nerve, through which the sense of Taste, as well as that of Touch, is exercised, in the parts of the tongue to which it is specially distributed, -which are those that possess both senses in the most acute degree; and that the Glosso-pharyngeal is subservient to the same functions in the parts supplied by it, being probably the exclusive channel, also, through which the impressions made by disagreeable substances taken into the mouth, are propagated to the Medulla Oblongata, so as to produce nausea and excite efforts to vomit. The latter nerve is also, as we have seen, the principal channel of the impressions that give rise to the reflex act of swallowing; with which the fifth pair is concerned in a much inferior degree (§ 897).

945. A considerable part of the impression produced by many substances taken into the mouth, is received through the sense of Smell, rather than through that of Taste. Of this, any one may easily satisfy himself, by closing the nostrils, and breathing through the mouth only, whilst holding in his mouth, or even rubbing between his tongue and his palate, some aromatic substance; its taste is then scarcely recognised, although it is immediately perceived when the nasal passages are reopened, and its effluvia are drawn into them. There are many substances, however, which have no aromatic or volatile character; and whose taste, though not in the least dependent upon the action of the nose, is nevertheless of a powerful character. Some of these produce, by irritating the mucous membrane, a sense of pungency, allied to that which the same substances (mustard, for instance) will produce, when applied to the skin for a sufficient length of time, especially if the Epidermis have been removed. Such sensations, therefore, are evidently of the same kind with those of Touch, differing from them only in the degree of sensibility of the organ through which they are received. But there are others which produce sensations entirely different from any that can be received through the skin, and which are properly distinguished, therefore, as gustative; such are common Salt, which may be considered as a type of the saline taste, Sugar, the type of the saccharine, Quinine of the bitter, and Tannin of the astringent, and Citric acid of the sour. All such substances, therefore, are said to possess sapid properties, exciting distinctive tastes, quite irrespectively of any aromatic or odoriferous properties which they may also possess, as well as of their stimulating action on the skin.

4. Of the Sense of Smell.

946. Certain bodies possess the property of exciting sensations of a peculiar nature, which cannot be perceived by the organs of taste or touch, but which seem to depend upon the diffusion of the particles of the substance through the surrounding air, in a state of extreme minuteness. As the solubility of a substance in liquid seems a necessary condition of its exciting the sense of Taste, so does its volatility, or tendency to a vaporous state, appear requisite for its having Odorous properties. Most volatile substances are more or less odorous: whilst those which do not readily transform themselves into vapor, usually possess little or no fragrance in the liquid or solid state, but acquire strong odorous

properties, as soon as they are converted into vapor,-by the aid of heat for example. There are some solid substances, which possess very strong odorous properties, without losing weight in any appreciable degree by the diffusion of their particles through the air. This is the case, for example, with Musk; a grain of which has been kept freely exposed to the air of a room, whose doors and windows were constantly open, for a period of ten years; during which time the air, thus continually changed, was completely impregnated with the odor of musk; and yet, at the end of that time, the particle was not found to have perceptibly diminished in weight. We can only attribute this result to the extreme minuteness of the division of the odorous particles of this substance. There are other odorous solids, such as Camphor, which rapidly lose weight by the loss of particles from their surface, when freely exposed to the air.

947. The conditions of the sense of Smell are very simple. The Olfactory nerve is minutely distributed over the Schneiderian membrane, which is itself highly vascular. The arrangement of the ultimate fibres of this nerve has not been ascertained. The Schneiderian membrane is kept constantly but moderately moist, by a mucous secretion from its surface; and this condition is essential to the acute perception of odors. If the mucous surface be too dry, as happens when the fifth pair is Fig. 160.

The Olfactory nerve, with its distribution on the septum nasi. The nares have been divided by a longitudinal section made immediately to the left of the septum, the right nares being preserved entire. 1. The frontal sinus. 2. The nasal bone. 3. The crista galli process of the ethmoid bone. 4. The sphenoidal sinus of the left side. 5. The sella turcica. 6. The basilar process of the sphenoid and occipital bones. 7. The posterior opening of the right nares. 8. The opening of the Eustachian tube in the upper part of the pharynx. 9. The soft palate, divided through its middle. 10. Cut surface of the hard palate. a. The olfac tory peduncle. b. Its three roots of origin. c. Olfactory ganglion, from which the filaments proceed that spread out in the substance of the pituitary membrane. d. The nasal nerve, a branch of the ophthalmic nerve descending into the left nares from the anterior foramen of the cribriform plate, and dividing into its external and internal branch. e. The naso-palatine nerve, a branch of the spheno palatine ganglion distributing twigs to the mucous membrane of the septum nasi in its course to (f) the anterior palatine foramen, where it forms a small gangliform swelling (Cloquet's ganglion) by its union with its fellow of the opposite side. g. Branches of the naso-palatine nerve to the palate. h. Posterior palatine nerves. i, i. The septum nasi.

paralysed, the sensation is blunted, or even destroyed; and the same effect is produced by the presence of too copious a secretion, as when

we are suffering under an ordinary cold.-The highest part of the nasal fossæ appears to be that, in which there is the most acute sensibility to odors; and hence it is, that, when we snuff the air, so as to direct it into this portion of the cavity, we perceive delicate odors, which would otherwise have 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 the Ruminants, which are warned by its means of the proximity of their enemies. The habit of attention to sensory impressions of this class, however, very much heightens their acuteness: hence in those who suffer under blindness and deafness conjointly, it is usually the principal means by which individuals are distinguished, and the presence of strangers recognised; and there are cases, in which individuals in a state of somnambulism have exhibited a degree of acuteness of smell, quite comparable to that which is characteristic of Deer, Antelopes, &c.

948. Besides ministering to the sense of Smell, by stimulating the secreting powers of its surface, the Fifth pair has another very important function, that of endowing the interior of the nose with common sensibility, and thus receiving the impression produced by acrid or pungent substances, which act upon it in the same way as they do upon the tongue. Such substances are felt, by the irritation they produce, rather than smell; and the sensation they occasion gives rise to the consensual act of sneezing, by which a violent blast of air is directed through the nasal passages, in such a manner as to clear them of the irritating matter, whether solid (as snuff), fluid or gaseous. Hence this action may be excited by the contact of an irritant with the Schnellerian membrane, 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 is retained.

5. Of the Sense of Hearing.

949. By this sense we become acquainted with the sounds produced by bodies in a certain state of vibration; the vibrations being propagated through the surrounding medium, by the corresponding waves or undulations which they produce in it. Although air is the usual medium through which sound is propagated, yet liquids or solids may answer the same purpose. On the other hand, no sound can be propagated through a perfect vacuum.-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 neighborhood of these bodies, than it would otherwise have been.

III. The sonorous vibrations are much more weakened in the transmission of solids to air; and those of air make but little impression on solids.

IV. 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 communication is rendered more easy by the intervention of a membrane extended between them.

The application of these conclusions, in the Physiology of Hearing, will be presently apparent.

950. It is on the Auditory nerve (commonly termed the Portio Mollis of the 7th pair), that the sonorous undulations make their impression; but we invariably find, that this impression is made through the medium of a liquid, contained in a cavity, on the walls of which the ultimate branches of this nervę are distributed. The simplest form of the organ of Hearing, such as we find in Cephalopods and in certain Fishes, consists merely of a cavity excavated in the solid framework of the head; which cavity is filled with liquid, and lined by a membrane on which the auditory nerve is distributed. These animals are inhabitants of the water; and the sonorous vibrations excited in this medium being communicated to the solid parts of the head, will be by them again transmitted to the contained fluid, without much diminution of their intensity; according to principles I. and II.-In certain Crustacea, however, whose organ of hearing is contained in the base of the antennæ, as well as in most Fishes, we find the auditory cavity or vestibule no longer entirely closed; but having an aperture on its external side, which is covered in by a membrane. Here the vibrations of the liquid within the cavity will be more directly excited by those of the surrounding medium, for if this be water, it will propagate its undulations into the cavity, with little interruption from the membrane stretched across its mouth; whilst, if it be air, the interposition of this very membrane will greatly assist in the transmission of the vibrations to the water of the auditory cavity, according to principle IV. In most of the animals which have the organ of hearing constructed upon this simple plan, the force of the vibrations of the fluid within the cavity is increased by several minute stony concretions (termed otolithes), which are suspended in it. These act according to principle II. Some traces of them are found in the higher animals; in which they are for the most part superseded, however, by an apparatus better adapted to augment the intensity of the sonorous vibrations.

951. This apparatus consists, in all Vertebrated animals above the inferior Reptiles, of the tympanum or drum, with its membrane and chain of bones; together with, in the mammalia, the external ear; which is adapted to direct itself, more or less completely, towards the point from which the sonorous vibrations proceed, and to give them a degree of preliminary concentration. The tympanic apparatus is interposed between the external ear and the membrane covering the foramen ovale, which is the entrance to the real auditory cavity; and its purpose is evidently, to receive the sonorous vibrations from the air,

and to transmit them to that membrane, in such a manner that the vibrations thus excited in the latter may be much more powerful, than they would be if the air acted immediately upon it, as in the lower Vertebrata. The usual condition of the Membrana Tympani appears to be rather lax; and, when in this condition, it vibrates in accordance with grave or deep tones. By the action of the tensor tympani it may be tightened, so as to vibrate in accordance with sharper or higher tones; but it will then be less able to receive the impressions of deeper sounds. This state we may easily induce artificially, by holding the breath, and forcing air from the throat into the Eustachian tube, so as to make the membrane bulge out by pressure from within; or by exhausting the cavity by an effort at inspiration, with the mouth and nostrils closed, which will cause the membrane to be pressed inwards by the external air. In either case, the hearing is immediately found to be imperfect; but the deficiency relates only to grave sounds, acute ones being heard even more plainly than before. There is a different limit to the acuteness of the sounds, of which the ear can naturally take cognizance, in different persons. If the sound be so high in pitch, that the membrana tympani cannot vibrate in unison with it, the individual will not hear it, although it may be loud; and it has been noticed, that certain individuals cannot hear the very shrill tones produced by particular Insects, or even Birds, which are distinctly audible to others.

This

952. Not only do we find the tympanic apparatus superadded, in the higher forms of the organ of Hearing, but also the Semicircular Canals, and the Cochlea.-The former exist in all Vertebrata, save the lowest Fishes; and in nearly every case, they are three in number, and lie in three different planes. Hence it has been supposed, with some probability, that they assist in producing the idea of the direction of sounds. The Cochlea does not exist at all in Fishes; and in Reptiles its condition is quite rudimentary. In Birds, this cavity is more completely formed, though the passage is nearly straight instead of spiral; of its real character, however, there can be no doubt, from its being divided, like the cochlea of Man, by a membranous partition, on which the ramifications of the auditory nerve are spread out. appendage has been supposed to be the organ that enables us to judge of the pitch of sounds; an idea which derives some confirmation from the correspondence between the development of the cochlea in different animals, and the variety in the pitch (or length of the scale) of the sounds which it is important that they should hear distinctly, especially the voices of their own kind.-That the Vestibule with the passages proceeding from it, constitutes the true organ of hearing, even in Man, is evident from the fact, that when (as not unfrequently happens) the tympanic apparatus has been entirely destroyed by disease, so as to reduce the organ to the condition of that in which no such apparatus exists, the faculty of Hearing is by no means abolished, although it is deadened.

953. The faculty of Hearing, like other senses, may be very much increased in acuteness by cultivation; but this improvement depends rather upon the habit of attention to the faintest impressions made