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

BOOK 1

To return to pain, the sharp sting of a cut is pure pain, but the burning pain of an inflammation is due to simultaneous stimulation of pain and warmth organs, and in a throbbing pain the reactions of the senseorgans are complicated by rhythmical pressure-changes due to the arterial pulse.

In particular, sensations of touch are nearly always due to a co-operation of different sense-organs. Occasionally the touch-organs of the skin come into play by themselves—a very light pressure, for example, may move the hairs, and so give rise to a pure touch sensation. But usually what we call “ touch” involves sensations of pressure and movement in the deeper structures.

Our muscles, tendons and joints are richly provided with sense-organs, chiefly sensitive to pressure, which supply the central nervous system with information about the position and movements of the limbs. These sense-organs also play an important part in feeling the shapes and textures of objects. Suppose, for example, that the finger is drawn along the table, the eyes being closed. The impression of a hard, flat surface depends chiefly on senseimpulses from the deeper structures; we know that it is hard, because we can press on it vigorously without deforming it, and we know that it is flat because in order to draw the finger along it, we have to move our muscles in a certain way. The sense of pressing on the table is due to organs lying in the deeper connective tissue in the finger, and that of movement to senseorgans in the muscles concerned—which are placed, not in the finger at all, but in the arm and shoulder. In this way, even the simplest impressions of touch are made up in reality of sensations from a number of different kinds of sense-organ, which are fitted together by the central nervous system, and interpreted as a single feeling.

In structure the organs responsible for sensation in the skin and muscles are comparatively simple. We have already noted that the touch-organs in the hair-follicles consist merely of the branching endings of nerve-fibres. The other organs are generally more complicated than this—they involve not only branching nerve-fibres, but special cells—but they do not compare in intricacy of design with the sense-organs that we shall study in subsequent sections. A great number of different kinds of sensory structures have been found in the skin and deeper structures, but unhappily very little is known at present about the way they work.

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THE SCIENCE OF LIFE

CHAPTER 3

For this reason we need not describe them here. :

We may, however, take note of a kind of sense-organ that is even more elementary in design than the touch-organs in the hairfollicles. Everywhere in the skin there is a network made by the branching ends of nerve-fibres, running among the cells without any evident order, and without any specialized accessory structures. When these fibres are stimulated they give rise to sensations of pain. The pain sense is of more importance to the organism than any other of the skin sensations, for while a message about the temperature of the air or about the hardness and shape of a neighbouring object may or may not be interesting to the central nervous system, a stab of pain always means immediate danger. The pain organs are very much less sensitive than their more elaborate neighbours, but, on the other hand, they are less specialized and can be excited by any stimulus, whatever its nature, if it is of sufficient intensity to injure living tissue. Slight warmth, for example, excites only the special warmth cells, but a temperature high enough to burn or scald is brilliantly painful. Pain is an alarm signal, and its biological importance is reflected in the anatomical fact that whereas warmth and cold and touch are senses confined to particular patches of skin, the pain network is unrestricted and pervades the whole surface of the body.

Internal Sensation. While we are on the subject of these scattered and rather mysterious sense-organs, we may note the feelings that arise from time to time in our thoracic and abdominal viscera. Normally, we are unaware of these independently working parts. There is a close correspondence between the parts of the body over which the sway of the central nervous system extends, and the parts from which it receives precise information. The mouth, for example, is controlled by the central government—chewing and swallowing are the results of impulses from the brain—but the oesophagus is an independent structure, driving mouthfuls down to the stomach by means of its automatic movements. Corresponding with this, we find that the mouth is very sensitive to the temperature, texture and chemical composition of our food, but that as soon as a mouthful is swallowed it ceases to stimulate sense-organs; it is handed over to the autonomous factories and passes at the same time out of our control and out of our ken. Generally speaking, whenever an abdominal organ sends a vivid