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

BOOK 1

structure of the nasal cavities, inspired air does not generally blow over the olfactory membranes ; if it did so, it would tend to clog them with its suspended dust, and to dry their moisture. The lower part of the nose is swept by this rhythmic gale, while in the upper regions the air is comparatively still. Normally, stimulating molecules can only reach these peaceful places by diffusion. But, when we are especially interested, a brisk sniff, accompanied by dilation of the nostrils, can upset the normal direction of the air-currents and waft a scent-charged breeze straight to the sensitive cells.

The organs responsible for the sense of taste are the ‘“‘taste-buds,’ egg-shaped clusters of cells distributed over the tongue and soft palate. In the process usually called “tasting” the organs of taste sensu stricto play a part which is as limited as that played by the sense-organs of the skin in

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only four kinds of pure taste sensations: Sweet, sour, bitter, and salt. Any particular taste-bud is specialized to respond to one only of these four tastes. The many and varied _ sensations arising from substances in the mouth are complexes, made up partly of the four elementary taste-sensations, and partly of other kinds of stimulus. The difference between mutton and ham, for example, is smelt, not tasted ; it is due to a diffusion of odorous molecules into the hinder parts of the nose, and most of the subtler discriminations usually attributed to the palate are in reality performed by the olfactory membranes. A serious cold, by clogging the nasal passages with mucus, can completely destroy this part of our sense of taste, and a man who shuts his eyes and holds his nose will be hard put to it to distinguish between a bit of apple and a bit of onion in his mouth. F ees substances may stimulate sensecells o, Supportin eens

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Fig. 43. A taste-bud from the tongue, magnified.

which are neither gustatory nor olfactory ; thus, mustard stimulates the warmth-ends, peppermint stimulates the

cold organs, and some substances produce a delicious tingling that is in fact a very feeble pain.

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THE SCIENCE OF LIFE

CHAPTER 3

Hearing. The sense-organs which have been described thus far perceive stimuli that are known to affect living substance directly. A fresh muscle isolated from the body can be made to twitch by a pinch or wetting with certain chemicals, and in a touch organ or a taste-cell this natural irritability to mechanical or chemical stimuli is exaggerated and taken advantage of. But the sense-organ to which we are now coming is more complicated than this. Sound consists of rhythmical pulsations of the air, and there is no evidence whatever that such pulsations can act directly on living protoplasm. Indeed, we shall find on examining the ear that the stimulus which excites the auditory cells is not a sound-stimulus but a touch-stimulus, for the ear is an elaborate machine so constructed that whenever sound-pulsations fall upon it, sensitive cells are touched. Herein lie the wonder and ingenuity of the ear; it is a definite extension of the faculties of living matter. A lowly organized creature, such as an earthworm or a polyp, can taste or feel, but it lacks the structures that convert soundwaves into stimuli capable of exciting protoplasm directly, and is therefore ignorant of the rich and varied world of sensations that sounds can evoke. An examination of the anatomy of the ear will make the point plain.

The organ of hearing consists of three parts—the outer visible ear, the middle ear with its ear-drum, and the inner ear. The inner ear is the true sense-organ, for it is here that the vibrations cause the excitation of sensory cells and the initiation of nervous impulses. The outer and middle ears are structures which collect sound vibrations and transmit them to the inner ear, making it possible for that elaborate and fragile device to lie safely embedded in the bone of the skull. Most mammals are provided with a natural ear-trumpet in the external ear, a hollow cone which can be turned about to face the direction from which sounds are coming, and which therefore makes feeble sounds more distinctly audible. From this ear-trumpet a short tube leads to the ear-drum, the boundary separating the outer and middle ears. In man this amplifying apparatus is poorly developed ; the external ear is a mere flap of no acoustic importance, and the power of moving it, except in a few gifted individuals, is absent. For this reason, although a man can distinguish sounds varying over a wide range of pitch, he is less able to discern feeble sounds than a dog or a horse,