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

BOOK 1

In its distended tissue-spaces it holds about a litre of blood—about one-fifth of the total volume in our bodies. During exercise, on the other hand, the muscle-fibres are permanently contracted, and its capacity decreases to only about one-third of the resting value. The reserve blood in the organ is therefore driven out into the rest of the circulation, and in this way the bloodsupply to the muscles is increased.

The spleen, then, is essentially a bloodreservoir, and during moderately severe exercise the contribution which it makes to the muscular blood-supply is sufficient. During very severe exercise, when every resource of the organism is called upon, the blood in the spleen may not be enough, and in this case the process is carried a step farther ; blood is taken from other organs, such as the digestive organs, and sent to the toiling muscles, the activities of the former being temporarily suspended. In a crisis, when a man or an animal has to struggle for his life, the whole circulation is concentrated into a blood-flow through his muscles. The portal veins, for example, are the capacious vessels which carry blood from the intestine to the liver, where absorbed food has to undergo various chemical adjustments before it is fit for general distribution. In an emergency, when the maximum possible muscular work is required, these veins contract; their normal function is suspended, that circuit is all but closed, and the blood-flow thus economized goes to swell the muscle supply. ‘The kidneys, again, hold a considerable volume of blood, and during severe exercise they also contract and, like the portal veins, restrict the demand. In the greatest muscular exertion of which a healthy man is capable, this latter process may be carried so far that the kidney-cells are actually injured from bloodloss—the kidney-cells have been sacrificed so that the whole organism can survive the crisis.

This is simply another reflection of the fact which has already been stressed—that food-supply and the excretion of wasteproducts are not processes of immediate urgency, for they can be temporarily suspended without serious loss to the organism. The respiratory function, on the other hand, is of immediate and critical importance ; when necessity demands, other functions are neglected and all the resources of the circulation are concentrated on this one vital need.

It is clear, then, that the thoracic and abdominal organs are controlled and regulated to suit the ever-varying needs of the

66

THE SCIENCE OF LIFE

CHAPTER 3

organism ; moreover, we have seen that this control can be exerted in either of two ways—by means of nervous influences, or by means of chemical substances in the blood. As everyone has heard, a nerve is a sort of protoplasmic telephone wire, so that the nerves to the various viscera constitute a telephone system by means of which the brain can communicate with them and modify their activities.

We may, however, look a little farther into chemical regulation before we leave it altogether and turn our attention to the nervous system.

§ 2 Chemical Messengers

Chemical regulation, as we have seen it operating through the carbon dioxide in the blood, plays a very large part in securing the harmonious co-operation of our viscera. Another example is afforded by the pancreas. This organ does not continually pour its digestive juice into the duodenum ; it only produces it when there is food to digest in that part of the alimentary canal. The mechanism by which the pancreas is suddenly activated at that particular moment is as follows: There are cells in the duodenum whose duty it is to secrete a particular substance, secretin—not into the digestive tube, but into the blood. This they do whenever food passes from the stomach into the duodenum. ‘The secretin is distributed by the blood all over the body, and in most places it has no effect at all; but when it reaches the pancreas it stimulates that organ and makes it produce its digestive juice.

The process of secreting particular substances into the blood is called internal secretion ; and the various organs which do this are called ductless glands, or endocrines, from the Greek for internal secretion. Many of the ductless (endocrine) glands produce their secretions continuously—we shall learn more about them when we are considering development and growth—but a few, like the ductless gland cells in the duodenum, do so only at particular moments, and are used for regulating other organs. Regulation by means of nerves is comparable to telephonic communication, because particular parts of the body are connected by definite nerve-fibres, and thus the nervous messages are sent only to the particular organs they concern. Regulation by means of an internal secretion is more like stamping ““ Buy British Goods ”’ on all the letters that go through the British post. The appeal is