It has been pointed out that the afferent or sensory nerves receive impressions at the surface of the body, and carry the impulses to nerve cells in the brain, where they give rise to sensation. The afferent nerves are the means by which the mind becomes acquainted with occurrences in the outer world, and also the channels along which the impulses pass to reflex nerve centres whence they are sent to different parts, without causing any sensation in the nerve cells of the sensorium.
The ordinary sensory nerves are in such relationship to the surface that they are affected by slight, mechanical and thermal stimuli, which throw them into activity and send impulses to the brain. But we are capable of appreciating many other impressions besides those excited by the ordinary sensory nerves. We feel the character of a surface by touch, and we distinguish between degrees of heat and cold, when the difference is far too slight to act as a direct nerve stimulus. We can appreciate light, of which no degree of intensity is capable of exciting a nerve fibre to its active state, or of stimulating an ordinary nerve cell in the least degree. We recognize the delicate air vibrations called sound, which would have no effect on an ordinary nerve ending. We can also distinguish several tastes; and, finally, we are conscious of the presence of incomprehensibly small quantities of subtle odors floating in the air. When the amount of the substance is too small to be recognized even by spectrum analysis, which detects extraordinarily minute quantities, Ave can perceive an odor by our olfactory organs.
There must, then, be a special apparatus for the reception of each of these impressions, in order that the nervous system may be accessible to such slender influences. In fact, special mechanisms must exist by means of which the quality of a surface, heat, light, sound, taste and odor are enabled to act as nerve stimuli. These nerve terminals are known as the special sense organs, the physiology of which is at the same time the most difficult and most interesting branch of study in Biological Science.
The nerve fibres which carry the impulses from the various organs of special sense do not differ from other nervous cords, so far as their structure and capabilities are concerned. The special peripheral end organs are connected with nerve cells in the brain, the sole duty of which is to receive impulses from a special sense organ and distribute them to the brain centres, so that they may cause a special sensation. By whatever means a nerve trunk from a special sense organ be stimulated, its impulse excites the special sensation usually arising from stimulation of the special organ to which it belongs. Thus, electric stimulation of nerves in the tongue causes a certain taste; mechanical or other stimulation of the optic nerve trunk gives rise to the sensation of flashes of light, and a distinct odor may be caused by the presence of a bony growth, pressing upon the olfactory nerve.
The capability of the nerve centres connected with the nerves of special sense to give rise to a special sensation, is called their specific energy. And the special influence, light, sound, etc., which alone suffices to excite the special peripheral terminal, and which the given terminal alone can convert into a nerve stimulus, may be called its specific or adequate stimulus.
Although we habitually think of the sensation as if coming from the surface where the stimulus is applied, it is really only developed in the centres in the brain. Thus we say we feel with our skin, hear with our ears, and see with our eyes, etc., whereas these are only the parts from which the nerve impulses, giving rise to the specific energy, pass to the feeling, hearing or seeing regions of our cerebral cortex. This is obvious from what has been already said of the nerve fibres of the special sense organs. If the nerve be cut, no sensation is excited, though adequate stimulus reach the organ of special sense; and if a stimulus be applied to the nerve trunk, a similar sensation is produced, as if the specific stimulation had operated on the special nerve terminal from which these fibres habitually carried impulses. This peripheral localization of cutaneous sensations is really accomplished in the mind, just as, by a mental act of a different character, the impressions communicated by the eye are projected into the space about us in our thoughts, instead of being referred to the retina, or thought of as being produced in the eye itself. This power of the sensorium to localize impressions to certain points of the skin, and to project into space the stimulation caused by the light reflected from distant objects, so as to get a distinct and accurate idea of their position, is the result of experience and habit, which teach each individual that when a certain sensation is produced, it means the stimulation of a certain point of the skin, and that the objects we see are not in our eyes, where the impulses start, but at some distance from us. We learn this from a long series of unconscious experiments carried on in our early youth by movements of the eyes with co-operation of the hands. Even the sensations which arise in the various centres of the sensorium, as the result of internal or central excitations, are, from habit, attributed to external influences, and thus we have various hallucinations and delusions, such as seeing objects or hearing sounds which may only depend on the excitation of certain groups of cells in the cortex of the brain.
The sensations produced in our nerve centres as the result of the afferent impulses coming from our special sense organs give rise to a form of knowledge called perception. Each perception helps to make up our knowledge of the outer world and of ourselves. Without this power of perception we could have no notion of our own existence and no ideas of our surroundings; in fact, we should be cut off from all sources of knowledge and be idiots by deprivation of all intelligence from without.
A complete special sense apparatus may. be said to be made up of the following parts:
1. A special nerve ending, only capable of being excited by a special adequate stimulus.
2. An afferent nerve to conduct the impulses from the special end organ to the nerve centre.
3. Central nerve cells, capable by specific energy of translating the nerve impulse into a sensation, which is commonly referred to some local point of the periphery.
4. Associated nerve centres, capable of perceiving the sensations, forming notions thereon, and drawing conclusions from the present and past perceptions, as to the intensity, position, quality, etc., of the external influence.