Sensory organ

“ As the sensory and motor systems became more complicated, the sensory portion of a cell would become separated by an increasing interval from the muscular part of a cell, and the two parts of a cell would only be connected by a long protoplasmic process. When such a condition as that was reached, the sensory portion of the cell would be called a ganglion-cell or terminal sensory organ, the connecting process a nerve, and the contractile portion of the cell a muscle-cell. ”

“ So long as the eye forms an integral part of the organism, every stimulus acting on the eye necessarily acts on the organism, and every reaction of the organ is necessarily conditioned by the state of the organism. Further, every stimulation of a sensory nerve necessarily affects the general sensorium, since the whole nervous system is structurally continuous and functionally co-operant. ”

“ The structure of the other sense organs {256}indicates their adaptation to a tactile function; in each case a nerve traverses the axis of the organ, and the nerve-fibrils terminate in sensory cells. Very probably the palps have a certain power of testing the food—a combination of the senses of taste and smell. ”

“ The "mind" is not, so to speak, upon the other side of the epithelium. On the contrary, the inner ends of the olfactory cells are connected with nerve-fibers, and these nerve-fibers, passing into the cavity of the skull, at length end in a part of the brain, the olfactory sensorium. It is certain that the integrity of each, and the physical interconnection of all these three structures, the epithelium of the sensory organ, the nerve-fibers, and the sensorium, are essential conditions of ordinary sensation. That is to say, the air in the olfactory chambers may be charged with particles of musk ”

“ The air-bladder, which is partially encased in a bony capsule, may be so reduced as to lose its hydrostatic functions and becomes transformed into a sensory organ, its outer exposed surface being connected with the skin by a meatus between the bands of muscle, and conveying the thermo-barometrical impressions to the auditory nerves ”

“ How is consciousness of these sensations brought about? No doubt, in a manner similar to that of the mediation of such sensations as odors and tones: through the stimulation of sensory neurons and the propagation of nervous processes toward the motor points of the body. The place of stimulation must be somewhere in our organs of nutrition, and thus these organs must be regarded also as a kind of sense organ. That the sensory function can be attributed to an organ in addition to another function has been proved by the example of the skin, muscles, and joints. ”

“ There is a single small eye-spot, a single organ of smell, but no hearing organs. It seems probable that this extremely ill-developed condition of the sensory system is due in some measure to degeneration, and is not a primitive characteristic. ”

“ In these cases, touching the mucous surface caused pain, and not a sensation of touch.
Organs of Touch.—Comparative Sketch.—The organs of touch present many varieties of form, from a simple filament of sensitive protoplasm to a highly complex end-organ connected with the commencement of a sensory nerve-fibre. The bodies of the lowest organisms are formed of contractile protoplasm, and mechanical con tact with any resisting substances causes a change of form. Here is the simplest kind of touch a response on the part of any portion of the surface of the body to a mechanical stimulus. ”

“ For illustration, it is perfectly evident that we can not understand what is taking place in an organ for receiving sensory impressions without first understanding its mechanism and the nature of the connections between it and the central part of the nervous system. ”

“ Just as men and animals in the world are only able to maintain their existence and constantly adapt themselves to changing circumstances by aid of the peripheral endings of their sensory nerves, so every organ, indeed every cell of every organ, can only maintain its place in the animal microcosm, and adapt itself to the activity of innumerable associates, as well as to the general life of the whole, by virtue of the fact that the peripheral end apparatus of its centripetal nerves possesses a specific excitability. ”

“ There is only one thing that a sense organ always and necessarily contains, and that is the termination of a sensory nerve. Without that, the sense organ, being isolated, would have no effect on the brain or muscles or any other {189} part of the body, and would be entirely useless. The axons of the sensory nerve divide into fine branches in the sense organ, and thus are more easily aroused by the stimulus. ”

“ The percept then is sensory and is constituted by primary sensory elements, or primary sensations, and by secondary sensory elements, or secondary sensations.
The character of the secondary sensory elements stands out clear and independent in the phenomena of synaesthesia, of secondary sensations. In the phenomena of synaesthesia we have a sensation of one sense organ followed, without an intermediary direct stimulation, by a sensation coming from another sense organ. ”

“ There is no instance in the human body of a direct connection between any sense organ and any muscle or gland. The nerve path from sense organ to muscle always leads through a nerve center. One {27} nerve, called the sensory nerve, runs from the sense organ to the nerve center, and another, the motor nerve, runs from the center to the muscle ”

“ In other words the excitation of any sensory organs sets up a series of sensory impulses which are transmitted along the sensory nerve fibres to the brain, where they are referred to the cerebellum or filing case, locating a set of associated impulses which travel outward from the motor area of the brain and result in the actions, or series of actions, which are necessary to produce a word. ”

“ It seems in place for me, therefore, to give in the briefest possible way a general idea of their structure, and to pass then at once to the points in which my work has led me to different conclusions from those of Claus and Schewiakoff. In brief, then, the knob of the sensory club consists of a thick, complex mass of nerve fibres, more or less imbedded in which lie the special sensory organs, surrounding the ampulla-like terminal enlargement of the canal. The surface between the special organs is covered with less specialized sensory epithelium. The sensory organs are seven in number. ”

“ If the nervous system were a mere bundle of nerve fibres extending between sensory organs and muscles, every muscular contraction would require the stimulation of that special point of the surface on which the appropriate sensory nerve ended. The contraction of several muscles at the same time, that is, the combination of movements towards one end, would be possible only if the appropriate nerves were severally stimulated in the proper order, and every movement would be the direct result of external changes. ”

“ Muscular sensations are really excited by irritation of sensory nerves passing from the muscles themselves. There are specialized spindle-like bodies in many muscles, and there are organs connected with tendons which are regarded as sensory organs by which pressures are communicated to sensory nerve-filaments. ”

“ And in the process of specialization the sensorium itself is subdivided into higher and lower nerve centers—centers of conscious thought and automatic transfer of impulse into motion. This transfer indicates the real nature of all forms of nerve action. All are processes of transfer of sensation into movement. The sensorium or brain has no knowedge except such as comes to it from the sense organs through the ingoing or sensory nerves. It has no power to act save by its control of the muscles through the outgoing or motor nerves. ”

“ But we must go farther, and ask whether the skin-nerve is ever motor, and whether the muscle-nerve is ever sensory? To answer this, we must first settle one or two points of physiology and anatomy. A nerve is sensory because it stimulates the Sensibility of its Centre, and not because its termination is in the skin. It is not the nerve which is sensitive, bat the centre. ”

“ Thus this most delicate and difficult course of investigation supports the view that much of the activity of the animal organism is provided for by an established connection between nerve cells respectively presenting the terminus in the brain for a sensory nerve, and the starting point for a motor nerve, or point of communication with such a nerve. From this conclusion, it follows that a very large amount of the activity which we witness in the case of animals, often attributed to instinct, or even to voluntary determination, is to be described as sensori-motor activity. ”

“ The body is cut across the middle to show the relative position of organs in transverse section. In the posterior half the alimentary canal has been bisected and removed from the left side, to exhibit the deeply placed nervous sheath (nn) and the male reproductive organs. br, Brain; dp, "diaphragm"; e, cerebral group of eyes; et, tentacular eye-group; gr, marginal groove; gm, true mouth; lg, lateral gut-branch; ln, longitudinal nerve stem; m, external mouth; mg, mg', main-gut, whole, and bisected; n, sensory nerve supplying the eyes; nn, nervous network lying on the ventral musculature ”

“ The arrangement of the nervous system is very much like that of a city telephone system. The sensory nerve cells all lead into a part of the nervous system to which is given the name of the Central Nervous System, just as the telephone wires all lead into a “central exchange.” ”

“ From what has been said, it is plain that a sensory nerve is one which conveys a sensory impulse from the peripheral or outer part of a nerve to the spinal cord or brain, and which is, therefore, termed afferent; and that a motor nerve is one which transmits an impulse from the nerve centre, or is efferent. So difference in structure, or in chemical or physical composition, can be discerned between the afferent and the efferent nerves. A certain period of time is required for the transmission of all impulses. ”

“ The sense organ, the nerve, and the sensorium, taken together, constitute the sensiferous apparatus. They make up the thickness of the wall between the mind, as represented by the sensation “muskiness,” and the object, as represented by the particle of musk in contact with the olfactory epithelium. ”

“ The complex nature of our higher nervous centres, which is required to bring about the proper adjustments of the infinite variety of our present sensory experiences to appropriate co-ordination with a nearly equal variety of possible muscular combinations, is too common a subject of modern knowledge to need more than mention here. This vast central co-ordination of sensory paths to motor paths has itself become a special function of our neural organism of major importance. ”

“ The eyes of these lowly creatures are not visible, and consist of single special cells scattered among the epidermal cells of the skin, and connected by means of a sensory nerve fibre with a little bunch of nervous matter in the body. ”

“ Those which carry the orders of the mind to the different organs are called the motory nerves; while those which bring back impressions which they receive are styled sensory nerves. If the sensory nerve leading to any part be cut, all sensation in that spot will be lost, while motion will remain; if the motory nerve be cut, all motion will be destroyed, while sensation will exist as before. ”

“ For, in the locomotive muscles, in the retina of the eye, and other organs of senses, no pain occurs from the absence of stimulus, nor any great accumulation of sensorial power beyond their natural quantity, since these organs have not been used to a perpetual supply of it. There is indeed a greater accumulation occurs in the organ of vision after its quiescence, because it is subject to more constant stimulus. ”

“ We must remember that the nerve junctions are exceedingly delicate things, consisting, as they do, of the interweaving of the almost inconceivably tiny featherings at the tips of sensory or connecting nerve cells with the equally tiny featherlike branches from the cell bodies of connecting or motor nerve cells. It is likely, also, that on account of their delicacy they are easily affected by the waste products that may be circulating about in the blood stream from the active muscles. ”

“ Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs—the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate ”

“ The sensory nerve cell, the recipient of the stimulation, like the muscle cell, the reagent to stimuli, has evolved from the primitive cell by greater and greater differentiation, both of structure and function. In the crustaceans, invertebrates, and lower vertebrates where motor reactions to stimuli are more or less complex and varied, the sensory aspect of the moment is probably correspondingly complicated,―organic connections are present giving rise to secondary sensory elements, constituting the material of perceptual life. ”

“ To come to closer quarters with the relationship which holds good between the external and internal elements, it appears that, when the stimulus evokes or intensifies the want or need, this is probably effected by efferent waves which call the organs or parts into tonic action, of which the animal becomes conscious through the afferent messages which come in from them to the sensory centres; in much the same way as the whiff from the kitchen takes effect on the salivary and other glands, and throws the organs of digestion into a felt preparedness for the fulfilment of their functions. ”