It is part of the essential nature of an animal to be receptive and responsive. The forces of nature rain their influence upon it; and it reacts to their influence in certain special ways. Other organisms surround it, compete with it, contend with it, strive to prey upon it, and occasionally lend it their aid. It has to adjust itself to this complex environment.

There are two kinds of organic response—one more or less permanent, the other temporary and transient. We have already seen something of the former, by which the tissues (the epidermis of the oarsman's hand, and the muscles of his arm) respond to the call made upon them. The response is here gradual, and the effects on the organism more or less enduring. This, however, is not the kind of response with which we have now to deal. What we have now to consider is that rapid response, transient, but of the utmost importance, by means of which the organism directly answers to certain changes in the environment by the performance of certain activities. The parts specially set aside and adapted to receive special modes of influence of the environment are the sense-organs. We human folk get so much pleasure from and through the employment of our sense-organs, that it is important to remember that the primary object of the process of reception of the influences from without was not the Æsthetic one of ministering to the enjoyment of life by the recipient organism, but the essentially practical one of enabling that organism to respond to these influences. In other words, the raison d'Être of the sense-organs is to set agoing suitable activities—activities in due response to the special stimuli.

In this chapter we shall consider the modes in which the special sense-organs are fitted to receive the influences of the environment, deferring to a future chapter the consideration of the resulting activities. For the present we take these activities for granted, observing them only in so far as they give us a clue to the sense-reaction by which they are originated. In this chapter, too, we shall deal, for the most part, with the physiological aspects of sensation. In all other organisms than ourselves, that is to say, than each one of us individually for himself, the psychological accompaniments of the physiological reactions of the sense-organs are matters of inference. Still, so closely and intimately associated are the physiological and the psychological aspects, that the exclusion of all reference to the latter would be impracticable, or, if practicable, unadvisable. What is practicable and advisable is to remember that, even if the two are mentioned in a breath, the physiological and the psychological belong to distinct orders of being.

In addition to the time-honoured "five senses," there are certain organic sensations, so called, which take their origin within the body. These are, for the most part, somewhat vague and indefinite. They do not arise immediately and in direct response to changes in the environment, but indicate conditions of the internal organs. Such are hunger, thirst, nausea, fatigue, and various forms of discomfort. Although they are of vital importance to the organism, prompting it to perform certain actions or to desist from others, they need not detain us here.

More definite than these, but still of internal origin, is the muscular sense. This, too, is of continual service to every active animal. By it information is given as to the energy of contraction of the muscles, and of the amount of movement effected—not to mention the rapidity and duration of the muscular effort. By it the position, or changes of position, of the motor-organs are indicated. It is obvious, therefore, that the sensations obtained in this way, some of which are exceedingly delicate, are an important guide to the organism in the putting forth of its activities. It is through the muscular sense that we maintain an upright position. It is through an educated and refined muscular sense that the juggler and the acrobat can perform their often surprising feats. Concerning the physiology of the muscular sense, we have at present no very definite knowledge. Some have held that we judge of muscular movements by the amount of effort required to initiate them; but it is much more probable that there are special sensory nerves, whose terminations are either in the muscles themselves or in the membranes which surround them.

We come now to the special senses. Of these we will take first the sense of touch. Through this sense we are made aware of bodies solid or liquid (or perhaps gaseous) which are actually in contact with the skin or its infoldings at the mouth, nostrils, etc. There are considerable differences in the sensitiveness of the skin in different parts of its surface; some parts, like the filmy membrane which covers the eye, being very sensitive, while others, like the horny skin that covers the heel of a man who is accustomed to much walking, are relatively callous. Different from this is the delicacy of the sense of touch. This delicacy is really the power of discrimination, and therefore involves some mental activity. But it is also dependent upon the distribution of the recipient end-organs of the nerve. The highest pitch of delicacy is reached in the tip of the tongue, which is about sixty times as delicate as the skin of the back. The power of discrimination is tested in the following way: The points of a pair of compasses are blunted, and with them the skin is lightly touched. When the points are close together, the sensation is of one object; when they are more divergent, each point is felt as distinct from the other. On the thigh and in the middle of the back, two distinct points of contact are not felt unless the compass-tips are about 2-1/3 inches (67.7 millimetres) apart. When the divergence is 2 inches, they are felt as one. With the tip of the tongue, however, we can distinguish the two separate points when they are only 1/25 of an inch (1.1 millimetre) apart. For the finger-tip the distance is about 1/12 of an inch (2 millimetres); for the tip of the nose, about 1/4 of an inch (6.8 millimetres); for the forehead, a little less than an inch (22.6 millimetres); and so on. Shut your eyes, and allow a friend to draw the compass with the points about 1/2 an inch apart, from the forehead to the tip of your nose, or (setting the points about 1/4 of an inch apart) from the ball of your thumb to the finger-tip. The increasing delicacy and power of discrimination is readily felt, and it is difficult to believe that the compasses are not being slowly opened.

It is beyond the purpose of this chapter to describe minutely the nature and structure of the nerve-ends in the sense-organs. This is a matter of minute anatomy, or histology. A full description of them as they occur in man will be found in any standard text-book of physiology; while Sir John Lubbock's "Senses of Animals" gives much information concerning, and many illustrations of, the minute structure of the sense-organs in the invertebrates. Here I can only touch very briefly on some of the more important points.

One of the larger nerves of the body (e.g. the sciatic nerve), consists of a bundle of nerve-threads collected from a considerable area; some of these (motor threads) end in muscles, others (sensory threads) in the skin or its neighbourhood. Each nerve-thread has a central axis-fibre, which is surrounded by a fatty, insulating medullary sheath, and this by a delicate primitive sheath. In some parts of the skin the sensory nerve-threads lose their medullary sheath, and end in very fine branches between the cells of the tissue. In other cases the cells near their termination are specially modified to form tactile cells, or tactile corpuscles, in contact with or surrounding the axis-fibre or its expansion (Fig. 23).

Hairs are delicate organs of touch, though, of course, this is not their only function. They act as little levers embedded in the skin.

Turning now to the vertebrate animals other than man, we find in them a sense of touch closely analogous to our own. As in us, so in them, the specially mobile parts are eminently sensitive and delicate; for instance, the lips in many animals, such as the horse, and the finger-like organ at the end of the elephant's trunk. In some of them special hairs are largely developed as organs of touch, as in the whiskers of the cat and the long hairs on the rabbit's lip. With the aid of these the rabbit finds its way in the darkness of its burrow; and it is said that, deprived of these organs, the poor animal blunders about, and is unable to steer its course in the dark.

The wing of the bat is very sensitive to touch; and it is supposed that it is through this sense that the bat is able to direct its course in the darkness of caves. Miss Caroline Bolton thus describes an experimental trial of this power of the bat at which she was herself present. A room, about twenty feet by sixteen, was arranged with strings crossing each other in all directions so as to form a network with about sixteen inches space between the strands. To each string was attached a bell in such a way that the slightest touch would make it ring. One corner of the room was left free for those who were present at the experiment. A bat, measuring about one foot from the tip of one wing to that of the other, was let loose in the room when it was quite dark, "and it was distinctly heard flying about all over the room, but never once did it touch a string or stop flying. It several times came quite near to the spectators, so that they could feel the vibration of the air in their faces. The experiment was continued for half an hour. Then, when the door was opened and light let in, the bat stopped flying, and settled down in the darkest corner." Now, here it may be said that, although the room was dark to human spectators, there may have been light enough for a bat to see his way. The cruel experiments of Spalanzani, however, who put out the eyes of bats and obtained a similar result, seem to show that the animal is guided by some sense other than that of sight.

The crustaceans and many insects are covered with a dense armour, and it might be supposed that in them there could be no sense of touch. But this sense is by no means absent. Seated on the tough integument are delicate little hairs, to the base of which a nerve-fibril passes through a perforation in the integument. These are specially numerous in the antennÆ of insects.

In yet lower organisms we know in some cases the manner in which they are sensitive to touch; but in a great number of cases, although observation shows that they are thus sensitive, we know nothing definite as to how the surface is specially fitted to receive the stimuli. Even the primitive amoeba, however, is sensitive in the sense spoken of on p.8; that is to say, it reacts under the influence of a stimulus.

Closely associated with the sense of touch is the temperature-sense. Goldschneider and others have shown that on the skin of the human hand, for example, there are special points that are sensitive to heat and cold. Some of these little specialized areas are sensitive to cold; others are sensitive to heat; and neither of these seem to be sensitive to pressure. It therefore seems probable that special nerve-fibrils are set apart for the temperature-sense; but of the manner in which these fibrils terminate little or nothing is known.

Let us note that this temperature-sense, unlike the sense of touch, may make us aware of distant bodies. It is, then, what we may term a telÆsthetic sense in contradistinction to a contact-sense. It is stimulated by a molecular throb; the throbbing body may be in contact, but it may be as distant as the sun, in which case the molecular pulsations are brought to us on waves of Æther. Whether these waves act directly on the nerve end-organs, or indirectly on them through the warming of the skin-surface in which they terminate, we cannot say for certain. But if the hand be held before a heated stove and be sheltered from the heat by a screen, the removal of the screen, even for the fraction of a second, gives rise to a strong stimulation of the temperature-sense, though the skin-surface be not appreciably raised in temperature. Hence it is probable that the end-organs are stimulated directly, and not indirectly.

Concerning the temperature-sense in the lower animals, nothing definite is known. But it is impossible to see our familiar pets basking in the sunshine, or a butterfly sunning itself on a bright summer's day, without feeling confident that the temperature-sense is a channel of keen enjoyment. As before mentioned, however, this is not to be regarded as the primary end in sensation. The primary end is not life-enjoyment, but life-preservation. And we must regard the temperature-sense as developed in the first instance to enable the organism to escape from the ill effects of deleterious heat or cold, and to seek those temperature-conditions which are most helpful to the continued and healthful fulfilment of the process of life.

The sense of taste is called into play by certain soluble substances, or liquids, which must come in contact with the specialized nerve-endings. Under normal circumstances, the sense of taste is closely associated with that of smell, the result of the combination of the two special senses being a flavour. The bouquet of a choice wine, the flavour of a peach, involve both senses; quinine involves taste alone; and garlic and vanilla are nearly, if not quite, tasteless,—what we call their taste is in reality their action on the organ of smell.

It is difficult to classify tastes. Sweet, bitter, salt, alkaline, sour, acid, astringent, acrid,—these are the prominent and characteristic varieties.

This sense is generally localized in or near the mouth; in us mainly in the tongue. One manner, but not the only manner, in which the nerves in this region terminate is in the minute flask-shaped taste-buds, which have near one end, where they reach the surface, a funnel-shaped opening, the taste-pore. They are made up of elongated cells, some of which near the centre are spindle-shaped, and are called taste-cells. They are found chiefly round the large circumvallate papillÆ; but in the rabbit and some other animals they are collected in the folds of a little ridged or pleated patch—the papilla foliata—on each side of the tongue near the cheek-teeth.

It is probable that the stimulation of the end-organs of taste is effected by the special mode of molecular vibration due to the chemical nature of the sapid substance. Mr. J. B. Haycroft, in a paper read before the Royal Society of Edinburgh,[EO] suggests that "a group of salts of similar chemical properties have their molecules in a similar vibrating condition, giving rise to similar colours and similar tastes." "Thus the chlorides and sulphates of a series of similar elements—called a group of elements by Mendeljeff—have similar tastes."

The delicacy of the sense of taste in man has been the subject of investigation by Messrs. E. H. S. Bailey and E. L. Nichols.[EP] They give the following table:—

The above figures represent means or averages of a great number of individuals. There was very considerable variation for some tastes. In the case of the bitter of quinine, the maximum delicacy was the detection of 1 part in 5,120,000 parts of water; the minimum 1 part in 456,000 parts of water. Except in the case of salt, the sense was more delicate in women than in men. It is not stated whether the men tested were smokers.

It does not seem necessary to say anything concerning the sense of taste in the lower mammalia.

In birds and reptiles the sense of taste does not appear to be highly developed. Parrots are, perhaps, better off in this respect than the majority of their class; and the ducks have special organs on the edges of the beak, which seem to minister to this sense. A python at the Zoological Gardens, partially blind owing to a change of skin, is said to have struck at an animal, but to have only succeeded in capturing its blanket. This, however, it constricted, and proceeded to swallow with abundant satisfaction.

It may here be mentioned that the scales and skin of many fishes are provided with sense-organs which very closely resemble the taste-buds of higher animals. They occur in the head and along the "lateral line" which runs down the side of the fish, and may be readily seen, for example, in the cod. Mr. Bateson's[EQ] careful observations at Plymouth gave, however, no indication of the possession of an olfactory or gustatory function, and their place in the sensory economy of the fish remains problematical. In or near the mouth similar end-organs are found to be somewhat variously developed in different fishes—on the palate and lips, on the gill-bars, more rarely on the tongue, and on the barbels of the rockling and the pout. How far any or all of these have a gustatory function remains to be proved.

Anglers and fishermen, however, from their everyday experience, and naturalists from special observations, do not doubt that fishes have a sense of taste. Professor Herdman's recent experiments on feeding fishes with nudibranchs[ER] (naked molluscs) seem to show, for example, that the fishes concerned, including shannies, flat-fish, cod, rockling, and others, have a sense of taste leading them to reject these molluscs as nasty. They show, too, that some of the nudibranchs (Doris, Ancula, Eolis) are protected by warning coloration.

Our knowledge of the sense of taste among the lower (invertebrate) animals is imperfect, and is largely based rather on observation of their habits than on the evidence of anatomical structure. Here, again, comes in the difficulty of distinguishing between taste and smell. But even if the caterpillars which refuse to eat all but one or two special herbs, or the races of bloodsuckers which seem to have individual and special tastes, are guided in part by an olfactory sense, there is much evidence which seems to admit of no alternative explanation. Moisten, for example, the antennÆ of a cockroach with a solution of Epsom salts or quinine, and watch him suck it off; or repeat F. Will's experiments on bees, tempting them with sugar, and then perfidiously substituting pounded alum. The way in which these little insects splutter and spit suggests that, whatever may be the psychological effect, the physiological effect is analogous to that produced in us by an exceedingly nasty taste. Here smell would seem to be excluded. Forel, moreover, mixed strychnine with honey, and offered it to his ants. The smell of the honey attracted them, but when they began to feed, the effect of the taste was at once evident.

The organs of taste in insects are probably certain minute pits, in each of which is a delicate taste-hair, which, in some cases, is perforated at the free end. They occur in the maxillÆ and tongue in ants and bees, and on the proboscis of the fly.

In many of the invertebrates, the crayfish and the earthworm, for example—to take two instances from very different groups—observation seems to show that a sense of taste is developed, for they have marked and decided food-preferences. Nevertheless, the existence of special organs for this purpose has not been definitely proved.

The sense of taste no doubt ministers to the enjoyment of life. But, presumably, it has been developed in subservience to the process of nutrition. Primarily, taste was not an end in itself, but was to guide the organism in its selection of food that could be assimilated. Nice and nasty were at first, and still are to a large extent, synonymous with good-for-eating and not-good-for-eating. With unwonted substances, however, its testimony may be false. Sugar of lead is sweet, but fatal. Brought to a new country, cattle often eat, apparently with relish, poisonous plants. Still, under normal circumstances, the testimony of taste is reliable.

The sense of smell is, to a large extent, telÆsthetic. It is true that the stimulation of the end-organs is effected by actual contact with the odoriferous vapour. But since this vapour may be given off from an odoriferous body at some distance from the organism, such as a flower or a decomposing carcase, it is clear that the sense gives information of the existence of such bodies before they themselves come in contact with us. Primitively, we may suppose that it was developed in connection with that sense of taste with which, as we have seen, it is so closely associated. In this respect smell is a kind of anticipatory taste. But it has now other ends, apart from those which are purely Æsthetic. In us it may serve as a warning of a pestilential atmosphere; in many organisms, such as the deer, it gives warning of the presence of enemies; in many again, and some insects among the number, it is the guiding sense in the search for mates.

The organ of smell in ourselves and in all the mammalia is the delicate membrane that covers the turbinal bones in the nose. It contains cells with a largish nucleus, around which the protoplasm is mainly collected. A filament passes from this to the surface, and ends in a fine hair or cilium (or a group of hairs or cilia in birds and amphibia); a second filament runs downwards into the deeper parts of the tissue, and may pass into a nerve-fibril.

In us and air-breathing creatures, the substance which excites the sensation of smell must be either gaseous or in a very fine state of division; but in water-breathers the substance exciting this sensation—or, in any case, one of anticipatory taste—may be in solution. The sensitiveness of the olfactory membrane is very remarkable. A grain of musk will scent a room for years, and yet have not sensibly lost in weight. Drs. Emil Fischer and Penzoldt found that our olfactory nerves are capable of detecting the 1/4,600,000 part of a milligramme of chlorophenol, and the 1/460,000,000 part of a milligramme, or about one thirty-thousand-millionth of a grain, of mercaptan. It may be that to such substances our olfactory sensibility is especially delicate. Not much is known concerning the manner in which the end-organs of smell are stimulated. As in the case of taste, it is probably a matter of molecular vibration; and Professor William Ramsay has suggested that the end-organs are stimulated by vibrations of a lower order than those which give rise to sensations of light and heat. He has also drawn attention to the fact that to produce a sensation of smell, the substance must have a molecular weight at least fifteen times that of hydrogen.

It is well known that the sense of smell is in some of the mammalia exceedingly acute. The dog can track his master through a crowded thoroughfare. The interesting experiments of Mr. Romanes[ES] show that, under ordinary conditions of civilized life, the smell of boot-leather is a factor, and the dog tracks his master's boots. In one case, the boots were soaked in oil of aniseed, but this to us powerful scent did not overcome the normal odour of the master's boots. Mr. W. J. Russell, in a subsequent number of the same periodical, describes how his pug could find a small piece of biscuit by scent, and this odour of biscuit was not overmastered by a strong smell of eau-de-Cologne. Deer-stalkers know well how keen is the sense of smell in the antlered ruminants.

We must not, however, be too ready to conclude, from these observations, that the olfactory membrane is absolutely more sensitive in such animals than it is in man. It may well be that, though they are so keen to detect certain scents, they are dull to those which affect us powerfully. It is quite possible that the odour of aniseed or eau-de-Cologne is—possibly from the fact that their end-organs are not attuned to these special molecular vibrations—out of their range of smell. Their special interests in life have led to the cultivation of extreme sensibility to special tones of olfactory sensation. Under unusual circumstances, man may cultivate unwonted modes of utilizing the sense of smell. A boy, James Mitchell, who was born blind, deaf, and dumb, and who was mainly dependent on the sense of smell for keeping up some connection with the external world, observed the presence of a stranger in the room, and formed his opinion of people from their characteristic smell. On the whole, therefore, we may, perhaps, conclude that the variations in sensitiveness are mainly relative to the needs of life.

In birds the sense of smell is but little developed, notwithstanding all that most interesting naturalist, Charles Waterton, wrote on the subject. Vultures seem unable to discover the presence of food which is hidden from their sight. Probably reptiles share with them this dulness of the sense of smell.

It has already been remarked that, in the case of aquatic animals, there is probably little distinction between taste and smell. It would be well, perhaps, to restrict the word "smell" to the stimuli produced by vapours or air-borne particles, and to use the phrase "telÆsthetic taste," or simply "taste," for those cases where the effects are produced through the medium of solution. In this case, however, the point to be specially noticed is that taste in aquatic animals becomes a telÆsthetic sense, informing the organism of the presence of more or less distant food. Thus, if you stir with your finger the water in which leeches are living, they will soon flock to the spot, showing that the telÆsthetic sense is associated with an appreciation of direction. If a stick be used to stir the water, they do not take any notice of it. Mr. W. Bateson[ET] has shown that there are many fishes, among which are the dog-fish, skate, conger eel, rockling, loach, sole, and sterlet, which habitually seek their food by scent (telÆsthetic taste), aided to some extent by touch, and but little, if at all, by sight. "None of these fishes ever starts in quest of food when it is first put into the tank, but waits for an interval, doubtless until the scent has been diffused through the water. Having perceived the scent of food, they swim vaguely about, and appear to seek it by examining the whole area pervaded by the scent, having seemingly no sense of the direction whence it proceeds." I venture to think that further observation and experiment may show that such a sense of direction does in some cases exist. Some years ago I was fishing in Simon's Bay, at the Cape, with a long casting-line. The sea was unusually calm, and the water clear as crystal. Beneath me was a clear patch of granite, two or three yards across, surrounded by tangled seaweed. Evening was coming on, and I was just going to put up my tackle when I saw a long dark fish slowly sail into the open space and take up his position at one side. My line was out, baited, I think, with a piece of cuttle-fish, and I tried to draw it into the clear space, but only succeeded in bringing it to within a foot or so of the side furthest from the fish. There it got hitched in the weed; but the fish being still undisturbed, I awaited further developments. After two or three minutes the fish slowly turned, crossed the pool, and remained motionless for a few moments; then he proceeded straight to the bait; and in a few minutes I had landed a dog-fish between four and five feet long. I did not then know that the dog-fish sought its food mainly or solely by scent (taste); but in any case I do not think in this instance he could have seen the bait, hidden as it was amid the seaweed.

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.

We must now pass on to consider the sense of hearing in animals. That the mammalia have this sense well developed is a matter of familiar observation, and in some of them, such as the horse and the deer, it is exceedingly acute. The form and movements of the external ear also enable many of the mammalia to collect and attend to sounds from special directions. The mammalia possess also the power of tone-discrimination, as is shown by the fact that our domesticated animals recognize different modulations of the human voice, and that wild creatures distinguish tones or noises of different quality. A Newfoundland dog, possessed by a friend of mine, always howled when the tenor D was struck on the piano, or sung. And ThÉophile Gautier reports that one of his cats could not endure the note G, and always put a reproving and silencing paw on the mouth of any one who sang it.

In birds the sense of hearing is not only very sensitive, but the power of discrimination is exceedingly delicate. No one who has watched a thrush listening for worms can doubt that her ear is highly sensitive. The astonishing accuracy with which many birds imitate, not only the song of other birds, but such unwonted sounds as the clink of glasses or the ring of quoits, shows that the delicacy in discrimination has reached a high level of development. In birds, however, the cochlear canal has not the same development that it has in mammals, and there are no arched rods—no organs of Corti.

Nothing special is to be noted concerning the sense of hearing in the reptiles, amphibia, and fishes. In all (with the exception of the lowly lancelet) the auditory organ is developed. We shall, however, presently see reason to question whether the possession of an "auditory organ," with well-developed semicircular canals, necessarily indicates the power of hearing. And Mr. Bateson's recent experiments at Plymouth[EX] seem to indicate that fishes are not so sensitive in this respect as anglers[EY] are wont to believe. "The sound made by pebbles rattling inside an opaque glass tube does not attract or alarm pollack; neither are they affected by the sharp sound made by letting a hanging stone tap against an opaque glass plate standing vertically in the water." Carp at Potsdam are, indeed, said to come to be fed at the sound of a bell. But Mr. Bateson well remarks that this "can scarcely be taken to prove that the sound of the bell was heard by them, unless it be clearly proven that the person about to feed them was hidden from their sight." There is clearly room for further observation and experiment in this matter.

Turning to the invertebrata, we find, even in creatures as low down in the scale of life as jelly-fish, around the margin of the umbrella in certain medusa, simple auditory organs. In some cases they are pits containing otoliths (minute calcareous or other bodies, which are supposed to be set a-dance by the sound-vibrations); in others there is a closed sac with one or more otoliths; in others, again, they are modified tentacles, partially or completely enclosed in a hood. All these are generally regarded as auditory, there being specially modified cells of the nature of hair-cells. We shall see, however, that another interpretation of organs containing otoliths is at any rate possible. For the present, we will follow the usual interpretation, and regard them as auditory.

Vesicular organs containing otoliths are found near the cerebral ganglia in some of the worms and their relations. But the common earthworm, though it appears to be sensitive to sound, does not appear to have any such organs.

Molluscan shell-fish are generally provided with auditory organs. In the fresh-water mussel it is found in the muscular foot. It can be more readily seen in the Cyclas, if the transparent foot of this small mollusc be examined under the microscope. It is a small sac containing an otolith. Mr. Bateson found that the mollusc Anomia "can be made to shut its shell by smearing the finger on the glass of the tank so as to make a creaking sound. The animals shut themselves thus when the object on which they were fixed was hung in the water by a thread." In the snail and its allies the auditory sac is found in close connection with the nerve-collar that surrounds the gullet. In the cuttle-fishes it is found embedded in the cartilage of the head.

Fig. 26.—Antennule of crayfish.

i.j., inner joint; o.j., outer joint; ol., olfactory setÆ; ol'., the same, enlarged; au.op., auditory opening in the basal division, which has been cut open to show au.s., the auditory sac; au.n., auditory nerve branching to the two ridges beset with auditory hairs; au.h., auditory hair, enlarged. (After Howes.)