Paradoxical Arousal of Cold Spots. — In areal stimulation another factor enters in when the temperature of the object lies between 45° to 50° C. If we select and mark out on the skin a certain number of cold spots and then stimulate each by a metal point at this temperature, a reaction to cold will be obtained. In other words, temperatures from 45° to 50° C. are ("inade- quate") stimuli for the cold spots. Thisi means evidently that when large areas are stimulated with objects possessing such temperatures, both warm and cold spots are functioning. In other words, our general reactions to such temperatures are due 54 PSYCHOLOGY to a complex stimulation, viz., the adequate oy normal stimulation of the warm spots and the inadequate or paradoxical stimulation of the cold.
3. The Pressure Sense. — The stimulus to the pressure sense is the deformation of the skin surface. In daily life objects such as wood, metal, jets of air or other gases, fluids, mechanical im- pacts upon the surface of the skin, pulling, wrinkling of the skin, touching a hair, etc., all produce deformation of the skin and hence serve as pressure stimuli. The best method for investi- gating this sense organ is to stimulate a small area with a series of hairs or bristles of varying length and thickness.2 If an area is gone over in a uniform way, as was suggested above for the temperature sense, it will be found that spots appear at which reactions may be obtained. It will be noted, further, that these spots are distinct from those of temperature. Distinctions should be made between hairy and non-hairy areas. Movement of the hair itself serves as a pressure stimulation. If the hair is shaved off and the position of each hair is marked and the area then gone over in a punctiform way, it will be found that there is a pressure spot on the skin to the windward of each hair (the hairs enter the skin at an angle). Every part of the skin is sup- plied with pressure spots with few exceptions. The cornea is said to contain no pressure spots. The parts most richly supplied with pressure spots are the tip of the tongue, the red part of the lips and the finger tips. Not only are the spots more numer- ous here, but their thresholds are lowest. The average number of spots per square centimeter is about 25, but they may drop to as low as 7 or go as high as 300.
4. The Pain Sense. — Any object which will prick, cut, burn or tear the tissue will serve as a stimulus for calling out pain reactions. It can be aroused by mechanical, thermal, electrical and chemical means. If a very small skin area is moistened thoroughly and gone over with a finely ground needle-point, the * Some investigators have used finely spun glass threads, since they are not affected by dampness and are always straight and do not alter their elasticity with use.
THE RECEPTORS AND THEIR STIMULI 55 pain spots can be located. They are far more numerous than any of the other cutaneous spots. Pain spots do not usually coincide with cold, warmth or pressure spots. The threshold for pain isi far greater than that for pressure. Stimulation of small areas shows that sensitivity to pressure is 1000 times greater than that to pain. The cornea is richly supplied with pain spots. Any stimulus applied to the cornea, if above the limen, will produce a strong reflex movement. The posterior portion of the mouth cavity and the back of the tongue are not well supplied with pain spots. In the mucous membrane of the cheeks (opposite lower second molar) pain spots are absent in many subjects.
End Organs Affected by Cutaneous Stimuli. — So far in this work we have not had occasion to study the nervous system in its relation to the sense organs on the one hand and to the muscles on the other. In Chapter IV this relationship will be taken up. All that needs to be said now is that every sense, be it the eye, ear, smell or taste, has within it highly modified sensory struc- tures which are affected by the characteristic stimuli for that sense. One must look upon them as the chemical laboratory in which the energy is liberated that initiates the nerve impulse. These sense-organ structures (cells) are usually not a part of the nervous system, but are highly modified epithelial structures around which the nerve fibers end.
In the putei- layer or epidermis of the skin we find the first sense structures. The nerve fiber loses its sheath or covering after passing into the epidermis and splits up into numerous branches which end in between the cells of the skin (the epithelial cells). The endings sometimes pierce these cells or end between any two in small nodules. The epidermis is very richly supplied with nerves ending in this manner. These are the so-called free nerve endings. In the upper layer of the true skin we find the complex corpuscles of Meissner and of Dogiel and the papillary endings of Ruffini, also the corpuscles or end-bulbs of Golgi-Maz- zoni. Finally in the deep layers of the skin we find the Pacini corpuscles and the corpuscles or cylinders of Ruffini. In addition 56 PSYCHOLOGY 56 PSYCHOLOGY Fia. 7. — Showing some varieties of termination of afferent nerves in skin, muscles. tendons and hairs. A, Pacinian corpuscle (after Dogiel); B, Krause's corpuscle from con- junctiva (after Dogiel); C, two Golgi-Mazzoni corpuscles connected with a single bifur- cated nerve fiber. The ramified fibers shown in black present numerous yaricosities varying in size and appearance (after Ruffini); D, Rufnni's corpuscle in which the nerve fiber enters at one end of the corpuscle. The nerve endings are extremely complicated (after Ruffini). E, "free terminations" in the epithelium (after Wetzius); F, section through a hair and hair sheath showing the nerve plexus (after Bohm); G, Meissner's corpuscle from the skin. These are exceedingly numerous in non-hairy skin areas (after Dogiel); H, ter- mination upon a tendon sheath (after Huber and DeWitt); I, muscle spindle (after Ruffini), In the drawing "a" refers to the axone, "t" to the terminations of a nerve fiber or axone.
THE RECEPTORS AND THEIR STIMULI 57 to these principal forms there are many other transitional ones. The hairs are also supplied with a highly specialized form of nerve ending and must be looked upon as veritable sense organs. Fig. 7 shows an assemblage of some of the forms of sensory structures most frequently met with in the skin.
Anatomical and physiological researches have not gone far enough for us to assign with certainty definite functions to the various sensory structures we find in the skin. It may be said with some slight probability that the Golgi-Mazzoni end-bulbs or corpuscles, and the corpuscles of Dogiel are selective to or thrown into action by cold stimuli; that the deep-lying cor- puscles of Pacini and Ruffini are probably thrown into action by warmth stimuli. The evidence for these assignments is slender. It has been found that the conjunctiva of the eye does not initiate responses to warmth stimuli. Anatomical examina- tion shows that the end-bulb type of ending is present but not the Pacini and Ruffini corpuscles. It has been found by experi- ment that the reaction time (time between application of the stimulus and the subject's reaction) to a warmth stimulus is longer than the reaction time to any other cutaneous stimulus. Since the Pacini and Ruffini corpuscles lie usually in the deepest layers of the skin, their relationship is understandable if those structures are selective to warmth.
The nerve plexuses around the hairs are the pressure sense organs in the hairy areas of the body. On the non-hairy areas, Meissner's corpuscles are probably the pressure organs. Where pressure sensitivity is most highly developed, as on the finger tips, the hand, the tongue and the red area of the lips, the Meissner corpuscles are very numerous. On the hairy areas there are very few of them.
Finally, there is some evidence that the free nerve endings in the epidermis are the sense organs selective to pain stimuli: the evidence for this is mainly one of distribution. The pain spots are very numerous, as we have observed. The only sensory structures in the skin in sufficient quantities to care for this dis- tribution are the free nerve endings.
58 PSYCHOLOGY II. THE KIN^STHETIC SENSE.
Area and Stimuli.— The bodily tissues in which kinaesthetic impulses originate are (1) the muscles, (2) the tendons and (3) the articular surfaces. In all of these tissues lie special sense organ structures which are described on page 59. On account of the location of these organs, few investigations have been made upon them alone. It is impossible to stimulate them, since they lie so deeply imbedded in the tissues of muscles and tendons, without at the same time stimulating the cutaneous organs lying above them. Hence it is with difficulty that we can define the stimulus which affects them. Some success has attended our efforts to more exactly define the stimuli by anaesthetizing (co- caine injection or ether spray) the cutaneous area. It has been found that when the cutaneous organs are thrown out of func- tion, heavy pressure upon the muscle or tendon will still bring out a response. If the pressure is made great enough, pain responses can be obtained. In a similar way it has been found that the sensory structures in the muscles can be stimulated by forcing a contraction of the muscle by means of an electric cur- rent.3 Likewise, the pinching, pulling and forced contractions of the muscles affect the sensory structures both in the tendons and in the surfaces around the joints. Kinaesthetic sense organs are generally stimulated by a movement of the tissues themselves under the influence of normal muscular contraction. This con- traction of the muscle stimulates at the same time the structures in the tendons and in the joint surfaces. This occurs most fre- quently in walking, talking, drinking, eating, etc.; that is, when- ever an explicit bodily movement is made. The student cannot grasp too early the fact that while the muscles, tendons and joint surfaces are motor mechanisms, they are at the same time highly important sense organs as well. The muscles usually have a definite tone; that is, they are neither fully contracted nor fully extended. Whenever a muscle either elongates or contracts or •Experiments upon animals, have shown that the compression of the tendon may set off a reflex in a distant muscle. Pinching the muscle may produce rise of arterial pressure. »>The knee jerk may 'be inhibited by pressing or otherwise stimulating one of the leg muscles.
THE RECEPTORS AND THEIR STIMULI 59 in any way changes its length or diameter under the influence of the motor nerves, the sensory endings in the muscles, in the ten- dons and in the joint surfaces are stimulated and can, as we shall show later (page 295), arouse new motor impulses, which in turn can arouse new kin^esthetic stimulation, this process being re- peated again and again until a series of related acts is executed. From this one will quickly grasp the fact that in the functioning of a perfect habit nearly everything is turned over to the kin- sesthetic system (page 296). It is, of course, obvious that the length of the muscle cannot be changed very greatly without at the same time arousing some cutaneous impulses, so that even in executing most habitual acts the cutaneous sense organs are involved. It is well for the student to consider just how an animal, deprived of all of its senses except the kinsesthetic, could execute an act (assuming, of course, that enough organic pro- cesses are functioning to keep the animal alive). It would prob- ably be surprising to see how well such an animal could get about, or even thread a complicated maze. In thinking over this it might be well to recall that even an extremely fine singer's voice is little disturbed by anaesthetizing the larynx with cocaine, which throws the cutaneous sense organs in the mucous surfaces out of gear, but does not affect the sense organs in the muscles and tendons.
Types of Kinaesthetic Sense Organ Structures. — In Fig. 7 are shown some of the more characteristic sensory endings serv- ing the kinaesthetic sense as a whole. The most highly specialized sensory nerve ending in the muscle itself is the muscle spindle. In the transitional portions between muscle fibers and tendon fibers, one finds the highly important musculo-tendinous cor- puscle of Golgi. In the muscle sheaths, tendinous sheaths and joint capsules one finds numerous structures resembling the Pacini corpuscles already met with in the skin, known as modi- fied Pacini corpuscles. It is well to contrast these sensory end- ings in the muscles with the motor nerve endings shown on page 161. Nothing further can be said about the special func- tions of these endings.
60 PSYCHOLOGY III. EQUILIBRIUM SENSE* Introduction. — No discussion of the kinaesthetic sense is com- plete without some reference to the sensory structures in the semicircular canals and in the utricle and saccule. The struc- ture and function of these organs are somewhat complicated, and it must/ suffice for our present purpose to give, only a few central facts. In each ear there are three canals: external, su- perior and posterior. The canals are set in the ear in approxi- mate conformity with the three dimensions of space. The canals, utricle and saccule are really hollowed out of the petrous bone. Fig. 8 shows a cast of the bony cavities. Inside of the bony cavity will be found a continuous membranous sac, which, in the canals, takes closely the general shape of the bony structure, while in the utricle and saccule the conformity is less close. In- side of this membranous sac is a fluid, the endolymph. Between the sac and the bony walls there is the perilymph. Each canal is dilated into an ampulla where it joins the utricle. It is in these ampullae that the vestibular branch of the eighth cranial nerve finds its termination. The ending of the nerve inside the membranous sac together with the epithelial cells or sense structures are called the crista acustica. Each cell ends in a long flexible hair jutting out into the endolymph. The hairs are held together by a mucous mass called the cupula, so that they are unable to move individually and freely in the endolymph; the nerve fibers end in close connection with these sensory cells. In the utricle and saccule there are similar sensory structures, the structure as a whole being called the macula acustica. There is one macula in the utricle and one in the saccule. The sensory cells to be found in the macula are shorter than those in the crista. In the macula the hairs are held together by a denser mass. Lying among the hairs are to be found small carbonate of lime particles called otoliths.
Stimulus to the Sense of Equilibrium. — Variations in the pressure of the endolymph caused by movements of the head serve as the adequate stimuli to the hair cells in the canals of the ear. The movements of the head must be great enough to dis- place the cupula, which as it is displaced stimulates the hair cells.
THE RECEPTORS AND THEIR STIMULI 61 a. 30fnm. lateral.
FIG. 8. — Showing the general form of the semicircular canals, utricle, saccule and cochlea, together with their nerve supply (from human embryo). 3, Ganglion of Scarpa containing the cell bodies giving rise to vestibular branch of VIII nerve. Note that three branches are given off near here, two enter the ampullae of the semicircular canals, the other enters the utricle; 2, branch entering saccule; 5, branch entering posterior canal; 4, vestibular division of the VIII nerve on its way to the brain; 1, the cochlear division of the VIII nerve entering the modiolus. The cell bodies giving rise to this nerve lie in the modiolus. (From the original drawing of Doctor Streeter.)
When these canals are stimulated there occur changes in mus- cular tone, probably of every muscle of the whole body. If the stimulation is intense the most characteristic reaction to be seen is nystagmus of the eyes, a rapid to and fro movement plainly 62 PSYCHOLOGY 62 PSYCHOLOGY observable without the aid of instruments, if the subject is very sensitive, or the stimulation is made more intense, he may vomit. The easiest way to observe the phenomena of stimulation of the semicircular canals is to hold the head of the subject almost vertical and rotate him with eyes closed and ask him to indicate the direction in which he is being turned. He does this cor- rectly so long as the rate is increasing. If the chair is stopped suddenly, the subject will state that he is rotating in the opposite direction. It will be found that after several rotations there is imparted also the rapid to and fro movement of the eyes men- tioned above. If the head is held down during rotation, or to one side, other than the external canals will be stimulated. If after 10 rotations in 20 seconds the subject is asked to straighten up, violent compensatory movements are made. We are thus forced to admit that the semicircular canals contain definite sense organs which are excited by rotary movements of the head in various planes. The sense organ can also be excited electri- cally and by the introduction of hot and cold water against the ear-drum when the subject is rotated.
Let us take a concrete case: When the head is upright, or bet- ter, inclined at an angle of 30°, the external semicircular canals are stimulated when the subject is rotated. If the subject is rotated to the left, there will result in both external canals at first a movement of the endolymph toward the right on account of the inertia of the fluid. This will cause horizontal nystagmus of the eye to the left. When rotation ceases, the endolymph keeps on moving to the left for a short time. This will give rise to a horizontal nystagmus to the right. Usually in rotation ex- periments the nystagmus during rotation is neglected and only the after-nystagmus observed. If a large number of persons are rotated 10 times in 20 seconds, the after-nystagmus will be found to endure on the average about 26 plus or minus 10 sec- onds. This test has been much used by the otologists for deter- mining lesions in the central nervous system. Recently the tests have been introduced into the aviation service by a non- scientific group of otologists connected with the War Department as a qualifying test. Unless the nystagmus falls within the range of 26 plus or minus 10 seconds, the candidate is rejected. In view of its highly complex nature, involving both habit and reflex mechanisms, and in view further of the fact that training THE RECEPTORS AND THEIR STIMULI 63 apparently reduces the time of duration of nystagmus, the test, so far as is known, really has no significance for aviation (they were given up by the British, French and Italian armies after a short time). There is no evidence to show that an individual who has a balanced nystagmus of, say, 15 seconds and below, or one of, say, 37 seconds and above, cannot learn to guide his plane and to manage it as successfully as an individual who shows a nystagmus falling within the 26 plus or minus 10 limit.
In stimulating canals with water, no reaction is obtained if the temperature is at the physiological zero. Above and below this temperature water will serve as a stimulus. The nystagmus is in one direction if the water is cold and in the opposite if the water is hot. Changing the position of the head changes the direction of the nystagmus on any of these caloric tests. In stimulating electrically, the positive pole may be placed over one ear, the negative over the other, or the ears may be stimulated separately by placing one electrode over the ear and the other upon some distant part of the body. When both ears are stimu- lated simultaneously, a very weak current suffices to bring out nystagmus. If the negative pole is over the ear, the nystagmus is towards that side; if the positive pole, away from that side.
Function of the Utricle and Saccule. — In regard to the stimulation of the utricle and saccule, there are no very sure results. It is supposed that their impulses contribute towards the orienting of the body along the line of gravity. This depends, of course, largely upon the tacjtual and kinaesthetic impressions, as is shown in the shuffling gait of the patient in locomotor ataxia where kinaesthetic impulses are interfered with. But in swim- ming when the body is totally immersed, these tactual impulses cannot function differentially; yet the normal subject so im- mersed can always correctly indicate the position of the body with respect to the vertical. Deaf mutes, whose1 utricle and sac- cule are not functional, are said to be unable to do this. The view has been advanced that the saccule and utricle contribute impulses which maintain the head equilibrium when the body is at rest and during progressive (non-rotary) movements of the body. They thus supplement the equilibratory functions of the semicircular canals, which operate mainly in head rotation. The hair cells in the utricle and saccule are supposed to be stimulated by the pressure of the otoliths. When the body is at rest in any 64 PSYCHOLOGY 64 PSYCHOLOGY position, the otoliths, being heavier than the endolymph, settle down in response to gravity, thus stimulating the hair cells. A change in the position'of the head will again change the position of the otoliths, which will again stimulate the hair cells. The semicircular canals, the utricle and the saccule must be looked upon as a vastly important organ connected through the cere- bellum with every striped muscle of the body. Any sudden movement of the head will thus initiate impulses which pass through the cerebellum and out to the muscles. It should be mentioned that the speech habits are apparently not connected with the functioning of the vestibular apparatus. The subject can give no verbal report about this operation. After they have functioned and brought about muscular activity, he may report, ' * I feel dizzy; my eyes jump; vision is blurred; I feel like falling to the right, etc."
IV. THE ORGANIC SENSE.
Area. — The area in which organic impulses originate com- prises in general those organs and tissues which lie in the thoracic, abdominal and pelvic cavities. The muscular tissue in which the impulses chiefly originate is largely of the unstriped or smooth variety (heart, diaphragm, etc., excepted), and hence is innervated on the motor side by the autonomic nervous sys- tem (page 154); but nearly all of these visceral structures are supplied with afferent or sensory nerves belonging to the spinal cord or brain. These nerves either end free or else in highly specialized structures like the Pacinian corpuscles. When stim- ulated, they arouse the neural impulses belonging to the organic sense. These impulses, like those from the skin and kinaesthetic sense organ, pass back to the central nervous system and initiate movements of the body as a whole. The organic impulses, as we shall see in a moment, are aroused whenever the body must have food, water, sex outlets, or be freed from waste products and nox- ious substances (such as calculi, infections, or from the effects of disturbed or torn internal tissue, etc.). Since the very exist- ence of the organism depends upon the adjustment of these conditions, the organic impulses exert a powerful influence upon the striped muscles of the arms, legs, etc., arousing the general THE RECEPTORS AND THEIR STIMULI 65 reactions necessary to bring about adjustment — such as will bring food, water, companionship of a member of the opposite sex, or make the organism free from substances which irritate it. If the environment is such that the objects which would bring out the adjustment are not at hand, the individual will oftentimes tend to take on the posture or attitude which he would take if he were eating or drinking. Kempf has recently called attention to the great prevalence of these postural attitudes in psychopathological cases, and to their very great variety and complexity.