THE Sensory system makes the animal aware, though not necessarily in the sense of "conscious", of conditions and changes both outside and inside its body. In the simpler animals only very general stimuli such as light or darkness, heat or cold can be perceived by the sensory system. In the higher animals detailed information about surroundings such as distance, size and colours of objects can be gained as a result of the specialization of the sensory organs and the elaboration of the nervous system.
The general sensory system.
Included in the general sensory system are the organs which are fairly evenly distributed through the dermis of the skin; hence any part of the skin is sensitive to touch, heat, pain and pressure. Examples of such sense organs are given.
It must be emphasized that, in general, a particular sense organ or sense cell can respond to only one kind of stimulus. Thus, a sense organ sensitive to touch will not be affected by the stimulus of heat; a cell which is sensitive to chemicals will not respond to pressure.
It is not yet certain just how specific some of the sense organs are in their responses, for the sensory-endings which produce the sensation of pain can be activated by a variety of stimuli such as pressure, heat and cold, and the ear lobe which contains only hair plexuses and free nerve endings can detect touch, heat, cold and pressure.
Certain regions of the skin have a greater concentration of sense organs than others. The finger tips, for example, have a large number of touch organs, making them particularly sensitive to touch. The front of the upper arm is sensitive to heat and cold. Some areas of the skin have relatively few sense organs and can be pricked or burned in certain places without any sensation being felt.
Stimulation and conduction of impulses. The sense organ or sense cell is connected to the brain or spinal cord by nerve fibres. When the sense organ receives an appropriate stimulus it sets off an electrical impulse which travels along the nerve fibre to the brain or spinal cord. When the impulse reaches one of these centres it may produce an automatic or reflex action, or record an impression by which the animal feels the nature of the stimulus and where it was applied.
The sense organs of one kind and in a definite area are connected with one particular region of the brain. It is the region of the brain to which the impulse comes that gives rise to the knowledge about the nature of the stimulus and where it was received. For example, if the regions of the brain receiving impulses from the right leg were eliminated or suppressed by drugs, no amount of stimulation of the sensory-endingss would produce any sensation at all, although the sense organs would still be functioning normally. On the other hand, if a region of the brain dealing with impulses from sense organs in the leg is stimulated by any means, the sensations produced seem to be from the leg. If a limb has been amputated the nerves from the stump may still send impulses to the region of the brain which formerly received them from the limb, and pain is felt as if the limb were still there.
Another important consideration is the fact that the impulses transmitted along the nerve fibres are fundamentally all exactly alike. It is not the sensations themselves that are carried but simply a surge of electricity, and this is so whether it is a heat organ or a touch organ that sets off the impulse. It is only in the brain that the stimulus is identified, according to the region of the brain which the impulse enters. For example, if the nerves from the arm and leg were changed over just before they entered the brain, stubbing one s toe would produce a sensation of pain in the arm or hand.
Intensity of sensation, A strong stimulus usually produces a more pronounced sensation than a weak stimulus. This is probably due to (a) the greater number of sense organs stimulated in the area, and (b) the stimulation of a number of sensory cells which do not respond at all unless the stimulus is intense, Many sensory organs are groups of cells, some of which are triggered off by the slightest stimulus while others'need a powerful stimulus to affect them. When these latter are activated, the stimulus is recognized as being stronger than usual.
Vigorous stimulation does not affect the quality or intensity of the electrical impulse travelling in the nerve fibres but increases the total number of these impulses reaching the brain.
Pain. Although we tend to regard sensations of pain as inconvenient and alarming, they have important biological advantages. By making animals respond quickly or automatically by reflex action they tend to remove the animal or the affected part from danger. Our response when touching Something unexpectedly hot affords a good example. If there were no sensations of pain, untold damage to tissues could result before one was aware of it. A sensation of pain is not essential For an efective reflex but it probably helps the animal to learn to avoid the same situation. Where pain occurs without producing a reflex action, as in tooth-ache, it serves as a warning hat all is not well in that region and gives an opportunity to seek advice or treatment.
Interrnal sense organs (proprioceptors). The tissues of the body also have sense organs. One kind, occurring within the muscles and tendons, responds to the degree of stretching. These sense organs enable an animal to learn to place Sims accurately in movement and to know their exact position without having to watch them. Sensory pain-endings ur in many internal organs, but not in the brain.
Special senses
Sight, hearing and balance, smell and taste are called the special senses. The relevant sense organs each consist of a great concentration of cells which are sensitive to one kind of stimulus. These sensory cells may be associated with structures at direct the stimulus on to the sensitive region, in the lining of the nasal cavity and on the tongue are groups of sensory cells that can be stimulated by chemicals which dissolve in the moisture overlying them. On the tongue these groups are called taste-buds; they lie mostly in the grooves round the bases of the little projections on the upper surface of the tongue.
Taste buds vary in their sensitivity to groups of chemicals which we describe as sweet, sour, salt or bitter. lhe chemicals producing similar taste sensations often have little in common, although a sour taste usually indicates an acid. It can be seen that the sense of taste is very limited and probably serves to distinguish only between food suitable and unsuitable for eating. The sensation of flavour has a much greater range and comes from the sense of smell. If the nose is blocked it is difficult to distinguish between many kinds of food which normally have quite distinctive flavours.
Smell. In the epithelium lining the top of the nasal cavity there are spindle-shaped cells with processes extending out into the mucus film that spreads over the epithelium. From these cells nerve fibres pass into the brain. The cells are stimulated by substances which dissolve in the moist lining of this region of the nasal cavity and so produce the sensation of smell. No satisfactory classification of smells, or explanation of how they are distinguished, has yet been made.
The sense of smell is easily fatigued, that is, a smell experienced for a long period ceases to give any sensation and we become unaware of it, though a newcomer may detect it at once.
Sight. The eyes are the organs of sight. They are spherical organs housed in deep depressions of the skull, called orbits, and are attached to the wall of the orbit by six muscles which can also move the eye-ball. The structure is best seen in a horizontal section as shown Hearing,
Structure and functions of the parts of the eye
The EYELIDS can cover and so protect the eye. Closing the eyelids can be a voluntary or reflex action. Regular blinking serves to distribute fluid over the surface of the eye and prevents its drying up.
The CONJUNCTIVA is a thin epithelium which lines the in- side of the eyelids, the front of the sclera and is continuous with the epithelium of the cornea.
The TEAR GLANDS open under the top eyelids. They secrete a solution of sodium hydrogencarbonate and sodium chloride and keep the exposed surface of the conjunctiva and cornea moist. They also wash away dust and other particles. An enzyme which is present in tear fluid has a destructive action on bacteria. Excess fluid is drained into the nasal cavity through the lachrymal duct which opens at the inside corner of the eyes.
The EYE MUSCLES are attached to the sclerotic at one end and to the wall of the orbit at the other. Their contractions can make the eye move from side to side and up and down.
The SCLERA IS a tough, non-elastic, fibrous coat round the eye-ball.
The CORNEA IS the transparent disc in the front part of the sclera. Light passes through the cornea into the eye. Since the cornea is a curved surface the light is refracted and the rays begin to converge
The CHOROID is a layer of tissue lining the inside of the sclera. It contains a network of blood vessels supplying food and oxygen to the eye. It is also deeply pigmented, the black pigment reducing the refiection of light within the eye.
The AQUEOUS AND VITREOUS HUMOURS are solutions of salts, sugars and proteins in water. The aqueous humour is quite fluid, the vitreous jelly-like. These liquids help to refract the light and produce an image on the retina. Their pressure outwards on the sclera maintains the shape of the eye.
The crystalline lens, the cornea and the conjunctiva are made of living cells which are quite transparent. They contain no blood vessels and must absorb their food and oxygen from the aqueous humour.
The CRYSTALLINE LENS. The cornea begins and the crystal- line lens continues the refraction of light so producing an image on the retina. The lens is held in position by the fibres of the suspensory ligament which radiate from its edge and attach it to the ciliary body. The shape of the lens can be altered by contraction or relaxation of muscles of the ciliary body.
The CILIARY BODY is the thickened edge of the choroid in the region round the lens. It contains blood vessels and muscle fibres some of which run in a circular direction, that is, parallel to the outer edge of the lens
The IRIS consists of an opaque disc of tissue. At its outer edges it is continuous with the choroid. In the centre is a hole, the pupil, through which passes the light that will produce an image on the retina. The contraction or relaxation of opposing sets of circular and radial muscle fibres in the iris increases or decreases the size of the pupil, so controlling the intensity of light entering the eye. The iris contains blood vessels and some- times a pigment layer that determines what is usually called the colour of the eyes. Blue eyes have no pigment, the colour being produced by a combination of the black backing. the blood capillaries and the white outer layers on the iris.
The RETINA. This is a layer of cells sensitive to light. There are two kinds of light-sensitive cell, called, according to their shape, rods and cones. Only the cones are sensitive to coloured light but the rods are more responsive to light of low intensity
The nerve fibres from these cells pass across the front of the retina and all leave at one point to form the optic nerve which passes through the skull into the brain.
The BLIND SPOT. In the region where the nerve fibres leave the eye to enter the optic nerve there are no light-sensitive cells
If part of an image falls on that region no impression is recorded in the brain. We are not normally aware of this "blank" in our field of vision, partly because it is compensated by the use of two eyes scanning the same field and partly because it never coincides with the image of an object on which we are concentrating.
The FOVEA is a small depression in the centre of the retina It contains only cones, and it is the region of the retina with the greatest concentration of sensory cells and, therefore, gives the most accurate interpretation of an image. When an observe
Concentrates on an object, or part of an object, its image is thrown on to the fovea. Only in this region is there details a appreciation of form and colour.
Image formation and vision. Light from an external object enters the eye. The curved surface of the cornea, the lens and the humours, refract the light and focus it so that "points of light from the object produce points of light on the retina. The image thrown on to the retina is real, upside-down, and smaller than the object. The light-sensitive cells are stimulated by the light falling on them, and impulses are fired off in the nerve fibres. These impulses pass along the optic nerve to the brain where, as a result, an impression is formed of the nature, size, colour and distance of the object. The inversion of the image on the retina is corrected in the optical centre of the brain to form the impression of an upright object.
The accuracy of the impression that the brain gains of the image depends on how numerous and how closely packed are the light-receiving cells of the retina, since each one can only record the presence or absence of a point of light and, in the cones, its colour. If there were only ten such cells, the image of a house falling on five of them would record an impression of its size, the fact that it was differently coloured at the top and bottom, and a vague representation of its shape. In a hawk's fovea there are about a million cones per square millimetre
Accommodation. With a rigid lens of definite focal length placed at a fixed distance from a screen it is possible to obtain a sharply focused image of an object only if the object is at a certain distance from the lens. The focal length of the lens in the eye can be altered by making it thicker or thinner. In this way light from objects from about 25 cm to the limits of visibility can be brought to a focus. This ability of the eye to alter its focal length is called accommodation.
The lens is surrounded by an elastic capsule and tends to shrink, becoming thicker in the centre, but the eye fluids pushing out on the sclerotic maintain a tension in the suspensory ligament that stretches the lens into a thinner shape. Thus, when the eye is at rest, the lens is thin and has a long focal length and is adapted for seeing distant objects. When a nearby object is to be observed, the ciliary muscles running round the ciliary body contract and so reduce the diameter of the latter. The ciliary body holds the suspensory ligament which pulls on the lens, so any reduction in its diameter reduces the tension in the suspensory ligament, and allows the lens to shrink and become thicker. A thicker lens has a shorter focal length, and light from a close object can be brought to a focus. Relaxation of the ciliary muscles allows the fluid pressure acting on the sclerotic to pull the lens back to its thin shape.
Control of light intensity. When the circular muscles of the iris contract, the size of the pupil is reduced and less light is admitted. Contraction of the radial muscles widens the pupil, so admitting more light. This is a reflex action set off by changes in the intensity of light. In poor light the pupils are wide open; in bright light the pupils are contracted. In this way the retina is protected from damage by light of high intensity, and in poor light the wider aperture of the pupil helps to increase the brightness of the image.
Colour vision. The mechanism by which we appreciate colour is not yet fully understood. One of the most straightforward theories that fits many of the observed facts suggests that there are three types of cone in the retina. They are sensitive to red, blue and green light respectively. The cones are stimulated maximally by their own particular wavelength of light so that, according to the kind of cell and the numbers which are stimulated, the brain receives an impression of colour. Among the mammals only the primates, e.g. the lemurs, apes, monkeys and man, can appreciate colour, The others see only black, white and shades of grey.
Stereoscopic vision. Each eye forms its own image of an object under observation, so that two sets of impulses are sent to the brain. Normally the brain correlates these so that we gain a single impression of the object. Since each eye "sees" a slightly different aspect of the same object the combination of these two images produces the sensation of solidity and the three-dimensional properties of the object.
If the eyes are not aligned normally., or if the centres of the brain dealing with sight impressions are dulled by alcohol, for example, the two sensory impressions from the eyes are ho properly correlated and we "see double"
Judgment of distance. For the eyes to focus an image from a nearby object they must be turned slightly inwards, directed towards the object. The eye muscles which control this movement have, in their tissues, sensory receptors which respond to the stretching of the muscles. Impulses reaching the brain from these receptors indicate the extent to which the eyes are converging and so give an impression of the distance of the object.
The stereoscopic vision described above also helps one to judge distance. It is very difficult to estimate distance accurately using only one eye.
Animals with their eyes set in the front of their heads and directed forwards have stereoscopic vision. This occurs chiefly in animals of prey and is an advantage in judging the distance of their prey before they leap or dive. Examples are lions and tigers among the mammals, hawks, owis and gannets among the birds, and pike among the fish. The judgment of distance in the apes is probably associated with their tree-dwelling habits.
Most of the animals with eyes at the sides of their heads can judge distance only by the apparent size of objects and by parallax. Parallax is the name given to the apparent movement of nearby objects against a background of distant objects when the head is turned from side to side. In most birds the overlapping fields of vision of the two eyes could give stereoScopic vision within an angle of 6-10 from the head
Animals with their eyes in the sides of their heads can usually see nearly all round, including objects directly behind them. The eyes of most mammals seem particularly sensitive to movement. These last two factors favour the rapid escape of animals such as deer which are likely to be preyed upon by others.
Animals with eyes facing forward have a more limited field of vision, but even a man is aware of objects within an angle of about 200°, although only those included in an angle of 2° from the eye will form an image on the fovea and so be observed accurately. This is considerably less than most people imagine and means, for example, that only about two letters in any word on this page can be studied in detail.