Ports of Entry - Sense Organs
Communication with the outside world is made possible by sense organs that form the most peripheral part of the nervous system.
They are essentially environmental organs that by way of the sensory nerve fibers with which they are connected receive and transmit impressions of what is happening in the surroundings. By themselves they do not sense or feel these happenings, for they are simply avenues of approach to the central nervous system which is the particular part of the body endowed with the power of sensation. When one, for example, “feels” a pin-prick in the finger tip at the point where the contact is made rather than in the brain where the sensation actually occurs, it is because he has become accustomed to refer all sensations to the point where the stimulus originates, and mistakes the gun for the man behind the gun. One does not see with a pair of glasses but through them.
It all goes back to remote evolutionary days when the function of sensation actually was located at the surface in that part of the body exposed directly to outside bombardment. Although the receptive sensory tissues of a living animal are originally ectodermal in origin and external in position, they do not remain in this exposed situation but in the course of time become for the most part buried within the protective body, in this way being shielded from direct impact with the strenuous and constantly changing outside world.
The withdrawal of the central nervous system from direct external influences necessitates the establishment of peripheral nerves in order to retain connection between the central nervous apparatus and the surrounding environment. Since nerve fibers in themselves are not so well adapted for the reception of outside messages as for the transmission of them, specialized sensory structures that are outposts, like sentries on guard around a camp, are required to keep in touch with the environment.
These intermediary structures between the organism and its surroundings are commonly known as “sense organs,” although it is preferable to call them receptors, since not all impressions received through them result in sensation. In fact sensation is only an incidental result of receptor excitation.
A stimulus, which is something that a receptor receives, is defined by Stiles as “a change, physical or chemical, which is capable of producing physiologic reactions.”
It may come from the outside in the form of environmental changes of various sorts, or it may arise from processes going on within the organism, as when the production of carbon dioxide in the blood stimulates the respiratory center in the medulla with the result that the muscles involved in breathing are set into action.
The essential thing about any stimulus is the element of change, or the fact that something different in kind or degree is taking place in the surroundings of an animal.
Constantly repeated or long enduring stimuli fail to register sensation, as for example when the hum or roar of city traffic fades with fatiguing repetition and becomes noticeable again only when there is a change in the character or intensity of the sounds produced. It is possible, therefore, to become oblivious to extraneous happenings that are irrelevant to certain sense organs involved, thereby gaining a high degree of efficiency in the reception of specific stimuli upon which the attention is focussed.
As pointed out long ago by Helmholtz, sense organs cannot give a true picture of the actual environment in which we live, but only of the changes taking place in it.
The continuous regularly repeated heart-beat, or the constant waving of cilia do not require a fresh stimulus for every muscular movement that they make. Neither do processes of growth and repair call for sensory stimulation.
Receptors are generally cells or organs that are specialized to take in impressions of a single kind, and no others (Muller’s Law). Such receptors are said to be “adequate” for the stimulus in question. The eye is adequate for light, and taste organs for the chemical stimulation of dissolved substances, but neither is adequate for the reception of sound waves. This exclusive adaptation to only one type of stimulation is a very great advantage since in this way protection is gained against unnecessary annoyance arising from stimuli other than the one to which the receptor in question has become adapted. The sensory elements of eyes and ears, for example, while retaining a clear pathway for exposure to the stimuli of light and sound respectively, are meanwhile protected from chemical and tactile stimulation, as well as from temperature changes. In the same way smell and taste organs, although in an ideal situation to be affected by gases and liquids, are not particularly exposed to mechanical contacts or to the impact of waves of sound or light.
Many receptors that are entirely adequate for their own kind of stimuli can gain relief from excessive or incessant bombardment of stimuli by being temporarily excluded from even their own particular sort of stimulation. Thus it is possible to shut out light for the time being by closing the eyes, and to cease from tasting things by keeping the mouth shut.
Sometimes a receptor is acted upon by a stimulus for which it is not adequate or appropriate. When menthol is rubbed into the skin a feeling of cold results instead of the usual chemical response of smell, or if a vibrating tuning-fork is placed on the tip of the tongue, instead of hearing a sound, the result is a tickling sensation which is a form of touch-response. In these instances the thermal sense endings in the skin are inadequate receptors for the chemical stimulation produced by menthol, and the tactile buds on the tip of the tongue are inadequate receptors for the sound waves from the tuning-fork. In both cases it is the central nervous system and not the stimulus or the receptor that is responsible for the particular sensation that is experienced. It is entirely possible for sensation to occur in the complete absence of a receptor, as in dreams, or when the stimulus comes in through a port of entry not adequate for it.
In any case the impulses that give rise to sensations do not originate with the receptors but are simply transferred through them to the sensory headquarters within. When we blame our innocent feet for being weary after tramping for hours through a museum or art gallery, the inaccessible brain within calls no attention to its guilty self but lets the illusion pass without protest.
When one realizes how easy it is to have sensory illusions of all sorts, and how variable the central judgment may be upon the “evidence of the senses,” not only in the case of different individuals reacting to the same circumstances but also of the same individual at different times, it is small wonder that lawyers have difficulty in extracting the truth even from perfectly honest sworn witnesses, since natural limitations compel us all to “see through a glass darkly.”
Protozoans have no hook-up between sense organs and a central nervous system, but nevertheless are in responsive communication with their environment, and may, therefore, be properly referred to as sensoria. The same is true of plants, which have no nerves, although responding definitely to environmental stimuli, such as light, temperature, moisture, and gravity. In fact irritability, or the capacity to respond to stimuli, is an essential universal property of protoplasm. The sensory receptors of higher animals are simply cellular structures that have specialized in this general endowment of irritability.
Response to stimuli resulting from environmental changes may take the form of (1) muscular contraction; (2) glandular secretion, or (3) even the more involved processes of psychic performance.
There are upper and lower limits in the receptive range of human sense organs. Thus, the range of vibrations which stimulate the human ear as sound lie approximately between 16 and 40,000 per second, while the range of the visible spectrum to the human eye is between 481 billion ether waves per second at the red end, and 764 billion at the violet end. Outside of these extremes human receptors are ineffective with respect to either sound vibrations or light waves, and except within such limits one can neither hear nor see.
Sensory Range in Animals
The sensory range of animals other than man is usually not only unlike that of man but may also extend beyond it. Cats, for example, are known to respond to higher tones than are perceptible to human ears; ants, subjected to a field in which light is broken up by a prism into components of the spectrum, arrange their exposed pupae with reference to ultra-violet light that is invisible to the human eye; dogs perceive a repertory of odors of which man knows nothing; and house flies distinguish the difference between cane sugar and saccharin which to the human tongue may taste alike.
Birds upon a battlefield, it is reported, are apparently not alarmed by the terrible crash of exploding cannon, continuing to sing and to fly about as usual since the whole unusual performance is quite beyond their customary range of reception. The picture of a mouse, however lifelike, awakens no interest when placed before the eyes of a hungry cat.
The character of any sensation depends not on the nature of the impression received but on the properties of the brain cells to which the impression is referred.
Owing to the specialized receptivity of different kinds of receptors for particular stimuli, and the largely variable subjective element that has so much to do with the use made of sense organs, it is extremely uncertain how far our knowledge about the sensory life of animals other than ourselves is dependable. Even in the case of other human beings with a sensory equipment apparently quite like our own, we can only infer what the sensations experienced by mutual stimuli are like.
It becomes necessary with regard to animals having receptors and a central apparatus unlike that of man to resort to observation of objective behavior resulting from the application of some particular stimulus, in order to make any reasonable guess as to how they “feel” in such experimental circumstances. Behavior, moreover, is by no means always a safe criterion on which to depend, for a responding animal may not do what man or some other animal might do under the same conditions. Similar movements do not necessarily express similar sensations. For example, when a friendly dog wags its tail, it portrays an entirely different state of mind from that of an excited tiger, lashing its tail back and forth. It is especially embarrassing for man, whose tail is only an evolutionary memory, to try to pass active objective judgment on the feelings that find expression in tail movement.
Naturally the more unlike man an animal is, the more uncertain must be the human interpretation of the world in which the animal lives. The classical story of Apuleius of The Golden Ass, in which the hero by witchcraft experiences an interlude of being an animal, suggests to the exploring biologist an illuminating line of imaginary research and speculation.
In general the sensory endings of afferent nerve fibers are either free or encapsulated. Free nerve endings terminate among the epithelial cells, connective tissue elements or muscle fibers without any specialized cells surrounding them. Encapsulated endings are enclosed by groups of cells of varying complexity. Rallying to the aid of the specialized sensory cells, which are the essential receptors, is an army of various accessory cells serving as nourishers, protectors, and magnifiers in the make-up of the complicated sense organs.
Ectodermal receptors may be scattered in integumentary sense organs, or be clustered together and surrounded by cells and tissues, largely of mesodermal origin, that are responsible for the elaborate differences which characterize the various sense organs.
As might be expected, the least morphological complication is found in receptors of the more primitive aquatic animals. The transition to land life, with consequent access to a wider range of stimuli, has been invariably accompanied by an increased differentiation in the structure of the receptor organs ; nevertheless among the highest land vertebrates the essential sensory receptor cells of elaborately specialized sense organs, such as the ear, are still bathed in fluid as are the more primitive receptors of aquatic forms.
In nearly all cases the sensory receptors are particularly protected from mechanical injury. Eyes are housed in sunken skeletal orbits; the sensory parts of the ears are entirely embedded within cartilage or bone of the cranial wall; the olfactory epithelium is safely concealed upon the inner wall of cavernous nasal pits; the organs of taste are inside the mouth cavity behind closing jaws; even the cutaneous organs of temperature, pain, touch, and pressure, which are located in the exposed skin, are in many instances either sunk below the surface, or surrounded by an individual barricade of non-sensitive, protective cellular materials.