the specific energy of the optic nerve of producing a sensation of light alone; that of the auditory nerve of only calling forth a sensation of sound; and the connection of this with cases in which, within the sphere of a sensory sensation, we discover several qualities of the same sensation.

In conclusion we will remark, that from daily experience we learn to distinguish one noise from another. We therefore call them grating, creaking, hissing noises. Their character depends partly upon the strength of the separate shocks, partly upon the rapidity with which they follow each other, but partly also upon the presence of real tones of various pitch which are intermingled with the noise. Low tones are often connected with a grating noise, and with a hissing noise generally very high tones, which impart to them their peculiar character. It is often by means of this intermixture of tones that we are able to distinguish the cause of the noise.

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PART IV.

SMELL AND TASTE.

CHAPTER I.

The Sense of Smell.

CERTAIN substances, in a gaseous form, when inhaled by the nostrils together with the air, create a sensation of smell. The substance smelt here comes into direct contact with the sensory organ, as is the case with taste, and excites it directly in a certain manner, the result of which is an entirely specific sensation of the senses. It would be quite impossible to include the description of the sense of smell with that of the other senses, merely as a species of touch or taste, for it differs from them quite as much as sight and hearing differ from each other, and is a sensation of quite a peculiar kind.

The action of the organ of smell is, therefore, due to a special nerve, the olfactory nerve, which differs from the others both in origin, position, and extension. It has its origin in the anterior portion of the cranium in a bulbous swelling, the olfactory ganglion, which is strongly developed in the lower animals. Its fibres spread

themselves out in the base of the skull, and force their way through the cribriform plate of the cribriform bone, which lies between the sockets of the eyes, by a great number of small apertures into the upper portion of the nose. This part of the nostril is itself divided into three mussel-shaped passages, which are covered by a mucous membrane.

The inferior, and, partly, the middle passage of the nostril serve principally for inhaling and exhaling the air, and are, therefore, called the respiratory region (regio respiratoria). Like the other air-passages in the wind-pipes and lungs, it is covered with cylindrical cells (epithelial cells) packed closely together, and at their free extremity provided with fine hairs, which, by a sort of waving motion, propel outwards all mucous secretion and dust.

The upper and, partly, the lower passages of the nostril are occupied by the sensory organ for the sensation of smell and have therefore been called the olfactory region (regio olfactoria). It is distinguished from the respiratory region by its yellow colour, caused by pigments, and, unlike the latter, is not covered with hairy epithelial cells, but presents a different organisation upon its surface.

The manner in which the olfactory nerve terminates in the mucous membrane of the nostril has only lately been discovered by Max Schultze. The analogy with other sensory organs would lead us to suppose that these nerve-fibres are furnished with peculiar terminal organs, capable of receiving sensory impressions.

The olfactory mucous membrane is also covered with cylindrical epithelial cells, as represented in fig. 91 e.

Lory

0

They present their broad end to the surface (o), but become
attenuated when traced inwards towards the underlying
network. Between them we find long rod-like filaments
(s) which in their lower part swell out into
a nut-shaped expansion, and then are pro-
longed into a fine fibre towards the interior.
Now, as this fibre has a great resemblance
to the finest nerve-fibres, and loses itself
near the terminations of the finest fibres of
the olfactory nerve, it very probably has
some connection with the olfactory nerve.
These formations have been termed by
Schultze olfactory cells. Very small fine
hairs have been observed upon the ends of
the small rods in some animals, when the
specimens have been perfectly fresh.

Fig. 91.

The odorous substances, which, together with the air inhaled, are brought into contact with the olfactory mucous membrane, and thus evidently act upon the terminal apparatus of the olfactory nerve, are not, however, directly received by the nerve-fibres, but by the olfactory cells, the fibres themselves very probably not being excited in the slightest degree by the greater number of odorous substances, certainly not by those in a gaseous form. We remember that neither are the optic nerve-fibres affected by the waves of light, nor the auditory nerve-fibres by the waves of sound, and shall here find ourselves justified in coming to a similar conclusion.

We must, therefore, suppose that the olfactory nerve would not be conscious of a pleasant odour, if we were to place its ramifications, at the point where they pass

through the cribriform bone, in an atmosphere of Eau de Cologne, nor would it be sensible of an unpleasant odour if we exposed it to an atmosphere of sulphuretted hydrogen. The action of odorous substances consists rather in their producing a change of condition in the terminal apparatus in the mucous membrane, the terminal apparatus then exciting the nerve-fibres whose terminations are situated in close proximity to them. The terminal apparatus alone, and not the nerve fibres, can receive impressions from an odorous substance, and, as is the case throughout the body, the nerve-fibres serve only to convey the message to the brain that an irritation has taken place.

The substances which we perceive through the organ of smell must be conveyed to the olfactory mucous membrane, together with the inspired air, in a gaseous form; and since all gases and vapours are more or less soluble in water, they penetrate the moisture of the mucous membrane in small particles. Their influence upon the terminal apparatus of the olfactory nerve with which they come in contact, is very probably of a chemical nature, for the smell of substances differs with their chemical composition. There is, however, a mechanical condition without which we could have no perception of smell-namely, a continual current of air through the nostrils, which is maintained by the process of breathing. If we hold our breath all sensation of smell ceases, even if we are in an atmosphere very strongly impregnated with odorous substances. Again, the sensation is strongest at the moment when we snuff up the air, and we, therefore, repeat it quickly several times in succession when we wish to perceive a delicate odour. The cause