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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

effect is due simply to disuse, for the size of a muscle depends on the frequency and vigour with which it is used ; the point is not clearly established. But, however this may be, it is evident that a muscle depends completely on the central nervous system both for the impulses that control it and for its general welfare, and that the latter organ is to be regarded not as a wilful and despotic government but as a vigilant, untiring leader.

Sensation and the Senses

If we could see the whole living web of the nervous system laid out before us, and if a nervous impulse was a visible thing, we should get a picture of continual thrilling and rippling activity. We should see a ceaseless succession of impulses flashing to the muscles, keeping them taut and ready for work, and spurring single muscles or groups of muscles to vigorous, disciplined activity. But also we should see an equally incessant series of impulses travelling in the opposite direction. Before we come to the central working of this marvellously complicated and accurate government we must consider those other impulses which bring information unceasingly from all the quarters of its realm.

For the web—unlike the telephone web of a city—is a double one. Any nerve contains fibres of two kinds, motor fibres that carry impulses outwards and sensory fibres that bring impulses inwards. These two kinds of fibre are bound up together to form nerves, their structure is similar and the physical nature of their impulses is similar, but they differ in the direction in which the impulses travel, and in the origins, destinations, and functional meanings of these impulses.

The sense-organs, the intelligence agents, in which the sensory nerve-fibres end, are of many and various kinds. ‘There are senseorgans scattered about in the muscles, in the joints, and indeed in every tissue of our bodies, reporting the state of affairs there. So the central government is informed of the positions and movements of our limbs, and sometimes, when there is discontent in the thorax or abdomen, it feels hunger or indigestion. These sense-organs, which watch and report on the other tissues, may be called internal sense-organs.

Contrasting with the internal sense-organs there is a second sort of sense-organ which takes note of circumstances external to the body state. This second group, the external sense-organs, may be further divided into two: there are some, such as the organs of feeling and taste, that report on objects and events in the immediate neighbourhood of our bodies, and there are others, the organs of sight, hearing, and smell, by means of which the central nervous system is informed of more distant events. The importance of this distinction is clear enough, for the contact organs can only detect things which are actually happening to the body, while the distance organs can detect a danger before it is actually upon the organism, or a remote attraction that can be moved towards and secured. They enable

Sensitive aaa Belt Hair

x Nerve Fig. 40. How our hairs feel (see text).

the government to foresee and prepare. For this reason, as we shall find, the distance organs are directly connected to the upper more independent and personal braincentres. Let us consider the structure and working of a very simple contact sense-organ, a hair. Our hairs feel. Ifa match is pressed against one of the fine hairs on the back of the hand we feel a tickling, and that is one of the most sensitive forms of touch. The hairs of a mouse are far more important than our scanty supply is to us, and the mouse, like the cat, has large and special whisker-hairs for guidance in the dark. A hair is a more or less cylindrical rod of horny dead cells ; it arises from and it grows by a supply of fresh cells from a ‘‘ root”’ lying fairly deep in the skin; it runs to the surface along a special tube, the ‘ hair-follicle,” and then