This section is from the book "Health", by W. H. Coefield.
Let us now consider the organs by which the brain and spinal cord are put into communication with the outer world, which receive impressions from the outer world (impressions which are, if you like, caused by contact of some kind or other with the outer world), and from which those stimuli are conveyed, by means of nerves, to the great nervous centres, producing what we call sensations.
But before speaking of the organs of the special senses, I want to draw your attention to the fact, that there are certain general sensations that have no particular locality; for instance, we have the sensations of hunger, thirst, restlessness, fatigue, and a number of other sensations which depend on states of the nervous system, sensations that do not belong to any particular part of the body, general sensations. Then there is the so-called muscular sense, which is localised in a special apparatus. People who are accustomed to deal with a particular kind of article are able, by taking up a certain quantity of it, to say very nearly its weight. That is due to a sense which has been called the muscular sense. I may make the matter simpler in this way. Suppose I put my hand on the table, and put this sheet of paper on it, 1 can feel the paper on it; that is due to one of the special senses ; but suppose I put on this piece of paper a weight, now I not only feel that there is something on my fingers, but I feel that there is something I have to resist, to hold up; and this sensation is due to the muscular sense, by means of which we .are able to judge of weights.
We will now pass on to the five special senses.
There is one of these special senses, and only one, which is not confined to the head. The sense of touch is possessed by the skin all over the body, so you see that the skin is not merely a great excreting organ, nor merely a covering which protects the softer organs beneath it, but is an important organ of special sense.
Now, in the skin, in the papillae of the skin, the elevations that are found on the surface of the true skin, underneath the scarf-skin or epidermis, the ends of the sensory fibres of the spinal nerves are found; these sensory fibres end in various ways in the papillae; and in the parts which are specially endowed with what we call tactile sensibility (as, for instance, the tips of the fingers), they end in little spindle-shaped bodies which are called tactile corpuscles.
I want you to see how this apparatus acts. When we touch an object we touch it with'part of our skin, and not with the ends of the sensory nerve fibres which belong to that part of the skin.
That is an exceedingly important thing to bear in mind; we touch an object with the epidermis, or scarf-skin that is in contact with the papillae beneath, in which these sensory fibres end When we touch an object the extremities of the 'nerves are not brought into contact with the object; and we shall find that this is true of all the organs of special sense; that the stimulus from the external world is not applied directly to the nerve itself, but to some modification of the epidermis or epithelium. If we lay bare some part of the skin, and the extremities of the sensory fibres of the nerves touch the objects, we do not get in our brain the sensation of touch, but that of pain. If we irritate in any way directly the sensory fibres of the nerves, we do not get the special sensation that these nerves ordinarily convey, but we get a sensation of pain.
Those nerves that belong to the sense of touch also convey other sensations of a somewhat different char-acter-the sensations of heat, cold, etc.; and the same thing is true of these also, if we apply the exposed ends of the sensory fibres of the nerves to heat or cold, we do not get the sensation of heat or cold, but the sensation of extreme pain; and the sensation is precisely the same whether we put them against an extremely hot or an extremely cold object Persons who touch an extremely cold object, and do not know what they are touching, think it is red hot.
Now, the sense of touch is not equally developed all over the different parts of the body; it is extremely developed, as you would expect, at the ends of the fingers, where the tactile corpuscles are very numerous. If you take a pair of compasses, and put their points a tenth or a twelfth of an inch apart, and touch the tip of your finger with them, you will feel the points of these compasses as two. If, on the other hand, you put them against the cheek, with the legs of the compasses half-an-inch apart, you will find that the two points feel like one, because the sense of touch in the skin of the cheek is developed to a much less extent than it is at the tips of the fingers. Similarly, if you put the compasses on the skin of the back of a person, with the points as much as 2\ inches apart, even then the two points cannot be distinguished from one another; so that, you see, the sense of touch is not equally developed all over the body. Although the skin of the body is an organ of special sense, the organ of touch, and although that sense is developed all over the body, it seems as if the sensory nerve fibres in different parts of the skin do duty for a much larger area of the skin in one part than in another.
Let us pass on from the sense of touch to the two senses that are very nearly akin to it-the senses of taste and of smell The sense of taste is, indeed, a very slight modification of the sense of touch. On the surface of the tongue, the special organ of this sense, there are a large number of papillae of three different kinds, many of them so large that you can see them perfectly well by putting your tongue out before a looking-glass. In these papillae, just as in the papillae of the true skin all over the body, the fibres of the two sensory nerves that belong to the special sense of taste' end; and they are not exposed on the surface of the mucous membrane covering the tongue ; their ends are in these papillae, which are covered by the epithelium, the fine membrane which covers all the internal organs of the body that communicate with the external air. They are covered with the epithelium just as the papillae of the true skin are covered with the epidermis or scarf-skin; and so, the objects that are tasted stimulate these nerves to convey certain sensations to the brain, which the brain interprets as taste; these objects do not touch the extremities of the nerves.