Although I am aware, and have already mentioned, that Mr. Bateson's observations do not support the view that the sense-organs of the lateral line minister to this telæsthetic sense, still I think that further observations and experiments may show that these sense-organs are "olfactory," and that the lateral development may be in relation to the appreciation of the direction in which the food lies. It is, however, a difficult matter to determine, and the few experiments I have made are so far inconclusive.
Much has been written concerning the sense of smell in insects. That they possess such a sense few will be disposed to doubt. The classical observations of Huber show that bees are affected by the smell of honey, and that the penetrating odour of fresh bee-poison will throw a whole hive into a state of commotion. He was of opinion that the impunity with which his assistant, Francis Burnens, performed his various operations on bees was due to the gentleness of his motions, and the habit of repressing his respiration, it being the odour transmitted by the breath to which the bees objected. Sir John Lubbock formed a little bridge of paper, and suspended over it a camel's-hair brush containing scent, and then put an ant at one end. She ran forward, but stopped dead short when she came to the scented brush. Dr. McCook introduced a pellet of blotting-paper saturated with eau-de-Cologne into the neighbourhood of some pavement-ants, who were engaged in a free fight. The effect was instantaneous; in a very few seconds the warriors had unclasped mandibles, relaxed their hold of their enemies' legs, antennæ, or bodies.
The correct localization of the sense of smell has been a matter of difficulty. Kirby and Spence localized it at the extremity of the "nose," between it and the upper lip. That the nose, they naïvely remark, corresponds with the so-named part in mammalia, both from its situation and often from its form, must be evident to every one who looks at an insect. Lehman, Cuvier, and others, misled by the fact that the organ of smell is in us localized at the entrance of the air-track, supposed that at or near the spiracles of insects were the organs of smell. Modern research tends more and more clearly to localize the sense of smell, as first suggested by Réaumur, in the feelers or antennæ, and in some cases also in the palps. If the antennæ of a cockroach be extirpated or coated with paraffin, he no longer rushes to food, and takes little notice of, and will sometimes even walk over, blotting-paper moistened with turpentine or benzoline, which a normal insect cannot approach without agitation. There can be little doubt that it is by means of its large branching antennæ that the male emperor moth (Saturnia carpini) is able to find its mate.[EU] If a collector take a virgin female into a locality frequented by these moths, he will soon be surrounded by twenty or thirty males; but if the moth be not a virgin, he will at most see one or two males. The sense of smell is thus delicate enough to distinguish the fertilized from the unfertilized female, and has associated with it a sense of direction by which the insect is guided to the right spot. Carrion flies whose antennæ have been removed fail to discover putrid flesh; and E. Hasse has observed that male humble-bees whose antennæ have been removed cannot discover the females. The sensory elements are lodged in pits or cones, which may be filled with liquid, peculiar sensory rods or hairs being associated with the nerve-endings. Of these pits the queen-bee has, according to Mr. Cheshire, 1600, the worker 2400, and the drone nearly 19,000, on each antennæ. On the antennæ of the male cockchafer, Hauser estimates the number to be 39,000.
In the aquatic crayfish there are, besides the long antennæ, smaller antennules, each of which has two filaments, an inner and an outer. On the under surface of most of the joints of the outer filament there are two bunches of minute, curiously flattened organs, which were regarded by Leydig, their discoverer, as olfactory. Observation, too, seems to confirm the view that the sense of smell (or telæsthetic taste) is located in the antennule. I tried on a crayfish the following experiment: When it was at rest at the bottom of its tank, I allowed a current of pure water (the water in which it lived) to flow from a pipette over its antennæ and antennules. The antennæ moved slowly, but the antennules remained motionless. I then took some water in which a cod's head had been boiled, and allowed some of this to stream over the antennæ and antennules. The former moved slightly as before, but the antennules were thrown into a rapid up-and-down jerky vibration, and shortly afterwards the crayfish began moving about the bottom of its tank. If only one antennule be thus stimulated, or stimulated to a higher degree than the other, the crayfish seems generally (but not always) to turn to that side in search of food. Mr. Bateson[EV] has shown to how large an extent shrimps and prawns seek their food by smell, and states that a prawn, though blind, will often find his way back to his proper place, and stay in it.
In the snail the anterior pair of "horns," or tentacles, are said to be olfactory. Near the end of each is a large ganglion, or nerve-knot, from which fibres pass to the surface, in which there are said to be developed sensory knobs. Snails, however, from which these tentacles have been removed are apparently still possessed of a sense of smell. Certain lobed processes round the mouth have been regarded as the seat of olfactory sensation, but this is doubtful. In the foot of the snail, the part on which it glides, there is a hollow gland, and in this there are special cells, each of which gives off a delicate rod, enlarging at the free end into a ciliated knob. These are regarded as sensory and, it may be, olfactory. In shell-fish like the mussel, in which the water is sucked in by an inhalent tube or siphon, and ejected through an exhalent siphon above it (see [Fig. 2], [p. 4]), there is at the entrance of the incoming current a thin layer of elongated cells which are described as olfactory, and are in association with a special ganglion. Olfactory depressions have been described in some worms. But in a great number of the lower invertebrates very little or nothing is known concerning a sense of smell.
Hearing is a telæsthetic sense. Through it we become aware of certain vibratory states of more or less distant objects. The vibrations of these bodies are transferred to the air or other medium surrounding the body, and are transmitted through the air or other medium to the ear. The sound-waves traverse the air at a rate of 337 metres (1106 feet) in a second; but they travel about four times as fast in water. If the vibration is periodic or regular, the sound is called a tone; non-periodic or irregular sounds are noises. The pitch of a tone is determined by the number of vibrations in a second. The lowest or gravest tone most of us can hear is that where there are about 30 vibrations in a second; twice this number give us a tone of an octave higher; twice this again, another octave; and so on. In musical composition, tones from about 40 to about 4000 vibrations per second are employed. This is a range of somewhat over six octaves. But many of us are capable of hearing sounds over a range of about ten octaves, that is to say, from 30 to 30,000 vibrations per second. The upper limit of hearing is, however, very variable. Some people are deaf to tones of more than 15,000 or 20,000 vibrations per second.[EW] Others may hear shrill tones of 40,000, or even in rare cases 50,000. I could as a boy hear the shrill squeak of a bat; now I am quite deaf to it. A friend of mine in South Africa was unable to hear the piping of the frogs in the pond, which was to me so loud as almost to drown the tones of his voice.
Apart from the pitch of a note is its quality. The same note struck on different instruments or sung by different persons has a different ring. This is determined by the number and intensity of overtones, or partials, which are associated with the fundamental tone. Suppose the deep fundamental tone of 33 vibrations be sounded; with it there may be associated overtones, eight or nine in number, all of which are simple multiples (twice, thrice, four times, and so on) of the fundamental 33. The effects of these on the organ of hearing fuse or combine with the predominant effect of the fundamental tone. In harmonious chords, also, two or more fundamental tones, with their accompaniment of partials, blend in sensation so completely that it requires a keen musical ear and some training to analyze them into their component elements.
The delicacy of discrimination of tones is greatest in the mid-region of hearing; and there is much individual variation in accuracy of ear. I have made experiments on many individuals to test their powers in this respect. I found some who were unable, in the mid-region of hearing, to state which was the higher of two notes sounded on a violin, the tones of which were separated by a major third, and in one case by a fifth. With notes on the piano the discrimination was more delicate, and yet more delicate when the notes were sung. In such cases tone-discrimination is deficient; and between these and the musician, who is stated to be able to distinguish tones separated by only 1/64 of a tone, there are many intermediate stages.
It is beyond my purpose to describe, in more than a very general way, the nature of the auditory apparatus of man. The vibrations of the air are received by the drum-membrane, which lies in the auditory passage. From this it is transmitted, by a chain of small bones, to the inner auditory apparatus. This consists of two small membranous sacs, with one of which three membranous looped tubes, the semicircular canals, are connected; with the other is connected a spiral tube, the cochlear canal. These membranous sacs and canals are filled with fluid, and are surrounded by the fluid which fills the bony cavity in which they lie. This bony cavity has two little windows, one oval and the other round, across each of which a membrane is stretched. The oval membrane is in connection with the chain of auditory bones; and when this is made to vibrate in and out, the membrane of the round window vibrates out and in. Thus the fluid around and within the membranous sacs and canals is set in vibration. And the parts are so arranged that the vibrations, in passing from the oval to the round membrane, must run up one side and down the other side of the cochlear canal. As they run down they set in vibration a delicate membrane which is supported on beautiful arched rods (the organs of Corti). And this membrane contains a number of special hair-cells, so called because they bear minute hair-like structures. These are the special end-organs of hearing. It has been suggested that the fibres of the membrane on the arched rods, which are of different lengths and may be stretched with differing degrees of tension, respond to vibrations of different pitch. Thus the hair-cells on that particular part of the membrane would be stimulated, and the note might be appreciated in its true position in the scale.