air is taken off, and it reaches the alveoli at a more comfortable temperature. More important than heat is the question of water-loss. “The alveolar walls are moist and their moisture plays an essential part in the exchange of gases between air and blood. A continuous draught of ordinary room air would rapidly dry up the delicate alveoli by evaporating their water. But the lining of the nasal cavities is also moist, and air is saturated with water-vapour as it passes through the nose ; thus in the alveoli it takes up very little.