that you cannot smell alcohol at all. If you fatigue the nose with ammonium sulphide, you will still be able to smell oil of anise, oil of turpentine, oil of lemon and eau de Cologne; but you will be unable to smell sulphuretted hydrogen, hydrochloric acid ( 7 parts of concentrated solution to 50 of water) and bromine (yo% solu- tion). These facts mean that the quality or complex of qualities in tincture of iodine is entirely different from that of oil of lav- ender, but contains or is contained in the alcohol quality or quali- ties. And so with the ammonium sulphide.
The method assumes that the sensitive cells, set in the mucous membrane of the nose, respond some to one quality of scent and some to another, so that when one cell-group is exhausted, others are still fresh. The assumption seems to be justified by the results of experiments upon taste (§ 15): the organs of the two senses are very similar.
Substantives like ' scent,' ' odour,' ' perfume,' ^ bouquet,' and adjectives like ' aromatic,' ' fragrant,' * redolent,' ' savoury,' are either quite general terms (corresponding to * bright,' '■ colour,' 'flavour,' in other sense departments) or refer to objects grouped together for practical purposes (cookery, the toilet, etc.). They do not help us towards a psychological classification of smell qualities.
§ 15. The Quality of Gustatory Sensations. — There are four qualities of taste: sweet, bitter, acid and salt. All the other ' tastes ' of which we speak in everyday life are complex perceptions.
Thus the ' taste ' of lemonade is made up of a sweet taste, an acid taste, a scent (the fragrance of lemon), a sensation of tem- perature and a pricking (cutaneous) sensation. The ' taste ' of lime-water is made up of a weakly sweet taste, a sensation of nausea (common sensation), a sensation of temperature and a biting (cutaneous) sensation. The ' taste ' of tea is made up of a bitter taste, a scent, a temperature sensation and an astringent (cutaneous) sensation. 'Tea tasters' and ^wine tasters* should rather be called * tea ' and ' wine smellers.'
§ 15- The Quality of Gustatory Settsations 55 Sensations of brightness, colour, noise, tone and (prob- ably) smell form unbroken series of qualities. We can pass gradually from black to white, through intermediate shades of grey; we can pass from bass to treble, without any break or interruption of the scale, etc. The sensations of taste, on the contrary, do not constitute a series; * sweet ' is not in any way nearer or more like ' acid ' than it is like *salt.' Each of the four qualities stands out distinctly by itself, so that if we did not know that all four came from the tongue, we might be disposed to think that they belonged to separate senses.
Tastes, however, resemble visual sensations in the fact that they contrast with one another. A red seen upon a bluish green background seems redder than would other- wise be the case; and white seen after black seems more brilliant. So an acid seems more sour after a sweet; and salt and sweet, if applied at the same time to different parts of the tongue, are salter and sweeter than they would be if sensed singly.
Method. — Seat yourself before a concave (enlarging) mirror, and put your tongue out. You will notice that the pink skin of the tip is dotted with redder, darker, and more transparent-looking flecks. These are the papiltce fiingiformes, little folds of mucous membrane, in which are planted the sensitive cells forming the organs of the nerve of taste. Dip fine camel's-hair brushes into the solutions which you wish to test, and apply them carefully to single papillae. The tongue must be dried, to prevent the solution from ' running '; and the nose closed, to prevent any interference of smell with taste proper. You will find that the only sensations obtainable are those of sweet, bitter, acid, and (weakly) salt.
To test contrast, fill the mouth with a sour solution; then spit this out, and fill with a sweet liquid: or brush sweet on one side of the tip of the tongue, and salt on the other. To prove the 56 The Quality of Sensation absence of simultaneous contrast in the domain of smell, place tubes containing different scents in the two nostrils, and sniif. You will either have a single smell throughout the experiment; or the two smells will be sensed alternately, neither affecting the other. It is doubtful whether there is a successive smell contrast. Tones and noises do not contrast with one another when simul- taneously heard; and probably do not, when heard successively. Musical contrast is affective (§ 56), not sensible.
§ 16. The Quality of Cutaneous Sensations. — The skin can be stimulated both mechanically (by pressure, a blow, tickling, etc.) and thermally (by the application to it of hot and cold objects).
( I ) Sensations co7inected zvitJi Mechanical Stimulation. — In the spheres of sight, hearing and smell there are far more sensation qualities than there are names to indicate them. On the other hand, language is rich in names for ' tastes '; but these names indicate, not simple sensation qualities, but mental processes which are really of a complex nature, and arise from the excitation of two or more senses. The mechanical cutaneous sense resembles that of taste in this respect. We are apt to speak of ' sensations ' of touch, resistance, impact, pain, tickling, etc., and to think of them as coming to us exclusively through the skin. In reality, these processes are all mixtures of cutaneous and organic sensations. There is only one quality of the mechanical cutaneous sense: the qimlity oi J^ressnre.
(i) Contact is simply a very light pressure: there is no differ- ence of quahty between the two experiences. (2) Hai'dness and Softness are primarily differences of the intensity of pressure. Often, too, they contain certain of the organic qualities connected with bodily movement, and sometimes qualities of temperature (a 'soft' is either a 'clammy' or a 'warm soft'). (3) Sharpness and Bluntness are primarily differences of the extent of press- § 1 6. The Quality of Cutaneous Sensations 57 ure. Sharpness often contains in it the common quahty of pain. (4) Roughness and Smoothness differ as interrupted and continuous extent of pressure. A full appreciation of either requires move- ment over the rough or smooth surface: if a rough or smooth ob- ject be pressed down upon the passive skin, no difference is sensed until the pressure becomes quite intensive. Then the observer realises that he has in the one case a continuous sensation of uniform intensity (smooth), and in the other a number of severe separate pressures, with or without Hght pressure over the inter- vening spaces (rough). (5) Wetness and Dryness are easily confused, if the conditions of the test allow the skin to remain passive. They are likely to differ in temperature: but we or- dinarily distinguish them by the different resistance which they offer to the moving hand. (6) Resistance is a complex percep- tion, containing organic sensations from muscle, sinew and joint, in various proportions, as well as the cutaneous quality of pressure. (7) Touch is active pressure, i.e., pressure ////i- the organic sensa- tions arising from movement. (8) Impact, if the stimulus is weak, is a sudden pressure, possibly mixed with the organic sensa- tion of tickhng. If the stimulus is strong, other organic sensations make their appearance in the perception. In either case, an emotion (surprise) is usually present. It is clear that these terms are neither all mutually exclusive, nor all sharply defined.
Many of the complexes resolved here into cutaneous and or- ganic qualities also contain, under ordinary circumstances, a vis- ual quality, remembered or imagined. Thus the differentiation of pressure and contact depends not infrequently on the fact that when we are pressed with any degree of force we can imagine what the object is which presses us, whereas contact (very light pressure) carries with it no visual idea of its stimulus.
Analysis, then, justifies our statement that there is only one quality of the mechanical cutaneous cause, the quality oi pressure.
Method. — Go over a portion of the skin very carefully with a pointed pencil of cork or pith. If you press lightly, you will find that there are certain spots from which you get a sharp, well-de- fined pressure sensation, while the intervening spaces are insensi- tive. If you press harder, you will receive more intensive pressure 58 The Quality of Sensation sensations from the '■ pressure-spots,' and a dull, diffused pressure sensation from the intervening spaces, which were insensitive to less severe stimulation.
The * pressure-spots ' occur where a sensory nerve-fibril termi- nates in the ctitis; where there are no nerves, the skin is insensi- tive. The dull sensation arising from intensive pressure upon insensitive areas is due to the spread of stimulation from the point pressed to neighbouring ^pressure-spots.' There is no dif- ference in quality between the ' sharp ' and * dull ' pressure sensa- tions.
(2) Sensations connected with Thermal Stimulation. — Language and scientific observation are here in agreement. Both alike recognise two qualities of the thermal cuta- neous sense: heat and cold. We may lay it down as a general rule that any stimulus whose temperature is above 34° C. (the average natural temperature of the healthy skin) gives rise to a sensation of heat, and that all stimuli below that temperature give rise to sensations of cold.
Method. — Take a hollow metal tube, brought to a sharp point at its lower end. Fill it with hot or cold water, and pass it lightly over a portion of the skin. Move slowly, but be careful not to fatigue the sense-organ. You will find that cold sensations, of very definite extent, flash out in response to the cold stimulus; while the hot point reveals definite spots or areas of the skin which are sensitive to warmth. Sometimes the same cutaneous spot will answer to stimulation both by heat and cold. The inter- vening spaces are insensitive to temperature.
The sensitive areas — ' hot ' and ' cold spots ' — of the skin are always found in the neighbourhood of blood-vessels. Like the pressure-spots, they only occur where a sensory nerve-fibril termi- nates in the attis. The nerve-ending underlying a pressure-spot, however, differs in form from that found beneath a temperature- spot.
The ' internal skin ' of the body, or raucous membrane, appears § I/. Muscular, Tendiiiotts, Articular Sensations 59 to be sensitive to pressure throughout its whole extent, but insen- sitive to temperature from the pharnyx downwards. A sudden draught of cold water is, undoubtedly, sensed internally: but accurate introspection localises the cold sensation not in the stomach, but in the body-wall. That is, the nerve-endings affected are the same as those which are reached by external cold-stimuH. The investigation of the temperature sense is exceedingly diffi- cult, since the skin adapts itself readily to wide differences of out- side temperature. Place the two hands in two bowls of cold water. Gradually heat the water in which the right hand is laid, and cool that in which you have put the left. You can alter the temperature of the two waters very considerably, and the two hands will still 'feel comfortable.' But now take the left hand out of the cooled water, and dip it into the bowl of heated water: the heat will seem so great as to be painful to it, though the right hand still experiences only an agreeable warmth. The two hands have become adapted to different temperatures.
II. Organic Sensatiojis § 17. The Quality of Muscular, Tendinous and Articular Sensations. — In the preceding Sections of this chapter, in which we have been dealing with the sensations of the special senses, we have taken it for granted that a single quality (red, hot, sweet) can be sensed by itself, independ- ently of all other qualities. This is not strictly true; for consciousness is always complex, always consists of more than one process (§ 43). But it is approximately true. We can attend so strongly to one simple impression that the sensations set up by other stimuli, active at the same time, are for all practical purposes non-existent; and we can, further, arrange the external conditions of our obser- vation in such a way as to bring the particular quality into unusual prominence.
6o The Quality of Sensation In the sphere of organic sensation, on the other hand, it is very difficult to detach any single sensation from those which ordinarily accompany it. The simple qualities are here so closely woven together that the psychologist does not even know what to look for, when he begins his analy- sis. And the sense-organs within the body are not sepa- rated as are those upon its surface: muscle and tendon, e.g., pass directly into each other. Nevertheless, careful experiments made during the last few years upon the nor- mal individual, and careful observations of pathological cases (anaesthesia or insensitiveness of particular internal organs), have thrown some light upon the nature of the elementary processes included under the general title of organic sensations.
(i) Miiscidar Sensation. — The 'voluntary' (striped) muscles of the body are supplied with sensory nerves. The fibrils of these nerves terminate among the muscle fibres just as the cutaneous nerve-fibrils terminate beneath a pressure-spot. Muscular contraction occasions a spe- cific sensation, the quality of which is indistinguishable from that of cutaneous pressure. Under ordinary circum- stances, this quality is too weak to be separately sensed.
Method. — Lay the arm comfortably upon a table or arm-rest, and keep it steady. Render a portion of the skin anaesthetic, by cocaine injection, ether spray, etc. Now press hard upon the skin, so as to flatten the underlying muscle; or stimulate the muscle electrically by a strong induction current, so that it contracts. You will have a faint, dull sensation, the seat of which appears to be the muscle.
When you are aroused from a dream by fright, you will notice a pressure in the region of the heart. This pressure is due to an abnormal excitability (hyperaesthesia) of the sensory nerves which terminate in the cardiac muscle.
Slight muscular fatigue also brings out the specific pressure quality of the muscular sensation.
(2) Tendinous Sensatio7t. — Like the muscles to which they are attached, the tendons are supplied with sensory nerves. The nerve-endings in tendon, however, are dif- ferent in form from those in muscle or skin. The specific quality of tendinous sensation is not the quality of press- ure, but that of tension or strain.
Method. — Lay your arm and hand, palm upwards, upon a table. Place a small ball, or other round object, in the palm, and close the fingers lightly round it. Note carefully the sen- sations which you are receiving from hand and arm. Now grasp the ball as tightly with the fingers as you can. You obtain, almost immediately, a new sensation, that of strain. This sensa- tion quality is different from any skin sensation (pressure, heat, cold) and from the muscular sensation observed in the preceding experiment.
The new quality might, however, proceed from the joints, since in curling your fingers over the ball you have altered the mutual pressure of various articular surfaces. You may easily assure yourself that it does not. Let your arm hang down loosely by your side. Attach a fairly heavy weight by a string to the fore- finger. The weight pulls the surfaces of the elbow and other joints apart; so that there is no pressure or friction of one sur- face against another. But you soon get the sensation of strain throughout the arm.
If the sensation of strain is different from any sensation obtain- able from skin or muscle, and is independent of stimulation of the joints, it must come from the tendons.
(3) Articular Sensation. — The surfaces of the joints are richly supplied with sensory nerves, whose endings resemble certain of those found in the skin. The quality of articular sensation, like that of muscular, is not distin- guishable from pressure. But the articular sensation is 62 The Quality of Sensation far more important than the muscular, since (as we shall see later, § 46) it is one of the two principal sources from which we obtain knowledge of the position or movement of the limbs.
Method.— -T'lQ. a moderately heavy weight by a string to the forefinger of the right hand. Lay a soft cushion on the floor, so that the striking of the weight upon it will not be heard. Close the eyes and lower the hand quickly, till the weight rests upon the cushion. At the moment of striking, you experience a push up- ward, as if the string had become rigid and were thrust against you. The push is localised within the arm. It is due to the spring back of the lower against the upper articular surfaces, which takes place when the limb is released from the pull of the weight. It can only be described as a pressure sensation.
§ 18. The Quality of the Alimentary Sensations. — Here we seem to find at least three new qualities: those of hunger, thirst and nausea. Each of these experiences is complex; but each appears to contain, in addition to sen- sations of pressure and temperature, a specific quality.
(i) Hunger is localised in the stomach. When the stomach has digested a mass of food, its walls begin to dry, and fall into folds or ridges. The dryness and folding somehow stimulate the nerve- endings in the mucous membrane of the stomach. There then arises the organic sensation of hunger. (2) Thirst is localised in the mouth and pharnyx. A dryness of the mucous membrane in this region somehow stimulates the nerve-endings. We do not know precisely in what way the thirst sensation is set up. It can be re- moved by painting the back of the mouth with a weak solution of citric acid. (3) The specific quality of nausea seems to be due to the pressure upon the nerve-endings in the oesophagus which occurs during the first stages of the vomiting reflex. It may be that this quality is that of pressure {cf. joint and muscle). Nausea usually contains sensations of taste, smell and giddiness; and its compo- nents are so intimately blended that analysis is exceedingly difficult.
§ 20. The Quality of the Static Sense 63 § 19. The Quality of the Circulatory, Respiratory and Sex- ual Sensations. — ( i ) Tickling, itching, tingling, pins and needles, feverishness, etc., are not simple sensation quali- ties, but complexes, made up of sensations of cutaneous pressure, sensations of temperature, and organic sensations called forth by alteration in the circulation of the blood. It is doubtful whether these organic sensations are novel qual- ities. (2) The action of the lungs, like that of the heart, does not normally excite sensation. But in the complex per- ceptions of freshness and closeness of the atmosphere, of breathlessness, of a 'bracing' air, in the 'feelmg' of suffo- cation, etc., the stimulation of nerve-endings within the lungs themselves apparently arouses a true respiratory sensation. (3) The sex organs furnish a specific sensation quality. They are also sensitive to pressure and temperature.
Tickling can be produced by moving a pencil, lightly or inter- mittently, over the palm of the hand; tingling by keeping the legs crossed at the knee until the upper one ' goes to sleep,' and then uncrossing. It is noteworthy that tingling can also be occa- sioned by a jar upon a nerve trunk, as is shown by the effect of a blow upon the elbow (' funny bone ').
§ 20. The duality of the Static Sense. — The internal ear consists of the cochlea (§ 13), the vestibule and the semi- circular canals. The vestibule is a membra- nous bag filled with endolymph. The ca- nals — also composed of membrane, and filled with endolymph ^ig. 4. - Diagram (schematic) of the internal ear, in longitudinal section, a, semicircu- are three semicn- lar canals; (^, cochlea; <r, basilar membrane; cular tubes, arranged d, vestibule.
64 The Quality of Sensation in the three planes of space. All three structures are con- tinuous; but the two last constitute a special organ, dis- tinct in function from the cochlea. They possess a special nerve, whose fibrils terminate in hair-cells, planted in the walls of the vestibule and at the bases of the canals.
There is good evidence for the hypothesis that the canals and vestibule constitute an apparatus which assists us to maintain our equilibrium and to estimate correctly our position in space. As we move our head or body in different directions, the endolymph washes against differ- ent groups of hair-cells, and the nerve-fibrils are thus stim- ulated. Ordinarily, the action of the apparatus is not attended by sensation {cf. heart and lungs); but if equi- librium is seriously disturbed, or our position in space abnormal, we receive from the vestibular nerve-endings the organic sensation of giddiness.^ Method. — Twirl round quickly upon your heels for a few seconds. When you stop, your equilibrium is uncertain, you stagger; and at the same time you ' feel dizzy.' Or place yourself in an abnormal position, upon your head, upon a narrow plank over a deep ravine, etc. Again you have the organic sensation of giddiness.
The evidence for the view that giddiness proceeds from the in- ternal ear is derived from three sources, (i) Psychological experi- ments have been made by the aid of the ' tilt-board ' and 'rotation table.' These are beds or tables, upon which the subject is laid at full length. The tilt-board can be slanted, see-saw fashion, in either direction, so that the subject's head or feet may be raised or lowered through any angle. The rotation table can be revolved, at any angle of inclination. The subject is tilted, or rotated, with eyes shut; and, this done, is required to open his eyes and judge of the position of certain objects shown in the field of vision. The results of such experiments point to the existence of a special 1 For another possible quality of the static sense cf. § 48.
mechanism, situated in the head, which subserves equihbration, etc. (2) Pathological observation helps us to determine what this organ is, and to bring it into connection with the sensation of giddiness. If patients with damaged vestibule and canals are twirled upon the rotation table or in a rotating chair, they neither become giddy, nor make any allowance for centrifugal force, as the normal subject does. (3) Vivisection confirms pathology. An animal whose canals have been cut cannot maintain its equi- librium.
It might be thought that as all three parts of the internal ear are continuous in structure, and as the mode of excitation of nerve-end- ings in vestibule and canals is the same as that in the cochlea, — in all cases it is the impact of water upon the hairs of sensitive cells, — the static sense should be numbered, with audition, among the special senses. But although the mode of excitation is the same, the cochlear stimulus — an air-wave — is externally origi- nated, while the stimulus to giddiness is internal, a change in the organ itself.
III. Common Sensation § 21. Pain. — Excessive stimulation of any sense-organ, or direct injury to any sensory nerve, occasions the common sensation of pain. A concrete pain, such as is excited by a dazzling light or a cut of the finger, contains three distinct factors: a sensation of special sense or organic sensation, the common sensation of pain, and a severe unpleasantness (§31). In extreme pain, the first of these factors is far out- weighed by the other two. But we always know that it is the finger which is cut, a tooth that is aching, the alimentary canal which is giving us colic pains, etc.; and this know- ledge of locality comes from a sensation of special sense or specific organic quality.
All the sensations and sensation complexes described above under the headings of organic and common sensations, together F 66 The Quality of Sensation with certain of the sensations of special sense (temperature, e.g.), were formerly called common sensations. It was beheved that they could be occasioned by the stimulation of any, or at least of more than one, group of sensory nerves. They were looked upon, that is, as ' common ' to several different sense departments. Our discussion shows that there is only one common sensation, in the strict meaning of the word, — pain. Hence it is best to speak of giddiness, strain, etc., as organic sensations.
Pressure comes very near being a common sensation, since it is producible by stimulation of nerve-endings in skin and mucous membrane, in striped muscle and in joint. We may regard press- ure and pain as the most primitive sensations of the organism, the first to appear in the course of mental evolution.
It must be noted that the existence of a common sensation does not invalidate our general definition of sensation. Pain is con- nected with excessive stimulation of any bodily sense-organ; but a particular pain is always seated in some definite organ.
There are many other names for complexes of organic and com- mon sensations, besides those noticed in the text. But there are no other specific sensation qualities. Fatigue, drowsiness, health, discomfort, etc., can easily be resolved (so far as they consist of sensations, § 32) into factors already mentioned.
Method. — It is difficult to assure oneself of the qualitative simi- larity of pains set up in different sense departments, because the presence of pain is extremely unfavourable to introspection. But the following method may be successfully employed. Press a blunt rod down upon your chest, until the pressure becomes pain- ful, and let an assistant, when you give the word, sound a painfully shrill tone upon a piston whistle. After a few trials you will be able to introspect well enough to convince yourself that the two pains have the same quality.
§ 22. The Total Number of Elementary Sensations. — Putting together the results of the foregoing Sections, wq obtain the following list of sensation qualities: § 22. Total NiLJuber of Elementary Sensations 6y Ear (audition) Nose.. Tongue Skin.. (Muscle. Tendon.
?
4 3 I) I I) Alimentary canal Blood-vessels Lungs...Sex organs Ear (static sense) All organs (pain) I? I I I More than 42,415 Each one of these forty thousand qualities is a conscious element, distinct from all the rest, and altogether simple and unanalysable. Each one may be blended or con- nected with others in various ways, to form perceptions and ideas. A large part of psychology is taken up with the determination of the laws and conditions which govern the formation of these sensation complexes.
The above list represents the full resources of the normal mind. It must not be supposed, however, that every normal individual has had experience of all the qualities enumerated. It is safe to say that no one, not even the most experienced psychologist, has seen all the possible visual qualities, heard all the possible tones, smelled all the possible scents, etc. The list is a summary of the results obtained by many observers in the course of minute inves- tigations of our capacity of discrimination in the various fields of sense.
Apart from this, a slight abnormality is much more common than is ordinarily supposed. Very many people are more or less colour-blind; they confuse red with green, or blue with yellow, or have a shortened spectrum, i.e., do not see the full number of red and violet qualities. Very many are partially tone-deaf, deaf to very deep or very high tones. Very many have a defective sense of smell, etc.
But when all allowances are made, the average number of con- scious elements must run into the tens of thousands. And the permutations and combinations even of 10,000 elements would give a very large stock of ideas.
CHAPTER IV The Intensity, Extent and Duration of Sensation