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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

broadcast to everybody, but only upon those already disposed to respond to it has it the intended effect.

As a further and very remarkable example of chemical regulation we may take the adrenal bodies—two small yellowish glands, weighing three or four grams each, and lying just above and in front of the kidneys. They are made of two kinds of tissue, having quite distinct functions, the one forming a thick capsule round the other. It is with the central part that we are now concerned. This tissue is used in mobilising the resources of the body to meet a sudden emergency. In a crisis when everything depends on muscular exertion—on escape from an enemy, for example, or on fighting and winning—a nervous message is sent to these two glands, which promptly pour their secretions into the blood-stream. This substance, adrenin, is swept round the body, and as it goes it affects different organs in different ways. It speeds up the heart and dilates the capillaries, it stimulates the sweat-glands so that the body may be cooled, it slows the movements of the digestive organs and contracts their blood-vessels, it makes the liver shed its stored glycogen so that the muscles may have a copious supply of fuel, it stands the hair on end, dilates the pupil and bulges the eye, so that the individual may be terrifying to look upon—in fact, it is a chemical Whip, a broadcast SO S, a tocsin which makes every organ play its part in the general mobilization.

§ 3 Man and Mouse as Individuals

But, as everyone knows, the main unifying organization in the complex activities of Mr. Everyman is his nervous system. So far we have been so busy inside of him, dealing with the details and problems of his continually more astounding mechanism, that we have given little heed to his behaviour as a whole. We have preferred to study how he does it, before we even thought of what he does. Before we have done with The Science of Life we hope to illuminate the activities of this gentleman up to their very highest level, to ask how far his comings and goings, his loves and hostilities, his laughter and tears, are spontaneous and how far they are as determinate as the swallowing action of his oesophagus when it is set going by a bolus of food. But here we simply want to consider behaviour so obvious and primitive —Tunning away from a mad dog, or looking

for drink when thirsty—that it seems almost below Mr. Everyman’s dignity to discuss him. Such behaviour as we are going to study here may be studied equally well in the case of his minor cousin, the mouse.

We have already seen that little creature, hunger-driven, entering its life about the house, while Mr. Everyman sleeps. Its sensitive nose quivers perpetually, and samples and appreciates the surrounding air. Meanwhile, its eyes and ears are

Three vitally important ductless glands, life size from a normal man—the right adrenal (above), from above his kidney, the thyroid (centre), from the front of his throat, and the pituitary (below), from the base of his brain.

fig. 36.

alert for the least intimation of cat, dog, or other enemy. It moves about in swift, eager runs, or sits up to listen. The warning squeak of a kindred mouse will instantly send it flying to cover. It is the smallest of common mammals, a poor fighter, and its strategy is hiding. Whenever it can, it travels in the dark behind the wainscot or under the floor, feeling its way through the obscurity with its extraordinarily sensitive