Document LpR3g0vXJjzyJpmZdkVVO6105

8 77,8c N; ^ -XS^*** /f/ MAR 27 ir V Health Bulletinfor teachers '** Special Sensibilities ight, hearing, taace, touch, and smell--the familiar "five senses"-- S were first classified as special sensibilities in the early part of the 19th century. At that time all the ocher less distinct sensations were lamped together as general sensibilities. Now touch is no longer called a "special" sense. Ic belongs among the sensations designated as somatic, or body, sensibilities which make us aware of pressure, pain, heat, and cold. The mechanism of sense perception consists of a vast system of sensory nerve endings, or "receptors," connected by devious but well-defined nerve paths with definite sensory areas in the cerebral cortex, where the different types of sensation are translated into conscious impressions. The simplest sort of diagram upon which an illustration of the sensory mechanism might be roughed in is that of a circle. On the periphery we should place dots representing the sensory nerve endings, and from them we should draw radii representing the nerve fibers. Somewhere along each radius we should place other dots to stand for the neurons, or nerve cells, through which the impulse Item a receptor is relayed to the center of the circle, or cerebral cortex. Actually this simple wheellike diagram would resemble a far more highly complicated web than any spider could ever spin if we attempted to show the devious routes, the cron roads, the relay stations by which the impulses finally reach the particular central areas where they "make sense." The Cause of a Sensation. The primary cause of a sensation of any kind is the reaction of a sensory receptor to a stimulus to which it it sensitive. The sensation lasts just as long as the sensory end organ reacts to the stimulus. The reaction may outlast the application of the stimulus just as a rocking chair may continue to rock after it has once been set in motion. This is true especially of the sensation of pain. On the other hand, reaction may cease while the stimulus is still being applied. This is true especially of the sensation of touch. The nerve fibers involved get used to or adapt themselves to the constant stimulus of mechanical pressure so that only single impulses, or at most a few, are discharged. The Intensity of a Sensation. The strength or weakness of a sensation apparently is connected with the number of impulses arriving at the central nerve-cell junctions (synapses). The greater the number of nerve fibers engaged in shunting impulses into a sensory area of the brain, the Volutt* XQ No. 7 MARCH 1941 l I 773 i MAR 2? is; -1 '/ more intense a sensation will be. Thus a pin prick in a finger hurts more than does the same stimulus applied to the middle of the back, because there are more sensory receptors per square inch of body surface on the finger than on the back. The intensity of a sensation depends also on the speed with which impulses follow one another along individual nerve fibers. In the conduction of impulses nerves obey the "all or nothing" law. Either they do not transmit impulses at all, or they transmit them as fast as they possibly can. The Character of a Sensation. The starting point of an impulse and its destination are what determine the character of a sensation, not the impulse itself. The impulses carried by sensory nerve fibers leading from light-wave receptors do not differ materially from the impulses carried by nerve fibers leading from sound-wave receptors. In she one ease you see, in the other you hear, because sensory nerve fibers leading from the retina of the eye end in the visual center of the cerebral cortex, and those from the inner ear in the auditory center. Although all impulses in them selves are more or less alike--no one knows exactly ithat they are like. The passage along a nerve fiber of an excitation process started by the stimu lation of a receptor, and associated with a series of electrical and chemical changes along the route, is the nearest we can come to defining a nerve impulse. Each sensory receptor is the station of dep^ju* for the passage of an impulse to the terminal station in the brain with which it is connected. Only one kind of stimulus can initiate the passage of an impulse from that particular itacoc. Just aa a ticket for Boston would not get a passenger past the gate to board a train for Miami, so the stimulus of light waves would not get a message started on die way to the auditory center. However, it does sometimes happen that a receptor is excited by an unusual stimulus. When that happens the sensation aroused is similar to the sensation aroused by the usual stimulus. Thus the usual stimulus of a receptor on the retina is light waves, but an unusual sdmulua, like a blow between the eyes, also givea rise to the senssrion of light. One secs stars! AweamoaaeJMawaidrt^wnpeieaMaiaieigeseehwoasiwwweiwdweaesdwwwIeowealeanapMaaaas aad-iatpwleewdeeiew*iawetiaaMaeaaMh*wMafcwaMatyesMhaiiLd^*a*4hdifhem)epad set lIll itTBlldW. Somatic Sensibilities. The so-called somatic, or body, sensations may be divided into paleo-sensibilities and neo-sensibilities. Paleo-sensibilities, which are the first sensations experienced by a new-born baby, are those of extreme degrees of pressure, pain, heat, and cold. They are all un pleasant and all vitally important in self-preservation. A hearty smack, as likely as not, made many of us draw our first breaths. The discomforts of coldness, heat, hunger, and wetness evoked the cries which brought us needed attention. Then, by degrees, neo-sensibilities became superim posed on the older ones to supply us with critical information regarding the outside world. These newer sensibilities include tactile sensations, those evoked by light contact with receptors in the skin, and kinesthetic sensations, those from receptors in the voluntary muscles, tendons, joints, and bone coverings. Tactile sensations give us the tense of touch and its refinements. Kinesthetic sensations furnish us with information con cerning the movements and position of the limbs, trunk, and head, and 32 S) yo jo. 33 V enable us to estimate resistance to muscular contraction when we lift or move objects. Thi l n fni-mr, Unprr, rnawesewe'CT, arid mights u<~ ubjcttew^Watttad^uiiaoWi The sensation of pain is served by naked nerve endings in the skin, tomes of the eye, and serous surfaces inside the body, such as the peri toneum and the pleura. Pain nerve endings are not selective, as other receptors are. They respond to any type of stimulus whether mechanical, chemical, or thermal, provided it is intense enough. Thtrsfeit) ih rsnnritn T" g' rs rh -- *hiwwd -Mm mrr spenifca yaaiissw Regardless of where s mechanical or chemical agent acts along a nerve, the sensation of pain evoked is interpreted by the brain as earning from the part of the body which it associates with pain impulses carried by the particular nerve fibers involved. That is, the brain is educated to localise pain. If anything happens to dcscasititc or remove the pain receptors in a particular area, it takes time for the brain to forget its old associations and to learn new ones. This explains why a person whose leg has been amputated often complains of pain in the foot or lower limb. Actually the eensationi arise in the severed nerves of the stump. The Stnst of Sight. When we see we have many sensations. Not only do we know that it is light, but also we recognize different intensities of illumination. By virtue of this ability we avoid obstacles and recognize moving objects. We can distinguish colors, outlines, and line details in the things we look at, and appreciate depth, volume, and perspective. The sensory receptors which mediate all these sensations are located in the retina of the eye. There are two kinds--rods and cones--named after their shapes. At the point where the optic nerve, which conducts impulses from the retina to the brain, enters the retina, rods and cones are absent. Hence this is a "blind spot," a point on the retina where light waves produce no visual sensations. Rods and cones arc arranged on the retina in a definite pattern. Beginning with a small ares near the center of the retina, only cones packed closely together art present. Then in a widening circle the cone* diminish progressively in number while the rods increase proportionately, until finally, at the outer rim, only rods remain. Distinct and accurate vision and die recognition of color arc localised near the center of the retina, while the ability to see in a dim tight is almoet entirely lacking. On the other hand, low light intensities and movements at any illumination are beat detected at the outer rim of the retina, while color perception and perception of detail art lacking. Thus ir it inferred that the function! of the cone apparatus are sharpness of vision and recognition of details and colors, erhile the rod apparatus, which is vary sensitive to tight, ia used \ for twilight vision. A rose-red light-sensitive pigment found in the rods is essential for norma] vision in dim light. This pigment is called visual purple. Since illumination of the eye causes bleaching of the visual purple, this sub stance must be regenerated when the eyes are in the dark. Vitamin A is in some way intimately associated with the bleaching-regeneration cycle of visual purple. Thus people who do not get enough vitamin A may have night blindness--that is, difficulty in seeing in dim lights-- a factor of significance in automobile driving accidents. 33 r -^6. `1 f Tiny up*ide-down images of objects in the outside world re projected on the retini ^ / by a series of re/racdve suffices forming the lent system of the eve. Diseurbinces of / vision most frequently occur through structural defects or muscular weaknesses which make it impossible for the lens system to focus images properly on the retini. This is the basis for the prescription of lenses to correct visual disturbances, ________ _____ The Seme of Hearing. When we heir we do not just hear a sound--we recognize even small differences in the pitch, loudness or intensity, and >v- Quality of the sound. We know whether it is disagreeable (noisy) or pleasant (musical), and we know usually what makes it. The organ of hearing, which mediates the sensations of sound, is called the organ of Corti and is located in the lymph-filled cochlea (inner ear). The interior of this bony "snailshell," which is about as big aa a pea, is subdivided by walls of membrane into three spiral "undersea" galleries, or chambers. Along the floor of one of these galleries (the seala media) lies the organ of Corti. This organ contains sensory sound receptors and hair cells. It is the stimulation of the hair cells by the tiny variations in pressure of the fluid surrounding them that excites the sound receptors to send impulses to the brain via the auditory nerve. All the other apparatus of the car it concerned teleiy with sound were conduction. Over a tvitem of delicate membrane* and tiny bonce impinging on one another the ecries of weves ict up in air by vibrating bodies in the outside world travel to the eochlet, where they are transformed into "wntcr" waves of the tarns frequency, amplitude, end form. Their frequency determines the pitch; their amplitude, the loudness; end their form, the qualicv of the sound heard. Impairment of hearing is usually the mult of conditions which affect the conduction of sound to the inner ear. Thus inflammation of the middle ear caused by the entrance of infected material from the nose and throat via the Eustachian cube, as a result of frequent or chronic colds, often produces conditions which interfere with sound conduction. The Sense of Taste. We hsve four primary taste sensations--sweet, bitter, sour, and salty. Each kind is mediated by the sensory receptors, or taste buds, on the tongue which sre sensitive to it. Tsste buds are chemically stimulated, but it is not known exactly what centers in the brain are reached by the impulses started off as a result of the stimulation. The great variety of different taste sensations actually experienced is due to a mixture of primary sensations (much aa a mixture of primary colon will produce different shades and tones), to the stimulation at the asms time of touch and temperature nerve endings on the tongue, end to confusion with sensations of smell. Many flavors arc really odors and disappear when the noae is "scuEad up" as a result of * cold. Personal variationa in the sense of taste occur, end taste "blindness" for one or mors of the primary sensations is common and hereditary. The Sense of Smell. The organs of amell, called the olfactory bulba, are located in the roof of the nose. They are composed of a collection of spindle-shaped rcceptora which are stimulated by the pressure of vapor molecules given off by volatile substances. It is possible to distinguish I I'llHgu, HI II S'amK BE mi. einpni'M School Health Bureau--Wslmss Division METROPOLITAN LIFE INSURANCE COMPANY 34 8778 MAP 77 79 i many different odors, but they are hard to classify except as being pleasant, unpleasant, or neutral. Since sensations of smell may be tied up intimately with other sense impressions from the outside world, it it no wonder that their ultimate destination in the brain it the most obscure of all the cerebral sensory areas. Put associations hsve t great deal to do with psychological reactions to odors of different kinds. Thus an odor may be very disagreeable to one person and pleaunt or neutral to aaotlier. The senses of taste and smell together determine whether or not a food is appetising. Forming pleasant associations with the taste and smell of food in childhood plays a leading role in creating a liking for the right kinds of food. q/V tM*'i / rr MiBDH mLr>. BULtjn'xv 'Dr. Amatronr ,, , . nrriw * --a vOr. Bhaatlay vDr. Dublin, Dr. Lotka i-^Pi' i Oapwv^tfc'* -'Dr. tally villas Abbot ^Dr. Shaptrd JDr. Burnatta t Dr. UeCoanaU Kiss fair (/School Buraau staff ,