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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

are not confined to changing temperatures, and we can feel extreme heat or cold even if the temperature is constant. But such feelings are not due to the action of the temperature sense-organs. On a hot day one has the moist sensation of perspiration and sensations due to the expansion of the blood-vessels, and the consequent more violent pulsing of blood in the skin, and on a cold day one has sensations caused by the response of shivering. The continual warm tingle of exposure to strong sunlight is a feeble stimulation of the pain-organs by ultra-violet light. None of these things concerns the temperature senseorgans, which are unaffected by constant temperatures.

Similarly with our other senses. In an aeroplane flight we speedily become accustomed to the steady roar of the engine and ordinarily we are unaware of the constant contact of our clothes, although a slight change of pitch in the former case or a slight displacement of the latter is instantly felt. The principle holds even for our eyes; the images of still objects make little impression, but moving objects catch the attention and are more distinctly seen. For this reason when we think we are looking fixedly at an object to see it distinctly our eyes are not really still at all; they quiver rapidly and almost imperceptibly from side to side so that the image may move about on the retina and always be falling on a new part of its sensitive surface. But we have dwelt long enough on the general aspects of sensation.

treatment, and examine the various senses one by one.

Pain, temperature and touch. he skin is able to feel four different kinds of sensationwarmth, cold, pain, and “touch” or deformation of the surface. It can be proved by a simple experiment that these four sensations correspond to four different kinds of sense-organs. If we dab about on the skin with the point of a warm pin, we find that the warmth can only be felt at scattered points; there are warm-sensitive areas with intervening areas where warmth is not perceived. Similarly with a cold pin ; the sense of cold is restricted to definite areas. Now the warm-sensitive areas and the coldsensitive areas do not correspond with each

other. Some points are sensitive to warmth and not to cold, some to cold and not to warmth, some to both, some to neither. Therefore there must be two distinct kinds of sense-organ, one for the warmth sensation and one for the cold sensation. One can proceed similarly with the sense of touch and get a different distribution; and finally the pain sense is not restricted to areas but is present everywhere, so here again the senseorgans must be distinct.

Tt will probably appear to the reader that the list of sensations just given is too short. We have spoken, for example, of a single

Fig. 41. Four types of “ touch” organ, magnified. We must pass to a more detailed 4 is responsive to changes in temperature, B and C to pressure, and D to

pain. Note the relative simplicity of the pain organ.

pain-sense, but there are several different qualities of pain. As a matter of fact, the rich variety of feelings that we get from our skin is due to the fact that a stimulus often affects more than one kind of sense-organ. Slight warming, for example, affects only the warm-sensitive organs, and slight cooling affects only the cold-sensitive organs, but extreme heat or cold may upset both at the same time. Thus it is probable that the distinct ‘‘ feel’? of a very hot bath, as compared with a warm bath, is due to simultaneous stimulation of warmth and cold organs. Generally speaking, we feel not simple sensations, but complexes of sensations.

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