This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Changes in many parts of our bodies are accompanied or followed by states of consciousness which we call sensations. All such parts (sensitive parts) are in connection, direct or indirect, with the brain by sensory nerve-fibres. Since all feeling is lost in any region of the body when this connecting path is severed, it is clear that all sensations, whatever their primary exciting cause, are finally dependent on conditions of the brain. Since all nerves lie within the body as circumscribed by the skin, one might be inclined to suppose that the cause of all sensations would appear to be within our bodies themselves; that the thing felt would be recognized as a modification of some portion of the person feeling. This is the case with regard to many sensations: a headache, toothache, or earache gives us no idea of any external object; it merely suggests to each one a particular state of a sensitive portion of himself. As regards many sensations this is not so; they suggest to us external causes, to properties of which, and not to states of our bodies, we ascribe them; and so they lead us to the conception of an external universe in which we live. A knife laid on the skin produces changes in it which lead us to think not of a state of the skin, but of properties of some object outside the skin; we believe we feel a cold heavy hard thing which is not the skin. Wo have, however, no sensory nerves going into the knife and informing us directly of its condition; what we really feel are the modifications of the body produced by the knife, although we irresistibly think of them as properties of the knife—of some object that is no part of the body. Let now the knife cut through the skin; we feel no more knife, but experience pain, which we think of as a condition of ourselves. We do not say the knife is painful, but that the finger is, and yet we have, so far as sensation goes, as much reason to call the knife painful as cold. Applied one way it produced local changes in the skin arousing a sensation of cold, and in another local changes causing a sensation of pain. Nevertheless in the one case we speak of the cold as being in the knife, and in the other of the pain as being in the finger.
With what are all sensitive parts of the body in connection ? By What nerve-fibres? How do we know that all sensations finally depend on the brain?
Why might we suppose that the causes of all sensations would seem to lie within the body? Name sensations merely suggesting to us a state of the body itself. What do some other sensations suggest to us? Illustrate.
Sensitive parts, such as the surface of the skin, through which we get, or believe we get, information about outer things, are of far more intellectual value to us than sensitive parts, such as the subcutaneous tissue into which the knife may cut, which only give us sensations referred to conditions of our own bodies. The former are called Organs of Special Sense; the latter are parts endowed with Common Sensation.
Common Sensations are quite numerous; for example, pain, hunger, nausea, thirst, satiety, and fatigue.
What is meant by " organs of some special sense"? What by parts endowed with common sensation? Name some common sensations.