The sensory endings most frequently stimulated are those lying in the diaphragm and other respiratory mechanisms, the heart and other circulatory mechanisms, the external peritoneum, the stomach and the entrance to the alimentary canal, the soft palate, and finally in those structures connected with sex and with the elimination of body waste. Probably the great majority of the afferent pain endings in the internal structures are never called upon to function in a normal individual.4 They become functional in disease; for example, the passage of gall stones, infections, etc. It should be mentioned that pain sensitivity seems to be lacking in the heart, the arteries and veins, in the spleen, pancreas, kidneys and lymphatic glands. The function- ing of the organic sense organ proceeds oftentimes without involv- ing language. We mean by this merely that if the subject is ques- tioned about what processes are going on, he finds it impossible or next to impossible to make any kind of a serviceable observation. It is true that there is a certain amount of language activity con- nected with their functioning; for example, people say that they are hungry, thirsty, have pain or colic. But every one must admit that the organic sensory motor processes are but poorly integrated with speech functions. On account of the wide dis- tribution of the sense organ structures and their difficulty of access, detailed experimental work is all but impossible. Some success has been obtained by indirect methods, by the swallow- 4 The pain endings in the periosteum of the bone and in the meninges of the brain and cord are most conveniently classified under the organic sense.
5 66 PSYCHOLOGY ing of rubber balloons which can then be*filled with warm or cold water, by the stimulation of structures during operations, and by conditioned reflex methods.
Organic Stimuli. — Notwithstanding the lack of complex language habits connected with organic impulses, one can see the result of their functioning in the clearest manner: in thirst, which is initiated by dryness of the soft palate; in hunger, initiated by the rhythmical muscular contractions of the stomach; in defe- cation, initiated by the pressure of the faeces upon the muscular walls of the large intestine; in micturition, initiated by the pres- sure of the urine upon the sphincters of the bladder; in sex activity, initiated, in part at least, by the pressure of the seminal fluid; in the pain reflexes due to internal pressure, infections, etc.; in hiccoughing, vomiting, etc., the stimuli to which are varied. As long as the organic impulses are normally aroused and the vegetative reflexes occur in an orderly way, the individual is said to have a good organic tone.
Many of the activities initiated by organic impulses are rhythmical in function, such as the heart-beat, respiration, hun- ger, the eliminative functions and sex activity. We see in the organic reflexes a possible basis for a time "sense." In highly complex animal communities one animal will do a definite thing, such as go for food, spell his mate upon the nest, at fairly regu- lar intervals. The same mechanisms are functioning in man when, fairly regularly even in the absence of a watch but guided by the rhythmical contractions of the stomach muscles, he drops his occupation and goes for food. The human being is more de- pendent upon these rhythms than he usually admits. Students become restless if held over the hour; guests become troublesome and distrait if dinner is delayed too long beyond their customary hour of dining; infants taught to feed at two-hour intervals awaken almost on the moment and cry lustily if food is not forthcoming.
The glands, so far as location is concerned, belong to the area in which organic impulses arise. That afferent or sensory nerves end in the glands is reasonably certain, but what the func- tion of such impulses is does not seem to be known. They are THE RECEPTORS AND THEIR STIMULI 67 possibly regulatory of the gland itself. Under the influence of autonomic motor impulses, the glands secrete, but these secre- tions have a wide general distribution. Such secretions and their functions are discussed in detail further on.
V. SENSE OF TASTE.
Area. — The sense of taste as a whole has been very well worked out, thanks largely to the work of Kiesow. The bodily areas sensitive to taste are much larger than is generally sup- posed, and relatively much larger in children than in adults. The taste buds, which are the organs of taste, are found dis- tributed fairly densely over the tip, sides and edges of the tongue. The median part of the dorsum of the tongue is lacking in taste buds in adults. The portion of the palate that lies above the uvula contains these structures, as do the anterior pillars of the fauces. Some are found in a portion of the posterior wall of the pharynx and in a portion of the epiglottis and of the larynx. They are lacking in the lips, hard FIG. 9.— Taste bud from the side , n wall of the circumvallate papilla of the palate, UVUla, tonsils, Cheeks and tongue: a, taste pore; b, nerve fiber „ „,, _. entering taste bud. (Herrick, "IntrolOWer SUrtace OI the tongue, and in duction to Neurology," W.B.Saunders Company.)
the gums.
Organs of Taste. — Each taste bud (Fig. 9) consists of a large number of modified epithelial cells constituting a pear-shaped organ 73/t to 8 1//, in height and about 40/x, in width. In addition to the sensory cells supporting cells are present. Each sensory cell is supplied with taste hairs. The whole structure is beaker or barrel shaped. A small pore, opening directly upon the surface, serves for the reception of the taste solutions. The nerve fibers are distributed directly to the taste buds. While the buds are the true organs of taste (corresponding to the hairs and corpuscles in the skin) one rarely finds them isolated upon the surface of the tongue, but grouped together around a so-called papilla.
68 PSYCHOLOGY 68 PSYCHOLOGY -Fig. 10 shows the tongue and its papillae.. With the exception of the 7 to 12 large circumvallate papillae at the base of the tongue forming the lingual V, the only other form of papilla having a taste function is the fungif orm. The latter are extremely numer- ous as can be verified by drying the tongue with a linen cloth Flo. 10. — Papillary surface of the tongue. 1, Ciroumvallate papillae forming the lingual V. 3, the fungiform papillae; 4, filiform papillee. (Modified from Sappey.)
and observing them in a mirror. The fine filiform and conical papillae serve merely to roughen the tongue for the better manip- ulation of food objects. About 400 taste buds lie in each of the circumvallate papillae, while a variable but much smaller number are attached to the fungiform.
It must be remembered that the tongue and oral cavity com- prise not only a taste organ, but a cutaneous and kinassthetic area THE RECEPTORS AND THEIR STIMULI 69 as well. Furthermore, taste as a whole functions in close con- nection with smell; consequently care must be exerted in making tests upon taste. In general the test fluids must be warmed to body temperature; they must be weak enough to avoid causing reflex tongue movement which would involve kinsesthetic factors; the nose must be tightly plugged up; finally, the individual pa- pilla must be stimulated by the fluid in such a way that the stim- ulus cannot spread and thereby arouse contact impulses. The fluid can best be applied by the use of a small camel 's-hair brush. When these precautions have been taken it is generally agreed that there are four separate sense organs: one responsive to sweet substances, one to bitter, one to salt and one to acid. Experi- menters in the field of taste have used the conditioned reflex method, but more largely the verbal report method.
When the tongue as a whole is investigated it is found that sensitivity to sweet substances is greatest at the tip and least at the base. The maximum sensitivity to bitter substances is in the region of the circumvallate papillae. Sensitivity to acid is greatest at the median part of the border. Finally, the sensitivity to salt is maximal at the apex and margin and minimal at the base. Apparently the stimuli to the taste buds have to be ade- quate. The mechanical arousal of the taste bud by electrical, thermal and contact stimuli has not been confirmed. An ade- quate stimulus is afforded when the underneath portion of the tongue is smartly tapped. The subject reports the presence of the stimulus of salt. This is due to the sudden pressure on the capillaries and the consequent release of small quantities of their contents.
The individual taste bud cannot be stimulated like the cold and warm spots, but if a large group of papilla? is gone over with the four taste stimuli, it is found that not all of them react to the four solutions. Kiesow examined carefully 39 papilla?. Four were insensitive to all four substances, the other 35 reacted as follows: 18 to salt 3 to salt exclusively; 26 to sweet 7 to sweet exclusively; 18 to acid 3 to acid exclusively; 13 to bitter 0 <to bitter exclusively.
70 PSYCHOLOGY Certain substances have the power of throwing taste buds out of gear. Gymnenic acid when applied to taste buds abolishes first, sensitivity of organs to sweet, then to bitter and finally to salt. It apparently does not affect the organs sensitive to acid. The gymnenic acid has no effect upon the tactile, temperature or pain sensibility of the tongue. Cocaine affects the cutaneous sensi- tivity of the tongue and finally gustatory sensitivity.
Some investigation has been undertaken for the purpose of obtaining taste contrasts and compensations. Distilled water apparently affects the organs sensitive to sweet after weak solu- tions of hydrochloric acid, caustic soda or potassium chloride have been used in the mouth. The use of a solution of sodium chloride and quinine apparently sensitizes the organs responsive to sweet substances to such an extent that a subliminal sugar solu- tion will produce a reaction. There is no good evidence at present for asserting that the presence of one taste solution will neutralize the effects of the others.
Many experiments have been undertaken for the purpose of discovering what peculiar chemical property substances must possess before they can serve as adequate taste stimuli. So far dependable results have not been obtained.
VI. OLFACTORY SENSE.
Area. — The area comprising the olfactory sense proper is very small. It consists of a small saddle-shaped membrane lining the roof and sides of each nasal cavity. The total area (right and left) sensitive to olfactory stimuli is about 5 square centimeters.
Stimuli. — The adequate stimuli to the sense of smell are gase- ous particles coming into direct contact with the olfactory mem- brane. Not all gaseous particles will produce olfactory response. The total number which will is unknown, but is very large. Many objects, such as arsenic, resins and metals, are non-volatile at ordinary temperatures, but they give off particles sufficiently fine to affect the end organs when heated. The question as to whether solutions containing known olfactory stimuli can arouse olfactory reactions when they come into direct contact with the olfactory membrane is not so well settled. According to more 1 THE RECEPTORS AND THEIR STIMULI 71 recent experimentation, it can be asserted that fluids containing odoriferous substances brought into direct contact with the olfac- tory membrane can serve as adequate stimuli. In making such experiments great care has to be exerted to keep bubbles of air from excluding the fluid from contact with the membrane. This seems to be in line with studies in comparative psychology, since it has been shown that certain fish even when blind are stimulated by distant food substances.
The membrane can possibly be stimulated inadequately by an electrical current, but there is no general agreement to this. Heat, cold and mechanical irritation seem not to affect it. At present we can say little about the chemical properties which a substance must have in order to affect the olfactory membrane. On the physical side, solubility in liquid and in gaseous air seem to be correlated with the property of arousing the membrane. Ability to arouse the membrane is possibly also to be correlated with the coefficient of absorption of heat rays.
When all has been said we know little about the nature and number of adequate olfactory stimuli. The various odoriferous substances have been classified with reference to the general similarity of response produced by them. This classification, which is given below, has little to recommend it: Class 1 — fruit odors — fruits, wine, ethers, beeswax; Class 2 — aromatic odors — spices, camiphor, cloves, ginger, anise; Class 3 — flower odors — flowers, vanilla; Class 4 — musk odors — amber, musk; Class 5 — leek odors — chlorine, iodine, hydrogen carbide, asafcetida; Class 6 — burned odors — roast coffee, tobacco smoke, creosote; Class 7 — hircine odors — caproic acid, cheese, sweat; Class 8 — foul odors — opium, laudanum, bugs; Class 9 — nauseous odors — carrion flowers, faeces.
The phenomena of fatigue and adaptation are most interesting in this field. The student of chemistry or anatomy ceases in a short time to react to the numerous odors surrounding him. Indi- viduals in large audiences housed in poorly ventilated rooms be- come adapted to the various odors and perfumes. Individuals just coming in react strongly to this situation. In the laboratory it is 72 PSYCHOLOGY possible to produce with more or less rabidity adaptation to any particular olfactory substance.
The perfumers have learned the art of combining smell stim- uli so as to produce what from a reaction standpoint are entirely new olfactory stimuli. The olfactory organ is undoubtedly peculiar in this respect. The perfumers have worked mainly with the first four classes. Experiments in the laboratory show that combinations of stimuli can be made in a similar way from any of the classes.
Smell compensation or physiological cancellation of stimuli has been sought for in the laboratory. In daily life there seems to be such a principle at work. Carbolic acid is used in the operating room, balsam of Peru is used to offset the effect of iodofonn. Creolin is used to counteract the smell in public toilets, tar to offset the odor of ozcena. Just what the cancellations mean psychologically is not known. The fact remains that we constantly use one odor to drown the stimulating value of another odor.
One of the most discussed problems in the recent war was the question as to how to cancel the smell effect of a lethal gas, or to impart to it the smell of a beneficent gas. This was desirable, for if the enemy could be induced to leave off his mask for even a short time, its deadly effect would have' been accomplished before the mask could be donned. Zwaardemaker maintains that certain smells can be made to cancel completely; that is, the stimuli can be introduced in such intensities by means of an olfactometer that no smell reaction can be obtained. He main- tains that the following cancel when the intensity relations are properly adjusted: cedar wood and rubber; benzoin and rubber; paraffin and rubber; rubber and wax; rubber and balsam of tolu; paraffin and wax. Complete cancellations of these kinds are so infrequently met with in nature that they play little role in the normal life of man.
The Smell Sense in Conjunction with Touch and Tempera- ture.—It is well to remember that many taste stimuli are at the same time smell stimuli. All of the delicate differential reac- tions that the human makes to wines, meats and viands of any THE RECEPTORS AND THEIR STIMULI 73 kind are made largely on the basis of the olfactory sense. Fur- thermore, cutaneous nerves are distributed to the nasal cavities and actually to the olfactory membrane itself. Even the anosmic reacts strongly to ammonia, ethers and many other substances when they are placed in the respiratory field. Hence we must consider that in many if not all cases a so-called olfactory stim- ulus is at the same time a tactual, or even a tactual and kin- aesthetic stimulus.
Structure of the Olfactory Organ. — As has been stated above, FIG. 11. — Ending of olfactory and other nerves in the outer wall of the nasal cavity. 1, Shows distribution of olfactory nerve (after Sappey).
the olfactory area is quite small. It is situated at the top of the nasal cavity with extension on the sides. It is out of the way of the regular respiratory tract or breathing passages. The streams of inspired and expired air pass just beneath it in both inspiration and expiration. No odoriferous substance will produce an olfac- tory response if the subject is prevented from breathing it. Or put it another way: in order to produce- an olfactory response, the odoriferous substance must be placed in the field from which the air is inspired. It is generally supposed that gaseous particles are given off from the stream of inspired or expired air which 74 PSYCHOLOGY by diffusion reach and stimulate the olf actoBgr sense organ. Fig. 11 shows in a general way the location of the membrane and its relation to the nasal cavity as a whole.
The structure of the individual olfactory elements in the mem- brane is rather different from that found in the sense organs in the skin. In the skin we find the nerve fibers ending around highly modified epithelial cells, the sense organ itself being this modified, non-nervous structure. Fig. 12 shows the olfactory cell or individual sense organ. The cell body is bipolar and lies in Olfactory hairs Supporting cells Olfactory ^±T cells Central \ processes of olfactory cells .Olfactory hairs Peripheral "process .,. Body of > +V cell with nucleus Central proceM C Flo. 12. — Cells from the olfactory mucous membrane. A, From the frog, B and C from man. The olfactory cells which are nerve cells are supplied with hairlets. The cells in between the olfactory cells are non-nervous supporting cells. The central process is really an axone which ends in the olfactory bulb of the brain. (From Herrick's ''Intro- duction to Neurology," W. B. Saunders Co.)
the membrane itself. The peripheral process of each cell con- sists of a number of hair-like structures which project into and slightly beyond the membrane. The other end of the cell gives rise to the nerve fiber (axone, page 114), which can be traced upward through the sponge-like bone to end around cells situ- ated in the olfactory bulb. The olfactory bulb is shown upon the ventral surface of the brain in Fig. 30, page 127.
THE RECEPTORS AND THEIR STIMULI 75 VII. AUDITION.
The Physical Side of Hearing. — Before entering upon the nature of auditory stimulation and the reactions such stimula- tions call forth, we may profitably glance for a moment at the physical nature of sounding bodies. We find first that some elastic bodies such as steel bars and tuning forks impart to the air when struck a simple pendular or sinusoidal wave motion, equally spaced waves of condensation and rarefaction. Depend- ing upon the length and the structure of such elastic bodies, and the energy with which they are actuated, we may have waves varying in frequency or length and in amplitude through a very wide range. Most elastic bodies, those, for example, used in musical instruments such as the string, vibrate when actuated not only as a whole but in parts as well. The wave motion im- parted to the air by such bodies becomes very complex. In such cases we speak usually of the lowest vibration rate given out by the body as its fundamental vibration (or note) and of the other frequencies as its partials. If we have a string vibrating as a whole 100 times per second, experiment will show that it vibrates in one-half its length one-third, one-fourth and one-fifth, etc., at the same time; so that a stretched string when plucked really gives a very complex stimulation. A large bank of resonators arranged so that each unit will} signal when its particular fre- quency appears: as a component part of a complex wave enables us to record the total set of vibrations which any given musical instrument will give out when any particular note is struck upon it. By this means it is possible to state with some degree of accu- racy the different sets of frequencies set up by two human voices sounding the same musical note. Keyed and fretted instruments differ greatly as regards the complex of vibration rates which they give out. This is the reason why our reactions are different, for example, when middle C is struck on the piano, the flute, the organ or the cornet; they all have the same fundamental vibra- tions, but they vary enough in their partials for us to learn to react differently to them. We can name the instrument from which it comes, or make one type of reaction to it when it appears on the piano and another type when it appears on the violin.
76 PSYCHOLOGY Such stimuli we call tonal. Attention is calfed to the fact that a simple tonal stimulus, such, for example, as would be offered by a note of 512 d.v. on the tuning fork, could differ in amplitude and in duration, but not in frequency. All tonal stimuli, how- ever, in daily life are complex, and the stimulus as a whole in- duces the reaction. The farmer drops work and goes to his food when the dinner horn blows; the mother gets up at night when her baby cries. Only in the laboratories and in the science of music are tonal stimuli controlled in the way we treat them below.
A very different type of vibration from those described above is set up by the tearing of paper or by the dragging of a chair along the floor. Here the elastic bodies do not impart an orderly timed disturbance to the air particles, nor do the tonal constitu- ents, which are undoubtedly present, endure oftentimes for more than a fraction of a second. The physical tracings of the air waves set up by such stimuli lack periodicity and regularity. Such bodies are said to give out aperiodic vibrations. We can conveniently group all such stimuli under the general term noise stimuli.
The Adequate Stimulus to Auditory Response. — The ade- quate stimulus for the excitation of the ear is ordinarily the air wave set up by the vibrations of elastic bodies such as a stretched string, a tuning fork or the human voice. In addition to the air waves which produce their effect upon the fluids of the inner ear by means of the chain of ossicles to be described below, the bones of the skull when placed in direct contact with the sound- ing body can also transmit the vibrations to the fluids and can arouse an auditory reaction. This can be tested by actuating a tuning fork and placing it between the teeth of the subject. We shall for reasons mentioned further on state that it is best to say that the stimulus to an auditory reaction is a wave motion (how- ever produced) in the fluids of the inner ear. In general we may say that such a motion may be imparted to this fluid by (1) air waves generated by the to and fro movement of elastic bodies, (2) by conduction through the bones, (3) by spasmodic or reflex movements of the tensor tympani and by the stapedius muscles — • two small muscles belonging to the middle ear structure, and (4) THE RECEPTORS AND THEIR STIMULI 77 possibly through congestion of any of the membranes of the ear, and (5) possibly through the clicking of the bones of the middle ear as discussed below in Helmholtz's theory of combination tones.
Beats. — Two forks having related frequencies, for example, one of 512 d.v. and the other 511 d.v., wrhen struck simultaneously offer a peculiar type of auditory stimulation. There is first a slow increase in the intensity of the stimulus, then a decrease in the intensity followed once each second by a momentary period of total absence of stimulation (this absence of stimulation is theoretical, since even though the two fundamental notes are in opposite phase and hence cancelled, the upper partials are not cancelled). The ear becomes very sensitive to such fluctuations in intensity. When the beats become very rapid they arouse antagonistic or avoiding reactions which appear when a per- former plays a mistuned interval.5 Reactions to Tonal Stimuli. — If we investigate the behavior of any individual when stimulated by simple periodic vibrations such as are afforded by a large set of tuning forks, we find that sensitivity to this form of stimulation begins at approximately 40 single vibrations per second and ends at 40,000. We often find this range shortened at one or both ends. W^ith advancing age, the range is almost always shortened at the upper end. We find, furthermore, great sensitivity to even slight differences in vibration frequencies. If one sets up a conditioned reflex (page 35) to the tone 512 d.v., any tone slightly greater or less will set off the reflex; but after training one finds that the tone 515 d.v. will not set it off, nor will one of 509 d.v. arouse it. By decreas- ing1 the vibration difference we can approach the differential threshold (D. L.). It has been affirmed that the differential threshold obtained by the conditioned reflex method does not differ markedly from that obtained by the verbal report method (we have not yet confirmed this). By the latter method a differ- ence of less than one-third of a vibration has been reported. This value, however, depends greatly upon what region in the scale we s It will be noted that so far as a mathematical calculation is concerned, the formula for determining- the frequency of beats is the same as that for determining the difference tones, page 78.
78 PSYCHOLOGY are working with. With musically untrained individuals the difference is very much greater, and with the so-called tone-deaf it is very much greater still. Occasionally upon making investiga- tions upon individuals with defective hearing one finds that they cannot respond to a given tone or group of neighboring tones, but that they respond normally to vibration rates of greater or less frequency.
Combination Tones. — One of the remarkable things noticed when a subject is stimulated by two simple ("pure") tones simul- taneously is the fact that he responds really to three tones (or more). If the tone of 1328 d.v. and the tone 1024 d.v. are sounded in the ear of an individual and he is asked to strike the forks which have been used in the stimulus, he will strike not only 1328 and 1024 but 304 as well, the " difference" tone. If he is musically trained, he may strike several others, for example, 720, 416, etc. There is a general law which shows these relations. If we let u stand for the highest frequency and I stand for the lowest frequency and D be used to designate the frequency which is not physically demonstrable, then Some investigators have reported another type of tone appearing in the stimulus complex, namely the so-called summation tone whose frequency with reference to the two primary or generating tones is u + I- It is doubtful whether this tone is present.
Theory of Origin of Combination Tones. — Just how do these tones arise? Helmholtz believed that when the middle ear is forced to respond to two primary tones simultaneously there is an asymmetrical movement of the ossicles resulting in a click which can be shown mathematically to have the frequencies called for in the observed facts. This periodic clicking of the bones be- comes a part of the combined wave motion imparted to the fluids of the inner ear. Thus, although there is no elastic outside body (or may not be) vibrating with the frequencies corresponding to the combination tones, nevertheless such waves are imparted to the middle ear. We shall find a similar case when we study the THE RECEPTORS AND THEIR STIMULI 79 eye: there the sense organ itself when stimulated in a certain way contributes a part of the stimulus which finally acts upon the sensory nerve endings (simultaneous contrast, page 111).
Reactions to Noise Stimuli. — The various reactions to noise stimuli have not received any great amount of study. Any tonal stimulus interrupted before two complete vibrations have been transmitted to the fluids of the ear is reacted to as a noise. Popu- lar language contains many words characterizing noise stimuli such as hiss, murmur, sigh, boom, bang, rumble, crash, etc. It is probable that noise stimuli are more potent arousers of emotional reaction than are tonal stimuli (page 199). Phylogenetically at any rate, sensitivity to differences in vibration frequency comes very late. We know from every-day life that noises have a tre- mendous significance in human behavior and that complicated reaction systems develop around noise stimuli. This is shown most clearly in the avoiding of automobiles and cars. Echoes and other sound reflections play a part in our responses, especially when vision is cut off. A great many stories have been written around blind detectives, and while the portrayed behavior is exaggerated, it is not without some foundation in fact. A mother has not the slightest difficulty after the first few days in approach- ing in the dark her own crying infant and picking it out from a large number of other crying children in the nursery. Differential sensitivity as well as liminal sensitivity grow very acute when an individual's occupation demands that he react to a world of noises. We cite the examples of the hunter who can by their cries name the various animals in the forest, and the Indian's delicate attunement and sensitivity to the slightest noise. Noises are the stimuli which are most important in daily life. Tonal stimuli are of significance mainly in the realm of music.