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

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

Everyman, we will say a little more about certain of its defensive activities. Apart from the graver dangers of fire and flood, war and traffic, it is continually being assailed by more insidious and deadly invaders.

In our account of the blood we described the police-activity of the white corpuscles (leucocytes or phagocytes—‘ white cells’ and “consuming cells”? are alternative names for them) which fight and devour those bacteria that are swallowed in the food or otherwise reach the blood-stream. At times this police work attains dimensions that make it comparable to warfare.

most layer has a winding appearance which suggests the Great Wall of China. The parallel is a good one; the skin guards the body from the Mongol hordes of bacteria. But if the Great Wall had been built of the mummified bodies of Chinamen the parallel would be closer.

Sometimes, through a cut or a scratch, a breach is made in the wall, and the bacteria lurking in the air or on our skins make an entry, whereupon there follows a battle royal in our tissues. The first response to any injury to the epidermis is a dilatation

The main frontier of the cell-empire

that we know as Mr. Everyman is

the skin, and we may devote a little

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attention to the organization of this =

first line of defence.

Fig. 34 presents a section through a small part of the skin and the underlying connective tissue. At the upper surface separating the tissues beneath from the outside air there is a layer of special cells, the epidermis. The deeper cells of this layer are constantly growing and dividing ; because of this continual multiplication they spread outwards towards the surface. But as they spread they become cut off from their blood-supply, for the capillaries do not extend into the epidermis. Only the deepest epidermal cells are properly nourished ; the others are all in various stages of starvation. The most superficial cells, being remotest from the blood-stream, actually die, the outermost layer being composed of dead cells.

Now these dead cells are dry and horny ; they form a tough rampart

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=z Se > SSS ag <= SS SSS S

oo Tess Se

Se Pe. Actively Multiplying Cell

Touch Sense Organ

between the outer world and the 7g. 34. A vertical section of skin, as seen through the

living tissue. These cell-corpses are ordinarily brushed or washed away as they die; but when they are allowed to accumulate, as under a bandage which is not undone for many days, they form dense white masses of flaky scurf or scarf-skin, showing to what an extent our bodies die daily. At the same time, by the active division of the deeper layers, new cells are produced to starve and die so that the rampart is renewed from within. The epidermal cells are the martyr volunteers of the cell community. It is their sole business to multiply and perish so that their corpses may protect the state.

In Fig. 35 the hard impervious outer-

microscope.

of the capillary vessels in that region. Chemical products of the injured stuff stimulate them to dilate. By thus increasing the local blood-flow reinforcements of white corpuscles are at once rushed to the affected spot.

If the process be watched in a transparent tissue, such as the web of a frog’s foot, the movements of these skirmishers can be traced. At the region of injury these defenders can be seen to leave the capillaries. They elbow their way, so to speak, between the living cells of the capillary wall and reach the intercellular spaces. Here they fall