the roof of the cavity, are distributed; and the extent of its surface is increased, by its being folded over certain projections from the walls of the cavity, which are termed spongy bones. Of these there are three in Man (g, i, k). Prolongations of this membrane are carried also into cavities hollowed out in the neighbouring bones, which are termed sinuses. Thus we have the frontal sinuses l, situated just above the nose, between the eyebrows; and the sphenoidal sinuses m, situated further back. There is also a large cavity hollowed out in the bone of the upper jaw on either side. The membrane which lines these is kept moist by its own secretion; and it is covered with vibratile cilia, the office of which seems to be to prevent that secretion from unduly accumulating, by conveying it over the surface of the membrane to the outlet.

507. The mechanism of the sense of Smell is very simple. When air charged with odoriferous particles passes over the membrane that lines the nose, some of these particles are delayed by the mucous secretion that covers it, and act upon the delicate sensory extremities of the olfactory nerve with which it is thickly set. The highest part of the nasal cavity appears to be that in which there is the most acute sensibility to odours; and hence it is that when we snuff the air, so as to direct it into the upper part of the nose, instead of allowing it to pass simply along the lower portion from the anterior to the posterior nares, we perceive delicate odours which would have otherwise escaped us. The acuteness of the sense of smell depends, in no small degree, upon the extent of surface exposed by the membrane lining the nasal cavity; and in this respect Man is far surpassed by many of the lower Mammalia, especially among the Carnivora, Ruminantia, and some Pachydermata. The extreme delicacy of this sense in Deer, Antelopes, &c., is well known, from its being a source of great difficulty to the hunter, who cannot advance near enough to attack them, except by stealing upon them in the direction contrary to that of the wind. In these animals it serves as the chief means by which they are warned of the proximity of their enemies; in the Carnivora, on the other hand, it serves to direct them to their prey. In general, however, it seems to have for its office to assist in directing animals to their food, and in warning them of the presence of noxious vapours.

508. Besides receiving the Olfactory nerve, the mucous membrane of the nose is supplied by branches of the Fifth pair; this nerve endows it with common sensibility, and also receives the impressions produced by acrid or pungent vapours, which act upon it in the same way as the corresponding fluids do upon the tongue. Such vapours are felt by the irritation they produce, rather than smelt; and the impression they occasion gives rise to the reflex action of sneezing, by which they are driven from the nose (§ 342). Hence this action may be excited by an irritating agent (such as snuff) after the olfactory nerve has been divided, if the branches of the fifth pair be entire; whilst it does not take place when the fifth pair is paralysed, even though the sense of smell may be retained. This sense loses much of its acuteness, however, when the branches of the fifth pair supplying its organ can no longer discharge their functions; for the membrane then becomes dry from the want of its proper secretion, and the odoriferous particles are consequently not properly applied to it.

509. Among animals that live in water, the olfactory organs cannot act to the like advantage; and we do not find much provision made for this sense. In the Whale tribe, the nostrils serve as the channels by which the water is expelled, that has been drawn-in through the mouth (§ 185); they are situated at the top of the head, and are known as blow-holes. In Fishes, the nasal cavity has no posterior opening; but the surface of its lining membrane is very much extended by its arrangement in folds, which are sometimes disposed in a radiated manner around a centre, and sometimes parallel like the teeth of a comb. There are many Invertebrated Animals, from whose actions it may be judged that they possess a delicate sense of smell, although the precise seat of it cannot be assigned. This is the case especially with Insects, Crustacea, and the higher Mollusca. The lining membrane of the airtubes of Insects appears to be delicately sensitive to irritating vapours (§ 443); but we have no evidence that it ministers to the sense of Smell properly so called.

Sense of Hearing.

510. By this sense we become acquainted with the Sounds produced by bodies in a certain state of vibration. The vibrations which sonorous bodies undergo, are communicated by

D D

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TRANSMISSION OF SONOROUS VIBRATIONS.

them to the air, producing in it a series of undulations or waves, by which the sound is conveyed to a great distance. These undulations spread more widely as they become more distant from the sounding body, just like the ripples produced on the surface of the water when we throw a stone into it; and in proportion as they spread, they become less powerful. This is the reason why Sound becomes less intense as the sounding body is more distant. Although air is the usual conducting medium for the sonorous undulations, liquids or solids may answer the same purpose. Thus if a person hold his head under water, whilst two stones are struck together, also under water, even at a considerable distance, he will hear the sound produced by the blow with extreme distinctness, and even with painful force. Or if the ear be laid against one end of a long piece of timber, whilst a scratch with a pin be made on the other, or a watch be laid upon it, even the faint sounds thus produced will be heard very distinctly. That a medium of some kind is necessary to convey the sonorous vibrations, is proved by the fact, that if a bell be made to ring in the receiver of an air-pump from which the air has been exhausted, no sound is heard, though its ringing becomes audible as soon as the air is allowed to re-enter.

511. It is a fact of much importance, in regard to the action of the organ of Hearing, that sonorous vibrations which have been excited and are being transmitted in a medium of one kind, are not imparted with the same readiness to others. The following conclusions have been drawn from experimental inquiries on this subject. I. Vibrations excited in solid bodies may be transmitted to water without much loss of their intensity, although not with the same readiness that they would be communicated to another solid. II. On the other hand, vibrations excited in water lose something of their intensity in being propagated to solids; but they are returned, as it were, by these solids to the liquid, so that the sound is more loudly heard in the neighbourhood of those bodies, than it would otherwise have been. III. The sonorous, vibrations of solid bodies are much more weakened by transmission to air; and those of air make but little, impression on solids. IV. Lastly, sonorous vibrations in water are transmitted but feebly to air; and those which are taking place in air are with difficulty communicated to water; but the com

TRANSMISSION OF SONOROUS VIBRATIONS.

403

munication is rendered much more easy by the intervention of a membrane extended between them.

512. The Auditory nerve, or nerve of Hearing, is adapted to receive and transmit to the brain the sonorous undulations produced in the surrounding medium by vibrating bodies. Now, it is obvious that it may be affected by these in various ways, especially in animals that inhabit the water. The vibrations excited in the liquid will be transmitted to the solid parts of the head, and thence to the nerve contained in it, without much interruption; and this independently of any special apparatus of hearing. Indeed, the simplest form of this apparatus is only designed to give increased effect to the vibrations thus excited in the solid parts of the head; for it consists merely of a cavity excavated in their thickness, which cavity is filled with fluid, and is lined by a membrane whereon the auditory nerve is minutely distributed. This is the condition of the organ of hearing in the Mollusca, where any such exists; and also in many of the Crustacea. In those of the latter class which chiefly inhabit air, however, this cavity is excavated in the surface of the shell covering the head, and is shut-in by a membrane which is exposed to the surrounding medium. According to the principle (IV.) mentioned in the last paragraph, the liquid contained in the chamber will be thrown into undulation by vibrations in air, as well as by those of water; so that those animals which possess this kind of apparatus are able to hear much better in air, than are those in which the cavity is completely shut-in by stony walls. Of the degree in which sonorous vibrations may be communicated to our own auditory nerves through the solid parts of the skull, we may easily satisfy ourselves by closing the ears carefully, and placing any part of the head against a solid body which communicates with the one in vibration. In this manner we may hear the sounds produced by the latter with considerable distinctness, though accompanied by an unpleasant jarring. A deaf gentleman was once agreeably surprised to find that, when smoking his pipe, with the bowl resting on his daughter's pianoforte, he could distinctly hear the music she was producing from it; and many deaf persons may be made to hear conversation, by holding a piece of stick between their own teeth, and placing it against the teeth of the person speaking.

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STRUCTURE OF ORGAN OF HEARING,

513. In animals which have the organ of hearing constructed upon the simple plan just described, the force of the vibrations of the fluid contained in the cavity is increased by several minute stony concretions suspended in it: these act according to the second principle just stated (§ 511). They are termed otolithes, or ear-stones; and some traces of them may be found even in Man and the higher animals.1

514. We see, then, that a cavity excavated in the solid walls of the head, covered-in externally by a membrane, having the auditory nerve distributed upon its walls, and filled with fluid, is the simplest form of the organ of hearing; and may be regarded as including all that is essential to the exercise of this function. No more complicated apparatus is to be found in any of the Invertebrata; and even in the lowest Fishes there is but little variation from this type. On the other hand, in Man and the higher Vertebrata we find a very complex structure, adapted to render the faculty much more perfect; enabling us to receive impressions which make us. aware, not only of the presence of a sounding body, but of its nature, its direction, the pitch and peculiar quality of the sound; and also, it is probable, taking cognizance of sounds much fainter than those which would be perceptible to the lower animals. Yet even in the most complicated forms of the organ of hearing, we shall find that the essential part is still the same as that which forms the whole organ in the lower tribes; and also that the faculty is retained, though in an inferior degree, when by disease or injury the accessory parts are prevented from acting.-To the structure of the Ear of Man we shall now proceed.

515. The organ of hearing in Man may be divided into three parts-the external, the middle, and the internal ear. The former is the fibro-cartilaginous appendage placed on the outside of the head, to receive and collect the sounds which are to be transmitted to the interior; the two latter divisions are excavated in a bone of remarkable solidity, the petrous (stony) portion of the temporal bone. The uses of the different

1 Vesicles containing otoliths which are kept in rapid movement within them by ciliary action, are found in immediate contiguity with the cephalic ganglia of the lower Mollusks, or are even imbedded in their substance; and these seem to constitute the most rudimentary form of an organ of hearing.