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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

sensory message to the brain, it means that something is seriously wrong and that the government is expected to do something about it.

The whole subject of sensation in the viscera is still obscure. We know very little of the elements into which visceral sensations can be analysed, or of the sense-organs that are responsible for them. It is clear that the arrangements are very different from those in the skin, and the differences seem to depend on the relative independence of the viscera to which we have just alluded. Surgeons find, for example, that the intestine can be cut or sewn without giving rise to any painful sensation whatever, but a strong contraction of its muscular wall or excessive distension of its cavity—the sort of thing that happens when its working is disturbedmay give rise to intense gripping pains.

There is, however, a possibility that the senseimpulses received from the viscera include messages other than these cries of distress. We saw that motor impulses to the voluntary muscles are not confined to the moments when contraction is desired ; they include continual encouragements, and unceasing influence that keeps the muscle fit and ready. In the same sort of way it may be that the government receives continual intimations from the chest and belly, that all goes well with the kidneys, or that there is a slight unrest in the stomach that may presage more serious trouble, and so on. In addition to our accurately-analysable sensations there are vague feelings that are at present imperfectly understood—feelings of vigour, appetite, discomfort, restlessness, weariness, oppression, and so on, that lead up to, and are evidently related to, emotional states. It may be that such feelings are to a large extent due to impulses from our viscera. It is a familiar fact that the condition of the viscera has a profound effect on the colour, so to speak, of the mind. When the stomach is empty, for example, the organism becomes restless before it feels definite hunger, and when the stomach is filled it becomes indolent and sleepy. On the other hand it may be that these feelings are not dependent upon messages received along sensory nerves, but upon changes in the chemical state of the nerve-cells, due to alterations of the way in which circulating

Fig. 42.

Two smell-cells and a supporting cell are seen.

blood is distributed between belly and brain or of the composition of the blood itself. We have at present no precise information about these vague sensations, and therefore we cannot profitably discuss them further ; we must simply note their existence and leave it at that.

Smell and Taste. Smell and taste are the chemical senses—that is to say, the stimulating agents in each case are not physical influences, such as temperature, pressure, or light, but chemical substances which act because of their molecular structure. In man, the chemical senses, and particularly smell, are poorly developed, being overshadowed by sight and by hearing, but in the lower mammals the sense of smell is very much more acute. In a dog, for example, the nose is probably quite as important a sense-organ as the eye or the ear.

Three cells from the sensitive membrane of the nose, highly magnified.

Surface is to left.

A man recognizes his fellows by their appearance, and by the noises they make, and the store-rooms of his memory are largely filled with labelled records of sights and sounds ; the memory of a dog, on the other hand, is more probably a sort of card catalogue of smells.

The mechanisms of smell and taste are fundamentally alike. In both cases the essential elements are cells, sensitive to particular substances, which dispatch impulses to the brain when those substances are present. In both cases, the stimulating substance has to be dissolved in water before it is effective. Thus, although even a weak tincture of quinine has a powerful taste, dry quinine powder, which dissolves slowly and sparingly in saliva, has hardly any taste at all; and similarly odorous substances have to dissolve in the moisture covering the nasal mucous membrane before they evoke any sensation of smell. _

The nostrils lead into two triangular cavities which are broken up and made labyrinthine by the scroll-like turbinal bones. The special cells that perceive odorous substances are found at the upper end of these cavities, and on the thin partition that divides the right nose cavity from the left. Owing to the complicated

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